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Water-dipole

-C=O, -NH, -OH

Similar

Water-nonpolar

Alkyl group

Much smaller

FIGURE 4.10 Organization of water molecules around ions in aqueous solutions.

toward the ion. At a sufficiently large distance from the ion surface, the water molecules are uninfluenced by its presence and have properties similar to those of bulk water (Franks 1973, Reichardt 1988). Alterations in the structural organization and interactions of water molecules in the vicinity of an ion cause significant changes in the physicochemical properties of water (Fennema 1996b). The water that is "bound" to an ionic solute is less mobile, less compressible, more dense, has a lower freezing point, and has a higher boiling point than bulk water. Most ionic solutes have a high water solubility because the formation of many ion-dipole bonds in an aqueous solution helps to compensate for the loss of the strong ion-ion bonds in the crystals, which is coupled with the favorable entropy of mixing contribution (Chapter 2).

The number of water molecules whose mobility and structural organization are altered by the presence of an ion increases as the strength of its electric field increases. The strength of the electrical field generated by an ion is determined by its charge divided by its radius (Israelachvili 1992). Thus, ions which are small and/or multivalent generate strong electric fields that influence the properties of the water molecules up to relatively large distances from their surface (e.g., Li+, Na+, H3O+, Ca2+, Ba2+, Mg2+, Al3+ and OH-). On the other hand, ions which are large and/or monovalent generate relatively weak electrical fields, and therefore their influence extends a much shorter distance into the surrounding water (e.g., K+, Rb+, Cs+, NH+, Cl-, Br-, and I-). The number of water molecules "bound" to an ion is usually referred to as the hydration number. Thus the hydration number of small multivalent ions is usually larger than that of large monovalent ions.

When an ionic solute is added to pure water, it disrupts the existing tetrahedral arrangement of the water molecules but imposes a new order on the water molecules in its immediate vicinity (Franks 1973, Reichardt 1988). The overall structural organization of the water molecules in an aqueous solution can therefore either increase or decrease after a solute is added, depending on the amount of structure imposed on the water by the ion compared to that lost by disruption of the tetrahedral structure of bulk water (Israelachvili 1992). If the structure imposed by the ion is greater than that lost by the bulk water, the overall structural organization of the water molecules is increased, and the solute is referred to as a structure maker (Robinson et al. 1996). Ionic solutes that generate strong electric fields are structure makers, and the magnitude of their effect increases as the size of the ions decreases and/or their charge increases. If the structure imposed by an ion is not sufficiently large to compensate for that lost by disruption of the tetrahedral structure of bulk water, then the overall structural organization of the water molecules in the solution is decreased, and the solute is referred to as a structure breaker (Robinson et al. 1996). Ionic solutes that generate weak electric fields are structure breakers, and the magnitude of their effect increases as their size increases or they become less charged (Israelachvili 1992).

The influence of ionic solutes on the overall properties of water depends on their concentration (Israelachvili 1992, Robinson et al. 1996). At low solute concentrations, the majority of water is not influenced by the presence of the ions and therefore has properties similar to that of bulk water. At intermediate solute concentrations, some of the water molecules have properties similar to those of bulk water, whereas the rest have properties which are dominated by the presence of the ions. At high solute concentrations, all the water molecules may be influenced by the presence of the solute molecules and therefore have properties which are appreciably different from those of bulk water. The solubility of biopolymer molecules in aqueous solutions decreases as the concentration of ionic solutes increases above a certain level, which is known as "salting out," because the solutes compete with the biopolymers for the limited amount of water that is available to hydrate them.

The presence of electrolytes in the aqueous phase of an emulsion can influence the interactions between the droplets in a variety of ways. First, ions can screen electrostatic interactions (Section 3.4.3) or the zero-frequency contribution of the van der Waals interaction (Section 3.3.4). Second, multivalent ions may be able to form salt bridges between charged droplets (Section 3.4.4). Third, the ability of ions to alter the structural organization of water influences the strength of hydrophobic interactions (Section 3.7). Fourth, ions can cause changes in the size of biopolymer molecules in solution, which alters the strength of steric and depletion interactions (Sections 3.5 and 3.6). Finally, the binding of hydrated ions to the surface of emulsion droplets increases the hydration repulsion between them (Section 3.8). The fact that ions influence the interactions between emulsion droplets in many different ways means that it is difficult to accurately quantify their effect on emulsion properties.

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