Abstract
Enthalpies of dilution of sodium n-alkanoate and cyclodextrin aqueous solutions with water as functions of their concentration were measured at 298 K. The enthalpy of transfer (ΔHt) of cyclodextrin (≈0.02 mol kg-1) from water to the aqueous solutions of the substrates was determined as a function of the substrate concentration. The cyclodextrins are hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and hydroxypropyl-γ-cyclodextrin (HP-γ-CD). The substrates (CnCO2Na) are sodium acetate to sodium decanoate. From the experimental data of the binary systems, the apparent molar relative enthalpies were calculated. The trends of ΔHt versus substrate concentration in the premicellar region were rationalized in terms of the substrate−cyclodextrin inclusion complex formation. The latter was not evidenced for HP-γ-CD with C3CO2Na and C5CO2Na and for HP-β-CD with C3CO2Na. The standard free energy for the complex formation decreases with the number of carbon atoms in the alkyl chain. Bot...
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