OSMOTIC-PRESSURE OF UNCHARGED AND WEAKLY CHARGED OIL WATER MICROEMULSION DROPLETS/

The osmotic pressure of uncharged and weakly charged spherical oil-in- water microemulsion droplets was measured as a function of the droplet volume fraction, phi, in the concentration range 0.061 less than or e qual to phi less than or equal to 0.44. The data from the uncharged dr oplets were analyzed in terms of a hard-sphere repulsion in combinatio n with a van der Waals attraction. The droplet size, as obtained from the analysis, shows a good agreement with previously published small-a ngle neutron and light scattering data. The droplets were made weakly charged by replacing a small fraction (0.01, 0.04, and 0.06) of the no nionic surfactant with ionic surfactant, retaining the droplet size. T he introduction of long-range electrostatic interactions resulted in a strong increase of the osmotic pressure. From comparison with model c alculations, solving the Poisson-Boltzmann equation in the cell model, it is concluded that essentially all the ionic surfactant adsorbs to the droplets for phi > 0.1. At lower phi, however, the fraction of non adsorbed ionic surfactant in the aqueous solvent becomes significant a nd increases with dilution. The degree of adsorption was also calculat ed within a molecular model, and a good agreement with the values obta ined from the osmotic pressure data was found. When adding a large amo unt of salt to the charged droplets, the relevant osmotic pressure dro ps back to its original value for the uncharged case, demonstrating th at the only influence of the ionic surfactant is the long-range electr ostatic interaction.