Binding constants of calcium and/or magnesium electrolytes and aggregation numbers in oil-in-water type microemulsions formed by an amphoteric surfactant

Abstract

The effects of anti-symmetric electrolytes (CaCl2, Ca(SCN)2, MgCl2, and/or Mg(SCN)2) and pH on the phase behavior, the ζ-potential, the hydrodynamic diameter and the surface charge density of an oil-inwater type (O/W-type) microemulsion formed in solutions of an amphoteric surfactant (N α,N α-dimethyl-N ɛ-lauroyllysine, DMLL)/n-octane/1-pentanol/brine have been examined. The formation of the microemulsion in the presence of CaCl2 and/or Ca(SCN)2 is of Winsor-type with an increase in the concentration of 1-pentanol. Particularly, microemulsion is not formed by the addition of Ca(SCN)2 in a pH region less than 2.6. The ζ-potential and the surface charge density of the microemulsion in the presence of CaCl2 decrease with an increase in pH and show slightly positive values in the isoelectric region (pH 5-7), while, in the presence of Ca(SCN)2, the ζ-potential and the surface charge density show negative values in the same region at which the net charge of DMLL molecules becomes almost zero. The hydrodynamic diameters in the presence of CaCl2 show a maximum value around pH 2.5, whereas, in the presence of Ca(SCN)2, the minimum value is around pH 5.5. Similar tendencies are recognized in results for the ζ-potential, the hydrodynamic diameter and the surface charge density of the O/W-type microemulsion in the presence of MgCl2 and Mg(SCN)2. A new formula to estimate the binding constants (K) of Ca2+, Mg2+, Cl−, and SCN− to the hydrophilic groups in DMLL molecules and the adsorption density of DMLL molecules on the oil/water interface (N) in the presence of antisymmetric electrolytes has been derived.K for Ca2+, Mg2+, Cl−, and SCN− was found to beK Ca=0.12M−1,K Mg=0.14 M−1,K Cl=0.0084±0.0016 M−1, respectively.N for DMLL molecules in the presence of CaCl2, Ca(SCN)2, MgCl2 and/or Mg(SCN)2 was found to be 0.50 nm−2, 0.38 nm−2, 0.44 nm−2, and 0.47 nm−2, respectively; and the surfactant (DMLL) numbers per O/W-type microemulsion droplet change from a few hundreds to a few thousands with changing pH. The larger the hydrodynamic diameter of the O/W-type microemulsion, the greater the number of DMLL molecules adsorbed on the O/W-type microemulsion surfaces.