Membranes segregate mixtures of molecules into their individual components. This segregation typically requires pore diffusion and relies on the sizes of the molecules being distinct; the size of the pores within the membrane is chosen so that small molecules (e.g., Helium) can pass through it while larger ones (e.g., Neon) cannot. The behavior of these graphene-based membranes does not follow this typical train-of-thought; water and helium are of similar size, yet these membranes allow facile permeation of water but blocks entirely the passage of He.
This is because these membranes do not rely on pore diffusion to segregate the molecules. Water permeates through these graphene-based membranes through an entirely different mechanism that relies on the intermolecular interactions between water molecules (i.e., hydrogen bonded networks of water). One He molecule does not interact strongly with another He molecule, and He molecules do not interact strongly with graphene. Water molecules, however, interact strongly not only with other water molecules but with graphene surfaces. This discrepancy in the intermoleculer and fluid-surface interactions is what fundamentally gives rise to this "strange" behavior of these graphene-based membranes.
This answer is, unfortunately, wrong. Or at best, incomplete. And they say the best way to get the correct answer on the internet is to post an incorrect one. In fact, the fact that this (11-year-old) paper was published to HN last night has been irking me, and I have thrice started, then abandoned, a comment explaining why GO work in this field is mostly useless, and why peoples' hopes in "low cost desalination" are moonshine based on a misunderstanding of the relevant thermodynamics.
First, this article is 11 years old. This is extremely old news. To the best of my knowledge, most of the serious research on GO has fizzled out, except as a random "might as well be pencil shavings" additive to enhance the perceived novelty of bad research. A favorite of mine: "Will Any Crap We Put into Graphene Increase Its Electrocatalytic Effect?" (2020) (https://doi.org/10.1021/acsnano.9b00184). It seems as if the field has moved on to trying to better understand how existing crosslinked polyamide desalination membranes can be better optimized for neutral solute rejection (something that graphene doesn't, and likely won't ever, but very good at separating), as mentioned towards the end of this comment.
The helium parts of your explanation are correct. However, the sections pertaining to water and the explanation of the molecular interactions as a mechanistic model which explains the different behaviors of He and H2O in the paper are wrong. In fact, water does not have strong interactions with graphene or other fullerenes. For example carbon nanotubes, water permeation is modeled as being nearly frictionless.
Water does have strong interactions with itself, true. The permeation of penetrant through a membrane is usually rationalized in terms of its permeability, which can be thought of as the product of how much material is in the membrane phase (as opposed to the external solution -- gas or liquid) and how fast the penetrant moves through the material. He doesn't interact strongly with materials, so it doesn't sorb very strongly into materials. It is also very small and doesn't form transient bonds with other atoms, so it tends to diffuse very quickly as well. Water is very condensable and tends to form stronger interactions with atoms, so it tends to sorb more and diffuse less. You can think of He as a 1-lane 100 MPH highway through a material and water as a 200-foot-wide moving sidewalk, in terms of mass conveyed per time.
However, the tendency of water to form hydrogen-bonded networks is not, strictly speaking, why the membranes in this study behaved the way they did. The actual answer is incredibly simple. The water condenses into and swells the graphene oxide, so the material is physically separated apart and allows water (and anything that water can carry with it) to penetrate through.
These membranes are made out of graphene oxide (GO), not graphene (a different material). GO is (obviously) an oxidized form of graphene. In an ideal model, you can think of GO as a flake of graphene with a bunch of oxide groups around the edge (=O, -OH, -COOH, etc.). Water permeates GO rapidly because it interacts strongly with the terminal oxide groups on the edges of the GO flake and (again, this is the key part) swells the GO flakes apart from one another considerably. The flakes are physically further apart, which allows the water to freely permeate. This mechanism, by which a condensable gas or vapor condense to form a liquid-like phase in a porous solid is known as capillary condensation.
The fact that the flakes are physically spaced further apart also allows other gases to permeate as well. As discussed in the article, the GO membranes are no longer helium-leak-tight when the helium gas is humidified. He permeates the membrane in large part by sorbing into the water which has condensed between the GO flakes (this type of sorption is described by Henry's law) and diffusing as a dissolved gas through the water channels formed through the swollen GO material. In water, the d-spacing of GO (the space between the flakes) goes from 3-5 Å (good for molecular sieving) to 1-2 nm (will let food dye pass through).
