ARENEDIAZONIUM SALTS - NEW PROBES OF THE INTERFACIAL COMPOSITIONS OF ASSOCIATION COLLOIDS .4. ESTIMATION OF THE HYDRATION NUMBERS OF AQUEOUS HEXAETHYLENE GLYCOL MONODODECYL ETHER, C(12)E(6), MICELLES BY CHEMICAL TRAPPING

Chemical trapping of water and terminal OH groups in nonionic micelles of hexaoxyethylene monododecyl ether, C(12)E(6), by an aryl cation fo rmed by spontaneous decomposition of an aggregate-bound 4-hexadecyl-2, 6-dimethylbenzenediazonium ion, 16-ArN2+, demonstrates that the interf acial region of a C(12)E(6) micelles is ''wet'' and that hydration of interfacial ethylene oxide groups depends on both surfactant concentra tion and temperature. Product yields are used to estimate hydration nu mbers of C(12)E(6) micelles by assuming that the selectivity of the re action in micelles is the same as that of its short chain analog 2,4,6 -trimethylbenzenediazonium ion, 1-ArN2+, in aqueous oligooxyethylene g lycol solutions. The hydration numbers are found to decrease gradually with increasing C(12)E(6) concentration at 40 degrees C from 3.5 at 0 .45% (0.01 M) C(12)E(6) to 2.5 in 54% C(12)E(6),just before the lamell ar phase region, and to 0.84 in 82.5% C(12)E(6), above the lamellar ph ase region. The hydration numbers also decrease linearly with increasi ng temperature in 0.01 M C(12)E(6) from 4.2 at 20 degrees C to 2.9 at 60 degrees C, passing through the cloud point at 50 degrees C. The val ues of the hydration numbers are in good agreement with some estimates made from water self-diffusion measurements. Chemical trapping is a r apid, straightforward method for estimating hydration numbers of aggre gates of nonionic surfactants that requires no information about their size and shape and that can be used in any fluid region of their phas e diagrams. Potential applications are briefly discussed.