How would you explain the difference between van der Waals interactions and hydrophobic interactions?

1 Answer
Dec 29, 2015

The hydrophobic force arises from the disruption of hydrogen bonds between water molecules, whilst van der Waals interactions are the result of instantaneous and induced dipoles .


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The Hydrophobic Force
Let us consider the shape of a water molecule: it is a bent molecule consisting of two #"O - H"# single covalent bonds that are separated by a bond angle of approximately #104.5^@#, with oxygen as the central atom and surrounded by #2# bonding pairs and #2# lone pairs.

Furthermore, the high electronegativity of oxygen means that the #"O - H"# bonds are very polar: it is this polarity, combined with the fact that oxygen has lone pairs, that allows for the existence of hydrogen bonds . These hydrogen bonds allow water molecules to form an extended three dimensional network :

Source: Wikipedia

If a non-hydrogen bonding surface is introduced, such as oil, this network is disrupted. As such, the water molecules redistribute themselves so that as few hydrogen bonds as possible are disrupted. This typically occurs more and more readily as the entropy, #S# of water is lost as it interacts with dissolved substances.

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Van der Waals Interactions

Van der Waals interactions arise from the temporary dipoles that occur due to random movement of electrons in the orbitals - the regions of space they may occupy - within an atom or molecule. The changing distribution of the electron cloud leads to certain areas of the species being slightly positive in charge (#delta^+#) whilst other areas are slightly negative in charge (#delta^-#).

Source: BBC

This instantaneous dipole distorts the electron clouds of local complexes, resulting in induced dipoles withing them. These induced dipoles then induce temporary dipoles in other local complexes. Of course, being temporary, this dipole instantly ceases to be as the movement of electrons causes the electron cloud to be redistributed; however, this results in a new instantaneous dipole and so the induction of new induced dipoles.