We Finally Know Why Oil and Water Don't Mix

CoveredTrax writes "Everyone knows oil and water don't mix. It's a simple concept, sure, but the hydrophobic interactions between fats and water are crucial to the mechanics of microbiology. The weird thing is, the base theories of chemistry suggest that there's no reason oil and water shouldn't mix, even though it's obvious that's not the case. Now there's an explanation: a team of chemical engineers at the University of California, Santa Barbara have defined an equation that measures a compound's hydrophobic character. It's the first such equation of its kind."

Comments

Huh?

[Anonymous Coward]

I read TFA, and I still don't know why oil and water don't mix. Frankly, I don't think these researchers do, either. They seems to have come up with some kind of empirical formula that describes the interactions without really understanding why they are happening.

Re:Huh?

[LordLimecat]

If science didnt believe there was a "why", it wouldnt bother with experiments in the first place. The why is what we are generally after-- what is the cause?

Re:Huh?

[MokuMokuRyoushi]

"Just because" is not an excuse I've ever heard a scientist use. In any case, I think OP actually has something; the discovery is a measurement of what's happening, not an explanation. It seems they still don't know why fat and polar compounds interact the way they do, and in fact are still baffled, because they see no reason for them not to interact normally rather than repel each other. However, we can now measure to what degree it occurs. Anybody want to correct me or clarify? I'm actually pretty interested here.

Re:Huh?

[rufty_tufty]

Not really.
Does gravity work because mass distorts space or because of gravitons? At the heart of it most science doesn't care why, but it does care what.
Now theories are proposed to postulate a why, but they're usually used to encourage more experiments.
Many of the previous whys have been proved wrong, or at least incomplete, bohr model of the atom Newton's universal gravitation, any theory of superconductivity; but it doesn't matter the experiments and results were real and the ideas produced by the models useful.
Can't remember who said this, but Asking why we do science is like asking why we have sex, sure sometimes something useful comes out, but that's not the reason we're doing it.

Re:Huh?

[Anonymous Coward]

The answer to "why" something is, is that God did it. All the time. Every time.

At least according to my grandfather. An overly simplified conversation regarding such went like so:

Him: Helium rises because God did it.
Me: Helium rises because it is lighter than the rest of our atmosphere.
Him: Why is it lighter?
Me: It has less mass.
Him: Why does it have less mass?
Me: Because it does?
Him: So you admit that God did it.

Re:Huh?

[Kjella]

Well there's two meanings:
Q: Why does the apple fall to the ground?
A: Gravity

Q: Yes, you've named the force and given me a formula to calculate it but why does the apple fall to the ground?
A: We don't know, and even if we ever find something more fundamental that explains gravity, then that again won't have a "why".

Science explains the "how", when you derive it from other things we often say "why". But if you want turtles all the way down, there's no "why", no reason the universe is this way and not some other way. It's purely descriptive of the way it is.

Re:Huh?

[LordLimecat]

Reproducibility implies an unchanging cause, that is, a why. If there isnt a cause to something, you will be unable to reproduce it.

Re:Huh?

[maxwell demon]

Well, I wonder how he would have coped with the following:

Him: Why is it lighter?
You: Because it has fewer nucleons.
Him: Why does it have fewer nucleons?
You: Because otherwise it would not be Helium.
Him: Why would it not be Helium?
You: Because we humans defined Helium that way.

Re:Huh?

[Mab_Mass]

Does gravity work because mass distorts space or because of gravitons? At the heart of it most science doesn't care why, but it does care what.

No, it is engineering that really doesn't care about why. Steel is stronger than wood, which is why we build big buildings out of steel and not wood, but to make a building, we don't need to understand why, only how it behaves.

Science is all about explaining why something happens. If someone could determine, conclusively, the mechanism of how gravity works, that would be a major scientific discovery. At that point, then, we'll how to ask why THAT happens, and thus science continues.

Entropy

[vossman77]

As I teach in my biochemistry class it is entropic cost of not separating them that causes their separation, but I have yet to really wrap my head around this study. Nonetheless, here are some links to the original research:

* Abstract: http://www.ncbi.nlm.nih.gov/pubmed/21896718
* PNAS (paywalled): http://www.pnas.org/content/108/38/15699

Re:Entropy

[digitalderbs]

I'd like to add a few points to this useful post, as a related expert.

As implied by the parent post, one of the biggest reason scientists care is because this is a dominating contribution to the folding of soluble proteins--proteins in water. The hydrophic effect has been understood for a long time (half a centery), including the fact that the entropic contribution to the free energy is proportional to the surface area change between two separate oil droplets and one. (This is the a-a(0) term in their equation.)

Their equation further adds contributions for the surface tension of the solvent (gamma) and an exponential decay term for the drying of water between the two two hydrophobic surfaces are they approach each other. Such phenomena have been well characterized in the last ten or so years by molecular dynamics simulations, and this appears to be an experimental confirmation of this effect.

