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 (TcpS ) of PNIPAm in aqueous solutions are explored. It is found that, despite of various structures of anions and cations, trans-TcpS under vis light irradiation are always higher than cis-TcpS under UV irradiation. Particularly, Hofmeister effect of anions on TcpS is roughly observed. For example, azobenzene with kosmotropic CO32- gives the lowest cis-Tcp while in use of typical chaotropic anions, such as ClO4- , azobenzene isomerization less affects values of Tcp 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 Tcp s while UV induces kosmotropic behaviors on TcpS . 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.