j. Soc. Cosmet. Chem., 43, 1-12 (January/February 1992) Dynamic rheologic measurement of the interaction between xanthan gum and colloidal magnesium aluminum silicate CHUN-REN CHEN and JOEL L. ZATZ, Department of Pharmaceutics, Rutgers College of Pharmacy, P.O. Box 789, Piscataway, N.J 08855-0789. Received September 13, 1991. Synopsis An oscillatory shear device was used to measure the dynamic rheology of dispersions of xanthan gum (XG), magnesium aluminum silicate (MAS), and mixtures of the two. XG exhibited linear behavior up to about 50% strain, while the linear range for MAS was much more restricted. Storage modulus (G') values for MAS were nearly constant over a wide range of frequencies and the loss tangent was low, indicative of rigid structure. The structure of XG depended on concentration, and no yield value was evident. XG effectiveness in reducing sedimentation is ascribed to liquid structure and high apparent viscosity at low shear. The addition of XG to MAS dispersions extended the strain at which G' remained constant. The loss modulus (G") increased in value as a function of XG concentration while G' was generally reduced at low frequencies but raised at high frequencies. The loss tangent was increased. These changes point to a reduction in structural rigidity and an increase in flexibility. INTRODUCTION Xanthan gum (XG), a polysaccharide produced by the bacterium Xanthomonas campestris, is used industrially as a thickener and dispersion stabilizer. The material of commerce differs from most other anionic polymers in that the viscosity of water solutions is nearly insensitive to a variety of salts over a wide electrolyte concentration range. Many of the attributes of XG relevant to its rheologic properties were summarized in a previous paper from our laboratory (1). The gum undergoes a transition from an ordered to disordered state at elevated temperatures, depending on the salt content and the ions present. Shear viscometry was used to determine the influence of concentration and several added salts on aqueous solutions of a commercial gum. Aqueous gum solutions at a concentration of 0.3% or more are very viscous at low shear the data suggest the existence of a gel-like structure that is disrupted by shear. At moderate and high shear, the shear stress-shear rate curve exhibits no hysteresis. The power law constant, which is inversely related to the degree of pseudoplasticity, drops as the gum concentration is raised.

2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The effect of salts, while relatively small, depends on gum concentration in solution. At 0.3% gum there is a negligible change in viscosity in the presence of up to 0.01 M sodium or calcium chloride. At higher gum concentrations, the viscosity is raised somewhat by salts, while at lower concentrations, the opposite occurs. Further information has been obtained from dynamic rheology measurements. Santore and Prud'homme explored the behavior of a 4.7% XG broth at shear rates as low as 10-5 s- • (2). Both dynamic viscosity (Xl*) and steady shear viscosity (x I) followed the power law over several orders of magnitude of shear rate. In the ultra-low shear range there was no indication of either a positive deviation (which would signify the existence of a yield value) or a negative deviation that is typical of most simple polymer solutions. Values of Xl* were higher than corresponding values of Xl (at the same shear rate), suggesting the existence of long-range order. Rochefort and Middleman utilized both oscillatory and steady shear measurements to assess the influence of salt and temperature on flow behavior of gum concentrations of up to 0.5% (3). The measurements confirmed the existence of an order-- disorder transition at about 55øC in water solutions containing low salt and at higher temper- atures with high salt present. In the presence of salt, the "healing" of structure disrupted by shear or temperature is rapid and essentially complete. Veegum ©, a colloidal magnesium aluminum silicate (MAS), is a smectite clay whose platelets are capable of aggregating to form a "house of cards" structure in aqueous dispersions (4). The dispersions are highly thixotropic they are disrupted by shear but then regain their structure over time. Unlike XG, the properties of these clay disper- sions are highly sensitive to electrolytes. Combinations of MAS and several polymers, including XG and carboxymethylcellulose, have been promoted as having rheologic properties superior to either of the materials alone. Mixtures of MAS and XG appear to be synergistic with respect to viscosity and yield value, based on steady shear measurements using a Brookfield viscometer (5). Thixotropy was eliminated within optimum mixtures, in which the XG:MAS ratio was 1:9 to 1:2. Furthermore, single-point viscosity of dispersions of the mixtures is affected much less by aging than are dispersions of MAS alone. Continuous shear viscometry is widely used and the flow curves obtained by this technique contain a great deal of information about the behavior of non-Newtonian materials. However, the initial structure of the material under investigation is broken down during the course of measurement. In many cases, this structure is the most important attribute under consideration. Dynamic small strain methods have the ad- vantage of deforming a sample without necessarily disrupting its structure. These methods provide information about elasticity as well as viscous flow. A popular method for measuring viscoelasticity in the linear region utilizes oscillatory shear (6). The complex shear modulus, G*, is defined by Equation 1. c•(t) G*(co) - [1] ?(t) In this equation, c•(t) represents the time-dependent shear stress and •(t) is the strain. G* may be divided into real and imaginary parts, G' and G", respectively. G' is called the storage modulus (or dynamic rigidity) and refers to energy stored because of elas-