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MDT ‘Nanotechnologies’

Nanomaterials and Nanotechnologies: Colloidal and Interfacial Aspects

Victoria Dutschk

University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands

[email protected]

Keywords: Nanomaterials; nanomaterial classes; preparation strategies; ‘smart’ emulsions and foams; encapsulation techniques; chemical toxicity

Abstract

The review on nanomaterials and nanotechnologies is aimed at serving as Training Materials for both – PhD candidates interested in the topic and employees of small and medium enterprises (SMEs). The review consists of 5 main parts – a brief introduction into nanotechnology and some advanced technologies based on, classification of nanomaterials, preparation strategies, the basics of surface forces acting between nanoparticles as well as cross cutting issues. Since nanoobjects and the corresponding nanosystems have been under consideration in colloid and interface science for many decades, the focus is put on nanomaterials as a part of colloids in the length scale of approximately 1–100 nanometer range. Besides physico-chemical properties and characterization of nanomaterials, some attention is put on potential hot spots in the nanomaterial life such as the highest risks areas and their intrinsic chemical toxicity if any. In the Section Nanotechnology: Cross cutting issues, nanomaterials carriers like ‘smart’ emulsions, foams, some sophisticated encapsulation techniques as well as ‘green’ manufacturing technologies are highlighted. The last Section provides some definitions and terminology based on the IUPAC Manual of Symbols and Terminology for Physico-Chemical Units.

MDT ‘Nanotechnologies’ Nanomaterials and Nanotechnologies: Colloidal and Interfacial Aspects

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Contents 1 Nanotechnology trends and applications to 2020

1.1 Nanotechnology: A brief introduction 1.2 Advanced technologies based on nanotechnology

2 Nanomaterials 2.1 Nano-oxide materials 2.2 Metallic nanoparticles

2.3 Semicondutor nanoparticls 2.4 Carbon nanomaterials 2.5 Macromolecules 2.6 Self-assembly 2.7 Nanocomposite assemblies

3 Preparation strategies and interaction forces

3.1 Preparation strategies 3.2 Van der Waals interaction 3.3 Electrostatic forces 3.4 Acid-base interaction 3.5 Steric interaction 3.6 Nanobiointeractions

4 Nanotechnology: Cross cutting issues

4.1 ‘Smart’ emulsions 4.2 ‘Smart’ foams 4.3 Encapsulation techniques 4.4 Combination of ‘smart’ colloid materials

5 Definitions and Terminology 6 References

1 NANOTECHNOLOGY TRENDS AND APPLICATIONS TO 2020

Nanotechnology, which is taken to mean research and development in nanometer scale science and related technologies is a burgeoning field worldwide. Most working definitions of nanoscience and nanotechnologies define the nanoscale in the size range of 1 to 100 nanometers. Nanotechnology has undergone an exponential increase in research and investment over the last couple of decade, with multiple potential applications proposed, in the information technology (IT), industry and medicine. Sales of products incorporating emerging nanotechnology are ecpected to rise less than 0.1% of global manufacturing output


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today, to 15% in 2014, approaching the size of the IT and telecom industries combined and exceeding biotechnology revenues by 10 times [1]. It is clear that as applications and uses of nanotechnology increase, so too will the exposure to nanoparticles and nanomaterials.

The worldwide interest in nanotechnology is based on the belief that the ability to understand and affect atomic and molecular interactions at the nanoscale is both prerequisite and an enabler for a host of technological capabilities from smart, multifunctional materials to designer drugs and new generations of information and communication systems [2]. Based on the developments in materials science, engineering and manufacturing, the following foresight suggest may be feasible in 2020 [2]: (i) fabrics that incorporate power sources, electronics and optical fibres; (ii) cloths that respond to external stimuli, such as temperature changes or the presence of specific substances; (iii) widespread adoption of ‘green’ manufacturing methods that substantially reduce the introduction of hazardous materials into commerce and the volume of hazardous waste streams; (iv) nanostructured coatings and composite materials with greatly enhanced strength, toughness, wear and corrosion resistance; (v) organic electronics for increased brightness of lightening systems and displays; (vi) water purification and decontamination systems based on nanostructured, activated membranes an filters; (vii) engineered multifunctional tissues grown in vivo from biodegradable scaffolds.

1.1 Nanotechnology: A brief Introduction The term ‘nanotechnology’ was introduced by Norio Taniguchi in 1974 [3]. In [4], Eric Drexler charted a direction for the future of nanotechnology research and development. His publication focused largely on one aspect of nanotechnology, molecular assembly, which could enable manufacturing and production through the bottom-up assembly of consumer goods and products, one atom at a time. This view expanded on the vision of Richard Feynman’s famous 1959 lecture at California Institute of Technology ‘There’s Plenty of Room at the Bottom’ [5].

Currently, the following criteria for defining nanotechnology are used:

- Research and technology development at the atomic, molecular or macromolecular levels, in the length scale of approximately 1–100 nanometer range;

- Creating and using structures, devices and systems that have novel properties and functions because of their small and/or intermediate size

- Ability to control or manipulate on the atomic scale.

Nanotechnology involves science and engineering of matter at the nanoscale where the properties may change with size or new properties may emerge.

Scientific advances in microscopy and related fields allow us to routinely observe and manipulate materials on the atomic or molecular scale. Nanotechnology, broadly viewed, has had a profound impact in virtually every scientific discipline in the physical, chemical, and biological sciences. Given the rapid growth of the discipline and the impact nanotechnology


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has already had in both science and industry. An increasing number of nanotechnology-enabled products have begun to appear in commercial goods. For example, sunscreens are using nanoscale particles to enhance protection from ultraviolet radiation, and nanoscale coatings for glass lenses and textiles are being used to improve wear resistance and in some cases provide added functionality. Other commercial sectors, such as computer-integrated circuits and catalysis for chemical processing, have been using nanotechnology for many years.

The transition of new and emerging nano-enabled technologies from the laboratory to commercial products is dependent on numerous factors, including

- integrating the device into products with characterized and reproducible properties - costs - scaling up the manufacturing or fabrication for commercial production - development of related technologies - market forces - consumer acceptance of nano-enabled technologies.

All of these factors will determine whether nanotechnologies will be able to move from the laboratory to the commercial market.

1.2 Advanced technologies based on nanotechnology

The following review is mainly based on Eric Landree’s studies published in [2].

One area in which nanotechnology is uniquely positioned to enable new capabilities is sensor technology. New methods of sensing and detection have enabled unprecedented levels of sensitivity (minimum detection limit) and selectivity (ability to detect specific chemicals or processes), as well as the ability to detect processes or events that were previously undetectable. These sensor devices are based on various emerging nano-enabled technologies - for example, functionalized metallic nanoparticles, functionalized nanowires and nanotubes, macroscopic materials with nanoscale features or surface treatments, and nanostructured mechanical systems. All of these techniques rely on measurable changes in the fundamental properties of the material or material system as a result of interactions that are detectable by virtue of their nanoscale properties.

Recent advances in nanotechnology suggest the potential for improvements not only in overall battery performance but also in an expanded range of materials that may be useful for battery and solar cell applications. Nanotechnology-enable power is another area of potential technologies. Scientists are actively pursuing nanotechnology and nanocomposites to improve the performance of battery electrodes. Much of the focus of this research is the integration of nanomaterials into conventional battery architectures. In addition, over the past decade many microelectromechanical (MEMS) devices have been developed and commercialized into numerous applications. MEMS are typically fabricated using semiconductor fabrication techniques (e.g., lithography, etching) that produce devices with individual features and


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components that are on the order of micrometers (e.g., one-millionth of a meter). More recently, a growing number of nanoelectromechanical systems (NEMS) have been envisioned and developed with the objective of further miniaturization. NEMS are electromechanical devices with features or components on the order of nanometers (e.g., one-billionth of a meter, or one-thousandth of a micron).

Like sensors, nanotechnology-enabled electronics seem to have the potential for many advances in the coming years. However, the path of integrating new nanoscience discoveries into commercial applications, particularly in the case of integrated circuit and chip design, is extremely challenging. Unlike many other areas in which nanotechnology is only just beginning to play a role, nanotechnology and nanotechnology-related processes have been enabling technologies within integrated processors since the 1990s. However, all of these technologies are at the very early stages of development.

Much research is still needed in producing these devices in large quantities and at the extremely tight tolerances required for the semiconductor industry.

Advances in nanotechnology-enabled sensors will have a large impact in the area of nanobiotechnology. Many of the advances in sensing biological systems (e.g., proteins and micro-organisms) and biomolecular phenomena (e.g., DNA identification and biological process) are enabled by improvements in nanoscale sensing. The sensors we are beginning to construct are on the order of biological processes one is interested in investigating, enabling new tests and experiments that were previously difficult, time consuming, or nonexistent. Research in nanobiotechnology not only has the potential to improve biological detection methods but also other aspects of biology, including drug discovery, drug delivery, surgical methods, biocompatibility, diagnosis, implants, and even prosthetics. Other active areas of nanobiotechnology research include spectroscopic tools for characterizing biological processes, biomimetic artificial nanostructures, materials for biomolecular sorting and sensing, and functionalized nanostructures for controlled drug delivery.

