On the Boiling of Pure Solvents: The Intermolecular Origins of Trouton’s Rule

Microscopic and macroscopic view of vaporizing liquid showing the relationship between the attractive and repulsive potentials. [Display omitted] •A lattice model describing the attractive and repulsive potentials in a pair of liquid molecules is used to derive Trouton’s rule.•The repulsive potential at the end of boiling is the cohesive energy at the start of boiling. A 4 % expansion of van der Waal’s liquid from the onset to the end of boiling is predicted.•The latent heat of vaporization obtained from this model liquid shows good agreement with experiment.•This model was also applied to non-Trouton rule obeying liquids, like those interacting with hydrogen bonding forces via hydroxyl groups. The latent heat of vaporization of calculated for these liquids also agrees with experiment. A lattice model describing the pairwise intermolecular interactions of a two-molecule system is used to derive Trouton’s rule. This model posits that the repulsive potential, which arises from the Pauli repulsion between two interacting molecules or atoms, being equal in magnitude and opposite to the attractive potential just before the end of boiling, facilitates the separation of the interacting molecules. By solving the resultant free energy equation at an equilibrium intermolecular distance, an expression for the attractive pair potential, and the corresponding cohesive energy can be obtained in terms of kBTB (where kB is the Boltzmann constant and TB is the boiling temperature). The resultant latent heat or enthalpy of vaporization, ΔH, is approximately equal to 10.2RTB. This value agrees well with Trouton’s rule which predicts a ΔH of 10-10.5RTB. This model was further applied to liquids interacting through hydrogen bonding whose ΔH deviates noticeably from Trouton’s rule. The calculated latent heat of vaporization thus obtained agrees well with experiments.