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 hydro- gen bonds (Israelachvili 1992, Evans and Wennerstrom 1994). For this reason, when a
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 (∆Htransfer) and entropy (∆Stransfer) of the system. The enthalpy change is related to the alter-
ation 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 (∆Gtransfer) depends on the relative magnitude of these two contri-
butions (Evans and Wennerstrom 1994):
∆Gtransfer = ∆Htransfer − T S∆ 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 ∆Htransfer 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).
TABLE 4.5
Temperature Dependence of the Enthalpic and Entropic Contributions to the Free Energy Change That Occurs When a Nonpolar Molecule (Benzene) Is Transferred from the Neat Liquid to Water
T (ⴗC) ⌬H (kJ mol–1) –T⌬S (J ⴗC–1 mol–1) ⌬G (kJ mol–1)
15 –0.15 18.89 18.75
25 2.08 17.30 19.38
30 3.16 16.50 19.66
35 4.37 15.55 19.92
Adapted from Baldwin 1986.
As the temperature is raised, the water molecules become more thermally agitated, and so their organization within the cage-like structure is progressively lost, which has consequences for both the enthalpy and entropy contributions. First, some of the partial charges on the water molecules face toward the nonpolar group and are therefore unable to form hydrogen bonds with the surrounding water molecules. Thus, the number of hydrogen bonds formed by the water molecules in the cage-like structure decreases with increasing temperature. At a certain temperature, the number of hydrogen bonds formed by the water molecules in the cage-like structure becomes less than that of bulk water. Below this temperature, the enthalpy associ- ated with transferring a nonpolar molecule into water is negative (favorable to transfer), but above it, it is positive (unfavorable to transfer). The enthalpy term therefore makes an increasing contribution to opposing the transfer of nonpolar molecules into water as the temperature rises. Second, the increasing disorganization of the water molecules surrounding a nonpolar molecule as the temperature is raised means that the entropy difference between the water molecules in the cage-like structure and those in the bulk water is lessened. Thus, as the temperature is increased, the contribution of the entropy term becomes progressively less important. In summary, at low temperatures, the major contribution to the unfavorable free energy change associated with transfer of a nonpolar molecule into water is the entropy term, but at higher temperatures it is the enthalpy term. Overall, the transfer of a nonpolar molecule from an organic solvent into water becomes increasingly thermodynamically unfa- vorable as the temperature is raised (Table 4.5).
The free energy associated with transferring a nonpolar molecule from an environment where it is surrounded by similar molecules to one in which it is surrounded by water molecules has been shown to be a product of its surface area and the interfacial tension between the bulk nonpolar liquid and water (i.e., ∆G = γ∆A) (Tunon et al. 1992). An aqueous solution containing a nonpolar solute can decrease its free energy by reducing the unfavorable contact area between the nonpolar groups and water, which is known as the hydrophobic effect. The strong tendency for nonpolar molecules to associate with each other in aqueous solutions is a result of the attempt of the system to reduce the contact area between water and nonpolar regions and is known as the hydrophobic interaction (Section 3.7). The hydrophobic effect is responsible for many of the characteristic properties of food emulsions, including the aggregation of proteins, the formation of surfactant micelles, the adsorption of emulsifiers at oil–water and air–water interfaces, the aggregation of hydrophobic particles, and the immis- cibility of oil and water.
The strength of the hydrophobic interaction between nonpolar substances in water is affected by the presence of ions in the aqueous phase separating them. Ions can either increase or decrease the structural organization of water molecules in an aqueous solution, depending
on whether they are structure makers or structure breakers (Section 4.4.1). As one of the major driving forces for hydrophobic interactions is the difference in structural organization (entropy) between the water molecules in the immediate vicinity of the nonpolar solute and those in the bulk water, then changes in the organization of the water molecules in bulk water alter its strength (Israelachvili 1992). Structure makers decrease the magnitude of the hydro- phobic interaction and therefore increase the water solubility of nonpolar solutes because the difference in structural organization is reduced, whereas structure breakers have the opposite effect. The strength of the hydrophobic interaction also depends on temperature, increasing as the temperature is raised.
4.5. SURFACTANTS