Heat-induced structural changes in the lichrosorb RP-18—water high-performance liquid chromatographic system and their impact on the thermodynamic adsorption data of benzene, nitrobenzene and phenol

Capacity ratio (K) data of benzene, nitrobenzene and phenol have been measured on liChrosorb RP-18 with water as the eluent at temperatures between 5 and 85°C. The influence of temperature on the structure of the RP-18 layer is discussed. Partial molar free energies, entropies, enthalpies and heat capacities of adsorption have been computed as a function of temperature by fitting Taylor's series expansions to the ln k data. These thermodynamic data appear to depend on (alterations of) the structure of the RP-18 layer. The effects of cavity formation and of hydrophobic hydration of benzene in water, and of the solute polarity on the strength of the adsorption, are discussed. It is shown that enthalpy/entropy compensation effects play an important, although not readily predictable, role in solute adsorption.