Local Solvent Density Research Articles

Dependencies of solvation-free energy and coordination number of ions on external electrostatic fields

An external electrostatic field (Eext) is applied to water clusters containing Na+, K+, and Ca++ to explore the dependencies of solvation-free energy (ΔGslv) and the coordination number (CN) of ions on Eext. The ΔGslv, radial distribution function [g(r)], and CN of ions are computed from molecular dynamics simulation trajectories. The results demonstrate that ΔEΔGslv [ΔGslv(Eext) − ΔGslv(Eext = 0)] increases with increasing Eext due to dielectric saturation. Eext altered the local solvent density, g(r, θ). However, the g(r) and CN of ions depended slightly on Eext.

Structural Order of Water Molecules around Hydrophobic Solutes: Length-Scale Dependence and Solute-Solvent Coupling.

It has been suggested that the structure and thermodynamics of the water molecules in the hydration layer of simple hydrophobic solutes undergo an order-disorder transition around a nanometer length-scale of the solute size. Using extensive atomistic molecular dynamics (MD) and replica exchange molecular dynamics (REMD) simulation studies, we have probed this order-disorder transition around model hydrophobic solutes of varying size and shape (spherical, planar, and linear), as well as flexible hydrophobic homopolymer chains (n-alkanes), where the conformational fluctuations are likely to create both spatial and temporal heterogeneity on the solvent accessible surface. We have explored the structural response of the water molecules in the hydration shell due to the local variations of the length-scale (or curvature) upon hydrophobic collapse and/or local conformational changes of these polymers. We have shown that the tetrahedral order of the water molecules in the hydration shell is practically independent of the polymer size in the extended state of the polymer due to the availability of a subnanometer cross-sectional length-scale, allowing the water molecules to form hydrogen bonds around the polymer chain. Beyond a certain length of the polymer chains, the collapsed states (associated with larger solute length-scale) start to induce disorder in the surface water molecules. We demonstrate that the local structure (both local number density and tetrahedral order) of the hydration layer is dynamically coupled to the local topology of the polymer. Thus, we envisage that in a flexible (bio)polymer, the hydration shell properties will be sensitive to the local conformational state of the molecule (both spatially and temporally), and the overall observed water structure and dynamics will be dependent on the topological/chemical heterogeneity, and the time-scale of fluctuations in the local curvature (length-scale) of the solvent accessible surface. Moreover, we have demonstrated the direct coupling between the local density fluctuations of water and the local hydrophobic collapse of the polymer. For the extended state of the polymer, the local solvent density fluctuation is practically independent of the solute coordinate (length-scale), and the hydrophobic collapse of the polymer is prompted by a "local dewetting" process induced by these fluctuations.

Temperature dependence of local solubility of hydrophobic molecules in the liquid-vapor interface of water.

One important aspect of the hydrophobic effect is that solubility of small, nonpolar molecules in liquid water decreases with increasing temperature. We investigate here how the characteristic temperature dependence in liquid water persists or changes in the vicinity of the liquid-vapor interface. From the molecular dynamics simulation and the test-particle insertion method, the local solubility Σ of methane in the liquid-vapor interface of water as well as Σ of nonpolar solutes in the interface of simple liquids are calculated as a function of the distance z from the interface. We then examine the temperature dependence of Σ under two conditions: variation of Σ at fixed position z and that at fixed local solvent density around the solute molecule. It is found that the temperature dependence of Σ at fixed z depends on the position z and the system, whereas Σ at fixed local density decreases with increasing temperature for all the model solutions at any fixed density between vapor and liquid phases. The monotonic decrease of Σ under the fixed-density condition in the liquid-vapor interface is in accord with what we know for the solubility of nonpolar molecules in bulk liquid water under the fixed-volume condition but it is much robust since the solvent density to be fixed can be anything between the coexisting vapor and liquid phases. A unique feature found in the water interface is that there is a minimum in the local solubility profile Σ(z) on the liquid side of the interface. We find that with decreasing temperature the minimum of Σ grows and at the same time the first peak in the oscillatory density profile of water develops. It is likely that the minimum of Σ is due to the layering structure of the free interface of water.

