porous materials

Water in hydrophobic and hydrophilic channels

~ moleculardreams ~ Leave a comment

Water. Always difficult to write something original about it, but let’s spend again a few words in celebration of this molecule. Water is present, or can be inserted in many porous hosts, like zeolites or MOFs. Not all of them love water. This time the question was: what does water do inside channels of similar size but different hydrophilicity?

We modelled the behaviour of water in two porous materials. The first one is zeolite L, which is hydrophilic. The second one is a metal organic framework, or MOF, which has pores of similar size, but less affinity to water. Our starting point was the X-ray structure of the two materials, shown below.

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X-ray structure of  ZL-MOF and zeolite L, viewed perpendicular to the channel axis. In both materials, the water positions (cyan spheres) are partially occupied

The water distribution inside the pores looks very nice and symmetric. Unfortunately, the water positions are only partially occupied. So, by using the experimental water content as input, we optimized the structure of these materials, and here’s what we got.

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Optimized structures of ZL-MOF (left) and zeolite L (right), viewed along the channel axis. The hydrophobic methyl groups of the MOF (in gray) protrude inside the channel and force the water molecules to arrange in well-separated rings. In the hydrophilic channels of zeolite L, a continous water structure is formed.

We found that water stabilizes both materials, and that the shape of the water clusters inside the channels depends on the affinity of the hosts to water.
While the hydrophobic host contains water rings, kept together by water-water hydrogen bonds, the hydrophilic host contains a continuous water tube, stabilized by interactions with the zeolite and also by hydrogen bonds.

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Optimized structures of ZL-MOF (left) and zeolite L (right) (front view). While the water rings in the MOF are dominated by water-water hydrogen bonds, the water molecules in zeolite L can interact very strongly also with the potassium cations (purple spheres) and the framework.

In the zeolite channels, some water molecules are surrounded by five strong hydrogen bonds. This structure is similar to water pre-dissociation complexes found in liquid water, and it might probably explain the high proton activity found in zeolite L.

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Water rings in ZL-MOF (left), and water pre-dissociation complex (in red) in zeolite L (right)

Zeolite L is a promising material for solar cell applications, but the high proton activity inside the channels might damage some of the organic dyes that are incorporated as guests. Now we have identified a possible cause of the problem, and this might be a first step to improve the performances of these materials. Also, we hope that the atomistic insight on the water rings inside the MOF could help to exploit this material as host matrix for new compounds.

Personally, I much enjoyed doing this work: there’s always something to learn about confined water! The “driving force” for starting this work was an invitation, so many thanks to Michael Fischer and Robert Bell for organizing the Special Issue “Modelling Crystalline Microporous Materials” in the Zeitschrift für Kristallographie. If you like porous materials and #compchem, please have a look at this issue, it has many beautiful contributions.  Also, thanks to ChemRxiv for hosting our preprint,  and to the 2019 Twitter #RSCPoster conference, where this work was first presented as  poster.

High pressure and small spaces create order from disorder

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Our work on ethanol and water in ferrierite, published here and blogged in my previous post,  has been recently covered by MRS Bulletin in an excellent news article – “High pressure and small spaces create order from disorder”  by science writer Tim Palucka. Some time ago, I had a very pleasant communication with Tim about the main ideas and results of the paper. That interview also helped me a lot to understand how science communication is done professionally. The piece by Tim really does a great job in explaining the scientific background, the main findings and the perspectives of our research – and, of course, all of us are so happy about it!

MRS Bulletin contains other interesting news articles, which are very useful to get a first impression about what’s going on in the many diverse areas of materials science –  we’re very proud to be featured there! Big thanks, therefore, to MRS Bulletin and Dr. Palucka for the awesome coverage, and to Prof. Gion Calzaferri for commenting on our work as an external expert. A pdf version of the news article is freely available at MRS Bulletin (Volume 42, Issue 3, pp. 176-177, DOI: https://doi.org/10.1557/mrs.2017.38 ), while the illustration showing the arrangement of water and ethanol in the zeolite is just here below:
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Many thanks also to my institution, @Uni_ Insubria, for issuing a piece on our research and sharing it on the social media. The Uni_Insubria release – including also an English translation, can be found at this Facebook link  (Italian version also at: Chemistry & Earth Science Department of Uni Modena-Reggio Emilia – that we thank as well).

