SOLVATION: PROCESS, DIFFERENCES WITH HYDRATION AND EXAMPLES - CHEMISTRY - 2025

The solvation is the physical and chemical bonding between solute particles and solvent in a solution. It differs from the concept of solubility in the fact that there is no thermodynamic equilibrium between a solid and its dissolved particles.

This union is responsible for the dissolved solids "disappearing" in view of the audience; when in reality, the particles become very small and end up "wrapped" in sheets of solvent molecules, making them impossible to observe.

Source: Gabriel Bolívar

A very general sketch of the solvation of a M particle is represented in the upper image. M can be either an ion (M ⁺) or a molecule; and S is the solvent molecule, which can be any compound in liquid state (although it can also be gaseous).

Note that M is surrounded by six molecules of S, which make up what is known as the sphere of primary solvation. Other S molecules at a greater distance interact by Van der Waals forces with the former, forming a sphere of secondary solvation, and so on until some ordering is not evident.

Solvation process

Source: Gabriel Bolívar

Molecularly, how is the solvation process? The image above summarizes the necessary steps.

The solvent molecules, which are blue in color, are initially ordered, all interacting with each other (SS); and purple solute particles (ions or molecules) do the same with strong or weak MM interactions.

For solvation to occur, both solvent and solute must expand (second black arrow) to allow solute-solvent (MS) interactions.

This necessarily implies a decrease in solute-solute and solvent-solvent interactions; decrease that requires energy, and therefore this first step is endothermic.

Once the solute and solvent have expanded molecularly, the two mix and exchange places in space. Each purple circle in the second image can be compared to the one in the first image.

A change in the degree of ordering of the particles can be detailed in the image; ordered at the beginning, and disordered at the end. As a consequence, the last step is exothermic, since the formation of the new MS interactions stabilize all the particles in the solution.

Energy aspects

Behind the solvation process, there are many energetic aspects that must be taken into account. First: SS, MM and MS interactions.

When the MS interactions, that is, between the solute and the solvent, are much higher (strong and stable) compared to those of the individual components, we speak of an exothermic solvation process; and therefore, energy is released to the medium, which can be verified by measuring the increase in temperature with a thermometer.

If, on the contrary, the MM and SS interactions are stronger than the MS interactions, then to “expand” they will need more energy than they gain once solvation is complete.

We speak then of an endothermic solvation process. This being the case, a drop in temperature is recorded, or what is the same, the surroundings are cooled.

There are two fundamental factors that dictate whether or not a solute dissolves in a solvent. The first is the enthalpy change of solution (ΔH _dis), as just explained, and the second is the entropy change (ΔS) between solute and dissolved solute. Generally, ΔS is associated with the increase in disorder also mentioned above.

Intermolecular interactions

It was mentioned that solvation is the result of the physical and chemical bond between the solute and the solvent; however, what exactly are these interactions or unions like?

If the solute is an ion, M ⁺, the so-called ion-dipole interactions (M ⁺ -S) occur; and if it is a molecule, then there will be dipole-dipole interactions or London scattering forces.

When talking about dipole-dipole interactions, it is said that there is a permanent dipole moment in M ​​and S. Thus, the δ- electron-rich region of M interacts with the δ + electron-poor region of S. The result of all these Interactions is the formation of several solvation spheres around M.

Additionally, there is another type of interaction: the coordinative. Here, the S molecules form coordination (or dative) bonds with M, forming various geometries.

A fundamental rule for memorizing and predicting the affinity between solute and solvent is: like dissolves like. Therefore, polar substances dissolve very easily in equally polar solvents; and nonpolar substances dissolve in nonpolar solvents.

Differences with hydration

Source: Gabriel Bolívar

How is solvation different from hydration? The two identical processes, except that the S molecules, in the first image, are replaced by those of water, HOH.

In the upper image you can see an M ⁺ cation surrounded by six H ₂ O molecules. Note that the oxygen atoms (in red color) are oriented towards the positive charge, because it is the most electronegative and therefore both have the highest negative density δ-.

Behind the first hydration sphere, other water molecules are grouped around by hydrogen bonds (OH ₂ -OH ₂). These are ion-dipole interactions. However, water molecules can also form coordination bonds with the positive center, especially if it is metallic.

Thus, the famous water complexes, M (OH ₂) _n, originate. Since n = 6 in the image, the six molecules are oriented around M in a coordination octahedron (the internal sphere of hydration). Depending on the size of M ⁺, the magnitude of its charge, and its electronic availability, this sphere can be smaller or larger.

Water is perhaps the most surprising solvent of all: it dissolves an immeasurable amount of solutes, is too polar a solvent, and has an abnormally high dielectric constant (78.5 K).

Examples

Three examples of solvation in water are mentioned below.

Calcium chloride

By dissolving calcium chloride in water, heat is released as Ca ²⁺ cations and Cl ^- anions solvate. Ca ²⁺ is surrounded by a number of water molecules equal to or greater than six (Ca ²⁺ -OH ₂).

Likewise, Cl ^- is surrounded by hydrogen atoms, the δ + region of water (Cl ^- -H ₂ O). The heat released can be used to melt masses of ice.

Urea

In the case of urea, it is an organic molecule with the structure H ₂ N – CO – NH ₂. When solvated, the H ₂ O molecules form hydrogen bonds with the two amino groups (-NH ₂ -OH ₂) and with the carbonyl group (C = O-H ₂ O). These interactions are responsible for its great solubility in water.

Likewise, its dissolution is endothermic, that is, it cools the water container where it is added.

Ammonium nitrate

Ammonium nitrate, like urea, is a solute that cools the solution after the solvation of its ions. NH ₄ ⁺ is solvated in a similar way to Ca ²⁺, although probably due to its tetrahedral geometry it has fewer H ₂ O molecules around it; and NO ₃ ^- is solvated in the same way as Cl ^- (OH ₂ -O ₂ NO- H ₂ O) anions.

References

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