Interpretation of activity coefficients in alkane systems

Abstract

The recent Flory theory of solution thermodynamics has been used to interpret activity coefficients for lower branched and normal alkanes at infinite dilution in n-C₁₈ and n-C₃₆(g.l.c. data of Cruickshank and collaborators and of Pease and Thorburn). Values of the X₁₂ parameter are found to be independent of the extent of branching of the lower alkanes. This is incompatible with the usual interpretation of X₁₂ in terms of different methyl and methylene contents of the components coupled with a large methyl-methylene interchange energy. The X₁₂ values correlate instead with equation of state properties of the lower alkanes. This indicates that X₁₂ has its origin in these systems mainly as a correction to an inaccurate assessment of the equation of state contribution in the activity coefficients. The more general Prigogine theory is used to give an empirical equation of state term for the alkane systems which correlates the data with X₁₂= 0, suggesting a small methyl-methylene interchange energy.