Looking Through the Magnifying Glass at Water

Title: Molecular hydrophobicity at a macroscopically hydrophilic surface

Authors: Jenée D. Cyran, Michael A. Donovan, Doris Vollmer, Flavio Siro Brigiano, Simone Pezzotti, Daria R. Galimberti, Marie-Pierre Gaigeot, Mischa Bonn, and Ellen H. G. Backus

Journal: PNAS

Year: 2019

Article: https://www.pnas.org/content/116/5/1520

Cover image: https://pixabay.com/en/water-on-glass-water-rain-sky-sun-2465919/

Researchers in this article seek to understand how water molecules arrange when in contact with a glass surface, which is important for understanding how it interacts with and can dissolve minerals. Water is a polar molecule, which means that the electrons are held more on the oxygen atom, creating a slight negative charge and leaving a slight positive charge on the hydrogens. This means that it can create complex structures when interacting with other atoms in a solution or on a surface. If the water is attracted to the surface, then it is hydrophilic, but if water is repelled from the surface, then it is hydrophobic. One example that is studied here, is on a piece of glass, which is made of silica.

Here researchers use a spectroscopic technique called sum-frequency generation where researchers generate light of a certain wavelength that allows them to probe the water molecules at the surface. They combine this spectroscopic technique with molecular dynamics simulations which allow them to calculate the movement of the water molecules using their electronic structure. They find that the water is weakly hydrogen bonded at the interface where the water meets the glass, which is like where it meets air.  Since these findings show that the water is not strongly hydrogen bonded to the silica atoms in the glass like often occurs with other dissolved salts, they are able to determine that the water is predominantly hydrophobic at the interface with glass.

Based on the location of the spectroscopic peaks the researchers can tell the type of interaction that is occurring for the OH functional group. For example, the peaks at 3200 cm-1 and 3400 cm-1 are hydrogen-bonded stretches vibration of the OH groups and the peak at 3660 cm-1 is a weakly hydrogen-bonded OH group. This weakly hydrogen bound group indicates the hydrophobic nature of the water surface interaction.

The results of the molecular dynamics simulations can be seen in Figure 1. The researchers can create simulated spectra based on the simulated molecular interactions which allow them to determine that the peaks observed in their spectra are due to a water monolayer at the direct interface with the glass. This is comprised of two distinct water OH groups based on the nature of the interaction with the molecules on the surface of the glass. This powerful technique has allowed researchers to visualize how the hydrophobic interaction arises through individual molecular interactions. This has allowed the researchers to conclude that the water arranges in heterogeneous patches that are hydrophobic in an otherwise hydrophilic surface.

Figure 1: Simulated spectra showing the different conformations of the water glass interactions. The black line is the simulated spectrum, while the green and red show the two different conformations of molecules that gives rise to the overall spectrum observed.

Overall, this research is important for understanding how water is absorbed on mineral surfaces and how these minerals can be dissolved in water. It is also important for our chemical understanding of water-mediated reactions and separating species from water. Water plays a vital role in our life on Earth and in shaping our environment, so it is crucial we understand the chemical nature of these interactions.

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