Micellization of Pluronic L64 in salt solution by FTIR spectroscopy

The micellization of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer (Pluronic L64: EO13PO30EO13) with the presence of various sodium halides (NaF, NaCl, NaBr and NaI) was studied by Fourier transform infrared (FTIR) spectroscopy. The critical micelle temperature (CMT) of Pluronic L64 in the salt solution decreased with the addition of salt and the anion effect followed the sequence: F− > Cl− > Br− > I−. The deconvolution method was used to resolve the overlapping bands in the CO stretching region. The deconvoluted spectra provide information about the microenvironmental changes of ethylene oxide (EO) and propylene oxide (PO) blocks in the salt solution. Both the PEO and PPO blocks are dehydrated at lower temperature in the salt solution. The standard enthalpies (ΔH0) and entropies (ΔS0) of micellization for copolymer, PEO and PPO blocks, decreased by the addition of salt. Thus, it is deduced that the salt-induced decrease of enthalpy of micellization contributes to the decrease of CMT. The results reveal the leading role of salt-water interaction in promoting the micellization of PEO-PPO-PEO copolymer on addition of salt.