Hydrophobic Effects and the Hydrogen Bond

A large number of soft matter systems exist in aqueous solution, and one of the most common driving forces for assembly in such systems is the hydrophobic effect. Water is a particularly interesting solvent, ubiquitous 

FIGURE 1.6 The hydrogen bond and molecular structures for water, the polar solvent methanol, and the nonpolar solvent hexane. 

If a different polar molecule is placed in water, it also may form hydrogen bonds with the water molecules and is therefore said to be hydrophilic. Nonpolar molecules, however, will not form these bonds, so instead the water molecules must arrange to form a cage-like structure around the nonpolar species. The formation of this water cage is entropi-cally unfavorable as the addition of a nonpolar molecules forces the water molecules to arrange in a more ordered state than they would if they were to enclose a polar molecule. As a result, there is a decrease in the local entropy of the system and therefore an energy cost to incorporating the hydrophobic molecule into the solution. 

FIGURE 1.7 The waxy coating on the surface cuticle of many leaves and flowers produces a hydrophobic surface. Water beads on the surface and runs off easily. 

FIGURE 1.8 An example of an amphiphilic molecule, sodium lauryl sulfate (SDS). This common detergent, found in most shampoos, has a hydrophilic head group and a hydrophobic tail. 

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