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

Abstract

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. © 2008 IOP Publishing Ltd.