Interactive behavior of caffeine at a platinum electrode surface

Interactive behavior of caffeine at a charged platinum/solution interface was investigated in a wide temperature range, from 295 to 333 K, in a phosphate buffer solution pH 7.0. It was shown that the amount of adsorbed caffeine (surface concentration) is directly proportional to the measured adsorption surface charge density resulting from caffeine oxidation to theophilline. At low temperatures, a monolayer of caffeine molecules laying in a flat orientation on the Pt surface is adsorbed, while at higher temperatures, conformational changes occur, resulting in tilting of the adsorbed molecules to allow for higher surface concentrations to be achieved. A highly negative Gibbs energy of adsorption, ranging from −51.1 kJ mol−1 at 295 K to −60.6 kJ mol−1 at 333 K, demonstrated a high affinity of caffeine for the Pt surface. Although the adsorption process was found to be endothermic (ΔHADS = 20 kJ mol−1), it was determined that a large positive change in the adsorption entropy (TΔSADS = 75 ± 3 kJ mol−1) represents the driving force for the strong interaction of caffeine with Pt.