Arenediazonium salts: New probes of the interfacial compositions of association colloids. 5. determination of hydration numbers and radial distributions of terminal hydroxyl groups in mixed nonionic CmEn micelles by chemicaltrapping

An aggregate-buund long tail arenediazonium ion. 4-hexadecyl-2,6-dimethylbe nzenediazonium ion, 16-ArN2+, was used as a chemical trapping reagent to es timate the hydration state and terminal hydroxyl, OH, group distributions w ithin the interfacial region of sets of binary mixed nonionic micelles of o ligooxyethylene alkyl monoether, CmEn, surfactants: C10E4/C12E6, C10E4/C16E 8, C10E5/C16E8, C12E6/C16E8, and C10E5/C12E5 16-ArN2+ decomposes spontaneou sly 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 et hers as products. Quantitative analyses of product yields by HPLC were used to estimate the hydration numbers and relative number densities of the ter minal OH groups of the two surfactants in the two inner layers of the inter facial region adjacent to the micellar core. The thickness of one layer is set equal to the length of one ethylene oxide unit. The radial distribution s of terminal OH groups and ethylene oxide units in the mixed micelles are described by using a simple radial one-dimensional random walk model withou t adjustable parameters. The calculated ratios of terminal OH groups in the two inner layers are in excellent agreement with those obtained from exper imental product yields. The calculated hydration numbers of the inner two l ayers ranged between 1.8 and 3.5, depending upon oligooxyethylene chain len gth, mixed micelle composition, and temperature. The average hydration numb er of the inner two layers is about 3 at 18 degrees C.