Under pressure, from chaos to order.

~ moleculardreams ~ 4 Comments

What happens to a liquid mixture when it is driven by pressure into an initially empty container? What if the container has an ordered pattern of molecular-sized pores? To answer this question, we prepared a sort of good vodka drink – three parts water, and one ethanol – and we injected it into the pores of an hydrophobic container – the zeolite ferrierite. As hydrophobic materials  don’t like water and don’t care about drinks, we had to be very drastic:  we used a diamond anvil cell. In this apparatus, the sample – the empty container and the mixture, in our case –  is compressed between the tips of two opposing diamonds and experiences huge pressures – about 10.000 times the normal atmospheric pressure.  At these conditions,  matter is subjected to unimaginable forces, comparable to internal atomic forces: this means that strange, unexpected phenomena could show up. Now, let’s combine the power of high-pressures  with the ordering effect of the pore matrix and see what happens to our mixture.

Just to start with, the water-ethanol mixture – the pressure-transmitting medium – enters the pores of the matrix.  But how do the molecules occupy the pores? You don’t need to be a chemist to know how it is difficult to separate alcohol from water. This is a critical issue also for sustainable processes – such as the production of biofuels.

Thanks to high pressure and to the porous matrix – and with the help of computational modeling – here we obtained the separation of ethanol from water, and the formation of a beautiful pattern of clusters.  The clusters – rows of ethanol dimers, and square water tetramers – occupy different regions of the host matrix and alternate like tiles forming a nice molecular mosaic – a “two-dimensional architecture” – inside the porous host.  What’s really exciting about it is that the ordered pattern, created by high pressure, also remained stable by bringing the material back to atmospheric pressure.  This means that using high pressures and porous hosts, we can create new materials, which are stable at normal conditions, and could potentially be exploited in applications.

The metamorphosis of the initial water-ethanol solution into a beautiful two-dimensional pattern remains somewhat mysterious. More in general, how organization arises from chaos is still one big question in science.  However, our molecular dynamics simulations show that water molecules, already inside the pores, can spontaneously self-organize in square tetramers:

The final result, is the formation of the stable two-dimensional architecture of water and ethanol clusters. As the movie shows, the molecules move, but the clusters do not break apart. – even upon returning to room pressure.

Perspectives

Disclosing the way in which molecules and nanoparticles assemble at high pressure conditions, under the guidance of a suitable matrix would be a great and intriguing challenge for future studies. Another one would be the actual production of technologically relevant materials through the combined use of pressures and suitable porous matrices.  These goals could be achieved only through a close collaboration between experiment and theory – a synergy which has been at the very origin of the present work.

In a wider perspective, understanding the behavior of matter at high pressures is  of central relevance in science, as explained in this excellent introductory feature article.  Pressure effects are ubiquitous, in chemistry, physics, earth and planetary sciences, as well as in many industrial processes and technological applications.  High-pressure conditions are also hypothesized to explain the origin of complex chemistry and life. The study of this exotic regime, so different from our everyday-life, may reveal plenty of phenomena which would be hard to imagine based on our experience.

Reference: Irreversible Conversion of a Water–Ethanol Solution into an Organized Two-Dimensional Network of Alternating Supramolecular Units in a Hydrophobic Zeolite under Pressure, by Rossella Arletti, Ettore Fois, Lara Gigli,  Giovanna Vezzalini, Simona Quartieri, and myself. Angewandte Chemie 2017 – DOI:  http://dx.doi.org/10.1002/anie.201610949
http://dx.doi.org/10.1002/ange.201610949

Special thanks to Andrea Stangoni (@andrea_stangoni), author of the cover artwork.  His image summarizes the ideas of our work much more beautifully than my blog post!

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And, of course, big thanks to Angewandte too! 🙂

Here are the official versions of the cover:

http://dx.doi.org/10.1002/anie.201700219
http://dx.doi.org/10.1002/ange.201700219

Hope you enjoyed the movies, both (equilibration and final) available at figshare.

Update:  the green open access (accepted article) version of this paper is now freely downloadable from the figshare repository.