Micellization of Dodecyltrimethylammonium Bromide in D2O as Probed by Proton Longitudinal Magnetic Relaxation and Chemical Shift Measurements

摘要

1H-NMR chemical shifts and relaxation rates of dodecyltrimethylammonium bromide (DTAB) were measured in aqueous solutions as a function of solute concentrations, Downfield chemical shifts were found on micellization for all protons. Critical micelle concentration (CMC), aggregation number (n), equilibrium constant (K) and chemical shifts of monomer and micelle (ヤmono, ヤmic) were obtained from chemical shifts measurement. In spherical DTAB micelle which is confirmed by the calculated value of surface area, the hydrocarbon chain had two gauche connections in opposite directions in average. When micelles were formed the relaxation rates of all protons were greatly increased, as their environment changed from water to liquid hydrocarbon. The variation of relaxation rates indicate that 1) part of the surfactants molecules in the micelles, i.e., the head groups, are exposed to the water, 2) molecular motion in the micellar state is more resticted than in monomer state and 3) the penetration of the water molecules into micelle interior reach to the メ position. A deformation from spherical to ellipsoidal micelles has been suggested for DTAB when the surfactant concentration is higher than 32.0 mM. An explanation of this possibility is given.