The Hydrophobic Effect Causes Nonpolar Molecules to Adhere to One Another

Because nonpolar molecules do not contain charged groups, possess a dipole moment, or become hydrated, they are insoluble or almost insoluble in water; that is, they are hy-drophobic. The covalent bonds between two carbon atoms and between carbon and hydrogen atoms are the most common nonpolar bonds in biological systems. Hydrocarbons— molecules made up only of carbon and hydrogen—are virtually insoluble in water. Large triacylglycerols (or triglycerides), which comprise animal fats and vegetable oils, also are insoluble in water. As we see later, the major portion of these molecules consists of long hydrocarbon chains. After being shaken in water, triacylglycerols form a separate phase. A familiar example is the separation of oil from the water-based vinegar in an oil-and-vinegar salad dressing.

Nonpolar molecules or nonpolar portions of molecules tend to aggregate in water owing to a phenomenon called the hydrophobic effect. Because water molecules cannot form hydrogen bonds with nonpolar substances, they tend to form "cages" of relatively rigid hydrogen-bonded pentagons and hexagons around nonpolar molecules (Figure 2-9, left). This state is energetically unfavorable because it decreases the randomness (entropy) of the population of water molecules. (The role of entropy in chemical systems is discussed in a later section.) If nonpolar molecules in an aqueous environment aggregate with their hydrophobic surfaces facing each other, there is a reduction in the hydrophobic surface area exposed to water (Figure 2-9, right). As a consequence, less water is needed to form the cages surrounding the nonpolar molecules, and entropy increases (an energetically more favorable state) relative to the unaggregated state. In a sense, then, water squeezes the nonpolar molecules into spontaneously forming aggregates. Rather than constituting an attractive force such as in hydrogen bonds, the hydrophobic effect results from an avoidance of an unstable state (extensive water cages around individual nonpolar molecules).

Nonpolar molecules can also associate, albeit weakly, through van der Waals interactions. The net result of the hydrophobic and van der Waals interactions is a very powerful tendency for hydrophobic molecules to interact with one another, not with water. Simply put, like dissolves like. Polar molecules dissolve in polar solvents such as water; nonpolar molecules dissolve in nonpolar solvents such as hexane.

▲ FIGURE 2-9 Schematic depiction of the hydrophobic effect. Cages of water molecules that form around nonpolar molecules in solution are more ordered than water molecules in the surrounding bulk liquid. Aggregation of nonpolar molecules reduces the number of water molecules involved in highly ordered cages, resulting in a higher-entropy, more energetically favorable state (right) compared with the unaggregated state (left).

Waters released into bulk Nonpolar Highly ordered solution substance water molecules

Waters released into bulk Nonpolar Highly ordered solution substance water molecules

Unaggregated state: Aggregated state: Water population highly ordered Water population less ordered

Lower entropy; energetically Higher entropy; energetically unfavorable more favorable

Unaggregated state: Aggregated state: Water population highly ordered Water population less ordered

Lower entropy; energetically Higher entropy; energetically unfavorable more favorable

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