Complexation of phosphine ligands with peracetylated β-cyclodextrin in supercritical carbon dioxide: Effect of temperature and cosolvent on the equilibrium constant

The interaction between peracetylated -β-cyclodextrin and tert-butyl and adamantyl functionalized triphenylphosphine derivatives was studied in supercritical carbon dioxide (scCO2) based solvent media by UV–vis spectroscopy. The equilibrium constant for a 1:1 complexation reaction was obtained from titration spectra both in pure carbon dioxide and in the presence of methanol as a cosolvent in the temperature range 308–323K to estimate the internal energy and entropy of the inclusion equilibrium. The values of the equilibrium constants were found significantly smaller than those obtained in aqueous solution with analogous phosphines and substantially independent of the nature of the phosphine and of the solvent medium. On the basis of the results that we have obtained in this study we do believe that a possible explanation of the unfavorable complexation equilibria in CO2 based solvent systems could be a distorsion of the conformation of the cyclic oligosaccharides arising from the vanishing of the hydrogen bond interactions between contiguous glucose units of the peracetylated cyclodextrin.