Electron spin echo modulation and electron spin resonance studies of sodium dodecylsulfate and dodecyltrimethylammonium bromide micellar solutions: Effect of urea addition

Abstract

Electron spin echo modulation (ESEM) and electron spin resonance (ESR) studies have been carried out for a series of x-doxylstearic acids (x-DSA, x=5, 7, 10, 12 and 16) and 4-octanoyl-2,2,6,6-tetramethylpiperidinyl-1-oxy (C8-TEMPO) spin probes in micellar solutions of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in H2O, D2O, and in presence of 2 M or 6 M urea and urea-d4. The analysis of the nitrogen hyperfine coupling constant, 〈A N 〉, and of the correlation time for the probe motion τ indicates that urea slightly decreases the polarity and strongly increases (from 20% to 100% depending on the surfactant and urea concentration) the microviscosity of the micellar interface. Modulation effects due to the interaction of the unpaired electron with urea and water deuteriums show that urea does not affect the U-bent conformation of the x-DSA probe in the micelle. The analysis of the deuterium modulation depth and the Fourier transformation of the two-pulse spectra show that urea interacts with the surfactant polar headgroups at the micelle surface. It is also found that the interaction of urea is greater with DTAB than SDS surfactant. These results support recent molecular dynamics and Monte Carlo calculations, and are in agreement with a direct mechanism of action in which urea replaces some water molecules that solvate the hydrophobic chain and the polar headgroups of the amphiphile.