Diverging electrophoretic and dynamic mobility of model silica colloids at low ionic strength in ethanol

• Electrokinetics of monodisperse charged silica spheres in ethanol were studied.
• Dynamic and electrophoretic mobility were compared at low and high salt.
• Electroacoustics agrees with laser Doppler electrophoresis at high ionic strength.
• At low ionic strength, dynamic mobility by far exceeds electrophoretic mobility.
• Is the model system not as simple as it seems, is the theory inadequate, or both?

Electroacoustics and laser Doppler electrophoresis were employed to measure the mobility of surface-modified silica colloids in ethanol as a function of the ionic strength. Sufficiently low volume fractions were chosen to exclude effects of interparticle interactions. At high ionic strength, the electrophoretic mobility μeμe is equal to the (electroacoustic) dynamic mobility μdμd at 3.3 MHz. However, the ratio μd/μeμd/μe increases significantly to ∼∼5 at low ionic strength. This increase may be related to the porous outer layer of the surface-modified silica spheres.

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