THERMODYNAMIC MODEL-FITTING OF THE CALORIMETRIC OUTPUT OBTAINED FOR AQUEOUS-SOLUTIONS OF OXYETHYLENE-OXYPROPYLENE-OXYETHYLENE TRIBLOCK COPOLYMERS

A high-sensitivity differential scanning calorimetry (HSDSC) study of aggregation transitions in dilute aqueous solutions of oxyethylene-oxy propylene-oxyethylene (EO-PO-EO) triblock copolymers (poloxamers) is r eported. The data have been analyzed using a previously described ther modynamic model (Armstrong, J. K.; et al. J. Chem. Res. 1994, 364) bas ed upon a mass action description of aggregation which has been furthe r 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 obviat ing the need for baseline fitting and as such provides a useful mechan ism for the analysis of the data. Model-fitting results are presented for aqueous solutions of various concentrations of the poloxamers P237 (EO62PO39EO62) and P333 (EO19PO56EO19) In addition model-derived resu lts are presented for a number of other poloxamer solutions. The therm odynamic data obtained are further used to produce phase diagrams of t he 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 obta ined for P333 complement spectroscopic data reported in the literature . The thermodynamic data obtained show a number of important trends. T he heat capacity change values obtained are invariably negative, point ing toward the loss of solvating water structure on aggregation. Two m easures of enthalpy are computed: the calorimetric enthalpy-obtained f rom integration of the calorimetric output-and the van't Hoff enthalpy -obtained from the change of the equilibrium constant characterizing a ggregation with temperature. Both these measure of enthalpy are positi ve. The computed entropy changes are Likewise positive, indicating tha t aggregation in these systems is an entropy-driven process. The van't Hoff enthalpy/calorimetric enthalpy ratio further indicates the aggre gation process to be cooperative. The temperature at which aggregation is half completed (T-1/2) varies with copolymer concentration. The co rresponding change in the van't Hoff enthalpy results from the tempera ture dependence of the enthalpy. Data are also obtained for aqueous so lutions of a further 12 EO-PO-EO block copolymers. Multiple linear reg ression 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 P O block in determining the size of the van't Hoff enthalpy. Finally an enthalpy-entropy compensation plot indicates that the same solvent-so lute interactions are responsible for the transitions in all the sampl es regardless of the copolymer composition and concentration.