Bulk Liquid Properties Research Articles

An Atomistic Model for Simulations of the General Anesthetic Isoflurane

An atomistic model of isoflurane is constructed and calibrated to describe its conformational preferences and intermolecular interactions. The model, which is compatible with the CHARMM force field for biomolecules, is based on target quantities including bulk liquid properties, molecular conformations, and local interactions with isolated water molecules. Reference data is obtained from tabulated thermodynamic properties and high-resolution structural information from gas-phase electron diffraction, as well as DFT calculations at the B3LYP level. The model is tested against experimentally known solvation properties in water and oil, and shows quantitative agreement. In particular, isoflurane is faithfully described as lipophilic, yet nonhydrophobic, a combination of properties critical to its pharmacological activity. Intermolecular interactions of the model are further probed through simulations of the binding of isoflurane to a binding site in horse spleen apoferritin (HSAF). The observed binding mode compares well with crystallographic data, and the calculated binding affinities are compatible with experimental results, although both computational and experimental measurements are challenging and provide results with limited precision. The model is expected to be useful for detailed simulations of the elementary molecular processes associated with anesthesia. Full parameters are provided as Supporting Information.

Revised Charge Equilibration Potential for Liquid Alkanes

We present a revised liquid alkane force field based on the charge equilibration formalism for incorporating electrostatic nonadditive effects arising from local polarization. The model is a revision of earlier work by Patel and Brooks, specifically addressing deficiencies in the dihedral potential, electrostatic, and Lennard-Jones (van der Waals) parameters of the force field. We discuss refinement of the alkane backbone torsion potential to match high-level ab initio relative conformational energetics for pentane, hexane, and heptane. We further discuss refinement of the electrostatic and Lennard-Jones (van der Waals) parameters to reproduce the experimental polarizability, liquid density, and vaporization enthalpy of hexane. Finally, we calculate bulk liquid properties including densities, vaporization enthalpies, self-diffusion constants, isothermal compressibilities, constant pressure heat capacities, and NMR T 1 relaxation times for a series of linear alkanes ranging from hexane to pentadecane based on the current revised model. We also compute free energies of hydration for pentane, hexane, and heptane. The revised force field offers a significantly improved overall description of these properties relative to the original parametrization. The current alkane force field represents a platform for ongoing development of a CHARMM (Chemistry at Harvard Molecular Mechanics) polarizable force field for lipids and integral membrane proteins.

The derivation of the GAB adsorption equation from the BDDT adsorption theory

SummaryThe BET‐like adsorption equation, popularly known as the GAB equation, was derived from the BDDT adsorption equation without changes to the bulk liquid properties of multilayer molecules. Without these changes the third parameter K in the GAB equation cannot be said to measure changes in the bulk liquid properties of the multilayer molecules. Instead K measures the range of application of the GAB equation and differentiates multilayer adsorption, when K > 0.5, from singular layer adsorption, when K ≤ 0.5. The GAB equation, derived from and based on the BDDT theory is argued, in this paper, to provide a greater insight into the mechanism of water adsorption than the same equation derived from and based on the BET theory.

A nonadditive methanol force field: Bulk liquid and liquid-vapor interfacial properties via molecular dynamics simulations using a fluctuating charge model

We study the bulk and interfacial properties of methanol via molecular dynamics simulations using a CHARMM (Chemistry at HARvard Molecular Mechanics) fluctuating charge force field. We discuss the parametrization of the electrostatic model as part of the ongoing CHARMM development for polarizable protein force fields. The bulk liquid properties are in agreement with available experimental data and competitive with existing fixed-charge and polarizable force fields. The liquid density and vaporization enthalpy are determined to be 0.809 g/cm3 and 8.9 kcal/mol compared to the experimental values of 0.787 g/cm3 and 8.94 kcal/mol, respectively. The liquid structure as indicated by radial distribution functions is in keeping with the most recent neutron diffraction results; the force field shows a slightly more ordered liquid, necessarily arising from the enhanced condensed phase electrostatics (as evidenced by an induced liquid phase dipole moment of 0.7 D), although the average coordination with two neighboring molecules is consistent with the experimental diffraction study as well as with recent density functional molecular dynamics calculations. The predicted surface tension of 19.66+/-1.03 dyn/cm is slightly lower than the experimental value of 22.6 dyn/cm, but still competitive with classical force fields. The interface demonstrates the preferential molecular orientation of molecules as observed via nonlinear optical spectroscopic methods. Finally, via canonical molecular dynamics simulations, we assess the model's ability to reproduce the vapor-liquid equilibrium from 298 to 423 K, the simulation data then used to obtain estimates of the model's critical temperature and density. The model predicts a critical temperature of 470.1 K and critical density of 0.312 g/cm3 compared to the experimental values of 512.65 K and 0.279 g/cm3, respectively. The model underestimates the critical temperature by 8% and overestimates the critical density by 10%, and in this sense is roughly equivalent to the underlying fixed-charge CHARMM22 force field.

