Interfacial thermodynamics of gallic acid adsorption on a chargeable hydrophobic surface.

The thermodynamics of adsorption of gallic acid (GA, 3,4,5-trihydroxylbenzoic acid) on the hanging mercury drop electrode (HMDE) surface was studied by temperature-dependent stripping voltammetry (TD-SV), at physiological pH 7.4. The thermodynamic parameters, e.g., Gibbs free energy, ΔG(ADS), enthalpy, ΔΗ(ADS) and entropy, ΔS(ADS), of adsorption have been determined at physiological temperatures 2-40 °C. Chemisorption of the radical species ≡[GA(OH)(2)(O(-))]* is the energetically important reaction. The thermodynamic data show a complex mechanism of adsorption of GA on the electrode surface, which is strongly dependent on temperature. At low-temperatures T<12 °C, adsorption is controlled by enthalpy, while at T>22 °C, adsorption is entropy driven. In the temperature range 12 °C and 22 °C, a combined enthalpy-entropy stabilization occurs. A mechanism is proposed which analyses the implication of thermodynamics to the interfacial adsorption of polyphenols with cell membranes under physiological conditions.