MD SIMULATION OF CaCl2 SOLUTION IN METHANOL-WATER
MIXTURES.
Emilia Owczarek and Ewa Hawlicka
Institute of Applied Radiation Chemistry, Department of Chemistry
Technical University, Zeromskiego 116, 90-924 Lodz, Poland
Introduction:
The calcium ion plays a key role in many biological processes. It is decisive for
the control of metabolism, muscle contraction and blood clotting in living bodies[1].
Hydration of Ca2+ ion affects a biological activity of many biochemically important
polyelectrolytes (e.g. dextran and heparin). Thus our attention is focused on a hydration
of calcium ion in water and water-alcohols mixtures.
MD simulation is an effective tool to investigate the structural and dynamical properties
of such systems, especially if the experimental methods are not decisive[2].
Potentials and details of the simulation:
The effective potential of ion-ion and ion-solvent molecule interactions may be
represented by the following form:
⎤
⎡
3 ⎢ Q ij
A ij
⎥
(1)
Vij (r) = ∑ ⎢
+
+ Bij ⋅ exp(− C ij ⋅ rij )⎥
n ij
r
j =1 ⎢ ij
rij
⎥⎦
⎣
where rij denotes distance between interacting sites. Qij terms represent the Coulomb
interactions, which are defined by the ion charges and the partial charges of three sites
of the PHH[3] methanol and BJH[4] water molecules. Parameters Aij, Bij and Cij, which
have no physical meaning, describe the non-Coulomb part of the potential energy.
Interaction between Ca2+ and methanol molecule were represented by the recently
derived effective potential [2] while for Ca2+ and water molecule by potential derived
by Probst et al.[5]. The effective potentials reported by Marx et al.[6] and by Hawlicka
et al.[7] respectively, were used for Cl--methanol and Cl--water molecules interactions.
The ion-ion interactions were described by the effective potential elaborated by Probst
et al.[5] and Dietz et al.[8]. All the potential parameters are given in Table 1.
Table 1. Parameters Qij, Aij, Bij and Cij for interactions of ion-solvent and ion-ion.
i α
Qiα
Aiα
Biα
Ciα
n
Reference
-1
n
-1
-1
-1
[kJ Ǻ mol ] [kJ Ǻ mol ]
[kJ mol ]
[Ǻ ]
Ca OW
Ca HW
-1832.6
916.28
-1572.6
626.39
2.5970 × 105
1.2022 × 105
3.4900
6.7900
2
2
5
5
Ca OM
Ca HM
Ca Me
-1667.3
972.58
694.70
-1372.6
933.29
-474.93
2.5970 × 105
8.3273 × 102
5.1660 × 104
3.4900
0.9600
2.7930
2
2
2
2
2
2
Cl OW
Cl HW
916.28
-458.14
9.3400
-68.270
1.1749 × 105
9.0290 × 104
2.6727
4.5420
2
2
7
7
Cl OM
Cl HM
Cl Me
833.61
-486.27
-347.34
127.00
-193.37
6.7657
1.4529 × 105
2.5086 × 104
5.9250 × 105
3.1999
3.3082
3.2984
2
2
2
6
6
6
Ca Ca
Cl Cl
Ca Cl
5557.6
1389.4
2778.8
-15198
-28672
-353.01
2.6010 × 106
9.1704 × 105
3.6608 × 105
4.4870
3.3863
3.0100
6
6
2
2
8
5
Two MD simulations were performed for standard NVE ensemble at 298K. The
concentration of CaCl2 was equal 0.52 and 0.50 M for mixtures containing 5 and 10
mol% of methanol in water, respectively. Thus the periodic cubes contained 400 solvent
molecules, 4 cations and 8 anions. Their lengths were calculated from experimental
densities at 298 K. Initial configurations were obtained by random placement of
particles in the cubic box. Ewald summation was applied for Coulomb interactions, and
the shifted force potential method was used for all non-Coulomb ones. The simulation
time step was 0.25 fs. After about 5 ps of equilibration, simulations of systems were
extended up to 150 ps. Velocities and coordinates of all sites were collected in 1 fs
intervals for half of the total time and in 10 fs intervals for the rest of simulation. In both
simulations the stability of the potential energy was better than 0.1%.
Results and discussion:
The nearest surrounding of the ions in methanol-water mixtures is described by
5 radial distribution functions (OM, OW, HM, HW, Me). The Ca2+O and Cl-O radial
distribution function are depicted in Fig. 1 and 2, respectively. As seen from Fig.1 the
Ca2+OM and Ca2+OW radial distribution function for methanol and water, respectively
are completely different. The Ca2+OW function exhibits a sharp peak at (0.240 ± 0.002)
nm. Its position is the same in both systems, 5 and 10 mol% of methanol in water and is
very close to that in aqueous CaCl2 solution (0.237 ± 0.002) nm. For all systems the
heights of these maxima are comparable. As seen from Fig. 1. the maximum for Ca2+OM
is not observed. This means that the methanol molecules do not enter the calcium ion
coordination shell.