The statement, however, that this paper finally describes the enigmatic hydrophobic effect is a gross PR overstatement.

I read TFA

[bryan1945]

And basically it says van der Waals' theory is wrong, and here is a new equation. That's pretty much it.
Anyone who knows about this stuff want to take a look at the equation, and see if it makes any sense? Not my area.

E(D)= -2i(a-a)e^(D-D)
where:
E = energy
D = distance
a = area of molecule

yeah, well, actually, they do mix.

[cellocgw]

Old news -- maybe you youngsters can't remember:

yes they do

Re:And here I thought

[TeknoHog]

I thought this too. Another practical reason is that the two liquids usually have different densities, so one will tend to float on top of the other.

I'm not sure about the stacking theory though. Long-chain molecules are not exactly straight, at least when in the liquid phase. If they were to stack neatly with each other, you would get a crystal. My impression is that polar interactions are generally stronger, so it is mainly water that squeezes out any non-polar intruders.

Re:I thought the reasons whre obvious?

[cmacdonald]

Right, but try and use those sentences to predict and calculate the magnitude of those forces. How is that going? The reason this seems to be significant is it allows us to model these forces beyond the the explanation of "the oil sticks together". Van der waals forces don't apply accurately here so we don't have a good tool to calculate these things. From the actual publication: "A quantitative and general model is derived for the interaction potential of charged bilayers that includes the electrostatic double-layer force of the Derjaguin-Landau-Verwey-Overbeek theory, attractive hydrophobic interactions, and repulsive steric-hydration forces. The model quantitatively accounts for the elastic strains, deformations, long-range forces, energy maxima, adhesion minima, as well as the instability (when it exists) as two bilayers breakthrough and (hemi)fuse. These results have several important implications, including quantitative and qualitative understanding of the hydrophobic interaction, which is furthermore shown to be a nonadditive interaction." While I wouldn't want to imply it's on the following scale, it's along the lines of the difference between "gravity pulls us down towards the earth" and Newton's Law of Universal Gravitation. Long time listener, first time poster. I apologize for not being able to make new lines somehow.

hang on a minute, I thought oil and water DO MIX.

[StormyWeather]

http://www.newscientist.com/article/dn3408-oil-and-water-do-mix-after-all.html

Re:how is baby formed

[Reverand Dave]

Hey, if you're not part of the solution, you're part of the precipitate.

Re:I thought...

[swanzilla]

FTA:

The model quantitatively accounts for the elastic strains, deformations, long-range forces, energy maxima, adhesion minima, as well as the instability (when it exists) as two bilayers breakthrough and (hemi)fuse. These results have several important implications, including quantitative and qualitative understanding of the hydrophobic interaction, and making Amish bread makers exercise more.

Partial credit awarded.

Let me take a crack at this...

[snoop.daub]

I work in the field on the theory/simulation side, and have actually had dinner and discussed research with Dr. Israelachvili a couple of times. I've only had a chance to skim the paper, but I think I can summarize it pretty well... by the time I've really absorbed it you folks will have moved on to the next shiny new story so I'd better do it now!

First of all, the report claims that the paper is all about how oil and water don't mix and makes a big deal about how we don't know how that works. For simple stuff like say water and a basic hydrocarbon like octane, that's really not true... it's all about what has already been said above, polar vs. nonpolar (electrostatics) and entropy.

Things get more complicated when you want to model something like an extended hydrophobic surface, or the interactions and formation of bilayer membranes like we have in a cell. It's been known from experiments since Dr. Israelachvili's work in the 80's that if you take two such surfaces (usually mica functionalized to make it hydrophobic) and bring them together in water, they will repel each other, up until at some point they very quickly strongly attract, expel the water between them and glue themselves together (also called "cavitation"). This is the sort of data shown in Fig. 2 in the paper. The connection with membrane formation is to describe how two membranes behave when they come close together, they have to do something similar to get close enough to fuse (figure 3).

Figuring out how to describe this behaviour from a theoretical standpoint has been very difficult! We know what all the parts have to be (hydrophobic,electrostatic, steric/Van der Waals, entropic) but haven't been able to put them together in the right way to describe all of the experimental data. What Jacob and his team have done here is found a nice way to 1) describe the hydrophobic interaction between extended surfaces mathematically (the equation above), 2) combine it with all the other parts (figure 4), and 3) show that the equation with a combination of fitted and measured parameters can fit the experimental data pretty well (Table 1). It's very nice work, definitely a step forward in our knowledge of hydrophobic surface and membrane interactions, and I'm going to make sure I study it more carefully soon!

I thought it was because of molecular bonds?

[Digital Vomit]

The weird thing is, the base theories of chemistry suggest that there's no reason oil and water shouldn't mix,

Huh? I thought oil and water don't mix because oils have primarily non-polar covalent bonds vs the ionic bonds in water molecules? That's what I was taught, and a quick google finds that this appears to be the generally accepted answer.