Thermodynamics of carbonaceous mesophase: II. General theory for nonideal solutions

Abstract

Carbonaceous mesophase is a discotic liquid crystal (LC) that forms as an intermediate during the fluid state carbonization of many organic precursors. The literature contains two distinct conceptual approaches to the description of carbonaceous mesophase formation: one based on orientational molecular potentials with ideal mixing (LC theory) and one based on nonideal mixing and finite solubility of pitch fractions of differing molecular weight and chemical composition (solubility theory). The present paper proposes the first general theory of liquid phase behavior in mesophase-containing pitches that integrates these two approaches. Submodels are developed for the individual terms in the general theory, drawing from regular solution theory, Flory–Huggins theory, and statistical theories of the liquid crystalline state. The resulting hybrid, integrated model is capable of describing the essential features of three separate literature data sets on pitch phase behavior. Sensitivity studies with the model indicate the importance of two distinct driving forces for mesophase formation: (a) poor solubility of high molecular weight aromatic molecules in the lower molecular weight fractions, and (b) reduction in the system free energy by molecular orientation of the larger, disk-like molecules accompanied by expulsion of some of the smaller, less oriented molecules to a separate isotropic phase. For the cases examined here, both contributions (orientational and excess mixing free energies) are significant and must be considered.