Christine Bonal

Interactions between lanthanide cations and nitrate anions in water Part 2 Microcalorimetric determination of the Gibbs energies, enthalpies and entropies of complexation of Y3+ and trivalent lanthanide cations

Microcalorimetry is applied to the study of the very weak complexation of Y3+ and trivalent lanthanide cations (La3+, Ce3+, Pr3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Lu3+) by nitrate anion in water at 298.15 K. For each cation, both the apparent association constant and enthalpy of reaction at an ionic strength of 2.0 mol kg-1 are extracted from the calorimetric data. The Gibbs energies, enthalpies and entropies of complexation of these lanthanide cations, to which are added the values previously found for Nd3+, Sm3+ and Eu3+, are extrapolated to infinite dilution using the Pitzer interaction model and the ionic strength effect previously observed with neodymium. An attempt is also made to extract the enthalpies of complexation from the available heats of dilution by comparing the nitrate and perchlorate heat data; the results agree quite well with the values found by microcalorimetry. The trends in the thermodynamic properties of association with the cation radius are examined. Some of the properties show breakpoints that delimit four subfamilies, a pattern that may possibly be attributed to a tetrad effect.

Calculation of the long-range interactions for interfacial properties

The molecular simulation of heterogeneous systems cannot be performed routinely. The results of such systems depend on the truncation procedures, size effects, long-range corrections (LRCs) to the thermodynamic properties and on the way of calculating the Coulombic interactions. We propose here to illustrate the impact of the truncation procedures on the mechanical equilibrium of the liquid–vapour interface of alkanes. The importance of the LRCs to the surface tension is established in alkanes, water, carbon dioxide and hydrogen sulphide liquid–vapour interfaces. The calculation of the electrostatic interactions in a slab geometry using a two-dimensional method and the standard three-dimensional Ewald summation technique is also reported.

Interactions between lanthanide cations and nitrate anions in water. Part 1.—Effect of the ionic strength on the Gibbs energy, enthalpy and entropy of complexation of the neodymium cation

Microcalorimetry has been applied for the first time to the study of the very weak complexation of the trivalent neodymium cation by the nitrate anion in water at 298.15 K. The apparent association constant and enthalpy of reaction at a given ionic strength are both extracted from the calorimetric data. The ionic strength, which is controlled by addition of sodium perchlorate, is varied from 2.0 to 0.5 mol kg–1. For comparison, the association constant is also determined by visible spectroscopy under similar conditions. The thermodynamic properties of complexation are extrapolated to infinite dilution using the Pitzer interaction model: this yields K= 4.0 ± 0.1 and ΔrH⊖=–1.2 ± 0.2 kJ mol–1. The microcalorimetric method and the data treatment are fully described.

Free Energy Calculations in Electroactive Self-Assembled Monolayers (SAMs): Impact of the Chain Length on the Redox Reaction

The free energy approach is used to study the effect of the relative chain length of the two constituents of electroactive self-assembled monolayers (SAMs) on gold. In this study, the ferrocene groups are exposed to the electrolyte solution. This situation is achieved by using shorter diluent alkanethiol chains. To this end, the mixed monolayers formed by the self-assembly of 11-ferrocenylundecanethiol and butanethiol FcC(11)S/C(4)S and of 6-ferrocenylhexanethiol and butanethiol FcC(6)S/C(4)S onto a gold surface are studied. Calculation of enthalpy and entropy differences are also performed using molecular simulations. Additionally, the electrochemical signatures of these systems are determined to allow a direct comparison with our calculations. The thermodynamic properties are discussed in terms of enthalpy and entropy changes. Two effects account for the thermodynamic behavior. The first one involves the ion pairing between the ferrocenium group and the perchlorate anion. The second one concerns the desolvation of the first hydration shell of the anions. Finally, this work is also completed with a microscopic description associated with an energy characterization of these SAMs as a function of the surface coverage under conditions close to experiments.

Environment effect on the redox properties of Self-Assembled Monolayers: a theoretical investigation of the nature of the supporting electrolyte

Gibbs free energy calculations have been performed to calculate the change in the simulated redox properties of FcC6S–/C12S–Au and FcC6S–/C4S–Au in three different electrolytes (NaClO4, Na2SO4 and NaPF6). The effect of surface ion association between the ferricinium cation and the electrolyte anions is investigated on the redox properties of SAMs. The microscopic description of the binary SAMs before and after oxidation indicates that the formation of surface ion pairs is strongly correlated with the hydration energy of the corresponding anion. The correlation between the desolvation of the anions and the ion-pairs formation is established. We have shown from the energy contributions that a general understanding of the thermodynamic behavior of these systems should be analyzed in the context of the enthalpy–entropy compensation effect.

