Acoustophoresis of electrolyte solutions and sedimentation profiles of screened charged colloids

Abstract

Acoustophoresis consists of applying an ultrasonic wave to a charged solution and measuring the induced electric field, originating from the local separation of charges. This effect, predicted by Debye in 1933, has been studied for electrolytes (Ionic Vibration Potential, IVP) and for colloids (Colloidal Vibration Potential, CVP).

In the first part of this work, we analyze the effect of interionic forces on IVP within the Mean Spherical Approximation, at the same level of description as for conductance of electrolytes. This analysis is required by the fairly high concentrations used in IVP experiments. Experimental data found in the literature are used to find the “acoustophoretic mass” of cations, which involves solvation water. The values obtained for alkali and alkali-earth cations are in good agreement with other cation solvation numbers.

In the second part, a method for the calculation of equilibrium sedimentation profiles of charged colloids, in the presence of added salt, is presented. It is found that the concentration of the colloidal particles obeys a non-linear equation, which can be solved by a numerical method. The main assumptions used are that the electrical neutrality condition is verified locally and that the colloids interact through a hard-sphere potential. The results are compared with recent experimental data. It is found that the agreement is excellent in the case of high screening (excess of salt), but becomes poorer when the salt concentration is decreased. The salt-free case is also approached.

We finally present our projects that describe acoustophoresis applied to colloidal suspensions where the two previous studies are used.