Gorbaty and Bondarenko (2016) in their article ‘Transition of liquid water into supercritical state’ focus on the experimental evaluation of transition of liquid water to the supercritical state. As it has been long believed, liquid and gas can be distinguished from each other if both exist simultaneously below the critical point (Marrone, 2013). This conclusion follows from the fact that there is no qualitative difference between both states of a substance. Similarly, there should be no difference between a dense supercritical fluid and a liquid. In fact, it is strictly defined that there are no quantitative differences between the different states of nature. On the contrary, many facts from the environment cast doubt on this kind of view. The article by Gorbaty and Bondarenko (2016), aims at showing there exist differences between a supercritical fluid, gas, and a liquid.
The author begins by presenting the argument that has been a major contention for many years. In the P-T diagram, the border between the liquid state and the supercritical fluid is considered to be a substance’s critical isotherm. According to the author, there are no explicit changes that have been observed in a state of a substance at crossing the critical isotherm along an isochore, isobar, or a random line. He further asserts that there is no difference between the state of liquid and supercritical fluid. Rather, there is only a conventional boundary. In an attempt to explain the mystery behind this statement, the author finds confusion in the notion that a liquid and a gas can only be distinguished if both forms coexist below the critical temperature, leading to misunderstandings about the supercritical state. The author points out the error in the understanding of the supercritical flow mechanisms. The fact that that lines of maximal fluctuations of thermodynamic quantities diverge above the critical point is a proof that there cannot be a prolongation of the two-phase equilibrium curve into the supercritical region. Citing research from other sources, the author asserts that there exist hydrogen bonds in the supercritical water. The intention of the paper is to answer two important questions: the condition under which the infinite network of hydrogen bonds vanish and the forms of hydrogen bonded finite clusters. Gorbaty and Bondarenko aim at presenting an experimental fact that tsignify changes in the state of a substance near the critical isotherm. The peculiar behavior is discovered in the peculiar behavior of water. According to Elsässer and Van den Akker (2013), the critical temperature of the water is 374oC, which is high if you consider the strong intermolecular bonds between hydrogen molecules. Therefore, it is easy to observe the behavior of hydrogen molecules as water changes from one state to another. Gorbaty and Bondarenko choice to use water in the study is justified since the bonds between hydrogen molecules clearly manifest themselves, providing a powerful probe for one to peep into the changed and reconfigured structure through the different states.
The author conducts an experimental research in which hydrogen molecules are observed as water changes from a liquid into the supercritical flow. According to Elsässer and Van den Akker (2013), the specific feature in the structure of water is that it is tetrahedral nature of arrangement of hydrogen molecules. Thus, each molecule can join the four nearest molecules through hydrogen bonding. This research aims observe the change in the behavior of bonds as temperature rise and reaches the peak. Gorbaty and Bondarenko (2016) define the peak as the point of separation between the vertices of more or less perfect tetrahedrons. It is observed that as water temperature rises, the number of relatively stronger hydrogen bond decreases and the bonds become longer as well as distorted. Moreover, it seems natural that the peak disappears while approaching the critical isotherm. The peak is observed to reappear above the critical isotherm and seems to grow with further increases in temperature. The same observations are made for aqueous solution. At this point, the only conclusion of the peak that can be suggested from the author’s experiment is that it corresponds to the rib of an incomplete or full tetrahedron that is built from the hydrogen-bonded molecules. Moreover, it is possible that the number of hydrogen bonds becomes larger above the critical temperature.
The probability of hydrogen bonding at different temperatures and in a wide range of pressures showed that it decreased in all ranges of temperature. The critical temperature probability amounted to 0.34, which is fairly close to the Stanly and Teixerra calculated value of 0.39 for the diamond crystal-like lattice. As a result, the observation led to the assumption that close to the critical isotherm there is the line of percolation threshold for water network. Further, the experiment revealed that above the percolation threshold, a substance is in the liquid state as a result of the spanning infinite cluster of hydrogen-bonded molecules holding the bonds together. Marre, Roig, and Aymonier (2012) support the observation with their analysis of hydrogen molecules. According to their observation, it is not possible to have such an infinite network of hydrogen bonds in the supercritical state. They assert that it is only clusters that have finite dimensions that can be found in the supercritical fluid. Following from the observations is the fact that the running of the finite cluster of hydrogen molecules leads to the appearance of ‘freely rotating’ monomers and aggregates molecules of water linked by the remaining hydrogen bonds. The explanation furthered in the research show that such aggregates play the effect of cooperativity of hydrogen bonds. Scientifically explaining the phenomenon, Wagner and Schreiner (2015) state that the molecules, as they seek a minimum of free energy, assemble in tetrahedral clusters of finite dimensions, organized in a certain order. This form possibly explains the reappearing of the main indication of the tetrahedral ordering of the water molecules above the critical isotherm. However, the research fails to put into consideration the fact that the probability of hydrogen bonding can be influenced by many other factors other than temperature and pressure. Such factors include cutoff energy, the perfection of the tetrahedral ordering, and the geometry of hydrogen bonds (Marrone, 2013). Nevertheless, these factors have no major impact on the results of the experiment. Actually, they are not considered in similar other experiments as they are assumed to be constants.
