Densities and heat capacities of water−substrate, water−cyclodextrin, and water−substrate−cyclodextrin systems were determined at 298 K. The substrates studied are sodium n-alkanecarboxylates (C n COONa) (from sodium acetate to sodium decanoate) and the cyclodextrins are hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin, (HP-β-CD), hydroxypropyl-γ-cyclodextrin (HP-γ-CD) and β-cyclodextrin (β-CD). The apparent molar volumes and heat capacities of C n COONa in water were calculated as functions of concentration. The standard partial molar properties agree with those obtained by using the additivity rule. HP-β-CD essentially does not affect the thermodynamic properties of C1COONa and C2COONa. Contrarily, the formation of the inclusion complex between cyclodextrin and substrate modifies the apparent molar properties. From the standard partial molar properties of the substrate in pure water and in water + HP-β-CD mixtures and literature values for the equilibrium constant for the inclusion complex formation, the standard partial molar properties of the complex ( ) were calculated. The increase of with the number of carbon atoms in the alkyl chain (n) is consistent with the solubilization of methylene groups in the hydrophobic cavity of the cyclodextrin and the expulsion of some water molecules from the cavity. To explain the dependence of on n, conformational effects are also invoked. Studies performed as functions of cyclodextrin concentration evidence that micellization occurs provided that all the cyclodextrin is almost complexed and that the dispersed surfactant concentration equals its critical micelle concentration in water. Data in HP-α-CD, HP-β-CD, HP-γ-CD, and β-CD indicate that the size cavity of the cyclodextrin strongly affects the thermodynamics of the inclusion complex formation while the nature of the hydrophilic shell of the cyclodextrin does not.

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 (ΔH t) 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 (C n CO2Na) are sodium acetate to sodium decanoate. From the experimental data of the binary systems, the apparent molar relative enthalpies were calculated. The trends of ΔH 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-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. Both enthalpy and entropy favor the HP-α-CD−substrate complex formation while governs the HP-β-CD−substrate and HP-γ-CD + C7CO2Na complex formation. For a given substrate, , and increase with the cavity size. The ΔH 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.

Enthalpies of dilution and osmotic coefficients of sodium dodecyl sulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB) in water + 18-crown-6 ether (CR) and water + β-cyclodextrin (CD) at a fixed cosolvent concentration were measured at 298 and 310 K, respectively, as functions of the surfactant concentration (m S). Enthalpies of transfer ΔH (W → W + S) of CR (0.03 m) from water to NaDS and DTAB aqueous solutions as functions of m S were also determined at 298 K. From the enthalpies of dilution the apparent (LΦ,S) and partial (L2,S) molar relative enthalpies of both surfactants were calculated. Despite CR forms inclusion complexes with the anionic surfactant only, the L2,S vs m S profiles are similar and the enthalpies of micellization are lower than those in water by about −5 kJ mol-1. In the case of CD as a cosolvent, the L2,S vs m S profile for DTAB is similar to that for NaDS in the postmicellar region but very different in the premicellar one. The trends in the premicellar region are discussed in terms of different solute−solute hydrophilic interactions other than encapsulation while those in the postmicellar region are discussed in terms of the micellization process. The enthalpies of micellization are very large because of the complexed monomers contribution. ΔH (W → W + S) data for CR in DTAB micellar solutions were fitted through an equation previously reported which permits simultaneously obtaining the distribution constant of the uncomplexed CR and its enthalpy of transfer from the aqueous to the micellar phases. The equations were reviewed for CR in NaDS micellar solutions to account for the CR complexation and for the distribution of both the complexed and uncomplexed CR between the aqueous and the micellar phases. The derived properties are briefly discussed. The osmotic coefficient (ΦS) vs m S curve of both surfactants in W + CD shows a minimum at m S equal to the CD concentration (m CD) and a maximum at m S = m CD + cmc. These peculiarities are ascribed to the inclusion complex formation between the macrocyclic compound and the apolar chain of the surfactant. The addition of CR to water leads to the shift of the osmotic coefficient toward lower values. This shift is not very important for DTAB while it is for NaDS for which negative ΦS values were obtained. Sodium perfluorooctanoate behaves like NaDS. Since the osmotic coefficients for NaCl in W + CR are close to those in pure water, the results are interpreted in terms of complexed CR solubilization in the micellar phase.

Apparent molar volumes V Φ ,S were determined for sodium octyl, decyl, and dodecyl sulfates in water at 2 and 19 MPa from 25 to 130 °C. The shapes of V Φ ,S vs the surfactant concentration curves depend on the surfactant alkyl chain, temperature and pressure. The standard partial molar volumes were calculated from data in the premicellar region whereas the partial molar volumes of the surfactant in the micellar phase were obtained from data in the postmicellar region. The partial molar expansibility and compressibility were evaluated from the dependence of the partial molar volume on temperature and pressure, respectively. Attention was focused to the expansibility and its pressure coefficient since studying the pressure effect on the expansibility is equivalent to studying the temperature effect on the compressibility. The hydrophilic and the hydrophobic (methylene group) contributions to the expansibility were evaluated. The comparison between the present and the alkyltrimethylammonium bromides data evidenced that, contrarily to the expectation, the pressure effect on the expansibility (or temperature effect on compressibility) is the same for the two polar heads in the micelles and independent of the nature solvent (micelles or water) for −SO4Na. The pressure effect on the methylene group expansibility does not depend on the polar head in the micelles whereas it does in water. At a given temperature and pressure, the volume of micellization ΔV m was calculated by assuming the pseudo-phase transition model. ΔV m decreases with temperature according to the negative expansibility of micellization. The temperature at which ΔV m assumes a null value depends on pressure and on the nature of the surfactant. In particular, at a given pressure, the inversion of the ΔV m sign occurs at lower temperature the longer the alkyl chain is. Moreover, for each surfactant, ΔV m shows a sign inversion at lower temperature by increasing pressure.