Thermodynamic Evaluation of the Interaction Driven by Hydrophobic Bonding in the Aqueous Phase

In the present study, the interaction between phenobarbital and activated carbons which is driven by hydrophobic bonding was evaluated. The Two-Mechanism Langmuir-Like Equation was proposed to describe the isotherms for phenobarbital adsorbing to activated carbons. The parameters in the Two-Mechanism Langmuir-Like Equation obtained from the nonlinear fitting of isotherms were used in the calculations of the differential Gibbs free energy for the hydrophobic bonding-driven interaction. Two thermodynamic models, the Modified Crisp Model and the van't Hoff Equation, were adopted to estimate the differential Gibbs free energy. And, comparing the differential Gibbs free energy obtained from the 2 thermodynamic relationships, it can be determined that an adsorbing phenobarbital molecule displaces 12 water molecules on the hydrocarbon surfaces of the activated carbons (hydrophobic bonding case). The difference between the estimates of the differential Gibbs free energy obtained by the Modified Crisp Model and by the van't Hoff Equation provides a new experimental method to calculate the number of solvent molecules displaced by an adsorbing solute molecule. This is a completely general technique for the hydrophobic bonding-driven interaction and is not limited to the systems studied. The calculated positive differential entropy confirmed that the adsorption process was entropy driven.