Dynamics of liquid4Hein confined geometries from time-dependent density functional calculations

We present numerical results obtained from time-dependent density functional calculations of the dynamics of $\mathrm{liquid}{}^{4}\mathrm{He}$ in different environments characterized by geometrical confinement. The time-dependent density profile and velocity field of ${}^{4}\mathrm{He}$ are obtained by means of direct numerical integration of the nonlinear Schr\"odinger equation associated with a phenomenological energy functional which describes accurately both the static and dynamic properties of bulk liquid ${}^{4}\mathrm{He}.$ Our implementation allows for a general solution in three dimensions (i.e., no symmetries are assumed in order to simplify the calculations). We apply our method to study the real-time dynamics of pure and alkali-doped clusters, of a monolayer film on a weakly attractive surface and a nanodroplet spreading on a solid surface.

Electrowetting: a model for contact-angle saturation

Electrowetting (EW) involves the application of an electric potential across a solid–liquid (SL) interface, which modifies the wetting properties of that interface by reducing the SL surface energy and induces a contact-angle change without altering either the bulk liquid or solid properties. Reversible contact angles are achieved at low potentials, while the application of high potentials results in contact-angle saturation and system instabilities. In the present work, an EW system consisting of a substrate (plate or rod) coated with a thin Au underlayer, a dielectric (parylene) layer of various thickness and a 100-nm layer of fluoropolymer (Teflon AF1600) is studied both theoretically and experimentally. A theoretical consideration of the EW effect, taking into account the detailed structure of the electrical double layer, and a model for the saturation of the contact angle observed at high applied potentials are presented. The theoretical treatment presented here confirms that the EW effect is, in the most part, due to polarization of the dielectric, as opposed to purely a rearrangement of the double layer, and explains why no dependence of the EW effect on the electrolyte concentration has been observed. The theory is only applicable over a limited potential range, and we have developed a model that predicts the potential at which this limit occurs. This is the potential at which the SL surface energy becomes zero. The limiting potential is dependent only on the bulk liquid and solid properties and is thus fixed for a given system. Experimentally, the limiting potential corresponds to the onset of contact-angle saturation, although variations in the exact angle of saturation are not uncommon due to the kinetic effects involved in a real system as opposed to a strict thermodynamic analysis. The model predicts that for an EW device in which an aqueous droplet can be forced to completely wet a hydrophobic surface, a surface with basically the same surface energy as the liquid is required.

Accommodation of Alcohols by the Liquid/Vapor Interface of Water: Molecular Dynamics Study

