**Solvent E**ff**ects on Catechol Crystal Habits and Aspect Ratios: A Combination of Experiments and Molecular Dynamics Simulation Study**

**Dan Zhu 1, Shihao Zhang 1, Pingping Cui 1, Chang Wang 1, Jiayu Dai 1, Ling Zhou 1,\*, Yaohui Huang 1, Baohong Hou 1,2, Hongxun Hao 1,2, Lina Zhou 1,2 and Qiuxiang Yin 1,2,3**


Received: 29 March 2020; Accepted: 16 April 2020; Published: 18 April 2020

**Abstract:** This work could help to better understand the solvent effects on crystal habits and aspect ratio changes at the molecular level, which provide some guidance for solvent selection in industrial crystallization processes. With the catechol crystal habits acquired using both experimental and simulation methods in isopropanol, methyl acetate and ethyl acetate, solvent effects on crystal morphology were explored based on the modified attachment energy model. Firstly, morphologically dominant crystal faces were obtained with the predicted crystal habit in vacuum. Then, modified attachment energies were calculated by the molecular dynamics simulation to modify the crystal shapes in a real solvent environment, and the simulation results were in agreement with the experimental ones. Meanwhile, the surface properties such as roughness and the diffusion coefficient were introduced to analyze the solvent adsorption behaviors and the radial distribution function curves were generated to distinguish diverse types of interactions like hydrogen bonds and van der Waals forces. Results show that the catechol crystal habits were affected by the combination of the attachment energy, surface structures and molecular interaction types. Moreover, the changing aspect ratios of catechol crystals are closely related to the existence of hydrogen bonds which contribute to growth inhibition on specific faces.

**Keywords:** solvent effect; crystal habit; aspect ratio; molecular dynamics (MD); surface structure
