PHYSICAL CHEMISTRY AND PRODUCT DEVELOPMENT 903 in an O/W emulsion stabilized with a soap solution, the --COO- ions will be protruding from the surface thus giving the particle a negative charge. An O/W emulsion stabilized with cationics will have a positive charge. A W/O or O/W emulsion stabilized with nonionics will have no charge, but adsorption of ions from the aqueous phase is possible. 2. The pH of the solution. This applies specifically to surfaces that contain both acidic and basic ionic groups, such as polymers or amphoteric surfactants. As previously stated, the dielectric constant, D, is dependent on the concentration of ions in solution. These ions can be adsorbed or chemi- sorbed onto the surface, thus giving it a charge. One must be very care- ful in noting that, as shown in Figs. 4 and 5, the increase in electrolyte concentration compresses the thickness of the double layer, thus tending to reduce the repulsive forces and hence lead to instability of the formula- tion (4). Caution must, therefore, be exercised in determining the exact electrolyte concentration required to give the suspension or emulsion the needed repulsive forces and permanent stability. In order of decreasing flocculating ability (5), the monovalent cation and anion progressions are Cs, Rb, NH4, K, Na, Li, and F, IOa, H2PO4, BrOa, C1, C1Oa, Br, NOa, C10•, I, CNS, respectively. Also, a very important element is the valency of the electrolyte as shown in Fig. 6. A trivalent electrolyte will compress the EDL more than a monovalent one and, therefore, require less of it in solution to accom- plish the exact same effect of a monovalent or divalent electrolyte at Figure 4. Effect of mole/liter concentrations of monovalent counter ions on double layer potential •k . •' 25' .•' Fzgure 5. Effect of mole/liter concentration of monovalent counter ions on surface charge of a particle

904 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 6. Effect of electrolyte concen- tration and valency on the double layer potential • of a pa•icle t0 higher concentrations (6). These ion effects can be systematized by referring to the Schulz-Hardy rule, which states that the valence of the ions having a ch•ge opposite to that of the hydrophobic particle appears to determine the effectiveness of the electrolyte in coagulating the particles. The flocculating value of efficiency, therefore, increases with the valence of the ions. Div•ent ions are 10 times as effective as monov•ent tri- valent •e 1,000 times as effective as monov•ent. This rule, however, is only valid for systems in which there is no chemical interaction between the flocculating electrolyte and the ions of the EDL. The zeta potenti• equation, previously described, shows that the EDL is dependent on the viscosity of the medium. This is an important and significant dependency because particle size then comes into play. An emulsion or dispersion which has smiler oil droplets would not, by it- self, guarantee stability. Simil•ly, a viscous solution, by itself, may •so fail. However, once deflocculation, or sedimentation, commences, a new set of phenomena takes effect namely, that the sedimentation rate, or creaming rate, will be governed by phenomena such as Stokes law, etc. The significance of this theory (7, 8) is that a good nonc•ing suspen- sion may be produced if the particle charge is maintained low and/or the electrolyte is maintained at a su•ciently high level. Therefore, ionic surfactants may induce flocculation by neutraliza- tion of the particle surface charge one finds that addition of excess sur- factant would lead to a charge reversal and a tendency for deflocculation. Nonionic surfactants, on the other hand, •equently increase defloccula- tion, which is a good feature especi•ly in cosmetic applications. Flocculation or defiocculation may •so be induced or retarded by other methods and a cosmetic chemist may have to use a suitable com- bination of ionics, nonionics, and electrolytes to produce the desired de- gree of deflocculation and, hence, stab•ity of the product. The knowl- edge of the zeta potenti• will help him achieve his go• easier and faster.