Real single ion solvation free energies with quantum mechanical simulation

Single ion solvation free energies are one of the most important properties of electrolyte solutions and yet there is ongoing debate about what these values are. Only the values for neutral ion pairs are known. Here, we use DFT interaction potentials with molecular dynamics simulation (DFT-MD) combined with a modified version of the quasi-chemical theory (QCT) to calculate these energies for the lithium and fluoride ions. A method to correct for the error in the DFT functional is developed and very good agreement with the experimental value for the lithium fluoride pair is obtained. Moreover, this method partitions the energies into physically intuitive terms such as surface potential, cavity and charging energies which are amenable to descriptions with reduced models. Our research suggests that lithium's solvation free energy is dominated by the free energetics of a charged hard sphere, whereas fluoride exhibits significant quantum mechanical behavior that cannot be simply described with a reduced model.Comment: 13 pages, 4 figure

Electrostatic solvation free energies of charged hard spheres using molecular dynamics with density functional theory interactions

Determining the solvation free energies of single ions in water is one of the most fundamental problems in physical chemistry and yet many unresolved questions remain. In particular, the ability to decompose the solvation free energy into simple and intuitive contributions will have important implications for models of electrolyte solution. Here, we provide definitions of the various types of single ion solvation free energies based on different simulation protocols. We calculate solvation free energies of charged hard spheres using density functional theory interaction potentials with molecular dynamics simulation (DFT-MD) and isolate the effects of charge and cavitation, comparing to the Born (linear response) model. We show that using uncorrected Ewald summation leads to unphysical values for the single ion solvation free energy and that charging free energies for cations are approximately linear as a function of charge but that there is a small non-linearity for small anions. The charge hydration asymmetry (CHA) for hard spheres, determined with quantum mechanics, is much larger than for the analogous real ions. This suggests that real ions, particularly anions, are significantly more complex than simple charged hard spheres, a commonly employed representation.Comment: 28 pages, 5 figure

Smoothed Dissipative Particle Dynamics model for mesoscopic multiphase flows in the presence of thermal fluctuations

Thermal fluctuations cause perturbations of fluid-fluid interfaces and highly nonlinear hydrodynamics in multiphase flows. In this work, we develop a novel multiphase smoothed dissipative particle dynamics model. This model accounts for both bulk hydrodynamics and interfacial fluctuations. Interfacial surface tension is modeled by imposing a pairwise force between SDPD particles. We show that the relationship between the model parameters and surface tension, previously derived under the assumption of zero thermal fluctuation, is accurate for fluid systems at low temperature but overestimates the surface tension for intermediate and large thermal fluctuations. To analyze the effect of thermal fluctuations on surface tension, we construct a coarse-grained Euler lattice model based on the mean field theory and derive a semi-analytical formula to directly relate the surface tension to model parameters for a wide range of temperatures and model resolutions. We demonstrate that the present method correctly models the dynamic processes, such as bubble coalescence and capillary spectra across the interface

Mass Density Fluctuations in Quantum and Classical descriptions of Liquid Water

First principles molecular dynamics simulation protocol is established using revised functional of Perdew-Burke-Ernzerhof (revPBE) in conjunction with Grimme's third generation of dispersion (D3) correction to describe properties of water at ambient conditions. This study also demonstrates the consistency of the structure of water across both isobaric (NpT) and isothermal (NVT) ensembles. Going beyond the standard structural benchmarks for liquid water, we compute properties that are connected to both local structure and mass density uctuations that are related to concepts of solvation and hydrophobicity. We directly compare our revPBE results to the Becke-Lee-Yang-Parr (BLYP) plus Grimme dispersion corrections (D2) and both the empirical fixed charged model (SPC/E) and many body interaction potential model (MB-pol) to further our understanding of how the computed properties herein depend on the form of the interaction potential

Remote Estimates of Ice Algae Biomass and Their Response to Environmental Conditions during Spring Melt

