ESR STUDY OF SODIUM DODECYL-SULFATE AND DODECYLTRIMETHYLAMMONIUM BROMIDE MICELLAR SOLUTIONS - EFFECT OF UREA

Electron spin resonance spectroscopy of cationic ((4-(trimethylammonio)-2,2,6,6-tetramethylpiperidinyl)-1-oxy iodide, Temp-TMA+) and nonionic spin probes ((4-octanoyl-2,2,6,6-tetramethylpiperidinyl)-1-oxy, C8-TEMPO, and x-doxylstearic acid with the nitroxide group in position x = 5, 12, and 16, along the stearic acid chain, 5-DSA, 12-DSA, and 16-DSA) has been studied in sodium dodecyl sulfate and in dodecyltrimethylammonium bromide solutions as a function of surfactant and urea concentration. The analysis of the nitrogen hyperfine coupling constant, 〈AN〉, and of the correlation time for the probe motion, τ, indicates that urea interacts with the surfactant polar headgroups of the micelle and penetrates below them. Urea slightly decreases the polarity and strongly increases (from 20% to 100% depending on the surfactant and on the urea concentration) the microviscosity of the micelle interface. These results support recent molecular dynamics and Monte Carlo experiments 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. © 1990 American Chemical Society.