A high-sensitivity differential scanning calorimetry (HSDSC) study of aggregation transitions in dilute aqueous solutions of oxyethylene-oxypropylene-oxyethylene (EO-PO-EO) triblock copolymers (poloxamers) is reported. The data have been analyzed using a previously described thermodynamic model et al. J. Chem. Res. 1994, 364) based upon a mass action description of aggregation which has been further elaborated to include the effect of changes in the heat capacity of the initial and final states. As a consequence the model incorporates the underlying changes in the heat capacity of the system, thus obviating the need for baseline fitting and as such provides a useful mechanism for the analysis of the data. Model-fitting results are presented for aqueous solutions of various concentrations of the poloxamers P237 (EO 62PO39EO62) and P333 (EO19PO56EO19). In addition model-derived results are presented for a number of other poloxamer solutions. The thermodynamic data obtained are further used to produce phase diagrams of the aggregation process as a function of concentration and temperature. Furthermore the calorimetric output is also used to compute critical micelle concentration and critical micelle temperature data. Data obtained for P333 complement spectroscopic data reported in the literature. The thermodynamic data obtained show a number of important trends. The heat capacity change values obtained are invariably negative, pointing toward the loss of solvating water structure on aggregation. Two measures of enthalpy are computed: the calorimetric enthalpysobtained from integration of the calorimetric outputsand the van't Hoff enthalpysobtained from the change of the equilibrium constant characterizing aggregation with temperature. Both these measure of enthalpy are positive. The computed entropy changes are likewise positive, indicating that aggregation in these systems is an entropy-driven process. The van't Hoff enthalpy/calorimetric enthalpy ratio further indicates the aggregation process to be cooperative. The temperature at which aggregation is half completed (T 1/2) varies with copolymer concentration. The corresponding change in the van't Hoff enthalpy results from the temperature dependence of the enthalpy. Data are also obtained for aqueous solutions of a further 12 EO-PO-EO block copolymers. Multiple linear regression analysis of the van't Hoff enthalpy normalized to 298.15 K as a function of PO and EO block length points to the importance of the PO block in determining the size of the van't Hoff enthalpy. Finally an enthalpy-entropy compensation plot indicates that the same solvent-solute interactions are responsible for the transitions in all the samples regardless of the copolymer composition and concentration.

The enhanced apparent solubility of naphthalene in aqueous solutions of several ABA block copolymeric surfactants has been measured using HPLC. The surfactants investigated combine, within their structure, a block of propylene oxide (PO) (the hydrophobic B block) sandwiched between two blocks of ethylene oxide (EO) (the hydrophilic A blocks). This commercially produced family of surfactants encompass a variety of materials which differ from each other in terms of block sizes and thus in terms of the balance of hydrophobic and hydrophilic forces. It is this balance which controls the ability of the surfactants to effect particular solubility enhancements. Substantial increases in solubility arise from the incorporation of naphthalene into block copolymer micelles. However the experimental data also point to solubility enhancements arising from naphthalene−surfactant interactions at surfactant concentrations below the critical micelle concentration (cmc). The apparent equilibrium constantdescribing the solute distribution between the aqueous phase and the surfactantthat characterizes this interaction correlates well with the surfactant cmc. It is concluded that the more hydrophobic EO−PO−EO copolymers produce micellar environments that are more favorable for naphthalene incorporation compared to the hydrophilic members of the family and that surfactant−naphthalene interactions below the cmc can substantially increase apparent aqueous solubilities. Experimental determinations of cmc values were measured by reductions in surface tension and by high-sensitivity differential scanning calorimetry and compared with the values estimated by naphthalene solubilization. The values obtained by the three methods are comparable. The data clearly demonstrate that cmc values are strongly dependent upon molecular composition and molecular size and that the cmc values are largest for small molecules with large hydrophilic blocks.

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