The critical constants of long-chain normal paraffins

Abstract

Because of the recent availability of the critical constants of normal alkanes up to octadecane, some modifications in the estimation procedures for the critical constants have become necessary. It has been shown that the equation of Ambrose for the critical temperature of normal alkanes leads to the result that as n → ∞, the limiting value for the critical temperature is equal to the limiting value for the normal boiling point and the limiting value for the critical pressure is 1 atm. Currently, the CH2 increment for the critical volume is considered constant. The recent data of Teja have shown that the CH2 increment increases indefinitely in a homologous series until the critical volume reaches its limiting value. This has made the current procedure for estimating the critical volume obsolete. Taking into account the new measurements of Teja, we have now developed new equations for estimating the critical constants. The limiting values for an infinitely long alkyl chain for Tb, Tc, Pc, and Vc have been found to be 1021 K, 1021 K, 1.01325 bar, and 18618 cm3 · mol−1, respectively. These new concepts have been applied to the estimation of various properties other than the critical constants.