Association colloids micelle formation

Association colloids - micelle formationGROUP 4Physical properties of surfactant solutionsSolutions of highly surface-active materials exhibit unusual physical properties. In dilute solution the surfactant acts as a normal solute (and in the case of ionic surfactants, normal electrolyte behaviour is observed). At fairly well defined concentrations, however, abrupt changes in several physical properties, such as osmotic pressure, turbidity, electrical conductance and surface tension. The rate at which osmotic pressure increases with concentration becomes abnormally low and the rate of increase of turbidity with concentration is much enhanced, which suggests that considerable association is taking place. The conductance of ionic surfactant solutions, however, remains relatively high, which shows that ionic dissociation is still in force.

Experimental study of micellesCritical micelle concentrations can be determined by measuring any micelle-influenced physical property as a function of surfactant concentration. In practice, surface tension, electrical conductivity and dye solubilisation measurements are the most popular. The choice of physical property will slightly influence the measured c.m.c., as will the procedure adopted to determine the point of discontinuity.Information concerning the sizes and shapes of micelles can be obtained from dynamic light scattering, ultracentrifugation, viscosity and low-angle X-ray scattering.Factors affecting critical micelle concentrations1. Increasing the hydrophobic part of the surfactant molecules favours micelle formation.2. Micelle formation is opposed by thermal agitation and c.m.c.'s would thus be expected to increase with increasing temperature.3. With ionic micelles, the addition of simple electrolyte reduces the repulsion between the charged groups at the surface of the micelle by the screening action of the added ions.4. The addition of organic molecules can affect c.m.c.'s in a variety of ways.Structure of micellesMicellar theory has developed in a somewhat uncertain fashion and is still in many respects open to discussion. Possible micelle structures include the spherical, laminar and cylindrical arrangements illustrated schematically. Living cells can be considered as micellar-type arrangements with a vesicular structure.Typically, micelles tend to be approximately spherical over a fairly wide range of concentration, but often there are marked transitions to larger, non-spherical liquid-crystal structures at high concentrations. Systems containing spherical micelles tend to have low viscosities, whereas liquid-crystal phases tend to have high viscosities. The free energies of transition between micellar phases tend to be small and, consequently, the phase diagrams for these systems tend to be quite complicated and sensitive to additives.

Spherical (anionic) micelle. This is the usual shape at surfactant concentrations below about 40 per cent,

Spherical vesicle bilayer structure, which is representative of the living cell,

(c) And (d) Hexagonal and lamellar phases formed from cylindrical and laminar micelles, respectively. These, and other structures, exist in highly concentrated surfactant solutionsSolubilisationSolubilisation is of practical importance in the formulation of pharmaceutical and other products containing water-insoluble ingredients, detergency, where it plays a major role in the removal of oily soil, emulsion polymerisation and micellar catalysis of organic reactions.In micellar catalysis, reactant must be solubilised at a location near to the micelle surface where it is accessible to reagent in the aqueous phase. The strong electrostatic interactions which are likely at this location may influence the nature of the transition state and/or reactant concentration; for example, cationic micelles may catalyse reaction between a nucleophilic anion and a neutral solubilized substrate.The Krafft phenomenonMicelle-forming surfactants exhibit another unusual phenomenon in that their solubilities show a rapid increase above a certain temperature, known as the Krafft point. The explanation of this behaviour arises from the fact that unassociated surfactant has a limited solubility, whereas the micelles are highly soluble. Below the Krafft temperature the solubility of the surfactant is insufficient for micellisation. As the temperature is raised, the solubility slowlyincreases until, at the Krafft temperature, the c.m.c. is reached. A relatively large amount of surfactant can now be dispersed in the form of micelles, so that a large increase in solubility is observed.SpreadingGROUP 4Adhesion and cohesionThe work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and form two separate liquid-air interfaces (figure 4.15a), and is given by the Dupre equation.

The work of cohesion for a single liquid corresponds to the work required to pull apart a column of liquid of unit cross-sectional area (Figure 4.15b)

Spreading of one liquid on another

When a drop of an insoluble oil is placed on a clean water surface, it may behave in one of three ways:1. Remain as a lens, as in Figure 4.16 (non-spreading).2. Spread as a thin film, which may show interference colours, until it is uniformly distributed over the surface as a 'duplex' film. (A duplex film is a film which is thick enough for the two interfaces - i.e. liquid-film and film-air - to be independent and possess characteristic surface tensions.)3. Spread as a monolayer, leaving excess oil as lenses in equilibrium,Monomolecular filmsGROUP 4The molecules in a monomolecular film, especially at high surface concentrations, are often arranged in a simple manner, and much can be learned about the size, shape and orientation of the individual molecules by studying various properties of the monolayer. Monomolecular films can exist in different, two-dimensional physical states, depending mainly on the magnitude of the lateral adhesive forces between the film molecules, in much the same way as threedimensional matter.Surface pressureThe surface pressure of a monolayer is the lowering of surface tension due to the monolayer - i.e. it is the expanding pressure exerted by the monolayer which opposes the normal contracting tension of the clean interface, or

y0 is the tension of clean interfacey is the tension of interface plus monolayerThe variation of surface pressure with the area available to the spread material is represented by a -A (force-area) curve. With a little imagination, -A curves can be regarded as the twodimensional equivalent of the p-V curves for three dimensional matter. (N.B. For a 1 nm thick film, a surface pressure of 1 mN m-1 is equivalent to a bulk pressure of 106 N m-2, or ~ 10 atm.)

The Langmuir-Adam surface balance (or trough) uses a technique for containing and manoeuvring insoluble monolayers between barriers for the direct determination of ir-A curves. The film is contained between a movable barrier and a float attached to a torsion-wire arrangement. The surface pressure of the film is measured directly in terms of the horizontal force which it exerts on the float and the area of the film is varied by means of the movable barrier.

The surface pressure of the film is determined by measuring the force which must be applied via a torsion wire to maintain the float at a fixed position on the surface (located optically) and dividing by the length of the float. For precise work, the surface balance is enclosed in an air thermostat and operated by remote control. With a good modern instrument, surface pressures can be measured with an accuracy of 0.01 mN m-1.THANK YOU