the hydrophobic effect

There are a lot of misconceptions about this effect. The word “hydrophobic” literally means “afraid of water” - but I hate this term - and you know I’m all for anthropomorphizing molecules - that’s not my issue here. Actually, issue(s) - I have a lot of issues :P but, when it comes to hydrophobes , here are a couple…⠀
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Firstly, I don’t think it’s “fair” to call them “afraid,” especially because, as we’ll see, the hydrophobes aren’t the ones with the problem in this relationship…. I prefer the interpretation “water-avoiding” - but even this doesn’t well characterize them - a better description would be water-avoided or water excluded. Because it’s not that hydrophobes are repulsed by water or anything - and water’s not even repulsed by them - they just don’t offer much in the way of favorable interactions. In contrast, water molecules offer lots of attractive interaction opportunities for other water molecules, so water molecules would much rather spend their time hanging out with other water molecules. ⠀
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So, the hydrophobes really don’t have much say in the interactions named after them - instead it’s the water optimizing itself that is the real source of the “hydrophobic effect”⠀
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Water molecules are really “sticky” towards each other because they’re highly polar - basically atoms (like the 2 hydrogens and the oxygen in H₂O) have smaller parts called subatomic particles - positive-charged protons & neutral neutrons in a dense central nucleus with a cloud of negatively-charged electrons whizzing around. Atoms can form bonds by sharing electrons, but they don’t always share fair. Oxygen is much more electronegative (electron-hogging) than hydrogen, so it pulls their shared electrons closer to itself, making the O partly negative (δ-) and the Hs partly positive (δ+). And opposite charges attract, so the H’s of one water molecule can hang out with the O of another. Each water molecule can form up to 4 “hydrogen bonds” (H-bonds) with other molecules. ⠀
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Unlike the strong covalent bonds holding the Hs to the O in each individual water molecule, these inter-molecular bonds are weaker, so they can stick and unstick. As long as water molecules have sufficient energy to temporarily break free of the bonds, the water molecules can move around & explore, breaking and forming interactions with other water molecules as they travel. These water molecules can occupy many different “states” and the term we use to describe this is high entropy (aka “disorder” or “randomness”)⠀
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But they can’t interact readily with hydrophobic molecules, which are characterized by being nonpolar (electrons are evenly distributed so there aren’t partly or fully charged regions) and thus don’t offer tantalizing charge opportunities. So each water molecule that has to be next to part of a hydrophobe has part of its stickiness “hidden” and is limited in its binding opportunities - it can occupy fewer “states” and thus has lower entropy (is less disordered). If those hydrophobes all cluster together, fewer water molecules will have to give up the “better” opportunities offered by water. ⠀

I’ve been talking about hydrophobes as whole molecules - but you can also have molecules where parts are hydrophobic but other parts are hydrophilic (can form positive interactions with water). And this is the case with proteins. The hydrophobic exclusion effect is actually the main driving force for protein folding. ⠀

much more here: http://bit.ly/hydrophobesarenotafraid