Ion condensation behavior and dynamics of water molecules surrounding the sodium poly(methacrylic acid) chain in water: A molecular dynamics study

Adsorption Isotherm and Mechanism of Ca2+ Binding to Polyelectrolyte

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.

Molecular dynamics study on the interaction of phosphorus building gypsum /surfactant composites

While surfactants are widely used in phosphogypsum, their interactions with the phosphogypsum-water interface remain unclear. This study investigates the impact of three types of surfactants, namely polycarboxylate-based surfactant (PCE-TPEG), naphthalene-based surfactant (NS), and melamine-based surfactant (MS), on the performance of phosphorus building gypsum (PBG). Additionally, a nanoscale model of the PBG-surfactant-water interface is constructed using molecular dynamics to elucidate the mechanisms underlying the interaction between different surfactants and PBG at multiple scales. The results demonstrate that all surfactants enhance the mechanical properties of PBG. PCE-TPEG exhibits the most pronounced improvement. In the model, PCE-TPEG molecules likely undergo comb-like adsorption, while NS and MS molecules tend to adsorb on both ends of the crystal plane. Changes in the potential difference between CaSO4·2H2O and H2O, as well as between CaSO4·2H2O and the surfactant, play a crucial role in adsorption. PCE-TPEG, NS, and MS molecules tend to spread horizontally in a vacuum state. With the addition of water molecules, they transition to spatial adsorption. Ca2+ easily interacts with -COO- and -SO3- groups, leading to reduced migration and flexibility of the main chain. The adsorption process of surfactants at the gypsum-water interface occurs spontaneously and Electrostatic forces are the main driving factor. This study contributes to a more comprehensive understanding on the behaviour of the phosphorus building gypsum/surfactant composites.

The Role of Sulphonic and Phosphoric Pendant Groups on the Diffusion of Monovalent Ions in Polyelectrolyte Membranes: A Molecular Dynamics Study

Lithium-ion consumption has risen significantly in recent years due to its use in portable devices. Alternative sources of lithium, which include the recovery from brine using the sustainable and eco-friendly electrodialysis technology, has been explored. This technology, however, requires effective cation-exchange membranes that allow the selective permeation of lithium ions. In this study, we have investigated, via molecular dynamics simulations, the role of the two common charged groups, the sulfonic and the phosphoric groups, in promoting the adsorption of monovalent ions from brine comprising Li+, Na+, Mg2+, and Ca2+ ions. The analysis of the mean square displacement of the ions revealed that Li+ and Na+ ions exhibit superior diffusion behaviors within the polyelectrolyte system. The O-atoms of the charged groups bind strongly with the divalent ions (Mg2+ and Ca2+), which raises their diffusion energy barrier and consequently lowers their rate of permeation. In contrast, the monovalent ions exhibit weaker interactions, with Na+ being slightly above Li+, enabling the permeation of Li+ ions. The present study demonstrates the role of both charged groups in cation-exchange membranes in promoting the diffusion of Li+ and Na+ ions, and could serve as a guide for the design of effective membranes for the recovery of these ions from brine.

Adsorption Conformation of Comb-Shaped Polycarboxylate Ethers on Ettringite (100) Surface: An Atomic Scales Simulation

The adsorption of five polycarboxylate ethers (PCEs) oligomers with different side-chain number and side-chain length on the ettringite (100) surface in explicit solution is studied by all-atom molecular dynamics (MD) simulations. The adsorption conformations, adsorption energies, the radial distribution functions (RDF) between PCEs and ettringite surface and density distribution profile of water perpendicular to the substrate are analyzed. After dynamic equilibrium, negatively charged carboxylate groups are absorbed on the surface of the disordered ettringite crystal and the side-chains are extended to solution. The influence of the number of side-chain on the adsorption strength of PCEs on the ettringite (100) surface is more significant than that of the length of side-chain. The less number of grafted side-chains, the weaker electrostatic shielding, the electrostatic interaction between PCEs and ettringite (100) surface is stronger, which is favorable for the adsorption. The conformation of adsorbed PCEs is closely related to the length of side chain. The greater the length of side-chains is, the larger will be the coverage area of PCE on surface and the degree of water reduction. The mechanism of adsorption of PCEs on ettringite surface is concluded and the major contribution to the adsorption includes ion pairs, hydrogen bonds and an entropic compensation.

