j. Soc. Cosmet. Chem., 32, 275-285 (September/October 1981) Rheological properties of magnesium aluminum silicate/xanthan gum dispersions PETER A. CIULLO, R. T. Vanderbilt Company, Inc., 30 IVinfield Street, Norwalk, CT 06855 Received January 12, 1981. Presented at the SCC Annual Meeting, New York City, December 12, 1980. Synopsis The ability to modify the rheology of magnesium aluminum silicate (MAS) with various ORGANIC THICKENERS has long been known and used to advantage. The widely used MAS/ carboxymethylcellulose combination, for example, has been the standard stabilizing system in liquid makeups for a number of years. Use of xanthan gum with MAS is now shown to even further extend the versatility of this smectite mineral. Comparative evaluations demonstrate how minor inclusions of xanthan gum in MAS dispersions provide synergism in both viscosity and yield value. The ability to modify the THIXOTROPIC nature of MAS is also shown, allowing pseudoplastic behavior through small additions of xanthan gum. For the best balance of properties, the most generally useful range of MAS:xanthan gum ratios in practice will be 9:1 to 2:1. This combination should provide formulas with good viscosity stability and smooth flow characteristics. The excellent yield value possible suggests superior stabilization of suspensions and emulsions. The combination of these properties makes the MAS/xanthan gum system especially well suited to stabilizing all types of fluid suspensions, lotions, and makeup. INTRODUCTION The refined smectite mineral, magnesium aluminum silicate (MAS), has for decades been popularly used as a thickener, suspending agent, emulsion stabilizer, and general texture modifier in a number of industries. The observed rheological properties of MAS that account for its utility are well described by the popular '*house of cards" model for the aqueous colloidal structure of smectites. Once the MAS has bee•n well dispersed and exists as discrete platelets, according to this model, the weakly positive platelet edges are attracted to the negatively charged faces. This attraction, coupled with face-face electrostatic repul- sion, is sufficient to establish a cohesive cubic network. This network determines dispersion properties. In addition to imparting viscosity, it is • responsible for suspend- ing and segregating the internal phase of emulsions and suspensions. The amount of force required to disrupt this network determines the yield value. Shear exceeding the yield value produces reduced dispersion viscosity. Increasing shear gives decreasing viscosity, as with pseudoplastic materials. Cessation of shear allows displaced faces and 275

276 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS edges to again be mutually attracted and reform the cubic network. Since this structure recovery takes a measurable amount of time, the dispersion is considered thixotropic. The proposed mechanisms and arguments favoring this view of colloidal smectite structure have been detailed by van Olphen (1). Specific applications of MAS to cosmetic and pharmaceutical products have been described by Carlson (2,3) and Ciullo (4). To date, the most widely used organic in combination with MAS has been sodium carboxymethylcellulose (CMC). This anionic gum provides synergistic viscosities with MAS, even when included at as little as 10% of the mineral's weight. Greater electrolyte tolerance and reduced rise in dispersion viscosity over time are also obtained. The most common use of the MAS/CMC combination is in liquid makeups where the MAS provides emulsion and suspension stabilization while the CMC contributes to smooth flow. The polyanionic polyheterosaccharide, xanthan gum (XG) is a particularly functional material which offers additional potential in modifying the properties and extending the usefulness of MAS. XG is a high viscosity thickener with yield value and high electrolyte tolerance. As a pseudoplastic thickener, solution viscosity is reduced in proportion to the amount of shear. When shear is removed, viscosity recovery occurs almost instantaneously. The colloidal structure corresponding to this behavior is not well characterized for this material. The commercial literature (5) cites the work by Rees (6,7) suggesting that double helices are formed by the polymer chains, which in turn form flexible aggregates. Yield value would be a measure of the force required to begin dissociation of aggregates, with shear thinning resulting from further dissocia- tion. In the absence of shear, reaggregation would immediately take place. The properties and uses of XG have been reviewed by Jeanes (8). While XG is widely used in industrial and food products, it is not as popular as the various cellulose derivatives for cosmetic and pharmaceutical applications. It has been found, however, that use of small amounts of XG with MAS provides a synergism in both viscosity and yield value. This has for a number of years been used to advantage in household and agricultural products. This likewise suggests considerable potential in the personal products field for stable flowable emulsions and suspensions. The present investigation was undertaken to demonstrate the effects on dispersion rheology of combining MAS and XG in various ratios. The properties of each combination were compared to the properties of separate preparations of each component. The synergism in viscosity and yield value and effect on MAS thixotropy were thereby demonstrated. All dispersions and solutions were suitably preserved due to the susceptibility of XG to microbial degradation on aging. EXPERIMENTAL MATERIALS The magnesium aluminum silicate (VEEGUM, R. T. Vanderbilt Co.), food grade xanthan gum (Keltrol, Kelco Div. of Merck), and paraformaldehyde, 95% (Matheson, Coleman & Bell) were used as received. Distilled, deionized water was used for all preparations. All dispersions and solutions were made in a Waring Commercial