colloid & interface science case study model answers distance learning course in cosmetic...
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Colloid & Interface Science
Case Study Model Answers
Distance Learning Course in Cosmetic ScienceSociety of Cosmetic Scientists
• Formulations were examples of lyophobic colloidal systems
• Dispersed and continuous phases are not compatible
• Interfacial properties are relevant • Size of interfacial area is important• Van der Waals forces will play a role at the interface
• We are creating new interface/interfacial area during the processing of the formulation• 2nd law of thermodynamics
Common Features
Liquid ( )Liquid ( )
Liquid ( )Liquid ( )
Liquid ( )Liquid ( )
Solid ( )Solid ( )
A broad diffuse boundary region separates the two immiscible liquids
The composition of the boundary region is not the same as the liquid/liquid or gas/solid interface. There is an abrupt transition from one phase to another at the point separating them
The interface
Characteristic Features Of Colloids
• Surface-to-volume ratio (S/V) is high• Potentially, colloidal systems may have interfacial areas comparable in
size to a football pitch! • 6 cm diameter jar containing 25 cm3 oil and 25 cm3 water respectively• Form emulsion droplets with a
diameter of 0.0001 cm• New interfacial area created
• 150,1681 cm150,1681 cm2 2 (~150 m(~150 m22))• S/V ratio: ~ 60,000S/V ratio: ~ 60,000
• 50,000 times increase in interfacial area!
Surface Area/Volume Ratio (S/V)
Oil
Water
d
Area of oil/water interface:Area = (d2/4)
Add emulsifier and shake to form particles with a diameter of x cm:Pvol = (4/3) (x3/8)
Number of particles (N) = V/Pvol Total surface area (S) = 4 (d2/4) NS/V Ratio = S/V V = volume of the continuous phase
S/ V ratio: variation with particle size
0
10000
20000
30000
40000
50000
60000
0.0001 0.001 0.01 0.1 1
Particle diameter (cm)
S/V
Rat
io
Volume = 25 cm3
Feel the force….
• The stability of cosmetic and personal care formulations (lyophobic colloids) are influenced by the following intermolecular interactions:
• Van der Waals attractive forces•Leads to product instability
• Electrostatic and steric interactions•Stabilise the dispersion
‘Do not underestimate the power of the force….’ – Darth Vader
Van der Waals Attractive Forces
• Forces with the greatest effect are :
• London Dispersion Forces or Universal Attractive Forces.
• Keesom or Orientation Forces (Dipole-Dipole Interactions), e.g. hydrogen bonding
• Debye Forces (Dipole Induced Dipole Interactions).
• Magnitude of the interactions affect properties such as surface/interfacial tension
Thermodynamics – The Fly In The Ointment
• Energy changes (G) during preparation of the dispersion is described by the 2nd law of thermodynamics
G = A – TS
• is the interfacial tension (emulsion), A is the ‘new’ interfacial area, T is temperature and S is the entropy contribution (mixing)
• Driving force for instability is determined by the magnitude of G.
• Reason why interfacial area plays an important role
Energy Changes : Emulsion Stability
Free Energy (G)
Time (t)TwoDroplets
One Droplet
FilmRupture
Rate is determined by the thinning and rupturing ofthe film separating the two droplets
Add emulsifiers to reduce interfacial tension and create ‘energy’ barrier (steric and electrostatic repulsions). Work needs to be done to overcome interactions (E)
Preferred pathway
E
Creaming Coalescence
Flocculation Sedimentation
Colloidal dispersion
Routes To Instability - Kinetic Mechanisms
Stokes’ Law - Predicting Phase Separation
For a spherical particle (dilute solution):
Rate = x = 2r2 (m - p) g t 9m
m = viscosity of the continuous phasem = density of continuous phase
p = density of dispersed phaser = radius of spherical particlet = time taken to move specified distance (x)g = acceleration due to gravity
Relevance – suspending pearlescent agents or pigments in cosmetic formulations
Stokes’ Law - Problem Solving
• Phase separation prevented by determining the mechanism
• Matching the density of the dispersed and continuous phase – ensure is small
• ‘Weighting’ the oil phase (changing the density)
• Increasing the viscosity
• Surfactant system (phase behaviour)
• Polymers
• Inorganics (clays, silicas)
• Shower gel & Liquid Foundation Formulations• Krafft point (viscosity problem) – anionic surfactants
• Alkyl sulphates are prone to become insoluble at low temperatures• Use hydrotrope
• Variation of viscosity with temperature• Micelle shape changes • Loss of rod micelle network (shower gel/shampoo)• Packing of the surfactant molecules within the micelle
Case Studies – Main Points To Remember
• Foaming problems caused by creaming of the conditioner from the formulation
• Will behave as an antifoam
• How can we stop the problem?
• Understand the properties of foam• Lyophobic colloidal dispersion• Polydisperse bubbles (cells)• Pressure differences (Laplace) are important• Drainage mechanisms (gravity, pressure pump)
Case Studies – Main Points To Remember
What is foam ?
