Microscopic solvent structure of supercritical carbon dioxide and its mixtures with methanol in the cybotactic region of the solute molecule

UV/VIS absorption spectroscopy was used to estimate the microscopic solvent polarities of supercritical carbon dioxide alone and its mixtures with methanol. It was found that the local composition of methanol in the cybotactic region of 4-nitroanisole was larger than the corresponding bulk concentration. With increasing pressure the methanol molecules were pressed out of the solvation shell of 4-nitroanisole by the carbon dioxide molecules. From the solubility measurements the solvation numbers of the carbon dioxide molecules around 4-nitroanisole were calculated as a function of fluid density.