Why Surface Area?
The need to determine the surface area of powders is well established. It influences many aspects of product performance; the hiding power of pigments, the activity of catalysts, the taste of food, and the potency of drugs, to name but a few. The most widely employed technique is that of gas adsorption; others include mercury porosimetry, gravimetric analysis and permeametry. The common feature of all these techniques is that they are only useful for analyzing dry powders.
However, the overwhelming majority of manufactured products involve suspensions of particulate materials in some fluid, either in the final state or at some stage of their production. While gas adsorption is useful to test incoming dry powder raw materials, it cannot provide information about the powder when dispersed in a liquid. To date this has not been possible other than to crudely estimate the surface area from particle size measurements or from time consuming adsorption isotherm or potentiometric/conductimetric titration measurements.
New Technological Breakthrough
Microtrac's XiGo Acorn Area Analyzer (XAA) is a revolutionary instrument designed to measure the volume fraction and surface area of nanoscale particles or droplets dispersed in a liquid. This new (patent pending) technique is based on nuclear magnetic resonance (NMR) and it offers many advantages in comparison with conventional surface area instrumentation.
Measurements can be made from start to finish in about 5 minutes. The optimal measurement parameters are determined automatically in software, so that no operator training is required. No sample preparation such as drying, or degassing is required, making XAA measurements orders of magnitude faster than any other technique. Since the samples are measured as dispersions, the XAA is ideal for characterizing emulsions.
Data comparison between two technologies
It is well known and accepted that the measurement of the particle size of broadly distributed asymmetric particles by different techniques will result in different measured values. Each technique provides different sensitivity to the various size fractions in a dispersion and varies in the method of “deconvolution” or analysis method.
A similar situation will arise when comparing surface area by NMR and BET. The NMR technique differs from BET in that the sample is measured in the dispersed or wetted state, i.e., in a liquid, while the BET technique measures the amount of gas adsorbed to the surface of dry particles. Thus, a priori, it should not be expected that the two techniques should necessarily give the same value for dispersion surface area.
Validation of the XAAA technique
Klebosol 30HB 25K (Rohm & Haas) is a nanoparticulate silica slurry used in the electronics industry as a polishing compound for silicon wafers. The silica particles are spherical, the dispersion is highly monodisperse, and is delivered as a 30% slurry. The surface area of these particles was determined to be 120 m2/g measured using titration by the manufacturer. Independent calculation of the surface area from the measured diameter (determined from dynamic light scattering) was 123 m 2/g. Direct measurement using the XAA gave 121 m2/g.
Measurements with the XAA on this concentrated dispersion took less than 5 minutes. In contrast gas adsorption would require separation of the particles from the liquid by drying, degassing, etc. a process that would take several hours. This could also promote particle aggregation, leading to an inaccurate determination of surface area.
The titration method is non-routine, requires sensitive instrumentation and skilled operators. Calculation from particle size is only possible because this material is spherical and monodisperse. Measurement by DLS also necessitates substantial dilution of the suspension. This is not always straightforward and can lead to particle aggregation.