Scientists have long sought the ability to recreate the manufacturing power of nature in the laboratory - that is, the ability to create something from the ‘bottom-up’, one atom at a time. Conventional manufacturing of many nanotechnology-enabled products (e.g., computer chips, MEMS devices) starts with a macroscopic amount of material from which small amounts of material are removed through cutting, etching, or grinding to produce nanoscale components, referred to as ‘top-down’ manufacturing. ‘Bottom-up’ manufacturing, also referred to as molecular manufacturing or molecular assembly, may be considered one of the ‘holy grails’ of nanotechnology. The growth and manufacturing of raw nanostructured materials, such as carbon nanotubes, nanoparticles, and more complex nanostructures, is a growing area for both academia and industry. More companies are beginning to develop and market nanostructured materials. However, researchers and engineers are still trying to improve the reproducibility and quality of individual nanostructured materials while scaling up manufacturing to increase the overall yield.


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3 NANOMATERIALS

Lyklema wrote in Fundamentals of Interface and Colloid Science [6] that ‘… a colloid is an entity, having at least in one direction a dimension between 1 nm and 1000 nm…’. The definition adopted by IUPAC, the International Union of Pure and Applied Chemistry, states that the size of colloidal particles is approximately between 1 and 1000 nm [7]. Nanocolloidal systems, also known as nanodispersed systems cannot be obtained by ‘top-down’ technologies, but by ‘bottom-up’ technology only. In colloidal systems, the surface-to-volume ratio for particles is always very high, and, therefore, this science is referred to colloid and interface science. Nanoobjects and the corresponding nanosystems have been under consideration in colloid and interface science for many decades:

- thin liquid films (one-dimensional nanosystems) - thin rods/filaments (two-dimensional nanosystems) - micellar and polymer solutions - colloidal sols.

Dispersions of metallic nanoparticles as three-dimensional nanosystems have been known and studied for approximately 150 years [8]. Nanoscience is the inborn property of colloid and interface science [9]. However besides claiming, on the basis of formal arguments that nanoscience is part of colloid science, it must be also appreciated that over the past decades a number of novel nanostructures have been developed, which cannot be found in the classical colloidal literature. Several industries and, not forget, financial sponsors have therefore embraced terms such as nanoscience, nanotechnology and others to suggest the impression of novelty [10].

Nanoparticles are small pieces of matter within the nanoscale range and constitute an important area of nanotechnology research and development. Kreuter [11] gave, for example, the following definition ‘Nanoparticles are colloidal particles ranging in size from 10 nm to 1000 nm, consisting of macromolecular materials in which the active principle is entrapped, encapsulated and/or adsorbed’. An obvious feature of nanoparticles is the extreme surface area-to-volume ratio that affects the physical and chemical behaviour of such systems. In [12], molecular dynamics simulations of titanium dioxide nanoparticles in the three commonly occurring phases – anatase, brookite, and rutile – are reported. The formation of a surface on these nanoparticles is important, as it is the main factor controlling the difference in behaviour between bulk and nanocrystalline titanium dioxide. The surface energies of finite size particles are different from the surface energies of planar surfaces, as the curvature of the finite size particles in addition to their shape has to be accounted for. In this work the surface energies as a function of phase and size were calculated. The surface energies of the nanoparticles were compared to the surface energies of planar surfaces. The surface energy increases as the diameter of the particles increases (see Figure 1) and approaches a maximum.


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Figure 1: Surface energies as a function of particle diameter at 300 K (redrawn from [12])

On this example it can be seen, that that the contribution of surface energy to the total energy of a nanoparticle is very significant while for large particles the contribution of the surface energy to the total energy is negligible. When the particles become larger the surface energy becomes less important. This is easily rationalized if we consider that the surface energy per surface unit does not drop significantly when size increases but that the contribution of surface energy to the overall energy becomes negligible when the particle size increases because the bulk of the particle dominates the overall energy. Fundamental is the identification of the physico-chemical properties of nanoparticles and establishment of links between such properties and their functional impacts.

According to the National Council on Nanotechnology, Rice University of Houston, USA [13], key findings about nanomaterials are summarized below:

- Subtle changes in structure, surface structure and composition of nanomaterals could dramatically affect electronic and chemical properties over the material’s life time. Many properties can be dramatically altered during nanomaterial synthesis and functionalization. It should be taken into account the dynamic nature of nanomaterials throughout their lifestyle.

- The best potential mechanisms to characterize nanomaterial properties t various stages of the life cycle are physical/chemical screens and select tests to determine chemical reactivity, surface charge, surface composition and solubility. In general, for the use of nanoparticles for further functionalization of materials, a set of screening tools is needed to correlate the functional properties of nanomaterials with their potential for pre-determined applications.

- For nanomaterials in a dry powder form, potential for exposure to high concentrations is greatest during the cleaning of synthetic reactors, bagging operations, surface

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functionalization and formulations areas of manufacturing. Nanomaterials bound in a liquid or in solid matrix have a lower potential to exposure than an unbound nanomaterial.

2.1 Nanomaterial classes

Smaller than microscale particles, yet larger than atoms and many molecules, nanoparticles occupy a transitional regime between classical and quantum physics, where physical and chemical properties are tuneable with changes in size, structure, composition, surface structure and surface composition. Nanoparticles diversity and tenability make it difficult to predict their behaviour.

Preliminary classes of nanoparticles are:

- Nano-oxide materials - Metallic nanomaterials - Semiconductor nanomaterials - Carbon nanomaterials - Macromolecules - Self-assembly.

2.1.1 Nano-oxide materials

- Titanium oxide - Zinc oxide - Cerium oxide - Iron oxide - Silicon oxide (silica) - Copper oxide - Zirconia - Alumina - Nickel oxide - Antimony pentoxide - Yttria - Hydroxyapatite - Barium sulphate - Calcium carbonate (paper mills, filler) - Pigments (manganese oxide) - Pigments in inks - Nano-clays

The properties affecting interactions between nanoparticles and other materials include size, shape, composition, structure, surface composition, chemical reactivity, solubility and surface charge of nanomaterial. While many of these properties can be characterized with existing experimental tools, the chemical reactivity and surface charge can be dramatically changed by


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small changes in composition or nanostructure occurring in synthesis, processing or by environmental conditions

Synthesis techniques include the following: combustion synthesis, plasma synthesis, wet-phase processing, chemical precipitation, sol-gel processing, mechanical and mechano-chemical processing, high-energy ball milling, chemical vapour deposition, laser ablation. Almost all applications are placed in a liquid or polymer with surfactants or coupling agents. The range of chemistries is quite broad. Common matrices include water, silicone, rubber, plastics, oils. The highest risk areas in the life of these nanomaterials are in the handling of powders: bagging and unbagging, maintaining pyrolysis reactors, cleaning bagging houses, accidental spilling.

Materials characterization should comprise the following as appropriate:

- chemical composition - aggregation/agglomeration state - number of particle per unit mass - physical form - median size and size distribution - surface area - surface charge - solubility/insolubility - state of dissolution - partition coefficient.

An example of the characterization of nano-silica using electrokinetic method is shown in Figure 2.

Figure 2: Surface charge of unmodified and modified nanosilica in terms of zeta potential [14]; see Section 3.3 Electrostatic forces

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2.1.2 Metallic nanoparticles

Common metallic nanomaterials and their applications are shown in Table 1.

Table 1: Common metallic nanomaterials and their applications

Nanomaterial Common applications Silver Antimicrobial uses Cobalt, nickel, iron Catalysts for carbon nanomaterial synthesis Platinum, palladium, rhodium Catalytic converters Gold Medical diagnostics Aluminium Propellant Copper Electronics Iron Metal doping

For metallic nanoparticles, the most important properties are electrochemical potential, surface area, surface texture, edge texture, size and size distribution. The most common synthesis technique for nanosilver and nanocatalysts is wet chemical precipitation. The steps after synthesis include functionalization, dispersion and formulation. Functionalization is done to enable dispersion, improve matrix compatibility, passivate the nanomaterial, reduce particle solubility, introduce reactive sites, minimize photocatalysis, improve biocompatibility. Common functional groups employed include: organic carboxylic acids, amines, phosphonates, mercaptans, organosilanes, siloxanes, inorganic oxide coatings. Dispersion mechanisms include: direct charging electrostatic techniques (MOH2+), silane treatment (M-OSiR), polymeric dispersal through steric interactions, surfactant bilayers. The latter if ionic can introduce particle charging.

The functionalized engineered nanomaterials are mixed with multiple materials depending on the application. The chemistry of formulations is quite complex. In most applications, the goal is to keep the particles in the formulation matrix. In respect to the nanotoxicity, nanosilver and nanocopper may have risk as ecotoxins in the disposal of waste.

2.1.3 Semiconductor nanomaterials

Fluorescent crystalline semiconductor nanoparticles, also known as QDs, are being developed for use in

- bio-labels and in vitro diagnostics (a possibility of bio-accumulation, i.e. critical nano-toxicity issue)

- optoelectronic applications as light-emitting diode (LED) displays - solar cells - Inks and paints for identification or brand protection.

Synthesis and processing of QDs generally involves the use of organic solvents, highly volatile metal precursors (e.g. dimethyl cadmium) and temperatures between 100-400°C. Under these conditions, explosion presents a possible hazard, though new processing techniques are beginning to utilize salt of metals rather than the highly volatile precursors.


