Electrochemically modulated liquid chromatography and the Gibbs adsorption equation.

Electrochemically modulated liquid chromatography uses a conductive material like porous graphitic carbon (PGC) as a chromatographic stationary phase and a working electrode. This dual functionality enables manipulation of separations by changes in the potential applied (E(app)) to the packing. Thus, by monitoring the retention factor (k') with respect to E(app), a chromatographic tool for examination of electrosorption processes can be devised. This novel capability is developed herein by examining the retention of charged aromatic compounds at PGC. The Gibbs adsorption equation and related formulations (e.g., the Lippmann equation) are used to determine interfacial excesses (Gamma) of these solutes in different supporting electrolytes, changes in interfacial tension (dgamma), the charge on the electrode (q(M)), and the potential of zero charge (PZC). Values of the PZC were also determined from plots of ln k' versus E(app). In this case, the dependence of ln k' reveals a shift in the PZC to more cathodic values as the strength of specific adsorption by anions as the electrolyte concentration increases. Together, these results provide insights into the retention mechanism and, more generally, to adsorption at electrified carbon electrodes. Extensions of this strategy as a probe of electrified interfaces with respect to mobile-phase composition, temperature, and pressure are briefly described.