6 JOURNAL OF COSMETIC SCIENCE Table I Composition and Apparent Viscosity ('q'4.2) of Microemulsion Gels at the Shear Rate of 4.2 s -• Isopropyl myristate Polysorbate 80 Glycerol Water qq'4.2 (% w/w) (% w/w) (% w/w) (% w/w) (Pa s) 32 48 4 16 46.5 3O 46 5 19 56.8 28 42 6 24 103.2 26 40 7 27 111.8 24 36 8 32 110.1 20 30 10 40 56.8 25 37 19 19 3.8 24 36 2O 2O 4.1 23 35 21 21 5.4 22 34 22 22 4.5 21 33 23 23 4.3 2O 3O 25 25 4.1 swelling, the viscosity of microemulsion gel decreases because the molecule cohesion decreases with an increase of aqueous phase. The maximum value of apparent viscosity was obtained when the total glycerol-and- water concentration was close to 40% w/w, for all preparations with varying glycerol- to-water and isopropyl myristate to polysorbate 80 mass ratios. This concentration was selected to determine the values of shear rate and shear stress on the upward curve, and the apparent viscosity was calculated at the shear rates of 4.2 s -•, 34 s -•, and 136 s -• The results (Table II) show a decrease in apparent viscosity, with an increase in glycerol- to-water mass ratio. This is because the concentration of water in aqueous phase deter- mines the water swelling of polysorbate 80, and for a constant total concentration of glycerol and water, the concentration of water decreases with an increase in glycerol- to-water ratio. Additionally, this decrease in apparent viscosity with the increase in glycerol-to-water ratio, at the concentration of aqueous phase giving the most viscous gel in all systems, leads to the conclusion that the presence of glycerol decreases the po- tentiality for the water swelling of polysorbate 80. Table II Apparent Viscosity of Microemulsion Gels with the Total Glycerol and Water Concentration of 40% w/w and the Constant Isopropyl Myristate to Polysorbate 80 Mass Ratio of 4:6 Apparent viscosity at the shear rates of: Glycerol-to-water 4.2 s -• 34.1 s -• 136.0 s -• ratio (Pa s) (Pa s) (Pa s) 0:10 172.0 72.5 21.3 1:9 137.6 63.9 16.0 2:8 110.1 50.3 13.7 3:7 99.8 36.2 12.8 4:6 68.8 25.1 7.9 5:5 41.3 8.5 4.2 6:4 17.2 3.8 1.0

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