Collision of rare-gas atoms on helium nanodroplets: Theoretical evidence for an efficient coagulation of heavy rare-gas atoms.

The coagulation of rare-gas atoms (RG = Ne, Ar, Kr, Xe, and Rn) in helium nanodroplets (HNDs) composed of 1000 atoms is investigated by zero-point averaged dynamics where a He-He pseudopotential is used to make the droplet liquid with proper energies. This method reproduces the qualitative abundances of embedded Arn+1 structures obtained by Time-Dependent Density Functional Theory and Ring Polymer Molecular Dynamics for Ar + ArnHe1000 collisions at realistic projectile speeds and impact parameters. More generally, coagulation is found to be much more efficient for heavy rare-gases (Xe and Rn) than for light ones (Ne and Ar), a behavior mainly attributed to a slower energy dissipation of the projectile in the HND. When coagulation does not occur, the projectile maintains a speed of 10-30 m s-1 within the HND, but its velocity vector is rarely oriented toward the dopant, and the projectile roams in a limited region of the droplet. The structure of embedded RGn+1 clusters does not systematically match their gas-phase global minimum structure, and more than 30% of RGn-RG unbound structures are due to one He atom located in between the projectile and a dopant atom.

Densities, Viscosities, Thermal Expansivities, and Isothermal Compressibilities of Carbonated Hydroalcoholic Solutions for Applications in Sparkling Beverages.

Densities, viscosities, isothermal compressibilities, and thermal expansivities of carbonated hydroalcoholic solutions relevant for sparkling beverages are evaluated by molecular dynamics simulations as a function of temperature and alcoholic degree. They are compared with available experimental data, among which new measurements of densities and viscosities are performed in that respect. The OPC water model seems to yield the most accurate results, and the choice of CO2 model has little influence on the results. Theoretical densities obtained with the OPC model typically deviate by ∼2 kg m-3 from experimental data. At low alcoholic degrees (<9% EtOH vol), experimental viscosities lie in between theoretical values derived from the Stokes-Einstein formula and the calculation of transverse current autocorrelation functions, but at higher alcoholic degrees (≥9% EtOH vol), the Stokes-Einstein relation leads to viscosities in quantitative agreement with experiments. Isothermal compressibilities estimated with a fluctuation formula roughly extend from 0.40 to 0.49 GPa-1 in close agreement with the experimental range of values. However, thermal expansivities are found to significantly overestimate experimental data, a behavior that is partly attributed to the low temperature of maximum density of the OPC model. Despite this discrepancy, our molecular model seems to be suitable for describing several transport and thermodynamic properties of carbonated hydroalcoholic solutions. It could therefore serve as a starting point to build more realistic models for carbonated beverages, from fizzy drinks to sparkling wines.

Clustering, collision, and relaxation dynamics in pure and doped helium nanoclusters: Density- vs particle-based approaches.

The clustering, collision, and relaxation dynamics of pristine and doped helium nanodroplets is theoretically investigated in cases of pickup and clustering of heliophilic argon, collision of heliophobic cesium atoms, and coalescence of two droplets brought into contact by their mutual long-range van der Waals interaction. Three approaches are used and compared with each other. The He time-dependent density functional theory method considers the droplet as a continuous medium and accounts for its superfluid character. The ring-polymer molecular dynamics method uses a path-integral description of nuclear motion and incorporates zero-point delocalization while bosonic exchange effects are ignored. Finally, the zero-point averaged dynamics approach is a mixed quantum-classical method in which quantum delocalization is described by attaching a frozen wavefunction to each He atom, equivalent to classical dynamics with effective interaction potentials. All three methods predict that the growth of argon clusters is significantly hindered by the helium host droplet due to the impeding shell structure around the dopants and kinematic effects freezing the growing cluster in metastable configurations. The effects of superfluidity are qualitatively manifested by different collision dynamics of the heliophilic atom at high velocities, as well as quadrupole oscillations that are not seen with particle-based methods, for droplets experiencing a collision with cesium atoms or merging with each other.

Toward In Silico Prediction of CO2 Diffusion in Champagne Wines.

