Water determines the structure and properties of many animate and inanimate objects, playing an essential role in biochemical processes. It is a universal solvent and a participant in many chemical reactions, including hydrolysis and redox reactions. It is now known that water is not a simple collection of scattered molecules, but a complex associated liquid. Water has structure because of the presence of intermolecular hydrogen bonds. This structure is sensitive to various external influences. These include, for example, electric fields capable of changing the position of individual molecules in the composition of water associates or 'clusters' (1). Changes in the properties of water under the influence of alternating electric fields (AEFs) are the subject of many experimental and model studies due to the prevalence of alternating current sources (2-4). In July 2019, an extensive model study by scientists from the University of Virginia in the USA of the effect of AEFs on the static and dynamic properties of water was published in the Molecular Physics journal.
An electrostatic field is a special form of matter that arises around a stationary electric charge. The field is invisible to the naked eye and impalpable. The field can be represented using field lines or 'lines of force'.
In a liquid state, water molecules form numerous hydrogen bonds (Hbs), thanks to which a unique tetrahedral short-range order appears in the bulk of the liquid. Scientists associate this phenomenon with some anomalous properties of water that distinguish it from other liquids. Hbs arise when an electron-deficient hydrogen atom from one water molecule interacts with the lone electron pair of an oxygen atom from a neighbouring molecule. Each H2O molecule can form four hydrogen bonds owing to the two uncompensated positive charges of the hydrogen atoms (the donors) and the two negative charges of the oxygen atom (the acceptors) (5). On average, one molecule has about 3.6 bonds at room temperature (6), seemingly coordinated in a tetrahedron (7). Due to the presence of Hbs, water molecules are oriented relative to each other and create a three-dimensional network that forms associates or 'clusters'. Hbs in liquid water are highly dynamic, their energy is low and the 'life span' is several picoseconds (10-12 sec.) (5,7,8). Unlike ice crystals, Hbs in liquid water are easily destroyed and form again quickly (9). This process leads to inhomogeneities in the structure of water and an uneven distribution of Hbs throughout the bulk volume. It has been shown that a corresponding dynamic equilibrium is established between ordered structures (associated molecules) and unordered structures (non-associated molecules) (10). This characterises water as an associated heterogeneous two-phase liquid with a short-range order.
Because of the large difference in electronegativity between the hydrogen and oxygen atoms, the electron clouds in an H2O molecule are strongly displaced towards oxygen. Thus, water molecules are polar and have a large dipole moment. This means that their interaction with an external electric field (EF) is strong. Under its influence, the water dipoles are reoriented or 'aligned' in accordance with the direction of the field, leading to structural and dynamic changes in the water. This is further complicated by the competing influences of the external EF and Hbs (11). Studying these changes can help understand the role of EFs in many biological systems, chemical reactions, and technological advances. In 2015, a detailed review of possible scientific and technological applications of the impact of external EFs on water was published (1).
It is well known that applying an external AEF leads to an increase in the mobility of water molecules (5, 12), due to reorientation of the dipoles. This can lead to some Hbs breaking and new ones forming, which, in turn, is associated with a decrease in the degree of water structuredness. The direction of the alternating current field changes twice during each period. For example, the direction changes 100 times at a frequency of 50 Hz. Each change in the field leads to a partial reorientation of the water molecules.
Various models have been developed to study the effect of AEFs on the dynamics of bulk water Hbs. Luzar and Chandler's model of Hb dynamics and reorientation of water molecules was published in 1996 (13), and the Laage and Hynes molecular jump model in 2006 (14). These two models are interrelated but have different perspectives (5,11). On the basis of these models, scientists from the University of Virginia, under the leadership of Dusan Bratko and Alenka Luzar, created a modified model to assess the effect of AEFs on the dynamics of Hbs in bulk water (5). Since it is still difficult to investigate the structure of water and the dynamics of Hbs directly, molecular dynamics (a method of numerical modelling) was used instead of real water. Molecular dynamics is the collective name for a family of computer algorithms used to study the structure and function of atomic or molecular structures. This approach is one of the most important methods in modern chemistry and biology. Molecular dynamics can greatly facilitate the task of understanding the behaviour of water molecules in EFs. In this study Luzar, Bratko, and their colleagues took three rigid water models: one of non-polarisable water (SPC/E) and two of polarisable water (BK3 and SWM4-NDP) (5). The simulation system was a cube with 24.85 Å sides, which contained 512 molecules and had a density of 0.998 g/cm3. The simulation time step was 1 femtosecond (10-15 sec.). The models were 'placed' in a high-frequency one-dimensional AEF with an amplitude of 0 to 0.2 V/Å and a frequency of 100 to 1000 GHz (with a period of 1 to 10 picoseconds). To determine the effect of AEFs on water, the scientists evaluated the degree to which the water molecule 'follows' the field's fluctuations. They also studied the effect of AEFs on the Hb network and the accompanying cleavage rates (5).
