Investigation of Adsorption Thermodynamics at Electrified Liquid-Solid Interfaces by Electrochemically Modulated Liquid Chromatography

A method to investigate electrosorption thermodynamics has been developed by the examination of the temperature dependence of solute retention by means of electrochemically modulated liquid chromatography (EMLC). This hybrid technique couples electrochemistry and liquid chromatography, which enables the manipulation of solute retention through changes in the potential applied (E_app) to a conductive stationary phase like glassy carbon, porous graphitic carbon, and boron-doped diamond. An understanding of the thermodynamics of the EMLC separation mechanism would further the interpretations of the rules to predict retention, which may also provide fundamental insights into sorption processes and the structure of the electrical double layer of importance to a number of other technologies (e.g., battery and fuel cells). This paper describes a study in which the dependence of retention for two naphthalene disulfonates (1,5- and 2,6-naphthalene disulfonates) was measured at different fixed values of E_app [-200 to +200 mV vs Ag/AgCl (sat'd NaCl)] for a glassy carbon chromatographic packing that was held at several different thermally equilibrated column temperatures (22-55 °C). These data were analyzed using the van't Hoff relationship to determine the enthalpic and entropic contributions to Gibb's free energy for the transfer of a solute between the mobile and stationary phases. The results for both solutes showed that (1) the retention increased as E_app moved to more positive values, (2) the retention at each value of E_app became smaller as the column temperature increased, and (3) the dependence of retention on temperature was stronger as the value of E_app became more negative. The first and second observations follow expectations for the dependence of the electrostatic interaction strength for anionic solutes on the charge on the electrode surface and for the temperature dependence of the exothermic transfer of a solute from the mobile phase to the stationary phase, respectively. The third observation indicates that retention actually becomes more endothermic as the value of E_app becomes more positive, which should conceptually cause a decrease rather than an increase in retention. This points to the somewhat surprising importance of entropy to the overall retention mechanism. That is, the increase in retention at increasingly positive values of E_app reflects the fact that the increase in the entropy for solute transfer has a stronger contribution to the transfer process than the concomitant increase in the endothermicity of transfer. The paper concludes by briefly examining mechanistic origins for the thermodynamic behavior of this system within the context of the electrical double-layer theory and the hydrophobic effect, and possible applications of this intriguing development as a tool for the investigation of electrosorption processes in energy, colloidal, and bioanalytical systems.