Osmosis questions

Factual Questions
1

Recently, in another thread, the “How Stuff Works” page on ‘reverse osmosis’ was cited. On that page, a couple of assertions are made which I don’t understand. I’m hoping that a good soul or two will clarify things for me.

Assertion #1
“Gore-tex fabric contains an extremely thin plastic film into which billions of small pores have been cut. The pores are big enough to let water vapor through, but small enough to prevent liquid water from passing.”

How does that happen? Isn’t a molecule of water in water vapor the same size as a molecule of water in liquid water? In fact, if anything, might not a molecule of water in water vapor actually be ‘larger’ (i.e. occupy more volume) than a molecule of water in liquid water (by virtue of its higher kinetic energy)?

Assertion #2
“On the salty side (of a semipermeable membrane), some of the pores get plugged with salt atoms, but on the pure-water side that does not happen. Therefore, more water passes from the pure-water side to the salty side, as there are more pores on the pure-water side for the water molecules to pass through.”

What? The reason that osmosis occurs is because the solute plugs the pores of the membranes on the saltier side? That there are more ‘open’ pores on the pure water side? I always thought that the fundamental reason for the movement of solvent across a semipermeable membrane is simply due to statistical or thermodynamic considerations, i.e. there are more possible states, or more disorder, when the solvent is found in on both sides of the membrane rather than finding it localized to one side.

Comments, please?

Thanks!

2

Question 1: I’m utterly unfamiliar with gortex, but inlecture on the properties of water frequently. Water, as a moleclue, has certain elctrostatic properties that keep each molecule in close association with other water molecules (cohesive attraction). This phenomenon is what gives water it incredible surface tension. When you put enough energy (such as heat) into the water, the water molecules eventually have this association (called hydrogen bonding) distrupted (eg boiling). When associations are disrupted, each water molecule is independent and no longer attached( through weak electric charges) to each other. This association explains capillary action, for example.

Is this what gortex is all about- can’t say. But water can have different properties based on these phenomena.

3

Osmosis across a membrane occurs because of statistics. Water molecules pass back and forth across the membrane without hinderence. Fewer molecules pass through from the salty side because there are fewer molecules of water on that side, so the net effect is to dilute the salty side. No plugging is required, and if you think about it plugging a pore wouldn’t cause dilution, because then how would water get through from the dilute side?

4

Thanks.

So, it seems as if #1 may actually be correct. That being said, I’m still not 100% convinced (what you say makes a lot of sense but, does it apply here?)

With respect to #2, looks like at least someone agrees with me. I’m a bit surprised that “How Stuff Works” is, apparently, so off base on this.

5

Sorry for all the typos in my answer, ye gods.

:smack:

6

Yes, #1 is correct, and I concur with IvoryTowerDenizen’s explanation. #2 – well, I suspect that the writer actually knew better, but was trying too hard to present naïve readers with a comprehensible explanation – i.e., dumbed it down.

However, I will say that this plugging does occur in reverse osmosis, and is the leading reason for replacing RO resin beds.

7

Water is a peculiar molecule. One of the few materials that has a solid that is less dense than its liquid phase, and the hydrogen bonding that raise its boiling temperature and give it surface tension.

So a drop of water is pretty cohesive, and will find it hard to squeeze through a small hole, yet free gaseous water molecules from evaporation will pass right through.

8

JoeH20- quick question, if I may:

Your username suggest water is something you like or know a lot about. You registered this month- to answer this question perhaps? If so… how did you find a random question about osmosis out among the big wide Internets?

I’m always amazed (and tickled!) when someone shows up with seemingly the perfect expertise for a random question! :slight_smile:

Anyway- carry on!

9

I registered this week after being introduced to the boards by a long time member, and my username comes from the fact that H2O rhymes with Joe, and because I study chemistry.

This thread just happened to catch my eye, but wasn’t my initial motivation for registering!

So, checking out the new boards as a place to hang out on the net. I guess I could come to be a permanent guest!

10

Welcome JoeH2O! (I like saying your name as Joe2O though :))
Always nice to have more Sciencey types around, I always learn quite a bit from them!

11

Welcome aboard!

12

KarlGauss, I think your prior supposition about osmosis is correct, as far as it goes. The explanation you quote is obviously wrong on the face of it, because the passage of something through a plug doesn’t care which end of the pore you plug. If this reference is going to make up explanations for things whose real explanations seem unappealing, they should at least make up ones that aren’t obviously wrong!

13

Joe pretty much nailed the water molecule part. Since water is highly polar (with distinct positively- and negatively-charged ends) it tends to be “sticky”, more so than most other (non-polar) molecules.

