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_M), PNIPAM displays lower critical solution temperature (LCST) type behavior, with the thermotransition temperature decreasing as x_M increased. Upon further increasing x_M, a cononsolvency region is identified at approximately x_M = 0.15, beyond which re-entrant swelling is observed in conjunction with a suppressed thermoresponse. In the presence of x_M = 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.