Capillary electrochromatography: theories on electroosmotic flow in porous media

In view of the present interest in capillary electrochromatography (CEC), theories dealing with electroosmotic flow (EOF) in porous media are reviewed with particular regard to the use of packed capillaries in CEC. Two of the models found in the pertinent literature are applicable to CEC and give simple analytical solutions, The first of the two models is based on von Smoluchowski's work as adapted and extended by Overbeek. It deals with EOF through packed capillaries under conditions of low electric field strength where the EOF varies linearly with the field strength because there is no polarization of the double layer. Overbeek's model originally developed for porous media of infinite dimensions was modified in an attempt to account for the wall effect that assumes importance in the packed capillary columns used in CEC. The second model proposed by Dukhin and his coworkers predicts EOF of at least an order of magnitude higher than that expected by classical theories. This ''electroosmosis of the second kind'' is believed to occur in columns packed with conductive particles like ion exchangers at high electric field strengths when the double layer is polarized and the EOF becomes a non-linear function of the applied voltage, Conditions necessary for electroosmosis of the second kind are likely to arise upon the further development of CEC when further enhancement of the speed of analysis is brought about at electric field strength higher than that employed at present. (C) 1997 Elsevier Science B.V.