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Current techniques generate significant volumes of waste solvent contaminated with heavy metals. Critical material properties are

- Chemical toxicity e.g. cadmium, mercury, selenium with inherent chemical toxicity - Energy levels i.e. a ground state (valence band) and an excited state (conduction

band), see Figure 3.

Figure 3: Differences between metal, semiconductor and insulator

In Figure 4, direct band gap transition is shown.

Figure 4: Direct band gap transition

As the size of the semiconductor nanoparticles decreases, the energy level in both bands become split and usually the separation between the conduction and valence band increases (band gap increase). For luminescent nanomaterials, e.g. ‘direct band gap’, the wavelength of light shifts to shorter wavelengths (higher energy) as the particle size decreases. In case of indirect band gap materials, they are not luminescent, but absorb energy above the band gap and create an electron (negative charge) and hole (positive charge). If the energy of electrons at or close to the surface is higher than the energy to excite oxygen, they could create some


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level of oxygen radicals. The work function of the material is the energy required to excite an electron from the Fermi level to vacuum, which depends on the dopant levels in the semiconductor.

2.1.4 Carbon nanomaterials

Common carbon nanomaterials include

- carbon nanotubes (single, double and multi-walled) - fullerenes - carbon nanofibers - graphene sheets - carbon black.

Even within these general categories, there is a great diversity among size, structure, physical aggregation forms and residual species associated with the carbon. CNTs and fullerenes are produced in relatively large quantities and have many applications. Carbon black is a useful reference material for which toxicology and epidemiology data are available. CNT production usually involves the reaction at high temperature of a source of carbon with a catalyst particle either (1) floating in a reaction fluid or gas, (2) in a porous micropaticle, or (3) on a surface. Fullerene production involves a carbon source reacted using either arc or combustion method. In many applications, nanoparticles are part of a composite material, and are strongly bound to the host polymer. On some materials such as single- and double-walled nanotubes, the carbon materials can stick tenaciously to each other to form large aggregates., and do not provide a ready source of nanoparticles on their own. In other applications like drug delivery, the carbon nanomaterials are treated to remain suspended in water and resist aggregation.

During manufacturing and synthesis of nanomaterials, processes considered able to generate high exposure included anything that disrupts the normal process, maintenance, cleaning, vacuuming and failure. CNT are produced in a multitude of different forms for subsequent use. Examples include powders of various size, pressed fibres, suspensions in fluids, dry coatings on surfaces. Waste disposal and unintended use are also considered to be potentially high-exposure scenarios. Because the CNT and fullerenes are made by combustion processes, contamination with carcinogenic polyaromatic hydrocarbons (PAH) is possible. CNT purification requires acids, and so acids waste containing trace amounts of CNT occurs.

Uses for CNTs and nanofibres are diverse, including high-performance, light-weight fibers, high-thermal-conductivity fibres, wires of low-loss electricity transmission, multifunction fibres and materials (enhanced polymers), imaging of diseases, treatment of diseases (ablation), scaffolds for biological applications, lithium ion batteries, fuel cells, coatings on films for electronic applications such as flat-panel displays, photovoltaic cells, touch panels, sensors, food packaging. Consideration of the wide range of uses, exposure via ingestion, inhalation and dermal absorption are conceivable.


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The intrinsic toxicity of carbon was recognized as being low, but factors that may enhance toxicity include particle size (small being more toxic) and contamination with other materials (metal contamination of CNT) and certain characteristics of site and shape. The following particles properties need to be characterized as a part of Environment, Health and Safety (EHS) process:

- particle surface area - redox activity - composition/contamination - solubility in water and organics - durability (biopersistence) - particle count - particle size distribution - defect density - length (aspect ratio) of CNT affecting inhalation, transport, filtering, toxicity - charge - degree of agglomeration - environmentally relevant characteristics.

In Figure 5, multi-walled CNTs interacting with polyether glycol (PEG) are shown. The contact angles between CNT and PEG were measured using scanning electron microscope (SEM) images. Figure 5: SEM images of MWCNTs interacting with PEG; the contact angles were measured to 25 ± 7° [15, 16]

2.1.5 Macromolecules

This class of nanomaterials is focused on materials generally engineered from organic molecules to have a precise size, shape and surface functionality. Such materials include dendrimers, dendrons and dendrigrafts at various generations, superbranched polymers and nanoengineered classical polymers. Dendrimers are polymeric macromolecules composed of


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multiple perfectly-branched monomers radially emanating from a central core (Figure 6). The number of branch points increases upon moving from the dendrimer core to its surface and defines dendrimer generation. The branched topology confers dendrimers with several unique properties for different materials applications.

Figure 6: Schematic representation of a dendrimer

Dendrons (Figure 7a) are monodisperse wedge-shaped dendrimer sections with multiple terminal groups and a single reactive function at the focal point. Hyperbranched polymers (Figure 7b) are polydisperse dendritic macromolecules that possess dendrimer-like properties but are prepared in a single synthetic polymerization step. They are imperfectly branched and have an average (rather than precise) number of terminal functional groups, but are more cost effective than the perfect dendrimer products. In applications tolerant of the imperfections, hyperbranched polymers will deliver the advantages of dendrimers at a much lower cost.

a b

Figure 7: Schematic representation of a dendron (a) and a hyperbranched polymer (b)

This class of materials have a number of measurable critical properties that are engineered into the design of macromolecules that may lead to increased EHS risks, including

- intrinsic chemical toxicity of monomers (acrylates and neurotoxins) - shape - size/molecular weight


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- surface area - surface chemistry: charge, intermolecular forces, chemical reactivity (redox index).

Techniques for the synthesis of macromolecules include:

- step-growth/chain polymerization (in liquid phase) - gas-phase polymerization - grafting - reactive co-extrusion - electrospinning.

Common formulation processes include

- batch mixing - ultrafiltration - ball milling/jet milling - extrusion/thermoforming - coating (spin, spray, dip) - microfluidics.

Common formulation chemistries include surfactants and additives (stabilizers, inhibitors, antioxidants). Common matrices in which the macromolecules can be found include aqueous/solvent solutions, creams/gels, solids/powders. Potential applications for macromolecules include delivery systems (drugs, platforms, therapeutics, nutraceuticals), bioassays, image contrast agents, inkjet printers, ion exchange resins, coatings, cosmetics, formulation viscosity modifiers etc.

For potential commercial applications of macromolecules, the liquid phase step-growth/chain polymerization techniques is the most commonly employed synthetic technique. there is less of a concern around potential for high exposure at the end of life of products containing macromolecules because of their high propensity for degradation in the environment. The greatest concern addressed is the difficulty in removing these materials at waste treatment plants.

For the use of macromolecules in applications, the following characteristics of the materials that need to be understood include

- material size and shape (including molecular weight) - polyvalency/dynamics - amphiphilic character - charge state - monomer chemistry - surface functional groups - formulation chemistry (accompanying compounds).


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The size of materials includes the nanometrics, hydrodynamic diameter and radius of gyration of the particles. The functional groups on the material surface influence the charge, hydrophobicity/hydrophilicity and receptor-specific characteristics of the material.

2.1.6 Self-assembly

Self-assembled nanomaterials are composed of even smaller nanoscale building blocks such as lipids and metal oxide nanoparticles and may include modifying components such as surfactants, inorganic materials and organic molecules. Self-organizing nanostructures are designed to assemble into ordered functional or structural units by maximizing colloidal, electrostatic and noncovalent properties and minimizing human intervention. Self-assembled nanoscale structures display interesting and potentially useful properties such as optical transparency, enhanced diffusive transport and structural flexibility. Examples of self-assembling include nanoemulsions, lattices, hollow spheres, tubes and capsules.

The common formulation process is emulsification with sufficient shear to produce nanoscale assembles. The common formulation chemistries may include surfactants as well as further additives (stabilizers, inhibitors, antioxidants). The common matrices in which self-assembles can be found include aerosol, solution, cream/gel.

In Figure 8, complex formation between a polyelectrolyte and oppositely charged surfactant is shown schematically.

Figure 8: Schematic of complex formation between a polyelectrolyte and oppositely charged surfactant (see also Section 3.5. Steric interaction)

Potential applications are primarily delivery systems such as therapeutics, contrast agents, nutraceuticals for cosmetics and personal care products.

Critical characteristics are


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- size and morphology dependent on conditions of synthesis: primary particle size of 10-1000 nm; agglomerate size: nanoscale to macroscale

- shape: compact, fibrous, tubular, multilamellar - surface chemistry: charge state: positive, negative or neutral; surface functionalized

for specific applications; e.g. targeting to a cell type or organ system - biological interactions:

o biocompatibility of lipids, the primary building blocks o toxicity derived from nonspecific adsorption of molecules from the

microenvironment or inappropriate functionalization o primary routes of exposure: lung and skin, ingestion ocular uptake is also

possible o uptake dependent on the biological context o simple or complex response to stimuli.

Concerning their toxicity, increased potential for stealth entry into undesirable locations in the body and inappropriate reassembly represents a significant concern.

2.1.7 Nanocomposite assemblies

Two forms of nanocomposite assemblies are discussed: nanoparticle polymer assembles and organic-inorganic hybrid assembles.

