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Adsorption Isotherm and Mechanism of Ca²⁺ Binding to Polyelectrolyte.

ABSTRACT: Polyelectrolytes, such as poly(acrylic acid) (PAA), can effectively mitigate CaCO₃ scale formation. Despite their success as antiscalants, the underlying mechanism of binding of Ca²⁺ to polyelectrolyte chains remains unresolved. Through all-atom molecular dynamics simulations, we constructed an adsorption isotherm of Ca²⁺ binding to sodium polyacrylate (NaPAA) and investigated the associated binding mechanism. We find that the number of calcium ions adsorbed [Ca²⁺]_ads to the polymer saturates at moderately high concentrations of free calcium ions [Ca²⁺]_aq in the solution. This saturation value is intricately connected with the binding modes accessible to Ca²⁺ ions when they bind to the polyelectrolyte chain. We identify two dominant binding modes: the first involves binding to at most two carboxylate oxygens on a polyacrylate chain, and the second, termed the high binding mode, involves binding to four or more carboxylate oxygens. As the concentration of free calcium ions [Ca²⁺]_aq increases from low to moderate levels, the polyelectrolyte chain undergoes a conformational transition from an extended coil to a hairpin-like structure, enhancing the accessibility to the high binding mode. At moderate concentrations of [Ca²⁺]_aq, the high binding mode accounts for at least one-third of all binding events. The chain's conformational change and its consequent access to the high binding mode are found to increase the overall Ca²⁺ ion binding capacity of the polyelectrolyte chain.

SUBMITTER: Mantha S

PROVIDER: S-EPMC10976897 | biostudies-literature | 2024 Mar

REPOSITORIES: biostudies-literature

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Adsorption Isotherm and Mechanism of Ca2+ Binding to Polyelectrolyte.

Mantha Sriteja S Glisman Alec A Yu Decai D Wasserman Eric P EP Backer Scott S Wang Zhen-Gang ZG

Langmuir : the ACS journal of surfaces and colloids 20240318 12

Polyelectrolytes, such as poly(acrylic acid) (PAA), can effectively mitigate CaCO3 scale formation. Despite their success as antiscalants, the underlying mechanism of binding of Ca2+ to polyelectrolyte chains remains unresolved. Through all-atom molecular dynamics simulations, we constructed an adsorption isotherm of Ca2+ binding to sodium polyacrylate (NaPAA) and investigated the associated binding mechanism. We find that the number of calcium ions adsorbed [Ca< ...[more]

PMID: 38497336

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Project description:For more than a century, monolayer adsorptions in which adsorbate molecules and adsorbing sites behave ideally have been successfully described by Langmuir's adsorption isotherm. For example, the amount of adsorbed material, as a function of concentration of the material which is not adsorbed, obeys Langmuir's equation. In this paper, we argue that this relation is valid only for macroscopic systems. However, when particle numbers of adsorbate molecules and/or adsorbing sites are small, Langmuir's model fails to describe the chemical equilibrium of the system. This is because the kinetics of forming, or the probability of observing, occupied sites arises from two-body interactions, and as such, ought to include cross-correlations between particle numbers of the adsorbate and adsorbing sites. The effect of these correlations, as reflected by deviations in predicting composition when correlations are ignored, increases with decreasing particle numbers and becomes substantial when only few adsorbate molecules, or adsorbing sites, are present in the system. In addition, any change that augments the fraction of occupied sites at equilibrium (e.g., smaller volume, lower temperature, or stronger adsorption energy) further increases the discrepancy between observed properties of small systems and those predicted by Langmuir's theory. In contrast, for large systems, these cross-correlations become negligible, and therefore when expressing properties involving two-body processes, it is possible to consider independently the concentration of each component. By applying statistical mechanics concepts, we derive a general expression of the equilibrium constant for adsorption. It is also demonstrated that in ensembles in which total numbers of particles are fixed, the magnitudes of fluctuations in particle numbers alone can predict the average chemical composition of the system. Moreover, an alternative adsorption equation, predicting the average fraction of occupied sites from the value of the equilibrium constant, is proposed. All derived relations were tested against results obtained by Monte Carlo simulations.

Dataset Information

Adsorption Isotherm and Mechanism of Ca²⁺ Binding to Polyelectrolyte.

Publications

Adsorption Isotherm and Mechanism of Ca<sup>2+</sup> Binding to Polyelectrolyte.

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