Study of the Effect of Added Salts on the Interfacial Dynamics of Model Charged and Uncharged Water-Soluble Polymers

Theories of electrolyte and polyelectrolyte solutions have traditionally been developed without any consideration of ion and polymer hydration, an assumption that leads not only to a great simplification of theory, but also to a highly idealized perspective of the thermodynamic and dynamic properties of these solutions, a situation that makes it impossible to address many of their observed properties. In particular, we mention in this context that the propensity of the addition of some ions at low concentrations to decrease the shear viscosity η of aqueous solutions (the so-called chaotropic ions), while other ions to increase η (kosmotropic ions), a phenomenon that correlates strongly with the Hofmeister series governing the influence of salts on the solubility and self-assembly of bio-macromolecules and colloidal particles. We address these poorly understood ion-specific, which have profound ramifications in diverse biology, medical science and technological applications, through the molecular dynamics (MD) simulations of model neutral polymers in aqueous salt solutions, Polyacrylamide (PAM) and Polyethylene Oxide (PEO). Careful attention is made to the capacity of our model to reproduce at least qualitive trends of added salt on the diffusion coefficient of water using a generally accepted water model TIP4P/2005 whose properties have been extensively developed previously. Reproducing these ionic specific trends in the mobility of water has been a recurrent problem in the MD simulations of aqueous solutions based a realistic model of water so our work on salt solutions represents an important advance on which we build or modeling of aqueous polymer solutions with added salt. Our simulations of our model polymers reveal an extended nanoscale dynamic hydration layer having a scale on the order of a nm in which the mobility Is perturbed from Its bulk value, but the dynamics within this shell is found to be perturbed by the specific salts and salt concentrations that we study.