158 JOURNAL OF COSMETIC SCIENCE Figure 4. Conformation of hydrophobically modified inulin (HMI) at the O/W interface. The alkyl chains are soluble in the oil, and the polyfructose loops extend into the aqueous phase. mation of HMI at the O/W interface is given in Figure 4, whereas that of PHS-PEO at the W/0 interface is given in Figure 5. It is clear from the above description of polymer configurations that for full character- ization of the process of adsorption, it is necessary to know the following parameters, namely, the amount of polymer adsorbed per unit area of the surface, r (mole m- 2 or mg m- 2 ), the fraction of segments in close contact with the surface, p, and the distribution of polymer segments, p(z), from the surface towards the bulk solution. It is essential to know how far the segments extend into the solution, i.e., the thickness of the adsorbed layer. It is important to know how these parameters change with polymer overage (concentration), the structure of the polymer, and its molecular weight. It is also essen- Figure 5. Conformation of PHS-PEO-PHS block copolymer at the W/O interface. The PEO chains are soluble in the water droplets, and the PHS chains extend into the oil phase.

EMULSION STABILIZATION 159 rial to know how these parameters change with the environment, rn such aspects as solvency of the medium for the chains and temperature. INTERACTION BETWEEN DROPLETS CONTAINING ADSORBED POLYMER LAYERS (STERIC STABILIZATION) When two droplets containing adsorbed polymer layers (with an adsorbed layer thick- ness, 0) approach a distance of separation, h, whereby these layers begin to overlap, i.e., when h 20, repulsion occurs as a result of two main effects (8). The first repulsive force arises from the unfavorable mixing of the polymer layers when these are present in a good solvent (i.e., the chains are strongly solvated by the medium). The unfavorable mixing of polymer solutions in good solvent conditions was considered by Flory (9), whose theory was applied to the present case of interparticle interaction. A schematic representation of the mixing of polymer layers on close approach is shown in Figure 6, which shows the situation when two droplets with polymer layers are forced to approach a distance, h, that is less than 23, forming an overlap region with a volume element, dV. Before overlap, the chains have a volume fraction, p2, and the solvent has a chemical potential, µ 1 a_ In the overlap region, the volume fraction of the chains is p/, which is higher than p 2 , and the solvent has a chemical potential, µ 1 13, which is lower than µ 1 a. This is equivalent to an increase in the osmotic pressure in the overlap region. As a result, solvent diffuses from the bulk to the overlap region and the two particles or droplets are separated, i.e. this results in strong repulsion. The latter is referred to as mixing or osmotic repulsion. Using the Flory-Krigbaum theory (9), one can calculate the free energy of mixing, G mix ' due to this unfavorable overlap, i.e., 6 G m ix 47T 2 ( 1 ) ( h) 2 ( h) = r1--. N - - X o - - + 23 + - kT 3V 1 �2 av 2 -3R 2 µ� Chemical potential of solvent Fi g ure 6. Schematic representation of the overlap of two polymer layers. (2)