On the cavity size distribution in water and n-hexane (original) (raw)
Related papers
Water: cavity size distribution and hydrogen bonds
Chemical Physics Letters, 2004
There are two sizes for water molecules: (a) the distance of closest approach between two hydrogen bonded molecules, 2.8 Å ; (b) the distance of closest approach between two non-hydrogen bonded molecules, 3.2 Å , corresponding to the van der Waals diameter of the oxygen atom. This fact is due to the bunching up effect of hydrogen bonds. Correspondingly, when the hydrogen bonding potential is turned off in computer models of water, the effective size of water molecules increases. It is shown by means of scaled particle theory calculations that this basic point has profound effects on the cavity size distribution if the number density is kept constant. The recognition of the bunching up effect of hydrogen bonds is a key factor in order to address the role played by hydrogen bonds in the partitioning of void volume in liquid water.
Free Energy of Cavity Formation in Liquid Water and Hexane
The Journal of Physical Chemistry, 1996
The difference between the work of forming a cavity in water versus organic solvents is believed to play an important role in making apolar solutes less soluble in water than in these solvents, a property commonly referred to as the hydrophobic effect. In this study, two methods are applied, using molecular dynamics simulations, to compute the free energy of forming spherical cavities in the water and hexane liquids. One, based on the free energy perturbation approach, involves gradually growing into the liquid a soft cavity, by turning on a repulsive potential. The other computes the likelihood of finding a natural cavity in configurational data of neat liquids. In addition, the free energy of cavity formation in the two liquids is evaluated by the scale particle theory. Using all three approaches, we investigate how this free energy is influenced by the different descriptions of the cavity-solvent system: the perturbation method considers soft cavities whereas the statistical approach and scale-particle theory deal with hard sphere cavities. Also the scale-particle theory uses a simplified representation of the solvent while the computational procedures use an atomic description. The results of the perturbation approach show that it is more costly to accommodate a cavity of molecular size in water than in hexane, in agreement with previous evaluations, based on the statistical approach. In hexane, we obtain a rather similar cavity size dependence of the free energy computed with the two simulation methods. In principle, this should also be the case for water. We find, however, significantly higher free energy values in water with the statistical method than with the perturbation approach. This result is confirmed by an analysis of the structure of water around the cavities. Ways of bringing the two calculations to converge to the same result are discussed.
Cavities in molecular liquids and the theory of hydrophobic solubilities
Journal of the American Chemical Society, 1990
Thermal configurational data on neat liquids are used to obtain the work of formation of hard spherical cavities of atomic size in six molecular solvents: n-hexane, n-dodecane, n-undecyl alcohol, chloroform, carbon tetrachloride, and water. These results are used to test a recent suggestion that the differences between nonaqueous solvents and liquid water in solvation of inert gases are not principally due to the hydrogen-bonded structure of liquid water but rather to the comparatively small size of the water molecule. The frequencies of Occurrence of cavities in liquid water can be meaningfully distinguished from those in the organic solvents. Liquid water has a larger fractional free volume, but that free volume is distributed in smaller packets. With respect to cavity work, water is compared to a solvent of the same molecular density and composed of hard spheres of the same size as the water molecule. That comparison indicates that the hard-sphere liquid finds more ways to configure its free volume in order to accommodate an atomic solute of substantial size and, thus, would be a more favorable solvent for inert gases. The scaled particle model of inert gas solubility in liquid water predicts cavity works 20% below the numerical data for TIP4P water at 300 K and 1.0 g/cm3 for cavity radii near 2.0 A. It is argued that the sign of this difference is just the sign that ought to be expected and that the magnitude of this difference measures structural differences between water and the directly comparable hard-sphere liquid. In conjunction with previous data, these results indicate that atomic sized cavities should be considered submacroscopic.
Theory of hydrophobicity: Transient cavities in molecular liquids
Proceedings of the National Academy of Sciences, 1992
Observation of the size distribution of transient cavities in computer simulations of water, n -hexane, and n -dodecane under benchtop conditions shows that the sizes of cavities are more sharply defined in liquid water but the most-probable-size cavities are about the same size in each of these liquids. The calculated solvent atomic density in contact with these cavities shows that water applies more force per unit area of cavity surface than do the hydrocarbon liquids. This contact density, or “squeezing” force, reaches a maximum near cavity diameters of 2.4 Å. The results for liquid water are compared to the predictions of simple theories and, in addition, to results for a reference simple liquid. The numerical data for water at a range of temperatures are analyzed to extract a surface free energy contribution to the work of formation of atomic-size cavities. Comparison with the liquid-vapor interfacial tensions of the model liquids studies here indicates that the surface free en...
