GUMS IN COSMETIC FORMUI,ATIONS 397 lower molecular weight. These products, although of equal viscosity, would differ in solid content and "body." A factor frequently not taken into account is the variation of polymer- stability with degree of polymerization. The stability of many hydrophilic gums varies inversely with the degree of polymerization, since the chance of a rupture along a polymeric chain increases as the chain becomes longer, provided there is random degradation. The author has found it possible to increase the viscosity-stability of certain hydrocolloid solutions as much as fivefold through replacement of a high molecular weight polymer by a low molecular weight type of the same polymer (6). The frequently observed higher stability of so-called "low quality" natural gums may be explained on the basis of a low degree of polymerization. The "low-quality" gums, designated as such because of low thickening capacity, consist of molecules containing relatively few repeating units. The depolymerization rate of these gums is usually smaller than that of the higher molecular weight species. This must be taken into account when choosing a suitable hy- drocolloid for a cosmetic product. Often a particular gum is rejected in favor of another gum because the latter is more stable. Such a comparison may be unjustified if a "high" polymer of the former was compared to a "low" polymer of the apparently more stable gum. The viscosity-stability of a cosmetic product may frequently be increased by using a low molecular weight grade instead of a high molecular weight grade of a given gum. Possible disadvantages of "low" polymers are the high concentrations necessary to obtain a desired viscosity and solubility limitations which may make the incorporation of the required quantity of gum impossible to attain. IMPURITIES Apart from such readily noticeable impurities as plant fragments, insect parts and mineral matter, other, less obvious impurities may be present in gums. Some of these impurities have a profound effect on the properties of the gum in which they occur. For example, many grades of sodium alginate used by the cosmetic industry contain appreciable amounts of residual calcium. In a study of five types of a pharmaceutical grade of sodium alginate obtained froIn three different manufacturers, Levy and Schwarz found residual calcium concentrations up to 1.3 per cent (7). Solutions prepared froIn such materials differ markedly in their properties from solutions of pure sodium alginate. The presence of calcium increases the viscosity of sodium alginate solutions due to partial cross-linking of the polymer. On addition of sequestering agents such as EDTA, citrates or sodium hexametaphosphate, a marked viscosity decrease takes place. The flow characteristics of sodium alginate, an essentially pseudoplastic system, are modified by the calcium present, which imparts the characteristic thixo-

398 JOURNAI, OF THE SOCIETY OF COSMETIC CHEMISTS tropic behavior of calcium alginate to the solution. Calcium alginate is more stable than sodium alginate, so that the stability of the solutions can increase with increasing calcium concentration. Thus the presence of some traces of calcium in sodium alginate may or may not be objectionable. One definite objection to the presence of calcium is related to a specific temperature effect which will be described in the following section. Another hydrophilic polymer that appears to be sensitive to trace im- purities is Carbopol 934 ©. This material, although reported to be unaffected by aging (8), was found to undergo rapid degradation in the presence of daylight. This degradation could be prevented by the addition of EDTA, suggesting a possible catalytic function of trace-metals in the degradation of the polymer (9). Carbopol 934 is also very sensitive to electrolytes and tends to be of limited usefulness in products which contain electrolytes or partially ionizable components. TEMPERATURE The exposure of hydrophilic polymers to elevated temperatures usually leads to some degradation and a decrease in the viscosity of their solutions. Cosmetic products may occasionally be subjected to limited periods of high or low temperature during shipment, storage or use. A tabulation of sev- eral annual mean temperatures in the United States (10) lists a maximum of 43øC. and a minimum of -25øC. It is evident that both the degradative viscosity changes due to prolonged exposure to elevated temperatures, and the reversible viscosity changes caused by limited periods of exposure to extreme temperatures, must be considered in choosing a suitable thickening agent for a given cosmetic product. For example, a tanning lotion likely to be exposed to direct and intense sunlight on the beach should contain a thickening agent which is somewhat resistant to high temperatures. In this case, the thixotropic clays are generally more useful than the hydro- philic gums. While aqueous solutions of most hydrophilic gums decrease in viscosity with increasing temperature, methylcellulose (11) and methylethylcellulose (12) solutions will eventually change to gels. The temperature at which such a change occurs depends on the concentration and the degree of poly- merization of the gum. In the case of iso-viscous solutions of methylcellu- lose, the temperature at which the sol-gel transformation takes place (the gel point) increases with molecular weight. For example, a 2 per cent solution of Methocel 400 © has a viscosity of 400 cps. under standard condi- tions. This solution will gel at 55øC. A 5 per cent solution of Methocel 25 © has the same viscosity, but its gel-point occurs at 47øC. (11). Parti,•l hydroxypropyl substitution in methylcellulose leads to products which are more resistant to thermal gelation. The nature of the solvent may affect the sensitivity of a polymer to heat.