The common formulation processes are

- self-assembly driven by thermodynamics and/or kinetics: o evaporate self-assembly o electrostatic self-assembly: nanoparticle assembly and layer-by-layer assembly

- aggregation prevented by charge repulsion and steric repulsion (polymer and surfactant coating).

Common formulation chemistries may include

- surfactants and polymers - organic molecules.

Common matrices in which nano-composite assemblies can be found include

- aerosol - solution - cream/gel - solid organic composites - ceramics.

Potential applications of nanocomposite assembles are their usage as delivery systems: therapeutics, contrast agents, nutraceuticals for cosmetics and personal care products. The environmental applications are energy harvesting and catalysis. Nanocomposite assembles are


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also used as structural materials and nanoceramics as well as rheological modifiers for nano-composites.

In Figure 9, an example of nanocomposite assemblies is illustrated.

Figure 9: Illustration of the formation of QDs/PEG-b-PDMAEMA nano-composite assemblies based on CdTe-TGA quantum dots [17]

The critical physical-chemical characteristics, size and morphology are dependent on condition of synthesis:

- primary particle size: 1 – 100 nm - assembly size: 10 – 1000 nm - agglomerate size: morphology is condition-dependent and size is microenvironment-

dependent.

Further characteristics are

- shape: compact, fibrous (e.g. self-assembling polypeptides), capsular structures, two-dimensional sheets, hollow spheres, crystals, wires.

- crystal structure: higher-order crystallinity - functionalized surface chemistry: charge state, coating composition (surfactant,

polymer, inorganic, e.g. native nanoparticle) - surface coating functionalized for specific applications - porosity and permeability.

Potential hot spots in nanocomposite assembly life cycle:

- toxicity of the nanocomposite assembly components during manufacture - toxicity and potential transformation of components during degradation and

disassembly - assemblies as inappropriate carriers of manufacturing byproducts, e.g. toxic solvents - inaccurate assembly that may inappropriately enhance systematic transport; e.g. across

the blood-brain barrier, or through the environment - exposure to materials over their life cycle – manufacture, use and disposal – that may

all present different, largely unknown issues for safety assessment - nano-composite assembles that bind non-specifically to entities in the environment

that would change shape, chemistry and propensity for environmental transport


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- unknown potential for and consequences of disassembly and inappropriate reassembly, which can occur in the body or in the environment and lead to inappropriate structures, unanticipated uptake and transport

- increased potential for stealth entry into undesired locations in the body - use of nanocomposite assemblies in industry that may lead to an industry sector

mismatch between the required material science expertise to create the nanocomposite assemblies and the application expertise.

As identified in [13], the largest information gap in assessing the life stage effects of nanomaterial is the lack of both descriptive and mechanistic data regarding the fate and effects of the materials in developing systems.

3 PREPARATION STRATEGIES AND INTERACTION FORCES

3.1 Preparation strategies

Two different approaches exists for preparing materials in the colloidal size range [10]:

(i) Start with big particles and break them down by brute force, such as milling (so-called dispersion or comminution methods). These procedures are routinely carried out in industries. A typical example is the comminution of pigments in the paint industry to obtain samples in a size range compatible with optical and rheological specifications. By dispersion method it is difficult to obtain homodispersed sols, although size distribution can be narrowed down by fractioning.

(ii) Prepare particles by precipitation from solution (condensation methods). Such procedures are relevant because in their path that leads to growth from dissolved molecules via small nuclei and embryos to larger particles. Nucleation is achieved by making the solute insoluble, say by a chemical reaction, a change in temperature or quenching by a nonsolvent. Over the past decades, colloid chemists have learnt to prepare homodispersed sols by controlling the rates of nucleation and growth, exploiting the GIBBS-KELVIN rule. This rule states that for nucleation a much larger super-saturation is needed than for the growth of existing nuclei. The origin of this difference results from the large surface-to-volume ratio of very small particles that, therefore, carry a high excess interfacial GIBBS energy and, hence, are better soluble.

It is obvious that nanoparticles serve well as condensation nuclei. A variant is the synthesis of inorganic nanoparticles in the cores of microemulsions, droplets that are homodisperse in nature. Another variant, allowing the particles preparation of a variety of specific shapes (cylinders, plates, discs) involves inhibition of the growth of certain crystal planes [18]. A critical embryo has the size at which GIBBS energy at constant pressure and temperature is a maximum.


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Actual bodies interact with one another and with their surroundings. As this takes place, the interface serves as a 'stage' where all the interactions are 'enacted' between the surfaces. In disperse and colloid systems, these forces are of prime importance because of very large surfaces. By surface forces are meant the interactions between the surfaces of solid and liquid phases; among these are molecular VAN DER WAALS forces, electrostatic forces, acid-base interactions, as well as interactions which can result in chemical bonds. Attracting surface forces are responsible for adhesion between two solids but also between a solid and a liquid.

For a plethora of technical processes like flotation, washing, sticking of various materials, knowledge of the surface forces is of crucial importance.

Also the formation of larger particles from minute powder particles is based on the action of surface forces. In that case the action of the surface forces is especially strong. Although the attracting forces decrease, due to the small contact area, the effect of other forces on these particles (such as gravitational force) decreases still more as the particle size diminishes.

For more details on the material presented in Sections 3.2 – 3.4 see [19] as well as citations therein.

The existence of intermolecular interactions has been anticipated for a long time. 200 years ago, CLAIRAULT and LAPLACE elaborated the view of an observation that intermolecular forces diminish with growing distance. NEWTON was likely the first to suggest that intermolecular attraction forces are of no mechanical nature but must be related to gravitation, electromagnetic interaction or even to still unknown interactions. In 1730 he wrote in particular: “There are therefore agents in nature able to make the particles of bodies stick together by very strong attractions. And it is the business of experimental philosophy to find them out.“ 200 years had to pass away to let his words attain these ends: all results worth mentioning in the field of intermolecular surface forces have been obtained in the 20th century.

3.2 VAN DER WAALS interaction

VAN DER WAALS-type forces are of electrostatic origin, universal and non-local. They act between all the molecules and macroscopic bodies. The intensity of this interaction should be distinguished from the chemical one which brings about molecule formation and displays the property of saturation as follows: two molecules attracting one another on grounds of the VAN

DER WAALS interaction are able to attract still further molecules.

Since molecular surface forces are based on the forces between the individual molecules it appears worthwhile to look at these forces more closely. VAN DER WAALS forces between the molecules depend on the polarisability of the molecules, i.e. on the extent of the possible charge displacements inside the molecule1. Molecules of various compounds may be

1 Molecules are no rigid structures. In a diatomic molecule, say H2, both hydrogen nuclei vibrate against one

another while the centre of gravity of the molecule holds still. When vibrating, the charge displacement varies periodically. According to MAXWELL ’s theory, the molecule should radiate like a vibrating electrical dipole.


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subdivided into polar molecules concentrating electrical charges at their ends and non-polar molecules. A polar molecule continuously disposing of two dipoles is called a dipole (also permanent dipole); the electrical properties of the dipoles are characterised by their dipole moments2.

In 1912 REINGANUM and later KEESOM considered the intermolecular interactions as electrostatic interactions of dipoles. Because of their non-symmetrical charge distribution, polar molecules possess the ability to attract or to repulse other polar molecules as well as to orient themselves in a certain way in the electrical field of other dipoles. The attraction between the permanent dipoles is called orientation interaction (also dipole-dipole interaction or KEESOM forces) and results in preferred reciprocal orientation of the molecules. This mechanism makes it possible to explain the attraction between the molecules of polar gases with, more or less, high temperatures (but not with very high temperatures, at which the interaction energy should become approximately zero (for more information see [19]). On this basis DEBYE, and later FALKENHAGEN detected the induction mechanism of interaction (also dipole-induced dipole interaction or DEBYE forces). According to it, the electric field generated by the dipole moment of the first molecule induces a dipole moment with the second molecule and vice versa. As a result, the average interaction energy no longer depends on the temperature. Induction and orientation interactions are due to the existence of permanent dipole or higher moments. Inert gas atoms have a symmetrical charge distribution which is why they are uncharged. However the intermolecular interaction appears here as well and is just as important as the interaction due to induction or orientation. Fluctuations of the electric field cause displacements of the electron cloud against the nucleus. For this reason, fluctuating dipole moments also appear in non-polar molecules inducing, in their turn, a charge displacement in their neighbour. Although the average of this dipole moment equals zero, the average of the dipole moment squared is different from zero. Consideration of this fact leads to a generalised theory of VAN DER WAALS interactions and to the term dispersion forces (also induced dipole-induced dipole interaction or LONDON forces).

The energy of the VAN DER WAALS interaction follows from quantum-mechanical calculations and equals

( )6d

BdU vdW −= ,

where d is the intermolecular distance and B a characteristic constant. The negative sign shows the attracting potential action.

With larger distances, a so called retardation effect emerges due to the electromagnetic nature of the intermolecular interaction, resulting in decreasing the attraction.

With smaller distances, repulsing forces dominate. In terms of quantum mechanics, wave functions overlap here, and the electrons are able to accept also the states of the electrons of 2 It has been ascertained that polymers having polar molecules with large dipole moments have excellent

adhesion properties.


