Alenka Luzar

Structure and hydrogen bond dynamics of water–dimethyl sulfoxide mixtures by computer simulations

The authors have used two different force field models to study concentrated dimethyl sulfoxide (DMSO)-water solutions by molecular dynamics. The results of these simulations are shown to compare well with recent neutron diffraction experiments using H/D isotope substitution. Even for the highly concentrated 1DMSO : 2H2O solution, the water hydrogen-hydrogen radial distribution function, gHH(r), exhibits the characteristic tetrahedral ordering of water-water hydrogen bonds. Structural information is, further obtained from various partial atom-atom distribution functions, not accessible experimentally. The behavior of water radial distribution functions, goo(r) and goH(r) indicate that the nearest neighbor correlations among remaining water molecules in the mixture increase with increasing DMSO concentration. No preferential association of methyl groups on DMSO is detected. The pattern of hydrogen bonding and the distribution of hydrogen bond lifetimes in the simulated mixtures is further investigated. Molecular dynamics results show that DMSO typically forms two hydrogen bonds with water molecules. Hydrogen bonds between DMSO and water molecules are longer lived than water-water hydrogen bonds. The hydrogen bond lifetimes determined by reactive flux correlation function approach are about 5 ps and 3 ps for water-DMSO and water-water pairs, respectively, in 1DMSO: 2H20 Mixture. In contrast, for pure water, the hydrogen bond lifetime ismorexa0» about 1 ps. They discuss these times in light of experimentally determined rotational relaxation times. The relative values of the hydrogen bond lifetimes are consistent with a statistical (i.e., transition state theory) interpretation.«xa0less

Resolving the hydrogen bond dynamics conundrum

This paper analyzes dynamic properties of hydrogen bonds in liquid water. We use molecular dynamics simulation to calculate different probability densities that govern the time evolution of the formation and rupture of hydrogen bonds. We provide analytical connections between these functions. Excellent agreement with our simulation results is observed. We prove transition state theory rate constant to be identical to the inverse of the associated mean first passage time (hydrogen bond lifetime). Hence, the analysis establishes its Arrhenius temperature dependence. We give the explicit relation between reactive flux correlation function for the relaxation dynamics of hydrogen bonds, and their first passage time probability densities. All the different observations in the existing literature, associated with various estimates of hydrogen bonding times in liquid water that are affected (or not affected) by particular bond criteria, as well as by different definitions of hydrogen bond lifetimes applied in sim...

A neutron diffraction study of dimethyl sulphoxide–water mixtures

Neutron diffraction with hydrogen/deuterium isotope substitution is used to investigate the structure of water in concentrated dimethyl sulphoxide (DMSO) aqueous solutions. Partial structure factors and pair correlation functions involving the hydrogen atom on the water molecule are determined. Water structure is not found to be strongly affected by the presence of DMSO. However, the percentage of water molecules which hydrogen bond to themselves is substantially reduced compared to pure water, with a large proportion of the hydrogens available for bonding associated with the lone pairs on the DMSO. These experimental findings are in good agreement with the assumptions made in the simple mean‐field type model for hydrogen‐bonded mixtures, developed by Luzar [J. Chem. Phys. 91, 3603 (1989)]. A general scheme for analyzing experimental data on the HH and OH pair correlation functions in terms of coordination numbers is presented. The hydrogen–hydrogen correlation in the solvent (water) is also used to discuss the interparticle correlations between solute (DMSO) particles.

Impact of urea on water structure: a clue to its properties as a denaturant?

A new investigation of the structure of urea-water solutions at a mole ratio of 1 urea to 4 water molecules is described. Neutron diffraction is used in conjunction with isotope labelling on the water and urea hydrogen atoms and on the nitrogen atom of urea. The diffraction data are analysed using the empirical potential structure refinement procedure to yield a set of site-site radial distribution functions and spatial density functions that are consistent with the diffraction data. The results are discussed in relation to recent and past X-ray and neutron diffraction experiments and theoretical studies of this system. It is found that urea incorporates readily into water, forming pronounced hydrogen bonds with water at both the amine and carbonyl headgroups. In addition the urea also hydrogen bonds to itself, forming chains or clusters consisting of up to approximately 60 urea molecules in a cluster. There, is however, little or no evidence of urea segregating itself from water, in marked contrast to a recent study of the methanol-water system. This behaviour is discussed in the context of the great propensity of urea to effect protein denaturation.

