The trade-off effect of KCl and NH4Cl on the hydrated structure in their mixed aqueous solutions

Abstract

The hydrated structure of KCl and NH4Cl mixed solution were studied by X-ray scattering, Raman spectroscopy and molecular dynamics (MD) simulation method in the range of 0.00%-28.0% mass fraction at 25 °C. Through the analysis of reduced pair distribution function [G(r)], it was found that when the concentration of KCl in the mixed aqueous solution decreased and the concentration of NH4Cl increased, the main peak of G(r) shifted from r = 2.81 Å to the right of the axis. In the absence of KCl, the position of the main peak was r = 3.15 Å. It was speculated that this phenomenon was caused by the main interaction changed from K-OW and OW-OW (r = 2.80 Å) to NO (r = 3.06 Å) and NH4+-Cl− (r = 3.3 Å). It is indicated by the Raman spectra, when the concentration of KCl is high and the concentration of NH4Cl is low, the destruction of the tetrahedral hydrogen bond network in the solution is more serious. In the MD simulation, the coordination number of OW-OW decreases with the increase of KCl concentration, indicating that the tetrahedral hydrogen bond network is severely damaged, which is concerned with the results of Raman spectroscopy.