Chain length dependence of the critical density of organic homologous series

Whether the critical density of organic compounds belonging to a certain homologous series increases or decreases with (increasing) molecular weight has been a challenging question over the years. Two sets of experimental data have recently appeared in the literature for the critical density of n-alkanes: Steeleʹs data (up to n-decane) suggest that critical density increases with carbon number and reaches a limiting value. On the other hand, the data of Teja et al., 1990 which cover a broader range of n-alkanes (up to n-octadecane), reveal a decreasing trend of the critical density after a maximum at n-heptane. Teja et al. have also presented critical density measurements for 1-alkenes (up to 1-decene) and 1-alkanols (up to 1-undecanol). These data follow the same decreasing trend with the molecular weight as n-alkanes. This trend is not in agreement with the predictions of most group-contribution methods used for the prediction of critical densities. We show in this short communication that the Rackett equation when applied (in three different forms) to the calculation of the critical density provides, in addition to a number of theoretical considerations, support for Tejaʹs data and for the particular decreasing trend of the critical density for the homologous series of n-alkanes, 1-alkenes and 1-alkanols. This conclusion indicates that special techniques, like the Rackett equation, or specific correlations based on Tejaʹs data, should be preferred for the estimation of the critical density of organic compounds, especially for the heavier members, compared with most property estimation group-contribution methods currently available.