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the neighbouring atoms. If free states of the comparable energy are available, this overlapping causes a decrease of the total energy. If there are no free states, the electrons can occupy only empty states of the higher energy. Thereby the total energy becomes higher, resulting in a repulsing force. Empirically, the BORN-MAYER potential is used

( )12d

AdU B = ,

where the constant A is the potential force. The expression is a good approximation because the rapid increase in potential dominates with decreasing distance. Adding the attracting VAN

DER WAALS potential to the repulsing potential, we obtain

( )

−

=−=612

6124

d

s

d

s

d

B

d

AdU ε ,

what is called LENNARD JONES potential3, where s is the equilibrium distance and ε the depth of the potential well in the equilibrium state.

In order to calculate now the VAN DER WAALS forces between two solids from the intermolecular contributions, suitable suggestions are to be made. Traditionally BRADLEY´s (1932) and HAMAKER ´s (1937) concept of the additivity of pair interactions is used, whereby the searched interaction force is obtained via summation/integration. However, this microscopic theory fails to account for many-particle interactions whose effect may be considerable in case of condensed media. DERJAGUIN s and ABRIKOSOWA´s papers in 1956 experimentally confirmed the non-additivity of these forces.

A further theoretical concept for determination of dispersion forces4 between two macroscopic bodies has been developed by LIFSCHITZ, later also by DZYALOSHINSKII and PITAEVSKII . Starting with the hypothesis that the force action between two macroscopic bodies is generated by fluctuations of the electromagnetic field, the authors calculated the interaction force without reverting to atomic parameters. The addivity gets lost in this quantum-mechanical description. Each particle newly entered in the system influences the already existing ones in a variety of fashions. However, BRADLEY´s and HAMAKER ´s theory, which suggests paired additivity and constant density across the entire volume, correlates well with the results obtained by LIFSCHITZ and collaborators, who used a continuous approach from the outset. This correlation is valid only for very small distances. Since we want to restrict ourselves to the effect of the VAN DER WAALS forces on adhesion, where we are concerned with very small distances between the bodies, we need only to look at the dispersion forces as a power function of the distance, according to BRADLEY and HAMAKER .

3 In the thirties, when the LENNARD-JONES potential had yet to be found in textbooks, investigators still

speculated about the potency of the distance with the attraction potential. LENNARD-JONES was likely to be the first to ‘guess’ potency 6.

4 These are generally much larger than the KEESOM and DEBYE forces.


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3.3 Electrostatic forces

Electrostatic loaded particles or bodies exert forces on one another. Such electrostatic forces are long-range surface forces, just as are VAN DER WAALS forces. Because the electrostatic component of the surface forces is only difficult to separate from the VAN DER WAALS

component when being measured directly and therefore is disregarded in this paper when calculating the thermodynamic adhesion energy, here we will only briefly review possible effects of the electrostatic forces in general and refer to specialised literature (for more information see [19]).

The contribution of the electrostatic forces to the adhesion is particularly large with the interaction between a polymer and a metal (or a different, conductive material) as well as between two polymers. These latter should be relatively far apart from one another in the triboelectric series of polymers; the more separated from one another the polymers in the HENNIKER series (for more information see [19]), the larger the corresponding potential difference and thus attraction force.

Depending on the medium, different force laws act:

(i) attraction of two electrical charges of opposite sign or repulsion of two charges of the same sign in the air, which are known to drop in inverse proportion to the square of the distance d (COULOMB law):

( )2

21

04

1

d

qqdF

coul⋅=

πε

where 1q and 2q are the surface charges and 0ε is the dielectric constant.

(ii) attracting or repulsing force in electrolytes as an exponential function of the distance (also in pure water or in the presence of an adsorbed water film):

( ) dDLVO eRdF κκεεπ −Ψ⋅⋅= 2

002

where κ−1 is the DEBYE length, ε the dielectric constant of the medium, ψ0 the interfacial material-medium potential and R the radius of the reduced curvature. However the latter equation is valid in the specified shape only for two surfaces of the same material.

Nanoparticles and larger colloids have the following similarity: to describe their interaction, it is mandatory to characterize their surfaces electrically [10]. Two types of surface charges have to be distinguished:

(i) The real surface charge caused by charges that are so tightly bound that they may be considered as belonging to the surface. Examples are H+ and OH− ions for oxides and covalently bound sulphate groups on polystyrene latex spheres. These


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charges can be approximately obtained by acid-base titration and conductometry, respectively. Another possibility, which is more typical for nanoparticles, is inclusion of ions of a special type in the solid matrix of the particle. The colloidal equivalent is that of clay plateles, which through isomorphic substitution (Si4+ → Al3+, Al3+ → Mg2+) acquire a negative bulk charge, compensated by cations outside the platelets. For such systems, ion exchange capacity (i.e.c.) can be experimentally determined. Surface charges are usually recorded as charge

densities (µC⋅cm−2).

(ii) Electrokinetic charges, obtainable from electrophoresis and other electrokinetic techniques. These techniques measure the electrokinetic or ζ-potential, which is readily converted in electrokinetic charges, using the theory of GOUY-CHAPMAN. For a review of electrokinetics, including the effect of particle size see [20].

3.4 Acid-base interaction

According to the FOWKES theory (1987), thermodynamic work of adhesion can be represented as a sum of two components:

abA

dAA WWW += ,

where dAW is the work done by non-specific VAN DER WAALS forces (mainly by dispersion

forces) and abAW the work done by specific interactions (so-called acid-base interactions).

Contrary to the work done by non-local dispersion forces, the component abAW of the work of

adhesion is due to the formation of local donor-acceptor bonds at the interface. Because of its local nature, such an interaction can occur only with a direct contact of both bodies. Furthermore, the adhesion of rather non-polar polymers, such as polyolefins or fluoroplastics, is only caused by the action of the VAN DER WAALS forces, whereas the contribution of the acid-base interactions to the total value of the work of adhesion of polar polymers can reach 70 to 80% (for more information see [19]).

The formation of an electric double layer in the contact area of two solid surfaces due to the formation of donor-acceptor bonds between them is the subject of DERJAGUIN s semi-empirical electric theory of adhesion. According to it, an exchange of electrons takes place at the contact site, which is due to different electronic structural levels of the contacting material.

Functional groups, such as hydroxyl group OH, carboxyl group COOH, benzol ring, nitrile group CN, and amino group NH2, act at the polymer surface as carriers of their adhesion activity much as they determine the mechanical properties of the polymer inside the latter5. The relation between the presence of the functional groups and the adhesiveness of some

5 E.g. the presence of COOH, OH, or NH2 groups which interact with polar groups of adjacent chains is

responsible for a higher mechanical strength of the polymer.


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polymer materials is certainly no accident. Based on investigations of the semiconductor-polymer system, KROTOVA and collaborators made up the following succession of donor-acceptor properties of the functional groups contained in polymers:

–CN > =CO > –COOR > –C6H5 > –OR > –OH > NH2 ,

whereby the donor-acceptor properties change from left (donor) to right (acceptor).

Moreover, the same authors concluded that, in vacuum, the electrostatic component is predominant for the adhesion of polymers. The question was raised by JACOBASCH (see in [19] for more details) as to the validity of this deduction for actual interacting systems, i.e. for the contact formation in humid atmosphere (whereby conduction of the charge carriers is possible) and it still remains open.

Generally speaking, functional groups in polymers may be made up in series according to their donor properties. Each previous member of the series functions as an electron pair donor with regard to the following member, which then acts as an electron pair acceptor.

Combining polymers, which are to interact with one another adhesively according to the acid-base principle, with consideration of the fact that the possibility of a non-symmetric electron density distribution in the contact area grows as the functional groups move away from one another in a series, makes it possible to derive the following empirical rule: to attain a good adhesion, thus great bond strengths, polymers must be purposefully combined with one another in such a way that their functional groups in a donor-acceptor series are as far as possible apart.

dm

df

dAW γγ ⋅= 2 ,

where f and m indicate the reinforcing material (fibre) and the matrix, respectively.

The acidity and basicity of a surface are due to the presence of polar groups, which can act as

an electron acceptor and electron donor. At present, constants AK and BK can be determined

using the inverse gas chromatography technique6 (IGC); corresponding values can be found in literature for many polymers and fibres. Knowing these constants, it is possible to calculate the contribution to the total work of adhesion of the energy attributed to the donor-acceptor bonds. According to FOWKES the following relation results:

abababA nHfW ⋅∆⋅−= ,

6 This sensitive technique was originally used in the eighties in France to analyse solid surfaces. Here, we are

dealing with interactions between a solid and the gas phase of low-molecular weight liquids. Based on measuring data, we can estimate the absorption enthalpy and then determine both the dispersion part and the thermodynamic work of adhesion caused by specific acid-base interactions.

In much the same way as conventional chromatography, IGC provides no absolute information on the chemical composition of a solid surface; data has to be correlated with the values obtained by means of spectroscopic techniques.


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where f is a correcting factor to transform enthalpy values into free energy values, taken

approximately equal to unity, and abH∆− the enthalpy of the interaction between the fibre and the matrix:

BfAmBmAfab KKKKH ⋅+⋅=∆− .

abn is the number of acid-base sites (eventual donor-acceptor pairs) per unit interfacial area. According to various estimates, this quantity is practically independent of the chemical nature

of both components and is equal to about 6⋅10-6 mol/m2 (3.6⋅1018 m-2).