Dynamics of Capillary Evaporation. I. Effect of Morphology of Hydrophobic Surfaces

Capillary evaporation (cavitation) has been suggested to be a possible source of long range interactions between mesoscopic hydrophobic surfaces. While evaporation is predicted by thermodynamics, little is known about its kinetics. Glauber dynamics Monte Carlo simulations of a lattice gas close to liquid–gas coexistence and confined between partially drying surfaces are used to model the effect of water confinement on the dynamics of surface-induced phase transition. Specifically, we examine how kinetics of induced evaporation changes as the texture of hydrophobic surfaces is varied. Our results provide guidelines for efficient manipulation of surface properties. We find that evaporation rates can be considerably slowed upon deposition of relatively small amount of hydrophilic coverage. The distribution of hydrophilic patches is however crucial, with the regularly spaced distribution being much more effective in slowing the formation of vapor tubes that trigger the evaporation process. To relate simulatio...

Combined Neutron Diffraction and Computer Simulation Study of Liquid Dimethyl Sulphoxide

The structure of liquid dimethylsulphoxide (DMSO) at 25u2009°C is explored using a combination of neutron diffraction with isotope substitution and computer simulation techniques. The potentials used in the computer simulation consist of Coulomb and 6‐12 Lennard‐Jones interactions for each of the carbon, oxygen, and sulfur sites on the molecule. To interpret the neutron diffraction data most effectively, it is necessary to refine both the molecular internal structure and the intermolecular contributions to the measured structure factors at the same time in order to separate the intermolecular terms correctly, because there is a large degree of overlap between intramolecular and intermolecular distances. This renders the data far more sensitive to the intermolecular forces than if this analysis were not performed. Direct comparison of neutron diffraction data and computer simulation results indicates that existing models of the molecular force field give a sensible description of the liquid structure, although...

Monte Carlo simulation of hydrophobic interaction

The solvation force between neutral surfaces in water is studied by running the grand canonical ensemble Monte Carlo simulations on the system containing up to 102 water‐like particles between parallel smooth plates immersed in the fluid. The water molecules are modeled like hard spheres with embedded point dipoles and orientation dependent adhesion potential. The pressure between the walls as a function of the distance displays oscillations resembling those observed experimentally, The overall attraction between the plates is observed despite the absence of direct wall–wall or wall–fluid attractive forces.

Dynamics of Capillary Evaporation. II. Free Energy Barriers

We investigate the free energy barrier of vapor tube formation in a metastable liquid confined between hydrophobic walls. The model we use is a lattice gas model with nearest neighbor interactions whose evaporation dynamics has been reported in the preceding paper (paper I). We apply transition state theory and a constrained umbrella sampling technique, taking as our transition state a vapor pocket in the middle of the liquid layer. The calculated transmission coefficients show that the size of a vapor pocket is indeed a reasonable order parameter to describe capillary evaporation. The umbrella sampling method gives estimates of free energy barrier for vapor tube formation that are within an order of magnitude agreement with direct Monte Carlo simulation runs. In all the cases studied, the estimated free energy barriers are much smaller than those predicted by a previous mean-field approach.

Single-particle dynamics in dimethyl–sulfoxide/water eutectic mixture by neutron scattering

We study water and dimethyl–sulfoxide (DMSO) dynamics in the eutectic DMSO/water mixture, 2H2O:1DMSO, by incoherent quasielastic and inelastic neutron scattering. A temperature range from room down to −27u200a°C is investigated. Both water and DMSO translational dynamics are significantly slowed in the mixture compared to pure liquids. They exhibit different diffusion dynamics, pointing to the absence of stable hydrogen bonded complexes. Further, the presence of a solute suppresses the dynamic anomalies of water: self-diffusion coefficients for water in the mixture, as well as residence times for jump diffusion reveal an Arrhenius temperature dependence, in contrast to the strong non-Arrhenius behavior in pure water. The density of vibrational states shows a shift of the characteristic librational peak in pure water towards higher energies in the mixture, reflecting hindrance of large amplitude proton motions. These results are compatible with computer simulations which predict longer hydrogen-bond lifetimes ...

A simple model for the intermolecular potential of water

A simple potential for the pair interaction of two water molecules is proposed. This model is intended for use in ionic solvation problems, and is a step beyond the ion dipole model proposed by Adelman, Deutch, and Blum to study molecular effects in solvation. It consists in adding to the hard spheres with a point dipole a narrow potential well with a tetrahedral octupole of the form v(θ,φ)=A(r)sinu2009θu2009sinu20092θu2009cosu20092φ. We test this potential and two improvements by running a Monte Carlo simulation on a system of 70 molecules. The results show that our model reproduces surprisingly well the essential features of the structure of water, namely the rather well defined tetrahedral structure of the first coordination shell, and the very poorly structured second and third coordination shells.