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The regulation of electroosmotic flow (EOF) in capillary electrophoresis is of great importance in many analytical applications. In this paper, the variation of EOF with the concentration of amphiphile is studied by adding a charged amphiphile to the background electrolyte (BGE) when using a C8-coated capillary. A theory relating the change in concentration of amphiphile in the BGE to the variation in observed EOF is presented. The basis of the theory is that the adsorbed amphiphile creates an electrostatic surface potential that is calculated from the linearised Poisson-Boltzmann equation. The adsorption isotherm of the amphiphile to the capillary surface is described by an electrostatic surface potential modified linear isotherm. By assuming that the ζ-potential in the Smoluchowski equation is proportional to the electrostatic surface potential, an equation describing the EOF as a function of amphiphile concentration is obtained. The proposed theory is experimentally tested by adding octane sulfonate, tetrabutylammonium or tetrapentylammonium ion to the BGE. It is shown that, in these cases, the proposed theory describes the EOF as a function of amphiphile concentration well and that, as expected, the proportionality constant between the electrostatic surface potential and the ζ-potential is less than one. Some practical aspects and problems that occur with this type of measurement are also discussed. |
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