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Thermodynamic model fitting of the calorimetric output obtained for aqueous solutions of oxyethylene-oxypropylene-oxyethylene triblock copolymers

Thermodynamic model fitting of the calorimetric output obtained for aqueous solutions of oxyethylene-oxypropylene-oxyethylene triblock copolymers

Paterson, Iain, Armstrong, Jonathan, Chowdhry, Babur and Leharne, Stephen (1997) Thermodynamic model fitting of the calorimetric output obtained for aqueous solutions of oxyethylene-oxypropylene-oxyethylene triblock copolymers. Langmuir, 13 (8). pp. 2219-2226. ISSN 0743-7463 (Print), 1520-5827 (Online) (doi:10.1021/la960432g)

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Abstract

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 (Armstrong, J. K.; 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 (EO62PO39EO62) 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 enthalpy-obtained from integration of the calorimetric output-and the van't Hoff enthalpy-obtained 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.

Item Type: Article
Uncontrolled Keywords: phase-behavior, micellization, water, transitions
Subjects: Q Science > QD Chemistry
Faculty / Department / Research Group: Faculty of Engineering & Science
Related URLs:
Last Modified: 14 Oct 2016 09:12
Selected for GREAT 2016: None
Selected for GREAT 2017: None
Selected for GREAT 2018: None
URI: http://gala.gre.ac.uk/id/eprint/4878

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