**7. H-bonds of the solvent molecules in the first coordination shell**

Comparison of the number of the solvent molecules in the first and second coordination shells suggests that in aqueous solutions and water rich mixtures, almost any water molecule in the primary shell of Mg2+ has at least two neighbours in the second shell. Such result seems to be consistent with the antidipole orientation of these molecules. The orientation of the water molecules in the first shell of Ca2+ is less restricted therefore it is not surprising that they have three neighbours in the second shell. Number of the second neighbours of Na+ is difficult to determine, because the second shell is not stable and the second peak of the Na+O radial distribution function is lacking.

With increasing methanol content the number of the neighbours in the second shells of Mg2+ and Ca2+ decreases rapidly and in equimolar mixture the water or methanol molecule in the first shell has only one neighbour in the second shell. A question is whether the molecules in the primary shell are hydrogen bonded with those in the second shell or in the bulk.

In MD simulation all pair interactions change continuously as a function of the separation and orientation of the molecules therefore there is no unambiguous definition of the hydrogen bond. Usually two definitions, either energetic or geometric, of the H-bond are considered. The energetic criterion of H-bond, based on the pair interaction energy, treats two molecules as H-bonded, if their interaction energy is less than –8 kJ. mol-1. This criterion coincides with the geometric definition, which considers two molecules as H-bonded, if the distances between two oxygens and between the hydrogen and oxygen of the H-bond acceptor do not exceed 0.350 and 0.250 nm, respectively, and if the angle between the OH intramolecular bond of the H-donor and the line connecting the oxygens is less than 30o (Hawlicka & Swiatla-Wojcik, 1998).

The average numbers of the H-bonds were computed in 0.001 ps intervals over the whole simulation runs. The average number of H-bonds per water molecule in pure water is <nHB>w=3.5. Addition of electrolytes, NaCl, MgCl2 and CaCl2 , reduces slightly this number.

The exchange of the water molecules between the Cl- shell and the bulk is fast. Though the size and composition of the anion shell in solutions of CaCl2, MgCl2 and NaCl are the same, the persistence of the shells is different. In solutions of CaCl2 and MgCl2 the anion shells are more flexible than those in NaCl solutions. In solutions of the alkali earth chlorides about 85% of the water molecules stay in the primary shell of the anion less than 5 ps. This means that the water residence time is shorter than the characteristic time of the water translations, about 6 ps (Hawlicka & Switla-Wojcik, 2000). This explains why

1986). In NaCl solution the lifetime of the anion shell is longer. About 75% of the water

The residence time of the methanol molecules in the Cl- shell is longer than that of the water

50 ps. Shorter lifetime, about 25 ps, has been found for the methanolic solution of NaCl. However even this shortest residence time, 25 ps, exceeds significantly the characteristic time of methanol translations, about 9 ps (Hawlicka & Switla-Wojcik, 2000). This explains why the hydrodynamic radius of Cl- in net methanol is greater than the radius in crystal

Comparison of the number of the solvent molecules in the first and second coordination shells suggests that in aqueous solutions and water rich mixtures, almost any water molecule in the primary shell of Mg2+ has at least two neighbours in the second shell. Such result seems to be consistent with the antidipole orientation of these molecules. The orientation of the water molecules in the first shell of Ca2+ is less restricted therefore it is not surprising that they have three neighbours in the second shell. Number of the second neighbours of Na+ is difficult to determine, because the second shell is not stable and the

With increasing methanol content the number of the neighbours in the second shells of Mg2+ and Ca2+ decreases rapidly and in equimolar mixture the water or methanol molecule in the first shell has only one neighbour in the second shell. A question is whether the molecules in

In MD simulation all pair interactions change continuously as a function of the separation and orientation of the molecules therefore there is no unambiguous definition of the hydrogen bond. Usually two definitions, either energetic or geometric, of the H-bond are considered. The energetic criterion of H-bond, based on the pair interaction energy, treats

coincides with the geometric definition, which considers two molecules as H-bonded, if the distances between two oxygens and between the hydrogen and oxygen of the H-bond acceptor do not exceed 0.350 and 0.250 nm, respectively, and if the angle between the OH intramolecular bond of the H-donor and the line connecting the oxygens is less than 30o

The average numbers of the H-bonds were computed in 0.001 ps intervals over the whole simulation runs. The average number of H-bonds per water molecule in pure water is <nHB>w=3.5. Addition of electrolytes, NaCl, MgCl2 and CaCl2 , reduces slightly this number.

