  Salty water is the most abundant electrolyte aqueous mixture on Earth,
however, very little is known about the NaCl-saturated solution interfacial
free energy. Here, we provide the first direct estimation of this magnitude for
several NaCl crystallographic planes by means of the Mold Integration
technique, a highly efficient computational method to evaluate interfacial free
energies with anisotropic crystal resolution. Making use of the JC-SPC/E model,
one of the most benchmarked force fields for NaCl/water solutions, we measure
the interfacial free energy of four different planes, (100), (110), (111), and
(11-2) with the saturated solution at normal conditions. We find high
anisotropy between the different crystal orientations with values ranging from
100 to 150 mJ/m2 and the average value of the distinct planes being 137(20)
mJ/m2. This value for the coexistence interfacial free energy is in reasonable
agreement with previous extrapolations from nucleation studies. Our work
represents a milestone in the computational calculation of interfacial free
energies between ionic crystals and aqueous solutions.
