RHEOLOGIC MEASUREMENTS 7 lOO .01 .01 [] [] [] [] [] [] [] [] [] 0 0 0 0 0 0 0 0 0 ß ß ß ß ß ß ß .... ...i , ...... i ....... i ß ß ß ß ,"" .1 1 10 100 (0 (Rad/s) Figure 4. Storage and loss moduli of two MAS dispersions. C), G' 1% O, G" 1% [2], G' 3% I, G" 3%. At all concentrations and frequencies, the increase in G" attributed to the presence of XG is greater than any positive change in G'. Consequently, the mixtures have a higher value of loss tangent than for MAS alone (Figures 10 and 11). This difference, along with the G' data described above, indicates that the mixtures are less rigidly structured than the clay by itself. As might be expected, the effects of the various XG concentra- tions studied are more profound in the dispersions containing 1% MAS rather than in the higher concentration (compare Figures 6 and 7, 8 and 9, 10 and 11). Based on these data, we may speculate on the nature of the interaction between XG and MAS. Structural attributes of MAS are related to the formation of particle assemblies with an open structure. At sufficient concentration, MAS units join to form a three- dimensional network that extends throughout the dispersion. Shear breaks the network, accounting for thixotropic behavior. MAS structure persists in the presence of XG since G' values are significant, although modified from those in pure MAS dispersions (Figures 8 and 9). A picture consistent with the data is one in which assemblies of aggregated MAS units and aggregated XG units coexist. In the presence of XG, the formation of a tight single-particle network extending throughout the system that can also accommodate aggregated XG molecules is unlikely. Furthermore, the G' dependence on frequency is altered in the presence of gum. When the system is at rest, MAS aggregates are weakly connected to each other. These junctions are broken at low shear, but the separated assemblies can withstand somewhat higher shear before disruption. At high frequencies, at which there is little

8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS o 100000 10000 1000 100 10 .01 .001 ß ß i ß ß i i i I i ß ß i i , i i I .Ol .1 1 Shear Stress (Pa) Figure 5. Apparent viscosity as a function of shear stress for two MAS dispersions and one xanthan gum solution. /•, 1% MAS A, 3% MAS ß, 0.4% XG. time for structural change to occur, the elasticity provided by the two components should be additive. Indeed, at 10 rad/s and above, the G' values in Figures 8 and 9 increase with XG concentration. According to this model, XG molecules in the mixed dispersions are joined in a manner similar to their arrangement in pure gum systems. This is in accord with the finding that dispersions containing both components remain in the linear viscoelastic region at higher strain values than dispersions containing only MAS (Figure 1). The presence of XG should also provide additional "smoothness" to the dispersions when it flows. CONCLUSIONS The substances discussed in this paper (MAS and XG) utilize different mechanisms of structure formation in aqueous dispersion. Nevertheless, depending on their ratio, both materials contribute to the viscous and elastic properties of the network that exists in essentially undisturbed mixtures. MAS provides rigidity XG augments the viscous component. The behavior of the mixtures is, in many respects, intermediate between that of the components. Compared to MAS alone, mixtures with XG exhibit a larger linear viscoelastic range, and higher values of G" and loss tangent. These changes, along with differences in the pattern of G', suggest a reduction in structural rigidity and an increase in flexibility.