Structural study of microemulsions of glycerol stabilised by cetyltrimethylammonium bromide dispersed in heptane + chloroform mixtures

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Abstract
Dynamic light scattering, viscometry and phase-study results are presented for cetyltrimethyl-ammonium bromide-(CTAB)-stabilised dispersion of glycerol in 50/50 v/v mixtures of n-heptane and chlorofom (glycerol-in-oil microemulsions). Up to ten moles of glycerol per mole of CTAB may be solubilised in 50/50 v/v heptane + chloroform mixtures at 35 °C. The resulting solutions consist of thermodynamically stable, discrete droplets of glycerol stabilised by the surfactant. The droplet size depends primarily on the mole ratio (R) of glycerol to CTAB according to the following equation: hydrodynamic radius/nm = 1.8(±0.4)+1.2(±0.2)R. The microemulsion phase separates into two phases if the temperature is decreased below a lower transition temperature or raised above an upper transition temperature. Attractive interactions between the droplets increase as the upper transition temperature is approached, but the droplet size remains constant. As the lower transition temperature is approached the droplets increase in size. This is attributed to loss of CTAB from the glycerol/oil continuous-solvent interface. Droplet swelling is also observed at very low surfactant concentrations. Viscosity data tentatively suggest that the extent of solvation of microemulsion particles increases with decreasing temperature or that the particles may be slightly non-spherical at low temperatures. The structural data are discussed with reference to the mechanisms leading to phase separation of the microemulsion system.

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