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.
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.
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.
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.
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.
Overwhelmed with the increasing flow of new scientific discoveries and related literature? You’re not alone. We live in the information overload era: too much to read, too little time, and life is short. Probably we’d need more readable, shorter papers too. Why writing a long one? Perhaps, it might connect disciplines which speak different languages but have much in common. Like material science and mineral science.
Let’s start from the first one.
You can make materials for solar cells, optical devices or medical sensors by trapping molecules or nanoparticles inside a “host”. Once there, molecules are no longer free to move, like in a gas or a liquid. This process, called “confinement”, brings to life new properties, which were not present in the individual molecules and are very useful in technology. Energy transfer or information storage, for instance, are made possible by the organization of the confined molecules.
The regular cavities of zeolites do a great job in organizing guest molecules
Tiny smart objects such as molecular machines, motors and diodes, make good use of self-organization processes, which create order from apparent disorder by exploiting interactions between molecules. This task gets easier when molecules are confined in regular pores. Think of a buzzing swarm of bees, first frantically hovering in the air, and then accommodated in a honeycomb.
Similar to honeycombs, regular patterns of pores like those in zeolites can orderly accomodate small molecules or clusters. But if you want to entrap, say, enzymes, peptides, or large nanoparticles, you must use materials with larger pores. Some porous silicas have large honeycomb channels, while the cavities of metal organic frameworks display an amazing variety of size and shape. With those nice architectures awaiting to be filled, ordering molecules might appear like an easy task.
As you imagine, things are more complex. Perfect order cannot be achieved. All cavities would need to be uniformly occupied by the guests. This is going to be very unlikely, because molecules move a lot even when they’re confined… like bees in a hive.
Molecules in nanocavities are sort of like bees in a honeycomb: they form an organized colony (Artwork: Andrea Stangoni)
About bees, I had direct experience… as a child, I used to observe my dad opening up his hives to inspect them. This gave me the chance to “study” the behaviour of these awesome creatures inside their honeycomb.
Bees do not occupy all hexagonal holes in the frame, and move continuously around, without any apparent pattern. Hence they’re not perfectly ordered. In spite of this, the colony is amazingly organized, and performs an impressive number of complex tasks…. not just honey production!
Similarly, guest molecules confined in porous cages are not rigorously ordered. Yet they are organized, and the resulting host-guest materials can perform useful functions, which were absent in the free molecules. They can, for example, absorb and transfer photons like the antenna systems of plants and bacteria.
Now, the question is: can we improve the organization of the molecules and the performances of the materials? Well, first we should know how the molecules occupy the cavities, their orientation, spacing and so on. Are the guests aligned? Are they attached to the pore walls? What happens if water enters the pores? To find those answers, you should use several different techniques: each experiment will give you some pieces to compose the puzzle. And yes, computational chemistry helps a lot to figure our what happens inside the pores. Yet this remains a very difficult problem.
This is where mineral science might help.
Regular patterns of cages are very common in the mineral world. Not long ago, for example, geologists found in Antartica a mineral with the same structure of zeolite Z-SM5, a well-known and widely used artificial industrial catalyst. That was indeed a big surprise! Natural zeolites are indeed amazing: their pores contain impressively stable structures formed by small molecules and cations. Just look at this water wire:
Water wire found in the channels of a natural zeolite
Contrary to what you’d expect, this chain is incredibly resistant to heat and pressure. First found in a rare mineral, it was named “one-dimensional ice”. But actually, our water wire “melts” at about 340 C inside the mineral framework! This is a great example of organized structure made by Nature. You can find many others: the most famous ones are perhaps gas hydrates. Several silica minerals have hydrate structures, which are also very common in man-made porous materials. Indeed, we should pay more attention to the close links between natural and artificial host-guest materials.
Natural porous minerals, the intriguing organization of their guests, and their response to mechanical stress can be an awesome source of inspiration in the quest of more robust and efficient materials. High pressure experiments with zeolites (and also some MOF’s) have already brought us new organized materials, along with many curious facts. But there’s so much yet to be discovered.
Perhaps, the problem with us (me included) and with our scientific era is that we don’t take enough time to relate with other disciplines. I’ve been so lucky to work with many awesome colleagues from the mineral, chemical and material science communities over the years, and it’s thanks to them that I wrote this review. One thing I learnt is that we should always try building bridges and strenghtening links between different fields because there’s nothing to lose, all to gain from a deeper exchange of ideas.
The title of this post is the literal translation of a proverb. The proverb means that Devil’s pot of wickedness sooner or later will boil – and, as there’s no lid, someone will see its content and reveal the truth. That’s the old innocent idea that, finally, justice will prevail over evil… well, I like it so much I use it as title. Rather than devils, this post is actually about pots and lids – of molecular size, of course.