• Dispersion of a gas in a liquid
• Trap gas by mechanical action (agitation)
• Can be a problem (industrial processes)
• Not stable (lyophobic colloid)….
• Foam is a collection of bubbles
• Stabilise using surface active agents – surfactants, polymers, particulates
Time
Gas bubbles trapped in liquid
Liquid drains from the films surrounding the gas bubbles (honeycomb structure)
Polyhedral structure is eventually formed
Life Cycle Of Foams
Foam Instability
• Gravitational force - drainage
• Capillary pressure (squeeze liquid from film separating bubbles) – liquid flows to regions of low pressure, i.e. separating cells (Plateau regions)
• Diffusion of gas across foam lamellae (bubble disproportionation)
• Leads to bursting of bubbles and rearrangement of foam lamellae
Foam Persistance
• Prevent drainage and diffusion of gas across foam lamellae (increase viscosity or retard fluid drainage by presence of liquid crystals)
• Polyelectrolytes bind to surfactant at interface – impart mechanical rigidity
• Close packing of surfactants at the interface• Maintain low interfacial tension
• Ionic surfactants (electrostatics) – can be screened by electrolytes and affect stability
• Annealing of foam lamellae by surfactant (Gibbs-Marangoni effect)
• Maintain equililibrium interfacial tension – foams can be deformed, i.e. stretchy
Film Elasticity Gibbs Marangoni Effect (Rubber Band)
• A =Area = Surface
tension
- - - - 1
-- -
--
- -- -
- -- --
112
f f
- - - - -
1
• Gravity thins lamellae
• Gibbs-Marangoni effect (combination of two separate processes) restores equilibrium (fills holes in the film) - lowers surface tension
• Concentration dependent (migration of surfactant to the interface from bulk solution)
A
d
dε A2
Foam Prevention - Antifoams
Oil
Oil
Oil spreads on the film and displaces surfactants O/L << Surface
Film thins and ruptures – result of change in interfacial tension between film and oil
Foam collapses
Air
Liquid
Air
Spreading
What happens when an oil drop is placed on a clean liquid surface?
Remains as a drop (lens on the surface)
Gas
LiquidOil
Or spreads as a thin (duplex) film
Oil layerLiquid
Gas
GL
OL
OG
Spreading• What happens when a liquid droplet (oil) is placed on a surface?
O
• It can reside as a droplet or….
S = GS - (OG + OS )
S is -ve S is + ve
The surface tension of the fluid (OG) <<< critical surface tension (CFT (GS)) for the liquid to spread along the interface (liquid or solid)
• We can predict whether the droplet will spread on the surface by considering the Initial Spreading Coefficient (S) interfacial tension ()
• The contact angle () of the fluid in contact with the surface will change over time
• Form a thin layer (spreading)
Characteristic Features Of Colloids
• The dispersed phase has an affect on the properties of the formulation, e.g. rheology or the phase volume (emulsions)
Monodisperse system (uniform droplets) : phase volume ~ 0.75 max Polydisperse system (non-uniform
droplets): phase volume > 0.75
Characteristic Features Of Colloids
Stratum corneum
Oil droplets
• Size matters!• Large oil droplets (macroemulsions) forms occlusive layer on surface of the substrate (e.g. skin) – delivery triggered by rubbing• Small oil droplets (microemulsions) penetrate surface of skin
• Improve deposition of silicones on hair, e.g. polydimethylsiloxane (PDMS)• Increase molecular weight (viscosity) or use cationic emulsifiers • Tailor particle size distribution
• Increase particle size to improve deposition• Deposition is poor for very small particulate sizes (microemulsions) though can be improved by presence of cationic polyelectrolytes and anionic surfactants (coacervates)
• Cosmetic foundation• Flocculation caused by insufficient dispersion of the solid particulates• Reduce particle size
• Interfacial properties become critical• S/V ratio increases
• Need to appreciate how dispersions behave and are made!• Wetting of the interface• Dispersant choice (anionic vs nonionic surfactants, or polymers)• Steric vs electrostatic stabilisation
Case Studies – Main Points To Remember
Properties Of Colloidal Dispersions
© BASF
Increase in surface area leads to better absorption properties, e.g. sunscreens
Dispersion• Surfactant (dispersant) wets the surface of the solid and
displaces any adsorbed fluids, e.g. gas.
• Solid disperses more readily in liquid.
Solid not wetted by surfactant
Wetting
• Why does a droplet of water refuse to form a film on a greasy surface?
• What causes a material to absorb a fluid, whilst another repels it?
• We are dealing with the properties of the interface and…
• Balancing the ‘driving’ forces of cohesion and adhesion• Cohesive forces are result of the Van der Waals interactions between the
molecules in the liquid• Adhesive forces are the result of Van der Waals interactions between the
molecules residing at the interface, i.e. fluid and substrate
• Wetting is purely: Adhesion >> Cohesion
Wetting
• Wetting is the displacement from a surface of one fluid by another
• Involves three phases - at least two must be fluids (liquid or gas) or a solid
• Wetting must take place before:• Spreading, dispersing and emulsification, e.g.
detergency (cleansing)
Wetting – the Young Equation
Spreading and wetting can be explained by the Young equation (1800’s).
