Study On The Polymorphic Behavior Of Nicosulfuron

Nicosulfuron is a kind of sulfonylurea herbicide,which was the only of sulfonylurea herbicides in gramineous weeds effects of herbicides.It is generally prepared as oil suspension agent to use,so the stability of the oil suspension agent is one of the most important factors to consider.To storage for a long time,the hydrate of nicosulfuron is commonly used and will prevent coalescence phenomena.In this study,a comprehensive crystallographic study of nicosulfuron polymorphs and their behavior was presented.The solubility of nicosulfuron was determined by dynamic method in the seven kinds of pure solvent system ranging from 298.15 K to 318.15 K,which is methanol,ethanol,propanol,isopropanol,n-butyl alcohol,acetonitrile and acetone and the solubility data was fitted by Apelblat model and NRTL model and the results showed that the Apelblat model is more suitable for this system.The dissolution thermodynamic data,including the gibbs free energy,the enthalpy change and entropy change,was calculated using the NRTL model.The data showed that the dissolution behavior of nicosulfuron was spontaneous and the spontaneous tendency will be more obvious if the temperature became higher.The Powder X-ray diffraction(PXRD),Differential scanning calorimeter(DSC),Thermogravimetric Analysis(TGA),microscope and other characterization methods were used to analyze the anhydride,hydrate and n,n-dimethylformamide(DMF)solvate of nicosulfuron.The transformation from the anhydride to the hydrate was monitored by the thermogravimetric analysis under different water content and temperature.The relationship between the initial supersaturation of nicosulfuron and the transformation from the anhydride to the DMF solvate was observated and analyzed.Dynamic vapor sorption(DVS)was used for observation of the gas-phase-mediated transformation from the DMF solvate to the hydrate,which is found to be dominated by the diffusion of water molecules and desorption of DMF molecules.The single crystal of DMF solvate and the hydrate were prepared and their structure data were obtained.The hydrate is triclinic crystal system,space group of P1,and DMF solvate is monoclinic system,space group of P21/c.The conformation and dimer structure of nicosulfuron in these two solvates are very similar,but the molecular arrangement of the DMF solvate is more loosely than the hydrate,because of the space steric effect of DMF molecules.Independent gradient model(IGM)and LMO Energy decomposition analysis(EDA)revealed that nicosulfuron dimer structure is dominated by dispersion energy and the solvent molecules combine with the solute molecules by electrostatic energy.The total energy of the molecule pair in the two crystal types is similar,but the energy distribution of hydrate is more uniform.