This type of separation (which is not necessarily what the authors were trying to do, admittedly) has been mechanistically and mathematically described in the literature for at least 80 years (e.g., for packed plugs of amorphous carbon studied by Barrer). Also, note that the capillary condensation effect observed here is mostly a function of the properties of the penetrant components, not of the GO itself, outside of how strongly GO interacts with the penetrants.
People have spent a lot of time trying to chemically stitch GO flakes together so that they don’t swell as much, but they haven't had much success. Several years ago, a group cast a piece of dry GO in epoxy, and showed that the films being physically constrained by the epoxy can have ion-selective flow edge-on (the membranes in this study are top-down), but this is more of a proof of physical concept than a practical implementation. Reduced GO (GO that's reduced back into just graphene, but now with more defects where the oxide residues were removed) can be used for gas separations and don't swell, but are hard to make (reducing conditions are not good for materials) and not particularly beneficial over polymeric materials. Single-pore graphene is still researched, but I think the interest in it is severely misguided, because:
Even if researchers were to succeed in making a high-flux high-rejection GO or graphene-based membrane for desalination, these properties membranes don’t address the real issues in water treatment. Instead, they are a showy material that appeals to metrics and gets the university PR photographer in the lab, rather than industrial partners.
Some huge issues off the top of my head include: 1) RO membranes for desalination aren't actually that inefficient, the energy cost of desalination is a large, but not even the major, operating expense, and efficiency gains to be had by using high-flux low-friction materials are minor (if we had a thermodynamically ideal desalinator, it would only use 2-3x less energy than existing technology, and GO/CNT/MOFs, etc., are unlikely to provide 200-300% improvements in efficiency) 2) practical membranes needs to be more fouling resistant and easier to clean to maximize productivity and efficiency over their lifespan, 3) any efficiency gains made by high-flux materials aren't really that 3) ultra-high flux materials tend to foul faster even if they can be more easily cleaned, 4) there are fluid dynamic reasons why ultra-high flux membrane materials (i.e., >10x current desalination membranes) are useless (you can look up "concentration polarization"), and 5) there's a much more critically pressing need to improve the rejection of other neutral compounds like urea, pharmaceuticals, NDMA (and other chloramine disinfection byproducts), and boron. Graphene, graphene oxide, carbon nanotube, and MOF based membranes for desalination are almost invariably focused on high salt rejection and high flux, which it turns out isn't really that hard to do. Neutral organic and inorganic molecules, however, tend to pass straight most membranes, requiring post-treatment.
To make a computer analogy, graphene-based membranes in the real-world applications are like several terabytes of ECC RAM hooked up to Babbage’s computation engines. They don't address the real needs or bottlenecks of separation processes.
Some reading for the curious (you will need to figure out how to access). The last two were written mostly by environmental engineers, the first two by chemical engineers.
Helium is very hard to store since it's a noble gas and so monatomic; I think it's smaller even than H2. So storing it (and H2 for that matter) is hard.
Strangely, their material is able to do that while also passing water, H20, which is a much larger molecule. They suggest in the summary that there's more going on than just the size of the porosity. Haven' read more.
Maybe someone else will know how useful this is for He or H2 in solution.. I'd imagine they'd vaporize out.
Might also be useful for drying H2 after electrolysis
And at the same time it will pass a thick oil... all due to surface tension, polarity, etc.
Which is to say don't think about the filter as a geometrical sieve that has holes that pass molecules solely based on their size (either it fits or not).
Plumbing for gaz and water follow different norms and pursue different sealing strategy. This distinction is omnipresent in gasket and screw thread engineering.
Has to do with how to define the diameter of a molecule/atom when you consider how it interacts with other matter.
Atoms are fuzzy since they are mostly electrons moving about, thus there are a few different ways to define diameter depending on how this cloud behaves.
At my work we use a gizmo called a helium mass spectrometer leak detector. We use it to test the hermetic seal of a welded part. It pulls a vacuum on a part or chamber and we spray helium at it and the machine looks for helium leaking through.
The units are weird: atm-cc/sec or torr liters/sec. 1x10-6 torr/liters sec is the limit for a crack large enough to let water through. Most customers require 1x10-7 or better depending. Some really exotic stuff that goes into satellites must seal to 1x10-9 or better. So helium is really small and sneaks past "holes" water and even other gases would not penetrate.
