Ion-ion correlation and charge reversal at titrating solid interfaces

Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titration and charge reversal (CR) experiments on silica surfaces by Dove et al. and van der Heyden it et al, we show that ion-ion correlations quantitatively explain why divalent counterions strongly promote surface charge which, in turn, eventually causes a charge reversal (CR). Titration and CR results from simulations and experiments are in excellent agreement without any fitting parameters. This is the first unambiguous evidence that ion-ion correlations are instrumental in the creation of highly charged surfaces and responsible for their CR. Finally, we show that charge correlations result in "anomalous" charge regulation in strongly coupled conditions in qualitative desagreement with its classical treatment.Comment: 4 pages, 4 figures, submitted to PR

Mechanisms and kinetics of C-S-H nucleation approaching the spinodal line: Insights into the role of organics additives

Wet chemistry C-S-H precipitation experiments were performed under controlled conditions of solution supersaturation in the presence and absence of gluconate and three hexitol molecules. Characterization of the precipitates with SAXS and cryo-TEM experiments confirmed the presence of a multi-step nucleation pathway. Induction times for the formation of the amorphous C-S-H spheroids were determined from light transmittance. Analysis of those data with the classical nucleation theory revealed a significant increase of the kinetic prefactor in the same order as the complexation constants of calcium and silicate with each of the organics. Finally, two distinct precipitation regimes of the C-S-H amorphous precursor were identified: i) a nucleation regime at low saturation indexes (SI) and ii) a spinodal nucleation regime at high SI where the free energy barrier to the phase transition is found to be of the order of the kinetic energy or less.Comment: Accepted in Cement and Concrete Research. 30 pages plus supplementary materials. arXiv admin note: substantial text overlap with arXiv:2111.0274

On the Charging Process of Minerals

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Thermodynamics and Molecular Mechanism of Al Incorporation in Calcium Silicate Hydrates

Quantitative description of thermodynamic and molecular mechanism of Al incorporation into calcium-silicate hydrates (C-S-H), the main binder in hydrated cement paste, is essential for development of novel cementitious materials with a lower CO₂ footprint. Thermodynamics integration based on ab initio molecular dynamic simulations was applied to estimate the Gibbs free energy of the Al exchange between different silica tetrahedral sites forming the dreierketten-chains at the C-S-H surface and aqueous Al­(OH)₄^– anions. The calculations confirm that the Al substitute for Si into bridging tetrahedral sites with an estimated equilibrium constant <i>K</i>^Al/Si ∼ 1. Al for Si substitution is further found to favor the cross-linking between adjacent chains of the same C-S-H layer. This result is in a good agreement with recent conclusions made from ²⁷Al MAS NMR spectroscopy results. Mesoscale Monte Carlo simulations were performed with the calculated <i>K</i>^Al/Si to interpret experimental observations of Al incorporation into C-S-H. The simulation results suggest that the chemical affinity of Al to C-S-H is controlled by electrostatic interactions and the Al­(OH)₄^–/Si­(OH)₃O^– aqueous molar ratio

Jellium and cell model for titratable colloids with continuous size distribution

International audienceA good understanding and determination of colloidal interactions is paramount to comprehend and model the thermodynamic and structural properties of colloidal suspensions. In concentrated aqueous suspensions of colloids with a titratable surface charge, this determination is, however, complicated by the density dependence of the effective pair potential due to both the many-body interactions and the charge regulation of the colloids. In addition, colloids generally present a size distribution which results in a virtually infinite combination of colloid pairs. In this paper we develop two methods and describe the corresponding algorithms to solve this problem for arbitrary size distributions. An implementation in Nim is also provided.The methods, inspired by the seminal work of Torres et al., are based on a generalization of the cell and renormalized jellium models to polydisperse suspensions of spherical colloids with a charge regulating boundary condition. The latter is described by the one-pK-Stern model. The predictions of the models are confronted to the equations of state of various commercially available silica dispersions. The renormalized Yukawa parameters (effective charges and screening lengths) are also calculated. The importance of size and charge polydispersity as well as the validity of these two models are discussed in light of the results

Electrostatics for a better understanding of the acid-base chemistry of montmorillonite

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Etude du transport et de la rétention de solutés neutres et ioniques par le modèle DSPM : membranes de nano- et d'ultra-filtration fine

La modélisation du transport de matière à travers une structure microporeuse chargée constitue une étape indispensable à la compréhension de la sélectivité d'une membrane de nanofiltration. Dans ce travail, un modèle reposant sur l'équation étendue de Nernst-Planck pour décrire le transport et l'équation de Donnan modifiée pour décrire l'exclusion électrique-stérique des espèces est développé. Outre l'analyse théorique des phénomènes de transport, le modèle permet de relier les propriétés électriques et structurales d'une membrane au taux de rejet de solutés. Le rayon de pore (rp) et la charge volumique (X) de membranes ont pu être déduits de la modélisation des taux de rejets de solutés neutres et ioniques. La confrontation des valeurs de X et de rp à celles déduites de mesures indépendantes ne permet pas de conclure définitivement quant à la validité du modèle. Les résultats mettent en évidence un phénomène de régulation de charge intervenant à l'intérieur des pores de la membrane.Modeling of the matter transport through a charged microporous medium is a necessary step to understand and control the selectivity of a nanofiltration membrane. In this work, a model based on the application of the extended Nernst-Planck equation and the assumption of a modified Donnan equilibrium at both membrane/solution interfaces is developed. Besides the analysis of transport mechanisms, the model allows to relate a membrane's microscopic structural and charge properties to the solute retentions. The effective pore radius (rp) and effective volume charge (X) of nanofiltration membranes could be then determined by fitting the experimental retention of neutral and charged solutes. The comparison of the predicted values of the membrane X and rp with those determined from independent measurements does not allow to conclude definitively on the validity of the model. Results show that a charge regulation phenomenon occurs inside the membrane pores.BESANCON-BU Sciences Staps (250562103) / SudocSudocFranceF