Solute-water attractions

Ashbaugh, H. S. and Paulaitis, M. E Effect of solute size and solute-water attractive interactions on hydration water structure around hydrophobic solutes. /. Am. Chem. Soc. 123, 10721-10728 (2001). 

In their study of krypton hydration, Durell and Wallqvist also reported a calculation of the enthalpy of hydration evaluated by the direct method of Eq. [31]." Both constant volume and constant pressure enthalpies were determined by varying the volume of the krypton solution. Their results are displayed in Table 1. The enthalpy of hydration in the constant volume case (—6.3 1.3 kcal/mol) is significantly more exothermic than in the constant pressure case (—3.4 1.3 kcal/mol). The latter number agrees very well with the experimental value of —3.3 kcal/mol, also obtained at constant pressure. The calculated enthalpies of solvation were decomposed into solute-water and water-water (solvent reorganization) terms. The solute-water contribution is comparable and favorable (—5.4 kcal/mol) in both the constant volume and constant pressure calculations. The solvent reorganization term, in contrast, shows a large ensemble dependence. In the constant-pressure case, the solvent reorganization term has a value of 2.0 1.3 kcal/mol. The overall favorable enthalpy of hydration of krypton at constant pressure therefore results from the solute-water attractions rather than from a... 

When ionic compounds dissolve in water, the ions in the crystal separate and move throughout the solution. When two such solutions are mixed, all types of positive ions in the new solution are attracted to all types of negative ions in the solution. Sometimes a reaction takes place. This reaction is called a double-replacement reaction. Double-replacement reactions are sometimes called ionic reactions. 

With the exception of rather small polar molecules, the majority of compounds, including drugs, appear to penetrate biological membranes via a lipid route. As a result, the membrane permeability of most compounds is dependent on K0/w. The physicochemical interpretation of this general relationship is based on the atomic and molecular forces to which the solute molecules are exposed in the aqueous and lipid phases. Thus, the ability of a compound to partition from an aqueous to a lipid phase of a membrane involves the balance between solute-water and solute-membrane intermolecular forces. If the attractive forces of the solute-water interaction are greater than those of the solute-membrane interaction, membrane permeability will be relatively poor and vice versa. In examining the permeability of a homologous series of compounds... 

Having disguised each particle of oil or grease, it can readily enter solution while sheathed in its water-attracting overcoat of surfactant. And if the oil particles enter the solution, then the oil is removed from the plate, and is cleaned. 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

The ability of an ionic solid to dissolve depends on its lattice energy, as well as the degree to which its ions can become hydrated. The lattice energy of an ionic crystal is a measure of the strength of its three-dimensional network of bonds. If these interactions are weaker than the solute-solvent attractions, the ionic bonds will be easily disrupted by water molecules. 

The solubility of solid compounds in water, as well as in other solvents, is determined by the competition between attractions in the solid state between molecules or ions and the solute-solvent attractions that occur in solution. solid that is more attracted to itself than to solvent molecules will not dissolve. A general... 

Hhyd) hydration enthalpy heat of hydration. The change in enthalpy for the process. A(g) A(aq)where the concentration of A in the aqueous solution approaches zero. Enthalpies of hydration for ions are always negative because strong ion-water attractions are formed when the gas-phase ion is surrounded by water. 

When a salt dissolves in water, the water molecules are attracted by ions in solution. This attraction is called... 

The physical meaning of Eq. [42] is tied to the interpretation of some of the terms, some being convoluted. The A term can be associated with dispersion interactions an increase in surface area suggests an increase in dispersion interactions (attractions) and, thus, increased solubility in octanol that in turn results in enhanced Pq/w values. A similar interpretation holds if one associates p /V with dipolarity/polarizability effects. The positive sign on the HBA term ( Umin) fot the solute suggests that the HBD acidity of water is less important than the HBA basicity of water for those molecules partitioning between phases. This implies that increased solute HBD acidity would increase the solute-water interaction. 

