The effect of NaCl on the eutectic phase behavior of aqueous poly(ethylene glycol) solutions

The phase behaviors of aqueous polymer solutions are known to be affected by the presence of ions even if the polymer itself does not have any charges. We studied the effect of salt (sodium chloride) on the eutectic phase behavior of non-ionic polymer, poly(ethylene glycol) (PEG) in aqueous solutions using differential scanning calorimetry. We observed that the addition of NaCl increased the liquidus temperature of PEG and decreased that of water. As a result, a steep rise (or fall) is induced in the liquidus around the eutectic point. A simple Flory-Huggins lattice model for the mixture (PEG-water-NaCl) was applied to the experimental results. The model quantitatively reproduced the change in the liquidus both with and without NaCl. The obtained interaction parameters suggest that the increase of the PEG melting temperature by NaCl can be understood as the depletion of NaCl around PEG, possibly due to the image charge repulsion.