As that’s not a Masterchef contest at the nanoscale, let’s get rid of the pot for the moment, and call it ‘container’. In the nanoworld there are many such containers, which can be filled with molecules. In this way, you can produce new materials with applications in various areas of technology: from solar energy to sustainability and human health.
Our containers are named zeolites – porous materials which are commonly used as adsorbents and catalysts in various commercial, industrial, and even medical applications as well as in our everyday life. Also, if you fill zeolites with dye molecules, you’ll get materialsable to capture and transfer solar energy very efficiently. You would do it much easier if you first know how their pores look like.
In particular, how do their entrances appear to an incoming molecule? This question is our “step one”, because this information is really hard to get from experiments.
Fortunately, modeling comes to the rescue…. and that’s one of the reasons why I love so much doing #compchem (computational chemistry)!!
Step 2 revealed that the channel openings expose hydroxyl groups, and look somewhat like this:
Entrance of zeolite L channel, showing the terminal -OH groups and the channel accessibility.
Those terminal hydroxils can be condensed with other molecules, carrying specific groups, hence new properties and functionalities. Among them, the possibility of “closing” the pores. Why is it so important?
Zeolites are resistant to heat and pressure, and act as a protective shield around the dye. But every “pot” needs a “lid”: plugging the zeolite pore entrances, so that the dyes, once included, cannot escape into the environment, would further enhance their stability. This has already been done experimentally, by attaching at the channel entrances peculiar molecules nicknamed “stopcocks”. They consist of two “parts”:
the “tail”, which can penetrate zeolite pores;
the “head”, which is too big to enter the pore and remains outside, thus blocking (at least partially) the channel opening.
Two typical stopcocks, one with a small tail, and the other with a long, bulkier tail, are shown below.
Such “molecular stoppers” do indeed a great job in preventing molecules to escape from zeolites. However, there were no clear ideas about how these stoppers were attached to the pore entrance, and how much space they occupied. This knowledge would help finding better “lids” for our zeolite “pots”. How do we get it? Of course by modeling, as sketched in step 3 and 4.
Here’s what we learned:
stopper molecules prefer to bind aluminum sites at the channel entrance;
the tail group always penetrates inside the pore, while the head stays outside;
the extent of blocking depends on the stopcock.
In particular:
– small-tailed stopcocks are like partially opened “lids” : no full closure – bulky-tailed stopcoks behave like “corks”: full closure
So the zeolite pore may be fully sealed by one bulky stopper, like a molecular cork on a Prosecco nano-bottle. On the contrary, one “lid” (small stopper) leaves our “pot” partially opened. Fortunately, there’s enough room to attach a second small stopper to the opening, that can now fully be closed.
And this brings us to step 5…
… which could well be the end of this story, first told some time ago. Thank you for reading it!
Anyway, there’s an epilogue, which is perhaps the nicest part (“dulcis in fundo“). Using such information, obtained from modeling, experimental colleagues recently trapped indigo (that’s, your denim’s blue) in zeolite L, and blocked the channel entrances with two small stopcocks. In this way, they made a new pigment, exceptionally resistant, with an amazingly beautiful blue color. For me #compchemist, that blue was simply….. the color of happiness.
Requiring subscription: i) Indigo in the nanochannels of zeolite L (Woodtli et al, Dyes and Pigments, 2018, 149, 456); ii) Invention of the stopcock (Maas & Calzaferri, Angew Chem 2002, 41, 2284); iii) Official version of our article: Structure of Nanochannel Entrances in Stopcock‐Functionalized Zeolite LComposites,Angewande2015, 54, 11112
When we fill porous materials with dye molecules of the right size, we obtain useful compounds for solar energy technology. These compounds can transfer solar energy efficiently because pores and channels fit to the dyes “like a glove”. In this way, molecules are forced to stay in line, and energy can easily pass from a molecule to the next one in the line. If we knew in detail the structure of the dye arrays, we’d have better chances to improve these compounds.
Unfortunately, the precise positioning of the molecules inside the pores is very hard to determine. Recently, we solved this problem for a class of particularly efficient dyes filling the channels of zeolite L. Key to success was diversity within the team, which favored the combination of multiple techniques involving both experiments and calculations.
The useful properties of these materials arise from the arrangement of dye molecules inside the porous host, which depends on the interactions among molecules and with the porous host. After this work, now it seems we understand a little better these complex materials. Indeed, our dyes are linear, symmetric and fit to the zeolite channels. Yet they adopt a slightly asymmetric positioning to maximize the interactions with the zeolite cations, which stabilize the compound.
