Solvation
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Solvation is the attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a solvent they spread out and become surrounded by solvent molecules. The bigger the ion, the more solvent molecules are able to surround it and the more it becomes solvated.
Polar solvents are solvents with a molecular structure that has a significant dipole moment. A dipole moment is created by localized more-negatively charged and localized more positively charged portions of the molecule. Examples of molecules with a high dipole moment are water or acetonitrile. These polar molecules can solvate ions because they can orient the appropriate partially charged portion of the molecule towards the ion in repsonse to electrostatic attractions. This stabilizes the system. Water represents the most common and well-studied polar solvent, but others exist, such as dimethyl sulfixoide, methanol, propylene carbonate, acetonitrile, ammonia, ethanol, and acetone, among others. These solvents can be used to dissolve inorganic compounds such as salts.
Solvation involves different types of intermolecular interactions: hydrogen bonding, ion-dipole and dipole-dipole attractions or van der Waals forces. The hydrogen bonding, dipole-charge, and dipole-dipole interactions occur only in polar solvents. Ion-ion interactions occur only in ionic solvents. The solvation process will only be thermodynamically favored if the overall Gibbs free energy of the solution is decreased compared to the free energy of the separated solvent and solid (or gas or liquid). This means that the change in enthalpy minus the change in entropy (multiplied by the absolute temperature) is a negative value, or that the Gibbs free energy of the system decreases.
For solvation to occur, energy is required to release individual ions from the crystal lattices in which they are present. This is necessary to break the attractions the ions have with each other and is equal to the solid's lattice free energy (the energy released at the formation of the lattice as the ions bonded with each other). The energy for this comes from the energy released when ions of the lattice associate with molecules of the solvent. Energy released in this form is called the free energy of solvation.
The enthalpy of solution is the solution enthalpy minus the separate systems enthalpy, while the entropy is the corresponding difference in entropy. Most gases have a positive enthalpy of solution. A positive enthalpy of solution means that the solute is less soluble at high temperatures.
Although early thinking was that a higher ratio of a cation's ion charge to the size, or the charge density, resulted in more solvation, this does not stand up to scrutiny for ions like Iron(III) or lanthanides and actinides, which are readily hydrolyzed to form insoluble (hydrous)oxides. As solids, these are obviously not solvated.
Enthalpy of solvation can help explain why solvation occurs with some ionic lattices but not with others. The difference in energy between that which is necessary to release an ion from its lattice and the energy given off when it combines with a solvent molecule is called the enthalpy change of solution. A negative value for the enthalpy change of solution corresponds to an ion that is likely to dissolve, whereas a high positive value means that solvation will not occur. It is possible that an ion will dissolve even if it has a positive enthalpy value. The extra energy required comes from the increase in entropy that results when the ion dissolves. The introduction of entropy makes it harder to determine by calculation alone whether a substance will dissolve or not. A quantitative measure for solvation power of solvents is given by donor numbers.
Note that solvation does not mean a reaction takes place. Adding NaCl(s) in water, for example, will only create a solution of sodium and chloride ions; you would only have solvation of the salt's ions. Adding the weak base ammonia to water, on the other hand, would be a reaction.
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[edit] References
- Dogonadze, Revaz R., et al. (eds.) (1985-88). The Chemical Physics of Solvation, 3 vols., Amsterdam: Elsevier. ISBN 0-444-42551-9 (part A), ISBN 0-444-42674-4 (part B), ISBN 0-444-42984-0 (part C).
