Speaker
Prof.
Carlos Vega
(Universidad Complutense)
Description
Among all the freezing transitions, that of water into ice
is
probably the most relevant to biology, physics, geology
or
atmospheric science. Computer simulations can be used
to
locate the coexistence conditions for a certain water
model.
Two procedures can be used to locate the coexistence.
In the
first one free energy calculations must be performed for
the
fluid and solid phases to locate the coexistence point.
Although the free energy of the fluid phase can be
determined easily, for the solid phase one must use
special
methods as for instance the Einstein crystal method. In
this
work we shall illustrate how to perform free energy
calculations for the solid phases of water (ices) using a
Molecular Dynamics package as GROMACS [1]. The
second
route to phase equilibrium is the direct coexistence
method,
where the two coexistence phases are located within
the
same simulation box. We shall present results
illustrating that
the direct coexistence method is efficient, not only for ice
Ih,
but for the rest of high pressure polymorphs of water.
Besides
we shall discuss two issues related to the use of direct
coexistence simulations: its stochastic character [2] and
in
the particular case of water the subtle issue of the
proton
ordering of ices (with no disorder, partial disorder or
complete
disorder) [3]. The direct coexistence method can also be
used
to analyze the melting point of finite size clusters of ice
embedded within a supercooled sample of liquid water.
In this
way the size of the critical cluster for the homogeneous
freezing of water can be evaluated. For temperatures
between -15 and -35 degrees below freezing the size of
the
critical clusters varies from 8000 molecules to 600. The
interfacial ice-water free energy can be estimated by
using
the expression of Classical Nucleation Theory for the size
of
the critical cluster (we obtained a value of around
29mN/m in
good agreement with experimental reported values).
After
determining the interfacial free energy, the free energy
barrier for nucleation of ice can be estimated. The free
energy barrier varies from 500kT at -15 Celsius to about
300kT at -20 Celsius. These high barriers strongly
suggest
that homogeneous ice nucleation is extremely unlikely
above
-20 Celsius and that freezing above this temperature
must be
necessarily heterogeneous.[4] The nucleation rate of ice
for
TIP4P/2005 at the locus of maximum compressibility of
supercooled water at room pressure (located on the
Widom
line) is very small so that the maximum in compressibility
in
this model can not be attributed to the transient
formation of
ice [5].
[1] J. L. Aragones, E. G. Noya, C. Valeriani and C. Vega J.
Chem. Phys. 139 034104 (2013).
[2] J. R. Espinosa and E. Sanz and C. Valeriani and C.
Vega
J. Chem. Phys. 139 144502 (2013).
[3] M. M. Conde and M. A. Gonzalez and J. L. F. Abascal
and
C. Vega J. Chem. Phys. 139 154505 (2013).
[4] E. Sanz and C. Vega and J. R. Espinosa and R.
Caballero-
Bernal and J.L.F. Abascal and C.Valeriani J. Am. Chem.
Soc.
135 15008 (2013).
[5] D. T. Limmer and D. Chandler, J. Chem. Phys. 138,
214504 (2013).
Co-authors
C. Valeriani
(Universidad Complutense)
E Sanz
(Universidad Complutense)
E. G. Noya
(Universidad Complutense)
J. L. Aragones
(Universidad Complutense)
J. L. F. Abascal
(Universidad Complutense)
J. R. Espinosa
(Universidad Complutense)
M. A. G. Gonzalez
(Universidad Complutense)
M. M. Conde
(Universidad Complutense)
