H-1-NMR STUDIES OF NONIONIC SURFACTANT ADSORPTION ONTO COLLOIDAL PARTICLES

Proton Nuclear Magnetic Resonance (NMR) spectroscopy was used to study the adsorption of a hexadecyl-terminated poly(oxyethylene) surfactant (nominal molecular weight of 1120 g/mol) onto hydrophobic polystyrene latex particles. The adsorption process affects the NMR response of the surfactant; various surfactant populations are represented by different features in the NMR spectra. An analytical method that utilizes surfactant systems with and without polystyrene latex particles was employed to determine the capability of NMR to observe adsorbed surfactant close to the particle surface. At the initial stages of surfactant adsorption, the oxyethylene chain interacts with the particle surface in a pancake-like conformation. At higher surfactant concentrations, surfactant molecules are bound to the particle surface and also exist as micelles or are free in solution; approximately one-third of the bound, 20-unit oxyethylene chains are near the surface and are not detected by NMR. Using a theoretical monomer density profile, laser light-scattering measurements, and the NMR results, an effective NMR detection limit of 1 nm from the particle surface has been calculated. (C) 1995 John Wiley & Sons, Inc.