the primary shell are hydrogen bonded with those in the second shell or in the bulk.

two molecules as H-bonded, if their interaction energy is less than –8 kJ.

molecules. In methanolic solutions of CaCl2 and MgCl2 the lifetime of the Cl-

**7. H-bonds of the solvent molecules in the first coordination shell** 

second peak of the Na+O radial distribution function is lacking.

in aqueous solution is like its radius in crystal (Hawlicka,

shell exceeds

mol-1. This criterion

the hydrodynamic radius of Cl-

(Hawlicka, 1986).

molecules stay in the shell about 19 ps.

(Hawlicka & Swiatla-Wojcik, 1998).

In all studied solutions the H-bond numbers are the same, <nHB>w=3.1. Thus this influence is slight and only in CaCl2 solutions it extends beyond the first coordination shells of the ions (Owczarek et al., 2007).

Differences between the electrolyte solutions appear when the H-bonds of the molecules in first coordination shell of the cations are compared. The Mg2+ and Na2+ ions are sixcoordinated and the angular distributions show, that all water molecules are properly oriented to form two H-bonds as H-donors. Though the average number of H-bonds per the water molecule in the Na+ shell is two, [<nHB>w]Na+=2, a detailed analysis shows that about 65% of the water molecules in the Na+ shell form 2 H-bonds, whereas the reminder of them has either one, about 15%, or three, about 20%, H-bonded neighbours. Most of the water molecules in the Mg2+ shell, about 70%, have two H-bonded neighbours, but 30% of the molecules form only one H-bond. Though both cations coordinate six water molecules the radius of Mg2+ shell is smaller, about 0.27 nm, than that of the Na+ shell, about 0.32 nm. Thus the Mg2+ shell must be more compact and there is probably not enough space for Hbonded neighbours of all water molecules. The water molecules in the Ca2+ shell have also less H-bonded neighbours. Most of them, about 80%, has only one H-bonded neighbour and only 20%of the molecules form two H-bonds with their neighbours in the second shell. The radii of Ca2+ and Na+ ions in crystal, 0.096 and 0.102 nm (Marcus & Hefter, 2004) and their shells, 0.34 and 0.32 nm respectively (see Table 3), are similar. The first shell of Ca2+ consists however of 10 water molecules. Though most of them are oriented properly to have the Hbonded neighbours the shell is compact and only 20% of the molecules have enough space for two H-bonded neighbours in the second shell.

In mixed solvent the number of H-bonds per water molecule in the shells of Na+ and Mg2+ ions remains unchanged. Such behaviour might be expected, because neither the coordination number nor the orientation of the molecules depends on the solvent composition. In methanol-water mixtures the water molecules from the first shells of Na+ and Mg2+ prefer the methanol molecules as H-bonded neighbours in the next sphere. Such preference can be understood, because the H-bond between the H-donor water molecule and the H-acceptor methanol molecule is energetically favourable (Palinkas et al. 1991).

As seen from Tables 3 and 4 the number of the molecules in the first and second shells of Ca2+ decrease with the increasing methanol content. The radii of both shells are, however, independent of the mixture composition. Thus the first shell becomes less compact. This improves the orientation of the water molecules as H-donors. In consequence all water molecules have two H-bonded neighbours. However they have water molecules as the Hbonded neighbours despite the unfavourable energy of H-bond between two water molecules. The methanol molecules appear in the first shells of Ca2+ in methanol rich solvents, when there is a lack of water to form the coordination shell. Their antidipole orientation causes that they have only one H-bonded neighbour.

The chloride ion is H-bond acceptor and in aqueous solution the water molecules form almost linear H-bond with Cl- shell. About 80% of the water molecules coordinated by the anion form three H-bonds with the neighbours in the bulk solvent. In mixed solvent the methanol molecules replace the water molecules in the anion shell. The molecules form the linear H-bond with Cl- therefore they are H-acceptors and have only one H-bonded neighbour in the bulk solvent.