Perylene-bisimide dye (cyan) in zeolite L (gray). The purple spheres represent the zeolite potassium cations
This work also suggests some possible ideas to improve these compounds by modifying either the porous container (the “host”) or the dye molecule (the “guest”). In my view, this is also a good example of how computational modeling may help to rationalize experimental results in apparent contrast with each other, yielding a consistent picture of a useful and intriguing material.
Gigli et al. (2018) “Structure and Host–Guest Interactions of Perylene–Diimide Dyes in Zeolite L Nanochannels”J. Phys. Chem. C 122, 6, 3401-3418
TheTwitter Poster Conference is an annual event organized by the Royal Society of Chemistry, which consists of sharing chemical research using tweets. You may take part either by tweeting an image of your poster or by commenting on other poster (doing both is better, in my view). As in a traditional conference, you see great research, are asked interesting questions, meet old friends, and come in contact with new colleagues or potential collaborators. Only, at the twitter conference this happens 24 hours non-stop on global scale; so, it’s a good idea to get there prepared!
I enjoyed so much my 2017 participation that I couldn’t miss the 2018 edition. Of course, it was awesome, and I am grateful to the organizers, the sessions’ chairs, and all participants, particularly those with whom I interacted. Indeed, I have learnt new stuff, seen exciting science, been inspired, without moving from my office and paying any conference fees. Really cannot ask for more. Many thanks to all of you!
Tweeting posters is not trivial. For optimal readability, you should keep into account, for example, that mobile phones have small screens, and that Twitter images are resized and cropped down – so, it would be better to prepare the poster in landscape format. These and other useful tips can be found in this excellent post. I came across it when the event was over, but it would surely be useful in the future.
Below you can find my poster, illustrating the fruitful collaboration between calculations and diffraction experiments at high-pressure conditions.
Besides water and ethanol in ferrierite (discussed in this post), the poster shows our new work on a host-guest compound of zeolite L and fluorenone dye under high pressure. These host-guest materials have excellent optical properties, useful for many applications, from solar cells to sensing in medical technology. Knowing their structure and working principles could help improve their performances – that’s why we try so hard to understand dye-zeolite composites at molecular level.
Basically fluorenone inside the channels of zeolite L forms a molecular ladder, which is very stable at room conditions because the carbonyl groups of the dye interact very strongly with the potassium cations of the zeolite.
Is this peculiar structure also stable under GPa pressures?
According to experiments and simulations, the answer is apparently yes! Our composite maintains its structure, and the interactions between the dye and the zeolite cations become stronger. The exceptional resilience of this material to compression highlights its outstanding mechanical properties. These are important to extend the application of dye-zeolite composites beyond room-pressure conditions.
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:
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!
How can a snake swallow a mouse bigger than its mouth?
Weird as it seems, questions like this emerge very often at the molecular scale. For example, we can fill porous materials with molecules larger than the diameter of the pores: in this way, we may obtain devices for energy and health applications. What makes this useful process possible? Flexibility is the key: both the porous host (the “snake”) and the molecule (the “mouse”) must deform for the process to occur. But here, contrary to the mouse-snake case, cooperation between the two partners is needed.
We captured the passage of a bulky molecule through the very narrow opening of one of these pores. We did this by computer simulations, because it is very hard to get such information experimentally. To get an idea of what we found, you don’t even need to read the paper – and i’m not kidding. Just look at the movie below!
What we’ve seen first, is that the pore is slightly larger at its entrance. This surely helps the molecule to go in.
Second: contrary to the mouse, which would escape the snake as fast as it could, the molecule is indeed “magically” drawn to the pore entrance – by electrostatic forces.
“So what?” – you may say.
Keep in mind that the molecule is still larger than the pore opening. No kind of “fatal attraction” could do the trick, in a world of rigid bodies.
We’ve found that the molecule can pass through the opening and slip inside the pore only because it’s flexible, and its motion is “in tune” with the vibrations of the porous matrix. All this factors cope to make the entrance process more favorable than the exit process – that’s why the molecule gets finally swallowed by the pore, and remains trapped inside the material.
For me, it was very nice to see how bulky molecules manage to pass through narrow openings and travel inside a porous material. But finding out the reason why they stay inside was, probably, even more exciting: because it explains how materials of this kind can form and remain stable. Which is exactly one of the things you may need, in the quest of easier and smarter ways to produce better materials.
As we have to give credit where credit is due, i must confess that i borrowed the mouse-and-snake idea used in this post. But you’ll never know from whom. Me neither: (s)he was an anonymous referee of the paper. I am very grateful to this person: i can hardly imagine a nicest way to sketch our work.
Update:
Many thanks, of course, also to ChemComm for the cover!
Molecular Dreams 