Impact of Chemically Specific Interactions between Anions and Weak Polyacids on Chain Ionization, Conformations, and Solution Energetics

Abstract

The presence of salts in a solution containing weak polyelectrolytes is known to modify both their titration behavior and conformations due to electrostatic screening. Instead, little is currently known about the changes induced by chemically specific interactions (e.g., charged hydrogen bonds, c-H-bonds). To investigate this aspect, we simulated the titration of weak polyacids with a primitive model and Monte Carlo methods in the presence of monovalent salts whose anions are capable of forming c-H-bonds with associated acid groups. The interaction between anions and weak polyacids (e.g., poly(acrylic acid)) substantially hampers ionization at low pH despite the somewhat limited number of coordinated anions, whereas it has a limited impact once pH > pK a + 2 due to a progressive anion decoordination. Importantly, the suppression of ionization appears extremely local in nature, with different chain segments differing in pK a by up to 1.3 units. As for chain conformations, c-H-bonds reduce the average sizes of polyacids independently of their structure as a consequence of multidentate binding or multiarm coordination in starlike species. Analyzing the length of chain segments with all monomers coordinated or uncoordinated has also evidenced that anion binding is extremely local in nature. The energetic analysis of c-H-bond formation suggests that polyacid chemical potential may be strongly lowered (up to −0.7 kcal/mol per monomer), the impact of such results on a few phenomena relevant for the physical chemistry of polyacid-containing solutions being analyzed in some detail.

Transition in solution dynamics of polyacrylic acid: An interplay between nonergodicity and triple mode relaxation

“…The scale of elongation is greatly affected by the degree of ionization and addition of external salt ions. Electrolytes or salt ions lead to screening of the electrostatic interactions between ionized groups and gradually reduce the polyelectrolyte effect 72–74 . At high salt concentrations, the salt ions completely screen out the coulombic (repulsive)—interactions among the charged backbone segments.…”

“…Electrolytes or salt ions lead to screening of the electrostatic interactions between ionized groups and gradually reduce the polyelectrolyte effect. [72][73][74] At high salt concentrations, the salt ions completely screen out the coulombic (repulsive)-interactions among the charged backbone segments. Thus there is likely transformation of polymer conformations impacting the solution dynamics.…”

Tuning Knotted Copolyelectrolyte Conformations via Solution Properties

“…Our results pave the way toward attaining a broader and finer external tunability of polymer properties by considering additional solvent- or solution-dependent features as well as the overall chain length. The latter may include the degree of solvophilicity of monomers belonging to different blocks or the presence of chemical specific interactions between monomers − or with mobile ions. , For instance, a lower solvophilicity of short alternating blocks, or strong hydrogen bonding, may pin one or more essential crossings, thus impacting the contour mobility of the knot . An alternative way to achieve such effect would be to confer weak acid and basic properties to different chain segments, obtaining polyampholytes with switchable (e.g., by means of pH stimuli) interblock interactions and possibly different effective block rigidities .…”