Ionic Interactions Are Attractions Between Oppositely Charged Ions
Ionic interactions result from the attraction between a positively charged ion—a cation—and a negatively charged ion—an anion. In sodium chloride (NaCl), for example, the bonding electron contributed by the sodium atom is completely transferred to the chlorine atom (Figure 2-7a). Unlike covalent bonds, ionic interactions do not have fixed or specific geometric orientations because the electrostatic field around an ion—its attraction for an opposite charge—is uniform in all directions. In solid NaCl, oppositely charged ions pack tightly together in an alternating pattern, forming the highly ordered crystalline array, or lattice, that is typical of salt crystals (Figure 2-7b). The energy required to break an ionic interaction depends on the distance between the ions and the electrical properties of the environment of the ions.
FIGURE 2-7 Electrostatic interactions of the oppositely charged ions of salt (NaCl) in crystals and in aqueous solution. (a) In crystalline table salt, sodium atoms are positively charged ions (Na+) due to the loss of one electron each, whereas chloride atoms are correspondingly negatively charged (Cl–) by gaining one electron each. (b) In solid form, ionic compounds form neatly ordered arrays, or crystals, of tightly packed ions in which the positive and negatively charged ions counterbalance each other. (c) When the crystals are dissolved in water, the ions separate, and their charges, no longer balanced by immediately adjacent ions of opposite charge, are stabilized by interactions with polar water. The water molecules and the ions are held together by electrostatic interactions between the charges on the ion and the partial charges on the water’s oxygen and hydrogen atoms. In aqueous solutions, all ions are surrounded by a hydration shell of water molecules.
When solid salts dissolve in water, the ions separate from one another and are stabilized by their interactions with water molecules. In aqueous solutions, simple ions of biological significance, such as Na+, K+, Ca2+, Mg2+, and Cl–, are hydrated, surrounded by a stable shell of water molecules held in place by ionic interactions between the ion at the center and the oppositely charged ends of the water molecules, which are dipoles (Figure 2-7c). Most ionic compounds dissolve readily in water because the energy of hydration—the energy released when ions tightly bind water molecules and spread out in an aqueous solution—is greater than the lattice energy that stabilizes the crystal structure. Parts or all of the aqueous hydration shell must be removed from ions in solution when they interact directly with proteins. For example, water of hydration is lost when ions pass through protein pores in the cell membrane during nerve conduction.
The relative strength of the interaction between two oppositely charged ions, A– and C+, depends on the concentration of other ions in a solution. The higher the concentration of other ions (e.g., Na+ and Cl–), the more opportunities A– and C+ have to interact ionically with those other ions, and thus the lower the energy required to break the interaction between A– and C+. As a result, increasing the concentrations of salts such as NaCl in a solution of biological molecules can weaken and even disrupt the ionic interactions holding the biomolecules together. This principle can be exploited to separate complex mixtures of interacting molecules such as proteins into their individual, pure components.