The effect of confinement and wall structure on the kinetics of isomerisation of n-butane

Abstract

We examined the effect of confinement on the torsional kinetics in n-butane by conducting equilibrium molecular dynamics (EMD) simulations of the adsorbed phase in a slit-pore geometry. Torsional rate constants were obtained with high precision using the powerful relaxation function method devised by Brown and Clarke (A direct method of studying reaction rates by EMD: application to the kinetics of isomerization in liquid n-butane, J. Chem. Phys. 92 (1990), pp. 3062-3073). Values of kGT, kTG and kGG were determined as a function of temperature, density and porewidth for two different slit pore models: one employing explicit moving atoms, the other employing a smooth potential, both models being simulated at two different wall surface densities. We observed that direct gauche-gauche transitions taking place via the trans well (i.e. double well jumps) are significant under all conditions. All rate constants show a linear variation with temperature when plotted in Arrhenius form but the pre-exponential factor has very different density dependence for the double well kinetics compared with the single well transitions (monotone decreasing and monotone increasing, respectively, in the bulk fluid).We also find clear differences between behaviour of the kinetics under confinement compared with the bulk condensed phase results; kGG being generally greater under confinement than for the corresponding bulk systems.