Specific Ion Effects of Azobenzene Salts on Photoresponse of PNIPAm in Aqueous Solutions

Ionic species are important to dominate phase separation behaviors of poly(N-isopropylacrylamide) (PNIPAm) in aqueous solutions. Herein, photoresponsive azobenzene-based salts with various ions are prepared and their photoresponsive ion effects on clouding temperatures (T-cpS) of PNIPAm in aqueous solutions are explored. It is found that, despite of various structures of anions and cations, trans-T-cpS under vis light irradiation are always higher than cis-T-cpS under UV irradiation. Particularly, Hofmeister effect of anions on T-cpS is roughly observed. For example, azobenzene with kosmotropic CO32- gives the lowest cis-T-cp while in use of typical chaotropic anions, such as ClO4-, azobenzene isomerization less affects values of T(cp)s. In another hand, azobenzene-based metallic salts containing lithium, sodium, and potassium cations also demonstrate photoresponsive Hofmeister effect. Trans-metallic azobenzene demonstrates a chaotropic effect on T(cp)s while UV induces kosmotropic behaviors on T-cpS. Additionally, ionic conduction of the solution along with photoresponsive phase separations is also investigated and PNIPAm aggregations induce a sharp reduction of ion conduction during UV light illumination.