MD Simulation of the Ion Solvation in Methanol-Water Mixtures 419

methanol. The influence of the ionic field on the H-bond lifetime is minor. The cationic field slightly stabilizes the persistence of the H-bonds. The lifetime of the H-bonds of the water and methanol molecules in the cation shells is by about 20% longer. The influence of the anion filed is opposite and the lifetime of the H-bonds is slightly shorter than in the bulk

Total spectral densities S() of water and methanol have been calculated as the sum of

3

*kT* <sup>0</sup>

*NN C*

 respectively. The normalized velocity autocorrelation function is defined as follows:

where Nt and N denote the numbers of the time averages and sites, respectively, vj

the velocity of the site j of the kind at the time ti and (0) *Cvv*

experimental half-width, about 260 cm-1 (Roberts et al. 2009).

1 *S S* () () 

where denotes the frequency of vibrations given in wave number (cm-1). The partial

*vv m c S C t ct dt*

where m is mass of the site, c and k denote the light velocity and Bolzmann's constant,

1 1 <sup>1</sup> ( ) () ( ) (0) *N Nt vv ji ji t vv i j C t vt vt t*

Basic frequencies obtained from MD simulation for the liquid BJH water are 1716 cm-1 and 3533 cm-1 for the HOH bending and OH stretching modes. These results agree reasonably with the experimental frequencies of bending, 1670 cm-1, and stretching, 3557 cm-1 (Falk & Walley, 1961). MD simulation reproduces correctly the shape of the density profile. As expected the sharper maximum, with the half-width about 200 cm-1, has been obtained for the bending vibrations. A broader band has been found for the OH stretching. Its half-with, about 300 cm-1 (Hawlicka & Swiatla-Wojcik,1997), is in good agreement with the

MD simulations of the liquid PHH methanol yielded accurate frequencies of the basic modes; 1055 cm-1 for the CO stretching, 1407 cm-1 for the bending of the COH angle and 3342 cm-1 for the OH stretching. These results are in very good agreement with the experimental frequencies, 1029, 1420 and 3337 cm-1, respectively (Lindgren et

Addition of electrolytes does not affect the CO stretching mode of methanol and their influence on bending modes, of methanol COH and water HOH, is minor (Stangret &

  (11)

 (ti) is

represents the normalization

<sup>2</sup> ( ) ( )cos(2 )

() of sites (= O, H, H for water and O, H, Me for methanol):

() have been obtained via Fourier transform of the normalized velocity

 

(9)

(10)

**8. Influence of the ions on intramolecular vibrations** 

solvent.

partial densities S

densities S

factor.

al, 1993).

autocorrelation function:

An interesting question is how does the ionic field influence a strength and persistency of the H-bonds. To describe this effect the average H-bond energy of the molecules in the ion shells was calculated and it was compared with the average H-bond energy computed for the mixed solvent. The average energy of two H-bonded molecules in liquid BJH water, <EHB>w= -17.3 kJ. mol-1, is of about 10% higher than the energy of two H-bonded molecules in liquid PHH methanol, <EHB>w= -19.2 kJ. mol-1. Such feature agrees with the difference of the binding energies of water and methanol dimers (Palinkas et al. 1991). In gas phase the binding energies for the dimer of unlike molecules depends on the configuration of the water and methanol molecules. The H-bond between the H-acceptor methanol molecule and H-donor water molecule is energetically favourable (Palinkas et al., 1991). Therefore the average energy of the H-bond in the methanol-water mixtures decreases with the increasing methanol content (Owczarek et al., 2009). In liquid mixtures the energies of two different configurations are slightly different and the energy of the H-acceptor methanol and Hdonor water is lower by about 7%.

The influence of the anionic field on the strength of H-bonds is negligible and the H-bond energy of the water and methanol molecules, coordinated by the Cl- ions, does not differ from the H-bond energy in the bulk solvent. A lack of the influence can be understood, because the charge density of the chloride ion is small, therefore the anion field does not polarise the solvent molecules.

The charge densities of the cations are higher, particularly of Mg2+, and their field polarises the solvent molecules. In such case a strengthening of the H-bonds might be expected. Indeed the energies of the H-bonds of the water molecules coordinated by the cations are lower than the energy of the H-bonds in the bulk solvent. As might be expected the influence of the Mg2+ field is the strongest one and the H-bond energy is lower by about 20%, than that in the bulk. The H-bond energy of the water molecules coordinated by Na+ and Ca2+ ions is lower, by about 10%, as compared with that in the bulk solvent. The influence of the Ca2+ and Na+ fields seems to be very similar, despite different charge densities, but it is worthy to stress that the second neighbours of these cations are different. The Ca2+ ion favours the water molecules in both shells and such H-bonds are weaker as compared with those between the water molecules in the Na+ shell and its second neighbours, the methanol molecules.

