Influence of additives on nucleation of vanillin: Experiments and introductory molecular simulations

Nucleation of vanillin (VAN) in 2-propanol/water in the presence of additives, viz., acetovanillone (AVA), ethyl vanillin (EVA), guaiacol (GUA), guaethol (GUE), 4-hydroxyacetophenone (HAP), 4-hydroxybenzaldehyde (HBA), and vanillic acid (VAC) is investigated experimentally and by molecular simulations. In the experimental work, the induction time for nucleation is measured at different temperatures and levels of supersaturation using a multicell apparatus. A large variation in the experimental data is observed, and this variation is analyzed by statistical methods. By classical nucleation theory, the induction time data are used to estimate the solid-liquid interfacial energy of vanillin for each VAN-additive system. At 1 mol %, the interfacial energy becomes lower in the presence of AVA, EVA, HAP, and VAC, while it becomes higher in the presence of the other additives. As the additive concentration increases from 1 to 10 mol %, the interfacial energy also increases. The interfacial energy ranges from 6.9 to 10.1 mJ m^-2. Molecular modeling, implemented in the program Cerius2, is used to simulate and examine the surface chemistry of the likely crystal growth faces of VAN (i.e., {0 0 1} and {1 0 0}). To evaluate the additive-crystal face interaction energy, two approaches are used: the surface adsorption method and the lattice integration method. Both experimental and molecular simulation results indicate that the additives studied are potential modifiers of the nucleation of VAN. However, a simple and clear relationship between the experimental values of interfacial energy and the calculated interaction energies for the most important crystal faces of VAN cannot be identified. The modeling does not concern the actual nucleation but rather the conditions of a growing surface and are based on several severe simplifications. Obviously, this simplistic approach does not sufficiently capture the influence of additives on the nucleation of vanillin.