NO QUERIES Simulating the vapour–liquid equilibria of large cyclic alkanes

Self-adapting fixed endpoint configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapour–liquid coexistence curves of cyclic alkanes from c-pentane to c-octadecane. In general, the critical temperatures and densities of the cyclic alkanes are substantially higher than those of their linear counterparts. Furthermore, the simulation data point to a maximum in the critical density for cyclic alkanes with about eight carbon atoms as also observed for the linear alkanes. 1. Introduction The molecular weight dependence of vapour–liquid equilibria (VLE) and the corresponding critical properties of flexible chain molecules are problems of fundamental and technological importance that have intrigued scientists for the past 50 years [1–14]. Experimental difficulties are caused by the fact that hydrocarbons are thermally unstable above about 600 K, i.e. only the vapour–liquid coexistence curves (VLCC) of hydrocarbons with fewer than 10 carbon atoms are readily accessible and special experimental techniques are required to measure the critical properties of higher-molecular-weight compounds [7, 15, 16]. However, the development of more efficient simulation approaches and more accurate theoretical treatments has enabled quantitative predictions for the critical properties of linear chain molecules [9–14]. As an outcome of these efforts, the chain length dependence of the critical properties of linear homopolymers (e.g., alkanes) is now well established [7, 9–14]. For normal alkanes, it is observed that (a) the critical temperature increases monotonically with increasing chain length, (b) the critical density shows an initial increase from methane to about n-octane, followed by a decrease for longer n-alkanes, and (c) the critical pressure is largest for ethane and thereafter decreases monotonically with increasing chain length [7, 9, 13, 14]. Unfortunately, measurements and predictions for branched alkanes with more than 10 carbons are rather scarce, but there are good indications that alkanes with short side chains follow the same trends as the linear alkanes [15–18]. Furthermore, for alkanes of equal molecular weight, the addition of methyl side chains (e.g., n-triacontane