]. Cosmet. Sci., 56, 267-277 (July/August 2005) The influence of ascorbic acid on the rheological properties of the microemulsion region of the SDS/pentanol/water system MARTA SZYMULA, Department of Radiochemistry & Colloid Chemistry, Faculty of Chemistry, Maria Curie Sklodowska University, M. Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland. Accepted for publication April 4, 2005. Synopsis The rheological properties of the microemulsion region of the SDS, pentanol, and water micellar system have been studied experimentally. The W/0 and 0/W microemulsions behave as typical Newtonian fluids: viscosity remains constant independent of the shear rate. For the bicontinuous system the shear-thinning effect is observed. The addition of ascorbic acid to the system caused a microemulsion viscosity increase, which was the highest in the bicontinuous region and at the lowest temperature. INTRODUCTION It is well known that rheology is an excellent tool for obtaining information about changes in colloidal structures and their interactions ( 1,2). Associated colloid molecules generally form spherical aggregates in the aqueous solution above the CMC concentra­ tion value (2). Under appropriate conditions of concentration, salinity, and temperature, in the presence of counterions, these spherical micelles grow in size or change their shape, resulting in the formation of rodlike structures or even long flexible micelles. For the dilute ionic surfactant solution, a simple way to transform spherical micelles to cylindrical micelles is to change the packing parameter, i.e., to decrease the effective headgroup area by adding salt, which diminishes the electrostatic repulsion between the headgroups and allows them to be located closer. With the increasing salt concentration, the cylindrical aggregates grow to flexible wormlike micelles and behave like polymers (3 ). The formation of such structures modifies the rheological properties of surfactant solutions significantly. This effect is more pronounced in the systems with cationic surfactants than in those containing nonionic and anionic surfactants (3-12). In emulsions the relationship between structure and rheology has been widely investigated (13-17). As follows from these investigations, the viscosity of a mesophase 267

268 JOURNAL OF COSMETIC SCIENCE structure, such as a lamellar phase, can be many orders of magnitude higher than that of an isotropic swollen micellar phase that can have a composition only slightly different from that of the mesophase. The viscosity of an emulsion is one of the most important factors when it is used as a delivery system in cosmetic and pharmaceutical applications. The present work is part of our wider studies of the influence of the microenvironment (i.e., type of micellar system) on the activity of antioxidants of varying hydrophobicity (18-23). In this context, it seems interesting to investigate how the addition of bioactive species to the emulsion system changes its rheological properties. Vitamin C, in the form of 1-ascorbic acid (AA), is the most popular antioxidant. It can protect tissues and cells against oxidative damage by free radicals and reactive oxygen­ derived species (18,24,25,27-31). Vitamin C also improves the elasticity of the skin and reduces wrinkles by its well-known ability to take part in proline and lysine hydroxyl­ ation in collagen biosynthesis. Because of its ability to suppress pigmentation of the skin and decomposition of melanin, it can be used as a whitening agent, removing age spots (28-30). For the reasons mentioned above, vitamin C has been used in cosmetic and dermatological products. Because of its hydrophilic character, ascorbic acid has a lower ability to penetrate into the skin. Therefore it is important to select a suitable carrier system to deliver it to the site of action. Microemulsions such as the colloidal drug (e.g., vitamins) delivery systems are preferred because they usually enhance penetration. After application to the skin, the microemulsion components interact with the intercellular lipids of the skin, resulting in changes in their liquid lamellar structure. This effect facilitates drug transport (31-34). Other advantages of microemulsions are thermody­ namic stability, simple technology of preparation, and high solubilizing power (3 5 ). Although the use of microemulsions seems to be suitable for cosmetic products, it should be noted that they are very often a low-viscosity Newtonian fluid (13). For this reason, they cannot be applied directly to the skin without modifying their viscosity. The usual way to solve this problem is to add suitable thickening agents. But first of all, depending on the final requirements for the product (moisturizing cream, nourishing cream, mas­ sage cream) and the skin conditions of the user, the emulsion type and its rheological properties ( cream or lotion) should be determined. In the literature one can find propositions of delivery systems of ascorbic acid and its derivatives, which are most convenient for cosmetics (25 ,26-34). Spiclin et al. (28,29) proposed as the delivery system for ascorbyl palmitate and sodium ascorbyl phosphate the microemulsion consisting of the medium-chain-length trigliceryde Mygliol 812 as the oil phase, the PEG 8 caprylic/capric glycerides (Labrasol) as the surfactant, the polyglyceryl-6-dioleate Plurol oleique as the cosurfactant, and water. Gallarate et al. (31) in their studies of ascorbic acid stability in emulsified systems for topical and cosmetic use applied O/W microemulsions, O/W and W/O emulsions, and a W/0/W multiple emulsion prepared using the nonionic, skin-compatible emulsifiers dodecylglucoside and cocoamide propylbetaine. As oil they applied isopropyl palmitate or cetearyl oc­ tanoate, and 2-ethyl-1,3-hexanediol was chosen as the cosurfactant. The authors under­ lined the possibility of increasing the stability of AA by choosing an appropriate type of emulsified system in which it is vehicled. The W/0/W multiple emulsion appears to be an interesting topical vehicle for AA, as it provides better stability of the vitamin over time than that given by the W/O emulsion. Han et al. (25) studied the phase behavior of alkanoyl-6-O-ascorbates/water/dodecane mixtures and butyl lactate and butanol as the cosurfactants as far as the application of vitamin derivatives in cosmetics is concerned.