In this study, we support previous work showing that a normalized difference index (NDI) using two spectral bands of transmitted irradiance (478 and 490 nm) can be used as a non-invasive method to estimate sea ice chlorophyll a (chl a) following a simple calibration to the local region. Application of this method during the spring bloom period (9 May to 26 June) provided the first non-invasive time series dataset used to monitor changes in bottom ice chl a concentration, an index of algal biomass, at a single point location. The transmitted irradiance dataset was collected on landfast first-year sea ice of Allen Bay, Nunavut, in 2011, along with the physical variables thought to affect chl a accumulation and loss at the ice bottom. Time series biomass calculated using the NDI technique adhered well to core based biomass estimates although, chl a values remained low throughout the bloom, reaching a maximum of 27.6 mg m-2 at the end of May. It is likely that warming of the bottom ice contributed to loss of chl a through its positive influence on brine drainage and ice melt. Chl a content in the bottom ice was also significantly affected by a storm event on 10 June, which caused extensive surface melt and a rapid increase in the magnitude of transmitted irradiance. Furthermore, the velocity of current, measured below the ice at the end of a spring neap-tidal cycle, was negatively associated with ice algae chl a biomass (the stronger the current, the less biomass). The NDI method to remotely estimate ice algal biomass proved useful for application in our time series process study, providing a way to assess the effects of changes to the sea ice environment on the biomass of a single population of ice algae.La présente étude vient appuyer d’anciennes études selon lesquelles un indice par différence normalisée (IDN) recourant à deux bandes spectrales d’éclairement énergétique transmis (478 et 490 nm) peut servir de méthode non invasive d’estimation de la chlorophylle a (chl a) de glace de mer suivant un simple étalonnage dans une aire locale. Le recours à cette méthode pendant la saison de l’efflorescence printanière (du 9 mai au 26 juin) a permis d’obtenir le premier ensemble de données non invasives en séries chronologiques dans le but de surveiller les changements se manifestant dans la concentration de chl a de la glace de fond, un indice de biomasse algale, en un seul point. Les données relatives à l’éclairement énergétique transmis ont été recueillies à partir de la glace de mer de rive de l’année à la baie Allen, au Nunavut, en 2011, en même temps que les variables physiques censées avoir des effets sur l’accumulation de chl a et sur la perte de glace de fond. Les données chronologiques relatives à la biomasse calculées à l’aide de la technique de l’IDN cadraient bien avec les estimations de la biomasse obtenues à l’aide d’échantillons, bien que les valeurs de la chl a restaient à la baisse pendant l’efflorescence, pour atteindre un maximum de 27,6 mg m-2 à la fin du mois de mai. Il est vraisemblable que le réchauffement de la glace de fond a entraîné la perte de chl a en raison de son influence positive sur l’égouttage de la saumure et la fonte des glaces. La teneur en chl a de la glace de fond a également été fortement touchée par un événement pluvio-hydrologique qui a eu lieu le 10 juin, événement qui a entraîné une importante fonte en surface et l’augmentation rapide de la magnitude de l’éclairement énergétique transmis. Par ailleurs, la vélocité du courant, mesurée sous la glace à la fin d’un cycle printanier de marée de mortes-eaux, a été négativement liée à la biomasse en chl a de l’algue glaciaire (plus le courant était fort, moins la biomasse était grande). La méthode de l’IDN en vue d’estimer la biomasse de l’algue glaciaire à distance s’est avérée utile dans le cadre de l’application de notre étude en séries chronologiques, car elle a présenté un moyen d’évaluer les effets des changements caractérisant l’environnement de la glace de mer sur la biomasse d’une seule population d’algues glaciaires

Minor versus major mergers: the stellar mass growth of massive galaxies from z=3 using number density selection techniques

We present a study on the stellar mass growth of the progenitors of local massive galaxies with a variety of number density selections with n≤1×10−4 Mpc−3 (corresponding to M*=1011.24 M⊙ at z=0.3) in the redshift range 0.3<z<3.0. We select the progenitors of massive galaxies using a constant number density selection, and one which is adjusted to account for major mergers. We find that the progenitors of massive galaxies grow by a factor of 4 in total stellar mass over this redshift range. On average the stellar mass added via the processes of star formation, major and minor mergers account for 24±8, 17±15 and 34±14per cent, respectively, of the total galaxy stellar mass at z=0.3. Therefore 51±20per cent of the total stellar mass in massive galaxies at z=0.3 is created externally to their z=3 progenitors. We explore the implication of these results on the cold gas accretion rate and size evolution of the progenitors of most massive galaxies over the same redshift range. We find an average gas accretion rate of∼66±32 M⊙ yr−1 over the redshift range of 1.5<z<3.0. We find that the size evolution of a galaxy sample selected this way is on average lower than the findings of other investigation

A consistent measure of the merger histories of massive galaxies using close-pair statistics I:Major mergers at z &lt;3.5

We use a large sample of $\sim 350,000$ galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies ($>10^{10}\ \mathrm{M}_\odot$) at $0.005 < z < 3.5$. We use a method adapted from that presented in Lopez-Sanjuan et al. (2014) using the full photometric redshift probability distributions, to measure pair $\textit{fractions}$ of flux-limited, stellar mass selected galaxy samples using close-pair statistics. The pair fraction is found to weakly evolve as $\propto (1+z)^{0.8}$ with no dependence on stellar mass. We subsequently derive major merger $\textit{rates}$ for galaxies at $> 10^{10}\ \mathrm{M}_\odot$ and at a constant number density of $n > 10^{-4}$ Mpc$^{-3}$, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at $z < 3.5$, accruing an additional 1-4 $\times 10^{10}\ \mathrm{M}_\odot$ in the process. Major merger accretion rate densities of $\sim 2 \times 10^{-4}$ $\mathrm{M}_\odot$ yr$^{-1}$ Mpc$^{-3}$ are found for number density selected samples, indicating that direct progenitors of local massive ($>10^{11}\mathrm{M}_\odot$) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at $z < 1$, but is 10-100 times smaller than the SFR density at higher redshifts.Comment: 26 pages, 18 figures, accepted to MNRA

Understanding the scale of the single ion free energy: a critical test of the tetra-phenyl arsonium and tetra-phenyl borate assumption

The tetra-phenyl arsonium and tetra-phenyl borate (TATB) assumption is a commonly used extra-thermodynamic assumption that allows single ion free energies to be split into cationic and anionic contributions. The assumption is that the values for the TATB salt can be divided equally. This is justified by arguing that these large hydrophobic ions will cause a symmetric response in water. Experimental and classical simulation work has raised potential flaws with this assumption, indicating that hydrogen bonding with the phenyl ring may favor the solvation of the TB anion. Here, we perform ab initio molecular dynamics simulations of these ions in bulk water demonstrating that there are significant structural differences. We quantify our findings by reproducing the experimentally observed vibrational shift for the TB anion and confirm that this is associated with hydrogen bonding with the phenyl rings. Finally, we demonstrate that this results in a substantial energetic preference of the water to solvate the anion. Our results suggest that the validity of the TATB assumption, which is still widely used today, should be reconsidered experimentally in order to properly reference single ion solvation free energy, enthalpy, and entropy