Solvation

Solving a sodium ion with water.

Solvation is the process of formation of interactions between molecules of a solvent with molecules or ions of a solute. In solution, the ions of the solute disperse and are surrounded by molecules of solvent, the same happens in the solvent molecules.

Difference Between Solvation, Dissolution, and Solubility

By IUPAC definition, solvation is an interaction of a solute with a solvent that leads to the stabilization of the solute species in solution. It can also refer to the solvated state, where an ion in a solution is complexed by molecules of the solvent. The concept of solvation interaction can also be applied to an insoluble material, for example, the solvation of functional groups on the surface of an ion exchange resin.

Dissolution is the process of adding the solute into the solvent or adding more solvent to a solution that already exists, solubility refers to the limit of solute that I can add to the solvent before a precipitate begins to form, that is say before it remains undissolved solute. Solvation explains what happens at the molecular level between the solute and the solvent.

By considering the units a clearer distinction can be made. The complexation can be described by the coordination number, and the stability constant of the complex. The typical unit for the rate of dissolution is mol/s. The unit for solubility can be mol/kg.

Solvents and intermolecular interactions

Polar solvents are those with a molecular structure that contains dipoles. Such compounds usually have a high dielectric constant. The polar molecules of these solvents can solvate ions because they can orient the partially charged portion of the molecule toward the ion in response to electrostatic attraction. This stabilizes the system. Water is the most common and well-studied polar solvent, but others exist, such as acetonitrile, dimethyl sulfoxide, methanol, propylene carbonate, ammonia, ethanol, and acetone. These solvents can be used to dissolve inorganic compounds like salts.

Solvation involves different types of molecular interactions: hydrogen bonding, ion-dipole, dipole-dipole interaction or London forces. The first three can be present only in polar solvents. Ion-ion interactions can only occur in ionic solvents (for example, in the melt phase). Solvation processes will only be thermodynamically favored if the Gibbs free energy of formation of the solution is less than the sum of the Gibbs free energy of formation of the solvent and the solute separately.

The conductivity of a solution depends on the solvation of its ions.

Thermodynamic aspects

For solvation to occur, the release of individual ions from the crystal lattice in which they are present is needed. It is necessary to break the attractions that the ions have among themselves, an attraction represented by the free energy of the solute network in its natural aggregation state. For this, it is obtained from the energy released when the ions of the solute network associate with the solvent molecules. The energy released in this way is known as the free energy of solvation.

The enthalpy change of solution is the enthalpy of formation of the solution minus the sum of the enthalpies of formation of the separate systems, while the entropy change is the corresponding difference in the entropies of formation. Most gases have a negative enthalpy of solution, which means they are less soluble at higher temperatures.

Although it was originally believed that a higher ion size/charge ratio, or charge density, implied higher solvation, this is not true for ions such as iron(III) or the lanthanides and actinides, which are rapidly hydrolyzed to form insoluble oxides.

The enthalpy of fertilization can explain why solvation occurs with some ionic lattices, while others do not. A negative value of solution enthalpy change corresponds to an ion that will dissolve, while a positive value means that solvation will not happen easily. A quantitative measure of the solvating power of solvents is the donor number.