A Study On The Impact Of Solvent And Additive On Crystal Habit Of Vanillin-series Compounds And The Control Mechanism

In the process of solution crystallization,the crystal habit of products will not only affect the subsequent processes such as filtration,drying,storage and transportation,but also affect the attributes of crystalline product such as bulk density and flowability,as well as the dissolution rate and mixing property.Therefore,crystal habit regulation is one of the most important topics in the industrial crystallization filed.However,the mechanisms of solvent and additive effect in crystal habit regulation are still not fully understood,hence resulting the lack of a universal screening guidance for solvent and additive,which makes the design and preparation of crystalline products with certain crystal habit are still heavily depended on the expensive high-throughput trial-and-error experiments.Vanillin-series compounds are important spice products,and the thin flake habit will bring a series of problems to the downstreaming process,hence the habit of vanillin-series compound have to be improved.Thus,this dissertation investigated the impact of solvent and additives on the habit of different vanillin-series products,and revealed the underlying mechanisms.The main contents in this paper are as follows:(1)The solubility data of vanillin-series compounds in various solvents were determined and fitted with the polynomial equation,the modified Apelblat model and the λh model,which provide fundamental data for following researches.The thermal dissolving parameters were calculated by the van’t Hoff equation,and results indicates that the dissolving process of vanillin-series products is an endothermic and entropy-drives process.(2)Considering the fact that the habit of ethyl vanillin in solution could not be predict accurately by the developed prediction models,the underlying solvent effect was investigated.Crystal surface analysis and molecular dynamics simulation results preclude the presumption that the habit change of ethyl vanillin is caused by the adsorption and inhibition of solvent.These analyses further confirm that the habit change of ethyl vanillin is caused by the impeded de-solvation process of local solvents,due to the ‘pocket’ structure on the surface.The habit of ethyl vanillin was successfully predicted by a new improved modified attachment energy model,which is improved by a correction factor p that based on the diffusion result of solvent.(3)The mechanism of solvent effect on the vanillin habit was investigated.A consistent molecular assembly pattern is confirmed in both crystalline solid and solution by proton nuclear magnetic resonance,electrostatic potential,crystal surface analysis and non-covalent interaction techniques,and the ‘hydroxyl-aldehyde’ hydrogen bond was confirmed to be the dominant molecular interaction.Adsorption locator results suggest that the change of vanillin habit was caused by the selective adsorption of solvent.Organic acid could be selectively adsorbed on the faces along length direction via a single hydrogen bond,and results in a thick plate habit;while,water could be selectively adsorbed on the faces along width direction via a double hydrogen bond,resulting needle-like crystals were obtained.(4)In view of the fact that the relationship between the habit control ability of polymer additive and polymer molecular weight is not fully understood,the effect of polymer additive polyvinyl pyrrolidone(PVP)on ethyl vanillin crystal habit was investigated in this dissertation.It was found that there are both concentration threshold and molecular weight threshold for habit modification ability of PVP,and a complementary relationship was revealed.Molecular dynamics simulation result shows that the habit change of ethyl vanillin was caused by the selective adsorption behavior of PVP.Furthermore,the low movement of PVP on crystal surface is found to be the key factor that affect the habit control ability of PVP.(5)The impact of structurally related impurity of 3-anisldehyde on isovanillin habit was studied,and the incorporation process of 3-anisldehyde on isovanillin faces was further investigated.The incorporation site of 3-anisldehyde on isovanillin is confirmed according to the energy and conformation change by using molecular simulation techniques.Additionally,the stable and meta-stable conformation of incorporated 3-anisldehyde were confirmed.The underlying mechanism was further revealed by adsorption locator analysis,result shows that the incorporated 3-anisldehyde makes the growth sites on(100)face disappeared,and results in the inhibition on(100)face,hence plate isovanillin was obtained.The results in this work are not only important to the solvent and additive screening,but also helpful to the design and operation of crystallization process of vanillin-series products.