The cavity elongation effect. Calorimetric studies of the complexes of long-chain carboxylic acids with methyl-α-cyclodextrin in aqueous solutions |
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Authors: | Giuseppina Castronuovo Marcella Niccoli |
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Institution: | (1) Department of Chemistry, University Federico II of Naples, Complesso Universitario di Monte S. Angelo, via Cintia, 80126 Naples, Italy |
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Abstract: | The formation of complexes between methyl-α-cyclodextrin (MαCD) and monocarboxylic acids from C5 to C12 or cycloalkanols has been studied calorimetrically at 298 K in aqueous phosphate buffer, pH 11.3. The forces involved in
the association process are discussed according to the signs and values of the thermodynamic parameters obtained: association
enthalpy, constant, Gibbs energy and entropy.
Methyl-α-cyclodextrin forms 1:1 inclusion complexes with monocarboxylic acids, characterized by a monotonous increase in the
values of enthalpies and association constants at increasing alkyl chain length. Association is characterized by negative
enthalpies and by positive entropies: that determines large association constants. That behaviour is compared to the unusual
trend in the values of the association constants shown by the parent α-cyclodextrin interacting␣with the same monocarboxylic
acids. The model proposed to rationalize the present data provides a cavity elongation effect. Namely, because of the presence
of the methyl groups on the outside, the cavity behaves as it were deeper than that of the parent cyclodextrin.
The association with cycloalkanols—cyclohexanol, cycloheptanol, cyclooctanol and 1-cylohexyl-ethanol—is characterized by lower
entropies, as determined by the␣enhanced negative contribution originating from the␣tighter fit of the guest into the cavity. |
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Keywords: | Methyl-α -cyclodextrin Monocarboxylic acids Calorimetry Inclusion complexes Hydrophobic interactions |
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