Microstructure of NH4Cl-NH3-H2O System Studied by ATR-FTIR,Raman and MD Simulation

The solubility of ammonium chloride in aqueous ammonia decreases then increases with increasing ammonia concentrations.The molecular mechanism behind this trend is unclear.In this study,ATR-FTIR and Raman spectroscopic techniques were used to determine the effect of ammonium chloride on hydrogen bonding in aqueous ammonia and ultimately explain the observed solubility trend.Spectral analyses were conducted in the wavelength range of 2500-4000 cm^-1 .The results showed that the addition of ammonium chloride endorses the formation of N-H…N hydrogen bonds between ammonium ions and ammonia molecules when the concentration of ammonia is greater than 10% ammonia.However,for concentrations lower than 10%,ammonium ions mainly bond to water molecules,leading to hydrogen bonds of the N-H…O type that are generally less stable than N-H…N bonds.The spectral analyses are confirmed by MD simulations.The results presented herein are useful in the development of novel techniques for the separation of ammonium potassium chloride.