Structure and Phase Behavior of Polyampholytes and Polyzwitterions

Abstract

Polyelectrolytes contain many units of charged moieties, often as either anionic or cationic. However, when a polymer comprises both anionic and cationic monomers, the result is a polyampholyte. Polyzwitterions are a type of polyampholyte that contain pairs of opposite charges within each monomer unit, ultimately leading to charge neutrality across the entire polymer structure. These special classes of ion‐containing polymers have many applications enabled by advances in synthesis. This article discusses the unique solution‐responsive behaviors of these ion‐containing polymers and how their chemistry is used to alter structure–property relationships.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

“…Some combinations of anions and cations increase the self-diffusion coefficient ( D ) of water, and others reduce it, while, correspondingly, the opposite trends arise for the shear viscosity (η) of the entire aqueous salt solution. − Ions are operationally termed chaotropic and kosmotropic , based on whether η is decreased or increased, an effect historically interpreted as being due to structure breaking and structure-making of the hydrogen-bonding network. ,, It is also implicit in these arguments, rationalizing specific ion effects, that an increase in solvent structural ordering and associated densification is responsible for this type of η enhancement so that density-based free volume models have been applied with some frequency to aqueous salt solutions . Such arguments have been repeatedly made despite the fact that increasing the density of water can, in some instances, increase water mobility. , Correspondingly, chaotropic ions are supposed to give rise to less ordering of the hydrogen-bonding network (due to a reduction of density in a free volume structural framework) to rationalize the lowering of η and the increase of D . ,, Regardless of the specific molecular mechanism, the differing behavior of chaotropic versus kosmotropic salts on the dynamical and thermodynamic properties of water have provided the basis of many applications with stimuli-responsive systems, controlled assembly, and a wide range of interfacial phenomena. − There is extensive and rapidly growing literature discussing the specific ion effects on the dynamics and thermodynamics of water that we do not attempt to summarize here. Many of the numerous applications and theories proposed to rationalize this phenomenon have been thoroughly reviewed by Gregory et al ., and we refer the interested reader to their work for this background information.…”

Molecular Dynamics Simulation of the Influence of Temperature and Salt on the Dynamic Hydration Layer in a Model Polyzwitterionic Polymer PAEDAPS

“…It has often been claimed that proteins are related to polyzwitterions, , and in accepting these arguments, it is natural to compare the solution properties of proteins to zwitterionic polymers for common observational trends in both classes of polymers. It is generally appreciated, for example, that the extended hydration layer of proteins can actively facilitate protein binding and folding, phenomena of profound biophysical importance. − Is there analogous behavior found in polyzwitterion polymers?…”