The effect of sodium cholate aggregates on thermoreversible gelation of PNIPAM.

Additives like salts and surfactants can alter the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM). The inclusion of a biological surfactant like sodium cholate (NaC) into PNIPAM could lead a better biocompatibility when the materials are used for biomedical applications. The phase transition behavior of PNIPAM was studied in presence of NaC. DSC study shows that the presence of NaC broadens the phase transition endotherm of PNIPAM, which is also accompanied by a small shift of the critical solution temperature (CST) to lower temperature. The results were compared with the optical measurements like, turbidity, DLS, fluorescence and rheology and it was observed that optical techniques are the best suitable for finding the onset temperature of gelation. The effect of the NaC bile salt is in contrast to the effect of conventional surfactants which are known to shift the CST to higher values, due to mutual solubilization. A study of fluorescence spectroscopic parameters like fluorescence anisotropy, spectral shift, intensity and DLS measurements suggest that a NaC-induced aggregation could be responsible for this unusual observation.