Hydration interactions arise from the structuring of water molecules around dipolar and ionic groups (in contrast to hydrophobic interactions, which arise from the structuring of water around nonpolar groups). Most food emulsifiers naturally have dipolar or ionic groups that are hydrated (e.g., -OH, -COO-, and -NH+), and some are also capable of binding hydrated ions (e.g., -COO- + Na+ ^ -COO- Na+). As two droplets approach each other, the bonds between the polar groups and the water molecules in their immediate vicinity must be disrupted, which results in a repulsive interaction (Figure 3.20) (Besseling 1997). The magnitude and range of the hydration interaction therefore depend on the number and strength of the bonds formed between the polar groups and the water molecules: the greater the degree of hydration, the more repulsive and long range the interaction (Israelachvili 1992). Just as with hydrophobic interactions, it is difficult to develop theories from first principles to describe this type of interaction because of the complex nature of its origin and its dependence on the specific type of ions and polar groups present. Nevertheless, experimental measurements of the forces between two liquid surfaces have shown that hydration interactions are fairly short-range repulsive forces that decay exponentially with surface-to-surface separation (Claesson 1987, Israelachvili 1992):
where A is a constant which depends on the degree of hydration of the surface (typically between 3 and 30 mJ m-2) and k0 is the characteristic decay length of the interaction (typically
between 0.6 and 1.1 nm) (Israelachvili 1992). The greater the degree of hydration of a surface group, the larger the values of A and In practice, it is often difficult to isolate the contribution of the hydration forces from other short-range interactions that are associated with mobile interfacial layers at small separations (such as steric and thermal fluctuation interactions) (Figure 3.20), and so there is still much controversy about their origin and nature. Nevertheless, it is widely accepted that they make an important contribution to the overall interaction energy in many systems.
At high electrolyte concentrations, it is possible for ionic surface groups to specifically bind hydrated ions to their surfaces (Hunter 1986, 1989; Miklavic and Ninham 1990). Some of these ions have large amounts of water associated with them and can therefore provide strong repulsive hydration interactions. Specific binding depends on the radius and valency of the ion involved, because these parameters determine the degree of ion hydration. Ions that have small radii and high valencies tend to bind less strongly because they are surrounded by a relatively thick layer of tightly "bound" water molecules and some of these must be removed before the ion can be adsorbed (Israelachvili 1992). As a general rule, the adsorbability of ions from water can be described by a lyotropic series: I- > Br- > Cl- > F- for monovalent ions, and K+ > Na+ > Li+ for monovalent cations (in order of decreasing adsorbability). On the other hand, once an ion is bound to a surface, the strength of the repulsive hydration interaction between the emulsion droplets increases with degree of ion hydration because more energy is needed to dehydrate the ion as the two droplets approach each other. Therefore, the ions that adsorb the least strongly are those that provide the greatest hydration repulsion. Thus it is possible to control the interaction between droplets by altering the type and concentration of ions present in the aqueous phase.
Hydration interactions are often strong enough to prevent droplets from aggregating (Israelachvili 1992). Thus, oil-in-water emulsions that should contain enough electrolyte to cause droplet flocculation through electrostatic screening have been found to be stable because of specific binding of ions (Israelachvili 1992). This effect is dependent on the pH of the aqueous phase because the electrolyte ions have to compete with the H+ or OH- ions in the water (Miklavic and Ninham 1990). For example, at relatively high pH and electrolyte concentrations (>10 mM), it has been observed that Na+ ions can adsorb to negatively charged surface groups and prevent droplets from aggregating through hydration repulsion, but when the pH of the solution is decreased, the droplets aggregate because the high concentration of H+ ions displaces the Na+ ions from the droplet surface (Israelachvili 1992). Nonionic emulsifiers are less sensitive to pH and ionic strength, and they do not usually bind highly hydrated ions. The magnitude of the hydration interaction decreases with increasing temperature because polar groups become progressively dehydrated as the temperature is raised (Israelachvili 1992). In summary, the importance of hydration interactions in a particular system depends on the nature of the hydrophilic groups on the droplet surfaces, as well as on the type and concentration of ions present in the aqueous phase.
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