Laser electrophoresis in surface electric double layer

Abstract

The dynamic scattering of laser light by dissolved charged macromolecules or suspended colloid particles in the surface potential well near the solid/liquid boundary is examined theoretically. This surface potential well is a combination of the long-range attraction potential of the electric double layer (EDL) and the short-range repulsion van der Waals potential. The frequency spectrum of the scattered light recorded in the heterodyne regime should contain peaks with the Doppler frequency shifts corresponding to the velocities of particle electrophoresis in the EDL. The wave reflected from the boundary surface may be used as the reference wave necessary in the heterodyne regime, and the intensity of the scattered wave collinear with the reflected wave should be recorded. Since field strengths in the EDL are about 10⁵ V cm^–1 such a method should have extremely high resolution. The case of the equilibrium reversible ‘partition’ of particles between the liquid film, where no force is present, and the surface layer is examined. In this case, the ‘partition coefficient’ defining the ratio of the bulk and surface fractions of particles may be about one, if the liquid film is thin enough (≈10^–3 cm). Also, in this case, the integral frequency spectrum should contain the bulk part corresponding to the particle diffusion and the surface part with the Doppler peaks, where electrophoresis of particles should be seen.