Interaction of Water with Nonpolar Solutes The Hydrophobic Effect

The attraction between a water molecule and a nonpolar solute is much weaker than that between two water molecules, because nonpolar molecules are incapable of forming hydrogen bonds (Israelachvili 1992, Evans and Wennerstrom 1994). For this reason, when a

Non-polar organized in solute tetrahedral structure

FIGURE 4.12 Schematic representation of the reorganization of water molecules near a nonpolar solute.

Non-polar organized in solute tetrahedral structure

FIGURE 4.12 Schematic representation of the reorganization of water molecules near a nonpolar solute.

nonpolar molecule is introduced into pure liquid water, the water molecules surrounding it change their orientation so that they can maximize the number of hydrogen bonds formed with neighboring water molecules (Figure 4.12). The structural rearrangement and alteration in the physicochemical properties of water molecules in the immediate vicinity of a nonpolar solute is known as hydrophobic hydration. At relatively low temperatures, it is believed that a "cage-like" or clathrate structure of water molecules exists around a nonpolar solute, in which the water molecules involved have a coordination number of four, which is greater than that of the water molecules in the bulk phase (3 to 3.5) (Israelachvili 1992). Despite gaining some order, the water molecules in the cage-like structures are still highly dynamic, having residence times of the order of 10-11 s (Evans and Wennerstrom 1994). The alteration in the organization and interactions of water molecules surrounding a nonpolar solute has important implications for the solubility and interactions of nonpolar groups in water (Tanford 1980, Dill 1990, Israelachvili 1992, Cramer and Tuhlar 1994, Fennema 1996b).

The behavior of nonpolar solutes in water can be understood by considering the transfer of a nonpolar molecule from an environment where it is surrounded by similar molecules to one where it is surrounded by water molecules (Tanford 1980). When a nonpolar solute is transferred from a nonpolar solvent into water, there are changes in both the enthalpy (A//transfer) and entropy (A5transfer) of the system. The enthalpy change is related to the alteration in the overall strength of the molecular interactions, whereas the entropy change is related to the alteration in the structural organization of the solute and solvent molecules. The overall free energy change (AGtransfer) depends on the relative magnitude of these two contributions (Evans and Wennerstrom 1994):

A ^"transfer = AHtransfer — ^A^transfer (4.7)

The relative contribution of the enthalpic and entropic contributions to the free energy depends on temperature (Table 4.5). An understanding of the temperature dependence of the free energy of transfer is important for food scientists because it governs the behavior of many food components during food processing, storage, and handling. At relatively low temperatures (<15°C), the number of hydrogen bonds formed by the water molecules in the cage-like structure surrounding the nonpolar solute is slightly higher than in bulk water, and so AHJransfer is negative (i.e., favors transfer). On the other hand, the water molecules in direct contact with the nonpolar solute are more ordered than those in bulk water, and so the entropy term is positive (i.e., opposes transfer). Overall, the entropy term dominates, and so the transfer of a nonpolar molecule into water is thermodynamically unfavorable (Tanford 1980, Israelachvili 1992).

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