The Hofmeister series is a metric of ion’s ability to affect the physical properties of aqueous processes. It has been shown that many aqueous systems follow the Hofmeister series. Although the Hofmeister effects are general phenomena in many different systems, the molecular level mechanisms of the Hofmeister series are not well understood. The effects of anions on uncharged systems are stronger than cations. Therefore, the studies of the effects of cations are more difficult than similar studies of anions. Herein, we study the phase behaviors of a triblock copolymer, poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) (PEO-PPO-PEO), in the presence of a series of fourteen monovalent and divalent cation chloride salts by using an automated melting point apparatus. The polymer undergoes a two-steps phase transition in the presence of all the cations studied. The first step phase transition occurs due to the formation of the micelles caused by the dehydration of PPO blocks, and the second step at a higher temperature is caused by the aggregation of micelles from dehydration of the PEO blocks. Cations can interact with the polymer in three ways: directly binding to the oxygen in the polymer, sharing one water molecule with the polymer in the hydration layer, or interacting with the polymer via two water molecules. We found that monovalent cations Na+, K+, Rb+, and Cs+ do not bind to the polymer, while Li+ and NH4+ and all the divalent cations investigated including Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Cu2+, Cd2+, bind to the polymer. It is suggested that hydration thermodynamics of cations was a key factor in determining how cations interact with the polymer and further modulate its phase behaviors. The mechanisms of cation-polymer interaction will be discussed in details.

Additional Abstract Information

Student(s): Tsung-Yu Wu, Jacob C. Lutter

Department: Chemistry and Biochemistry

Faculty Advisor: Dr. Yanjie Zhang

Type: Oral

Year: 2014

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