MOLECULAR-DYNAMICS SIMULATION OF LIQUID ALKANES .1. THERMODYNAMICS AND STRUCTURES OF NORMAL-ALKANES - N-BUTANE TO N-HEPTADECANE

We present results of molecular dynamic (MD) simulations for the therm odynamic and structural properties of liquid n-alkanes, from n-butane to n-heptadecane, using three different models I-III. Two of the three classes of models are collapsed atomic models while the third class i s an atomistically detailed model. Model I is the original Ryckaert an d Bellemans' collapsed atomic model [Discuss. Faraday Soc. 1978, 66, 9 5] and model II is the expanded collapsed model which includes C-C bon d stretching and C-C-C bond angle bending potentials in addition to Le nnard=Jones and torsional potentials of model I. In model III all the carbon and hydrogen atoms in the monomeric units are represented expli citly for the alkane molecules. Excellent agreement of the results of our MD simulations of model I for n-butane with those of Edberg et al. [J. Chem. Phys. 1986, 84, 6933], who used a different algorithm confi rms the validity of our algorithms for MD simulations of model II for 14 liquid n-alkanes and of models I and III for liquid n-butane, n-dec ane, and n-heptadecane. The thermodynamic and structural properties of models I and II are very similar to each other and the thermodynamic properties of model III for the three n-alkanes are not much different from those of models I and II. However, the structural properties of model III are very different-from those of models I and II as observed by comparing the radial distribution functions, the average end-to-en d distances and the root-mean-squared radii of gyrations.