LA PONITE CLAY--A SYNTHETIC INORGANIC GELLING AGENT 251 Table IV Effect of organic concentration on plastic viscosity (cP) at 2 •o clay concentration Compound Methanol Ethanol isoPropanol Acetone Ethylene G. Glycerol Concentration 10 20 25 30 40 50 60 70 2.2 2.4 3.4 14.6 $.0 5.1 s.$1 I 2.3 3.8 4.4 5.5 6.3 4.0 1.9 4.3 4.5 4.7 4.7 3.5 5.8 6.6 6.4 4.1 7.3 10.8 16.1 Sorbitol Concentration % W/W 14 28 42 56 70 6.3 10.6 5:4 lO.8 Table V Effect of electrolyte concentration on plastic viscosity (cP) at 2 • clay concentration System 40 % Methanol 25 % Methanol 40 % Ethanol 25 % Ethanol 40 •o isoPropanol 25 •o isoPropanol 40 % Acetone 50 •o Ethylene glycol 60 •o Glycerol 56 • Sorbitol Water 0 0.1 3.7 3.7 3.0 3.5 5.l 5.2 4.6 4.7 6.3 6.4 4.4 5.7 4.3 4.6 6.6 7.2 16.1 15.7 24.4 24.2 2.3 2.5 NaC1 me g-1 0.2 0.4 S S 3.8 3.8 5.7 S 4.9 5.0 S S 6.1 5.7 4.8 4.3* 7.1 7.1 16.2 14.8 18.1 23.4 2.9 3.4 Notes: * indicates that visible flocculation occurred S indicates gross flocculation and some settlement. 0.8 S 4.0 S 4.1' S 5.4* S 7.2 16.2 3.1' DISCUSSION The three properties studied, yield value, plastic viscosity and clarity, react differently to changes in the conditions, but are, of course, inter- related. Certain general trends can be observed in the results. 1. In all systems the yield value and plastic viscosity increase greatly with clay concentration, while the clarity, indicating a low degree of flocculation, does not alter significantly.

252 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The increase of yield value is caused by the presence of a greater con- centration of structure-forming elements. Since the plastic viscosity is measured under high shear, when the interparticle bonds are broken down, the Einstein equation may be applied to it as a first approximation, i.e. the plastic viscosity should increase linearly with the volume concentration of the disperse phase. The small change of clarity indicates that, although there is inter- particle bonding throughout the volume, flocculation, that is, the for- mation of small, densely packed groups of particles, does not occur. 2. When the concentration of the organic compound is increased, the yield value at first drops a little, then increases very rapidly, and finally decreases rapidly. At the same time, the clarity first improves, then remains nearly constant, until, at the peak yield value, flocculation starts. These effects, including the initial surprising changes, may be explained as follows:- It is well known that clay minerals of the smectite class absorb polar organic compounds on their silica surfaces. MacEwan (1) has reported that the organic liquids arrange themselves in up to three molecular layers between oriented clay lamellae. The low yield value of Laponite CP dis- persions in distilled water suggests that the particles are almost completely dispersed. The adsorption of small amounts of polar organic compound causes some oriented aggregation of the lamellae, and thus an effective reduction of the number of particles available for structure formation. The yield value, therefore, drops. Since only a limited amount of polar organic compound can be accommodated, (the maximum is approximately equal to the weight of the clay), at higher concentrations there is always a surplus in the medium. The effect of polar organic compounds on clay dispersions has been well described by van Olphen (2), whose arguments are used below. The thickness of the electrical double layer decreases when the dielectric constant of the medium is reduced, and such a reduction may be achieved by the presence of water-miscible solvents, such as alcohols or acetone. Consequently, the range of particle repulsion is reduced, allowing stronger interparticle bonding, that is, an increase of the yield value. At high con- centrations, the repulsive forces have a very short range and gross floccu- lation by Van der Waals forces can occur the yield value, therefore, goes through a peak, and the optical density suddenly increases. The changes of plastic viscosity are largely caused by the variations of the viscosity of the medium. The contribution from the clay is not con- stant, but increases. This still agrees with the Einstein equation if we