Calorimetric study of sodium n-alkanoate-modified cyclodextrin-water ternary systems

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 (Delta H-t) of cyclodextrin (approximate to 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-alpha-cyclodextrin (HP-alpha-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), and hydroxyprepyl-gamma-cyclodextrin (HP-gamma-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 Delta H-t 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-gamma-CD with C3CO2Na and C5CO2Na and for HP-beta-CD with C3CO2Na. The standard free energy for the complex formation (Delta G(C)(o)) decreases with the number of carbon atoms in the alkyl chain. Both enthalpy (Delta H-C(o)) and entropy (T Delta S-C(o)) favor the HP-alpha-CD-substrate complex formation while T Delta S-C(o) governs the HP-P-CD-substrate and HP-gamma-CD + C7CO2Na complex formation. For a given substrate, Delta G(C)(o), Delta H-C(o) and T Delta S-C(o) increase with the cavity size. The Delta H-t versus f(s)m(s) trends for the micellar substrate solutions were interpreted in terms of the substrate-cyclodextrin complex formation and the shift of the micellization equilibrium.