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Lund, Sweden

The pair interaction between two charged colloidal particles, in the presence of a polyelectrolyte as well as simple salt, is analyzed theoretically. Of particular interest is the way in which such a combination of salts can be used to induce a strong, long-range attraction, with at most a minor free energy barrier. We show that the nature of the simple salt is highly relevant, i.e., 2:1, 1:1, and 1:2 salts generate quite different particle interaction free energies at the same overall ionic strength. We adopt several different theoretical levels of description. Defining simulations at the primitive model level with explicit simple salt as our reference, we invoke stepwise coarse-graining with careful evaluations of each approximation. Representing monovalent simple ions by the ionic screening they generate is one such simplification. In order to proceed further, with additional computational savings, we also develop a correlation-corrected classical density functional theory. We analyze the performance of this theory with explicit spherical particles as well as in a flat surface geometry, utilizing Derjaguin's approximation. The calculations are particularly fast in the latter case, facilitating computational savings of many (typically 5-7) orders of magnitude, compared to corresponding simulations with explicit salt. Yet, the predictions are remarkably accurate, and considering the crudeness of the model itself, the density functional theory is a very attractive alternative to simulations. © 2012 American Chemical Society.

Forsman J.,Chemical Center | Woodward C.E.,University of New South Wales
Langmuir | Year: 2010

We investigate the Derjaguin approximation by explicitly determining the interactions between two spherical colloids using density functional theory solved in cylindrical coordinates. The colloids are composed of close-packed Lennard-Jones particles. The solvent particles are also modeled via Lennard-Jones interactions. Cross interactions are assumed to follow the commonly used Lorentz-Berthelot (LB) mixing rule. We demonstrate that this system may display a net repulsive interaction across a substantial separation range. This contradicts the Hamaker-Lifshitz theory, which predicts attractions between identical polarizable particles immersed in a polarizable medium. The source of this repulsion is traced to the LB mixing rule. Surprisingly, we also observe nonmonotonic convergences to the Derjaguin limit. This behavior is best understood by decomposing the total interaction between the colloids into separate contributions. With increasing colloid size, each of these contributions approach the Derjaguin limit in a monotonic manner, but their different rates of convergence mean that their sum may display nonmonotonic behavior. © 2010 American Chemical Society.

Labbez C.,University of Burgundy | Pochard I.,University of Burgundy | Jonsson B.,Chemical Center | Nonat A.,University of Burgundy
Cement and Concrete Research | Year: 2011

The surface charge density of C-S-H particles appears to be one of the key parameters for predicting the cohesion strength, understanding the ion retention, the pollutant leakage, and admixture adsorption in hydrated cement pastes. This paper presents a Monte Carlo simulation of the surface-ions interactions that permits the prediction of surface charge density (σ), electrokinetic potential (ζ) and ions adsorption of mineral surfaces in equilibrium with a given electrolyte solution. Simulated results are compared to experimental data obtained by titration, electrokinetic potential measurements and ions uptake in the case of C-S-H suspensions. An excellent agreement is found between simulated and experimental results. The wide spread idea that calcium is a potential determining ion in cement paste systems appears to be incorrect. Instead, the pH controls the charging behaviour of C-S-H nano-particles. This paper also shows to what extent the electrostatic interactions contribute to the measured Ca/Si ratio. © 2010 Elsevier Ltd.

Delhorme M.,Laboratory Interdisciplinaire Carnot de Bourgogne | Labbez C.,Laboratory Interdisciplinaire Carnot de Bourgogne | Jonsson B.,Chemical Center
Journal of Physical Chemistry Letters | Year: 2012

Anisotropic interactions in colloidal suspensions have recently emerged as a route for the design of new soft materials. Nonisotropic particles can form nematic, smectic, hexatic, and columnar liquid crystals. Although the formation of these phases is well rationalized when excluded volume is solely at play, the role of electrostatic interactions still remains unclear and even less so when particles present a charge heterogeneity, for example, clays. Here, we use Monte Carlo simulations of concentrated suspensions of charged disk-like particles to reveal the role of Coulomb interactions and charge anisotropy underlying liquid crystal formation and structures. We observe a vast zoo of exotic structures, going from hexatic to columnar phases, which are shown to be controlled by the charge anisotropy. The particle volume fraction at which these phases start to form is found to decrease with increasing Coulomb interactions and charge anisotropy, which suggests a route to tune the structure of aqueous liquid crystals. © 2012 American Chemical Society.

Roos B.O.,Chemical Center | Pyykko P.,University of Helsinki
Chemistry - A European Journal | Year: 2010

Quasi-relativistic Douglas-Kroll CASPT2 calculations are reported for the title molecules, mainly to provide primary data for a fit of double-bond covalent radii. Indeed, a well-developed σ2π2 double bond is identified in all cases. For Eu and Yb, however, it is an excited state. The main valence orbitais of all Ln ions are 6s and 5d. In the σ bonds, more 5d than 6s character is found at the Ln. The Ln=C bond lengths show a systematic lanthanide contraction of 13 pm from La to Lu. An agostic symmetry breaking is demonstrated for Ce but its effect on the Ln-C length is small. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

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