Specific Material Recognition by Small Peptides Mediated by the Interfacial Solvent Structure

We present evidence that specific material recognition by small peptides is governed by local solvent density variations at solid/liquid interfaces, sensed by the side-chain residues with atomic-scale precision. In particular, we unveil the origin of the selectivity of the binding motif RKLPDA for Ti over Si using a combination of metadynamics and steered molecular dynamics simulations, obtaining adsorption free energies and adhesion forces in quantitative agreement with corresponding experiments. For an accurate description, we employ realistic models of the natively oxidized surfaces which go beyond the commonly used perfect crystal surfaces. These results have profound implications for nanotechnology and materials science applications, offering a previously missing structure-function relationship for the rational design of materials-selective peptide sequences.

Wetting and drying scenarios of ionic solutions

We review recent theoretical results obtained for the wetting and drying behavior of ionic solutions near a charged solid substrate. Three levels of modeling ionic solutions are considered: the primitive model, where the solvent is replaced by a dielectric continuum, the ‘semi-primitive’ model where the solvent is represented by hard spheres with a Yukawa attraction and a dielectric permittivity is introduced, which depends on the local solvent density, and the ‘civilized’ model, where the solvent molecules are dipolar hard spheres with a Yukawa attraction. Calculations on the primitive model are based on a square gradient functional for the solvent, combined with a Poisson–Boltzmann description of the ions; the discrete solvent models are treated within a multi-component density functional theory, combining Rosenfeld's fundamental measure description of excluded volume effects with the mean-field approximation for the Yukawa and electrostatic interactions. Qualitative agreement is found between the predictions of the three models, while those of the two discrete solvent models agree quantitatively. The relative size of anions and cations is shown to have a crucial influence on interfacial properties, as observed experimentally. A novel drying scenario is predicted near a charged wall. New results are reported for the variation of the surface tension γ with ion concentration c. Contrary to the observed behavior of aqueous ionic solutions, the liquid/vapor surface tension is found to systematically decrease with c, presumably because our solvent models are insufficient to describe water, due to the neglect of strong hydrogen bonding. The solid/liquid surface tension is, in contrast, shown to be a non-monotonic function of c and to possess a minimum.

A study of solvation of benzaldehyde and cinnamaldehyde in CO 2 by molecular dynamics simulation

The solvation of benzaldehyde and cinnamaldehyde in CO 2 was simulated at 323 K at a density range from 0.135 to 0.807 g/cm 3. It was observed that the spatial distribution of CO 2 molecules around solutes closely follows the negative electrostatic potential in the solute molecules. The solvent density distribution maps and radial distribution maps at different densities provided the details of the local solvent density augmentation around each solute. In order to understand the difference of the local density augmentation, the interaction energies between CO 2 molecules and solutes were discussed.

Microscopic density functional theory of wetting and drying of a solid substrate by an explicit solvent model of ionic solutions

Classical density functional theory (DFT) of inhomogeneous fluids is applied to an explicit solvent ‘semi-primitive’ model (SPM) of ionic solutions to investigate the influence of ionic solutes on the wetting behaviour of a solvent in contact with a neutral or charged planar substrate. The SPM is made up of three species of hard sphere particles with different diameters, interacting via an attractive Yukawa potential to model excluded volume and cohesion. The solvent particles are neutral, while the monovalent anions and cations are oppositely charged. The polar nature of the solvent is modelled by a continuum dielectric permittivity linked to the local solvent density. All three species interact with the impenetrable substrate via an attractive external potential. While excluded volume effects are accurately described by a Rosenfeld ‘fundamental measure’ free energy functional, the short range Yukawa attraction and Coulombic interactions are treated within the mean-field approximation. The ionic solutes are found to have a significant impact on the wetting behaviour of the solvent, in particular on the wetting temperature. Strong electric fields, or long-ranged (weakly screened) Coulombic forces are shown to have the propensity to change the wetting transition from second to first order. The cation–anion size asymmetry leads to charge separation on the liquid–vapour interface of the solution, which in turn can induce a drying transition on the liquid side of liquid–vapour coexistence.

