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Insane in the membrane, part 1:  Oil and water

Page history last edited by Bill Dorland 2 years, 8 months ago Saved with comment



First, imagine a box full of two types of non-interacting gases such as helium (He) and neon (Ne) in equal amounts. 


1. Draw a picture of how you would imagine they would be distributed in a box.





2. Based on what you know about entropy, can you justify why this would be the state that you observe?






In your study of biology, you've probably encountered "hydrophobic interactions": some nonpolar molecules, or nonpolar parts of molecules, seem to be repelled by water.  This phenomena plays an important role in the structure of proteins, and next week we'll see how it relates to the formation of lipid bilayers:  the membranes that form the boundary of all cells (as well as some organelles within cells).  You've also encountered hydrophobic interactions in everyday life when you've observed that oil doesn't mix with water; they separate when you try to put them together.


Today you'll use the Second Law of Thermodynamics to understand where this interaction comes from.


3.  Draw (in a way that makes sense to you) what it would look like at the molecular level (a) for oil and water to mix, and (b) for oil and water to separate (don't worry about correctly representing the molecular structure of the oil, you can just use squares for oil molecules and triangles for water molecules.) 








Oil molecules are basically long hydrocarbon chains. These are nonpolar molecules.  The picture below shows an oil molecule with a carbon backbone (dark circles) and hydrogen atoms (white circles) coming out off of the backbone.




Water, in contrast, is a polar molecule:  the electrons are closer to the oxygen atom than to the hydrogen atoms, and the molecule is bent so that both hydrogens are to one side, so the hydrogen end is positive and the oxygen end is negative. This results in hydrogen bonding: the reason that water is a liquid at room temperature (even though hydrogen and oxygen are gases). The picture below shows water molecules forming hydrogen bonds with one another where the large red circles represent oxygen, and the smaller white circles represent hydrogen.




4.  Oil molecules can appear to get "stuck" inside surrounding water molecules (and, since an oil molecule is considerably larger than a water molecule, quite a few water molecules are required to surround a single oil molecule).  Using what you know about hydrogen bonding between water molecules, draw a picture showing a hydrogen-bonded "cage" of water molecules within which an oil molecule is apparently stuck.






5.  What effect does the formation of these hydrogen-bonded cages around oil molecules have on the entropy of the whole oil/water system (as compared to the situation where nothing "sticks" to anything and everything moves around freely)?  Why?






6.  Compare what will happen to the entropy of the whole system in the following two scenarios: A) if the oil molecules are all clumped together and the clump is surrounded by water and B) if the oil molecules are spread out evenly throughout the water, each individual oil molecule surrounded by water? 







7.  Explain from the perspective of entropy and the Second Law why it is that oil and water are likely to separate.






8.  Your answer to question #2 may have suggested that oil and water would mix due to entropy, but in #7 you found that maximizing entropy would indicate that oil and water separate. How can you reconcile these two different answers?



Note: We have neglected to explicitly consider the energetic interactions between oil and water.  Stay tuned... we'll add those interactions in next week!



Vashti Sawtelle, Ben Geller, Julia Gouvea, and Chandra Turpen 2/1/12

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