What are hydrophobic interactions?

The term "hydrophobic interaction" (or hydrophobic) refers to the tendency of nonpolar compounds to self-associate in an aqueous environment. Such self-association is not driven by mutual attraction or what is sometimes incorrectly referred to as "hydrophobic bonding." Self-association minimizes the disruption of energetically unfavorable interactions between the surrounding water molecules.

    Since hydrogens from nonpolar groups—such as the methylene groups of hydrocarbons—do not form hydrogen bonds, they affect the structure of the water around them. Water molecules adjacent to a hydrophobic group are restricted in the number of orientations (degrees of freedom) that allow them to participate in the maximum number of energetically favorable hydrogen bonds. The maximum formation of multiple hydrogen bonds, which maximizes enthalpy, can only be maintained by increasing the order of adjacent water molecules, with an accompanying decrease in entropy.

    The second law of thermodynamics establishes that the optimum free energy of a hydrocarbon-water mixture is a function of both the maximum enthalpy (due to the formation of hydrogen bonds) and the minimum entropy (maximum degrees of freedom). Thus, nonpolar molecules tend to form droplets in order to minimize exposed surface area and reduce the number of water molecules whose freedom of movement is restricted. Similarly, in the aqueous environment of the living cell, the hydrophobic portions of biopolymers tend to be entombed within the molecular structure or within a lipid bilayer, which minimizes contact with water.