Pouring oil on troubled waters – scientists solve secrets of the water-oil interface

August 4, 2008
Pouring oil on troubled waters – scientists solve secrets of the water-oil interface
Understanding the oil-water interface will be useful in many areas, including designing better detergents.

(PhysOrg.com) -- When oil and water are poured together they meet each other head-on to form a strong and rigid boundary between each other, says new research into how interactions between oil and water work, out this week in Physical Review Letters.

This discovery contradicts previous research which suggested that when oil and water meet, a tiny layer of water vapour, invisible to the human eye, forms between them, keeping them apart and creating a weak and fluctuating boundary between the two substances.

Oils are hydrophobic substances which means they repel water, and cannot mix with water. This is illustrated by the way any kind of oil and water remain separate if they are poured into the same cup.

Scientists are interested in understanding exactly how this separation works because these oil-water boundaries play a key role in many chemical and biological processes, from the design of detergents to the function of oily biological membranes, such as the walls of human cells which enclose the watery contents of the cell.

However, analysing the structure of the oil-water interface is very difficult because it fluctuates, moves around and changes as the oil and water themselves move and flow.

The team behind the new study have used computer simulations of water and oil to produce, for the first time, a clear picture of the oil-water interface, without the blurring and lack of clarity that is caused by the movements of the liquids. The computer models show that there is no thin layer of water vapour between the oil and the water as had been predicted – instead the two liquids were shown to be in direct contact with each other along the length of a boundary which was strong and robust, and not weakly fluctuating as expected.

Dr Fernando Bresme from Imperial College London's Department of Chemistry, one of the authors on the new paper, explained the significance of their findings, saying: "This study is one step towards a greater insight into the relationship between oily substances and water at the molecular level - an area of fundamental science which is relatively little-understood, but which has enormous potential for industry, medicine and nanotechnology.

"It was very interesting to see that our results suggest there is no tiny gap between oil and water when they meet. Despite having a reputation for not liking each other, it seems the opposite is true: they may not be able to mix but they come into full contact with each other at a strong interface. Perhaps they like each other more than we previously thought."

Citation: 'Intrinsic Structure Hydrophobic Surfaces: The Oil-Water Interface', Physical Review Letters, online publication 1 August 2008.

Provided by Imperial College London

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4 / 5 (4) Aug 04, 2008
So, the researchers are claiming to have contradicted an older theory based on computer simulations that may or may not accurately describe the system? I can only assume that the researchers have tried to approximate physical observation, right? I mean, I can write a computer program to tell me anything I want to know.

4 / 5 (4) Aug 04, 2008
mattytheory! Excellent news dude! Congratulations on the upcoming Nobel. :) lol

If only I had thought of that.
1 / 5 (1) Aug 05, 2008
mattytheory, you took the words out of my mouth. 1 to you.
not rated yet Aug 06, 2008
Have you read the paper mattytheory? That usually comes first before forming an opinion.
1 / 5 (1) Aug 07, 2008
Frankly, I never heard about air gap between oil and water interface. It would result into large amount of gas occluded by tiny oil droplet emulsion, which is apparent nonsense. But when somebody's sufficiently stupid to pay such "research" from our taxes....
not rated yet Aug 07, 2008
I think you misunderstand Alexa...

It is not so much an 'air gap' but a region of decreased density between the two phases. This region would supposedly have been on the order of 1/2 molecular diameters. The point is that it is very difficult to get an idea of what the interface is like from either experiments or computer simulations because the region of interest is very small and is subject to thermal fluctuations. Where does the water phase end and the oil phase begin?

The original theory is a very old one and not too bad. If oil and water do not mix, what does the interface look like? Naturally you might expect a region of decreased density between the two phases. This has been thought for a long time.

Its also worth adding that this is a very controversial area of research...surprisingly. There have been some interesting papers written in Nature on this topic.
5 / 5 (1) Aug 08, 2008

No, I have not read the paper, as I do not have a subscription to the journal from which this article is cited. However, if you can understand the point that any computer simulation is inherently limited by the programmer's depth of understanding for that which is to be simulated, then you will see why this claim is a bunch of baloney. In logic, conclusions are only as strong as the prepositions upon which they are founded. This is not to say that the conclusions are necessarily false, but, that until you can demonstrate the validity of the assumptions involved, they aren't necessarily correct either. It doesn't matter how close their model is to reality because our understanding of how these two different molecules interact in mass is not complete; there could be some completely unknown effect that cannot be accounted for in their simulations simply because it is not known. Therefore, any conclusions drawn based on these incompletely validated assumptions should be questioned. Though dripping with satire, this is the point I was trying to convey.

I don't think the researchers are right or wrong, I just think it is hasty to base claims on computer simulations.
not rated yet Aug 08, 2008
I'm not sure I understand your comment mattytheory but I will try my best to respond.

There is nothing mysterious about computer simulations of this type. The recipe is very simple:
classical mechanics statistical mechanics thermodynamics

These types of computer simulations have become an integral part of condensed matter physics simply because they work. You can both explain and predict experimental data. This has been the case for decades. If you are acquainted with this field you will understand that the accuracy of the model is not the problem. The models work pretty well infact, this has been demonstrated countless times over the last couple decades. The difficulty is to extract meaningful information from a system with complex dynamics. The main complicating factor are the capillary waves.

How can one probe the 'real' microscopic structure at the interface? Can we quantify the fluctuations in density at the interface? These are the real challenges. Issues to do with the models were resolved a long time ago.

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