NMR solvent relaxation studies of Na+-montmorillonite clay suspensions containing non-ionic polymers

Nuclear magnetic resonance (NMR) relaxation studies were carried out for Na+-montmorillonite aqueous suspensions containing non-ionic surfactants and polymers. The mobility of the solvent was determined by the relaxation time, or specific relaxation rate constant with respect to pure solvent. The adsorption of non-ionic surfactants or polymers onto Na+-montmorillonite reduces the mobility of the solvent molecules in contact with the clay surface and hence enhances the specific relaxation rate. The maximum value for the relaxation rate constant at approximately half of the surfactant particle coverage was interpreted to be due to the formation of ‘hemi-micelles’ on the clay surface, where the ethylene oxide chains are in closest proximity with the surface. For the surfactants studied, the relaxation rate constant was found to increase with the length of the ethylene oxide chain. The subsequent decline of this value was explained by a change in configuration of the surfactant ‘hemi-micelles’ to ‘oblate-shaped’ micelles on the surface of Na+-montmorillonite. A steady increase was found for the relaxation rate constant as a function of the adsorbed amount of polymer, followed by a plateau value. This indicated that the ‘train density’ or fraction of bound polymer had reached a maximum value, the remaining amount of polymer was adsorbed in ‘loops’ and ‘tails’ on the clay surface. However, a significantly higher molecular weight polymer showed an altogether different trend, indicating that a significant rearrangement on the surface took place. The bound fraction at the surface was estimated as a function of the adsorbed amount, and increased with decreasing molecular weight of the polymer.