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.

The effect of polyethylene oxide (PEO) or polypropylene oxide (PPO) oligomers of various molecular weight (Mw) as well as of triblock copolymers, based on PEO and PPO blocks, on aqueous laponite RD suspensions was studied with small-angle neutron scattering (SANS). The radius of gyration (RG) increases for low M w whereas the opposite occurs for larger Mw. This behavior is explained on the basis that an effective R G is given by two contributions: (1) the size of the particles coated with the polymer and (2) the interactions between the laponite RD particles which are attractive for small and repulsive for large polymers. The SANS curves in the whole Q-range are well described by a model of noninteracting polydisperse core+shell disks, where the thickness of the polymer layer increases with the Mw. The adsorbed polymer is in a more compact conformation compared to a random coil distribution while the fraction of the polymer in the shell formed around the laponite RD particles is nearly independent of Mw. For increasing laponite RD amounts, at a given polymer composition, the thickness of the polymer slightly changes. In some cases, where also gelation is sped up, a structure factor with attractive interaction was employed which allowed to evaluate the attractive forces between the laponite RD particles. The gelation time was determined for mixtures at fixed copolymer and laponite RD concentrations. Surprisingly, it is observed that gels are formed despite the fact that the binding sites of the laponite RD particles are almost covered but the polymer size is too small to prevent aggregation. The gelation rate is correlated to structure and thermodynamics of these systems. Namely, when the balance between the steric forces and the depletion attractive forces undergoes an abrupt change the gelation time also undergoes a sharp variation. For lower and comparable Mw, PPO speeds up the gelation more efficiently than PEO while for higher Mw the gelation kinetics is slowed down again. Interestingly, copolymers of PEO and PPO blocks do not induce gelation in the time-window where the homopolymers do.

A thermodynamic model which enables the properties of aqueous copolymer/surfactant mixtures to be fit quantitatively was proposed. Namely, a relationship between the properties of transfer of the unassociated copolymer from water to the aqueous surfactant solutions (∆Y t ) and the surfactant concentration was derived. The model was based on the idea that ∆Y t can be expressed in terms of the following contributions: (1) interaction between monomers of copolymer and surfactant, (2) displacement of the monomer-micelle equilibrium induced by the copolymer, (3) formation of the surfactant-copolymer aggregation complex, and (4) formation of the mixed micelles. Such a model was applied to most literature data relative to the systems formed by (ethylene oxide) 13 -(propylene oxide) 30 -(ethylene oxide) 13 (L64) or (ethylene oxide) 75 -(propylene oxide) 30 -(ethylene oxide) 75 (F68) copolymers and hydrogenated surfactants, i.e., sodium octanoate, sodium decanoate (NaDec), N-octylpyridinium chloride, and N,N-dimethyloctylamine-N-oxide. New enthalpy data of both L64 and F68 in decyltrimethylammonium bromide were analyzed, as well. The appropriate comparison among the parameters generated by the fitting processes gave physical insights on the mechanisms of binding between the copolymer and the surfactant according to the quantitative model. The quantities obtained from the minimizing procedure were used to predict ∆Y t of the L64/NaDec system in conditions not yet investigated. The agreement between the calculated values and the new experimental points was satisfactory.

A calorimetric study was performed to focus attention on the interactions between copolymers and anionic surfactants in aqueous solutions. Three aspects were analyzed: (1) the hydrophobicity of the surfactant, (2) the change of the copolymer molecular weight, and (3) the nature of the hydrophilicity of the copolymer. To this purpose, the family of sodium alkanoates (sodium octanoate through sodium dodecanoate) and the triblock copolymers EO 76 PO 29 EO 76 (F68), EO 103 PO 39 EO 103 and EO 132 PO 50 EO 132 were investigated. Comparing F68 and EO 13 PO 30 EO 13 (L64), previously studied by us, provided information on the effect of the copolymer hydrophilicity. The experimental data were analyzed by means of a thermodynamic model which assumes that in the surfactant dilute region monomers of surfactant interact with unimeric copolymer leading to the formation of surfactant-copolymer aggregation complexes; when micelles do form, they can solubilize the copolymer, and then, the copolymer-micelle aggregates formation takes place. Data modeling provided useful parameters which allowed us to discriminate the various effects contributing to the formation of such aggregates. The thermodynamic properties for the formation of both the surfactant-copolymer aggregation complexes and copolymer-micelle aggregates were not consistent with the expectation; despite the more hydrophilic sites available in F68, the enthalpy and entropy for F68 were larger and the free energy more negative than those of L64. Moreover, the role of the copolymer molecular weight was important although the ratio between the number of the hydrophilic and the hydrophobic sites of binding is constant. An effort was done to verify whether these findings are representative of the physical interactions between copolymer and surfactant or reflect the large difference in the molecular masses. Therefore, the thermodynamic properties were normalized for the number of EO and PO units. They straightforwardly showed that L64 has a larger hydrophobic character than F68 and the copolymer molecular weight plays a small role.