Arenediazonium salts: New probes of the compositions of association colloids. 7. Average hydration numbers and Cl- concentrations in the surfactant film of nonionic C12E5/octane/water macroemulsions: Temperature and NaCl concentration effects

A surface-active arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazoni um ion, 16-ArN2+, was used as a chemical trapping probe of the concentratio ns of weakly basic nucleophiles in the surfactant film between the aqueous and oil domains of opaque relatively long-lived C12E5/octane/water macroemu lsions as a function of temperature and added NaCl. The results demonstrate that the chemical trapping method "sees" the composition of the oligooxyet hylene or interfacial layer on the aqueous side of the surfactant film. Pro duct yields from trapping of the available nucleophiles, H2O, the terminal OH group of C12E5, and Cl-, were used to estimate average hydration numbers of all aggregates present in the macroemulsions as a function of increasin g temperature and added NaCl. At 19.9 degreesC, the average hydration numbe r in the macroemulsions is 2.5, compared to a 70% larger value of 4.2 in C1 2E6 micelles at 20 degreesC obtained previously by chemical trapping. Avera ge hydration numbers decrease with increasing temperature and with added Na Cl at constant temperature as macroemulsion structure changes from oil-in-w ater type through the balanced point to water-in-oil type. These results ar e consistent with the oriented wedge theory of macroemulsion stability. Che mical trapping estimates of Cl- concentrations within the interfacial layer of the surfactant film of the macroemulsions show that the molarity of Cl- in the aqueous region of the interfacial layer is always about 10% greater than the Cl- molarity in the aqueous domain. Thus, Cl-, and probably Na+, move freely between the bulk aqueous domain and the surfactant film of the macroemulsions, contradicting an assumption of adsorption/depletion model f or the effect of lyotropic salts on the hydration of the interfacial layer. These results demonstrate the potential of chemical trapping for probing s urfactant film compositions of opaque macroemulsions.