Hydrophobic Interactions

Compared to the other major forms of colloidal interaction, the contribution of hydrophobic interactions to emulsion stability has largely been ignored by emulsion scientists. Nevertheless, this type of interaction is of great importance in many types of foods and has recently been shown to promote droplet flocculation in protein-stabilized emulsions (Monahan et al. 1996, Demetriades et al. 1997b). Hydrophobic interactions are important when the surfaces of the droplets have some nonpolar character, either because they are not completely covered by emulsifier (e.g., during homogenization or at low emulsifier concentrations) or because the emulsifier has some hydrophobic regions exposed to the aqueous phase (e.g., adsorbed proteins). The origin of the hydrophobic interaction is the ability of water molecules to form relatively strong hydrogens bonds with each other, but not with nonpolar molecules (Chapter 4). Consequently, the interaction between nonpolar substances and water is thermodynami-cally unfavorable, which means that a system will attempt to minimize the contact area between these substances by causing them to associate (Tanford 1980, Israelachvili 1992, Israelachvili and Wennerstrom 1996, Alaimo and Kumosinski 1997). This process manifests itself as a relatively strong attractive force between hydrophobic substances dispersed in water and is responsible for many important phenomena which occur in food emulsions, such as protein conformation, micelle formation, adsorption of surfactants, and the low water solubility of nonpolar compounds (Chapter 4).

One of the major reasons that hydrophobic interactions were ignored in the past is that there were no theories available to predict their magnitude and range. The complex nature of their origin, which depends on changes in the interactions and structural organization of a large number of water molecules in the vicinity of the nonpolar groups, means that it is extremely difficult to develop mathematical theories from first principles (Israelachvili 1992, Paulaitis et al. 1996). Nevertheless, recent advances in the development of sensitive instruments for measuring the forces between macroscopic bodies have enabled researchers to develop empirical equations to describe the magnitude and range of hydrophobic interactions (Israelachvili and Pashley 1984, Pashley et al. 1985, Claesson 1987, Claesson and Christenson 1988, Rabinovich and Derjaguin 1988). These experiments have shown that the hydrophobic interaction between nonpolar surfaces is relatively strong and long range and that it decays exponentially with surface-to-surface separation. Considerable progress in understanding the nature of hydrophobic interactions has also been achieved using computer simulations (Paulaitis et al. 1996).

The interdroplet pair potential between two emulsion droplets with hydrophobic surfaces separated by water is given by (Israelachvili and Pashley 1984):

FIGURE 3.19 An attractive hydrophobic interaction arises between emulsion droplets when their surfaces have some hydrophobic character, where ^ is approximately equal to the fraction of the droplet surface which is nonpolar.

where y is the interfacial tension between the nonpolar groups and water (typically between 10 to 50 mJ m-2 for food oils), ^ is a parameter that varies between 0 and 1 which takes into account the fact that only part of the droplet surface is hydrophobic, and is the decay length of the interaction (typically between 1 to 2 nm) (Israelachvili 1992). This equation indicates that the magnitude of the hydrophobic interaction increases as the surfaces become more hydrophobic (i.e., ^ tends toward unity). Experiments have shown that for bare nonpolar surfaces, the hydrophobic attraction is stronger than the van der Waals attraction up to separations of 80 nm (Israelachvili 1992).

When hydrophobic surfaces are covered by amphiphilic molecules, such as small-molecule surfactants or biopolymers, the hydrophobic interaction between them is effectively screened and the overall attraction is mainly due to van der Waals interactions (Israelachvili 1992). Nevertheless, hydrophobic interactions are significant when the surface has some hydrophobic character (e.g., if the surface is not completely saturated with emulsifier molecules, if it is bent to expose the oil molecules below [Israelachvili 1992], or if the emulsifier molecules have some hydrophobic regions exposed to the aqueous phase [Demetriades et al. 1997b]). Experiments have shown that the hydrophobic interaction is not directly proportional to the number of nonpolar groups at a surface, because the alteration in water structure imposed by nonpolar groups is disrupted by the presence of any neighboring polar groups (Israelachvili 1992). Thus it is not possible to assume that ^ is simply equal to the fraction of nonpolar sites at a surface. As a consequence, it is difficult to accurately predict their magnitude from first principles.

Hydrophobic interactions become increasingly strong as the temperature is raised (Israelachvili 1992). Thus, hydrophobic interactions between emulsion droplets become more important at higher temperatures. Because the strength of hydrophobic interactions depends on the magnitude of the interfacial tension, any change in the properties of the solvent which increases the interfacial tension will increase the hydrophobic attraction. The addition of small amounts of alcohol to the aqueous phase of an emulsion lowers y and therefore reduces the hydrophobic attraction between nonpolar groups. Electrolytes which alter the structural arrangement of water molecules also influence the magnitude of the hydrophobic effect when they are present at sufficiently high concentrations (Christenson et al. 1990). Structure breakers tend to enhance hydrophobic interactions, whereas structure promoters tend to reduce them (Chapter 5). Variations in pH have little direct effect on the strength of hydro-phobic interactions, unless there are accompanying alterations in the structure of the water or the interfacial tension (Israelachvili and Pashley 1984).

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