GUMS IN COSMETIC FORMULATIONS 399 While the viscosity of aqueous solutions of Carbopol 934 is only slightly affected by temperature variations, solutions of the same polymer in glyc- erin are extremely temperature-sensitive (8). A special temperature effect may be found with aqueous solutions of sodium alginate containing residual calcium. When such solutions are frozen and thawed, their viscosity may increase as much as thirteenfold (2). This is due to a shift in the equilibrium between the calcium residue present in ionic form and that which is bound to alginate. On freezing, pure water separates as ice from the solution and causes an increase in the effective concentration of calcium ions. This leads to the formation of more calcium alginate in its typical cross-linked configuration which in turn results in a viscosity increase and a change in flow characteristics. The reversal to the previous equilibrium is very slow. Such viscosity-dependent properties as the pourability of a product may be seriously affected. SEQUESTERING AOEN'rS The addition of sequestering agents to solutions of hydrophilic gums may have various effects. In the case of sodium alginate, the sequestration of residual calcium causes a viscosity decrease while the addition of a suitable sequestering agent to Carbopol 934 prevents its degradation in the presence of daylight. When citrates, EDTA or hexametaphosphate are added to tragacanth, this gum breaks down rapidly, probably due to the removal of calcium by the sequestering agent (13). For this reason, citrate buffers cannot be used with tragacanth solutions if long-term stability is de- sired. INTERACTION WITH POLY-HYDROXY COMPOUNDS Many cosmetic products contain one of several polyols in addition to a hydrophilic gum. The choice of the polyol is usually based on its humec- rant, solvent and flow characteristics. However, the effect of the polyol on the properties of the gum should also be considered. In the case of methyl- cellulose, various polyols differ in their effect on the gel point (14). Sorbitol and glycerol lower the gel point considerably, so that products incorporating one of these polyols in combination with methylcellulose may, in hot weather, become cloudy or even change from a fluid, easily pourable liquid to a semisolid, opaque gel. On the other hand, propylene glycol and the lower molecular weight polyethylene glycol s actu ally raise the gel point of methyl- cellulose and help maintain a clear and liquid product even at relatively high temperatures. The addition of a polyol to aqueous solutions of a hydrophilic gum usually leads to an increase in the viscosity of such solutions. The effect of glycerol on the viscosity of a large number of gums has been tabulated by Segur and co-workers (17). When the concentration of a polyol is sufficient to inter-

400 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS fere with the hydration of a gum, an otherwise pseudoplastic solution may become thixotropic (18). Propylene glycol is especially useful because of its preservative properties. It has been stated (15), that propylene glycol appears to have a specific inhibitory effect against bacteria and most molds, while glycerol acts merely through an osmotic mechanism and will not inhibit micro/Srganisms unless present in high concentration. In one investigation, sodium carboxy- methylcellulose jellies prepared with propylene glycol retained their original viscosity, while those containing glycerol showed a decrease in viscosity during the same period (16). HYDROtEN Io• Co•eE•TRATtO• The pH exerts a profound influence on the viscosity and stability of many hydrophilic polymers. It has been reported that the viscosity of acacia changes markedly with pH, the maximum viscosity being obtained near neutrality (19). Some workers (20) have found that the maximum vis- cosity of tragacanth occurs at pH 8. On further investigation (13) it could be shown that this holds true only for an initial period of time. The pH for optimum long-term stability is about 5. At a pH below 4 or above 6, the stability of the gum is adversely affected. In general, statements that certain gums are stable over a wide pH range should be accepted with con- siderable reserve careful study often shows that the optimum pH range is quite restricted. PRESERVATJVES Lately a great deal of attention has been focused on the possible interac- tion of natural and synthetic gums with preservatives. Definite evidence of complexing between methylcellulose and the p-hydroxybenzoates has been demonstrated (21). Tragacanth was found to exert a strong neutral- izing effect on the bactericidal activity of chlorobutanol, the p-hydroxy- benzoates and benzalkonium chloride (22). According to other workers, it is possible to overcome this neutralizing effect by using adequate concen- trations of the preservative agent at the optimum pH range (23). The interaction between methyl p-hydroxybenzoate and methylcellulose, Car- bopol 934, and PVP has also been reported (24). Cationic preservatives should, of course, not be used in conjunction with anionic gums. SUMMARY Some effects of manufacturing methods, degree of polymerization, im- purities, temperature, sequestering agents, polyols, hydrogen ion con- centration and preservatives on the properties of hydrophilic gums have been described. These factors must be taken into account when formulat- ing cosmetic products containing hydrophilic gums.