-alkanes from the vapor phase: Experiments and Monte Carlo simulations*

The n-alkanes do not nucleate easily from the vapor phase, and, when rapidly cooled in a supersonic expansion, saturation levels on the order of 10,000 are typical. [1, 2] The nucleation rates measured for n-pentane through n-heptane, at temperatures ranging from ~109 K to 168 K, are on the order of 10¹⁷ cm^-3s^-1, increasing slightly with decreasing temperature. Despite the high degree of supercooling, Monte Carlo (MC) simulations suggest that for n-pentane through n-heptane the critical clusters remain liquid like under experimental conditions, whereas n-octane and n-nonane adopt more ordered structures. For all three alkanes, the scaled experimental and simulated nucleation rates are offset by ~3 orders of magnitude. Explicitly accounting for the surface tension difference between the real and model substances, however, increases the offset to ~ 6 orders of magnitude, equivalent to a a formation free energy difference of ~13k_BT . This difference could be reduced by changing the cutoff criterion or by using model potentials that better describe n-alkane behavior.


[1] David Ghosh, Dirk Bergmann, Regina Schwering, Judith Wölk, Reinhard Strey, Shinobu Tanimura, Barbara E. Wyslouzil, “Homogeneous nucleation of a homologous series of n-alkanes (C_iH_2i+2, i=7-10) in a supersonic nozzle,” J. Chem. Phys., 132, 024307 (2010).
[2] Kehinde Ogunronbi, Aliasghar Sepehri, Bin Chen, and Barbara E. Wyslouzil, Vapor phase nucleation of the short-chain n-alkanes (n-pentane, n-hexane and n-heptane): Experiments and Monte Carlo simulations, Journal of Chemical Physics, 148, 144312 (2018)