Low-field (LF) NMR Solvent Relaxation as a Convenient Tool to Determine the Hansen Solubility Parameters (HSP) of Powders and the Factors Affecting Such Values

The selection of appropriate solvents into which inorganic and organic sub-micron particles can be dispersed is important for product manufacturability and performance. Molecular-level interactions determine solvent suitability but are difficult to measure.
The Hansen Solubility Parameter (HSP) methodology was originally developed to quantify the solubility of polymers in solvents (and solvent blends) but has been applied to powders. The traditional experimental approach of visual observation of dispersion stability is slow and subjective. Analytical centrifugation (AC) provides a quantitative approach but has practical limitations.
Solvent relaxation NMR measurements – using a small, benchtop low-field NMR spectrometer - are shown to be a fast indicator of solvent suitability, with sensitivity to the same solvent-particle intermolecular forces (as well as the dynamics) with which HSP are concerned but they do not suffer from the limitations of the AC technique. A structured approach to relaxation measurements with a selection of both good and poor solvents yields the HSP for a given powder particle surface.
We have found that a number of factors – experimental and computational - can critically affect the determination of the specific values for the HSP of powders. This explains the variation of values found in the literature for the individual polar-dipolar (δP), dispersive (δD) and hydrogen bonding (δH) contributions (to the total cohesive energy for the particle-solvent interaction) for the same materials. We suggest that it is unlikely that an absolute value of HSP can be determined. However, when comparing powders, relative differences are valid, but no matter which methodology is used it is crucial to adopt a rigorous and consistent Standard Operating Procedure (SOP). When done in this manner LF-NMR relaxation could be used to assess batch-to-batch variations of particulate materials in the dispersed state.