Using this approach, it was shown that with a sufficiently strong AEF (more than 0.1 V/Å), the orientation of water molecules as a whole aligns with the direction of the field, i.e. it changes after the direction of the field changes. However, not all molecules 'follow' the field, and the equalisation amplitude decreases as the field frequency increases. Thus, at the AEF intensity of 0.2 V/Å and a frequency of 200 GHz, the maximum alignment is about 60%, whereas at a frequency of 500 GHz it decreases to less than 35%. In other words, the faster the field changes (the higher its frequency is), the fewer water molecules follow it, and most of them will only achieve incomplete reorientation. The delay in the alignment of water molecules with respect to the vibrations of the AEF is because the molecules need time to break Hbs.
The next step of the scientists was to assess the change in the number of Hbs in water under the influence of the AEF. It was found that despite there being a significant reorientation of water molecules under the influence of the AEF, the average number of Hbs decreased only slightly (less than 10%), even under the action of strong AEF. This is an interesting observation, since water molecules cannot be reoriented without breaking at least some of their Hbs (15). The preservation of the Hb count under these conditions may mean that they should break and form almost at the rate that Hbs switch between different proton acceptors. The retention of the number of hydrogen bonds suggests that the tetrahedral structure of water does not change significantly under the influence of AEFs. This assumption was confirmed by the findings of studies on the structural organisation of water based on distribution functions. The results of studying the oxygen-oxygen radial distribution functions and oxygen-triplet angular distributions in a wide range of AEF frequencies and intensities showed only insignificant violations of the hydrogen bond tetrahedral network.
How is it then that the reorientation of water molecules occurs under the influence of an AEF? Scientists have shown that, despite the preservation of the water's structure in its short-range order, under the influence of an AEF the dynamic properties of the system change significantly, and these changes are non-monotonically dependent on the frequency of the AEF and are more pronounced at a higher field intensity. The constant reorientation of water molecules with a change in the direction of the AEF can accelerate the processes of breaking Hbs and switching protons (hydrogen atoms) from one acceptor (an oxygen atom) to another. The rate that Hbs break at rises with an increase in the AEF frequency up to 200 GHz. Above 200 GHz, a further increase in frequency leads to a decrease in the Hb breaking rate and the switching rate constant. This means that molecules can no longer respond to faster field changes. In this case, the AEF duration is not long enough to force the molecules to break and switch their Hbs; therefore, most of them are retained. At AEF frequencies of about 200 GHz, the period of field oscillations is comparable to the average time it takes for a proton to switch from one acceptor to another. In other words, the dynamics of Hbs under the influence of an AEF correlates with the characteristic Hb breaking and switching times. This explains why the Hb switching rate peaks at field frequencies close to 200 GHz (16). At low AEF frequencies, disturbances are too rare, and at high frequencies the duration of the oscillations is too short to cause a significant response. In the latter case, despite an increase in the number of attempts to reorient the water molecules, a reduction in the 'thinking time' (inversely proportional to the oscillation frequency) ultimately leads to less pronounced Hb dynamics.
The influence of an alternating electric field (AEF) on the orientation of dipoles in water models.
The angular distribution of the dipole moments in water is shown relative to the AEF direction as a function of time, yellow and black denote the maximum and minimum probability of molecular alignment respectively. Time along the X-axis is normalised to the AEF period. Solid lines: sin (2πνt), show the AEF phase. The distribution is narrower at a probe frequency of 200 GHz compared to 500 GHz and at a field strength of 0.2 V/Å compared to 0.1 V/Å.
The above tendency is similarly reflected in such parameters related to Hb kinetics as the rates of translational and rotational diffusion in water. Scientists have demonstrated that although each water molecule does not lose its bond with its neighbours, more pronounced Hb dynamics leads to more rapid water molecule diffusion (5).
Thus, having conducted extensive model studies on the effects of AEFs on the structure and dynamic properties of water, scientists from the University of Virginia have made important findings. According to their data, AEFs at a wide range of intensities and frequencies do not likely affect the structure of water in the short-range order: the tetrahedral organisation is preserved. At the same time, AEFs within a certain window of frequencies significantly change the dynamic properties of the system, accelerating the rate of Hb cleavage and switching protons between different acceptors. The 'size' of this window is determined by the average time it takes for a proton to switch from one acceptor to another. Under these conditions, due to the rapid replacement of broken Hbs with new ones, the total number of Hbs when reorienting water dipoles under the influence of AEFs decreases insignificantly. In the future, the team of scientists plans to expand the study to include the effect of AEFs on water molecule dynamics at the air-water interface, where the need for optimal Hb coordination imposes additional restrictions on the orientation of molecules.
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