The role of polymer chain sequence and multivalent salts in weak polyampholyte brushes

Understanding charge regulation in weak polyampholytes, where each polymer chain contains both acidic and basic groups, is essential for designing advanced materials for biological and industrial applications. Although extensive research has been conducted on the charge regulation mechanisms of weak polyampholytes in bulk solution, the behavior of weak polyampholyte brushes remains largely unexplored.

We studied two types of polyampholyte brush chain sequence: a diblock and an alternating sequence, using Single-chain Ising Density Functional Theory (SciDFT), Ising Density Functional Theory (iDFT), and Molecular Dynamics (MD) simulations with the Grand-Reaction Monte Carlo (G-RxMC) method. Consistently, our findings indicate that altering the sequence while maintaining the same overall conditions – such as the same number of acid and base groups and their pKa values, pK_A^acid =  pK_A^base = 7, grafting density, and salt concentration – does not change the total charge of the polyampholyte but significantly affects the degree of ionization and brush thickness due to conformational changes. Additionally, we observed a pronounced change in molecular conformations: the chains remain stretched at very low or high pH values, whereas they collapse when the pH value is equal to the pKa values of the acid and base groups,  pH = pK_A^acid =  pK_A^base = 7. Furthermore, we investigated charge regulation behavior as a function of grafting density as well as in the presence of multivalent salts. These insights shed new light on the stimuli-responsive properties of weak polyampholyte brushes, providing valuable guidance for designing synthetic surfaces that control adsorption of biomolecules, wettability, or friction.