André Bellemans

Molecular dynamics of liquid n-butane near its boiling point

Liquid n-butane has been simulated near its boiling point by the method of molecular dynamics. The computed self-diffusion coefficient D is approximately equal to 6.0 × 10−5 cm2/s; the corresponding velocity autocorrelation function displays practically no cage effect, differing markedly from argon and other simple fluids in this respect.

Molecular dynamics of liquid alkanes

The method of molecular dynamics is applied to the simulation of liquid systems of n-alkanes. The model used is a semi-rigid one with fixed C—C bonds and C—C—C angles. In addition to the static and dynamic properties usually deduced for monatomic fluids from such computer experiments, the configurational properties and the internal relaxation behaviour of the alkane chain are also studied. The results of two different simulations of n-butane and of one simulation of n-decane are analysed. The usefulness and the limitations of such computer experiments are discussed briefly.

The rotation-translation coupling in diatomic molecules

The coupling between the translational and rotational motions of single diatomic molecules in the liquid state is investigated for both the homo- and heteronuclear cases. ‘Experimental’ data are provided by computer simulations using the method of molecular dynamics. Their analysis is made in terms of low order time correlation functions of the translational velocity V of the centre of mass and the rotational velocity ω; these correlation functions are defined in two different ways: (a) with respect to the fixed laboratory axis and (b) with respect to a moving reference frame attached to the molecule itself. It is observed, at the level of the correlation matrix {V, ω}, that the rotation-translation coupling appears only indirectly in presentation (a) while direct evidence is provided in presentation (b). Further, this second presentation displays qualitative differences between homo- and heteronuclear molecules. These results should help in clarifying the validity of various stochastic equations modellin...

Statistical mechanics of surface phenomena: I. A cluster expansion for the surface tension

Abstract The grand partition function of a finite system of interacting molecules is expressed in the form of a cluster expansion in such a way that all surface contributions are retained. A new type of cluster integrals is introduced which contain all surface contributions ( superficial cluster integrals). It is then possible to separate the grand partition function into a volumic term related to the so-called irreducible cluster integrals. From this last term expressions for the surface tension and the specific adsorption are derived in the form of power series into the number density of the homogeneous part of the system. The change of the surface tension of a solvent in the presence of a solute is also studied.

Phase transitions in two-dimensional lattice gases of hard-square molecules

The thermodynamic properties of lattice gases of hard‐square molecules on a two‐dimensional square lattice are investigated with special emphasis on the occurence of phase transitions. Three particular systems (called A, B, and C, respectively) are considered, corresponding to exclusion cores including (a) first neighboring sites, (b) first and second neighboring sites and (c) first, second, and third neighboring sites. Case A has already been extensively studied by several authors so that the original contribution of this paper essentially concerns Cases B and C. Three methods for computing the thermodynamic properties of these systems have been used: (a) the matrix method of Kramers and Wannier, (b) low‐ and high‐density series, and (c) an approximate method due to Rushbrooke and Scoins. The situation from the point of view of phase transitions seems to be as follows: Case A presents a second‐order continuous transition, Case B remains doubtful, although a first‐order transition seems to be ruled out, C...

Theorem of Corresponding States for Polymers

A theorem of corresponding states for polymers (r‐mers) is derived for condensed states (liquid and solid). This theorem permits to correlate the thermodynamic properties of an homologous series of r‐mer molecules. It gives satisfactory results when applied to the n hydrocarbons series.

Molecular dynamics simulation of the diffusion of n-butane and i-butane in silicalite

Diffusion of the two isomers of butane in silicalite is simulated by molecular dynamics at 300 K and low loading; it appears that n-butane diffuses much more rapidly, due to its elongated trans conformation, and that the vibrations of the silicalite lattice significantly contribute to increase the diffusion coefficient.

The shear viscosity of n-butane by equilibrium and non-equilibrium molecular dynamics

The shear viscosity of n-butane has been computed by the method of molecular dynamics (MD), by both the Green-Kubo technique and a non-equilibrium method employing a weak fictitious external field inducing a symmetric velocity gradient (pure deformational flow). In this last case, non-equilibrium segments, starting from equilibrium configurations, were averaged, the subtraction technique being used to measure the signal out of noise. The stress autocorrelation functions, provided independently by these two techniques, agree remarkably well with each other over the period of time investigated, i.e. the length of the non-equilibrium segments (0·76 ps). This function presents a long positive tail at two definitely different thermodynamical states: a dense liquid and a supercritical fluid. This tail, which is difficult to interpret, extends over 3·0 and 1·5ps respectively and gives a non-negligible contribution to the viscosity coefficient. In this particular case where the correlation time is relatively long...

The surface tension of krypton, methane and their mixtures

Abstract The surface tension of methane has been measured from 91 to 116°K while the surface tension and the molar volume of krypton have been measured from 116 to 122°K. The surface tension of five Kr-CH 4 mixtures was also determined, showing an excess surface tension of about -0.45 dyn/cm at 116°K for an equimolecular mixture. (Incidentally the molar volume of the Kr-CH 4 mixtures was shown to be additive). The molecular theory of surface tension proposed by Englert-Chwoles and Prigogine predicts an excess surface tension nearly twice as high as the observed value.

The molar volumes of liquid methane and deuteromethane

Abstract The molar volumes of liquid CH 4 and CD 4 have been measured between 98 and 112°K with a relative accuracy of about 2·10 −4 . The molar volume of CH 4 is about 1% larger than that of CD 4 over all this temperature range. This difference is tentatively interpreted as resulting from (a) a translational quantum effect and (b) small differences in the intermolecular forces.