Growth-induced polarity formation in two-component crystals of organic molecules: A statistical analysis

On the basis of a layer-by-layer growth model, growth-induced polarity formation is investigated in two-component crystals (solid solutions) of polar H (host) and non-polar G (guest) or polar H and G molecules, with native H and G crystals being assumed centrosymmetric. These two types of chemical systems are compared with one another in terms of a statistical analysis being comprised by an investigation of polarity for a large number of molecular interaction energies chosen randomly within an assumed but realistic parameter space for energies. The study focuses on a statistical estimate of the potentiality of each type of chemical system to evolve macroscopic polarity. Results are obtained by means of Monte Carlo simulations. Regardless of the type of G molecules (non-polar or polar), solid solution formation significantly increases the probability to obtain polarity in comparison with single-component crystals of polar H molecules only. However, a very high degree of vectorial alignment of polar molecules is only likely for ordered structures HG, but not for statistically uncorrelated distributions of H and G molecules as in real solid solutions. Within the scope of the present description, polarity formation is governed mainly by longitudinal interactions in systems with non-polar G molecules, whereas it is also strongly influenced by lateral interactions in systems with polar G molecules.