The Cl-OM radial distribution function shows a sharp peak at (0.320 ± 0.002) nm.
This distance is shorter than in methanolic CaCl2 solution (0.327 ± 0.002) nm[2]. The
height of Cl-OM function in methanol-water CaCl2 solution is almost three times as high
as in the case of methanolic CaCl2. As seen from Fig. 2. the Cl-OW radial distribution
function is worse pronounced. This maximum, located at about (0.320 ± 0.002) nm is
significantly lower than in the case of Cl-OM function in spite of much higher
concentration of water than methanol. That leads to the conclusion that methanol
molecules in the Cl- coordination shell are more fixed as compared with water ones.
This is probably due to the hindered rotation.
The first coordination shell of calcium ion consists of 9 or 10 (nW=9.7) water
molecules but it does not contain methanol molecules. The first coordination shell of the
chloride ion is completely different. Even in very diluted solution of methanol (5 mol%)
the coordination shell of Cl- contains a few (nM=1.6) methanol molecules and about 6
water molecules. As the amount of methanol increases the number of methanol
molecules in the Cl- coordination shell doubles (nM=3.0) whereas the number of water
molecules decreases to about 4÷5 (nW=4.4).
18
16
14
12
2+
gCa O(r)
10
8
6
4
2
0
0,0
0,2
0,4
0,6
0,8
1,0
r, nm
Fig. 1. Ca2+O radial distribution function for methanol (solid) and water (dashed) in 5
mol% methanol in water CaCl2 solution.
20
10
-
gCl O(r)
15
5
0
0,0
0,2
0,4
0,6
0,8
1,0
r, nm
Fig. 2. Cl-O radial distribution function for methanol (solid) and water (dashed) in 5
mol% methanol in water CaCl2 solution.
In order to investigate the hydrogen bonds network numbers and lifetime of the
H-bonds were computed separately for whole system and for the ion coordination
shells. A geometric criterion of the H-bond was used. That defines the distance between
oxygens of the nearest neighbours ROO, the distance between the hydrogen and oxygen
of the H-bond acceptor ROH, and the angle φ between the OH intramolecular bond of the
H-donor and the line connecting the oxygens [9].
In the simulated systems the number of H-bonds per one solvent molecule is
independent of methanol concentration and is equal <nHB>M=2 and <nHB>W=3 for
methanol and water, respectively. In calcium hydration shell the significant decrease of
<nHB> is observed. These values are 1.1 and 1.9 for 5 and 10 mol%, respectively. For
the chloride ion coordination shell H-bond numbers are slightly lower than for the
whole solutions. These results <nHB>M=1.7 and <nHB>W=2.4, for methanol and water,
respectively are closer to the average HB-numbers, as compared with the results
obtained for cation.
The average lifetime of H-bonds is equal (0.518 ± 0.002) ps and (0.412 ± 0.002)
ps for 5 and 10 mol%, respectively. The calcium ion field probably hinderes a rotation
of water molecules causing a slight elongation of the H-bond lifetime. The lifetime of
H-bonds in the chloride ion coordination shell is either comparable (for 5 mol%) or
slightly shorter (for 10 mol%) than for whole system. Our results show that the addition
of CaCl2 stabilizes the hydrogen bonds network in methanol-water mixtures [9] and this
effect is more observable in the case of 5 mol% solution.
Acknowledgement
Financial Support of calculations, done in Interdisciplinary Centre for Mathematical and
Computational Modeling (ICM) of Warsaw University, Project Number G27-17, is
gratefully acknowledged.
References:
[1] G. L. Zubay, Biochemistry, Fourth ed., W.C. Brown Publishers, 1998
[2] E. Owczarek, E. Hawlicka, J. Phys. Chem. B, in press
[3] G. Palinkas, E. Hawlicka, K. Heinzinger, J. Phys. Chem., 91 (1987) 4334
[4] P. Bopp, G. Jancso, K. Heinzinger, Chem. Phys. Lett., 98 (1983) 129
[5] M. M. Probst, T. Radnai, K. Heinzinger, P. Bopp, B. M. Rode, J. Phys. Chem. 89
(1985) 753
[6] D. Marx, K. Heinzinger, G. Palinkas, I. Bako, Z. Naturforsch. 46a (1991) 887
[7] E. Hawlicka, D. Swiatla-Wojcik, Chem. Phys. 195 (1995) 221
[8] W. Dietz, W. O. Riede, K. Heinzinger, Z. Naturforsch. 37a (1982) 1038
[9] E. Hawlicka, D. Swiatla-Wojcik, Chem. Phys. 232 (1998) 361