Microcalorimetric study of microemulsions as potential drug delivery systems. I. Evaluation of enthalpy in the absence of any drug.

Two microemulsions with the same oil/water/surfactant mixtures, but differing in the surfactant (lecithin or Aerosol AOT) have been studied. The enthalpy variations (ΔH > 0) upon addition of separate doses of the co-surfactant (butanol) to a fixed mixture of the other 3 components were measured calorimetrically in a closed system. A linear relationship between ΔH and molar fraction of added co-surfactant was found at the molar fractions below the one required for microemulsion formation. A marked variation in the linearity (minimum in endothermicity) was found at the microemulsion formation. An exothermic contribution due to the microemulsion formation was hypothesized also on the basis of data obtained when the microemulsion was prepared upon addition of water to oil, surfactant and co-surfactant. Addition of co-surfactant in the absence of surfactants yielded different ΔH values.