Microemulsions: Evolving technology for cosmetic applications
A. Jayakrishnan , K. Kalaiarasi , D. O. Shah

Synopsis

The solubilization of hydrocortisone by microemulsions based on the combination of sodium stearate and sodium myristate with various alcohols and hydrocarbon oils was investigated by spectroscopic means at room temperature. The alcohols employed were n-butanol, n-pentanol, n-hexanol, and n-heptanol. The oils ranged from n-hexane ( C ₆ ) to n-hexadecane ( C ₁₆ ). It was observed that an increase in chain length of alcohols affected the solubility adversely. Changes in surfactant concentration, oil chain length, or water-to-oil ratios in the microemulsions did not have any significant effect on the solubility of the steroid. The solubilization capacity of the microemulsions was comparable to that of pure alcohols.

The formulation of a microemulsion using pharmaceutically acceptable surfactants and hexadecane oil has been achieved. Brij® 35 and Arlacel® 186 were employed as surfactants, while isopropanol was incorporated as the cosurfactant. The microemulsion was stable upto 70°C without undergoing phase separation. Viscosity data suggested that the microemulsion obtained was the water-in-oil type. Electrical conductivity measurements were interpreted in terms of the percolation theory of charge transport. The solubility of hydrocortisone in the system was determined by gravimetric means and was found to be much greater than in isopropanol on a volume basis of alcohol incorporated in the system. As observed in the case of sodium stearate microemulsions, the solubilization capacity was independent of the oil chain length and water-to-oil ratios in these microemulsions. Replacing water by 0.9% NaCl solution did not affect the solubility of hydrocortisone in microemulsions. Possible applications of microemulsions in skin care and cosmetic areas are mentioned.

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