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

o o •{, , , , , , , , , , , , • • , .... •- -•/, , , .... , ...... o 2• so 7• lOO o • ,o • •oo •+w • s •+w • 0 25 50 75 100 0 25 50 75 100 G+W e s G+W f s Figure 1. Ternary phase diagram of the system isopropyl myristate (O), polysorbate 80 (S), glycerol (G), and water (W), showing area of existence of microemulsion gel, at 40øC, for glycerol-to-water mass ratios of: (a) 0:10, (b) 1:9, (c) 2:8, (d) 3:7, (e) 4:6, and (f) 5:5. Tie-lines of the (c), (d), and (e) diagrams, along which microemulsion gels were investigated, represent systems containing (c) 4:6, (d) 5:5, and (e) 4:6 of O:S.