90 JOURNAL OF COSMETIC SCIENCE Of the physicochemical assays used to typify formulations for topical administration, rheological analysis provides information that is useful for developing suitable formu- lations and forms of dispensing with maximal stability. The present study was under- taken to characterize the surfactants DC Q2-5200 (laurylmethicone copolyol) and Abil WE 09 (cetyldimethicone copolyol-polyglyceryl-4-isostearate-hexyl laurate). The evapo- ration-freezing properties of these compounds was investigated to determine which was most appropriate for use in topical formulations (5,6). Glycols are used together with silicones to increase substantiveness. We therefore in- vestigated the effect of different concentrations of propylene glycol, polyethylene glycol, and glycerin on rheological properties and evaporation/freezing behavior. MATERIALS AND METHODS MATERIALS Dow Corning Q2-5200: laurylmethicone copolyol (CTFA). Alkyldimethicone copolyol, a nonionic oily external phase emulsifier for cosmetic w/o formulations, was supplied by Dow Corning Ltd. (Brussels, Belgium). Abil WE 09: cetyldimethicone copolyol-polyglycerol-4-isostearate-hexyl laurate (CTFA). Poly- siloxane polyalkyl polyether copolymer, a nonionic oily external phase surfactant, was supplied by Th Goldschmidt Ag (Essen, Germany). Polyethylene glycol (PEG 400), glycerin, and propylene glycol (PG) were supplied by Merck (Darmstadt, Germany). METHODS Evaporation and freezing tests were done in accordance with standard DIN-53170 (7) for the evaporation of volatile components in oil, in accordance with standard UNE 55- 902-85 (8) for the determination of volatile substances and water content, and in accordance with the recommendations of Pharm Eur V. 4.5.5 for vaporization residues of essential oils (9). All values are expressed as the mean of three determinations performed under identical experimental conditions. Rheological measurements were taken with a Brookfield DV-II+ rotatory digital vis- cosimeter (Barcelona, Spain). All assays were done in triplicate at 15 ø, 30 ø, 40 ø, and 50øC. RESULTS AND DISCUSSION EVAPORATION ASSAYS Single compounds. The silicones DC Q2-5200 and Abil WE 09 were tested at 15 ø, 40 ø, and 60øC. Figures 1A and B shows the results of evaporation tests with time. Evapo- ration was more complete in silicone DC Q2-5200 than in Abil WE 09, although viscosity was greater in the former (4500 mm2s -• at 25øC). Evaporation of both silicones was lower than that of water regardless of the temperature tested. After ten days at 15øC, no evaporation had occurred in Abil WE 09 water loss

BEHAVIOR OF SILICONE SURFACTANTS 91 • ABILV•.0g 15"C • ABILWE0g 40"C .,F'•-• ABILWE 0g i ! ! ! i 0 1 2 3 4 5 6 i i ! i 14- n 12- o [----I DC Q2-5200 15"C , i i i i i i i 0 1 2 3 4 5 6 7 ! ! ! 9 10 Figure 1. A. Percent evaporation of Abil WE 90 during ten days at 15 ø, 40 ø, and 60øC. B. Percent evaporation of DC Q2-5200 during ten days at 15 ø, 40 ø, and 60øC. was 4% at 40øC, and 14% at 60øC. In DC Q2-5200, evaporation at 15øC was 8% at 40øC evaporation was 16%, and at 60øC it was 18%. These results may reflect differ- ences in the chemical composition of the two silicones. At 15 øC all three glycols increased in weight, the greatest increase occurring in glycerin, as a result of its greater hygroscopicity. At higher temperatures the glycols showed different behaviors. Evaporation was fastest in PG and was complete after 24 hours. However, PEG 400 and glycerin evaporated less readily. After 10 days at 60øC, evapo- ration in glycerin was 40% and evaporation of PEG was 15%. Silicone-glycol mixtures. We then tested mixtures of silicones and glycols. The latter can