Study on adsorption and desorption of urea in lignosulfonate with molecular simulations

The adsorption and desorption behavior of urea molecules in lignosulfonate has been studied through molecular simulation methods in this work. Firstly, the conformations of urea clusters around two small molecular models: methanesulfonic acid anion and methanol were optimized by ab initio calculations. Calculation results showed that through H-bonds, the number of urea molecules adsorbed around the sulfonic group was more than that of urea molecules adsorbed around the hydroxyl group. Secondly, a series of binary systems which were composed of the urea molecules and the lignosulfonate with the different contents of sulfonic groups were built. We calculated the diffusion constants of urea molecules from NVT molecular dynamics (MD) simulations for the built lignosulfonate–urea systems at 298 K. Calculation results suggested that the interaction between the sulfonic groups and the urea molecules was stronger than that between the hydroxyl groups and the urea molecules in the binary composite systems. Moreover, a series of NVT MD simulations were also performed for the ternary systems composed of the lignosulfonate, the urea and the water molecules, which was to learn how the water affect the diffusivity of urea in the ternary systems. Simulation results obtained can match well with the results of ab initio calculations and experimental observations.