HOFMEISTER ANION EFFECTS ON THE PHASE BEHAVIORS OF PEO-PPO-PEO TRIBLOCK COPOLYMERS

The Hofmeister series, first noted in 1888, ranks the relative influence of ions to affect the physical behavior of a wide variety of aqueous processes ranging from protein folding and enzymatic activity to colloidal assembly and protein crystallization. This behavior is more pronounced for anions than cations and is quite general. The typical order for the anion series is as follows: CO32- > SO42- > S2O32- > H2PO4- > F- > Cl- > Br- ~ NO3-> I- > ClO4- > SCN-. Ions on the left are called kosmotropes, which tend to precipitate proteins from solution and prevent protein unfolding, whereas ions on the right are chaotropes, which increase solubility and promote the denaturation of proteins. Chloride is usually considered the dividing line between these two types of behavior. In this presentation, the effects of sodium salts with 10 anions on the phase behaviors of a poly-(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer are investigated by using an automated melting point system. It is found that the ability of anions to change the phase transition temperatures followed the Hofmeister series. Chaotropic anions work through changing the interfacial tension at the polymer/aqueous interface and in enhancing the polymer hydration by ion binding. The phase transition of the polymer in the presence of kosmotropic anions correlates directly to the hydration entropy of the anions. As a consequence, the polymer shows a two-step phase transition in solutions containing chaotropic anions while display a single-step phase transition in the presence of kosmotropic anions. The mechanisms of how ions interact with the polymer and further modulate its phase behaviors will be discussed.

Additional Abstract Information

Student(s): Branden A. Deyerle, Emigdio E. Turner

Department: Chemistry and Biochemistry

Faculty Advisor: Dr. Yanjie Zhang

Type: Oral

Year: 2012