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."

Re:Huh?

[Anonymous Coward]

you mean like newton and gravity?

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 [nih.gov]
* PNAS (paywalled): http://www.pnas.org/content/108/38/15699 [pnas.org]

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.

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!