Solvation: Explanation, Factors, and Applications

In solution formation, when a solute is added to a particular solvent, the solvent particles start interacting with solute particles due to the presence of similar attractive forces.  These attractive forces are present in all types of substances. For example, London dispersion and dipole-dipole forces.  If the forces between solute and solvent are strong enough, then the solvent surrounding the solute particle is known as the solvation process. 

In particular, in solid-liquid solutions, the solvation mainly consists of two parts. In the first step, the solid particles, if have poles or charges, start binding with polar solvents to release energy. That energy is the driving force of dissolution. This energy is known as solvation energy. The greater the solvation energy, the greater would be the chances of solution formation. 

In the second step, this energy is utilized to break the lattice (regular arrangement of ions). The energy required to break that arrangement is lattice energy. The difference between lattice energy and solvation energy is used to calculate the enthalpy of the solution.

Solid solute particles are arranged in a regular pattern. The new attractive forces formed between the poles of solute and solvent release energy. That energy is the driving force of the dissolution of solute in a solvent to form a solution. 

Solvation is different from solubility as in the solvation process solvent molecules start encapsulation of salute particles whereas, solubility is the quantitative measurement of the maximum solute dissolved at a given temperature at dynamic equilibrium.

Solvation and hydration

If the solvent used is water, the process of solvation is known as hydration. When an ionic compound is added to water, it starts interacting with water molecules as shown in the figure below.

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The hydration energy released by this interaction is used to break down the lattice. 

Thermodynamics of solvation process

The energetics of solvation is based upon the formation and breakage of bonds. When one mole of solute is dissolved to form an infinitely dilute solution, the heat absorbed or released is known as enthalpy of solution. When a solute is added, it is in a pure state. In order to get it dissolved in the solvent, it should increase the intermolecular or interionic distance to easily interact with the solvent. The intermixing of solute and solvent comprises an endothermic stage in which solute particles are separated and an exothermic process where solute-solvent gets connected.

ΔHsoln= H(solution)– H(components)

Where,

  • ΔHsoln is the net enthalpy change after solution formation
  •  H(solution) is the enthalpy of solution before the formation
  • H(components) is the enthalpy of components before solution formation

Factors affecting the solvation process

The key factors affecting solubility are somehow similar to the factor for solvation. This is so because solubility is initiated through solvation. It can be enhanced by increasing the possible solute-solvent interactions. The greater the collisions of particles of solute, the greater the chances of solvation. The factors which are responsible to increase the rate of solvation are:

  • Temperature

In the case of solid solutes, the kinetic energy of the particle is directly proportional to the number of collisions of solute-solvent particles. Hence, at higher temperatures, the solvation process is faster.

If the solvation of gases in liquid is under consideration, by Henry’s law, at higher temperatures, the gas particles may easily escape and the solvation decreases.

  • The surface area of solute

The surface area of a solid solute can be increased by dividing it into small particles. Granular particles are more likely to solvate than a single piece.

  • Concentration of solvent

The concentration of solvent greatly influences solvation. For example, upon the addition of solute in a solvent initially, the rate of solvation is much higher because the number of solute particles is easily surrounded by the solvent. The solvation rate decreases near saturation.

  • Stirring

Stirring is the most simple method to increase the rate of solvation. In labs, magnetic stirrer plates are used for this purpose. In industries, motors are attached to agitators for thoroughly mixing solutes.

Applications of solvation

Ions in the solution and their solvation process play vital roles in many fields of chemistry. Some applications of this phenomenon are given here:

Electrochemistry

The electrolytes being used in batteries and capacitors are studied before being employed to make these devices economical and efficient. The solvation rate is important for ions regarding their dynamics and to avoid aggregation.

Nuclear waste management

The nuclear fuel after completing its process, nuclear power plants are reprocessed to extract the enriched uranium or plutonium left unreacted through the ion solvation process.

UO2 (aq) +NO3 (aq) + organic wates → UO22-NO22-(aq) + organic layer

In solvent extraction

The extraction of the desired product from a solution by distributing it in two-layer of solvents is worked on the principle of solvation. The rate of solvation in the organic phase or aqueous one determines the distribution.

in controlling reaction rate

Choice of solvent is one the most crucial factor that affects the rate of SN2 reactions. If this reaction takes place in a polar protic solvent, it forms a solvation shell due to hydrogen bonding with the attacking nucleophiles. As this reaction is a bimolecular single-step mechanism, the rate of SN2 reaction is dependent on the both nucleophile and substrate. Hence, the rate can be controlled with a solvent.

Concepts Berg

What is solvation used for?

It is the first step in solution preparation.

Are solvation and dissolution different?

Solvation and dissolution are the same processes.

What is the solvation effect?

When a solute particle is added to the solvent and it starts dispersing into it. This process of solvation (caging) of solute particles into the solvent is known as a solvent effect.

What is the difference between solvation and dissociation?

In the context of dissolution of solute, the solvation process is the dispersion of a solute in the solvent whereas dissociation is the complete separation of ionic particles in the electrolytic process. In solvation, the molecules go apart but still have some ionic solute interactions. 

What is solvent in chemistry?

The solvent is a component of a solution present in a greater quantity.

Is solvation ionic or covalent?

Solvation can be both ionic and covalent, depending upon the type of solute-solvent interaction. It works on the principle of “like dissolves like”. For example, benzene is a nonpolar solvent, it dissolves nonpolar (covalent) compounds such as oils and waxes. On the other hand, if the solute is ionic and the solvent is a polar compound like water then the solvation is ionic or polar. For example, if the solute is a molecular compound and solvent solvates it through polar interactions such as sucrose solution.

What is the solubility of a substance?

It is the maximum amount of a substance that can be dissolved in a particular solvent at a given temperature. For example, the solubility of NaCl is 35g/100ml of water at R.T.P.

What is the solvation effect in amines?

The solvation effect of amines is more prominent in water than in any other solvent. The effect is due to hydrogen bonding.

What are three ways to increase the rate of solvation?

The three ways to increase the solvation are:

  1. By Agitation
  2. By Increasing the temperature for solid solute
  3. By increasing surface area

What is solvation energy?

Whenever a bond is formed, energy is released. The energy released when a solvent starts interacting with solute particles is called solvation energy. In particular, for water, this energy is known as enthalpy of hydration.

What drives the dissolution process?

The driving force for the dissolution process is the solvation energy.

Why is the heat of the solution of hydrated salts endothermic?

The heat of the solution is mainly dependent upon two factors. These are lattice energy and solvation energy. If the lattice energy is greater than the heat of solvation then the solvation process is exothermic and vice versa.

How does precipitation depend on solvation energy?

Precipitation occurs when a supersaturated solution allows cooling. It depends upon the solvation energy and lattice energy.

Is a solution a physical or a chemical change?

A solution formation is a physical change because the solvation is temporary and this effect can be eliminated by removing the solvent.

Why is CO2 solvation in water exothermic?

It is due to strong polar hydrogen bonds formed between water’s hydrogen and oxygen of carbon dioxide.

Why does solvation help in dissolving water, oil, and alcohol soluble flavors?

Flavors, essence, fragrances, and scents are ester compounds. They are the polar end of the C=O bond. They are soluble in polar solvents like water and alcohol through solvation. They also have nonpolar alkyl chains, which allow them to dissolve in oils.

Reference books

  • Solvation Dynamics: A Notion of Charge Injection By Chang Q Sun
  • Solvation Thermodynamics By Arieh Y. Ben-Naim

Reference links

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