Solvent and Ion-Mediated Behavior of a Thermoresponsive Brush: Specific Ion Effects in Methanol-Water Electrolytes.

In this study, specific ion effects are explored in methanol-water mixtures, which play a critical role in a diverse range of applications, including protein solubilization and supercapacitors. Spectroscopic ellipsometry and neutron reflectometry are employed to investigate the solvent- and ion-mediated behavior of a poly(N-isopropylacrylamide) (PNIPAM) brush, a well-known thermoresponsive polymer. In the absence of ions and at low methanol mole fractions (x), PNIPAM displays lower critical solution temperature (LCST) type behavior, with the thermotransition temperature decreasing as x increased. Upon further increasing x, a cononsolvency region is identified at approximately x = 0.15, beyond which re-entrant swelling is observed in conjunction with a suppressed thermoresponse. In the presence of x = 0.10 electrolytes, the observed specific ion effects adhere to a forward Hofmeister series. Strongly solvated ions, such as Cl and Br, decrease the LCST of the brush. In contrast, poorly solvated ions, such as SCN and I, lead to more swollen brush profiles and an increase in the LCST. We hypothesize that the stability of water-methanol clusters plays a crucial role in governing polymer solvation, providing insights into the fundamental interactions within mixed solvent systems. Moreover, a theoretical ion that does not impact the swelling or structure of a PNIPAM brush is proposed.