arXiv:0903.3914v1 [cond-mat.soft] 23 Mar 2009
Effect of concentration on the thermodynamics of sodium chloride aqueous solutions
in the supercooled regime
D. Corradini, P. Gallo∗and M. Rovere
Dipartimento di Fisica, Universit` a “Roma Tre”
Via della Vasca Navale 84, I-00146 Roma, Italy
Molecular Dynamics simulations are performed on two sodium chloride solutions in TIP4P water
with concentrations c = 1.36mol/kg and c = 2.10mol/kg upon supercooling. The isotherms and
isochores planes are calculated. The temperature of maximum density line and the limit of mechani-
cal stability line are obtained from the analysis of the thermodynamic planes. The comparison of the
results shows that for densities well above the limit of mechanical stability, the isotherms and iso-
chores of the sodium chloride aqueous solution shift to lower pressures upon increasing concentration
while the limit of mechanical stability is very similar to that of bulk water for both concentrations.
We also find that the temperature of maximum density line shifts to lower pressures and temper-
atures upon increasing concentration. Indications of the presence of a liquid-liquid coexistence are
found for both concentrations.
PACS numbers: 65.20.Jk,De,64.60.My
The properties of aqueous ionic solutions besides being
of undoubtful importance in chemical physics1and elec-
trochemistry2, are relevant in many other fields of science
including biology and biophysics3, geophysics4, and even
atmospheric modeling.5In the supercooled region, ther-
modynamic properties of solutions are also of interest for
the cryopreservation of organs and food.6,7,8From a more
fundamental point of view an improved understanding of
the thermodynamics of these systems upon supercooling,
can help to shed light on the open questions on bulk liq-
It is well known that water presents, in the supercooled
region, peculiar thermodynamic behavior.10,11,12,13,14In
particular, the most striking effects are the existence of
a temperature of maximum density (TMD) line and the
divergence of the isothermal compressibility KT, of the
isobaric specific heat cP and of the coefficient of thermal
expansion αP. The origin of this anomalous behavior is
still a matter of large interest and debate in the liter-
ature.11Several theoretical15,16,17,18,19,20,21,22and com-
puter simulation23,24,25,26,27,28,29,30,31papers have shown
the presence in the supercooled region of water of a
liquid-liquid (LL) critical point. Experimental signatures
of this critical point have been also found.32The sec-
ond critical point of water would be the end point of
the coexistence line between a low density liquid (LDL)
and a high density liquid (HDL). In this framework, the
anomalous properties of water arise as a consequence of
the presence of the LL critical point. Furthermore in this
picture, the limit of mechanical stability (LMS) is non-
reentrant and the TMD line is knee-shaped and avoids
∗Author to whom correspondence should be addressed; e-mail: gal-
to cross the LMS line.
Aqueous ionic solutions have been extensively stud-
with particular emphasis on the hydration struc-
ture.33,34,35,36,37,38,39,40,41,42,43,44,45Many studies in the
supercooled regime deal with the glass transition phe-
nomenon (see Ref. 1 and references therein) while the
detailed comparison of the thermodynamic behavior of
the aqueous solutions with respect to bulk water in the
mild supercooled regime still lacks a thorough investi-
gation. Calorimetric experiments have shown that from
low to moderate concentration of ions several thermody-
namic properties of aqueous solutions are dominated by
In this paper, we present a Molecular Dynamics (MD)
simulation study of the thermodynamics of two sodium
chloride aqueous solutions, in the following denoted also
as NaCl(aq), in the supercooled regime. This work is an
extension of a previous study performed on bulk water
and on a NaCl(aq) solution with low salt concentration.47
The concentrations of salt in the solutions studied
in the present work are c = 1.36mol/kg and c =
2.10mol/kg. For both systems we study the isotherms
in the P −ρ plane and the isochores in the P −T plane.
The analysis of those thermodynamic planes leads to the
determination of the LMS and TMD lines. Moreover we
present the trend of the potential energy as a function
of density, at a low temperature. We will compare the
results of the present simulations with results on bulk
water and c = 0.67mol/kg NaCl(aq) studied in our pre-
vious work.47We also perform a comparison of the results
with what found for water confined in a hydrophobic en-
vironment of soft spheres.48
The paper is organized as follows. In Sec. II we explain
the details of the model and the computer simulation
setup. In Sec. III we show and discuss the thermody-
namic behavior. Conclusions are drawn in Sec. IV.
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