The electrophoretic mobility of an uncharged particle.

HYPOTHESIS: It has been claimed that uncharged particles can have negative electrophoretic mobilities, and so a negative mobility need not imply a negative particle charge. We show that although a steady electrophoresis may be possible for the uncharged infinite slabs studied in Molecular Dynamics simulations, it is not possible for a finite particle. EXPERIMENTS AND THEORY: An uncharged particle may initially move when the field is turned on, but our analysis shows that this motion ceases as charges of opposite sign build up on the front and back of the particle. Uncharged particles may move in alternating electric fields, but their mobility is predicted to increase with electrolyte conductivity. Experimentally, however, the mobility of hexadecane oil drops in water at pH 9 decreases with increasing NaCl concentrations. FINDINGS: Our results are consistent with the usual compression of the double layer with added salt, and with the traditional interpretation, that hydrophobic particles have negative mobilities because they are negatively charged. Uncharged particles may have a transient mobility but it is quickly quenched by polarisation of the double layer.

Ultrasonic attenuation by nanoporous particles. Part II: experimental.

In Part I of this paper O'Brien presented a new theory that describes the dissipation of ultrasonic energy by porous colloidal particles in suspension. In this paper we present experimental measurements on several such suspensions and compare the resulting ultrasonic attenuation spectra with O'Brien's theory. We find that microporous colloids (e.g. zeolites) and mesoporous colloids (e.g. porous silicas) both show greater attenuation of ultrasound than would solid particles of the same size and effective density, as predicted by O'Brien. Fitting theoretical spectra to the experimental data provides information about the porosity and pore diameters in the particles. This makes the technique potentially suitable as a convenient method of characterizing nanoporous materials, especially those already in suspension.

The Dynamic Mobility of Colloidal Semiconducting Antimony-Doped Tin(IV) Oxide Particles.

The dynamic mobility spectra of suspensions of semiconducting tin(IV) oxide particles doped with antimony have been measured with the technique of electroacoustics. The magnitude of the complex mobility decreases essentially monotonically with increasing frequency, just as for a nonconducting (dielectric) particle under the same conditions. Unlike the case for a dielectric particle, however, the magnitudes at low frequency increase with increasing conductivity. The phase angle behavior is also different from that of a normal dielectric particle. The change in the phase angle behavior is most obvious at low suspension conductivity and high frequency where the phase angles showed a much smaller phase lag than at high conductivities. Reasonable agreement was found between the experimental mobility and the theoretical dynamic mobility spectra obtained with O'Brien's theory for the enhanced permittivity of semiconductors. Copyright 2001 Academic Press.