ELECTROACOUSTIC CHARACTERIZATION OF COLLOIDS WITH UNUSUAL PARTICLE PROPERTIES

The AcoustoSizer (Matec Applied Sciences) has been used to follow the electroacoustic behaviour of oil-in-water emulsions, stabilised by an anionic surfactant, under various treatments. The property which is me asured in an electroacoustic experiment is the particle dynamic mobili ty, which is a complex quantity having both a magnitude and a phase an gle. For many systems it is possible to use the dynamic mobility to es timate the zeta potential and the size of the particles. In the emulsi on system, at low surfactant concentrations, the estimated zeta potent ials were very high (up to almost 200 mV (negative)). A comparison is made between the zeta values obtained on the emulsion system and the a nticipated measurements using d.c. methods. Whilst the d.c. measuremen t is ambiguous, the electroacoustic measurement is unequivocal in ascr ibing a high zeta potential to the measured data. The dynamic mobility in this case shows a phase lead superimposed on the more usual phase lag caused by particle inertia. Unusual electroacoustic phase behaviou r is also displayed by semiconductor particles, but in that case the o rigin of the effect is in the very large values of the effective parti cle dielectric permittivity, which is proportional to the particle siz e. For the 4 mu m semiconducting silicon particles used in this study, the phase lag is large (50 degrees) at low frequencies and becomes sm aller as the frequency rises. (C) 1997 Elsevier Science B.V.