To describe an influence of the ionic field on a persistence of the H-bonds a lifetime of Hbonds of the molecules in the first shells was computed and compared with that in binary solvent. From among various concepts of the H-bond lifetime an approach proposed previously (Rappaport, 1983) was adopted. The concept of so-called 'continuous lifetime' takes into account only the unbroken H-bonds. This means that the H-bond once broken and then renewed is neglected. The lifetime of H-bonds was computed from the time correlation function R(t), defined above by the equation (7). In these calculations N and N denote the number of ions and the H-bonds, respectively, and ij(t) is the step function. If the solvent molecule *j* was H-bonded than ij(t)=1 and otherwise ij(t)=0. The calculations of the R(t) functions were performed for at least 500 randomly chosen initial configurations. The H-bonds were monitored in 0.001 time intervals. This short time interval is consistent with hindered rotations of the solvent molecules (Roberts et al., 2009), which may destroy the Hbond. The R(t) functions can be fitted to the first-order exponential decay.

At room temperature the continuous lifetime of H-bonds in pure water is about 0.3 ps, it increases linearly with the increasing methanol content and reaches about 1.5 ps in pure

An interesting question is how does the ionic field influence a strength and persistency of the H-bonds. To describe this effect the average H-bond energy of the molecules in the ion shells was calculated and it was compared with the average H-bond energy computed for the mixed solvent. The average energy of two H-bonded molecules in liquid BJH water,

the binding energies of water and methanol dimers (Palinkas et al. 1991). In gas phase the binding energies for the dimer of unlike molecules depends on the configuration of the water and methanol molecules. The H-bond between the H-acceptor methanol molecule and H-donor water molecule is energetically favourable (Palinkas et al., 1991). Therefore the average energy of the H-bond in the methanol-water mixtures decreases with the increasing methanol content (Owczarek et al., 2009). In liquid mixtures the energies of two different configurations are slightly different and the energy of the H-acceptor methanol and H-

The influence of the anionic field on the strength of H-bonds is negligible and the H-bond energy of the water and methanol molecules, coordinated by the Cl- ions, does not differ from the H-bond energy in the bulk solvent. A lack of the influence can be understood, because the charge density of the chloride ion is small, therefore the anion field does not

The charge densities of the cations are higher, particularly of Mg2+, and their field polarises the solvent molecules. In such case a strengthening of the H-bonds might be expected. Indeed the energies of the H-bonds of the water molecules coordinated by the cations are lower than the energy of the H-bonds in the bulk solvent. As might be expected the influence of the Mg2+ field is the strongest one and the H-bond energy is lower by about 20%, than that in the bulk. The H-bond energy of the water molecules coordinated by Na+ and Ca2+ ions is lower, by about 10%, as compared with that in the bulk solvent. The influence of the Ca2+ and Na+ fields seems to be very similar, despite different charge densities, but it is worthy to stress that the second neighbours of these cations are different. The Ca2+ ion favours the water molecules in both shells and such H-bonds are weaker as compared with those between the water molecules in the Na+ shell and its second

To describe an influence of the ionic field on a persistence of the H-bonds a lifetime of Hbonds of the molecules in the first shells was computed and compared with that in binary solvent. From among various concepts of the H-bond lifetime an approach proposed previously (Rappaport, 1983) was adopted. The concept of so-called 'continuous lifetime' takes into account only the unbroken H-bonds. This means that the H-bond once broken and then renewed is neglected. The lifetime of H-bonds was computed from the time correlation function R(t), defined above by the equation (7). In these calculations N and N denote the number of ions and the H-bonds, respectively, and ij(t) is the step function. If the solvent molecule *j* was H-bonded than ij(t)=1 and otherwise ij(t)=0. The calculations of the R(t) functions were performed for at least 500 randomly chosen initial configurations. The H-bonds were monitored in 0.001 time intervals. This short time interval is consistent with hindered rotations of the solvent molecules (Roberts et al., 2009), which may destroy the H-

At room temperature the continuous lifetime of H-bonds in pure water is about 0.3 ps, it increases linearly with the increasing methanol content and reaches about 1.5 ps in pure

bond. The R(t) functions can be fitted to the first-order exponential decay.

