P. Mark Rodger

The intermolecular potential of chlorine

We present an anisotropic atom-atom effective pair potential for chlorine and compare it with a wide range of experimental data. The crystal structure, lattice frequencies, liquid structure factor, and several thermodynamic properties of the liquid are all well reproduced. Reasonable agreement with experiment is also obtained for the structure factor of the gas, and for the second virial coefficient over a range of temperatures.

Inhibition of crystal growth in methane hydrate

Computer modelling has been used to investigate the viability of using a non-toxic, water-soluble polymer, polyvinylpyrrolidone (PVP), to inhibit gas hydrate formation. Monte Carlo calculations have been used to study the adsorption of monomer, dimer, tetramer and octamer PVP units on different {001} surfaces of a type I hydrate; various polymer tacticities have also been considered. Adsorption has been found to occur predominantly through the formation of two hydrogen bonds between the pyrrolidone oxygen and the water surface, and thus the location of adsorption sites depended on the availability of pendant hydrogens on the hydrate surface. PVP chains were generally found to lie flat on the surface, although there was some evidence of loops forming for the octamer. The results indicate that inhibition via adsorption of PVP at hydrate growth sites is viable, but that the main factors influencing the adsorption are inherently statistical.

Intermolecular interactions in halogens: Bromine and iodine

Abstract Anisotropic atom-atom pair potentials are used to describe a variety of condensed-phase properties of bromine and iodine. The potential developed for bromine gives a good description of the experimental crystal structure, lattice frequencies and liquid structure factor, and of the published thermodynamic properties of the liquid. The shortage of experimental data for iodine makes it difficult to develop and test a good pair potential, but we have still managed to reproduce the observed structures of the liquid and crystal. The need for more extensive and more accurate experimental data on the physical properties of the condensed phases of these molecules is emphasized.

Mechanisms for Stabilising Water Clathrates

Abstract Molecular dynamics computer simulations are used to examine various possible mechanisms for stabilising water clathrates. It is found that the attractive interactions between guest and host molecules are not sufficient to explain the clathrates stability; nor is there any significant dynamic coupling between the guest and host molecules in the clathrate. Instead, it appears that the repulsive guest-host interactions provide the mechanical stability of the clathrate, and that they are even sufficient to define its structure. Some of the thermodynamic implications of these observations are discussed.

Effect of available volumes on radial distribution functions

The traditional method of analyzing solution structuring properties of solutes using atom–atom radial distribution functions (rdfs) can give rise to misleading interpretations when the volume occupied by the solute is ignored. It is shown by using the examples of O(4) in α‐ and β‐D‐allose that a more reliable interpretation of rdfs can be obtained by normalising the rdf using the available volume, rather than the traditional volume of a spherical shell. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 363–367, 1998

A local harmonic study of clusters of water and methane

In previous work we have extended the local harmonic model (LHM) to molecular systems and applied it in a study of the bulk properties of gas hydrates. This local molecular harmonic model (LMHM) has several other interesting applications in the study of gas hydrates. In this paper we present one such application: the study of cluster growth towards nucleation. A range of cluster sizes has been studied from 20 to 184 water molecules. Free energy minimisation has been applied using the LMHM and the geometry of the resulting clusters compared to the gas hydrate crystal geometry. Using composite chemical potentials for the solid, gas and interfacial phases a method has been devised to estimate the critical cluster size during nucleation.

Characterisation of the {111} growth planes of a type II gas hydrate and study of the mechanism of kinetic inhibition by poly(vinylpyrrolidone)

This work has developed techniques for studying the mechanism by which a hydrate growth inhibitor, i.e. poly(vinylpyrrolidone)(PVP), acts. The topology of a series of {111} faces of the type II clathrate hydrate, the postulated growth plane, have been studied in detail and used to calculate hydrogen-bond energy surface. N-Ethylpyrrolidone (NEP), the monomeric unit, has been used to sample adsorption on one of the {111} surfaces. The adsorption sites were found to be located around the large partial cavities, at positions determined by the location of the pendant hydrogens. There is an added van der Waals attraction between the NEP and the hydrate surface which is optimal when the pyrrolidone occupies the partial large (16-hedra) cavities on the hydrate surface.

Computer modelling studies of the water-structuring properties of carbohydrates and the sweetness response

Abstract Molecular dynamics calculations have been carried out for a variety of carbohydrates in aqueous solvent to ascertain their water-structuring capabilities. Radial distribution functions have been calculated for oxygen, carbon and hydrogen atoms in a range of different environments. These functions are able to reveal quite different and distinctive behaviour of the solvent around the atom types and in particular can distinguish between hydrophilic and hydrophobic hydration effects. The arrangements of water molecules around the different hydroxide groups in the sweet tasting α- d -mannopyranose and the bitter tasting anomer β- d -mannopyranose have been calculated and the radial distribution functions show significant differences between hydroxyl groups in the same molecule and the equivalent hydroxyl groups in different molecules. Methods are assessed for the summation of hydrogen bonds around the carbohydrate oxygen atoms. The results of the molecular dynamics calculations are consistent with previous experimental and theoretical data about the taste properties of these two molecules.

A Molecular Dynamics Study of the Mechanism of Kinetic Inhibition

Abstract: Over the past 10 years interest in the use of kinetic inhibitors for preventing gas hydrate blockages has gained momentum. This interest stemmed from the discovery of a protein in the fish Winter Flounder that depressed the freezing point of water as well as retarding the rate of ice nucleation and growth, so allowing the fish to survive in Arctic conditions. In this paper we present results of a series of molecular dynamics simulations aimed at investigating the action of a family of sulfonate compounds whose structures are related to a group of known quaternary amine kinetic inhibitors.

Configuration‐Biased Monte Carlo Simulations of Poly(vinylpyrrolidone) at a Gas Hydrate Crystal Surface

Abstract: In this paper we report the use of Monte Carlo simulation methods to study the properties of both isotactic and atactic PVP near a hydrate surface for polymers with molecular weights up to 12,000 Daltons. Information is presented about the conformation and particle size distribution found for PVP bound to a hydrate surface, and about the adsorption sites and energies involved. These results are found to correlate with the behavior already identified for the vinylpyrrolidone monomer. In particular, the same adsorption sites are evident. In the polymeric case the adsorption of any one unit is less optimal than was found for the monomer, but the trains of adsorbed polymer were found to involve partial occupancy of a succession of monomeric adsorption sites.