3.5 Steric interaction

Steric interaction refers to the influence of added macromolecules on colloid stability. The mechanisms come in a variety of forms, depending on the nature of the macromolecules and its interactions with the particle. The first distinction is between adsorbing and nonadsorbing polymers. In the latter case, around each particle a zone devoid of polymer develops, the depletion zone. Overlap or depletion zone gives more space for the polymers, which is therefore entropically favourable and hence leads to depletion flocculation [20]. It is a weak phenomenon in the sense that GIBBS energy is much less than that of adsorbing polymers, when the first polymers completely cover the particle surface and when this layer repels the free polymer in solution (good solvent quality).

With regard to adsorbing macomolecules, distinction can be made between uncharged polymers and polyelectrolytes. Adsorption of polymers is entropically unfavourable because the number of available confirmations in the adsorbed state is much less than in the free state. Therefore, a minimum critical segment adsorption (GIBBS) energy is required for attachment. Once this threshold is surpassed, the binding of a macromolecule is very strong, because many segments can attach. However, polymers rarely adsorb completely flat: that would again be entropically very unfavourable. They form besides trains (a few attached segments in series), also loops and tails. Typically, most of the adsorbed weight is in the loops; only a few percent is in the tails. It is also typical, that a large fraction of an adsorbed polymer is still in contact with the solvent. For this reason, solvent quality plays an important role: the poorer the solvent quality, the easier it is for the polymer to form loops and the higher the adsorbed amount [20]. All of this is valid for nanoparticles. As a trend, for fully polymer-covered particles, displaying no desorption upon particle approach, the interaction is attractive in a poor solvent and repulsive in a good one.

The difference between colloids and nanoparticles is mostly a matter of the relative sizes of particle and polymer.

Polyelectrolyte adsorption and the ensuring influence on stability:

- Attachment of polyelectrolytes to surfaces is not a purely electrostatic effect; besides this, there is the same specific adsorption of segments b ‘chemical’ forces, i.e. by hydrophobic binding.


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- Electrolytes play a role: they screen electric interactions, both the attractive and repulsive ones. For very high salt concentrations, the polyelectrolyte behaves as an uncharged polymer.

- Adsorbed polyelectrolytes form very thin layers because interchain repulsion inhibits loop formation. Typically, plateau adsorption of polymers amount to a few mg⋅m−2,

whereas for polyelectrolytes it is rather a few tenths of mg⋅m−2, - The trend is that long-distance polyelectrolyte stabilization is of an electrostatic nature,

whereas at shorter distances steric elements and partial desorption also play a role. This interaction is called electrosteric interaction.

3.6 Nanobiointeractions

It is now recognized that nanparticles of even quite convetnional materials, when they become smaller than 100 nm can enter the biological cell [21]. When they are smaller, than 40 nm, they can enter the cell nucleus, and when they become smaller than 30 nm they can assess much of the body, even crossing the blood-brain barrier. The new effects arise in part because at the size scales nanoparticles can utilize the intrinsic transport mechanisms, such as endocytos, of living matter that permit proteins to be trafficked to different locations.

The capacity of nanoparticles to enter previously inaccessible locations in the body has led to some concerns about their potential toxicity. It has also been recognized that nanoparticles may provide a new direction for therapy, leading to the emergence of the field of nanomedicine.

The colloidal science world has long known that particles in suspension are not generally in their monodisperse form. If there are forces between particles as described in Sections 3.2 – 3.5, they tend to attract each other and form aggregates by processes called flocculation or coagulation.

Figure 10: Schematic representation of how particle properties can influence dispersion stability and aggregation; above: Simple particle that aggregates under the influence of BROWNian motion, and which binds an added biopolymer; in the middle: Charged particles have an intrinsic charge repulsion that prevents them from aggregating to some extent. Addition of an oppositely charged biopolymer can result in charge reversal; below: Grafting of a hydrophilic polymer at the surface of particles can result in a steric hindrance, which also

acts against particle aggregation. In this case, there is reduced interaction with the added biopolymer [21].


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There is also an obvious effect of the concentration of particles on the colloidal stability (Figure 10) as a result of entropy effects – the more space that particles have, the less likely they are to come into contact with one another as a result of BROWNian motion and to stick to one another.

The aggregation process is strongly influenced by the properties of the solvent and the presence of other species in the solution. This has particular relevance for nanoparticles in biological fluids, which contain large numbers of proteins, lipids, polysaccharides and other biopolymers. Due to very significantly larger surface area presented by nanoparticles, adsorption of proteins and other biomolecules can be expressed to have a much greater impact on the properties of the particles and dispersion (see Figure 11), and indeed, significant work is being devoted to the elucidation of the impacts of interaction with proteins on nanoparticle behaviour and toxicity (for more details see [22, 23]).

Figure 11: Schematic representation of protein (biomolecule) adsorption to nanoparticles in complex biological solutions such as plasma or tissue culture media on their aggregation behaviour, and the consequent need to

work in dilute solutions to ensure good control of the dispersion [21].

In addition to adsorption at particle surfaces, there is another potential effect from the presence of biopolymers in a dispersion that results in the formation of an attraction between the particles due to depletion forces. Addition of a non-adsorbing polymer to a particle dispersion leads to the particles experiencing an effective depletion attraction, whereby the polymer is excluded from the region between the surfaces of two nearby particles, leading to an increased osmotic pressure pushing particles together.

4 NANOTECHNOLOGY: CROSS CUTTING ISSUES

Since nanomaterials as modifiers for functionalization of textile and other materials are usually bound in a liquid medium such as emulsion, dispersion, suspension or foam, the following review on ‘smart’ and ‘green’ interfaces should give an insight into nanotechnology methods available. The review below being the content of Chapter 4 was submitted to the Advances in Colloid and Interface Science.

Material science has evolved from studies of inert building materials to designing functional materials. Many functional materials, such as piezoelectric materials, shape memory alloys, optical fibres, electrorheological fluids, have a unique ability to respond to stimuli, displaying ‘smart behaviour’. Stimuli may be stress, strain, temperature, pressure, an electric field, incident photons etc. Smart behaviour occurs when a material can sense some stimulus from its environment and react to it in a useful, reliable, reproducible and usually reversible manner, i.e. thermochromic behaviour. Smart materials have been the focus of much attention as researchers seek ever more functionality in materials [24]. Since many reactions occur at


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surface and interfaces, surfaces and interfaces play a crucial role in the development and use of smart materials. New technologies exploiting smart interfaces include reversibly adherent polymer melts, removable inks, healable surfaces and interfaces, contact adhesives etc. At liquid/solid interfaces, for example, lab on chip technologies, biomaterials and sensors require tunable interfacial dynamics in layers physically adsorbed at liquid/solid interfaces [25]. For more details on available techniques see Section 4 further below.

On the other hand, there is a strong need for redesigned manufacturing processes, so-called green manufacturing. Green manufacturing is based on the substitution of raw materials and the design of processes to either eliminate waste streams or ensure that they are non-toxic and environmentally friendly. Such green manufacturing techniques include, for example, synthetic catalysts, bio-base processes, non-chlorine-based water purification, biodegradable polymers [26]. In the past three decades, scientific advance in chemistry, biology, physics, engineering and medicine have altogether revolutionized drug delivery [27]. Smart drug delivery systems have been developed to provide highly controlled drug release rates in response to stimuli, such as pH or temperature changes, small molecules, enzymatic reactions, light, magnetic fields or radio frequencies. The example of smart drug delivery shows the intimate relationships between developments in biotechnology, nanotechnology and materials technology. These relationships have continued to grow and mature over the past decade to an extent that integration and cross-functionality of advanced technologies has become the rule rather than the exception.

As the subject of “smart and green” is very broad, it will be restricted here to (i) smart dispersions – materials with liquid/liquid and liquid/solid interfaces with a green aspect (manufacturing or applications); (ii) smart foams – materials with air/liquid interfaces displaying smart behaviour, and (iii) encapsulation materials such as micro-/hydrogels, and (iv) their combinations. Additionally, some ‘green’ processes for manufacturing smart materials are mentioned briefly.

4.1 ‘Smart’ emulsions

Solid particles have been identified as a new type of emulsifying agents in addition to surfactants and amphiphilic polymers, going back to the pioneering studies by Ramsden [28] and Pickering [29] more than a century ago. Such emulsions are known as Pickering emulsions, where solid particles of intermediate wettability in the size range from several nanometers to several micrometers attach to liquid-liquid interfaces and provide emulsion stability. If solid particles with additional functionality are used [30], the emulsions are considered as smart materials – they have additional functionality (UV protection) and change their behaviour depending on external conditions. It was demonstrated, that emulsion, stabilized by both – nanoparticles and emulsifiers – showed a different behaviour in dependency on external strain, i.e. sometimes it exhibited thixotropy or antithixotropy and sometimes both of them.

Pickering emulsions can be used to synthesize core-shell nanocomposite particles, where the polymer serves as the core and inorganic particle serve as the shell [31]. Such materials


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provide a new class of supramolecular building blocks, exhibiting unusual, unique properties with potential applications in drug and gene deliveries, enzyme immobilization, colloidal nanocatalysts, chemical sensing, functional coating. Emulsion-based synthetic strategies for preparation of amphiphilic core-shell particles were recently reviewed in [32].

