j. Soc. Cosmet. Chem., 37, 329-350 (September/October 1986) Stabilization of oil-in-water emulsions by gums JOEL L. ZATZ and BERNARD K. IP, Rutgers University, College of Pharmacy, Busch Campus, P.O. Box 789, Piscataway, NJ 08854. Received January 9, 1986. Presented at the Annual Scientific Seminar of the Society of Cosmetic Chemists, New York, June 5-6, 1985. Synopsis Model emulsions containing 10% mineral oil, oleth 3, oleth 10, methylparaben, propylparaben, water, and several concentrations of selected gums were prepared. A two-step manufacturing procedure, designed to prevent variations in initial particle size in emulsions containing the sanhe emulsifier concentration, was employed. Median particle size, measured by an electronic sizing technique, was inversely related to emul- sifter concentration. There was no significant change in median particle size of the emulsions after storage for over one year. All of the emulsions were flocculated to sonhe extent. Initial viscosity values of emulsions containing ionic polymers were a function of emulsifier concentration, due, we believe, to residual ions. Emulsion viscosity changed with tinhe in many cases. The logarithm of creaming rate was a linear function of gum concentration, making it possible to compare different gums quantitatively. The order of effective- ness in retarding creaming was xanthan gum carboxymethylcellulose, high viscosity methylcellulose, 4000 cp. Inclusion of high concentrations of sodium sulfate and/or storage at elevated temperature (45øC) decreased emulsion stability markedly. Under these conditions, the rate of oil separation was generally inversely related to polymer concentration. INTRODUCTION Naturally occurring macromolecules, such as gums, have been used as stabilizers in food, cosmetic, and pharmaceutical emulsions for many years (1,2). Stabilization was attributed to the formation of rigid films at the interface between oil droplets and aqueous solutions of the macromolecules (3,4). Biswas and Haydon (5) investigated the viscoelastic properties of the adsorbed film formed by various proteins, such as albumin, pepsin, lysine, and arabinic acid. They found that stability of the droplets against coalescence increased with increased film viscosity and thickness. Polymers that are not surface-active may influence emulsion stability by affecting rheo- logical behavior of the external phase so as to minimize or slow the process of creaming. Beneficial rheological characteristics include pseudoplastic behavior and/or the presence of a yield value which confers high viscosity at rest while permitting such high shear activities as shaking and pouring. Xanthan gum is a high-molecular-weight natural polysaccharide produced by the fer- mentation of Xanthomonas campestris in a glucose medium. The polymer was originally isolated and characterized by Jeanes and coworkers (6). The xanthan molecule exhibits a high solution viscosity with increasing ionic strength the effect of salt on solution 329

330 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS viscosity depended on the xanthan gum concentration (7). Milas and Rinando (8) inves- tigated the xanthan conformation and concluded that the gum underwent chain melting at a certain temperature, which depended on the ionic strength but not on polymer concentration. Heat promotes the transition to a denatured structure at low ionic strength the xanthan conformation is stabilized at high ionic strength (9). Holz- warth proposed that the xanthan molecule underwent a transition from native --- dena- tured -- renatured form as salt was added (9). The native molecule has a stiff chain conformation and is arranged in a double or triple helix. Xanthan gum solution is extremely pseudoplastic (10) and shows little evidence of thixotropy, except perhaps at low shear (7). The stiff polymer chains form transient aggregates (11) and force is required to dissociate these aggregates, resulting in shear thinning. In the absence of shear, reaggregation takes place almost immediately. Vis- cosity of aqueous xanthan gum solutions is relatively insensitive to salts and changes in temperature (7, 12). The inability of xanthan gum to lower the surface tension of water (13) suggests that its functionality as an emulsion stabilizer is not due to an interfacial mechanism, but depends instead on bulk phase rheological considerations. The present investigation was undertaken to study the effect of concentration of xanthan gum and other polymers on emulsion droplet coalescence and sedimentation rate (or creaming). Previous work has shown that emulsion properties are sensitive to manufacturing tech- nique (14) and the concentration of emulsifier used (15). Since we were concerned with effects of formulation on emulsion stability, we employed a standardized manufacturing procedure. 10% mineral oil-in-water emulsions containing several concentrations of nonionic emulsifiers were investigated. A difficulty in studying emulsion stabilization by polymers is that viscosity effects during manufacture may influence the initial particle size, upon which many properties of emulsions and the rate of creaming are strongly dependent. The comparison of a series of emulsions containing different polymers or different polymer concentrations becomes very complicated if particle size is a variable. We chose a consistent manufacturing procedure designed to avoid this problem. EXPERIMENTAL MATERIALS Distilled water was used to prepare the emulsions. Light mineral oil (Class III B, Say- bolt viscosity 125/135, Lot #710466, Fisher Scientific Co., Fair Lawn, NJ) was of laboratory grade. The surfactants were commercial samples of oleth 3 and oleth 10 (Volpo 3, Lot #660 Volpo 10, Lot #7178 both fragrance grade Croda Inc., NY). Xanthan gum (Keltrol, Lot #80834A, Kelco, Division of Merck & Co., Inc., San Diego, CA) was of food grade (M.W. = 13-50 million (16)). Sodium carboxymethyl- cellulose (Fisher Scientific Co., Fair Lawn, NJ) was CMC-7HSP (M.W. = 700,000 (17)). Methylcellulose (Methocel, Dow Chemical Company) was A4M premium U.S.P. grade (M.W. = 86,000 (18)). Methyl-p-hydroxylbenzoate (Fisher Scientific Co., Fair Lawn, NJ), propyl-p-hydroxylbenzoate (Eastman Kodak Co., Rochester, NY 14650),