An integral equation theory for inhomogeneous molecular fluids: The reference interaction site model approach

An integral equation theory which is applicable to inhomogeneous molecular liquids is proposed. The "inhomogeneous reference interaction site model (RISM)" equation derived here is a natural extension of the RISM equation to inhomogeneous systems. This theory makes it possible to calculate the pair correlation function between two molecules which are located at different density regions. We also propose approximations concerning the closure relation and the intramolecular susceptibility of inhomogeneous molecular liquids. As a preliminary application of the theory, the hydration structure around an ion is investigated. Lithium, sodium, and potassium cations are chosen as the solute. Using the Percus trick, the local density of solvent around an ion is expressed in terms of the solute-solvent pair correlation function calculated from the RISM theory. We then analyze the hydration structure around an ion through the triplet correlation function which is defined with the inhomogeneous pair correlation function and the local density of the solvent. The results of the triplet correlation functions for cations indicate that the thermal fluctuation of the hydration shell is closely related to the size of the solute ion. The triplet correlation function from the present theory is also compared with that from the Kirkwood superposition approximation, which substitutes the inhomogeneous pair correlation by the homogeneous one. For the lithium ion, the behavior of the triplet correlation functions from the present theory shows marked differences from the one calculated within the Kirkwood approximation.

Solvent density inhomogeneities and solvation free energies in supercritical diatomic fluids: A density functional approach

Density functional theory is used to explore the solvation properties of a spherical solute immersed in a supercritical diatomic fluid. The solute is modeled as a hard core Yukawa particle surrounded by a diatomic Lennard-Jones fluid represented by two fused tangent spheres using an interaction site approximation. The authors' approach is particularly suitable for thoroughly exploring the effect of different interaction parameters, such as solute-solvent interaction strength and range, solvent-solvent long-range interactions, and particle size, on the local solvent structure and the solvation free energy under supercritical conditions. Their results indicate that the behavior of the local coordination number in homonuclear diatomic fluids follows trends similar to those reported in previous studies for monatomic fluids. The local density augmentation is particularly sensitive to changes in solute size and is affected to a lesser degree by variations in the solute-solvent interaction strength and range. The associated solvation free energies exhibit a nonmonotonous behavior as a function of density for systems with weak solute-solvent interactions. The authors' results suggest that solute-solvent interaction anisotropies have a major influence on the nature and extent of local solvent density inhomogeneities and on the value of the solvation free energies in supercritical solutions of heteronuclear molecules.

Solvation in supercritical water

Solvation in supercritical water under equilibrium and nonequilibrium conditions is studied via molecular dynamics simulations. The influence of solute charge distributions and solvent density on the solvation structures and dynamics is examined with a diatomic probe solute molecule. It is found that the solvation structure varies dramatically with the solute dipole moment, especially in low-density water, in accord with many previous studies on ion solvation. This electrostrictive effect has important consequences for solvation dynamics. In the case of a nonequilibrium solvent relaxation, if there are sufficiently many water molecules close to the solute at the outset of the relaxation, the solvent response measured as a dynamic Stokes shift is almost completely governed by inertial rotations of these water molecules. By contrast, in the opposite case of a low local solvent density near the solute, not only rotations but also translations of water molecules play an important role in solvent relaxation dynamics. The applicability of a linear response is found to be significantly restricted at low water densities.

極短パルスレーザー分光法による超臨界流体中のダイナミクス

Solvation dynamics and rotational relaxation in supercritical fluids have been examined using the techniques of the subpicosecond laser spectroscopy. The fluorescence up-conversion technique has been employed to detect the fluorescence anisotropy decay of coumarin 153 in supercritical CO2 (310 K) and CHF3 (302, 310 K) over a wide density range. The rotational correlation times in supercritical CHF3 exhibit the remarkable increase at 0.6-0.7 times ρc, which implies that the local solvent density augmentation slows down the rotational dynamics in supercritical fluids. This conclusion is supported by computer simulations based on a 2-site model of fluoroform. The solvation dynamics of the probe 4-dimethylamino-4'-cyanostilbene have been measured in supercritical CHF3 at 310 K using the optical Kerr-gated emission technique, which appears to have captured most (∼90%) of the dynamics occurring over the accessible density range between 1.4-2.0ρc. The measured spectral response exhibits a bimodal decay with ∼0.6 ps and ∼9 ps components and only a modest density dependence.

