Thermodynamic approaches to the study of adsorption from solution

The primary objective of this research is to further the understanding of the energy uptake/release associated with the adsorption of small drug molecules to activated carbon surfaces and to address the discrepancies between the various methods of measuring the enthalpy (heat) of binding. This was accomplished by both employing calorimetric studies on adsorption and development of adsorption models that account for the shortcomings of the previous ones. The conclusions reached in this work are general and are expected to possess wide applications in the fields of thermodynamics, biochemistry, and medicinal chemistry. The adsorption of phenobarbital on commercial activated carbons from aqueous solutions was used to demonstrate the proposed models and mechanisms of interaction. The adsorption of fluoxetine, primidone, procaine, and barbituric acid was used to study the dynamics of the solvent molecules surrounding the solute molecules during adsorption.

Significant discrepancies were found between the calorimetric value of the enthalpy of phenobarbital binding and the value from the Van’t Hoff method. The Van’t Hoff method utilizes equilibrium constants which were obtained using the Two-Mechanism Langmuir-Like Equation (TMLLE). A new modified TMLLE model that accounts for solvent displacement from the binding site is proposed, and the analysis shows that the impact of solvent displacement is minimal on the Van’t Hoff enthalpy.

The enthalpy of phenobarbital binding was investigated calorimetrically under varying conditions of temperature, buffer compositions, and pH. The enthalpy showed great variability based on the experimental conditions. The value of the intrinsic enthalpy was unchanged. Comparing the intrinsic calorimetric enthalpy with the Van’t Hoff enthalpy shows that the two methods agree closely for all activated carbons tested. Finally, the dynamics of hydration shells surrounding the solute molecules were studied using molecular dynamics simulations. The water-solute interface is associated with a disturbance in the translational modes of motion resulting in slow-moving water molecules that are packed less ideally. This disturbance correlates very well with the extent of entropy-driven adsorption. Simulation methods provided a good basis for quick predictions for the capacity of entropy-driven ad-sorption for different drug molecules.