Investigating the Role of Solvent-Solute Interaction in Crystal Nucleation of Salicylic Acid from Organic Solvents

In previous work, it has been shown that the crystal nucleation of salicylic acid (SA) in different solvents becomes increasingly more difficult in the order: chloroform, ethyl acetate acetonitrile, acetone, methanol, and acetic acid. In the present work, vibration spectroscopy, calorimetric measurements, and density functional theory (DFT) calculations are used to reveal the underlying molecular mechanisms. Raman and infrared spectra suggest that SA exists predominately as dimers in chloroform, but in the other five solvents there is no dear evidence of dimerization. In all solvents, the shift in the SA carbonyl peak reflecting the strength in the solvent-solute interaction is quite well correlated to the nucleation ranking. This shift is corroborated by DFT calculated energies of binding one solvent molecule to the carboxyl group of SA. An even better correlation of the influence of the solvent on the nucleation is provided by DFT calculated energy of binding the complete first solvation shell to the SA molecule. These solvation shell binding energies are corroborated by the enthalpy of solvent-solute interaction as estimated from experimentally determined enthalpy of solution and calculated enthalpy of cavity formation using the scaled particle theory. The different methods reveal a consistent picture and suggest that the stronger the solvent binds to the SA molecule in solution, the slower the nucleation becomes.