Characterization of aggregation phenomena by means of acoustic and electroacoustic spectroscopy

Aggregation phenomena change the particle-size distribution, replacing smal l particles with larger aggregates. Measuring this evolution of particle si ze is an apparent way to characterize aggregation phenomena. It is often de sirable to perform this measurement in an intact, concentrated, dispersed s ystem, Until recently, this kind of measurement was impossible, but the sit uation 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 calcula tes the corresponding particle size. An ultrasound pulse also disturbs the particle double layer. As a result, the particles generate an electric curr ent, the so-called colloid vibration current (CVI). An electroacoustic spec trometer measures this current and calculates the zeta potential. We have s uggested in our previous papers that combined acoustic and electroacoustic spectroscopy provides the most reliable and complete characterization of co ncentrated 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 whe re the system is not stable. It is found that the system loses stability wh en the zeta potential becomes less than 30 mV, We prove that a lognormal di stribution is not adequate for characterizing unstable systems compared wit h the performance achieved with a bimodal distribution. (C) 1998 Elsevier S cience B.V. All rights reserved.