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Properties of Water and Hydrophobic Effect (Part II)
Water is an immensely important natural solvent as it participates in almost every biological process. The properties of water help direct reaction pathways and in some instances help determine the final three-dimensional structure of the biological molecule (i.e. protein, DNA). There are two important properties of water that you should consider. First of all, water is a polar molecule - the highly electronegative oxygen atom pulls the majority of the electron density away from the hydrogen atoms and towards itself. This gives the hydrogen atoms a partial positive charge and the oxygen atom a partial negative charge. The asymmetric separation of charge gives water a net electric dipole moment, which means water is a polar molecule. Because water is polar, water molecules can bond together via strong intermolecular forces called hydrogen bonds. The small size of the positively charged hydrogen atom of one water molecule allows it to get very close to the partially negative oxygen atom of another. This small distance increases the attractive electromagnetic force and makes the hydrogen bond a strong intermolecular bond. These two properties of water, namely its polarity and its ability to form strong hydrogen bonds with other polar molecules is what leads to the hydrophobic interactions. Due to the polarity of water, water can dissolve other polar molecules because it can readily form hydrogen bonds with them. However, if we place a non-polar molecule into the water, the water molecules will form a cage around the non-polar molecule. This is not a very favorable process because it traps the water molecules and limits their ability to form hydrogen bonds. If we place two non-polar molecules, then those two non-polar molecules will aggregate together. This aggregation process is favorable because it decreases the total surface area of the non-polar molecules and it decreases the number of water molecules that are trapped in the cage. The interactions between the non-polar molecules when they are placed into water is called hydrophobic interactions and this effect is known as the hydrophobic effect.
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