NMR Investigation of Water in Salt Crusts: Insights from Experiments and Molecular Simulations

The evaporation of water from a bare soil is often accompanied by the formation of a layer of crystallized salt, a process that must be understood in order to address the issue of soil salinisation. Here, we use nuclear magnetic relaxation dispersion measurements to better understand the dynamic properties of water within two types of salt crusts: sodium chloride (NaCl) and sodium sulfate (Na2SO4 ). Our experimental results display a stronger dispersion of the relaxation time T1 with frequency for the case of sodium sulfate as compared to sodium chloride salt crusts. To gain insight into these results, we perform molecular dynamics simulations of salt solutions confined within slit nanopores made of either NaCl or Na2SO4 . We find a strong dependence of the value of the relaxation time T1 on pore size and salt concentration. Our simulations reveal the complex interplay between the adsorption of ions at the solid surface, the structure of water near the interface, and the dispersion of T1 at low frequency, which we attribute to adsorption-desorption events.