Application of NMR Relaxation to Determine the Hansen Solubility Parameter (HSP) of Particles - poster
Ravi Sharma¹, Shin-Ichi Takeda² Dave Fairhurst³, Stuart Prescott⁴, Terence Cosgrove⁵
1 Mageleka, Inc., Winter Park, Florida, USA
2 Colloid-Techno-Consultants,Ltd., Osaka, Japan
3 Colloids Consultants, Ltd., Aiken, Fl, USA
4 University of New South Wales, Australia
5 Bristol University, UK
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The ease with which particles can be dispersed in a liquid is dependent on the details of the liquid to be used and the surface chemistry of the particles. The particle wettability, as controlled by dispersion forces, dipole-dipole forces and hydrogen bonding is the primary influence. Being able to predict the appropriate liquids to use for a given particle is of considerable interest to the formulator, and there has been much interest in semi-empirical routes to selecting the optimum solvent (or solvent mixture) for pigment particles.

The Hansen Solubility Parameters (HSP) approach, originally devised to study polymer-solvent compatibility, can be applied to solid materials. Determination of the particle HSP, traditionally by visual observation, is subjective, time-consuming, error-prone, and is only qualitative. Extinction profiles measured over centrifugation time are more quantitative; the influence of density and viscosity of different solvents employed must accounted for and the solids concentration must be chosen with care.

NMR relaxation measurements are sensitive to the same intermolecular forces between solvent and particles with which HSP are concerned. We will show relaxation data obtained using five different nano-size zinc oxide and aluminium oxide powders, each having a completely different surface chemistry, ranging from a high polar component (hydrophilic) to a lower polar component (hydrophobic).

Results suggest that a straightforward, quantitative, and fast instrumental approach to determining the HSP of a nanomaterial is feasible and, further, that NMR relaxation measurement can discriminate between suspensions that may initially appear similar but exhibiting different long-term colloidal stability. In addition NMR relaxation data may potentially be used as a method for quality control of particles. NMR relaxation measurements (made using a low cost, low field table-top device) can be made at almost any industrially relevant solids concentration without requiring further sample preparation; any hydrogen-containing solvent can be used.

The ability to project solid-liquid interactions obtained by NMR relaxation into Hansen space is powerful, is much simpler and easier than sedimentation and can potentially provide formulators with a time-saving method to optimize and select the liquid composition (solvent plus surfactant additives) for desired particle suspension performance.