Molecular Dynamics Study on Mechanical Properties of Interface between Urea-Formaldehyde Resin and Calcium-Silicate-Hydrates

Microcapsule based self-healing concrete can automatically repair damage and improve the durability of concrete structures, the performance of which depends on the binding behavior between the microcapsule wall and cement matrix. However, conventional experimental methods could not provide detailed information on a microscopic level. In this paper, through molecular dynamics simulation, three composite models of Tobermorite (Tobermorite 9 Å, Tobermorite 11 Å, Tobermorite 14 Å), a mineral similar to Calcium-Silicate-Hydrate (C-S-H) gel, with the linear urea-formaldehyde (UF), the shell of the microcapsule, were established to investigate the mechanical properties and interface binding behaviour of the Tobermorite/UF composite. The results showed that the Young's modulus, shear modulus and bulk modulus of Tobermorite/UF were lower than that of 'pure' Tobermorite, whereas the tensile strength and failure strain of Tobermorite/UF were higher than that of 'pure' Tobermorite. Moreover, through radial distribution function (RDF) analysis, the connection between Tobermorite and UF found a strong interaction between Ca, N, and O, whereas Si from Tobermorite and N from UF did not contribute to the interface binding strength. Finally, high binding energy between the Tobermorite and UF was observed. The research results should provide insights into the interface behavior between the microcapsule wall and the cement matrix.

Influence of Calcium Binding on Conformations and Motions of Anionic Polyamino Acids. Effect of Side Chain Length

Investigation of the effect of CaCl2 salt on conformations of two anionic poly(amino acids) with different side chain lengths, poly-(α-l glutamic acid) (PGA) and poly-(α-l aspartic acid) (PASA), was performed by atomistic molecular dynamics (MD) simulations. The simulations were performed using both unbiased MD and the Hamiltonian replica exchange (HRE) method. The results show that at low CaCl2 concentration adsorption of Ca2+ ions lead to a significant chain size reduction for both PGA and PASA. With the increase in concentration, the chains sizes partially recover due to electrostatic repulsion between the adsorbed Ca2+ ions. Here, the side chain length becomes important. Due to the longer side chain and its ability to distance the charged groups with adsorbed ions from both each other and the backbone, PGA remains longer in the collapsed state as the CaCl2 concentration is increased. The analysis of the distribution of the mineral ions suggests that both poly(amino acids) should induce the formation of mineral with the same structure of the crystal cell.

Effect of sodium gluconate on molecular conformation of polycarboxylate superplasticizer studied by the molecular dynamics simulation

Sodium gluconate (SG) has been accepted as one of the main additional components in polycarboxylate superplasticizer (PCE) system, due to its excellent retarding effect. While the negative effect on dispersion of PCE was reported in the literature, the reason was not completely revealed. In this study, molecular dynamics simulation was used to investigate the mutual influence between SG and PCE in calcium hydroxide (CH) solution. Radial distribution function (RDF) was used to analyze the effects of SG on the complexation of PCE with Ca²⁺. Radius of gyration (Rg) was adopted to characterize the conformations of the backbone and side chains of PCE in CH solution. Finally, several adsorption and dispersion models were proposed. The results showed that the presence of SG would perturb adsorption of PCE, which was one of the main reasons that affected the dispersion ability of PCE. SG could preferentially combine with Ca²⁺ so that less amount of Ca²⁺ is available for combination of PCE, and this could extend the main chain of PCE and show advantage for PCE adsorption. Besides, adding SG could squeeze the side chains of PCE, which would put a negative effect on the dispersion. These findings gave deeper insight into understanding the dispersion mechanism of PCE-SG system.

Binding of calcium cations with three different types of oxygen-based functional groups of superplasticizers studied by atomistic simulations

This work investigated interactions between calcium cations (Ca²⁺) and three common types of oxygen-based functional groups of concrete superplasticizers using density functional theory (DFT) calculations and all-atom molecular dynamics (MD) simulations. The three common types of oxygen-based functional groups were modeled as three hypothetical, low-molecular-weight organic molecules, each containing a methyl-terminated oxyethylene dimer and an adsorbing head of two oxygen-based functional groups, and are referred to as carboxylate, sulfonate, and phosphate groups, respectively, following the usual terminology in the field of concrete admixtures. Our DFT results show that the binding strength of the three groups with calcium cations follows (from high to low) phosphate>carboxylate>sulfonate, and both the electrophilic attack and the chemical reactivity of the three groups contribute significantly to the binding strength. The MD simulation results indicate that the adsorption of the three small molecules on the calcite (1 0 4) surface in aqueous solution shares a similar pattern in the sense that just two oxygen atoms of two adjacent anchor groups adsorb on the calcium atoms on the top layer of the crystal. The adsorption strength among the three types of functional groups follows the same order as the binding strength obtained from DFT calculations; both results corroborate a similar rule-of-thumb established by experiments. Furthermore, interactions of the three types of groups with water molecules suggest that strong hydrogen-bonding interactions exist in those systems. Graphical abstract Binding of calcium cations with three different types of oxygen-based functional groups of superplasticizersᅟ.