OilLiquid (or air)
Substrate
= contact angle = surface tension
OL
OSSL
At equilibrium:
OS OS + + OLOLCOS COS -- SLSL = 0 = 0
Pigment Dispersions
Increase in interfacial area
Input of energy – high shear, grinding, milling
Initial wetting of agglomerates by dispersant
Breakdown of agglomerates
Aggregates of primary particles
Primary pigment particles
Electrostatic Interactions – The Electrical Double Layer
-ve
Cation
Surface potential
Stern layer
Zeta potential ()
Electric Potential ()
Zeta potential ()
Stern layer
Surface potential
Distance (x)
Boundary of double layer in contact with the solution (‘slipping plane’)
Electrical double layer described by Guoy Chapman or Stern models
– magnitude affected by pH
Potential energy (VT)
Primary minimum
Van der Waals attractive interactions
Particle Separation (X)
X
Repulsive electrostatic (electrical double layer) interactions
Resultant interaction
Energy barrier
-ve
+ve
A B
DLVO Theory – Electrostatic Stabilisation
VT = Vv + VR
VR
Vv
Steric Stabilisation - Oil In Water (O/W) Emulsion
OilOil
Oil droplets stabilised by anchored polymer chains
Polymer chains act as ‘barrier’to coalescence.
Steric Stabilisation – Performance Engineering
• Molecular weight and chemical structure are important
• Dispersing agents• Anchor to substrate to provide stability
(hydrophobic or ionic interactions with surface)• Conformation is important (loops & tails)• Electrostatic/steric stabilisation• Select dispersant for the application, e.g.
molecular weight
• Problems: • Poor adsorption (solvent quality), e.g. depletion
flocculation• Particle size is very small, bridging flocculation
may become an issue – assess particle size distribution (photon correlation spectroscopy (PCS)
‘Comb’ polymer
Bridging flocculation
Reduce particle size
Pigment
TailLoop
TrainOil phase
Water phase
Steric Stabilisation – Conformation Effects
Hydrophobic group
Radius of gyration
Polymer ‘brush’
Polymer ‘mushroom’
Polymer chains extend into solvent owing to interactions with neighbouring molecules at high concentrations
Steric Stabilisation – Conformation Effects
Compression of the polymer chains prevents the particles from coalescing and flocculating
Limited penetration of the polymer chains occurs during collision
Adsorbed layers of polymer are fully extended into the solvent
HO
H1
Solvent concentration gradient between bulk phase and adsorbed polymer layer. Polymer prefers solvent and particles are forced to part, allowing the chains to be solvated
Steric Stabilisation - Solvent Effects‘The Good, The Bad And The Theta!’
• ‘Good’ solvent• Polymer chain segments extended in solvent producing an open configuration (polymer is miscible).
• ‘Bad’ solvent• Polymer chain collapses into a more compact form.• Transition occurs at the theta (q) temperature• Polymer separates from solution, e.g. cloud point of PEGs
‘Good’ solvent ‘Bad’ solvent
Stabilisation Method – Pro’s and Cons
Need to add stabilising agent (polymer) Not reversible Sensitive to temperature changes (solvent quality) Operates in aqueous and non-aqueous systems
Easier to control Reversible Change ionic strength Predominantly aqueous based
Steric Electrostatic
The Krafft Point
• The Krafft phenomena is the temperature dependent solubility of ionic surfactants
• Below the Krafft point the surfactant exists as hydrated crystals - turbid appearance at low temperature
• Krafft point increases with increasing chain length
• Addition of salting out electrolytes increases the Krafft point
The Krafft Point
• Krafft point is lowered by branched chains
• Unsaturation (double bonds)
• Insertion of EO groups between alkyl chain and the head group - alkyl ether sulphates have lower Krafft points than alkyl sulphates
• Hydrotropes - enhance solubility of surfactants in water, e.g aryl sulphonates, short chain (C8/10 phosphate ester, APG...), amphoteric surfactants
Summary
• Use principles of colloid and surface chemistry to solve the problem
• Identify causes and their effect on the formulation – evaluate/performance indicators
• Problems can be caused by more than one process• Need to bear in mind….
‘Nae cannae change the laws of physics’
Montgomery Scott
Thermodynamics rules ok!
Solutions…
• More than one solution….
• Increase the viscosity of the continuous phase
• Polymers, surfactants….
• Adapt the formulation e.g. Krafft point, tolerant to water hardness…
• Reduce level of oils (emollients) if they are suspected of acting as a defoamer or remove them completely
• Replace immiscible components, e.g. compatibility issues
• Evaluate performance (rheology, tests…)
• Carry out storage tests…
Summary
• Use the INCI listings on back of products as a guide• Review patents
• Raw materials - careful selection what you put in is what you get out!
• Contact raw material manufacturers!
www.solvay.com