Differential Scanning Calorimetry Investigation of the Effect of Salts on Aqueous Solution Properties of an Amphiphilic Block Copolymer (Poloxamer)

Abstract

Aqueous solution properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO) amphiphilic copolymer (Pluronic L64: EO13PO30EO13) were studied in the presence of various alkali halide salts (LiCl, KCl, NaCl, NaBr, and NaI), sodium thiocyanate (NaSCN), and urea ((NH2)2CO). Differential scanning calorimetry (DSC) was employed for the determination of both the unimer-to-micelle transition (critical micellization temperature, CMT) and the phase separation (cloud point, CP). DSC is particularly useful in the case of Pluronic L64 where the detection of the CMT by optical techniques is hindered by the presence of a hydrophobic impurity. The presence of LiCl, KCl, NaCl, and NaBr decreased both CMT and CP (in the order Cl- > Br- and Na+ > K+ > Li+), whereas addition of NaSCN and urea resulted in a CMT and CP increase (in the order NaSCN > urea). NaI appeared to be an intermediate case as it decreased the CMT but increased the CP. Variation of the anion type (rather than the cation) is a more effective means of modulating the CMT and CP. This is the first study where CMT and CP values were simultaneously determined, and led to the important observation CMT(no salt) − CMT(salt) = CP(no salt) − CP(salt). Both the micellization and the phase separation of the PEO−PPO−PEO copolymer in water are endothermic; the micellization (microphase separation) enthalpy was much larger than the (macro-) phase separation enthalpy (demonstrating the dominance of the PPO−water interactions over the PEO−water interactions) and increased with increasing NaCl and NaBr concentrations and decreasing NaI and urea concentrations. The salt effects on the solution behavior of the PEO−PPO−PEO polymer were correlated to the ion radius and the solvation heat of the salts.