Surprising Nanomechanical and Conformational Transition of Neutral Polyacrylamide in Monovalent Saline Solutions

Polyacrylamide (PAM) is one of the most important water-soluble polymers that has been extensively applied in water treatment, drug delivery, and flexible electronic devices. The basic properties, e.g., microstructure, nanomechanics, and solubility, are deeply involved in the performance of PAM materials. Current research has paid more attention to the development and expansion of the macroscopic properties of PAM materials, and the study of the mechanism involved with the roles of water and ions on the properties of PAM is insufficient, especially for the behaviors of neutral amide side groups. In this study, single molecule force spectroscopy was combined with molecular dynamic (MD) simulations, atomic force microscope imaging, and dynamic light scattering to investigate the effects of monovalent ions on the nanomechanics and molecular conformations of neutral PAM (NPAM). These results show that the single-molecule elasticity and conformation of NPAM exhibit huge variation in different monovalent salt solutions. NPAM adopts an extended conformation in aqueous solutions of strong hydrated ion (acetate), while transforms into a collapse globule in the existence of weakly hydrated ion (SCN^-). It is believed that the competition between intramolecular and intermolecular weak interactions plays a key role to adjust the molecular conformation and elasticity of NPAM. The competition can be largely influenced by the type of monovalent ions through hydration or a chaotropic effect. Methods utilized in this study provide a means to better understand the Hofmeister effect of ions on other macromolecules containing amide groups at the single-molecule level.