Spatially-resolved magnetic resonance study of the dissolution interface between soaps and water

The developing interfacial region between a soap bar and water has been studied using a suite of spatially resolved NMR techniques. Stray field imaging (STRAFI) allowed the dynamics of water ingress into a shop-bought, commercial soap to be followed. A simplistic analysis of the data shows the ingress to be a Fickian process (∝t^1/2) in the first 4 h. The T₂ contrast employed in the STRAFI method is not sufficient to resolve detail of the mesophase formation at the interface. However, double quantum filtered ²H spectroscopy at different positions in the interfacial region allowed water concentration (and mesophase distribution) to be mapped over the first 120 h of dissolution. A simple model shows good agreement with the water concentration data. In the isotropic soap solution above the interfacial region, J-cyclic cross polarization was used to selectively interrogate the CH₂ ¹H of the soap alkyl chains and, in combination with a pulsed field gradient measurement of self-diffusion, suggests a micellar solution in which the hydrodynamic radius of the micelles is ~5nm.