Arenediazonium Salts: New Probes of the Interfacial Compositions of Association Colloids. 4.1-3 Estimation of the Hydration Numbers of Aqueous Hexaethylene Glycol Monododecyl Ether, C12E6, Micelles by Chemical Trapping

Abstract

Chemical trapping of water and terminal OH groups in nonionic micelles of hexaoxyethylene monododecyl ether, C12E6, by an aryl cation formed by spontaneous decomposition of an aggregate-bound 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, demonstrates that the interfacial region of a C12E6 micelles is “wet” and that hydration of interfacial ethylene oxide groups depends on both surfactant concentration and temperature. Product yields are used to estimate hydration numbers of C12E6 micelles by assuming that the selectivity of the reaction in micelles is the same as that of its short chain analog 2,4,6-trimethylbenzenediazonium ion, 1-ArN2+, in aqueous oligooxyethylene glycol solutions. The hydration numbers are found to decrease gradually with increasing C12E6 concentration at 40 °C from 3.5 at 0.45% (0.01 M) C12E6 to 2.5 in 54% C12E6, just before the lamellar phase region, and to 0.84 in 82.5% C12E6, above the lamellar phase region. The hydration numbers also decrease linearly with increasing temperature in 0.01 M C12E6 from 4.2 at 20 °C to 2.9 at 60 °C, passing through the cloud point at 50 °C. The values of the hydration numbers are in good agreement with some estimates made from water self-diffusion measurements. Chemical trapping is a rapid, straightforward method for estimating hydration numbers of aggregates of nonionic surfactants that requires no information about their size and shape and that can be used in any fluid region of their phase diagrams. Potential applications are briefly discussed.