In [33], nanoporous adhesive latex coatings and inkjet deposited latex microstructures, containing concentrated viable but non-growing microorganisms were presented. Such non-toxic (low biocide or biocide-free) latex emulsions with carbohydrate porogens which were used for generating nanopores can be used for smart coatings. When rehydrated, these bioactive coatings can be used for multi-step oxidations, reductions, as biosensors, in biofuel cells or high intensity industrial biocatalysts. Another application of smart emulsions in biotechnology was described in [34]. The authors functionalized biotinylated oil-in-water emulsion droplets with biotinylated single-stranded DNA oligonucleotides using streptavidin as a linker. It was shown the components of this linking systems to be stable and to induce sequence-specific aggregation of binary mixtures of emulsion droplets.

In recent years, environmental awareness has increased dramatically leading to a quest of new surfactants with biodegradable and biocompatible properties. Non-conventional, eco-friendly surfactants have attracted attention for personal care products acting as a new class of emulsifiers. New eco-friendly surfactants, e.g. glycerol ethers, have been synthesized [35, 36] attaining very low critical micelle concentrations and low surface tensions. In addition, they combine the advantages of glycerol ether surfactants and amino acid lipopeptides and due to their ether bond they are very stable at high temperatures and can endure pH variations.

Finally, a smart design of oil-water interfaces and their behaviour in food science and technology was recently reviewed [37]. The development of nutrition and healthy food products requires specific microstructures for improved dispersability and bioavailability of bioactives and probiotics. The generation of microstructure for fat-reduced ice-cream is shown in Figure 12.

Figure 12: Generation of microstructure for fat-reduced ice cream, taken from [37].


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Food microstructures are also useful for chemical stability and for protecting probiotic bacteria during drying, shelf life and application. Suitable microstructures based on emulsions are need for foods with increased amount of fibres, whole grains, vegetables, fruits and proteins to minimize negative sensorial effects.

4.2 ‘Smart’ foams

Foams are of outstanding importance in materials chemistry and daily life. They can be used as sacrificial templates for production of macroporous materials, in food and cosmetics or for a large variety of other applications [38]. In [39], switching reversibility between ultrastable and unstable foams is described. Foams produced using multi-lamellar structure (12-hydroxy steric acid and the counterions ethanolamine and hexanolamine for dispersing) were shown to be outstandingly stable over months (ultrastable). Interfaces were optimally and extremely rapidly covered as in the case of low-molecular-weight surfactants. After heating, the multi-lamellar tubes melt into micelles depending on the nature of the counterions. The authors offer a versatile and simple way to produce temperature-tuneable foams, which are completely reversible.

Another example of the fabrication of smart foams with temperature-tuneable stability was shown in [40]. The authors provided direct evidence of catanionic monolayer formation from catanionic vesicular dispersions. The mechanical behaviour of the layers resembles that of soft glass materials. The foams made from vesicle dispersions are very stable against Ostwald ripening and coalescence due to the extremely high compression rigidity of the catanionic monolayer. Their temperature behaviour is similar to that of the vesicle bilayer, melting at 55°C. When the dispersion surface is well-covered, successive layers at vesicles are jammed underneath, yielding a very thick surface layer.

4.3 Encapsulation techniques

One of the first applications of microencapsulation was dedicated to carbon-free self-copying paper that was commercialized in 1968. Nowadays, encapsulation is used in pharmaceutics, cosmetics, food, agriculture and for chemical trapping and delivery. Microencapsulation involves encapsulating liquid or solid substances in tiny thin-walled natural or synthetic drops. Encapsulation allows moisturisers, therapeutic oils or insecticides to be incorporated into textiles and is also used in thermo-chromic and photo-chromic fabrics, which change colour with changes in temperature and light.

Methods of encapsulation based on emulsions – colloidosomes – as well as their release mechanisms are reviewed in [41], see Figure 13. An alternative to sustained release of microcapsules by shell rupture or their dissolution is to employ swellable shells, which are formed by incorporating responsible materials into the shells. Such materials undergo reversible transitions when stimulated by external conditions such as pH, ionic strength, light or temperature. Shell swelling initiates the release of microcapsules.


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Figure 13: Strategy for fabrication of enzyme-loaded colloidosomes. The same approach can be used in encapsulating any other hydrophilic ingredient. Adapted from [42].

Generalized flowsheet for the fabrication of colloidosomes, ball-like 3D aggregates and composite aggregates from emulsion droplets dispersed in a suspension of uniform particles is shown in Figure 14.

Figure 14: Fabrication of colloidosomes from emulsion droplets. Adopted from [43].


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The colloidal behaviour of microgels with switchable properties is strongly affected by interparticle interactions. In [44], smart microgels focusing on Janus and oscillating microgels were described. Janus microgels show anisotropic shapes and chemical/physical properties. Oscillating microgels show autonomous swelling/deswelling behaviour.

Green synthesis of a temperature sensitive hydrogel in supercritical carbon dioxide (sc-CO2) was described in [45] for possible applications in drug delivery, tissue engineering, smart membranes with tunable permeability. Over the last decades, a class of smart hydrogels exhibiting unique biomimicking functions was discovered and demonstrated by the authors of [46] – thermoresponsible volume phase transitions similar to sea cucumbers, self-organization into core-shell hollow structures similar to coconuts, shape memory as exhibited by living organisms, and metal ion-mediated cementing similar to marine mussels. It was demonstrated how the concept of balancing hydrophilic and hydrophobic forces could be exploited for designing chemically cross-linked hydrogels with self-healing properties.

Lung diseases are commonly treated by inhalation of aerosolized medicines. To achieve pharmacological action, micrometer-sized drug particles – after their deposition on the surface of bronchial mucus – have to penetrate the viscous layer and reach cells of the lung tissue. Mucus is often overproduced and more viscous in the disease what significantly reduces the rate of drug migration to the cell surface. Some concepts of new inhalation powders acting as carriers of pulmonary medicines were tested and, finally, special functional carrier particles have been designed and obtained by a spray drying technique [47]. Another smart medical application is described in [48], where modifications of medicines with laser beams in bulk and droplets is suggested in order to identify new ways for fighting multiple drug resistance acquired by bacteria. It is a promising alternative to modify existing medicines by exposing them to laser radiation and to obtain photoreaction products which may have bactericide effects. Phenothiazines are such candidates and when exposed to laser beams undergo reactions that result in the degradation of the parental compound and the formation of new species.

In recent studies [49, 50] the properties of pH/thermo-responsive polyelectrolyte microgels were investigated with regard to a surface functionalization of textiles. Microgels were prepared to have their pH/thermo-responsiveness expressed within the physiological pH and temperature range. They consisted of pH/thermo-responsive microparticles of poly(N-isopropylacrylamide-co-acrylic acid) either alone or complexed with the pH-responsive natural polysaccharide chitosan (see Figure 15).


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Figure 15: Cryo-SEM images of cross-sections of PNIAA microparticles (a) and complexes of PNIAA microparticles and chitosan (b) in freshly prepared microgels; taken from [50].

The studies revealed that the studied thermo-responsive microparticles and their complexes with chitosan undergo a volume-phase transition from swollen and hydrophilic to de-swollen and hydrophobic at temperatures close to the average human body temperature. Kinetics measurements showed that this transition is completed in approximately 15 min. Furthermore, the polyelectrolyte complexes exhibit a change of surface charge from positive to negative values at pH 6, i.e. within the physiological pH range. The size difference between chitosan/PNIAA complexes and PNIAA microparticles alone is decreased from ca. 45% to less than 20% as microgels passed from a hydrated to a dry state below their lower critical solution temperature. The PNIAA thermo-responsiveness kinetics was found to be both temperature- and pH-dependent. Finally, complexes were found to be surface active, with their surface activity lying in between those of chitosan and PNIAA. The information obtained about hydrophilicity/hydrophobicity aspects of the studied systems is considered essential, as these systems are intended to be used for surface functionalization of polyester textiles.

Figure 16: Poly(N-isopropylacrylamide-co-acrylic acid)/Chitosan polyelectrolyte complexes for fabrication of pH/thermo-responsible microgels intended for surface functionalization of textiles. Microgels were prepared to have their pH/thermo-responsiveness expressed within the physiological pH and temperature range (skin); taken

from [49]. LCST is the lower critical solution temperature.

Other examples of improved microstructures for textile functionalization were shown in [51, 52, 53]. The target was the design, synthesis, evaluation and characterization of environmentally friendly bactericide composites on textiles like polyester and on flexible thin low cost polymers like polyethylene. The long-range operational stability of films and


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surfaces like glass, metal was to be uniform and able to disinfect and preclude pernicious biofilm formation under mild conditions. Innovative composite anti-pathogenic films were prepared involving nano-particle preparation and optimization to obtain improved microstructures leading to a more effective bactericide action than the materials available today using green chemistry, i.e. no residues.

In the review [54], the most commonly used polymers for the preparation of aqueous-based soft and core-shell responsive particulate systems are summarized. For example, depending on targeted applications, researchers achieved ways of controlling particle adsorption at air/water or oil/water interfaces, or transport of particles across the respective interface. This holds large potential for formulating smart emulsion and foam products in cosmetics and food industries. Furthermore, these nanoparticle properties can be extremely useful in biological membranes/barriers and in the design of drug delivery vesicles.