As to the “plugged pores” explanation, that is not so far off-base as it may seem. Rather than visualizing a cork in a bottle, think of it as more like a “check valve”. In an osmotic cell, with a semi-permeable membrane seperating a solute (salt is a good choice) in solution from a purer solvent (water), there will be a net pressure on the “salt side” that is higher than the “water side”, yet water molecules continue (on average) to migrate across the membrane to dilute the salt.

Part of the answer is statistical, as Bill Door pointed out—more water molecules are striking the membrane from the water side than from the salt side, so, there will be a net flow to the salt side.

However, due to the higher pressure, the water molecules on the salt side strike more frequently, and with higher energy, than those on the water side. Another part of the answer is that the salt molecules (which are too large to pass through the pores) strike the membrane and are held in place by the differential pressure, which blocks the pores from that side to passage of the water molecules. On the water side, there is a “Bell” distribution of energies, with some water molecules having more than enough energy to “push” the salt molecules out of the way and pass through.

Since only the higher-energy water molecules make it through, and what we measure as “pressure” is the average energy of a fluid striking the walls of the container, an osmotic cell can build up a substantial pressure difference, and still have water migrating “uphill”.

This is why, in “reverse osmosis”, very high pressures must be maintained to drive the water from the “salt side” to the “water side”.

A very detailed and math-intensive discussion of osmosis may be found at:

There, a significant factor in the discussion is that the solute (salt) dissociates into charged ions, which, while they may be smaller than water molecules, electrostatically attract water “hangers-on”, much like groupies, making their effective diameter much larger than water, and also reducing the net concetration (and partial pressure contribution) of water molecules on the salt side.

As to my qualifications, I have worked in the Water-Treatment field for sixteen years [hence my user name, DHMO (Di-Hydrogen Mon-Oxide, or H2O, Water)], and have an extensive chemistry and engineering background.

14

Your explanation and the pdf monograph are very interesting and very informative. Thanks. A few comments/questions:

In the pdf document, there is mention of the “concentration of solvent”. Although that’s a helpful concept, can we really talk about the “concentration” of a solvent? When you really think about it, it doesn’t make sense, i.e. only solutes can have a “concentration”, no? I know that when I first started thinking about osmosis, whenever I found myself thinking in terms of the “concentration” of water, I figured I must be going down the wrong track.

Although I didn’t understand much of the osmosis monograph (the pdf), I still found it reassuring to see that were lots of different models for considering the mechanism, each offering its own insights.

You said:

This also makes sense. But, there is also a distribution of energies on the salt side. I assume the reason that doesn’t allow for water to pass from salt side to water side is the presence of salt ions blocking the pores in such a way that they can be pushed off the pore by water coming from the water side but held against the pore by water coming from the salt side. Does that make sense? As you said, a “check valve” not a cork. I guess it’s also like a door that can be pushed open in only one direction, with pushing from the other direction shutting it even tighter.

Finally, the ‘bell’ distribution of water molecule energies pushing the door open in only one direction is reminiscent of “Mawwell’s Demon”. Does that mean if we take the notion of the bell distribution of water molecule energies far enough, we may run into paradoxical conclusions (from an entropy or information standpoint)?

Again, thanks!

15

On any subject I have any reasonable expertise in, I find that “How Stuff Works” website is no better than most other websites compiled by non-experts. In other words, frequently off-base, sometimes badly. I wouldn’t consider it a reliable source for anything.

16

In this context, “concentration” refers to the number of particles per unit volume. In a liquid, such as water, that number will depend, in part, on the size of the particles (molecules).

[emphasis added—DHMO]

Of course. In almost any natural phenomenon, there will be a bell-curve distribution of the relevant property. In the diagram provided in the paper [Figure 1, p 6/39], a U-shaped vessel with a semi-permeable membrane at the midpoint, and a concentrated solution on the right will have the water level measurably higher on the right. This gain in potential energy is reflected in the average energy of the molecules.

At some point, equilibrium is established, where the osmotic pressure forcing the water molecules to the salt side, and the increase in potential energy on the salt side forcing water molecules back, is balanced. The energy and thermodynamic properties are complex, and, as can be gleaned from the paper, still in some dispute. The author laments that in common physics classes, the Fact of osmosis is explained, without sufficient explication of the How and Why.

Also, I don’t think there is much cause to worry about a “Maxwell’s Demon” scenario in this case. The distribution of energies on the two sides of the membrane are sufficiently explained by the laws of thermodynamics.

17

Wait, you mean we even have DHMO in our water treatment centers? How can such irresponsible behavior be allowed? Something must be done, think of all the children who might end up exposed to DHMO as a result!

18

dnftwte!
:wink: