Thermodynamic Evaluation of Activated Charcoal as a Poison Antidote by High-Performance Liquid Chromatography I: Derivation and Validation of an Equation for Gibbs Free Energy of Liquid–Solid Adsorption

An in vitro method utilizing high-performance liquid Chromatography (HPLC) was developed in order to investigate the adsorptive process between activated charcoal and various drugs and toxic chemicals by measuring their Gibbs free energy of adsorption from various acetonitrile: water mobile phases. This report details the derivation and validation of the equation for calculating the Gibbs free energy of liquid-solid adsorption via HPLC. The derived equation incorporates the following experimental parameters: specific surface area of the adsorbent, specific retention volume of the solute, molar volume of the mobile phase, and surface concentration of the solute in a predefined standard state. This equation was validated by means of a closed thermodynamic cycle composed of three segments. Each segment represents a different physical process: gas-solid adsorption of methyl iodide on activated charcoal, gas-liquid solution of methyl iodide in n-hexadecane, and liquid-solid adsorption of methyl iodide on activated charcoal from n-hexadecane. The Gibbs free energy for each of these thermodynamic processes was determined by the appropriate chromatographic technique. Since the cycle did not balance because it did not account for the interaction of n-hexadecane and activated charcoal, it was altered to include a gas-liquid-solid chromatographic technique. When the Gibbs free energies of solution and gas-solid adsorption determined by this chromatographic technique were incorporated into the cycle, the resulting imbalance was only 0.213 kJ/mol (1.1%), thereby validating the derived equation.