Carbon dioxide diffusion is the main physical process behind the formation and growth of bubbles in sparkling wines, especially champagne wines. By approximating brut-labeled champagnes as carbonated hydroalcoholic solutions, molecular dynamics (MD) simulations are carried out with six rigid water models and three CO2 models to evaluate CO2 diffusion coefficients. MD simulations are little sensitive to the CO2 model but proper water modeling is essential to reproduce experimental measurements. A satisfactory agreement with nuclear magnetic resonance (NMR) data is only reached at all temperatures for simulations based on the OPC and TIP4P/2005 water models; the similar efficiency of these two models is attributed to their common properties such as low mixture enthalpy, same number of hydrogen bonds, alike water tetrahedrality, and multipole values. Correcting CO2 diffusion coefficients to take into account their system-size dependence does not significantly alter the quality of the results. Estimates of viscosities deduced from the Stokes-Einstein formula are found in excellent agreement with viscometry on brut-labeled champagnes, while theoretical densities tend to underestimate experimental values. OPC and TIP4P/2005 water models appear to be choice water models to investigate CO2 solvation and transport properties in carbonated hydroalcoholic mixtures and should be the best candidates for any MD simulations concerning wines, spirits, or multicomponent mixtures with alike chemical composition.

Unveiling Carbon Dioxide and Ethanol Diffusion in Carbonated Water-Ethanol Mixtures by Molecular Dynamics Simulations.

The diffusion of carbon dioxide (CO2) and ethanol (EtOH) is a fundamental transport process behind the formation and growth of CO2 bubbles in sparkling beverages and the release of organoleptic compounds at the liquid free surface. In the present study, CO2 and EtOH diffusion coefficients are computed from molecular dynamics (MD) simulations and compared with experimental values derived from the Stokes-Einstein (SE) relation on the basis of viscometry experiments and hydrodynamic radii deduced from former nuclear magnetic resonance (NMR) measurements. These diffusion coefficients steadily increase with temperature and decrease as the concentration of ethanol rises. The agreement between theory and experiment is suitable for CO2. Theoretical EtOH diffusion coefficients tend to overestimate slightly experimental values, although the agreement can be improved by changing the hydrodynamic radius used to evaluate experimental diffusion coefficients. This apparent disagreement should not rely on limitations of the MD simulations nor on the approximations made to evaluate theoretical diffusion coefficients. Improvement of the molecular models, as well as additional NMR measurements on sparkling beverages at several temperatures and ethanol concentrations, would help solve this issue.

Fragmentation dynamics of Ar4He1000 upon electron impact ionization: Competition between ion ejection and trapping.

The fragmentation upon electron impact ionization of Ar4He1000 is investigated by means of mixed quantum-classical dynamics simulations. The Ar4 + dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple Ar4 + electronic surfaces and possible transitions between them. Helium atoms are treated individually using zero-point averaged dynamics, a method based on the building of an effective He-He potential. Fast electronic relaxation is observed from less than 2 ps to ∼30 ps, depending on initial conditions. The main fragments observed are Ar2 +Heq and Ar3 +Heq (q ≤ 1000), with a strong contribution of the bare Ar2 + ion, and neither Ar+ nor Ar+Heq fragments are found. The smaller fragments (q ≤ 50) are found to mostly come from ion ejection, whereas larger fragments (q > 500) originate from long-term ion trapping. Although the structure of the trapped Ar2 + ions is the same as in the gas phase, trapped Ar3 + and Ar4 + are rather slightly bound Ar2 +⋯Ar and Ar2 +⋯Ar⋯Ar structures (i.e., an Ar2 + core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release Ar3 +Heq or Ar4 +Heq (q ≤ 50) in the gas phase and/or induce strong helium droplet evaporation. Finally, the translational energy of the fragment center of mass was found to be suitable to provide a clear signature of the broad variety of processes at play in our simulations.

Rayleigh limit and fragmentation of multiply charged Lennard-Jones clusters: Can charged clusters provide clues to investigate the stability of electrospray droplets?

The fragmentation of multiply charged clusters composed of N≤1000 Lennard-Jones particles augmented with electrostatic interactions is explored by classical Monte Carlo and molecular dynamics simulations with the stated goal of establishing possible analogies with electrospray droplets. Clusters with few charge carriers are shown to be only subject to particle ejection and their Rayleigh limit can be estimated by quantifying the loss of charged particles. On the contrary, uniformly charged clusters can both evaporate particles and undergo fission, making them better candidates to model electrospray droplets. Critical charges delimiting regions of instability of these clusters are defined from the calculation of lower order multipole moments and asymmetry parameters based on the knowledge of moments of inertia. The first discontinuity of quadrupole moments and asymmetry parameters is related to cluster elongation before twofold fission and the corresponding charge is deemed to be a good estimate of the Rayleigh limit. Octopole moments are negligible about this charge, their discontinuities arising at higher charges when threefold fissions are allowed. The size dependence of these critical charges is qualitatively predicted from Rayleigh's formula and the expression of surface energy advocated in liquiddrop models. Deviations below 15% are commonly achieved when comparing Rayleigh limits extracted from experimental data with theoretical predictions based on Monte Carlo simulations or liquiddrop models for a set of eleven atomic and molecular liquid clusters. Although manifold fission of uniformly charged clusters is unlikely close to the Rayleigh limit, successive asymmetric fissions are found to occur in conjunction with other fragmentation mechanisms, including the expansion of ring-shaped structures, at charges more than twice as large as the Rayleigh limit.

