A solute vapor pressure study of complexes between benzene and caffeine in aqueous solutions

Highly precise vapor pressure-solubility measurements have been obtained for aqueous solutions of caffeine at temperatures in the range 15 to 45°C. Values of the fugacity of benzene at known total concentrations of benzene and caffeine have been calculated and used to derive equilibrium constants for the interaction of benzene molecules with monomers and aggregates of caffeine. Data are quite precisely represented by a mass action model in which benzene attaches to associated caffeine species with a binding constant that increases in direct proportion to the molecular weight of the caffeine aggregate. An equilibrium constant for the reaction benzene monomer+caffeine monomer-benzene·caffeine is reported at each temperature. The role of hydrophobic association effects in stabilizing the various complexes is considered.