Spectroscopy Focus


The Physical Characterisation of Suspensions and Slurries


Keith Sanderson, Mageleka Inc VP EMEA Business Development Owner Sanderson Technology Ltd Dr David Fairhurst, Colloid Consultants, Professor Terry Cosgrove, University of Bristol


A signifi cant number of commercial products involve, either in the fi nal state or at some stage of their production, suspensions of particulate materials dispersed into liquid vehicles often at high volume fraction. Quality dispersions are needed in applications as diverse as cosmetics, pharmaceuticals, and ceramics, but it is often under-appreciated that, in order to be properly formulated, these suspensions or slurries must be analysed as they are prepared and without dilution. The characteristics of the solid-liquid interface control dispersion behaviour and can infl uence, among other things, important processes such as adsorption, fl otation, and, in concentrated suspensions, rheological behaviour. Here we introduce a new methodology of studying concentrated dispersions in-situ.


NMR (Nuclear Magnetic Resonance) relaxation is a well-established method for characterising liquids and solids and more recently dispersions [1]. The methodology is straightforward and the combination of highly customisable digital radio frequency devices and the miniaturisation of affordable magnet systems (made using rare earth composites) has made it possible to create small desk-top spectrometers with advanced features that are useful for routine laboratory measurements. The basic NMR experiments have also


spin-relaxation rate and both can be used in these experiments and each are each sensitive to molecular motion and the local magnetic environment that the spins are in. Although there are many nuclei that can be used we focus here on protons. For convenience, we defi ne a relaxation number, R, which is the normalised relaxation rate and is dimensionless. The R value depends on many factors but those relevant to this work are the value of the pure solvent relaxation rate, the chemical composition of the interface, the available surface area and the strength of solvent or solute adsorption. We essentially measure an average relaxation assuming that the solvent can interact with all the available surface by a process of fast exchange. In this way, the measured R values are proportional to the surface area (A) and the plots of R vs A are linear and have a slope which depends on the surface interactions (Figure 1). This then provides a very useful tool to probe the available interface in dispersions and adsorption with minimal sample preparation. Measurement can be made typically in a few seconds so that dynamic changes such as adsorption or displacement rates of additives , settling, fl occulation etc. can be followed.


been refi ned to take into account limitations of magnet homogeneity and stability. There are two relaxation processes - R1


, the spin-lattice relaxation rate and R2


Techniques based on NMR relaxation avoid these dilution problems, and are able to make measurements on suspensions and slurries prepared at almost any industrially relevant concentration. This not only removes the practical need for dilution - no matter what the application – but also allows monitoring of any dilution process. Importantly, the technique is non-invasive and non-destructive so that samples - at whatever concentration - can be saved and re-measured at some future date. This allows examination of any time- dependent behaviour.


Figure 1: NMR relaxation numbers for a series of concentrated dispersions.


NMR relaxation of the dispersion medium in a suspension is sensitive to some of the key parameters discussed above – including particle concentration, surface area surface structure and porosity as well as solvent type, and solute chemical composition. For example, Figure 1 shows the variation in the NMR relaxation number as a function of particle surface area and type. Samples with higher solvent surface affi nity (silica cf latex) and elemental composition (alumina cf silica) give very different relaxation numbers (normalised to the solvent) and can be used as diagnostic tool and for quality control without the need for dilution. Figure 2 shows the effect of adsorption of surfactant from solution on the relaxation number (normalised to the bare particle dispersion); this can be used to optimise the best surfactant concentration for optimum stability.


The following illustrates a typical scenario where a concentrated dispersion is measured by conventional means and the problems that can arise.


Figure 3, on the left, depicts any concentrated suspension as it is made or formulated


, the spin-


Figure 2: NMR Surfactant Isotherm.


Figure 3: Schematic of a concentrated suspension or slurry at initial contact and at equilibrium.


Figure 4: Schematic of diluted suspension at initial contact and equilibrium.


LABMATE UK & IRELAND - JULY 2020


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