Monitoring the presence of nano-particles in dispersions having a broad particle size distributions can be a problem for many measurement techniques because large particles or even aggregates of the smaller particles can mask the presence of the sought after nano-particles. The ability of many existing techniques to detect the nano-particles when present in broad polydisprse systems is largely unknown, yet it is critical for proper selection of the measuring technique for characterizing a particular nano-dispersion. Acoustic Spectroscopy is already a known and proven tool for studying nano-particles in systems with a narrow size distribution.  The purpose of this paper is to evaluate the sensitivity of Acoustic Spectroscopy for determining the nano-particle content of very polydisperse systems.
We used eight different ZnO powders from different manufacturers to prepare 5%wt dispersions, each dispersed in water. The stability of the each dispersion was optimized by pH adjustment and addition of sodium hexametaphosphate as determined by maximizing the measured ζ-potential.  According to the acoustic measurement, the median size of these different ZnO dispersions varied from 200 nm to 700 nm. Independent TEM photographs in general confirmed the size variation between the samples.  Independent DLS measurements failed to provide particle size data correlating with TEM.  The Acoustic measurements further showed that the each dispersion contained a different relative content in the nano-particle fraction. The precision with which the nano-particle fraction could be determined was better than 2% of the total solids loading for all samples. In order to verify consistency of this measurement we performed mixing study by adding dispersion with the largest nano-particles content to the dispersion with the smallest nano-particles content, in small increments. This test confirms that Acoustics sensitivity threshold is about 2% of nano-particles in the broad polydisperse dispersions of dense metal oxide particles.