Report

Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water

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Science  18 Jul 2003:
Vol. 301, Issue 5631, pp. 347-349
DOI: 10.1126/science.1084801

Abstract

The effects of ions on bulk properties of liquid water, such as viscosity, have suggested that ions alter water's hydrogen-bonding network. We measured the orientational correlation time of water molecules in Mg(ClO4)2, NaClO4, and Na2SO4 solutions by means of femtosecond pump-probe spectroscopy. The addition of ions had no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This result shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.

It is generally assumed that ions dissolved in liquid water have a strong effect on the hydrogen-bond structure of the liquid. Some ions are considered to enhance the hydrogen-bond structure (“structure makers”), others to weaken it (“structure breakers”). During the last century, this concept has been widely accepted and has become textbook knowledge (1-5), and it is assumed to be an important factor determining the solubility of proteins in ionic solutions (6). However, the concept of structure making and breaking is based on macroscopic properties only [e.g., viscosity (7) and entropy of solvation (4)]. Various techniques have been used to study aqueous ionic solutions at a molecular level (8). However, none of these techniques has been able to provide detailed information about the effect of ions on hydrogen bonding in liquid water. With x-ray and neutron diffraction, coordination numbers of the solvation shell of ions and the mean distance from the ions to the waters in the first hydration shell have been determined (9-11). However, these techniques do not give dynamical information and therefore do not provide information on the stiffness of the hydrogen-bond network. The reorientation time of water molecules in the pure liquid has been measured with nuclear magnetic resonance (NMR) (12, 13). However, when applied to ionic solutions, NMR lacks specificity, because the average reorientation time of all the water molecules (8) is measured, meaning that the dynamics of the water molecules in the solvation shell of an ion cannot be distinguished from the dynamics of “bulk” water molecules. The properties of ionic solutions have also been studied with molecular dynamic (MD) simulations, but the results obtained are inconclusive as to the long-range effect of ions on the hydrogen bonding of water molecules. A number of MD results have been supportive of the concept of structure making and structure breaking (14-16), but other MD studies do not confirm this picture (17-20).

To study the effect of ions on the hydrogen-bond interactions in water, we measured the orientational correlation time of bulk water molecules for several aqueous salt solutions using femtosecond mid-infrared pump-probe spectroscopy. With this method, the dynamical behavior of different kinds of water molecules in the liquid can be observed on time scales shorter than the exchange time between the solvation shells and the bulk liquid. We used independently tunable pump and probe pulses with a pulse duration of 200 femtoseconds, a typical energy of 10 μJ (pump) and 1 μJ (probe), and a central wavelength tunable between 2.5 and 4.5 μm. To measure the orientational correlation time, OH groups in the liquid were anisotropically excited by the linearly polarized pump pulse. The polarization of the probe pulse was rotated 45° with respect to the polarization of the pump pulse, and the absorption changes parallel (Δα) and perpendicular (Δα) to the pump polarization were detected simultaneously as a function of delay τ between the pump and probe pulses. The anisotropy is defined as Embedded Image Because the effects of vibrational relaxation are divided out, the decay time of this parameter represents the orientational correlation time of the water molecules (21), which is a measure for the stiffness of the hydrogen-bond network. By measuring at different HDO concentrations, we verified that the influence of Förster energy transfer (22) on the anisotropy decay was negligible.

In a first experiment, we studied the orientational relaxation of aqueous solutions of Mg(ClO4)2. We chose Mg(ClO4)2 for two reasons. First, the absorption bands of the OH/OD groups that are hydrogen-bonded to water molecules and those that are hydrogen-bonded to ClO4 are quite distinct. In Fig. 1, the absorption band of the OD-stretching vibration of a solution of 3 M Mg(ClO4)2 in HDO:H2O is shown. The peak at 2500 cm1 is attributed to bulk OD groups hydrogen-bonded to other water molecules, because it also exists in the OD spectrum for pure HDO:H2O. The other peak, which is located at 2640 cm1, is attributed to OD groups hydrogen-bonded to the anions, because the height of this band depends on the anion concentration and not on the cation concentration (2). Hence, the orientational dynamics of bulk water molecules and of the water molecules hydrogen-bonded to the anion can be studied separately. Second, Mg2+ is considered to be a strong structure-making ion: The viscosity of 1 M Mg(ClO4)2 is about 30% higher than the viscosity of pure water. The Stokes-Einstein expression for orientational diffusion (23) predicts that the reorientation time scales with the viscosity of the liquid. This relation was derived for macroscopic solutes, but has been found to work surprisingly well for molecules embedded in a homogeneous liquid (24). Hence, if the higher viscosity of a Mg(ClO4)2 solution is caused by a long-range enhancement of the hydrogen-bond network (i.e., a more or less homogeneous change in the stiffness of the hydrogen-bond network), the reorientation time of individual water molecules in a concentrated Mg(ClO4)2 solution should be much larger than the reorientation time of water molecules in the pure liquid.

Fig. 1.

Absorption band of the OD-stretching vibration of a solution of 3 M Mg(ClO4)2 (≡ 6 M ClO4) in HDO:H2O. The absorption spectrum shows two maxima due to the presence of two distinct absorption components, namely, OD groups hydrogen-bonded to ClO4 ions, peaking at 2640 cm1, and OD groups hydrogen-bonded to H2O molecules, peaking at 25001 (2). The OD groups bonded to H2O molecules belong to HDO molecules that are either in the bulk or in the solvation shells of the cations and anions with the OD groups pointing away from these ions.

