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

Enthalpies of dilution of sodium n-alkanoate and cyclodextrin aqueous solut ions 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 deter mined as a function of the substrate concentration. The cyclodextrins are h ydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), hydroxypropyl-beta-cyclodext rin (HP-beta-CD), and hydroxyprepyl-gamma-cyclodextrin (HP-gamma-CD). The s ubstrates (CnCO2Na) are sodium acetate to sodium decanoate. From the experi mental data of the binary systems, the apparent molar relative enthalpies w ere calculated. The trends of Delta H-t versus substrate concentration in t he premicellar region were rationalized in terms of the substrate-cyclodext rin 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 f ree energy for the complex formation (Delta G(C)(o)) decreases with the num ber of carbon atoms in the alkyl chain. Both enthalpy (Delta H-C(o)) and en tropy (T Delta S-C(o)) favor the HP-alpha-CD-substrate complex formation wh ile 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 th e substrate-cyclodextrin complex formation and the shift of the micellizati on equilibrium.