Chemical Physics Letters, 2007
A very simple statistical geometric approach is devised that allows the derivation of a formula for the work to create a spherical cavity in a liquid consisting of spherical molecules in the small cavity size limit. This formula exactly corresponds to the general expression for the work of cavity creation provided by scaled particle theory. The origin of the success of the present statistical geometric approach needs further insight.
On the microscopic structure of liquid water
Molecular Physics, 2011
The radial distribution function of liquid water, as obtained by the computer simulations of several classical models of water, is reexamined herein and shown to display two intriguing features. These consist of a compact 'three-peaks structure' over three molecular diameters, which is followed by an apparent loss of the packing correlations beyond R c % 9 Å. This is in contrast to simple liquids for which the correlations decay continuously with distance. This structure is reproduced for many widely used classical force field models of water and by scattering experiments as well. It is also preserved in aqueous binary mixtures of organic solvents, even up to equimolar mixture in some cases. The analysis of the structure factor highlights the role played by the competition between the packing effect and the hydrogen bonding interactions. This analysis, in terms of competition of two length scales, is also supported by a simple core-soft model, which reproduces the structural features outlined above.
The Journal of Chemical Physics, 2011
Commonly, the confinement effects are studied from the grand canonical Monte Carlo (GCMC) simulations from the computation of the density of liquid in the confined phase. The GCMC modeling and chemical potential (μ) calculations are based on the insertion/deletion of the real and ghost particle, respectively. At high density, i.e., at high pressure or low temperature, the insertions fail from the Widom insertions while the performing methods as expanded method or perturbation approach are not efficient to treat the large and complex molecules. To overcome this problem we use a simple and efficient method to compute the liquid's density in the confined medium. This method does not require the precalculation of μ and is an alternative to the GCMC simulations. From the isothermalisosurface-isobaric statistical ensemble we consider the explicit framework/liquid external interface to model an explicit liquid's reservoir. In this procedure only the liquid molecules undergo the volume changes while the volume of the framework is kept constant. Therefore, this method is described in the N p n AV f T statistical ensemble, where N is the number of particles, p n is the normal pressure, V f is the volume of framework, A is the surface of the solid/fluid interface, and T is the temperature. This approach is applied and validated from the computation of the density of the methanol and water confined in the mesoporous cylindrical silica nanopores and the MIL-53(Cr) metal organic framework type, respectively.
Final report [Molecular simulations of complex fluids in confined geometrics]
2002
This report was prepared as an account of work sponsored by the United State Government. Neither the United States or the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, m a k any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefilness of any information, apparatus, product or process disclosed or represents that its use would not infiinge privately-owned rights. 1 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any kgal liabiiity or mponsibility for the accuracy, completenets, or usefulness of any information, apparatus. product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spccific commnCia1 product, process, or KNice by W e name, trademark, manufacturer. or othemik docs not necessarily constitute ot imply its endorsement. recommendation. or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect thosc of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Summary Sandia National Laboratories. The objectives of this work are to develop new methodologies for fast and accurate simulations and apply simulations to various problems of interest to DOE. The success of this work will address several deficiencies in the Sandia's capabilities in the area of molecular simulations. In addition, it provides educational opportunities for students and will enhance the scienceand technology capabilities at Kansas through partnership with the national laboratories.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2002
We recently developed a new molecular dynamics (MD) simulation approach for studying fluids confined between two solid substrates and in equilibrium with bulk fluids. Using this method, we perform MD simulations to investigate the influence of molecular structure on the properties of confined Lennard-Jones (LJ), n-decane, and 2,2-dimethyloctane fluids. Under confinement, spherical LJ particles and linear n-decane form layers parallel to the surfaces, while asymmetric 2,2-dimethyloctane forms a 'pillared-layered' structure consisting of both parallel and perpendicular molecules. As surface separation is varied, the number of spherical LJ and symmetric n-decane molecules changes in a step-wise manner, while the number of 2,2-methyloctane molecules varies in a smooth fashion due to pillar molecules gradually switching between parallel and perpendicular orientations. Concomitant configurational transitions cause oscillatory solvation forces, with force maxima corresponding to well-layered configurations. The double branches in 2,2-dimethyloctane reduce the densities and structural changes in the layers adjacent to the surfaces, causing solvation forces and force oscillations to be less pronounced than those of linear chains. Concerning dynamical properties, the translational diffusivity, computed with Einstein relation, and the shear viscosity, computed with Green-Kubo method, both oscillate, the former out of phase and the latter in phase with respect to force oscillations. Better-ordered films having higher densities exhibit lower translational diffusivities but higher shear viscosities. At disordered states, bulk diffusivities and viscosities are recovered. Asymmetric, branched 2,2-dimethyloctane has lower diffusivities due to its bulky t-butyl group, and weaker diffusivity and viscosity oscillations due to its reduced ordering and configurational differences.