CO2 diffusion in champagne wines: a molecular dynamics study.

Although diffusion is considered as the main physical process responsible for the nucleation and growth of carbon dioxide bubbles in sparkling beverages, the role of each type of molecule in the diffusion process remains unclear. In the present study, we have used the TIP5P and SPC/E water models to perform force field molecular dynamics simulations of CO2 molecules in water and in a water/ethanol mixture respecting Champagne wine proportions. CO2 diffusion coefficients were computed by applying the generalized Fick's law for the determination of multicomponent diffusion coefficients, a law that simplifies to the standard Fick's law in the case of champagnes. The CO2 diffusion coefficients obtained in pure water and water/ethanol mixtures composed of TIP5P water molecules were always found to exceed the coefficients obtained in mixtures composed of SPC/E water molecules, a trend that was attributed to a larger propensity of SPC/E water molecules to form hydrogen bonds. Despite the fact that the SPC/E model is more accurate than the TIP5P model to compute water self-diffusion and CO2 diffusion in pure water, the diffusion coefficients of CO2 molecules in the water/ethanol mixture are in much better agreement with the experimental values of 1.4 - 1.5 × 10(-9) m(2)/s obtained for Champagne wines when the TIP5P model is employed. This difference was deemed to rely on the larger propensity of SPC/E water molecules to maintain the hydrogen-bonded network between water molecules and form new hydrogen bonds with ethanol, although statistical issues cannot be completely excluded. The remarkable agreement between the theoretical CO2 diffusion coefficients obtained within the TIP5P water/ethanol mixture and the experimental data specific to Champagne wines makes us infer that the diffusion coefficient in these emblematic hydroalcoholic sparkling beverages is expected to remain roughly constant whathever their proportions in sugars, glycerol, or peptides.

Unveiling the Interplay Between Diffusing CO2 and Ethanol Molecules in Champagne Wines by Classical Molecular Dynamics and (13)C NMR Spectroscopy.

The diffusion coefficients of carbon dioxide (CO2) and ethanol (EtOH) in carbonated hydroalcoholic solutions and Champagne wines are evaluated as a function of temperature by classical molecular dynamics (MD) simulations and (13)C NMR spectroscopy measurements. The excellent agreement between theoretical and experimental diffusion coefficients suggest that ethanol is the main molecule, apart from water, responsible for the value of the CO2 diffusion coefficients in typical Champagne wines, a result that could likely be extended to most sparkling wines with alike ethanol concentrations. CO2 and EtOH hydrodynamical radii deduced from viscometry measurements by applying the Stokes-Einstein relationship are found to be mostly constant and in close agreement with MD predictions. The reliability of our approach should be of interest to physical chemists aiming to model transport phenomena in supersaturated aqueous solutions or water/alcohol mixtures.

Structure and stability of charged clusters.

When a cluster or nanodroplet bears charge, its structure and thermodynamics are altered and, if the charge exceeds a certain limit, the system becomes unstable with respect to fragmentation. Some of the key results in this area were derived by Rayleigh in the nineteenth century using a continuum model of liquid droplets. Here we revisit the topic using a simple particle-based description, presenting a systematic case study of how charge affects the physical properties of a Lennard-Jones cluster composed of 309 particles. We find that the ability of the cluster to sustain charge depends on the number of particles over which the charge is distributed-a parameter not included in Rayleigh's analysis. Furthermore, the cluster may fragment before the charge is strong enough to drive all charged particles to the surface. The charged particles in stable clusters are therefore likely to reside in the cluster's interior even without considering solvation effects.

Charge localization in multiply charged clusters and their electrical properties: some insights into electrospray droplets.

The surface composition of charged Lennard-Jones clusters A(N) (n+), composed of N particles (55 ≤ N ≤ 1169) among which n are positively charged with charge q, thus having a net total charge Q = nq, is investigated by Monte Carlo with Parallel Tempering simulations. At finite temperature, the surface sites of these charged clusters are found to be preferentially occupied by charged particles carrying large charges, due to Coulombic repulsions, but the full occupancy of surface sites is rarely achieved for clusters below the stability limit defined in this work. Large clusters (N = 1169) follow the same trends, with a smaller propensity for positive particles to occupy the cluster surface at non-zero temperature. We show that these charged clusters rather behave as electrical spherical conductors for the smaller sizes (N ≤ 147) but as spheres uniformly charged in their volume for the larger sizes (N = 1169).