Arenediazonium Salts: New Probes of the Interfacial Compositions of Association Colloids. 5.1−4 Determination of Hydration Numbers and Radial Distributions of Terminal Hydroxyl Groups in Mixed Nonionic CmEn Micelles by Chemical Trapping

Abstract

An aggregate-bound long tail arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, was used as a chemical trapping reagent to estimate the hydration state and terminal hydroxyl, OH, group distributions within the interfacial region of sets of binary mixed nonionic micelles of oligooxyethylene alkyl monoether, CmEn, surfactants: C10E4/C12E6, C10E4/C16E8, C10E5/C16E8, C12E6/C16E8, and C10E5/C12E5. 16-ArN2+ decomposes spontaneously to generate an aryl cation that is trapped by water and by the terminal OH groups of the two nonionic surfactants to give phenol and alkyl aryl ethers as products. Quantitative analyses of product yields by HPLC were used to estimate the hydration numbers and relative number densities of the terminal OH groups of the two surfactants in the two inner layers of the interfacial region adjacent to the micellar core. The thickness of one layer is set equal to the length of one ethylene oxide unit. The radial distributions of terminal OH groups and ethylene oxide units in the mixed micelles are described by using a simple radial one-dimensional random walk model without adjustable parameters. The calculated ratios of terminal OH groups in the two inner layers are in excellent agreement with those obtained from experimental product yields. The calculated hydration numbers of the inner two layers ranged between 1.8 and 3.5, depending upon oligooxyethylene chain length, mixed micelle composition, and temperature. The average hydration number of the inner two layers is about 3 at 18 °C.