Molecular dynamics computer simulations have been used in conjunction with statistical mechanical perturbation theory to examine the process by which water, ethanol, and ethylene glycol (1,2-ethanediol) molecules are transported from the vapor phase into bulk water. The calculated energetics for solvation in the bulk liquid and surface properties such as the surface tension of water and orientations of ethanol and ethylene glycol adsorbed at the water interface agree well with the corresponding experimental data. Currently, the uptake of trace species by water droplets is generally modeled by decoupling the process of mass accommodation at the interface from gas- and liquid-phase diffusion and then coupling the independent processes using an electrical resistance model. In the resistance model, the mass-accommodation coefficient is a measure of the competition between the kinetics of solvation into the bulk liquid and the kinetics of desorption back into the gas phase. Interpreting experimental uptake rates of a variety of solute molecules by water using the resistance model requires mass-accommodation coefficients that are less than 0.5. Mass-accommodation coefficients less than 0.5 imply that the rate of desorption is greater than solvation, thus suggesting that the free energy of activation for desorption is less than that for solvation. The calculated equilibrium free-energy curves for transporting water, ethanol, and ethylene glycol molecules across the liquid/vapor interface and into bulk water exhibit barriers to solvation that are considerably smaller than those implied by the resistance model. Nonequilibrium solvation or dynamical solvent effects on the calculated activation free energies have also been estimated and are shown to be too small to account for the large difference in comparison with the resistance model. In addition, the temperature dependence of this barrier for ethanol has been calculated. Although this dependence agrees with that predicted by the resistance model, the heights of the calculated barriers are again much lower than those predicted by the model.

Ab initio study of gas-phase sulphuric acid hydrates containing 1 to 3 water molecules

Sulphuric acid has a tendency to form hydrates, small clusters containing a few water molecules, in the gas phase. Hydrate formation has a stabilising effect on the vapour as the pressure of sulphuric acid drops (relative to unhydrated vapor), decreasing the nucleation rate. In classical nucleation theories the hydration energies and the hydrate distribution are predicted assuming that hydrates can be described as liquid droplets having thermodynamic properties of bulk liquid. To obtain a better understanding of the structures and formation energies of the smallest clusters, we have performed ab initio density functional calculations of the mono-,di-, and trihydrates. The hydrogen bonds between the molecules are found to be strong. The more water molecules the hydrate contains, the clearer ring-like structure is formed. Comparison to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the smallest clusters too well. The energy barrier for proton transfer reaction H2SO4⋅H2O →HSO4−⋅H3O+ for mono- and dihydrate is high, and protonisation is unlikely to occur, but in trihydrate the protonisation has almost occurred and the barrier is very low. We also studied the singly protonised monohydrate, and found that while sulphuric acid forms H bonds with the OH parts, the hydrogen sulphate ion tends to bind with the O (S=O) part, and the second proton stays tightly in the ion.

Non Equilibrium Molecular Dynamics Simulation of Coupled Heat- and Mass Transport Across a Liquid/Vapor Interface

Abstract The transport properties of bulk liquid, gas and at the gas/liquid interface were studied for two binary Lennard-Jones/spline mixtures by use of nonequilibrium molecular dynamics. One of the mixtures was an ideal isotope mixture, the other a non-ideal mixture. The simulations gave the thermal conductivity, mutual diffusion coefficient, heat flux, mass flux, and the changes in these quantities across the interface. The local entropy production was expressed in terms of fluxes and thermodynamic forces, and numercial estimates are given. It was shown that the largest contribution to the total entropy production occurs in the vapor phase under the chosen conditions. We expect, however that if the mass flux were larger, the major contribution to the entropy production would come from the liquid phase.

Interfacial forces

The past few years have witnessed tremendous advances in experimental and theoretical techniques for probing both the static and dynamic properties of surfaces at the ångstrom level. Here we review how these advances have furthered our fundamental understanding of adhesion−particularly those processes that contribute to energy dissipation during adhesion and separation (loading–unloading cycles), and friction−particularly how the dynamic (e.g., viscous and rheological) properties of liquids in ultrathin films differ from the bulk liquid properties, and how this affects the friction forces of lubricated contacts. The emphasis will be on ‘‘ideal’’ surfaces and interfaces, i.e., surfaces that are molecularly smooth, and interfacial films that are no more than a few molecular layers thick.

