Insights into solvent-dependent nucleation behavior of benzoic acid from metastable zone widths

Abstract

Currently, how the solvents and temperature affect the solution species and rate-determining step in the nucleation process, thus altering the nucleation behavior remains elusive. In this contribution, by employing metastable zone width (MSZW) experiments, the modified Sangwal’s theory and the molecular dynamic simulations, we explore nucleation behavior of benzoic acid (BA) in polar and non-polar solvents. We find that in methanol, the solid–liquid interfacial tension γ decreases with increasing the saturation temperature, in contrast, the value of γ increases with increasing the saturation temperature in chloroform. Furthermore, in the classical nucleation framework, we conclude that the critical nuclei size and critical Gibbs free energy monotonous decline as the driving force increases in methanol, which implies that the interfacial tension is independent of the driving force. On the contrary, the relationship between critical nucleation parameters and driving force indicates that the interfacial tension changes with the driving force in chloroform. By employing FTIR, we find that dimerization of BA is restricted in methanol, and BA always exists as the monomer even near the nucleation point, which implies that desolvation is the rate-determining step in the nucleation process. While in chloroform, we demonstrate that the form of monomers and dimers coexist in solution. As the saturation temperature increases, the equilibrium is broken and moves toward the monomer. It implies that the rate-determining step of the nucleation process of BA may change from molecular rearrangement to desolvation with increasing the saturation temperature in chloroform.