On the temperature and pressure dependences of cavities in the dielectric continuum picture

Cavity size at equilibrium and its variations with thermodynamic conditions are studied in the dielectric continuum framework of solvents. By employing Gibbs' theory of dividing surfaces, the fluctuating cavity description of Kim [H.J. Kim, J. Chem. Phys. 105, 6818 (1996)] is extended to include effects related to the local solvent density inhomogeneity near the cavity. The resulting theory is applied to study cavity size variations with temperature and pressure in dipolar and nondipolar solvents. Model calculations show that the cavity size tends to increase with temperature along an isobar and decrease with pressure along an isotherm.

Measuring the Change in the Intermolecular Raman Spectrum during Dipolar Solvation

We demonstrate a method to directly measure the change in the spectrum of intermolecular solvent fluctuations as a function of time after electronic excitation of a solute, and this method is applied to the dye Coumarin 102 (C102) in acetonitrile. The complete intermolecular response is captured following resonant excitation with time domain third-order Raman spectroscopy. In a previous report, we introduced this method and used it to probe one point in the intermolecular response as a function of time after solute excitation (Underwood, D. F., Blank, D. A. J. Phys. Chem. A 2003, 107 (7), 956). Here we extend this approach to recover the change in the entire intermolecular response as a function of time. To our knowledge the results provide the first direct measurement of the difference in the equilibrated intermolecular response after excitation of a solute and its evolution during a dipolar solvation event. Excitation of C102 results in a significant increase in the solvent-solute interaction due to a large increase in the dipole moment. The observed change in the intermolecular response is consistent with a rapid change in local solvent density, with intermolecular kinetic energy transfer changing the response on longer time scales. Evolution of the response exhibits a strong frequency dependence and suggests changes over longer distances at longer delay times. The measured change in the spectrum of solvent fluctuations represents a direct experimental confirmation of the breakdown of linear response and confirms predictions from molecular dynamics simulations.

Local Solvent Density Augmentation around a Solute in Supercritical Solvent Bath: 1. A Mechanism Explanation and a New Phenomenon

A recently proposed partitioned density functional (DF) approximation (Phys. Rev. E 2003, 68, 061201) and an adjustable parameter-free version of a Lagrangian theorem-based DF approximation (LTDFA: Phys. Lett. A 2003, 319, 279) are combined to propose a DF approximation for nonuniform Lennard-Jones (LJ) fluid. Predictions of the present DF approximation for local LJ solvent density inhomogeneity around a large LJ solute particle or hard core Yukawa particle are in good agreement with existing simulation data. An extensive investigation about the effect of solvent bath temperature, solvent-solute interaction range, solvent-solute interaction magnitude, and solute size on the local solvent density inhomogeneity is carried out with the present DF approximation. It is found that a plateau of solvent accumulation number as a function of solvent bath bulk density is due to a coupling between the solvent-solute interaction and solvent correlation whose mathematical expression is a convolution integral appearing in the density profile equation of the DF theory formalism. The coupling becomes stronger as the increasing of the whole solvent-solute interaction strength, solute size relative to solvent size, and the closeness to the critical density and temperature of the solvent bath. When the attractive solvent-solute interaction becomes large enough and the bulk state moves close enough to the critical temperature of the solvent bath, the maximum solvent accumulation number as a function of solvent bath bulk density appears near the solvent bath critical density; the appearance of this maximum is in contrast with a conclusion drawn by a previous investigation based on an inhomogeneous version of Ornstein-Zernike integral equation carried out only for a smaller parameter space than that in the present paper. Advantage of the DFT approach over the integral equation is discussed.

Ionic solvation in polar supercritical fluids: an integral equation study

A theoretical study of ion solvation in a polar supercritical solvent is reported. The solute–solvent microstructure is computed from the nonlocal integral equation theory and is shown to be in good agreement with simulation results. The nonlinear behavior of the solvation free energy of the ion in a highly compressible supercritical regime is rationalized in terms of a strong electrostriction effect. It is shown that the accuracy of the Born expression for the solvation energy can be substantially improved by taking into account the local solvent density augmentation around the ionic solute.