Toward a prediction of the redox properties of electroactive SAMs: a free energy calculation by molecular simulation.

We report free energy calculations of FcC(6)S-/C(4)S-Au and FcC(6)S-/C(12)S-Au binary self-assembled monolayers (SAMs) formed by one ferrocenylhexanethiolate chain and alkylthiolate chains. We demonstrate that the free energy perturbation methods are able to reproduce the positive shift of the redox potential when the coadsorbed butanethiolate C(4)S chains are replaced by dodecanethiolate C(12)S chains. The different contributions to the Ewald summation involved in the perturbation process are thoroughly described. We complete the study by a microscopic description of the binary SAMs before and after oxidation. The molecular dynamics (MD) simulations evidence that the formation of the ion-pair between the ferricinium and a single perchlorate anion of the supporting electrolyte is more favored in FcC(6)S-/C(12)S-Au SAM.

Molecular simulations of grafted metal-chelating monolayers: methodology, structure and energy

We report molecular dynamics simulations of monolayers of copper complexes covalently attached to a graphite surface in water. Simulations are performed to calculate the structural (density profiles, distribution functions), dynamical (diffusion coefficients) and energetical properties of typical systems presenting a finite length in the third dimension. The results are discussed as a function of the method used for computing the long-range Coulombic interactions. Three different methodologies are used: a two-dimensional method (HKE) more extensively in simulation time, the standard Ewald summation technique (EW3D) and the corrected Ewald method (EW3DC). From a computational viewpoint, the comparison between methods is essential to establish the set-up conditions and it represents a pre-requisite work before further investigating these grafted systems. We complete this study by providing a molecular description (structure, energy, diffusion, hydration) of the grafted monolayers of copper complexes and of the water molecules.

Molecular dynamics description of grafted monolayers: effect of the surface coverage.

Molecular dynamics simulations of monolayers of metal-chelating ligands grafted onto a graphite surface in water are carried out to calculate structural (density profiles, radius of gyration, and asphericity coefficients), dynamical (diffusion coefficients), and energetical properties as a function of the surface coverage. The purpose is to provide a better understanding of the dependence of various properties of these monolayers on the surface coverage. A critical value of the surface coverage from which all structural properties derive a limiting value has been established. It also appears that the chains rather adopt an elongated conformation along the direction normal to the surface from this critical surface coverage. The hydrogen-bonding structure and dynamics of water molecules are reported. An ordered structure of water in the region close to the terminal groups of the grafted molecules is shown at a relatively high surface coverage. This ordering is similar to that observed in the case of water in interaction with a solid surface.

Atomistic and energetic descriptions of self-assembled monolayers of differently endgroup-functionalized alkanethiols adsorbed on the gold substrate by using molecular simulations

In this paper, we employ molecular dynamics (MD) simulations to obtain microscopic structures and the energy picture of FcC11S-/X-C11S-Au as a function of the terminal groups (e.g., X = CH3, COOH, COO−, CH2NH2, CH2OH). A perturbation formalism has been performed to calculate the macroscopic redox properties of FcC11S-/X-C11S-Au as a function of the terminal groups. Since the shift as a function of the endgroups is fairly modest, it appears that the magnitude of the change in the simulated redox properties seems to vary with the polarity of the terminal groups of the alkanethiol coadsorbate. To deeply investigate the molecular scale architecture and its effect on the redox properties, the tilt angle, the film thickness and also the distribution of dihedral angles in the trans and gauche configurations have been determined as a function of the terminal groups. From this microscopic description, we have underlined the tendency of the carboxylate-functionalized alkanethiols to deviate from well-arranged structures. The results reported herein underscore the role of H-bonding in the structural properties. A link between the number of intra-layer hydrogen bonds and the disorder of monolayers is clearly established. This work is also completed with an energetic characterization of the SAMs.

Methodological approaches for the free energy calculations in electroactive SAMs

We compare the Free Energy Perturbation (FEP) and Thermodynamic Integration (TI) approaches in slab-geometries where the electrostatic interactions are handled with the standard three-dimensionally Ewald summation technique. The comparison between FEP and TI is made through energy distributions in the analysis of the phase space sampling between the forward and backward directions, the reversibility of the perturbation, the number of windows and the consistency of the free energy decomposition into individual components. We report here free energy calculations in order to predict the shift in the redox potential in self-assembled monolayers (SAMs) as the coadsorbed chain length is changed. The reproduction of the free energies with respect to the electrolyte is tested on neutral and charged simulation cells.