Silicas are used ubiquitously in a wide range of applications. Uses include surface coatings, rubber reinforcement, toothpaste abrasives and as thickening agents. Depending on how a silica has been manufactured (synthetic) or processed (natural), the surface properties range from strongly hydrophilic because of the surface silanol (-SiOH) groups to strongly hydrophobic - surface siloxane groups, -Si-O-Si-, have ether-like properties. In addition, commercial silicas can have an enormously greater specific surface area as compared to their apparent geometrical surface. Hence, surface physical and chemical characterization can be challenging. This is important since in formulation the ability to tailor properties of a silica to suit an application is of great value. NMR solvent relaxation times are sensitive to the available (wetted) surface area in a dispersion of particles in a fluid and, also, the specific nature of the particle’s interfacial characteristics. We will present data, using a small, bench-top low-field NMR device, illustrating how such measurements can be used to: (i)) characterize the strength of interaction between water and particle surface functional groups (ii) examine the effect of additives such as electrolytes and surfactants (iii) discriminate between blends of different silicas (iv) monitor chemical synthesis and degradation of silicas Importantly, in contrast to BET measurement, we show that NMR data can be obtained directly on silica hydrogels in minutes with no sample preparation. Finally, NMR relaxation measurements can provide a quick and facile method to differentiate silica suspensions undergoing settling. The results are seen to be consistent with more traditional methods of investigating stability such as the tapped bulk density (TBD), and light scattering estimates of the median particle size (d50 values).

Using low-field NMR relaxation to optimise particulate dispersions of silica

ACS Spring 2022

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