The change in number densities of aqueous solutions of alkali chlorides should be
qualitatively predictable. Typically, as cations get bigger the number density of the
solution decreases. However, aqueous solutions of lithium and sodium chloride exhibit
at ambient conditions practically identical number densities at equal molalities despite
dierent ionic sizes. Here, we provide an atomistic interpretation of this experimentally
observed anomalous behavior using molecular dynamics simulations. The obtained
results show that rigidity of the Li+ rst and second solvation shells and associated
compromised hydrogen bonding result in practically equal average water densities in
the local hydration regions for Li+ and Na+ despite dierent sizes of the cations. In
addition, in more distant regions from the cations, the water densities of these two
solutions also coincide. These ndings thus provide an atomistic interpretation for
matching number densities of LiCl and NaCl solutions. In contrast, the number density
dierences between NaCl and KCl solutions, as well as between LiCl and KCl solutions
behave in a regular fashion with lower number densities of solutions observed for larger
cations.