mol-1, is of about 10% higher than the energy of two H-bonded molecules

mol-1. Such feature agrees with the difference of

<EHB>w= -17.3 kJ.

in liquid PHH methanol, <EHB>w= -19.2 kJ.

donor water is lower by about 7%.

polarise the solvent molecules.

neighbours, the methanol molecules.

methanol. The influence of the ionic field on the H-bond lifetime is minor. The cationic field slightly stabilizes the persistence of the H-bonds. The lifetime of the H-bonds of the water and methanol molecules in the cation shells is by about 20% longer. The influence of the anion filed is opposite and the lifetime of the H-bonds is slightly shorter than in the bulk solvent.

### **8. Influence of the ions on intramolecular vibrations**

Total spectral densities S() of water and methanol have been calculated as the sum of partial densities S () of sites (= O, H, H for water and O, H, Me for methanol):

$$S(\alpha) = \sum\_{\alpha=1}^{3} S^{\alpha}(\alpha) \tag{9}$$

where denotes the frequency of vibrations given in wave number (cm-1). The partial densities S () have been obtained via Fourier transform of the normalized velocity autocorrelation function:

$$\mathbf{S}(\boldsymbol{\alpha}) = \frac{2m\_{\alpha}c}{kT} \int\_{0}^{v} \mathbf{C}\_{vv}^{\alpha}(\mathbf{t}) \cos(2\pi \boldsymbol{\alpha} \mathbf{t}) d\mathbf{t} \tag{10}$$

where m is mass of the site, c and k denote the light velocity and Bolzmann's constant, respectively. The normalized velocity autocorrelation function is defined as follows:

$$\mathbf{C}\_{vv}^{\alpha}(t) = \frac{1}{N\_t N\_{\alpha} \mathbf{C}\_{vv}^{\alpha}(0)} \sum\_{i=1}^{N\_t} \sum\_{j=1}^{N\_{\alpha}} v\_j^{\alpha}(t\_i) \cdot v\_j^{\alpha}(t\_i + \Delta t) \tag{11}$$

where Nt and N denote the numbers of the time averages and sites, respectively, vj (ti) is the velocity of the site j of the kind at the time ti and (0) *Cvv* represents the normalization factor.

Basic frequencies obtained from MD simulation for the liquid BJH water are 1716 cm-1 and 3533 cm-1 for the HOH bending and OH stretching modes. These results agree reasonably with the experimental frequencies of bending, 1670 cm-1, and stretching, 3557 cm-1 (Falk & Walley, 1961). MD simulation reproduces correctly the shape of the density profile. As expected the sharper maximum, with the half-width about 200 cm-1, has been obtained for the bending vibrations. A broader band has been found for the OH stretching. Its half-with, about 300 cm-1 (Hawlicka & Swiatla-Wojcik,1997), is in good agreement with the experimental half-width, about 260 cm-1 (Roberts et al. 2009).

MD simulations of the liquid PHH methanol yielded accurate frequencies of the basic modes; 1055 cm-1 for the CO stretching, 1407 cm-1 for the bending of the COH angle and 3342 cm-1 for the OH stretching. These results are in very good agreement with the experimental frequencies, 1029, 1420 and 3337 cm-1, respectively (Lindgren et al, 1993).

Addition of electrolytes does not affect the CO stretching mode of methanol and their influence on bending modes, of methanol COH and water HOH, is minor (Stangret &

MD Simulation of the Ion Solvation in Methanol-Water Mixtures 421

The change of the OH band of the water molecules coordinated by the calcium ions is dramatic. The OH density profile is very broad; moreover it consists of two bands, centred at 3200 and 2740 cm-1, respectively. As mentioned above the water molecules in the Ca2+ shell show two different orientations: most of 10 water molecules in the Ca2+ shell exhibit the antidipole orientation, but the dipole moment of some molecules is tilted by about 45o. Probably the split of the OH band reflects the vibrations of the water molecules differently

The cation field affects remarkably the stretching OH of the methanol molecules. The OH stretching bands, computed for the cation shells, are broader than that in pure methanol and shifted to lower frequencies. The red-shifts increases in order Na+ (–57cm-1) > Mg2+ (– 432cm-1) > Ca2+(-800 cm-1). One should notice that the OH band for the Ca2+ shell is not split,

