por what is the solvation?
The solvation It is the physical and chemical union between solute and solvent particles in a solution. It differs from the concept of solubility in 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 spectators; when in reality, the particles become very small and end up «wrapped» by sheets of solvent molecules, which makes it impossible to observe them.
A very general sketch of the solvation of a particle M is represented in the image above. M can be either an ion (M+) or a molecule; and S is the solvent molecule, which can be any compound in a liquid state (although it can also be a gas).
Note that M is surrounded by six S molecules, which make up what is known as primary solvation sphere. Other S molecules at a greater distance interact by van der Waals forces with the first ones, forming a secondary solvation sphere, and so on until some ordering is not evident.
solvation process
Molecularly, what is the solvation process like? The image above summarizes the necessary steps.
The solvent molecules, colored blue, are initially ordered, all interacting with each other (S—S); and solute particles (ions or molecules), colored purple, do the same with strong or weak M—M interactions.
For solvation to occur, both solvent and solute must expand (second black arrow) to allow solute-solvent (M—S) 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 the 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 unordered at the end. As a consequence, the last step is exothermic, since the formation of the new M—S interactions stabilizes all the particles in the solution.
energy aspects
Behind the solvation process, there are many energetic aspects that must be taken into account. First: S—S, M—M, and M—S interactions.
When the M—S 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 into the environment, which can be verified by measuring the increase in temperature with a thermometer.
If, on the other hand, the M—M and S—S interactions are stronger than the M—S interactions, then they will need more energy to “expand” than they gain once solvation is complete.
One speaks then of an endothermic solvation process. This being the case, a drop in temperature is recorded, or what is the same, the surroundings cool down.
There are two fundamental factors that dictate whether or not a solute dissolves in a solvent. The first is the enthalpy change of solution (ΔHdis), as just explained, and the second is the entropy change (ΔS) between the solute and the 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 union between the solute and the solvent; however, how exactly are these interactions or unions?
If the solute is an ion, M+, so-called ion-dipole interactions (M+—S) occur; and if it is a molecule, then there will be dipole-dipole interactions or London dispersion 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 of thumb to memorize and predict the affinity between solute and solvent is: equal dissolves equal. Therefore, polar substances dissolve very easily in equally polar solvents; and nonpolar substances dissolve in nonpolar solvents.
Differences Between Solvation and Hydration
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 H2O molecules. Note that the oxygen atoms (in red) are directed towards the positive charge, because it is the most electronegative and therefore has the highest negative density δ-.
Behind the first sphere of hydration, other water molecules are grouped around it by hydrogen bonding (OH2—OH2). These are interactions of the ion-dipole type. However, water molecules can also form coordination bonds with the positive center, especially if it is metallic.
Thus, the famous water complexes, M(OH2)n, originate. Since n=6 in the image, the six molecules are oriented around M in a coordination octahedron (the inner sphere of hydration). Depending on the size of M+, the magnitude of its charge, and its electronic availability, said sphere can be smaller or larger.
Water is perhaps the most amazing solvent of all: it dissolves an immeasurable number of solutes, is too polar a solvent, and has an abnormally high dielectric constant (78.5 K).
Solvation Examples
Three examples of solvation in water are mentioned below.
Calcium chloride
When dissolving calcium chloride in water, heat is released as the Ca2+ cations and Cl– anions solvate. The Ca2+ is surrounded by a number of water molecules equal to or greater than six (Ca2+—OH2).
Similarly, Cl– is seen surrounded by hydrogen atoms, the δ+ region of water (Cl–—H2O). The released heat can be used to melt ice masses.
Urea
In the case of urea, it is an organic molecule with the structure H2N–CO–NH2. Upon solvation, the H2O molecules form hydrogen bonds with the two amino groups (–NH2—OH2) and with the carbonyl group (C=O—H2O). 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 solvation of its ions. NH4+ is solvated in a similar way to Ca2+, although probably because it has a tetrahedral geometry it has fewer H2O molecules around it; and NO3– is solvated in the same way as Cl– (OH2—O2NO—H2O) anions.
References
Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning.
Belford R. (sf). Solvation Processes. Chemistry LibreTexts. Retrieved from: chem.libretexts.org
Surf Guppy. (nd). The Process of Solvation. Recovered from: surfguppy.com