title: Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series.
abstract: Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macromolecular precipitation are sensitive to the nature and concentration of the ions present in solution. Herein, the effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temperature gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, analysis of solvent isotope effects and of the changes in LCST with ion concentration and identity showed multiple mechanisms were at work. In solutions containing sufficient concentrations of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
authors.forename: Yanjie
authors.initials: Y
authors.lastname: Zhang
authors.forename: Steven
authors.initials: S
authors.lastname: Furyk
authors.forename: David E
authors.initials: DE
authors.lastname: Bergbreiter
authors.forename: Paul S
authors.initials: PS
authors.lastname: Cremer
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry*; D046911:Macromolecular Substances / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry*; D012995:Solubility; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D000180:Acrylic Resins; D046911:Macromolecular Substances; D014867:Water; C052970:poly-N-isopropylacrylamide; D012964:Sodium