ELECTROPHORETIC TRANSPORT IN POROUS-MEDIA - A VOLUME-AVERAGING APPROACH

Volume-averaging methods are applied to develop expressions for the ef fective electrophoretic mobility and dispersion coefficients in porous media as functions of media structure and electrical field. Case 1 ac counts for electrophoretic transport in a two-phase system where both phases may be electrically conductive and the medium is uncharged, and case 2 adds hydrodynamic pressure-driven flow to one of the two phase s. The results for case 1 suggest that the electrical field induces di spersion and that the ratio of the dispersion coefficient in the porou s medium to free solution diffusivity is not in general equal to the c orresponding ratio of electrophoretic mobilities. Calculations for a s quare unit cell where the fluid is conductive and the obstacles are no t conductive show that as the electrical field increases the effective dispersion coefficients increase; however, the effective mobilities a re independent of the electrical field. This finding supports recent M onte Carlo studies that suggest the basic assumption of the classical theory of gel electrophoresis is incorrect. The results for case 2 ind icate that the electrical field will affect the mean retention time in the column and induce additional dispersion through coupling with the hydrodynamic flow field.