pH-Controllable Enthalpically-Driven Interfacial Hydration of Strong Polyelectrolyte Brushes

Understanding the thermodynamic driving force behind the pH-controllable interfacial hydration of strong polyelectrolyte brushes (SPBs) is crucial yet challenging. Using the poly(styrenesulfonate) (PSS) brush as a model system, we elucidate this thermodynamic mechanism by employing advanced femtosecond sum frequency generation vibrational spectroscopy (SFG-VS). SFG-VS identifies the hydrogen bond interactions between the bound hydronium counterions and the grafted PSS chains, along with the ordering of water molecules at the PSS brush surface. As pH increases, more hydrogen bonds are formed between the interfacial water molecules and the PSS brush accompanying the disruption of the integrated hydrogen-bond network inside the brush, concurrently enhancing the ordering of interfacial water molecules, thereby leading to the interfacial hydration of the PSS brush with negative changes in both enthalpy (ΔH < 0) and entropy (ΔS < 0). Consequently, the pH-controllable interfacial hydration of the PSS brush is consistent with enthalpically driven hydration. Moreover, this pH-controllable, enthalpically driven interfacial hydration may extend to other SPB systems, enabling the key properties of SPBs pH-responsive. This work provides a thermodynamic perspective on the pH-controllable interfacial hydration of SPBs, and will pave the way for novel applications of SPBs in material science.