Helium, like hydrogen are really small and can penetrate many common materials, even metal (see hydrogen embrittlement). They're also difficult to remove from a vacuum chamber and even bounce backwards through high vacuum pumps. This is actually used in the leak detector and they measure this "reverse flow" of helium through the turbo vacuum pump into the mass spectrometer. Helium don't give a fuck.
So knowing that helium is really tiny and penetrates lots of things that big fat bulky water cant means this material exhibits exotic properties that are the complete opposite of what we normally observe. That's really interesting for sure.
Do you have a good reference or search terms for that reverse flow thing? It sounds crazy. Searching ["reverse flow" of helium vacuum pump] doesn't seem to be doing anything for me, at least as a layman.
It sort of seems like a bag that can hold a small object, like a golf ball, but something larger like a watermelon would "fall through" the bag without it tearing/breaking.
Pretty interesting stuff but I'm not too aware of the research field, just live in a place that's wet.
Desalination by reverse osmosis requires certain force to push water through the filters which is dictated by osmotic pressure. It is unavoidable.
To imagine it, think about it this way: to overcome osmotic pressure you only need to pump the water above the filter to a certain height. The height you need is only dictated by the magnitude of the pressure and not the filter. And as the water flows, you only need to top up the water column which is your only energy expenditure. The only thing that the filter dictates is how quickly the water will flow -- which is how much filter area you need to filter a certain amount of water in a unit of time. But pumping the water to certain height requires always exactly the same amount of energy regardless of the properties of the filter.
For sea water the pressure is 25-33 bar (at 25C) which translates to column of water 250-330 m high.
In practice building a column 1/3km in height would not be practical so you can start imagining other configurations. For example, you could come up with a piston that is pushed by a spring to maintain roughly constant pressure and you inject water from time to time to maintain piston position.
But you will always be working against this pressure you have to overcome for the sea water to start getting separated from salt in one go, or you could use multiple stages to reduce pressures at the cost of more complex processing. Whatever you do there is minimum cost in energy you have to expend that does not depend on the filter.
But there is still a possibility for cheaper/lower maintenance filters and so studying materials that can pass water and stop everything else can still provide a lot of value.
“We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.”
It seems to suggest the filter is composed of layers spaced exactly to match water molecule size ("two-dimensional" -- meaning space between two layers, "monolayer of water" -- meaning only single layer of molecules of water fits between the sheets).
It seems other molecules can't pass because there is already water blocking passage or the moment the molecule finds itself into filter the channel narrows down ("reversible narrowing of the capillaries in low humidity").
I honestly don't like the terminology used and the way some people try to make themselves sound more intelligent by making sentences more complex than necessary.
Also I am not sure "low humidity" is a good term to use when we are talking monolayers. Humidity is a statistical term (and also assumes presence of air) and what happens in this filter has nothing to do with statistics. Is the space around a molecule other than water "low humidity" region? Dunno... seems a bit forced to me.
You'll note, I hope, that I said "new science" which might mean such a thing is possible. Yes I agree that osmotic pressure is the thing you have to overcome. Unless somehow there's some new understanding of the world that supersedes our knowledge of osmotic pressure.
This "new science" would have to disprove everything we know about thermodynamics. No, not possible in the regular sense of the world.
Sure, we might find everything we know about physics is wrong but if this happens it will most likely serve to explain why things behave certain way rather than give us superpowers to ignore thermodynamics.
Imagine two columns of water in a single U-shaped pipe, with a selectively permeable membrane at the bottom. One column contains salt water. The columns will have unequal levels, because of the osmotic pressure.
If you had a different type of membrane that had a different pressure, you could swap it out and see the water level change.
Repeatedly swapping the two membranes would allow you to pump water up the column, and you could siphon off this energy to create a perpetual motion machine, with some extra energy to spare.
Current seawater desalination technology runs at about 2x the osmotic pressure power consumption, the current membranes require quite some pressure to push even pure water through.
Of course. You want a high enough water flow to get the installation to filter lots of water. You have to have pressure above osmotic pressure. You can increase the flow of water through filter either by increasing pressure or by adding more filters.
"The trends from the analysis presented in this paper show that in both seawater and brackish desalination
we are approaching thermodynamic limitations."
Capilaries don't have to be one dimensional. Capilaries just need to be narrow enough.
Honestly, the wording of this entire thing is a bit forced. IMO the word "capillary" is suggesting that surface tension has anything to do with it which probably does not. They also use "low humidity" to describe a place in the 2d channel where there are other particles. For me "humidity" is a statistical concept and is also supposed to be about proportion of water vapour and air.
