Molecular Dynamic Simulation Of The High-Concentration Reactive Dyes In Aqueous Solution

Molecular dynamics (MD) simulation is one of new modern research methods for chemical study. On one hand, it can be used to simulate and study the processes and phenomena which are hard to be measured by experiments, such as the dynamic behavior of molecules on surfaces, the characteristic of molecular movement, aggregation structure of chemical substances, and stability of mechanical processes; on the other hand, it is possible to lower the development cost and shorten the research and development cycle of new chemical products.In this study, the tendency of reactive dye molecules to self-aggregate in aqueous solution has been studied by molecular dynamic simulation. Optimal molecular simulation software were chosen firstly. Then, the characterization methods of molecular aggregates were explored. Finally, the effects of solution environment on the aggregation of reactive dyes were studied. This study investigated how the dyes aggregate in the solutions under different conditions, how the additives suppress the dyes to aggregate, and the interaction between the dyes and additives.Molecular dynamic simulations of concentrated aqueous dye solutions were performed. It was shown that the degree of the dye molecules to form aggregations was increased with the increase of dye concentration. The aggregates existed in a dynamic equilibrium, but there were stable dye monomers existed in all systems. Due to the aromatic ring structure of the dye molecules, parallel stacking facing one another occurred with slight shifts along the long axes of the aromatic rings.Molecular dynamic simulations of different temperature aqueous dye solutions were performed. It was found that the temperature promoted the dye molecules to form aggregates at high concentration, but after reaching a certain temperature, the disaggregation occurred because the temperature could provide the energy that required for the disaggregation. Hydrogen bonding and hydration layer played a role in the aggregation under different temperature conditions.Molecular dynamic simulations of aqueous dye solutions added electrolytes were performed. It was shown that the aggregation degree of reactive dye molecules was higher after adding the electrolytes. The main reason was that the Na+ions changed the electronegativity of the dye molecules, so the repulsion between molecules became weaker, which promoted the aggregation of the dye molecules. Hydrogen bonding and hydration layer did not play a major role in the aggregation. Molecular dynamic simulations of aqueous dye solutions added additives were performed. It was shown that the urea promoted the dye molecules to disaggregate. After adding the urea, the H-bond between the dye molecules was not the main reason for the occurrence of disaggregation. Disaggregation occurred mainly due to the replacement of the water molecules by the urea molecules and the formation of the hydrogen bonds between urea molecules and the dye molecules. Under certain concentration of urea solutions, urea could increase the number of water molecules in the hydration layer, then the dye molecules were less prone to aggregate. However, after increasing the concentration of urea to an extent, urea had no further impact on the hydration layer that surrounded the dye molecules.The above results demonstrated the aggregation of dye molecules under different conditions, except the temperature, which is basically consistent with the results from practical experiment. In the subsequent study, in order to obtain better results, the targeted experiments can be designed under the guidance of the molecular simulation. Therefore, the molecular simulation can guide and improve the practical experiments and complete the development and improvement of the dyeing technology of high concentration of reactive dyes.