Schematic Representation Of Mixed Film Formation

Water

Epithelial Cells Seen Men Urine

Oil droplet Water droplets in an oil droplet

Water

Figure 7.10 Photomicrograph of emulsions: (a) an oil-in-water (o/w) emulsion, (b) a water-in-oil (w/o) system, (c) a water-in-oil-in-water emulsion in which the internal water droplets can be seen in the larger oil droplets.

Oil droplet Water droplets in an oil droplet

Water

Figure 7.10 Photomicrograph of emulsions: (a) an oil-in-water (o/w) emulsion, (b) a water-in-oil (w/o) system, (c) a water-in-oil-in-water emulsion in which the internal water droplets can be seen in the larger oil droplets.

during manufacture, aids the dispersal of the oil into droplets of a small size and helps to maintain the particles in a dispersed state (Fig. 7.11) Unless the interfacial tension is zero, there is a natural tendency for the oil droplets to coalesce to reduce the area of oil-water contact, but the presence of the surfactant monolayer at the surface of the droplet reduces the possibility of collisions leading to coalescence. Charged surfactants will lead to an increase in negative or positive zeta potential and will thus help to maintain stability by increasing VR. Nonionic surfactants such as the alkyl or aryl polyoxyethylene ethers, sorbitan polyoxyethylene derivatives and sorbitan esters and polyoxyethylene-polyoxypropylene-polyoxyethylene block copolymers are widely used in pharmaceutical emulsions because of their lack of toxicity and their low sensitivity to additives. These nonionic stabilisers adsorb onto the emulsion droplets and, although they generally reduce zeta potentials, they maintain stability by creating a hydrated layer on the hydrophobic particle in o/w emulsions. They effectively convert a hydrophobic colloidal dispersion into a hydrophilic dispersion.

In w/o emulsions the hydrocarbon chains of the adsorbed molecules protrude into the oily continuous phase. Stabilisation arises from steric repulsive forces as described in section 7.2.2. Emulsions are more complex than suspensions, because of the possibility (a) of movement of the surfactant into either the continuous or disperse phase, (b) micelle formation in both phases, and (c) the formation under suitable conditions of liquid crystalline phases between the disperse droplets.

It is usually observed that mixtures of surfactants form more stable emulsions than do single surfactants. This may be because complex formation at the interface results in a more 'rigid' stabilising film. Certainly where complex films can be formed, such as between sodium lauryl sulfate and cetyl alcohol, the stability of emulsions prepared with such mixtures is high. Theory has not developed to

Oil Adsorption Films
Figure 7.11 Surfactant films at the water/oil interface in o/w and w/o emulsions: (a) formation of a monomolecular film at the oil/water interface for the stabilisation of o/w emulsions; (b) stabilisation of w/o emulsions by the oriented adsorption of divalent soap salts (not to scale).

an extent that it can readily cope with mixtures of stabiliser molecules. Complex formation between surfactant and cosurfactants in the bulk phase of emulsion systems is dealt with in section 7.3.5, as this frequently leads to semisolid systems of high intrinsic stability.

7.3.2 HLB system

In spite of many advances in the theory of stability of lyophobic colloids, resort has still to be made to an empirical approach to the choice of emulsifier, devised in 1949 by Griffin. In this system we calculate the hydrophile-lipophile balance (HLB) of surfactants, which is a measure of the relative contributions of the hydrophilic and lipophilic regions of the molecule. Values of the effective HLB of surfactant mixtures can be calculated.

The HLB number of a surfactant is calculated according to an empirical formula. For nonionic surfactants the values range from 0 to 20 on an arbitrary scale (see Fig. 7.12). At the higher end of the scale the surfactants are hydrophilic and act as solubilising agents, detergents and o/w emulsifiers. To maintain stability, an excess of surfactant is required in the continuous phase; hence, in general, water-soluble surfactants stabilise o/w emulsions and water-insoluble surfactants stabilise w/o emulsions. Oil-soluble surfactants with a low HLB act as w/o emulsifiers. In the stabilisation of oil globules it is essential that there is a degree of surfactant hydrophilicity to confer an enthalpic stabilising force and a degree of hydrophobicity to secure adsorption at the o/w interface. The balance between the two will depend on the nature of the oil and the mixture of surfactants; hence the need to apply the HLB system. The HLB of polyhydric alcohol fatty acid esters such as glyceryl monostearate may be obtained from equation (7.14)

where S is the saponification number of the ester and A is the acid number of the fatty acid. The HLB of polysorbate 20 (Tween 20) calculated using this formula is 16.7, with S = 45.5 and A = 276.

Typically, the polysorbate (Tween) surfactants have HLB values in the range 9.6-16.7; the sorbitan ester (Span) surfactants have HLBs in the lower range of 1.8-8.6.

For those materials for which it is not possible to obtain saponification numbers, for example beeswax and lanolin derivatives, the HLB is calculated from

where E is the percentage by weight of oxy-ethylene chains, and P is the percentage by weight of polyhydric alcohol groups (glycerol or sorbitol) in the molecule.

Hydrophilic (water soluble)

(water dispersible)

Hydrophobic (oil-soluble)

0 0

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