Activity coefficients of hydrocarbons C4 to C8 in n-octadecane at 35°C. Gas-chromatographic investigation

Abstract

Activity coefficients at effectively infinite dilution γ^∞₁₃ of various hydrocarbons in n-octadecane at 35°C have been derived from gas-liquid chromatographic retention volume data extrapolated to zero gas pressure. The carrier gas was in most cases nitrogen, or occasionally argon; but three n-alk-1-enes were examined with hydrogen, nitrogen and argon to give a consistency check; the log γ^∞₁₃ values from the three carrier gases agree within 0.002. “Mixed” second virial coefficients B₁₂ have been calculated for every hydrocarbon + carrier gas system. The activity coefficients are discussed in terms of the separability of the configurational (entropy of mixing) part and the interactional (energy of mixing) part. The energy of mixing part is analyzed in terms of the Hildebrand-Scatchard solubility parameter; the alkane systems are analyzed also by considering different types of contact points; and finally thc n-alkane systems are re-analyzed in terms of interactions between “end groups” and “middle groups”. The last analysis is marginally the most successful. The mixed second virial coefficients are compared with the values predicted by the principle of corresponding states, using the Hudson-McCoubrey combining rule to estimate the mixed pseudo-critical temperatures, together with the McGlashan-Potter reduced equation of state. The agreement is in most cases within the experimental uncertainty. Other combining rules are less successful.