Nuclear quantum effects in electronically adiabatic quantum time correlation functions : Application to the absorption spectrum of a hydrated electron

A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multi-level electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high-energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of the spectrum. It does not appear sufficient, however, to resolve persistent deviations in the slow Lorentzian-like decay part of the spectrum in the intermediate 2-3 eV region

Polarization effects at the surface of aqueous alkali halide solutions

The polarizability of ions, with its strong influence on their surface affinity, is one of the crucial pieces of the complex puzzle that determines the surface properties of electrolyte solutions. Here, we investigate the electrical and structural properties of alkali halide solutions at a concentration of about 1.3 M using molecular dynamics simulations of polarizable water and ions models. We show that capillary fluctuations have a dramatic impact on the sampled quantities and that without removing their smearing effect, it would be impossible to resolve the local structure of the interfacial region. This procedure allows us to investigate in detail the dependence of the permanent and induced dipoles on the distance from the interface. The enhanced resolution gives us access to the surface charges, estimated using the Gouy-Chapman theory, despite the Debye length being shorter than the amplitude of capillary fluctuations

Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions

Amphiphilic surfactants are changing the surface tension of solutions by adsorbing at their surfaces. In general, however, little is known about the actual distribution of the surface tension across the interface, as well as about the extent of the contribution of different moieties to the surface tension. Here, we consider the liquid–vapor interface of the solutions of five different amphiphilic molecules, representative of anionic, cationic and nonionic (alcoholic) surfactants. We investigate, by means of molecular dynamics simulations, the contribution of various chemical species and moieties to the surface tension distribution in these aqueous solutions at various surface coverages. We find that the headgroups of alcoholic surfactants give a negligible contribution to the surface tension. The opposite is true for ionic surfactants, whose effect depends on their “hardness” within the Hofmeister series, even though there is a large compensation between ions and counterions. In addition, we find that water molecules contribute negatively to the surface tension when they are hydrating the ionic headgroups and counterions, instead of being exposed to the vapor phase

Stability of the high-density Jagla liquid in 2D : sensitivity to parameterisation

We computed the pressure-temperature phase diagram of the hard-core two-scale ramp potential in two-dimensions, with the parameterisation originally suggested by Jagla[E. A. Jagla, Phys. Rev. E 63, 061501 (2001)], as well as with a series of systematically modified variants of the model to reveal the sensitivity of the stability of phases. The nested sampling method was used to explore the potential energy landscape, allowing the identification of thermodynamically relevant phases, such as low- and high-density liquids and various crystalline forms, some of which have not been reported before. We also proposed a smooth version of the potential, which is differentiable beyond the hard-core. This potential reproduces the density anomaly, but forms a dodecahedral quasi-crystal structure at high pressure. Our results allow to hypothesise on the necessary modifications of the original model in order to improve the stability of the metastable high density liquid phase in 3D

Fracture Properties of Kerogen and Importance for Organic-Rich Shales

International audienceOil and gas produced from organic-rich shales have become in the last ten years one of the most promising sources of unconventional fossil fuels. The oil and gas are trapped in rocks of very small permeability, but hydraulic fracturing enables to operate those reservoirs with competitive costs. The global reserves of shale oil and gas that are potentially recoverable are equivalent to tens of years of world con- sumption. However, hydraulic fracturing is facing many challenges regarding the productivity but also the security and the environment. One of those challenges is to un- derstand how the fractures propagate underground. The propagation depends on the mechanical stress prevailing in the reservoir and on the fracture properties of the rocks. Regarding the fracture properties, the oil and gas indus- try developed brittleness indicators to distinguish between brittle rocks (containing mostly calcite and silica) and duc- tile rocks (containing a significant proportion of clay and kerogen). During fracturing, a brittle rock shatters easily leading to a well-distributed network of fractures, whereas a ductile rock deforms instead of shattering leading to few fractures and in some situations acting as a barrier to the fracture propagation. In this work, we study the role of kerogen in the ductility of shale. The ultimate objective is to develop a fine understanding of the fracture properties of shales

Insight into liquid polymorphism from the complex phase behavior of a simple model

We systematically explored the phase behavior of the hard-core two-scale ramp model suggested by Jagla [Phys. Rev. E 63, 061501 (2001)] using a combination of the nested sampling and free energy methods. The sampling revealed that the phase diagram of the Jagla potential is significantly richer than previously anticipated, and we identified a family of new crystalline structures, which is stable over vast regions in the phase diagram. We showed that the new melting line is located at considerably higher temperature than the boundary between the low- and high-density liquid phases, which was previously suggested to lie in a thermodynamically stable region. The newly identified crystalline phases show unexpectedly complex structural features, some of which are shared with the high-pressure ice VI phase

Adsorption of Methylamine on Amorphous Ice under Interstellar Conditions. A Grand Canonical Monte Carlo Simulation Study

The adsorption of methylamine at the surface of amorphous ice is studied at various temperatures, ranging from 20 to 200 K, by grand canonical Monte Carlo simulations under conditions that are characteristic to the interstellar medium (ISM). The results are also compared with those obtained earlier on crystalline (<i>I_h</i>) ice. We found that methylamine has a strong ability of being adsorbed on amorphous ice, involving also multilayer adsorption. The decrease of the temperature leads to a substantial increase of this adsorption ability; thus, considerable adsorption is seen at 20–50 K even at bulk gas phase concentrations that are comparable with that of the ISM. Further, methylamine molecules can also be dissolved in the bulk amorphous ice phase. Both the adsorption capacity of amorphous ice and the strength of the adsorption on it are found to be clearly larger than those corresponding to crystalline (<i>I_h</i>) ice, due to the molecular scale roughness of the amorphous ice surface as well as to the lack of clear orientational preferences of the water molecules at this surface. Thus, the surface density of the saturated adsorption monolayer is estimated to be 12.6 ± 0.4 μmol/m², 20% larger than the value of 10.35 μmol/m², obtained earlier for <i>I_h</i> ice, and at low enough surface coverages the adsorbed methylamine molecules are found to easily form up to three hydrogen bonds with the surface water molecules. The estimated heat of adsorption at infinitely low surface coverage is calculated to be −69 ± 5 kJ/mol, being rather close to the estimated heat of solvation in the bulk amorphous ice phase of −74 ± 7 kJ/mol, indicating that there are at least a few positions at the surface where the adsorbed methylamine molecules experience a bulk-like local environment