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 measured in an electroacoustic experiment is the particle dynamic mobility, which is a complex quantity having both a magnitude and a phase angle. For many systems it is possible to use the dynamic mobility to estimate the zeta potential and the size of the particles. In the emulsion system, at low surfactant concentrations, the estimated zeta potentials were very high (up to almost 200 mV (negative)). A comparison is made between the zeta values obtained on the emulsion system and the anticipated measurements using d.c. methods. Whilst the d.c. measurement is ambiguous, the electroacoustic measurement is unequivocal in ascribing 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 behaviour is also displayed by semiconductor particles, but in that case the origin of the effect is in the very large values of the effective particle dielectric permittivity, which is proportional to the particle size. For the 4 mu m semiconducting silicon particles used in this study, the phase lag is large (50 degrees) at low frequencies and becomes smaller as the frequency rises. (C) 1997 Elsevier Science B.V.