Inclusion complexes of triphenylphosphine derivatives and peracetylated-β-cyclodextrin in supercritical carbon dioxide

The supramolecular chemistry of peracetylated-β-CD (perAc-β-CD) as a host for triphenyphosphine derivatives has been studied in supercritical (scCO₂) using UV absorption spectroscopy. It was found that the association constant in scCO₂ at 40°C and 300 bar is 10 to 1000 times smaller compared to analogous systems in aqueous solvent. Studies of the thermodynamics of the inclusion process found an enthalpy of association of -30 kJ/mole and an entropy of -55 J/moleK. This difference with respect to water is attributed to the absence of the hydrophobic effect in scCO₂ due to the much smaller polarity of scCO₂ versus water. To further explore the effect of the solvent on the association constant, values of solvent polarity intermediate between the limits of scCO₂ and water could be useful. To this end, methanol is added to the scCO₂ as a cosolvent to modify the overall polarity. By taking advantage of the tunability of the supercritical phase, a wide range of solvent polarity can be accessed by varying the temperature (35 to 50°C), pressure (200 to 400 bar), and mole fraction of methanol. This study is pursued to obtain a better comprehension of the inclusion process in CO₂-based supercritical fluids to make the association constant of the phosphine-perAc-β-CD system comparable to those observed in aqueous systems.