Fine analysis of the effect of NH3 on the microstructure of mixed KCl and NH4Cl aqueous solutions

In this study, a solution system of KCl-NH4Cl-NH3-H2O with different mass fractions was prepared at room temperature using x-ray scattering, Raman spectroscopy, and molecular dynamics simulations. From x-ray scattering, it was obtained that the peak near Q = 2.50 Å-1 in the F(Q) function changed from a flat-topped blunt peak to a bimodal peak as the concentration of ammonia increased. This change indicated that increased ammonia altered the hydrogen bonding network within the mixed solution. In the G(r) function, the peak near 3.25 Å enhances with the increase in ammonia concentration, suggesting a higher occurrence of N(NH4+)-N(NH3) interactions. Raman spectroscopy findings demonstrated that in the KCl-NH4Cl aqueous mixture, the area of DDAA-type hydrogen bonds increased as KCl concentration decreased and NH4Cl concentration increased. This suggests that KCl disrupts DDAA-type hydrogen bonds more significantly than NH4Cl. The situation was reversed when ammonia was added to the system, implying that KCl damages the DDAA-type hydrogen bonding structure less than NH4Cl when NH3 is present in the solution. Molecular dynamics simulations indicated that the coordination number of K-Cl increases with ammonia concentration, as ammonia's lone pair of electrons can bind to NH4+ to stabilize the [NH4(H2O)m-n(NH3)n]+ complex. This study elucidates the underlying microscopic mechanisms behind the decrease in KCl solubility and the increase in NH4Cl solubility upon increased ammonia.