BINDING CONSTANTS OF INORGANIC ELECTROLYTES AND AGGREGATION NUMBERS OF OIL-IN-WATER-TYPE MICROEMULSIONS WITH AN AMPHOTERIC SURFACTANT

The pH dependence of the zeta-potential and the particle size of the oil-in-water (O/W) type (lower phase) microemulsion forming in solutions of N(alpha,N(alpha)-dimethyl-N(epsilon)-lauroyllysine (DMLL)/1-pentanol/octane/brine has been examined. The zeta-potential changes from positive to negative values with increasing pH. The particle sizes of the microemulsion droplets show two maxima, at pH 3 and pH 8, with increasing salinity. Moreover, the surface charge densities become negative in regions where the net charge of DMLL molecules becomes almost zero (the so-called isoelectric region). Applying formulas described in this article, the adsorption of chlorine ions to the hydrophilic groups of the amphoteric surfactant (DMLL) microemulsion is shown to be greater than that of sodium ions. The binding constants of Na+ and Cl- to the surface of DMLL microemulsions are found to be 0 M-1 < K(Na) < 0.91 M-1, 0.20 M-1 < K(Cl), < 1.11 M-1, and K(Cl) - K(Na) = 0.20 M-1, respectively; the adsorption density of DMLL molecules on the surfaces of O/W-type microemulsions increases as the pH approaches that of the isoelectric region; the larger the particle size of the microemulsions, the greater the number of surfactant molecules adsorbed on microemulsion surfaces.