Chem. Pharm. Bull. 53(9) 1097—1102 (2005)

mulation of semisolid bases, generally with a significant influence on their rheological properties. When organic amines are employed as neutralizing agents of carbopols, it is possible to gel many semipolar liquids and mixtures of these liquids with water. Compatibility of the polymer and non-polar solvents depends on the formation of ion pairs with the amine. Although alcohols are useful for increasing the solubility of non-polar drugs, their use as cosolvents with hydrophilic polymers is often limited by potential physical incompatibilities among them. The rheological behavior of polymer solutions is determined not only by the polymer–polymer conformation and entanglement, but also by the concentration and polymer-solvent effects. Consequently, the compatibility of the gelling agent and non-aqueous solvent determines the alcohol content to be used in designing and developing technologically adequate hydroalcoholic gelled bases. Carbopols are synthetic cross-linked acrylic acid polymers used widely in the pharmaceutical and cosmetic industries. In recent years hydrogels obtained from these polymers have increasingly been employed as vehicles for pharmaceutical preparations. This kind of polymers are known for producing gels that depend on the pH of the medium. Neutralization causes dissociation of the carboxylic groups and expansion, through electrostatic repulsion, of the polymer chains, which form a three-dimensional network to gel the medium. For this reason, the pH of the medium is one of the most important parameters in their formulation and future characterization. The rheological and pharmacotechnical properties of these systems are determined by the extent of structural development in the network. Cross-linked acrylic acid polymers are already well-known for their unique rheological properties, such as the ability to impart vertical cling and a “no-drip” rheology. These unique properties can enhance the performance characteristics of applications. This kind of polymers also provides a shear thinning rheology and the ability to recover viscosity immediately after shearing has ceased, thus enabling the gel to be applied or squeezed out of a bottle easily. In addition, they are odour free and have a high buffering capability due to the large number of carboxylic acid groups on the polymer, which helps to maintain a stable pH in the finished product and during its use. Carbopol Ultrez 10, a carbomer polymer, is considered to be a universal polymer capable of replacing two or more carbomer polymer grades currently in use to formulate lotions, creams and gels. It is a special case of nonlinear, branched polymer chains which interconnect to form a threedimensional network structure. In dispersions consisting of network polymers the dominant structural feature is (water) swollen microgels. Polymer microgel particles increase the viscosity of a solution essentially through two mechanisms: high crosslink density and high microgel rigidity. It is interesting to note here that both high crosslink density and high microgel rigidity mechanisms, as present in Carbopol Ultrez 10, yield the most efficient carbomer polymers for viscosity building. An interesting crossover phenomenon occurs because of the dual nature of viscosity building through carbomer polymers. Low swelling carbomer polymers (e.g. Carbopol Ultrez 10, C-934) show less viscosity at lower polymer concentrations; however, the situation is reversed at higher polymer concentrations. This illustrates the unique rigid particle nature of Carbopol Ultrez 10 resin compared to the traditional C-940 and C-934 resins. In fact, the most efficient data reported in this paper show that the Carbopol Ultrez 10 carbomer polymer is capable of replacing either of these resins in certain dermopharmaceutical product formulations. The rheological parameters of Carbopol Ultrez 10 resin compared to the traditional C-941, C-940 and C-934 resins and quantitative values for each parameter have been published by BFGoodrich Company. Increased interest in these polymers for the elaboration of cosmetic and pharmaceutical formulations, and particularly the advantages and new nature of Carbopol Ultrez 10, justify an exhaustive rheological study of the gelled topical forms of the polymer as a function of neutralization and the September 2005 Chem. Pharm. Bull. 53(9) 1097—1102 (2005) 1097