Adsorption of organic molecules on mineral surfaces studied by first-principle calculations: A review

First-principle calculations, especially by the density functional theory (DFT) methods, are becoming a power technique to study molecular structure and properties of organic/inorganic interfaces. This review introduces some recent examples on the study of adsorption models of organic molecules or oligomers on mineral surfaces and interfacial properties obtained from first-principles calculations. The aim of this contribution is to inspire scientists to benefit from first-principle calculations and to apply the similar strategies when studying and tailoring interfacial properties at the atomistic scale, especially for those interested in the design and development of new molecules and new products.

Molecular insight on the binding of NNRTI to K103N mutated HIV-1 RT: molecular dynamics simulations and dynamic pharmacophore analysis

Regardless of advances in anti-HIV therapy, HIV infection remains an immense challenge due to the rapid onset of mutation instigating drug resistance. Rilpivirine is a second generation di-aryl pyrimidine (DAPY) derivative, known to effectively inhibit wild-type (WT) as well as various mutant HIV-1 reverse transcriptase (RT). In this study, a cumulative 240 ns of molecular dynamic (MD) simulations of WT HIV-1 RT and its corresponding K103N mutated form, complexed with rilpivirine, were performed in solution. Conformational analysis of the NNRTI inside the binding pocket (NNIBP) revealed the ability of rilpivirine to adopt different conformations, which is possibly the reason for its reasonable activity against mutant HIV-1 RT. Binding free energy (MM-PB/GB SA) calculations of rilpivirine with mutant HIV-1 RT are in agreement with experimental data. The dynamics of interaction patterns were investigated based on the MD simulations using dynophores, a novel approach for MD-based ligand-target interaction mapping. The results from this interaction profile analysis suggest an alternate interaction between the linker N atom of rilpivirine and Lys 101, potentially providing the stability for ligand binding. PCA analysis and per residue fluctuation has highlighted the significant role of flexible thumb and finger sub-domains of RT in its biological activity. This study investigated the underlying reason for rilpivirine's improved inhibitory profile against mutant RT, which could be helpful to understand the molecular basis of HIV-1 RT drug resistance and design novel NNRTIs with improved drug resistance tolerance.

Effect of methyl groups on conformational properties of small ionized comb-like polyelectrolytes at the atomic level

Comb-like polycarboxylate ether (PCE) molecules with different content of methyl groups substituted on backbone and different location of methyl groups substituted on the side chains, respectively, were designed and were studied in explicit salt solutions by all-atom molecular dynamics simulations. Methyl groups substituted on the backbone of PCE have a great effect on the conformation of PCE. Stiffness of charged backbone was not only affected by the rotational freedom but also the electrostatic repulsion between the charged COO^- groups. The interaction of counterions (Na⁺) with COO^- groups for PCE3 (with part of AA substituted by MAA on the backbone) was stronger and the screen effect was great, which decided the smaller size of PCE3. The interaction between water and COO^- groups was strong regardless of the content of AA substituted by MAA on the backbone. The effect of methyl groups substituted on the different location of side chains on the conformation of PCE was less than that of methyl groups substituted on the backbone. The equilibrium sizes of the four PCE molecules with methyl groups substituted on the side chains were similar. Graphical Abstract Effect of methyl groups on conformational properties of small ionized comb-like polyelectrolytes at the atomic level.

Investigation of pH-induced conformational change and hydration of poly(methacrylic acid) by analytical ultracentrifugation

Analytical ultracentrifugation was performed on poly(methacrylic acid) (PMAA) with a series of weight average molar masses (Mw) in aqueous solutions as a function of pH. The scales of the sedimentation coefficient (s) and the diffusion coefficient (D) to Mw at infinite dilutions were obtained at different pH values, indicating that PMAA chains adopt a collapsed structure at low pH values, and stretch at pH higher than 5.2. Our results show that the sedimentation coefficient exhibits a minimum at pH ∼ 6.0, presumably due to the effect of the conformational change and the hydration state of PMAA chains. When pH increases from 6.0 to 8.5, PMAA chains with high molar mass shrink a little bit, presumably because the sodium ions act as a bridging agent between nonadjacent carboxylate groups. Furthermore, the weight average molar mass of PMAA at pH 8.5 increases by one fold than that at pH 4.0, indicating the condensation of sodium ions and the increase in the number of hydration water molecules around carboxylate groups at high pH values.