Fig. 2 shows the measured decay of the anisotropy parameter R for 0 M, 1 M, and 3 M Mg(ClO4)2 in HDO:H2O solutions. In these measurements, the OD.. .O band was pumped and probed at the 0 → 1 transition. The decay of R turns out to be independent of the salt concentration, which means that the orientational correlation time τor for the water-bonded OD groups is not affected by the presence of Mg2+ and ClO4 ions. From fits of the data to a single exponential, we found a value of 2.5 ± 0.1 ps at all concentrations. This value agrees well with the orientational correlation time of pure liquid water as measured with NMR (12, 13) and terahertz spectroscopy (25, 26). These results show that there is no measurable effect of a high concentration of Mg2+ and ClO4 ions on the reorientation of water-bonded OD groups, implying that the strength of the hydrogen-bond interactions of these water molecules is not affected by the presence of these ions.

Fig. 2.

Logarithmic plot of the anisotropy parameter R as a function of time delay for different concentrations of Mg(ClO4)2 dissolved in H2O with 4% HDO. The OD stretch vibration is pumped and probed at 4 μm (0 → 1 transition). For each solution, the orientational correlation time of the bulk water molecules is 2.5 ± 0.1 ps. The very fast decay during the first few hundreds of femtoseconds after the pump excitation is caused by a coherent artifact. The data have been fitted monoexponentially in the delay range between 0.5 and 5 ps and are shifted vertically with respect to each other for clarity.

To test whether our findings for Mg(ClO4)2 also hold for other salts, we measured the orientational dynamics of water molecules for solutions of NaClO4 and Na2SO4. For a solution of NaClO4 in HDO:D2O, the OH.. .OD2 and OH.. .ClO4 absorption components are again well separated, and thus the orientational dynamics of the water-bonded OH groups can be studied in a similar selective way as for an aqueous solution of Mg(ClO4)2. In the case of Na2SO4, the OH groups that are hydrogen-bonded to the anion absorb near 3400 cm1, which is similar to the water-bonded OH groups. Therefore, the response of the water molecules that are hydrogen-bonded to other water molecules cannot be spectrally separated from that of the water molecules that solvate the SO42– ion. However, at low concentrations, there are many more water-bonded OH groups than anion-bonded OH groups, so that the measured response will be strongly dominated by the water-bonded OH groups. The results for pure water, 6 M NaClO4, and 1 M Na2SO4 (Fig. 3) show that the anisotropy decay of water-bonded OH groups is not affected by the presence of high concentrations of ClO4, SO42–, and/or Na+ ions.

Fig. 3.

A logarithmic plot of the anisotropy R as a function of delay for different concentrations of NaClO4 and Na2SO4 dissolved in 0.5% HDO:D2O. The pump and probe frequencies are 3400 cm1 and 3150 cm1, corresponding to the 0 → 1 and 1 → 2 vibrational transitions of water-bonded OH groups, respectively. We used the OH stretch vibration as a label. The hydrogen bond in the first excited state of the OH stretch vibration is somewhat stronger than in the vibrational ground state (28). As a result, the reorientational correlation time constant is longer when the 1 → 2 transition is probed than when the 0 → 1 transition is probed (3.2 versus 2.5 ps). The data are fitted to a single exponential and are shifted vertically with respect to each other for clarity.

If not by a structure-making effect, how can some salts, such as Mg(ClO4)2, make liquid water much more viscous without affecting the hydrogen-bond network of the bulk water molecules? Because the viscosity effects are not caused by changes in the bulk liquid, they must be associated with the water molecules that are directly interacting with the ions. The distinct character of the absorption bands of water-bonded and anion-bonded OH/OD groups for solutions of NaClO4 and Mg(ClO4)2 (Fig. 1) allows for a selective study of the orientational dynamics of the water molecules in the first solvation shell of the ClO4 ion. We studied the orientational dynamics of these water molecules by pumping and probing the OH.. .ClO4 band for a solution of NaClO4 dissolved in HDO:D2O. As shown in Fig. 4, the decay of R is much slower for these anion-bonded water molecules, with a time constant of 7.6 ± 0.3 ps. This result implies that the ions with their solvation shells should be regarded as rigid spheres on a picosecond time scale, so that the viscosity can be modeled by describing the solution as bulk liquid water with suspended spheres. At low concentrations, the viscosity of such a liquid with small suspended spheres is given by the Einstein equation (27). It follows from this equation that a 30% increase of the viscosity, as observed for a 1 M Mg(ClO4)2 solution, can be obtained with a solution of 1 M suspended spheres that have a radius of about 3.6 Å. This radius is similar to the radius of an ion and its first solvation shell of water molecules. Hence, the increase in viscosity upon adding ions to liquid water can be fully explained from the rigid nature of the solvation structure formed by the ion and its first solvation shell.

Fig. 4.

A logarithmic plot of the anisotropy R as a function of time delay for a solution of 6 M NaClO4 in 0.5% HDO:D2O for two different combinations of pump and probe frequencies. At a pump frequency of 3400 cm1 and a probe frequency of 3150 cm1, a single exponential decay is observed that represents the orientational dynamics of OH groups bonded to D2O molecules (2). At a pump frequency of 3575 cm1 and a probe frequency of 3325 cm1, a biexponential decay is observed. The initial rapid decay results again from the reorientation of OH groups bonded to D2O molecules. After a few picoseconds, these OH groups have relaxed (29), and the observed anisotropy is purely determined by OH groups that are hydrogen-bonded to ClO4 ions. These OH groups clearly show a much slower reorientation than the water-bonded OH groups. The time constants of the reorientation are determined by a fit of the data to a single exponential in the delay range between 0.5 and 3 ps for the 3400 cm1-pump data (water-bonded OH groups) and between 2.5 and 6 ps for the 3575 cm1-pump data (ClO4-bonded OH groups).

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