Magnetic resonance studies in aqueous systems. Part 3.—Electron spin and nuclear magnetic relaxation study of interactions between manganese ions and micelles

Abstract

The e.s.r. spectrum of Mn²⁺ ions in sodium dodecyl sulphate solutions and in manganese dodecyl sulphate solutions shows line-broadening and an intensity loss above the critical micelle concentration. Similar results are obtained in lamellar phase systems. N.m.r. relaxation enhancement studies of water protons show that the local viscosity at the micelle surface is not much different from that in bulk solution. Loss in signal intensity arises from Mn²⁺ ions being partly dehydrated, or hydration sheath distortion on binding to the micelle surface. Spin—spin broadening, caused by high local concentrations of symmetrically hydrated Mn²⁺ ions, may contribute to the loss in intensity at low concentrations of sodium dodecyl sulphate micelles, or in manganese dodecyl sulphate solutions above the critical micelle concentration. Line-broadening results from interaction of the symmetrically hydrated Mn²⁺ ion with other ions near the micelle. Evidently, Mn²⁺ ions exist in two environments at the micelle surface; loss in signal intensity is caused by Mn²⁺ ions in the Stern layer of the micelle and line-broadening by those in the diffuse layer. The increase in intensity loss and line-broadening with concentration of sodium dodecyl sulphate is attributed to adsorption of “free” Mn²⁺ ions into the Stern and diffuse layers of the micelle, as the concentration of micelles increases. From the results it is possible to estimate the concentrations of Mn²⁺ ions bound in the Stern and diffuse layers of the micelle and those “free” for each concentration of surfactant.