Nanoparticles were used in a recent study [55] to create pH-sensitive switchable interfaces based on a layer-by-layer film formation of haemoglobin. Smart interfaces were created which are switchable between the state ‘on’ and ‘off’ controlled by the pH value of the solution. The films were fabricated by alternatingly adsorbing haemoglobin and silver nanoparticles on the surface of a chitosan modified glassy carbon electrode.

In another study [56], the synthesis, characterization and film-forming properties of two-component polymer nanoparticles that undergo a reversible morphology transformation in water as a function of pH was described. The authors found a remarkable and reversible morphology transformation from a core-shell structure in basic to a uniform blend in acidic solution. It was also shown, that the protonated form of the oligomer accelerates polymer diffusion in latex films. These materials are intended for coating applications.

The growing request in innovative materials with low environmental impact and high performances often finds interesting solutions in multiphase systems, whose properties are strongly dependent on their microstructure. Flow induced microstructures of multiphase fluids were investigated in [57, 58, 59]. The non-NEWTONian behaviour of one of the phases can strongly influence the deformation of the droplet of the disperse phase or induce the migration under simple shear flow of the inclusions leading to flow focusing and to the formation of ordered micro structures. Even in the simple case of an emulsion of two NEWTONian immiscible fluids in a simple parallel plate shear flow cell, the formation of alternating regions of high and low volume fraction of dispersed phase droplets can be observed. A similar experimental approach, based on flow visualization, has been used to investigate the deformation under shear of surfactant multilamellar vesicles which play a key role in the formulation of many industrial products, such as detergents, foodstuff, and cosmetics.

4.4 Combination of ‘smart’ colloid materials

Stimuli-responsible emulsifiers that allow controlled stabilization and destabilization of emulsions could strongly enhance the opportunities for enzyme-catalytic reactions in biphasic media. It was recently shown [60] that smart microgels respond to changes in external stimuli


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such as temperature and pH value by changing their size. Softness and hydrophobicity can be used as stabilizers for emulsions, also called ‘Mickering’ emulsions. Compared to PICKERING emulsions, which are stabilized by rigid nanoparticles, microgels are soft gel particles that deform at an oil-water interface. The authors demonstrated a proof-of-concept that smart emulsions prepared by stimuli-responsible smart microgels provide unique opportunities for biocatalysis in two-phase systems. Furthermore, microgels can be tailored to enable reversible stabilization and breakage of emulsions under conditions that meet the requirements of enzymatic reaction and allow simple product separation and recycling of biocatalysts and emulsifier.

In another study, the structure of microgel packing at an oil/ water interface was investigated by using cryo-SEM [61]. It was shown, that the structure of interfacial microgel layers strongly depends on pH of the microgel dispersion. It is an evidence that emulsion stability is independend of the particle-packing density. Some structural changes induced by the interface were observed, leading to interconnections between individual interfacial microgels. These findings showed that microgels behave at oil/water interfaces quite differently compared to solid particles used for PICKERING emulsions.

A combination of two concepts – PICKERING emulsions and sol-gel chemistry – was demonstrated in [62] in order to elaborate for the first time stable wax and silica core-shell capsules. These capsules can be stored and used in both – dispersed and dried – states. The fabrication procedure can be generalized and applied to various oils – alkanes, block paraffin, triglycerides. This generalisation opens a large filed of applications in pharmaceutics, cosmetics, food since triglycerides are biocompatible and eatable oils. It was shown, that the temperature of release can easily be controlled by the choice of the oil and that the way the oil is released can be tuned by the choice of the continuous phase. And the kinetics of release can be accelerated by an external hydrodynamic field. More complex capsules comprising multiple compartments with a high potential as pharmaceutics for facilitating multitherapies can be elaborated.

5 DEFINITIONS AND TERMINOLOGY

The definition and terminology are taken from the Manual od symbols and terminology for physicochemical quantities and units of the International Union of Pure and Applied Chemistry (IUPAC) [7].

Absorption The transfer of a component from one phase to the other is often called absorption.

Adsorption Adsorption is the enrichment (positive adsorption) or depletion (negative adsorption) of one or more components in an interfacial layer. The material in the adsorbed state is called the adsorbate, while that present in one or other (or both) of the bulk phases and capable of being adsorbed may be distinguish as the adsorptive. The term adsorption may also be used to denote the process in which molecules accumulate in the interfacial layer.


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When used in this sense, its counterpart desorption denotes the converse process, i.e. eth decrease in the amount of adsorbed substance.

Adsorption from fluid mixtures Adsorption from fluid mixtures is said to have occurred only when there is a difference between the relative composition of the liquid in the interfacial layer and that in the adjoining bulk phase(s) and observable phenomena result from the difference.

Aggregate An aggregate is, in general, a group of particles which may be atoms or molecules held together in any way: a colloidal particle itself (e.g. a micelle) may be regarded as an aggregate.

Breaking of a foam The breaking of a foam involves the coalescence of gas bubbles. Coalescence of solid particles is called sintering.

Coagulation or flocculation When a sol is colloidally unstable, i.e. the rate of aggregation is not negligible, the formation of aggregates is called coagulation or flocculation.

Coalescence is the disappearance of the boundary between two particles usually droplets or bubbles in contrast or between one of these and a bulk phase followed by changes of shape leading to a reduction of the total surface area. If coalescence is extensive it leads to the formation of a macrophase and the emulsion is said to break.

Colloid The term ‘colloid’ may be used as a short synonym for colloidal system.

Colloidal The term ‘colloidal’ refers to a state of subdivision, implying that the molecules or polymolecular particles dispersed in a medium have at least in one direction a dimension

roughly between 1 nm and 1 µm, or that in a system discontinuities are found at distances of that order.

Colloidal dispersion A colloidal dispersion is a system in which particles of colloidal size of any nature (e.g. solid, liquid or gas) are dispersed in a continuous phase of a different composition or state. The name dispersed phase for the particles should be used only if they have essentially the properties of a bulk phase of the same composition. Colloidal dispersion may be lyophobic (hydrophobic if the dispersion medium is an aqueous solution) or lyophilic (hydrophilic).

Colloidal electrolyte is an electrolyte which gives ions of which at least one is of colloidal size.

Colloidally stable means that the particles do not aggregate at a significant rate: the precise connotation depends on the type of aggregation under consideration.

Counterions Ions of low relative molecular mass, with a charge opposite to that of the colloidal ion.

Cream is the highly concentrated emulsion formed by creaming of a dilute emulsion.


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Detergent A detergent is a surfactant (or a mixture containing one or more surfactants) having cleaning properties in dilute solution

Emulsion In an emulsion, liquid droplets and/or liquid crystals are dispersed in a liquid.

Emulsifier An emulsifier is a surfactant which when present in small amounts facilitates the formation of an emulsion.

Foaming agent is a surfactant which when present in small amounts facilitates the formation of a foam or enhances its colloidal stability by inhibiting the coalescence of bubbles.

Gel A gel is a colloidal system with a finite, usually rather small, yield stress.

Latex A latex is an emulsion or sol, in which each colloidal particle contains a number of macromolecules.

Monodisperse If all particles in a colloidal system are or (nearly) the same size, the system is monodisperse; in the opposite case the systems are heterodisperse.

Nucleating agent A nucleating agent is a material either added to or present in the system, which induces either homogeneous or heterogeneous nucleation. The rate of nucleation is the number of nuclei formed in unit time per volume.

Polyelectrolyte A polyelectrolyte is a macromolecular substance which on dissolving in water or another ionizing solvent dissociate to give polyions (polycations and polyanions) – multiply charged ions – together with an equivalent amount of ions of small charge and opposite sign.

Sedimentation Sedimentation is the setting of suspended particles under the action of gravity or centrifugal field. If the concentration of particles is high and interparticle forces are strong enough the process of sedimentation may be called subsidence.

Sediment Sediment is the highly concentrated suspension which may be formed by the sedimentation of a dilute suspension.

Soap A soap is a salt of a fatty acid, saturated or unsaturated containing at least eight carbon atoms or a mixture of such salts.

Sol A fluid colloidal system composed of two or more components may be called a ‘sol’, e.g. gold sol, an emulsion, a surfactant solution above the critical micellization concentration, an aerosol.

Specific surface area The specific surface area is defined as the surface area divided by the mass of the relevant phase.

Surface, interface A boundary between two phases is called a surface or interfaces. In some instances the word ‘surface’ is limited to its geometrical meaning while interface is used to


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describe the thin three-dimensional layer – surface layer or interfacial layer between the phases in contact.

Surface active agent (surfactant) is a substance which lowers the surface tension of the medium in which it is dissolved and/or the interfacial tension with other phases, and, accordingly, is positively adsorbed at the liquid/vapour and/or at other interfaces.

Suspension In a suspension, solid particles are dispersed in a liquid: a colloidal suspension is one in which the size of the particles lies in the colloidal range.

Thermodynamically stable or metastable means that the system is in a state of equilibrium corresponding to a local minimum of the appropriate thermodynamic potential for the specified constraints on the system (e.g. Gibbs energy at constant T and p).

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