Rotational Dynamics of Coumarin 153 in Supercritical Fluoroform

Subpicosecond fluorescence anisotropy decay curves have been measured using the fluorescence up-conversion technique to examine the rotational dynamics of coumarin 153 (C 153) in supercritical fluoroform (T = 302 and 310 K). For reduced densities (p, = ρ/ρ c ) above 0.9, the rotation times of C153 increase with density. For reduced densities lower than 0.9, the rotation times increase with decreasing density and has a maximum at a density near p, = 0.5. The comparison with the extrapolation of the data in polar aprotic solvents indicates that the local solvent density around the solute exceeds about 4 times the bulk density at a density near ρ r = 0.5, which is consistent with those obtained from the steady-state electronic spectral shifts.

UV spectral shift of benzene in sub- and supercritical water

UV absorption spectra of benzene have been measured over the wide range of temperature and pressure from the ambient state to the supercritical state ( T = 400 °C and P = 40 MPa). The analysis of the spectral shift of benzene in water relative to that in the gas indicates that at T = 380 and 390 °C the local solvent density around benzene is likely to be depressed below the bulk density for densities near the critical density. It is found that π-hydrogen bond between benzene and water becomes evident with lowering temperature below T = 340 °C.

A model for sedimentation in inhomogeneous media. I. Dynamic density gradients from sedimenting co-solutes

Macromolecular sedimentation in inhomogeneous media is of great practical importance. Dynamic density gradients have a long tradition in analytical ultracentrifugation, and are frequently used in preparative ultracentrifugation. In this paper, a new theoretical model for sedimentation in inhomogeneous media is presented, based on finite element solutions of the Lamm equation with spatial and temporal variation of the local solvent density and viscosity. It is applied to macromolecular sedimentation in the presence of a dynamic density gradient formed by the sedimentation of a co-solute at high concentration. It is implemented in the software sedfit for the analysis of experimental macromolecular concentration distributions. The model agrees well with the measured sedimentation profiles of a protein in a dynamic cesium chloride gradient, and may provide a measure for the effects of hydration or preferential solvation parameters. General features of protein sedimentation in dynamic density gradients are described.

Why do co-solvents enhance the solubility of solutes in supercritical fluids? New evidence and opinion.

The effects of two polar co-solvents, chlorodifluoromethane and acetone, on the solubility and enthalpy of a solution of 1,4-naphthoquinone in supercritical (SC) CO2 were studied. We found that the dissolution process becomes less exothermic in the presence of the co-solvents relative to that in pure CO2, although the solubility is enhanced significantly by the co-solvents. This indicates that the increase in the solubility by adding co-solvents results from the increase of the entropy of solution. On the basis of the unexpected results we propose a new mechanism for the solubility enhancement of the solute by the co-solvents in supercritical fluids (SCF); this should be applicable to cases in which the local density of the SC solvent around the solute and the co-solvent is larger, and the co-solvent associates preferentially with the solute. The results are also very important for the understanding of other fundamental questions of SCF science, such as the effect of co-solvents on the thermodynamic and kinetic properties of the reactions in SCFs.

Integral Equation Theory for the Interactions between Passivated Nanocrystals in Supercritical Fluids: Solvophobic and Solvophilic Cases

A study of the interactions between nanocrystals in supercritical fluids for the solvophobic and solvophilic cases is presented. The model for the nanocrystal consists of a highly polarizable core and a passivation layer, both of which are uniformly composed of Lennard-Jones particles. A nonlocal integral equation theory is applied to study the excess and total potential of mean force between the nanocrystals. The effects of solvent density, and thickness and density of passivation layer, on the total potential of mean force and on solubility of nanocrystals in solutions is discussed. Important differences observed between solvophobic and solvophilic cases are analyzed in terms of the local solvent density profile around the nanocrystals. Interesting characteristic features, such as the disappearance of anomalous behavior previously reported by us for nonpassivated attractive solutes, are predicted. Certain qualitative feature observed in experimental studies of nanocrystal interactions in supercritical s...