The enthalpy of transfer (ΔH t) of hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and β-cyclodextrin (β-CD) from water to the aqueous C6F13CO2Na and C7F15CO2Na solutions were determined in the pre- and post-micellar regions. The behavior of the macrocycles is system specific. Generally, the magnitude of the enthalpy is influenced by several factors:  (1) the alkyl chain length of the surfactant, (2) the cyclodextrin cavity and its alkylation, (3) the interactions between the free cyclodextrin and the free surfactant, (4) the host−guest equilibrium constant, (5) the host/guest stoichiometry, and (6) the micelle-cyclodextrin (free and/or complexed) interactions. As far as the premicellar region is concerned, HP-α-CD does not form the host−guest complexes. β-CD and HP-β-CD in the aqueous C7F15CO2Na solutions form host−guest complexes of 1:1 stoichiometry; β-CD shows a larger binding affinity toward the surfactant as a compensative effect between the more negative enthalpy and entropy. Besides 1:1 complexes, HP-β-CD in aqueous C6F13CO2Na solutions forms complexes of 1:2 stoichiometry (1 cyclodextrin:2 surfactants). Their presence was evidenced by the minimum in the ΔH t vs the surfactant concentration (f S m S) trend. The equation derived to take into account both 1:1 and 1:2 complexes equilibria was successfully applied to the present data and those of HP-α-CD/sodium alkanoate systems previously studied by us. As far as the postmicellar region is concerned, HP-α-CD was treated like an additive, which distributes between the aqueous and the micellar phases. An equation was proposed to rationalize the enthalpy data dealing with the cyclodextrins exhibiting inclusion complex formation. It was based on the following phenomena:  (1) formation of 1:1 and 1:2 complexes in the aqueous phase, (2) distribution of free cyclodextrin, 1:1 complex, and 1:2 complex between the aqueous and the micellar phases, and (3) shift of the micellization equilibrium induced by the cyclodextrin. As a general feature, cyclodextrin (free and/or complexed) shows affinity toward the micelles because of the favorable interactions between the carboxylate head in the hydrophilic shell and the hydroxyl groups of the cyclodextrin. C6F13CO2Na micelles compared to C7F15CO2Na exhibit a slightly larger affinity toward HP-α-CD controlled by more negative enthalpy and entropy changes. A single mechanism governs the interaction between the C7F15CO2Na micelles and the 1:1 complexes of HP-β-CD/surfactant and β-CD/surfactant, as the standard free energy, enthalpy, and entropy of transfer of the two complexes from the aqueous to the micellar phases are identical. The 1:2 complex (1 HP-β-CD:2 C6F13CO2Na) weakly binds to the micelles according to the unfavorable interactions between the micellar surface and the doubly charged complex.

Volume and enthalpy of transfer of hydroxypropyl-α-cyclodextrin (HP-α-CD) and hydroxypropyl-γ-cyclodextrin (HP-γ-CD) from water to the aqueous solutions of sodium alkanoates (sodium hexanoate, sodium decanoate and sodium dodecanoate) were determined at 298 K. The cyclodextrin concentration was kept constant, and that of the surfactant was varied in order to analyze both the pre- and postmicellar regions. The experimental data in the premicellar region were consistent with the formation of 1:1 and 1:2 (1 cyclodextrin:2 surfactants) inclusion complexes, with the exception of the HP-α-CD/sodium dodecanoate system which presented only the 1:1 complexes. The mechanism of the 1:2 complexes formation of HP-α-CD/surfactant is different from that involving HP-γ-CD. The quantitative analysis of the experimental data in the post-micellar region supplied parameters indicating that the cyclodextrin−micelles forces are ion−dipole (carboxylate head/hydroxylic group) in nature. The present results combined with the literature ones clarify the effect of the cavity size of the cyclodextrin as well as the hydrophobicity of the surfactant on the cyclodextrin-dispersed surfactant and cyclodextrin−micelle interactions.

The adsorption thermodynamics of copolymers, based on ethylene oxide (EO) and propylene oxide (PO) units, at the laponite (RD) clay/liquid interface was determined at 298 K. The copolymer nature was tuned at molecular level by changing the hydrophilicity, the architecture and the molecular weight (Mw) keeping constant the EO/ PO ratio. Polyethylene (PEGs) and polypropylene (PPGs) glycols with varying Mw and their mixture were also investigated to discriminate the role of the EO and the PO segments in the adsorption process. Enthalpies of transfer of RD, at fixed concentration, from water to the aqueous macromolecule solutions as functions of the macromolecule molality were determined. They were treated quantitatively by means of a model based on two equilibria: (1) one-to-one binding between the macromolecule and the site on the solid and (2) two-to-one binding following which one macromolecule interacts with another one adsorbed onto the solid. The good agreement between the equilibrium constants obtained from calorimetry and those determined from kinetic experiments confirmed the reliability of the experimental and theoretical approaches. Almost all of the systems investigated are highlighted by the one-to-one binding; the L35 and 10R5 systems present both equilibria. The insights provided by the thermodynamics of adsorption of their homopolymers onto RD were fruitful in obtaining detailed information on the nature of the forces involved between RD and the copolymers. The data obtained in the present work clearly evidenced that for comparable polymer Mw, PPG is more suitable in building up a steric barrier around the RD particles and, indeed, exhibits several advantages and no drawbacks. Moreover, the parent copolymers may properly functionalize the RD surface by exploiting both their high affinity to the solid surface and the ability to self-assemble onto it as L35 and 10R5 clearly showed.

The enthalpy and the volume of transfer (∆Yt) of the unassociated (ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 (L64) from water to the aqueous sodium alkanoate solutions as functions of the surfactant concentrations (mS) were determined at 298 K. The surfactants studied are sodium hexanoate, sodium heptanoate, sodium octanoate, sodium undecanoate, and sodium dodecanoate. As a general feature, for the short alkyl chain surfactants, ∆Y t describes an S-shaped curve in the range of mS analyzed whereas for the more hydrophobic surfactants the ∆Yt vs mS trends exhibit maxima which appear at mS values very close to the critical micellar concentration in water. The experimental properties were quantitatively treated by means of a thermodynamic model, recently proposed by us, which assumes that the processes of micellization and the formation of both the surfactant-copolymer aggregation complex and the micelle-copolymer mixed aggregate take place simultaneously. The thermodynamics for the surfactant/copolymer aggregation complex formation states that both the compounds release water molecules from their nonpolar moieties which interact through the van der Waals forces generating a hydrophobic microenvironment in the aggregation complex similar to that in the micellar state. Conformational variations of the copolymer produced by the attachment of the surfactant molecules induce the formation of some hydrogen bonds between the ether oxygen of the ethylene oxide units and water. The thermodynamics of transfer of L64 from the aqueous phase to the sodium alkanoate micelles was also determined. The interactions in the mixed micelles and the surfactant-copolymer aggregation complex are similar. However, due to the large size of the micelle, the conformational effects caused by the L64 solubilization in the micelle are quite significant.

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.