Roles of Enthalpy, Entropy, and Hydrogen Bonding in the Lower Critical Solution Temperature Behavior of Poly(ethylene oxide)/Water Solutions

Abstract

Analysis of the temperature and composition dependence of the excess thermodynamic properties for oligomers of poly(ethylene oxide) (PEO) in aqueous solution obtained from molecular dynamics simulations reveals that the lower critical solution temperature (LCST) behavior of PEO/water solutions is enthalpy-driven. At lower temperatures, the formation of ether−water hydrogen bonds results in a very favorable ΔG EX consistent with the consolubility of PEO and water. Increasing temperature leads to a dramatic reduction in favorable ether−water interactions due to break-up of ether−water hydrogen bonding, while relatively persistent water−water hydrogen bonding maintains the energetic penalty associated with disrupted water−water interactions upon insertion of the ether. The entropy gain associated with the break-up of ether−water hydrogen bonds and reduction in the structure of hydrating water with increasing temperature is insufficient to offset the unfavorable enthalpic effects associated with the break-up of the ether−water hydrogen bonds.