  Studying the freezing of saltwater on a molecular level is of fundamental
importance for improving freeze desalination techniques. Here, we investigate
the freezing process of NaCl solutions using a combination of x-ray diffraction
and molecular dynamics simulations (MD) for different salt-water
concentrations, ranging from seawater conditions to saturation. A linear
superposition model reproduces well the brine rejection due to hexagonal ice Ih
formation and allows us to quantify the fraction of ice and brine. Furthermore,
upon cooling at T = 233 K we observe the formation of NaCl$\cdot$2H$_2$O
hydrates (hydrohalites), which coexist with ice Ih. MD simulations are utilized
to model the formation of NaCl crystallites. From the simulations we estimate
that the salinity of the newly produced ice is 0.5% mass percent (m/m) due to
ion inclusions, which is within the salinity limits of fresh water. In
addition, we show the effect of ions on the local ice structure using the
tetrahedrality parameter and follow the crystalite formation by using the ion
coordination parameter and cluster analysis.
