Hofmeister Effects of Anions on Self-assembled Thermogels
Description
Thermogels are smart, temperature-responsive soft biomaterials with numerous biomedical applications. They possess high water content, and can spontaneously gelate by forming non-covalent physical crosslinks between their constituent amphiphilic polymers when warmed. However, despite the ubiquity of salts in biological fluids and buffer media, the influence of salts on thermogelling polymers and the overall physical properties of the resulting hydrogels are poorly understood. Herein, we elucidate the effects of common inorganic salts on the gelation and micellisation properties of a thermogelling polymer containing poly(ethylene glycol), poly(propylene glycol) and poly(caprolactone) components. The identity of the salts’ anions and their concentrations were found to exhibit significant effects on the thermogel properties, in some cases being able to decrease the sol-to-gel phase transition by up to 10 oC. We demonstrate that these notable influences are likely brought about by the changes in solvation of both the polymer’s hydrophobic and hydrophilic segments, as well as by direct interactions of poorly-hydrated anions with the hydrophobic polymer segments. Our findings demonstrate that the effects of salts on amphiphilic thermogelling polymers are non-negligible, and hence need to be taken into account for engineering and optimisation of thermogel properties for different biomedical applications.