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Papers by Christophe Labbez

Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titrat... more 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

Lecture Notes in Computer Science, 2007

The charge state of molecules and solid/liquid interfaces is of paramount importance in the under... more The charge state of molecules and solid/liquid interfaces is of paramount importance in the understanding of the reactivity and the physico-chemical properties of many systems. In this work, we propose a new Monte Carlo method in the grand canonical ensemble using the primitive model, which allows us to simulate the titration behavior of molecules or solids at constant pH. The method is applied to the charging process of colloidal silica particles dispersed in a mono-valent salt solution of various concentrations and pH. An excellent agreement is found between experimental and simulated results.

We present a theoretical investigation of a model clay dispersion in 1-1 salt solutions varying t... more We present a theoretical investigation of a model clay dispersion in 1-1 salt solutions varying the particle volume fraction, ionic strength as well as the charge distribution on the clay platelets. The platelets are modeled as discs with charged sites distributed on a hexagonal lattice. The edge sites can be positively charged while the remaining sites are negative giving rise to a charge anisotropy. Simulations are carried out using a Monte Carlo method in the canonical ensemble. The interactions between the platelet sites are described with a screened Coulomb potential plus a short range repulsive potential. Simulations show a complex phase behavior. When the charge anisotropy is strong, a repulsive liquid phase is found at low volume fraction and ionic strength. When increasing the latter an attractive liquid phase forms. At these volume fractions the platelets aggregate in an "Overlapping Coins" configuration. With increasing volume fraction the dispersion becomes unstable and the pressure goes through a van der Waals loop. A liquid crystalline phase, Smectic B, forms in the thermodynamically unstable region. On the other side of the van der Waals loop a stable gel phase is found. A phase separation between a liquid and a gel is thus predicted. The threshold value of the volume fraction at which the phase separation occurs is found to increase with the salt concentration. The gel structure is a mixture of "Overlapping Coins" and "House of Cards" configurations. When the charge anisotropy is intermediate, no phase separation occurs. Instead, a gel forms from a sol of clusters of individual particles randomly oriented that progressively growth with the volume fraction. These results are discussed in light of experimental observations on clay suspensions.

We present a theoretical investigation of the titratable charge of clays with various structural ... more We present a theoretical investigation of the titratable charge of clays with various structural charge (σ b ): pyrophyllite (σ b =0 e 3 nm -2 ), montmorillonite (σ b =-0.7 e 3 nm -2 ) and illite (σ b =-1.2 e 3 nm -2 ). The calculations were carried out using a Monte Carlo method in the Grand Canonical ensemble and in the framework of the primitive model. The clay particle was modeled as a perfect hexagonal platelet, with an "ideal" crystal structure. The only fitting parameters used are the intrinsic equilibrium constants (pK 0 ) for the protonation/deprotonation reactions of the broken-bond sites on the lateral faces of the clay particles, silanol, dSiOþ H þ f dSiOH, and aluminol, dAlO -1/2 þ H þ f =AlOH þ1/2 . Simulations are found to give a satisfactory description of the acid-base titration of montmorillonite without any additional fitting parameter. In particular, combining the electrostatics from the crystal substitutions with ionization constants, the simulations satisfactorily catch the shift in the titration curve of montmorillonite according to the ionic strength. Change in the ionic strength modulates the screening of the electrostatic interactions which results in this shift. Accordingly, the PZNPC is found to shift toward alkaline pH upon increasing the permanent basal charge. Unlike previous mean field model results, a significant decrease in PZNPC values is predicted in response to stack formation. Finally, the mean field approach is shown to be inappropriate to study the acid-base properties of clays.

ABSTRACT Cement paste is a complex heterogeneous, polyphasic and reactive material with a relativ... more ABSTRACT Cement paste is a complex heterogeneous, polyphasic and reactive material with a relatively low stability range of the different phases. The pH and ionic concentrations of the pore solution are high or very high, circumstances that have made it difficult to set up accurate experiments at the nanometric scale. The nanometric dimensions of the main hydrate of cement pastes, i.e. calcium silicate hydrate (C-S-H), and the rather limited knowledge of the physico-chemical properties of the C-S-H/solution interface have also added to the experimental difficulties. As a consequence, it is only recently that the molecular interactions responsible for the cohesion of hydrated cement were elucidated. In this contribution, we show, by comparing simulation results to experimental data of surface charge titration, electrophoresis and Atomic Force Microscopy conducted on C-S-H, how a simple self consistent electrostatic model, solved using a Monte Carlo method in the grand canonical ensemble, can predict both the charging process and the zeta potential reversal of C-S-H as well as the attraction between C-S-H nanoparticles.

ABSTRACT Among the hydrates constituting a cement paste, C-S-H is the main component, it represen... more ABSTRACT Among the hydrates constituting a cement paste, C-S-H is the main component, it represents at least 60 % of the fully hydrated paste. C-S-H has a lamellar structure similar to that of a rare mineral, tobermorite. The germination and growth of the C-S-H particles result in increasing mechanical resistance (strength, cohesion). Mechanical experiments show that the intrinsic properties of the paste remain the same throughout the hydration process. Indeed, despite its very high compression strength properties the hydrated cement paste retains the same very low critical strain (~10 -4) characterizing a small elastic limit of the material. More recently, AFM experiments has revealed that CSH particles, immersed in a solution that contains Ca2+ and OH- ions, attract each other [1] at short range (2 nm) provided that the pH exceeds 11. These two experimental facts suggest that the cohesion is due to short range forces between C-S-H and anhydrous C 3S grains and between C-S-H themselves. Under normal cement conditions, high pH and salt concentrations, C-S-H particles carry a strong negative surface charge density due to the titration of their surface silanol groups. The high surface charge density together with divalent calcium ions in the pore solution makes the cement paste a strongly coupled system. In such systems, it was demonstrated that the DLVO theory based on the mean field Poisson-Boltzmann equation (PB), becomes qualitatively wrong. Indeed, the PB equation predicts a double layer repulsion where it is in fact attractive because it neglects the ion-ion correlations [2]. Delville [3] was the first to suggest that the strong attraction induced by ion-ion correlations could explain the cohesion of early cement paste. In this contribution, we will show, by comparing simulation results to experimental data of titration, electrophoresis and Atomic Force Microscopy, how a simple self consistent electrostatic model solved using a Monte Carlo method in the grand canonical ensemble, can predict the charging process, ion adsorption and the zeta potential reversal [3] of C-S-H as well as the interaction forces [4] between C-S-H particles. [1] C. Plassard, E. Lesniewska, I. Pochard, and A. Nonat, Nanoscale Experimental Investigation of Particle Interactions at the Origin of the Cohesion of Cement, Langmuir 21 (2005) 7263-7270 [2] Guldbrand, L.; Jönsson, B. H. Wennerström, P. Linse, Electrical Double Layer Forces. A Monte Carlo study, J. Chem. Phys. 80 (1984) 2221-2229 [3] A. Delville, R. J. M. Pellenq, A. Caillol, Monte Carlo (N,V,T) Study of the Stability of Charged Interfaces: A Simulation on a Hypersphere J. Chem. Phys. 106 (1997) 7275-7286 [4] C. Labbez, B. Jönsson, I. Pochard, A. Nonat, B. Cabane, Surface Charge Density and Electrokinetic Potential of Highly Charged Minerals: Experiments and Monte Carlo Simulations on Calcium Silicate Hydrate J. Phys. Chem. B 110 (2006) 9219-9230

Lecture Notes in Computer Science, 2007