2 JOURNAL OF COSMETIC SCIENCE ing, especially when they are predestined for skin application. Viscosity measurements show that the concentration of components plays an essential role in the microemulsion gel structure (12). In the present work microemulsion gels were prepared with isopropyl myristate as the oil phase, polysorbate 80 as surfactant, glycerol as co-surfactant, and water as the continuous phase. Stable microemulsion gel regions were designated, and the influence of the glycerol-to-water mass ratio on the concentration boundaries within which mi- croemulsion gels exist was determined and discussed. Microemulsion gels' rheological behavior was characterized, and their apparent viscosity was determined as a function of water and glycerol concentration. EXPERIMENTAL MATERIALS Isopropyl myristate (98% pure), polysorbate 80 (polyoxyethylene 20 sorbitan monoole- ate, Tween © 80), and glycerol 99% were bought from Sigma Chemicals Co. (Saint Louis, MO). All water was distilled from an all-glass apparatus. PHASE STUDIES Pseudo-ternary phase diagrams were constructed for isopropyl myristate/polysorbate 80/glycerol/water systems at 40øC. Each diagram was characterized by a fixed glycerol- to-water mass ratio of 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, or 9:1. The boundaries of the microemulsion gel domains were determined by slow titration of the aqueous glycerol solutions in mixtures of isopropyl myristate and polysorbate 80. All titration experiments were performed in stirred water-jacketed beakers maintained at 40 + iøC. Each sample was assessed visually for clarity, stability, and flow. Clear, homogenous, and optically isotropic samples of high viscosity, which could not flow even when the container was turned upside down, were deemed to be within the microemulsion gel region (11). Samples prepared with compositions within this area were stable for at least six months at room temperature. RHEOLOGICAL STUDY The rheological properties of prepared microemulsion gels were studied using two cone- and-plate viscometers: Ferranti-Shirley (Ferrant Ltd., Moston, U.K.) and HAAKE VT 24 (HAAKE Mess-Technik GmbH u. Co., Germany). A Ferranti-Shirley viscometer was used with the medium cone (radius 2 cm and angle 20' 40"). The viscometer was calibrated with Bayer silicones 100 M and 1000 M (Caelo Caesar and Loretz, Hilden). The temperature of the plate was maintained constant at 37 _+ 1øC by a water circulator. The speed of rotation was increased from 0 to 100 rev/min -• over a period of one minute and subsequently decreased to 0 rev/min -• in the same time interval. Rheograms were plotted automatically on an X-Y recorder (Philips, Japan).

MICROEMULSION GELS 3 A HAAKE viscometer was used with the PK1, 1 ø cone (radius 1.4 cm and angle 1ø). The temperature of the plate was maintained stable at 37 + iøC by a water circulator. The speed of rotation was increased from 0 to 22.6 rev/min -1 over a period of one minute and subsequently decreased to 0 rev/min -• in the same time interval. Rheograms for the microemulsion gels of varying concentration were obtained after preparation and storage for 24 hours at 25øC, using the Ferranti-Shirley viscometer for high values of shear rate and the HAAKE viscometer for low values of shear rate. RESULTS AND DISCUSSION PHASE STUDIES Phase studies showed the existence of microemulsion gel regions in which: (a) the glycerol-to-water mass ratios were between 0:10 and 6:4, (b) the total concentrations of glycerol and water solutions in the system were between 15% and 50% w/w, and (c) the isopropyl myristate to polysorbate 80 mass ratios were between 1:9 and 6.5:3.5. Stable transparent gels outside the above limits could not be prepared under the experimental conditions of this study. A glycerol-to-water mass ratio of 2:8 produced the largest microemulsion gel region (Figure 1). The amount of glycerol and water solution required to form microemulsion gels in- creased as the glycerol content increased. For example, microemulsion gels were pre- pared, at an isopropyl myristate to polysorbate 80 mass ratio of 4:6, using total glycerol and water concentrations between 22% and 35% w/w, 23% and 43% w/w, 25% and 45% w/w, 25% and 45% w/w, 30% and 48% w/w, and 37% and 50% w/w, when the glycerol-to-water mass ratios were 0:10, 1:9, 2:8, 3:7, 4:6, and 5:5, respectively. An explanation of this observed increase in the amount of the glycerol and water solution required to form microemulsion gels, as the glycerol concentration is increased, might be the reduction in the concentration of water, which determines the swelling of poly- sorbate 80. Oil-in-water microemulsion regions (13), together with microemulsion gel regions, were obtained in the systems with glycerol-to-water mass ratios of 4:6, 5:5, and 6:4. These microemulsions were formed in regions with a higher aqueous phase concentration than in gel regions. According to Artwood and Florence (14), inversion of a water-in-oil microemulsion takes place upon addition of water via a viscoelastic gel stage, which is composed of a hexagonal array of water cylinders at lower water concentrations and a lameliar array of swollen bimolecular leaflets close to the oil-in-water microemulsion boundary. RHEOLOGICAL STUDY Graphs of shear rate against shear stress values (rheograms) showed pseudoplastic flow of prepared microemulsion gels. The thixotropy of examined gels was found to depend on the sweep time, and a sweep time of one minute was selected for all measurements. Figure 2 shows the rheograms of microemulsion gels with constant mass ratios of