After making the assumptions, the author raises one important question –Why had initial studies and calculations not revealed the surprising effect? One of the reasons the author points out is that former methods did not take into consideration other concepts of cooperativity of hydrogen bonds. Therefore, results from such calculations are close to the statistical distribution of molecules differing in the number of hydrogen bonds. This analysis leads to a major conviction from the experiment that, at high supercritical temperatures, the molecules with three or four hydrogen bonds are practically absent. However, these are not observations from the research. There are no observations shown from the computer simulations that indeed these molecules are not there, considering that the probability of bonding of hydrogen molecules above the critical temperature is very low. However, the author is not far wrong in making the assumption. An analysis of a different work by Elsässer and Van den Akker (2015) complements the study assumptions. The author states that there is a second peak of the pair correlation function where all hydrogen molecules bond above 220oC.
The author proceeds to examine other works of similar nature that have been conducted on the similar topic. One of the research attempted to explain the controversy by defining a deep minimum in the temperature trend on the height of the first peak near the critical temperature. The study inferred the assumption that the infinite hydrogen bonds do not exist near the critical point. Clearly, the above discussion illustrates that there is no way the unusual behavior of the experimental pair correlation functions can be explained theoretically or through calculations. In this regard, the article deviates from experimental research to speculative modeling.
Through speculative modeling techniques, the author assumes that hydrogen bonds form a square two-dimensional network with separation between nodes. At ambient temperatures, the author assumes that these separations correspond to the peaks in the correlation function of the incomplete and distorted networks of hydrogen bonds. The first peak, which is 1.41L (diagonal of a square), should be present in the pair correlational factor function of such a distorted network. However, as the temperatures increase, and approaches the critical isotherm, the number of hydrogen bonds decreases. It follows that the square nearest orders become more distorted, and large holes start appearing in the network. The observation explains the consequence of cooperativity effect. In addition, the loss of hydrogen bonds in some places within the network lowers the energy for bonding, making it easier for bonds in the neighborhood to be destroyed. The bonds that remain become longer and bent making the peak at 1.4 L to disappear. Nevertheless, the infinite cluster of hydrogen bonds still exists. The infinite clusters disintegrate above the critical isotherm, which is just below the speculation point leaving behind only non-bonded molecules and H-bonded groups of molecules. The tiny pieces of the initial molecule retain their free borders since they are not stretched during the process. In that case, nothing can prevent the initial structure from being restored. Thus, the peak at 1.41 L reappears. The speculative model is only used to emphasize the findings of the study where the first and second peak mentioned can be explained in the same way. Hydrogen bonds are greatly distorted close to the critical isotherm and as a result, the height of the peak reaches its minimum. At higher temperatures, the finite clusters join together to form the normal geometry. Demonstrations from the picture require the reader to bear in mind that hydrogen bonds and finite clusters have a very short lifetime. Generally, they are not stable at in the different zones of space due to changes in the parameters of thermodynamics. Such a state can hardly be called liquid-like. Thus, the experiment and the speculative model sum up the explanations of the other for the behavior of the experimental pair correlation functions.
There are many pieces of evidence that support the observations and the discussions mentioned in the article. For example, Elsässer and Van den Akker (2015) demonstrate that the temperature derivative of sound velocity in water shows minimums on the critical isotherms in different ranges of pressure. From the study, the coefficient of isotope distribution undergoes something likened to a jump when near the critical temperature. Since reactions in the natural system are slow, the study took 50 days. However, as seen from the natural system, maximums of concentration takes place in the regions of critical isotherm for all the components. Additionally, the reactions can be reversed and still the same maximum appear in the reverse reaction at decreasing temperatures. The observations are important in validating the details of the study, and its practical application in as much as it may serve as a model for supercritical chemical technologies in industries.
To sum up, the results of the study are both helpful and resourceful. They give a unique approach to solving the problem at hand. However, there is room for further development in the study. With the growth in the computational technology and techniques, better conformity with the results of experiments may be achieved. However, at the present state, all the methods used in attempting to reproduce the experimental correlation functions at different ranges of temperatures are not successful. Even the data on the behavior of the molecules at the first peak is controversial. This peak corresponds to the point where hydrogen bonds separate from oxygen atoms, and not breaking of hydrogen bonds as illustrated. In some studies, the height of the first peak increases at some thermodynamic conditions, while in others it decreases (Marrone, 2013). The study barely gives an explanation relating to the controversy. Thus, the conception described in the article is hypothetical and further experimental and theoretical validation is needed.
Elsässer, T., & Van den Akker, H. (Eds.). (2013). Ultrafast hydrogen bonding dynamics and proton transfer processes in the condensed phase (Vol. 23). Springer Science & Business Media.
Gorbaty, Y., & Bondarenko, G. V. (2016). Transition of liquid water to the supercritical state. Journal of Molecular Liquids.
Marre, S., Roig, Y., & Aymonier, C. (2012). Supercritical microfluidics: Opportunities in flow-through chemistry and materials science. The Journal of Supercritical Fluids, 66, 251-264.
Marrone, P. A. (2013). Supercritical water oxidation—current status of full-scale commercial activity for waste destruction. The Journal of Supercritical Fluids, 79, 283-288.
Wagner, J. P., & Schreiner, P. R. (2015). London Dispersion in Molecular Chemistry—Reconsidering Steric Effects. Angewandte Chemie International Edition, 54(42), 12274-12296.