Jean-François Dufrêche

Understanding the phase behaviour of pheromones and other messaging molecules remains a significant and largely unexplored challenge, even though it plays a central role in chemical communication. Here, we present all-atom molecular dynamics simulations to investigate the behaviour of bombykol, a model insect pheromone, adsorbed at the water–air interface. This system serves as a proxy for studying the amphiphilic nature of pheromones and their interactions with aerosol particles in the atmosphere. Our simulations reveal the molecular organization of the bombykol monolayer and its adsorption isotherm. A new soft-sticky particle equation of state accurately describes the monolayer's behaviour. The analysis uncovers a two-dimensional liquid–gas phase transition within the monolayer. Collective adsorption stabilizes the molecules at the interface and the calculated free energy gain is approximately 2kBT. This value increases under lower estimates of the dew surface concentration, thereby enhancing pheromone adsorption onto aerosols. Overall, our findings hold broad relevance for molecular interface science, atmospheric chemistry and organismal chemical communication, particularly in highlighting the critical role of phase transition phenomena.

The structure and dynamics of extractant-based aggregates strongly influence metal ion extraction and third-phase formation in liquid–liquid systems. Here, classical molecular dynamics simulations were used to study aggregates composed of Eu(NO3)3, water, and the extractants DMDOHEMA or DMDBTDMA in two linear n-alkanes, n-heptane, and n-dodecane. Micelle organization and solvent interactions were analyzed through radial distribution functions and potentials of mean force. Polar and apolar micellar radii were similar for both extractants, but diluent penetration into the apolar region was greater in n-dodecane than in n-heptane. Extractant chains generally wrapped around the polar core, with orientations primarily driven by entropy, except for DMDBTDMA in n-dodecane, where stronger diluent–extractant interactions were observed. Penetrating diluent molecules are oriented roughly perpendicular to the micelle, following the same behavior as the extractant chains. These results reveal that the apolar regions of the aggregates are highly dynamic, with chain motions becoming nearly isotropic beyond the polar core. Solvent molecules readily penetrate the micelle interior, challenging the conventional view of reverse micelles as rigid, star-like structures. Overall, this study demonstrates how the interplay between the extractant and diluent governs aggregate structure and dynamics, offering molecular-level insights into the factors controlling ion extraction and third-phase formation.

We develop a quantitative connection between mesoscopic simulations and macroscopic kinetic descriptions for reactive nanoparticle solutions. In concentrated solutions, effective forces, such as electrostatic or van der Waals interactions, modify local concentration profiles around each reactant, their encounter frequencies, and activation free energies. While microscopic simulations provide atomistic insight and macroscopic kinetic theories describe bulk behavior, the intermediate mesoscopic regime remains poorly characterized. Using reactive Brownian dynamics, we simulate nanometric particles interacting through van der Waals and Coulombic forces and undergoing stochastic charge-transfer reactions. We define macroscopic observables—mean concentrations, rate laws, and equilibrium constants—in terms of mesoscopic statistical averages and extract effective kinetic parameters directly from simulation trajectories. We further assess the extent to which classical theories, such as Smoluchowski diffusion-controlled association and Debye–Hückel electrostatics, reproduce the fitted constants. Two macroscopic models are introduced: a minimal one-reaction description and a physically motivated three-step scheme that separates encounter formation from reactive conversion. Comparison between the two clarifies how mesoscopic interactions shape macroscopic parameters.

A detailed analysis of the structure and speciation of La³⁺ clusters in the 0.1 mol L^-1 La(NO₃)₃ salt methanol (MeOH) solution has been performed by means of molecular dynamics (MD) simulations. The time distribution and NO₃^-/MeOH ligand composition of these clusters have been computed using graph theory techniques. These analyses revealed the formation of branched-like polynuclear clusters in the solution, the predominant clusters being the 3, 7, and 8 La³⁺ clusters. In these clusters, the La³⁺ cations are bound by "monodentate" nitrate bridges. Moreover, the mechanism of aggregation of the La³⁺ clusters has been examined with the development of a 3-step model. Finally, the origin of the aggregation process has been identified by estimating the binding constant for the ion pair La³⁺-NO₃^- using the Bjerrum theory of dilute solutions, with pK° = 5.32 at 25 °C. The low value of the dielectric constant of methanol promotes the binding of the ion pair La³⁺-NO₃^- and the nitrato-bridging polymerization, resulting in the formation of clusters.

Here we examine the question of the chemical models widely used to describe dense solutions, particularly ionic solutions.

In this paper, we propose a new nonpolarizable force field for describing the Ln³⁺ (Ln = lanthanide) series based on a 12-6-4 Lennard-Jones potential. The development of the force field was performed in pure water by adjusting both the ion-oxygen distance and the hydration free energy. This force field accurately reproduces the Ln³⁺ hydration properties through the series, especially the coordination number that is hardly accessible using a nonpolarizable force field. Then, the validity and the transferability of the current force field were evaluated for two different systems containing Ln³⁺ in various solvents, namely, 0.1 mol L^-1 La(NO₃)₃ salts in methanol and Eu(NO₃)₃ salts in solvent organic phases composed of DMDOHEMA molecules in n-heptane. The good agreement between our simulations and the data available in the literature confirms the accuracy of the force field for describing the lanthanide cations in both aqueous and nonaqueous media.

The ionic liquid tributyltetradecylphosphonium chloride ([P _4,4,4,14 ]Cl) forms micelles in water, with a very low CMC, below 1 wt%. The solution is macroscopically homogeneous, even large amounts of [P _4,4,4,14 ]Cl in water do not induce any phase separation. The ternary system [P _4,4,4,14 ]Cl/NaCl/H ₂ O instead displays a LCST (Lower Critical Separation Temperature) behavior, being monophasic at low T and experiencing phase separation when T is increased. This phenomenon has been ascribed to the T -increased adsorption onto the micellar surface of these additional chloride ions. The lowering of the repulsive interactions between micelles finally allows coalescence and thus phase separation. In this work, we explore the impact of NaCl addition onto the phase separation, at fixed T . Specific chloride electrode allows the determination of chloride counterion adsorption for different samples in the phase diagram, all of them being single-phase. A simple theory based on the Poisson–Boltzmann equation and with charge regulation is proposed. The only fitted parameter is the chloride adsorption constant. It enables to model the different populations of ions in the solution and at the micelle surface in different conditions. Considering the effective charge of the micelles with respect to the ionic strength of the solution, it moreover provides a key element in the prediction of phase separation.