402 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •3 3: 1,2 o i I I I 2 3 SHEAR RATE, s -= Figure 5. Shear stress versus shear rate for a 0.5% aqueous xanthan gum solution. Scan time for a complete cycle was 8 min. Numbers indicate the order of execution of the shearing cycles, and arrows indicate the direction of shear rate change. (Reproduced from reference 16 with permission of the copyright owner, the American Pharmaceutical Association.) In a recent study, the sedimentation of aqueous suspensions of sulfamerazine (10%) was monitored (17). As shown in Figure 6, the logarithm of sedimentation rate (obtained from the early portion of the sedimentation curve) was a linear function of xanthan gum concentration. Each increase in gum concentration of 0.1% resulted in about a tenfold Z .1 .01 ß DO1 ' O .1 .2 .3 .4 XANTHAN GUM CONC. (% W/V) Figure 6. Sedimentation of 10% sulfamerazine suspensions containing various concentrations of xanthan gum.

SUSPENSION STABILITY 403 decrease in sedimentation rate. Relationships of this type make it possible to determine the gum concentration needed to obtain any desired sedimentation rate. They also facil- itate comparisons between different polymers in terms of their efficiency in retarding sedimentation. Combination of xanthan gum with magnesium aluminum silicate increased the yield value (18) relative to that of the same materials used separately. Augmented network formation, due to charge-charge interactions between the gum and clay, was postu- lated. CAKING During manufacture of a suspension, particles are separated from each other as a result of wetting and dispersion. By placing a charge on particle surfaces, or through other means, particle dispersion can be maintained as long as the particles remain suspended. However, sedimentation brings particles together at the bottom of the container. If there is mutual repulsion, the particles within the sediment align themselves so as to stay as far apart as possible. This situation leads to efficient packing of the particles as the liquid medium is gradually expressed from the sediment. Under these conditions, the sediment eventually becomes highly compacted with (iron- ically) maximal contact between particles and it is very difficult (or impossible) to redisperse such a suspension by mild agitation, such as hand shaking. Formation of a non-redispersible sediment is referred to as "caking" or "cementing." Caking cannot be remedied once it occurs, but it can be anticipated and prevented. Three approaches can be taken for this purpose: 1) Formation of a non-sedimenting colloidal dispersion. 2) Formation of a non-sedimenting coarse dispersion based on stabilizer rheology (structured vehicle). 3) Controlled flocculation of particles. COLLOIDAL DISPERSIONS There are few cosmetic systems that would be candidates for formulation as a particulate colloidal system. Aside from the difficulty of producing particles that are small enough, the main drawback is that kinetic stability in colloidal dispersions requires that they be highly dilute, containing less than 1% dispersed phase. Colloidal dispersions in water are highly sensitive to small amounts of added electrolyte. Hydrophilic polymers may serve to protect hydrophobic colloids against salt fiocculation. NON-SEDIMENTING COARSE DISPERSIONS We have discussed some aspects of rheology related to sedimentation. If sedimentation can be stopped altogether, then caking can be prevented. Whether this is a feasible alternative depends on several factors. The agent(s) chosen must be functional at the desired pH and in the presence of other formulation components. The resulting vis- cosity must be compatible with the desired sensory and use characteristics of the product.