Poly(ethylene oxide)poly(propylene oxide)poly(ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces: thermodynamics, structure, dynamics, and modeling

Abstract

The association properties of poly(ethylene oxide)- block-poly(propyleneoxide)- block-poly(ethylene oxide) (PEOPPOPEO) copolymers (commercially available as Poloxamers and Pluronics) in aqueous solutions, and the adsorption of these copolymers at interfaces are reviewed. At low temperatures and/or concentrations the PEOPPOPEO copolymers exist in solution as individual coils (unimers). Thermodynamically stable micelles are formed with increasing copolymer concentration and/or solution temperature, as revealed by surface tension, light scattering, and dye solubilization experiments. The unimer-to-micelle transition is not sharp, but spans a concentration decade or 10 K. The critical micellization concentration (CMC) and temperature (CMT) decrease with an increase in the copolymer PPO content or molecular weight. The dependence of CMC on temperature, together with differential scanning calorimetry experiments, indicates that the micellization process of PEOPPOPEO copolymers in water is endothermic and driven by a decrease in the polarity of ethylene oxide (EO) and propylene oxide (PO) segments as the temperature increases, and by the entropy gain in water when unimers aggregate to form micelles (hydrophobic effect). The free energy and enthalpy of micellization can be correlated to the total number of EO and PO segments in the copolymer and its molecular weight. The micelles have hydrodynamic radii of approximately 10 nm and aggregation numbers in the order of 50. The aggregation number is thought to be independent of the copolymer concentration and to increase with temperature. Phenomenological and mean-field lattice models for the formation of micelles can describe qualitatively the trends observed experimentally. In addition, the lattice models can provide information on the distribution of the EO and PO segments in the micelle. The PEOPPOPEO copolymers adsorb on both airwater and solidwater interfaces; the PPO block is located at the interface while the PEO block extends into the solution, when copolymers are adsorbed at hydrophobic interfaces. Gels are formed by certain PEOPPOPEO block copolymers at high concentrations, with the micelles remaining apparently intact in the form of a “crystal”. The gelation onset temperature and the thermal stability range of the gel increase with increasing PEO block length. A comparison of PEOPPO copolymers with PEOPBO and PEO PS block copolymers and C i E j surfactants is made, and selected applications of PEOPPOPEO block copolymer solutions (such as solubilization of organics, protection of microorganisms, and biomedical uses of micelles and gels) are presented.