LECITHIN-BASED MICROEMULSIONS - PHASE-BEHAVIOR AND MICROSTRUCTURE

The conditions under which lecithin can effectively mix oil and water into homogeneous solution are discussed. Lecithin is slightly too lipophilic in water-oil systems to spontaneously form the zero mean curvature amphiphile layers needed for balanced microemulsions, but by adjusting the polarity of the polar solvent balanced microemulsions may be prepared. It is found that, by adding a short-chain alcohol as cosolvent, lecithin forms microemulsions at low amphiphile concentrations and over wide ranges of solvent compositions. An investigation of the phase behavior of the system lecithin-1-propanol-water-n-hexadecane is presented. At 1 wt % of lecithin, three liquid phases, i.e., aqueous alcohol solution, oil, and microemulsion, form over wide ranges of solvent ratios. About 2.3% lecithin is the minimum amount needed to form microemulsions at all mixing ratios. The propanol concentration in the microemulsions is in the range 10-15 wt % of the aqueous solvent, the propanol concentration slightly decreasing with increasing oil content. Solution microstructure was investigated from molecular self-diffusion coefficients, and it was found that, depending on the propanol concentration, either discrete droplet or bicontinuous microemulsions may form. With decreasing propanol concentration there is a gradual transition from oil droplets in a water continuum, to bicontinuous structure, and then to a structure of water droplets in oil. This is compared with the analogous sequence of structures of other systems on varying some other parameter (temperature, salinity, surfactant composition).