Hofmeister salt effects on the LCST behavior of poly(2-oxazoline) star ionomers

Abstract

Effects of pH, ionic strength, and type of salt (variation of anions) on the cloud-point temperature (T(CP)) of thermoresponsive poly(2-ethyl-2-oxazoline) star ionomers are investigated. T(CP) s can be varied in a wide range, from 10 to >95 °C, corresponding with the Hofmeister series and the ionization of the core. Addition of salt weakens the influence of pH. For star copolymer with an amine core similar trends are observed as for star copolymers with a carboxylic acid core, although the presence of a chaotropic salt affords an inversion of the effect of pH, that is, T(CP) s are lowest at pH 3, where the amine groups are fully protonated. This effect is likely due to the specific binding of chaotropic anions to the amine and ammonium groups, which is not observed for carboxylic acid groups.