Particles in colloidal systems are typically decorated by adsorbed surfactants or polymers to help impart stability to the dispersion or to provide desired functionality. Such functionalisation requires measurement of the total surface area of the particles in the dispersion or rapid characterisation of the adsorption, desorption or competition between surface-active species at the solid-liquid interface.

Time-domain NMR measurement (i.e. T₁ and T₂ solvent-relaxation) can be sensitive to the nature of the solid-liquid interface in dispersions, with surface chemistry and surface coverage changing the relaxation rate of the solvent molecules near the surface. Solvent relaxation measurements now form the basis of routine, non-destructive measurements of the specific surface area of silica and titania dispersions along with the volume-fraction of particles. For example, we have recently illustrated the sensitivity of these measurements by looking at the onset of depletion flocculation of silica nanoparticles upon addition of small amounts of poly(styrene sulfonate).

Solvent relaxation is also sensitive to water molecules trapped in near-surface polymer and surfactant layers, permitting facile measurement of a pseudo-adsorption-isotherm. Competitive adsorption may be studied in the same way, illustrating the adsorption of one polymeric species followed by the displacement of that polymer by another.

We have recently developed a miniature, low-field NMR instrument for performing these measurements. The instrument is small, low-powered, portable and can be used without being an NMR expert. It is conceivable that instrumentation like this could be used for routine analysis or on-line in a manufacturing setting. The application of this NMR technique, both with large-scale research NMRs as well as small, portable NMRs, will be discussed with reference to these examples.