RHEOLOGY OF MAS/XANTHAN GUM DISPERSIONS 279 Single Point Viscosity (cps) 2:1 MAS/XG 1:1 MAS/XG o 4:1 MAS/XG O-- • 9:1 MAS/XG 19:1 MAS/XG . 29:1 MAS/XG MAS •o- '- ' I ' I 1do o,1 1 lO Week Month Time (days) Figure 1. Three percent magnesium aluminum silicate/xanthan gum dispersions showing progressive flattening of the MAS aging curve with increasing levels of XG. ratios, thixotropic dispersions are still obtained. With 10% addition of the gum (9:1 ratio), the dispersion is rendered pseudoplastic and nonthixotropic. This is likewise true for the 4:1, 2:1, and 1:1 blends. Due to the inherently high viscosity and yield value of XG alone, it is not obvious that a synergism truly exists. To clarify this, separate MAS and XG dispersions were made at solids corresponding to those used in each blend. These results are presented in Tables I through IV. The calculated viscosity synergism factors (VSF) by Equation 1 and yield synergism factor (YSF) by Equation 3 provide a simple, illustrative indication of actual, synergistic values as opposed to expected additive values. In these tables, a general increase in viscosity synergism is noted as the dispersions age, indicating a colloidal structure becoming progressively more complex. There is, however, an interesting difference in VSF values depending on the type of viscosity determination used. Single point values are derived from a stationary rotating spindle allowed to turn for 6 min. This allows equilibration of colloidal structure breakdown and rebuilding. On a practical basis, the single point values relate to the working viscosity of the system, i.e., while being spread, poured, sprayed, etc. Helipath values are obtained from an inverted-T spindle cutting a slow helical path. The crosspiece constantly encounters new material and leaves that which its previous motion has

280 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Shear Rate (Viscometer Spindle RPM) 60- 20 40 gO 8'0 1(•0 Shear Stress (Viscometer Dial Reading) Figure 2. Typical rheograms denoting (A) a thixotropic system, (B) a pseudoplastic system. disrupted. The combination of slow speed and helical path minimize non-Newtonian effects and provide a relative measure of the apparent viscosity under low shear or static conditions. In using these blends in actual formula development, single point derived values generally better reflect observed performance. Possible means of interaction of smectites with polysaccharides in aqueous systems has been summarized by Theng (10). Although considerations of possible platelet-gum molecule colloidal configurations to explain the observed effects are beyond the scope of this paper, it is assumed that interaction takes place mainly through attraction of anionic carboxylate groups of the gum to the slightly positive smectite platelet edges. Some hydrogen bonding to silicate oxygens on platelet faces may also occur, but this is probably minor due to electrostatic repulsion of carboxylate groups and the negative faces. The strongest interaction between MAS and XG is seen to occur at ratios from 9:1 to 2:1, with the 4:1 ratio especially pronounced by both VSF and YSF figures. In this range, the association of polymer chains with MAS platelets progressively modifies the MAS cubic network into a more complex and flexible structure. This becomes less pronounced at the 1:1 ratio. In the areas of cosmetics and pharmaceuticals, the modification of magnesium aluminum silicate rheology caused by inclusion of xanthan gum will normally be associated more with the derived yield value synergism than viscosity synergism. The latter can likely be obtained as easily and at lower cost with similar MAS/CMC blends. The observed yield value gain, however, has not been readily obtainable with the addition of organic thickeners other than XG. In effect, this yield value synergism allows greater lattitude in balancing viscosity with stability in suspensions and emulsions. For example, this may enable suspension stability at lower viscosity than would otherwise be possible. Smooth flow properties without need of agitation, even