MELTING BEHAVIOR IN BINARY COMPOUNDS - INCLUSION-COMPOUNDS AS EXAMPLES OF CONGRUENT VS INCONGRUENT MELTING

The melting behaviors of three inclusion compounds (ethanol-Dianin's compound, CCl4-Dianin's compound, and CBr4-hexakis(phenylthio)benzene) and their parent compounds (Dianin's compound and hexakis(phenylthio)benzene) have been examined by differential scanning calorimetry. All three inclusion compounds were found to melt incongruently, at temperatures slightly less than the melting points of the parent compounds. To examine factors that determine whether melting takes place congruently or incongruently, Gibbs energies of these and two other inclusion and their parent (host) compounds were calculated. The most important factor in melting behavior of inclusion compounds is the relative Gibbs energy of the pure host lattice compared with the inclusion compound: lower Gibbs energy of the inclusion compound leads to congruent melting, and lower Gibbs energy of the host lattice leads to incongruent melting. This is discussed in terms of guest-host interactions and structural considerations in these systems. These considerations are generally applicable to melting behavior of any binary compound.