The water and methanol molecules, coordinated by the cations, favoured the antidipole orientation towards the ion therefore they form less H-bonds than the molecules in net liquid component. This means that in aqueous solution about 14% of the water molecules in the Na+ shell, more than 40% of the molecules in the Mg2+ shell and about 80% of molecules in the and Ca2+shell have one non-bonded OH group. In methanolic solutions most of the molecules coordinated by the cations have also non-bonded OH group. The frequencies of the non-bonded OH groups of water and methanol are higher. Thus the observed red-shift of the OH frequency of the molecules coordinated by the cations is usually ascribed to the strengthening of the H-bonds (Lindgren et al, 1993; Stangret & Gampe, 2002; Kristiansson & Lindgren, 1995). This might be the consequence of a polarisation (Collahan et al. 2010) of

The BJH water and PHH methanol molecules are flexible thus their geometry and, in consequence, their dipole moments can be changed. Indeed the dipole moments of the water and methanol molecules coordinated by the cations are greater of about 10% than that of the molecules in the net solvents. However the increase of the dipole moment does not influence remarkably the H-bond energy. The Na+ filed does not influence the H-bond energy, whereas the field of Ca2+ and Mg2+ lowers the H-bond energy, but this decrease, by about 20%, thus it cannot be responsible for the large red-shifts observed in MD

MD simulations of the electrolyte solutions improve the understanding of a nature of the solvation in methanol-water mixtures. Despite very similar interactions of the ions with both solvent components the compositions of the ion shells and the bulk solvent may be different. This leads to the conclusion that the solvation of ions does not depend only on the ionsolvent interactions, but it is affected by the interactions between the solvent molecules. Particularly in highly associated solvents a strong tendency to prevent the H-bonded network of the solvent competes with the ion-solvent interactions and it may lead to a

The H-bonds between the H-donor water and H-acceptor methanol molecules are energetically favourable. The molecules in the cationic shells exhibit the antidipole orientation, which favours the H-donor water molecules, whereas the almost linear H-bond

because all methanol molecules prefer similar orientation (see Figure 6 c).

orientated towards Ca2+.

the solvent molecules.

simulation.

**9. Conclusions** 

selective solvation of the ions.

Gampe, 2002). Only stretching OH vibrations of water and methanol are very sensitive to the local environment of the molecules. These modes have been used to investigate the ionic solvation. The experimental spectra are composed, however, of several components. Even in the diluted solution of the electrolyte three components of the OH band must be considered: vibrations of molecules coordinated by cations and anions, as well as the vibrations of the bulk molecules. In concentrated solutions the spectra become more complex, because a solvent shared ion pair cannot be neglected. Therefore it is difficult to interpret the experimental spectra and to deduce the contributions of the anions and cations without additional assumptions. The MD simulation may provide additional information, because the frequency of the OH stretching can be computed independently for the solvent molecules in the ion coordination shells.

Density profiles of the OH stretching bands of water and methanol in the coordination shells of the ions in aqueous and methanolic solutions are displayed in Figure 9. For comparison the OH bands computed for pure water and pure methanol are also shown.

Fig. 9. Density profile of the OH stretching band of water (a) and methanol (b) in net solvents () and coordination shells of Na+ (solid), Mg2+ (dashed), Ca2+() and Cl- (dotted).

As seen the influence of the Cl ion on the OH frequency of water and methanol is minor. Despite almost linear H-bond between the anion and water molecule the red-shift of the OH frequency, observed for the water molecules in the Cl- shell, is minor (OHw = -27 cm-1). Moreover the influence of the Cl- ion on the OH frequency of the methanol is opposite and a small blue-shift (= +25 cm-1) is found.

The influence of the cations on the OH vibrations of the water and methanol molecules is remarkable. The frequencies of the OH stretching of the water and methanol molecules in the cation shells are shifted to lower wave numbers. This agrees with the red-shift of the molecules in the cation shells, deduced from experimental infrared spectra in HDO (Kristiansson & Lindgren, 1995; Roberts et al., 2009) . As seen from Figure 9 the OH bands of water molecules in the Na+ and Mg2+ shells are broader than in pure water and the basic modes are shifted to lower wave numbers by – 125 and 265 cm-1, respectively. Usually the broad OH mode, observed in pure water, is ascribed to large distribution of the configurations of the H-bonds (Lindgren et al, 1993).