The intermolecular attraction between like molecules in the liquid state, such as the water-water attraction based on hydrogen bonds, is called cohesion. The attractive interaction between a liquid and a solid phase, such as water and the walls of a glass capillary (a cylindrical tube with a small internal diameter), is called adhesion. When the water-wall adhesion is appreciable compared with the water-water cohesion, the walls are said to be wettable, and water then rises in such a vertical capillary. At the opposite extreme, when the intermolecular cohesive forces within the liquid are substantially greater than is the adhesion between the liquid and the wall material, the upper level of the liquid in such a capillary is lower than the surface of the solution. Capillary depression occurs for liquid mercury in glass capillaries. For water in glass capillaries or in xylem vessels, the... 

Studies of ionic solutions have been overwhelmingly aqueous in the hundred years or so in which they have been pursued. This has been a blessing, for water has a dielectric constant, s, of 80, about ten times larger than the range for most nonaqueous solvents. Hence, because the force between ions is proportional to 1/e, the tendency of ions in aqueous solutions to attract each other and form groups is relatively small, and structure in aqueous solutions is therefore on the simple side. This enabled a start to be made on the theory of ion-ion attraction in solutions. 

If a positive potential is applied to the metal, as shown in Fig. 10.3, the ionization of the surface atoms will be promoted, and thus more metal ions will be produced at the surface. In the solution, water molecules, positive ions (cations), and negative ions (anions) drift around. The adsorbed layer of positive metal ions attracts nearby water dipoles in a preferential direction. The negative ions in the solution near the anode surface are also attracted toward the surface. The adsorbed fixed layer and the negative ion layer (Fig. 10.3) together are the so-called electrical double layer. Details about the double layer are available elsewhere [3]. Electrochemical reactions and mass transport for further electrochemical dissolution occur and pass through this double layer. 

Sodium hydroxide caustic soda), NaOH, is a white hygroscopic (water-attracting) solid, which dissolves readily in water. Its solutions have a smooth, soapy feeling, and are very corrosive to the skin (this is the meaning of caustic in the name caustic soda). Sodium hydroxide is made either by the electrolysis of sodium chloride (Chap. 

The dielectric constant of liquid water at room temperature is 80. This means that two opposite electrical charges in water attract each other with a force only 1/80 as strong as in air (or a vacuum). It is clear that the ions of a crystal of sodium chloride placed in water could dissociate away from the crystal far more easily than if the crystal were in air, since the electrostatic force bringing an ion back to the surface of the crystal from the aqueous solution is only 1/80 as strong as from air. It is accordingly not surprising that the thermal agitation of the... 

Positive-ion-to-negative-ion attractions are straightforward and strong interactions so strong, in fact, that they tend to dominate when they are present. As we pointed out in our discussion of precipitation, when opposite ions find each other, they can form an ionic bond and come out of solution. Ionic attractions do not, however, always prevail. Water molecules can keep ions apart by forming cages around the ions. These water cages are the result of dipole-to-dipole and dipole-to-ion attractions. 

Finally, linear complexes with coordination numbers of 2 are known, especially for ions with d ° configurations such as Cu, Ag", Au, and Flg. The central Ag atom in a complex such as [Ag(NFl3)2] in aqueous solution strongly attracts several water molecules as well, however, so its actual coordination number under these circumstances may be greater than 2. 

In the formation of a solution the attractions among the particles in the original phases (solvent-to-solvent and solute-to-solute attractions) are broken up and replaced, at least in part, by new solvent-to-solute attractions. Unlike a compound, a solution has its components present in variable proportions and cannot be represented by a chemical formula. Equations for dissolution reactions do not include the solvent as a reactant. They indicate the original state of the solute in parentheses on the left side of the equation and identify the solvent in parentheses on the right side. For example, solid (s) sucrose dissolves in water to give an aqueous (aq) solution of sucrose ... 

Most of the water on Earth is not pure, but rather is present in solutions. Water is difficult to keep pure because it is an excellent solvent for a variety of solutes. Water is such a versatile solvent that it is sometimes called the universal solvent. Its ability to act as a solvent is one of its most important physical properties. As you will see, it is again the attraction of water molecules for other molecules, as well as for one another, that accounts for these solvent properties. 

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