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Volume of hatched section (rov) = —— (3r, — x) = P (5 — H )2 (3a + 35 — 5 + H)

Vov = Total volume of overlap = 2rov

Figure 7.9 The model used in the derivation of equation (7.7): particles of radius a with adsorbed layer of thickness d approach to a distance H between the particle surfaces; r1 = (a + ¿) and x is the distance between the surface and the line bisecting the volume of overlap.

Reproduced from R. H. Ottewill, in Nonionic Surfactants (ed. M. J. Schick), Marcel Dekker, New York, 1967.

theta temperature) at which the polymer and solvent have no affinity for each other. Thus when T = 6, B tends to zero, and the stabilising influence of the hydrated layer disappears, as hydration is lost. Heating thus reduces A Gm in this case. Additives which salt out the macro-molecules from solution will have the same effect.

The requirement for the strict applicability of equations (7.1), (7.2) and (7.4) and subsequent equations is that the particles are monosized, which is rarely the case with conventional pharmaceutical emulsions and suspensions. Where particles of two radii, a1 and a2, interact, equation (7.2) is modified to

Similarly, VR is expressed by an analogue of equation (7.4), namely

where and are the surface potentials of particles 1 and 2.

These equations have been applied not only to the study of suspensions but to the reversible interaction of microbial cells with a solid substrate such as glass before permanent adhesion occurs due to the formation of polymeric bridges between cell and glass.

7.3 Emulsions

Emulsions - liquid dispersions usually of an oil phase and an aqueous phase - are a traditional pharmaceutical dosage form. Oil-in-water systems have enjoyed a renaissance as vehicles for the delivery of lipid-soluble drugs (e.g. propofol). Their use as a dosage form necessitates an understanding of the factors governing the formulation and stability of oil-in-water (o/w) and water-in-oil (w/o) emulsions, multiple emulsions (w/o/w or o/w/o) and microemulsions, which occupy a position between swollen micelles and emulsions with very small globule sizes. Photomicrographs of o/w, w/o systems and multiple emulsions are shown in Fig. 7.10. It is also possible to formulate nonaqueous or anhydrous emulsions, that is oil-in-oil systems and even multiple oil-in-oil-in-oil systems.

7.3.1 Stability of o/w and w/o emulsions

Adsorption of a surfactant at the oil/water interface, by lowering interfacial tension

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