EMULSION STABILITY 167 other values of isopropyl myristate to polysorbate mass ratio, the boundaries of the oil-in-water emulsion domain corresponding to the chosen value of the glycerol:water mass ratio were determined. The dispersion was continuously stirred with a magnetic stirrer in a water-jacketed beaker at 40 ø + iøC. Samples prepared with compositions within the oil-in-water emulsion regions were stable for at least two days at 25 ø + iøC. PREPARATION OF SAMPLES FOR PARTICLE SIZE MEASUREMENTS Two series of oil-in-water emulsion samples were prepared in order to assay the influence of glycerol and polysorbate 80 concentration on emulsion stability. In the first series the mass ratio of glycerol to water varied from 1:9 to 9:1, while the concentrations of polysorbate 80 and isopropyl myristate were maintained at 5% and 20% by weight, respectively. In the second series the concentration of polysorbate 80 varied from 0.1% to 20%, while the concentration of isopropyl myristate was maintained at 20% by weight and the mass ratio of glycerol to water was maintained at 4:6. The emulsions were prepared by adding the required amount of the glycerol solutions of known concentration to mixtures containing weighed amounts of isopropyl myristate and polysorbate 80. The dispersions were continuously stirred, in a beaker at 40 ø + iøC, with a magnetic stirrer at 1300 rev/min for 15 minutes, and the emulsions formed were stored at 25 ø _+ iøC. RHEOLOGICAL STUDY The rheological behavior of emulsions was performed at 25 ø + 1 øC using a HAAKE VT 24 viscometer with a double-cap sensor system NV. The speed of rotation was increased from 0 to 22.6 rev/min over a period of one minute and subsequently decreased to 0 rev/min in the same time interval. Ubbelohde suspended-level viscometers were used to carry out measurements of lower values of the viscosity. The viscometer, with an emulsion flow time between 100 s and 1000 s, was placed in a water bath at a constant temperature of 25 ø + iøC, and the flow time of the emulsion was measured. The viscosity was calculated from the flow time and the density of the emulsion. Density measurements were carried out at 25 ø + 1 øC using a density bottle of 10 mi. PARTICLE SIZE MEASUREMENTS Measurements were performed at 25 ø + iøC using a Coulter counter (model ZM) with a tube of 70 tam. The instrument was calibrated using latex spheres with a singlet number median diameter of 5.99 ism. The sample container was turned upside down seven times, and one drop of the sample was dispersed in a 150-ml solution of 0.9% NaC1. The latter had been already filtered through Millipore filters HA (0.45 tam) and GS (0.22 ism). The number of strange particles in the electrolyte was determined with a blank assessment.

168 JOURNAL OF COSMETIC SCIENCE The number of droplets in the diluted emulsion was counted in ten size ranges and a volume-number mean diameter was calculated according to the following equation (10): where n i is the number of droplets of diameter di. CREAMING MEASUREMENTS Creaming was measured by a visual technique. An amount of 10 ml of each formed emulsion was stored in a closed volumetric cylinder of 10 ml at 25 o + 1 øC. The volume of the formed cream was measured one month after the preparation. RESULTS AND DISCUSSION EMULSION AREAS Ternary phase diagrams, presented in Figure 1, show the influence of the glycerol:water mass ratio on the area of existence of stable oil-in-water emulsions. Stable oil-in-water emulsions are formed with any mixture of glycerol and water. Stable oil-in-water emul- sions with an isopropyl myristate concentration higher than 75 % could not be prepared under the experimental conditions of this study. Also, systems with a polysorbate 80 concentration higher than 20% could not be prepared. Phase studies on these systems, for the polysorbate 80 concentration higher than 20%, showed the existence of oil-in- water microemulsion and microemulsion gel regions (11). VISCOSITY The rheological behavior of emulsions was determined 24 hours after the preparation. The results with the HAAKE viscometer showed that the viscosity of each emulsion remains stable for different values of shear rate, namely that the examined emulsions were NewtonJan systems. The viscosity of emulsions with a value under 20 mPa s was confirmed with Ubbelohde suspended-level viscometers. The results are shown in the last columns of Tables I and II. An increase in viscosity with an increase in glycerol concentration in the external phase of the emulsions (Table I) is expected, because glycerol, as a very viscous liquid, increases the viscosity of an emulsion exponentially when added in the external phase. There are more viscous agents than glycerol in order to increase the viscosity of the external phase of an emulsion, such as methylcellulose, but glycerol is more convenient when the form is destined for topical pharmaceutical, as well as cosmetic, use (12). The decrease in the viscosity of the emulsions, with the increase in the polysorbate 80 concentration, at surfactant concentrations up to 1% by weight (Table II), is attributed to the decrease in droplet size with the increase in the surfactant concentration. On the other hand, at concentrations higher than 1%, the polysorbate 80 does not remain just