J.G. Powles

Orientational correlations in molecular liquids by neutron scattering Carbon tetrachloride and germanium tetrabromide

Neutron diffraction structure factors for liquid carbon tetrachloride and germanium tetrabromide have been measured and are analysed to determine the molecular centre positional correlation function allowing for molecular angular correlation. The analysis shows quite unequivocally that appreciable molecular orientational correlations must exist in these liquids but that it may only extend to nearest neighbours. This is thought to cause the pair distribution function for the molecular liquids to be markedly different from that for atomic liquids such as argon and the difference is discussed.

Proton spin-lattice relaxation in liquid water and liquid ammonia

The proton spin-lattice relaxation time has been measured at 20·8 Mc/s for a series of solutions of water in heavy water and solutions of ammonia in heavy ammonia for the temperature range from the melting point to the liquid-vapour critical temperature. Measurements have also been made for water over limited temperature ranges at several fixed densities. The contributions to the spin-lattice relaxation time from direct dipolar and spin-rotation interactions have been separated. The spin-rotation interaction contribution appears to be the same for H2O as for HDO and also as between NH3, NH2D and NHD2 and this result is justified. The correlation times for molecular re-orientation, τd, and for molecular angular velocity, τsr, are derived from the results and in so doing some support for the Hubbard [12] relation betweent τsr and τd is adduced. It is found that at the critical temperature τsr≪τd which contrasts with other liquids for which it is usually found that τsr≪τd. The spin-rotation interaction cons...

Non-destructive molecular-dynamics simulation of the chemical potential of a fluid

We have evaluated the chemical potential by molecular-dynamics simulation for a Lennard-Jones (L-J) fluid and for a Lennard-Jones shifted-force (L-J, sf) fluid over a wide range of temperature and density by Widoms [6] particle-insertion-energy method. We have also investigated, in some detail, more recent methods [10, 11] using the energy of a real particle, and combinations of both methods using the energy-distribution functions. We find that these methods are accurate, convenient and economical but are no better than the straightforward Widom method. We confirm that the molecular-dynamics method is as good, if not better, than the corresponding Monte-Carlo techniques and the grand-canonical Monte-Carlo method. We present results for the properties of the L-J, sf3 fluid, in particular the liquid-vapour co-existence curve, including the critical point, and compare them with those for the L-J fluid and for liquid argon. The method is of general application and, in particular, may be directly used for mol...

Deuteron spin lattice relaxation in benzene, bromobenzene, water and ammonia

The deuteron spin-lattice relaxation times, T 1Q, have been measured for benzene, water and ammonia from the melting point to the liquid-vapour critical temperature and for bromobenzene over a smaller temperature range. The results are compared with previously measured proton spin-lattice relaxation times. It is shown that the deuteron T 1Q values agree with the proton intramolecular T 1 values for benzene, thus confirming the separation of the dipolar and spin-rotation contributions performed by Powles and Figgins [1]. A value of the electric quadrupole coupling constant, e 2 qQ/th, is deduced, which agrees with the value for the molecule in the solid. For bromobenzene a value of e 2 qQ/h=175±10 khz is obtained in a similar way. For water and ammonia a solution is found to the classic N.M.R. problem of separating the inter and intra-dipolar interaction contributions to the dipolar relaxation rate. The two contributions are about equal, in agreement with theory. The separation of the spin-rotation contrib...

Molecular motion in liquid benzene by nuclear magnetic resonance

The proton spin-lattice relaxation time, T 1, has been measured for a series of mixtures of benzene in perdeuterobenzene for the liquid in equilibrium with its vapour over the temperature range from below the normal freezing point up to the critical temperature. The two contributions to T 1 due to interactions within the molecule (T 1 intra) and between molecules (T 1 inter) have been separated and are found to be very different in magnitude and in variation with temperature. The variation and magnitude of T 1 inter correlates well with other translational motion dependent properties such as self diffusion and viscosity. The correlation of T 1 intra with other re-orientation dependent properties such as deuteron T 1 and Rayleigh scattering is poor. The observed variation in T 1 intra and in particular the broad maximum at higher temperatures is then interpreted as due to a combination of dipolar and spin-rotation effects. This interpretation results in good agreement between the activation energies for re...

A computer simulation of the classic experiment on osmosis and osmotic pressure

A novel computer simulation technique for studying fluids in confined geometries has been developed and used to replicate Pfeffer’s experiment on osmosis in semipermeable membranes in 1877. Our results confirm the validity of van’t Hoff’s famous relationship for osmotic pressure over a wide range of concentrations, and also clearly establish its validity even for molecular systems. We believe this is the first theoretical validation of this result for such a wide range of concentrations, where no explicit assumption of ideality is made for the interactions of the solute molecules.

A computer simulation for a simple model of liquid hydrogen chloride

The results of a computer simulation of fluid hydrogen chloride are reported for an intermolecular potential consisting of four cut-off, shifted-force atom-atom Lennard-Jones 12–6 interactions. The parameters are fixed using the experimental density of the coexisting liquid and a satisfactory fit is obtained. The simulation is then used to predict the internal energy and the self diffusion constant for the liquid, and the internal energy, pressure and specific heat of the supercritical fluid, all of which are in reasonable agreement with experiment. It also gives moderately good agreement with the measured mean squared torque for the liquid and for the second virial coefficient of the dilute gas. Values of the self diffusion constant for the supercritical fluid are predicted. The nuclear pair-distribution functions are predicted and discussed for the liquid, the supercritical fluid and the dilute gas.

Exact Analytic Solutions for Diffusion Impeded by an Infinite Array of Partially Permeable Barriers

We give an exact solution for the macroscopic diffusion of particles in one dimension impeded by any set of plane, parallel partially permeable barriers. Explicit results are given for one barrier, and for an infinite system of uniformly spaced barriers, for any initial position of the particles. This model can be generalized from slits to square tubes, to cubic pores, etc., and is proposed as a convenient model, with disposable parameters, for fitting to experiment. Various convenient and physically illuminating approximations are discussed. An alternative, and sometimes more convenient, way of computing the exact solution for complicated systems of pores is also given.

The correlation of molecular orientation in liquid water by neutron and X-ray scattering

The neutron diffraction structure factor for heavy water at 22°c has been measured for momentum transfers, ħQ, up to a Q of 13 a -1. The results have been interpreted by abstracting the molecular structure effects as far as possible and using models for the correlation of orientation of pairs of molecules ranging from completely uncorrelated orientation to that for adjacent molecules in ice I. Neither these nor any of the popular models for the structure of water, nor a non-hydrogen bonded model, fit the neutron diffraction data over the whole range of Q. Nevertheless, information about the molecular-centres structure factor is obtained, especially for Q up to about 3 a -1. The method of analysis is generalized and applied to x-ray diffraction data and in particular to x-ray data for liquid water. It is shown that this method of analysis has some advantages over the conventional one. The effect of correlation of molecular orientation is less than that of molecular structure on x-ray scattering by water an...

A computer simulation study of fluids in model slit, tubular, and cubic micropores

Computer simulation studies have been carried out, using a novel method, to examine the behavior of fluids in various confined geometries, including, slit pores, square and cylindrical tubular pores, cubic pores, and pores with rough walls. The method used to model these pores allows for the permeability of the pore wall to the confined fluid to be controlled precisely between the impermeable and totally permeable limits, while at the same time maintaining the atomic nature of the pore wall. These systems have been studied with several models for the pore wall for a wide range of state conditions. The results obtained for nonuniform density distributions, wall permeabilities, and diffusion coefficients are examined in detail.