  We consider and compare the structural properties of bulk TIP4P water and of
a sodium chloride aqueous solution in TIP4P water with concentration c = 0.67
mol/kg, in the metastable supercooled region. In a previous paper [D.
Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)] we found
in both systems the presence of a liquid-liquid critical point (LLCP). The LLCP
is believed to be the end point of the coexistence line between a high density
liquid (HDL) and a low density liquid (LDL) phase of water. In the present
paper we study the different features of water-water structure in HDL and LDL
both in bulk water and in the solution. We find that the ions are able to
modify the bulk LDL structure, rendering water-water structure more similar to
the bulk HDL case. By the study of the hydration structure in HDL and LDL, a
possible mechanism for the modification of the bulk LDL structure in the
solution is identified in the substitution of the oxygen by the chloride ion in
oxygen coordination shells.
