Characterization of aggregation phenomena by means of acoustic and electroacoustic spectroscopy

Abstract

Aggregation phenomena change the particle-size distribution, replacing small particles with larger aggregates. Measuring this evolution of particle size is an apparent way to characterize aggregation phenomena. It is often desirable to perform this measurement in an intact, concentrated, dispersed system. Until recently, this kind of measurement was impossible, but the situation has improved dramatically with the availability of ultrasound-based spectroscopy. An ultrasound pulse interacts with dispersed particles while propagating through the dispersed system, thereby attenuating. An acoustic spectrometer measures this attenuation for a set of frequencies and calculates the corresponding particle size. An ultrasound pulse also disturbs the particle double layer. As a result, the particles generate an electric current, the so-called colloid vibration current (CVI). An electroacoustic spectrometer measures this current and calculates the ζ potential. We have suggested in our previous papers that combined acoustic and electroacoustic spectroscopy provides the most reliable and complete characterization of concentrated dispersed systems. We show in this paper that this technique is able to determine not only the isoelectric point but also a range of pH where the system is not stable. It is found that the system loses stability when the ζ potential becomes less than 30 mV. We prove that a lognormal distribution is not adequate for characterizing unstable systems compared with the performance achieved with a bimodal distribution.