Gampe, 2002). Only stretching OH vibrations of water and methanol are very sensitive to the local environment of the molecules. These modes have been used to investigate the ionic solvation. The experimental spectra are composed, however, of several components. Even in the diluted solution of the electrolyte three components of the OH band must be considered: vibrations of molecules coordinated by cations and anions, as well as the vibrations of the bulk molecules. In concentrated solutions the spectra become more complex, because a solvent shared ion pair cannot be neglected. Therefore it is difficult to interpret the experimental spectra and to deduce the contributions of the anions and cations without additional assumptions. The MD simulation may provide additional information, because the frequency of the OH stretching can be computed independently for the solvent

Density profiles of the OH stretching bands of water and methanol in the coordination shells of the ions in aqueous and methanolic solutions are displayed in Figure 9. For comparison the OH bands computed for pure water and pure methanol are also

Fig. 9. Density profile of the OH stretching band of water (a) and methanol (b) in net

solvents () and coordination shells of Na+ (solid), Mg2+ (dashed), Ca2+() and Cl- (dotted).

As seen the influence of the Cl- ion on the OH frequency of water and methanol is minor. Despite almost linear H-bond between the anion and water molecule the red-shift of the OH frequency, observed for the water molecules in the Cl- shell, is minor (OHw = -27 cm-1). Moreover the influence of the Cl- ion on the OH frequency of the methanol is opposite and a

The influence of the cations on the OH vibrations of the water and methanol molecules is remarkable. The frequencies of the OH stretching of the water and methanol molecules in the cation shells are shifted to lower wave numbers. This agrees with the red-shift of the molecules in the cation shells, deduced from experimental infrared spectra in HDO (Kristiansson & Lindgren, 1995; Roberts et al., 2009) . As seen from Figure 9 the OH bands of water molecules in the Na+ and Mg2+ shells are broader than in pure water and the basic modes are shifted to lower wave numbers by – 125 and 265 cm-1, respectively. Usually the broad OH mode, observed in pure water, is ascribed to large distribution of the

molecules in the ion coordination shells.

small blue-shift (= +25 cm-1) is found.

configurations of the H-bonds (Lindgren et al, 1993).

shown.

The change of the OH band of the water molecules coordinated by the calcium ions is dramatic. The OH density profile is very broad; moreover it consists of two bands, centred at 3200 and 2740 cm-1, respectively. As mentioned above the water molecules in the Ca2+ shell show two different orientations: most of 10 water molecules in the Ca2+ shell exhibit the antidipole orientation, but the dipole moment of some molecules is tilted by about 45o. Probably the split of the OH band reflects the vibrations of the water molecules differently orientated towards Ca2+.

The cation field affects remarkably the stretching OH of the methanol molecules. The OH stretching bands, computed for the cation shells, are broader than that in pure methanol and shifted to lower frequencies. The red-shifts increases in order Na+ (–57cm-1) > Mg2+ (– 432cm-1) > Ca2+(-800 cm-1). One should notice that the OH band for the Ca2+ shell is not split, because all methanol molecules prefer similar orientation (see Figure 6 c).

The water and methanol molecules, coordinated by the cations, favoured the antidipole orientation towards the ion therefore they form less H-bonds than the molecules in net liquid component. This means that in aqueous solution about 14% of the water molecules in the Na+ shell, more than 40% of the molecules in the Mg2+ shell and about 80% of molecules in the and Ca2+shell have one non-bonded OH group. In methanolic solutions most of the molecules coordinated by the cations have also non-bonded OH group. The frequencies of the non-bonded OH groups of water and methanol are higher. Thus the observed red-shift of the OH frequency of the molecules coordinated by the cations is usually ascribed to the strengthening of the H-bonds (Lindgren et al, 1993; Stangret & Gampe, 2002; Kristiansson & Lindgren, 1995). This might be the consequence of a polarisation (Collahan et al. 2010) of the solvent molecules.

The BJH water and PHH methanol molecules are flexible thus their geometry and, in consequence, their dipole moments can be changed. Indeed the dipole moments of the water and methanol molecules coordinated by the cations are greater of about 10% than that of the molecules in the net solvents. However the increase of the dipole moment does not influence remarkably the H-bond energy. The Na+ filed does not influence the H-bond energy, whereas the field of Ca2+ and Mg2+ lowers the H-bond energy, but this decrease, by about 20%, thus it cannot be responsible for the large red-shifts observed in MD simulation.