The molecular weight and temperature dependence of polymer surface tension: Comparison of experiment with interface gradient theory

The Cahn-Hilliard density gradient theory was used in conjunction with the Flory, Orwoll, and Vrij (FOV) and the Sanchez and Lacombe (SL) equations of state theories to predict surface tensions (γ) with bulk pressure-volume-temperature data. Wide temperature and molecular weight (MW) ranges for each polymer were studied in an attempt to clarify previous discrepancies reported between polymer and low MW liquids and the predictions of density gradient theory. Surface tensions were measured using the modified Wilhelmy technique for both nonpolar and highly polar polymers including poly(dimethyl siloxanes), polyethylenes, polystyrenes, poly(ethylene oxides) (PEO), and poly(propylene oxides) as a function of temperature. Included in the study were PEOs with different chain end functionalization. With one adjustable parameter for a given polymer series, the agreement of FOV theory was better than 1% in terms of the temperature and MW dependence indicating that bulk liquid properties govern surface tension and that the density gradient formalism adequately describes the phenomena. With no adjustable parameters the FOV theory predicts γ to better than 8% for all polymers and oligomers. From the comparison between experiment and theory it is found that the contribution of surface configurational entropy is negligibly small. Some deficiencies of the SL equation of state are traced to its description of the entropy of the system based on the lattice construction.

Variation of longitudinal acoustic velocity at gigahertz frequencies with water content in rat-tail tendon fibers.

AbstractBrillouin scattering was used to determine the longitudinal acoustic velocity along the axis of collagen fibers as the relative humidity decreased from 95 to 0%. Between 0 and 86%, the velocity decreased linearly at a modest rate. Above 86%, the decrease was steep. The changing mass of the absorbed water accounts for most of the velocity change in the lower humidity range. The remaining deviation is probably due to variation of the elastic coupling between molecules. At low humidity, the adsorbed water is known to be attached to the collagen molecules, whereas the additionally adsorbed water at high humidity has the properties of bulk liquid. The high‐humidity sonic velocity variation is ascribed to the presence of free water. It is possible to identify the five water regimes of Pineri et al. [(1978) Biopolymers 17, 2799–2815] with the variations of the sonic velocity with water content.

Continuum Model of Helium Desorption Kinetics

Helium desorption is treated in a model where the film is assumed to have the thermodynamic properties of bulk liquid and the vapor is described by kinetic theory. Equations for energy and mass conservation determine the dynamics of the system. The model resolves some puzzling features of a number of experiments, and predicts a thermally activated desorption time constant in reasonable agreement with experiment, without adjustable parameters.

Intramolecular and intermolecular interactions in methyl substituted pentanes as revealed by high energy CH-stretching overtone spectra

The overtone spectra of pentane and 12 methyl substituted pentanes are measured in the liquid phase in the region of the pure local mode overtones corresponding to ΔvCH = 3, 4, 5, and 6. The local mode structure of the spectra is discussed. The local mode frequencies ωCH and diagonal local mode anharmonicities XCH for the methyl oscillators are obtained from an analysis of the spectra. The change in ωCH and XCH with increased methyl substitution is related to a barrier to large amplitude vibrational motion arising from steric crowding. The height of this barrier is obtained from dissociation energies calculated from ωCH and XCH by taking the local methyl CH stretching potential to be a Lippincott–Schroeder potential. The methyl steric interactions are described in terms of nonbonded Lennard-Jones and Morse potentials. The parameters for these nonbonded potentials are calculated from the barrier heights and structural information obtained from molecular models for their least strained conformations. These parameters are used to identify a positional dependence in methyl steric hindrance. Force constants evaluated from the nonbonded potentials are correlated with bulk liquid properties.

Dielectric Properties of Wool-Water Systems. II. 26 000 Megacycles

The complex dielectric constant for wool-water systems has been determined at a frequency of 26 000 Mc by a cavity resonator method similar to the method employed at 3000 and 9300 Mc. At 26 000 Mc, measurements on single wool fibers and at relative humidities as high as 98% have proved feasible. The complex dielectric constant of the wool-water system increases with increasing water content. Curves for the dielectric constant and loss factor of the wool-water systems at 26 000 Mc are presented and analyzed in terms of a new theory of water sorption by wool. According to this theory, which is based upon a theory originally proposed by Cassie, the water sorbed by wool consists of three distinct molecular species: (1) localized water irrotationally bound to polar sites in the wool, (2) mobile water with bulk liquid properties, and (3) intermediate water with a potential energy of absorption which lies between those for localized and mobile water. The analysis of the measurements at 26 000 Mc for the higher relative humidities in terms of this theory has resulted in a more thorough description of the properties of the sorbed water than was possible for the data obtained at 3000 and 9300 Mc.