The influence of cosolvents on hydrophilic and hydrophobic interactions. Calorimetric studies of parent and alkylated cyclomaltooligosaccharides in concentrated aqueous solutions of ethanol or urea.

Heats of dilution in water and in aqueous 7 mol kg(-1) urea and 3 mol kg(-1) ethanol of binary solutions containing cyclomaltohexaose, cyclomaltoheptaose, cyclomaltooctaose, 2-hydroxypropyl-cyclomaltohexaose (HPαCD), 2-hydroxypropyl-cyclomaltoheptaose (HPßCD), methyl-cyclomaltohexaose (MeαCD), methyl-cyclomaltoheptaose (MeßCD) and 2-hydroxypropyl-cyclomaltooctaose (HPγCD) have been determined at 298.15K by flow microcalorimetry. The purpose of this study is to gain information about the influence of urea and ethanol, which have different effects on water structure, on hydrophilic and hydrophobic interactions. The pairwise interaction coefficients of the virial expansion of the excess enthalpies were evaluated and compared to those previously obtained for binary solutions of cyclomaltohexaose and cyclomaltoheptaose. The particular behaviour of cyclomaltooligosaccharides in water is put in evidence with respect to that shown by simple oligosaccharides. The values of the interaction coefficients greatly change in dependence of the solvent medium. They are negative in water for unsubstituted cyclomaltooligosaccharides, and positive for the alkyl-substituted ones, thus marking the major role of the hydrophobic interactions. In concentrated aqueous ethanol, coefficients are negative, while they are positive in concentrated aqueous urea. Urea solvates the hydroxyl group provoking the attenuation of hydrophilic and hydrophobic interactions. Instead, the presence of the cosolvent ethanol, which lowers the relative permittivity of the medium, enhances the strength of hydrophilic interactions.

Thermodynamics of inclusion complexes of natural and modified cyclodextrins with propranolol in aqueous solution at 298 K.

The association constant, standard Gibbs energy, enthalpy and entropy for formation of inclusion complexes of propranolol, a beta-blocker, with various natural and modified cyclodextrins have been determined by calorimetry at 298 K. Both natural and methyl-modified alpha-cyclodextrins do not form complexes, while beta- and gamma-cyclodextrins do. Complexing ability of 2-hydroxypropyl-beta-cyclodextrin depends on the average substitution degree. For gamma-cyclodextrin, hydrophobic interactions play the major role in binding the guest. The association of natural and modified beta-cyclodextrins is ruled by van der Waals interactions and hydrogen bonding because of the tighter fit of the guest into the cavity. Decreasing pH determines increasingly negative values of the association enthalpies.