Abstract
This paper demonstrates the influence of surface charge chemistry on the application of nuclear magnetic relaxation measurements (NMR relaxometry) for the in situ determination of particle surface area, in the presence of high electrolyte concentration. Specifically, dispersions of titania, calcite and silica with and without 1 M KCl were investigated. The addition of salt, showed no significant change to relaxation measurements for titanium dioxide; however, a significant rate enhancement was observed for both calcite and silica systems. These differences were attributed to counterion layers forming as a result of the particles surface charge, leading to an increase in the relaxation rate of bound surface layer water. Further, changes appeared to be more pronounced in the silica systems, due to their larger charge. No enhancement was observed for titania, which was assumed to be due to the particles being at their isoelectric point, with no resulting counterion layer formation.Solvent relaxation was further used to successfully determine the surface area of particles in a dispersion using a silica standard reference material, with results compared to Brunauer-Emmett-Teller (BET) and spherical equivalent estimations. Two different dispersions of titanium dioxide, of different crystal phases, were shown to have NMR surface area measurements in good agreement with BET. Thus showing the technique was able to measure changes in surface charge when surface chemistry remained relatively similar, due to the reference silica material also being an oxide. In contrast, the NMR technique appeared to overestimate the calcite surface areas in reference to BET, which was assumed to occur due to both better dispersion in the liquid state of nanocrystallites and potential ion enhancement from the solubility of the calcite. These results highlight the potential of this technique as a fast, non-destructive and non-invasive method for dispersion analysis, but also show the competition between surface area and surface chemistry interactions on measured relaxation rates.
Highlights
Particulate suspensions are present in numerous industries including pigments, cosmetics, pharmaceuticals and foods, and frequently, are at high concentrations [1]
For application, there are imperative questions as to what extent these effects may dominate or confound the use of these measurements for specific surface area analysis, greatly limiting the potential of NMR relaxation for on-line surface area measurements of concentrated slurries, for example [30]. To better understand these related phenomena, this paper presents NMR relaxation measurements in various systems, with changes in both particle surface area and interfacial chemistry
Electrolyte effects on relaxation NMR were investigated for a range of particle systems of differing particle charge
Summary
Particulate suspensions are present in numerous industries including pigments, cosmetics, pharmaceuticals and foods, and frequently, are at high concentrations [1]. The surface area of a particle is often determined for dry powders by Brunauer-EmmettTeller (BET), whereby, nitrogen gas molecules physically adsorb onto a particle surface in layers [2,3]. BET is not capable of determining the total particle surface area for particles in solid-liquid (S-L) dispersions. Mercury porosimetry and gravimetric analysis are alternative techniques for the measurement of surface area, they require dry powders [6]. It is well known that the drying of suspensions frequently leads to aggregation and agglomeration [7], resulting in measurements that are unreliable and misrepresentative of the particle surface area in a S-L dispersion [8]
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