Solvation

ignitable under 100 degrees Celsius. Other solvents, like methanol, will burn invisibly until they catch other materials on fire.

Certain ethers, like tetrahydrofuran and diethyl ether, can form organic peroxides with light and oxygen exposure. These organic peroxides are very explosive. This can happen even with a minimum of light involved. Because they have a high boiling point, they can concentrate during distillation and can form a crystalline substance that will explode if disrupted, such as when turning the jar lid as it precipitates on the lid of a jar. This is a bigger problem in laboratories that do not use up the parent solvent quickly. Ethers need to be stored in an airtight container away from light and air.

SOLVATION

This describes the interaction of a solvent with whatever dissolves in it. When the solvent is water, ionized substances interact strongly with the solvent with the characteristics of the solution based on the fact that the solvent and solute are polar. Ions are surrounded with a concentric shell of solvent into “solvation complexes”. There is bond forming (as in ionic bonding), van der Waals forces, and hydrogen bonding that happens in solvation. Solvation in water is known as hydration.

Solvation is different from solubility. Solvation is a kinetic process that is quantified by its rate, while solubility helps to quantify the dynamic equilibrium state that is achieved when the rate of precipitation is the same as the rate of dissolution. The dissolution rate is listed in moles per second, while the units for solubility is moles per liter, milligrams per milliliter, etcetera.

The different types of bonding that can take place in solvation include hydrogen bonding, ion-dipole connectivity, and the different van der Waals forces, which include dipole-induced dipole interactions, dipole-dipole interactions, and induced dipoleinduced dipole interactions. Exactly which forces take place depend on the nature of the solute and solvent. Less polar solutions of course do not have ion-dipole bonding as a general rule because there are no ions; solvents without dipole moments do not engage much in van der Waals forces.

The polarity of the solvent is the most important factor in determining how well it solvates a particular solute. From basic chemistry, you should remember that the polarity is determined by the dipole moment, which is the partial “charge” of different aspects of the molecule because of different electronegativity in different parts of the molecule. Something with carbon and hydrogen atoms only that is symmetric will have no “pull” on the part of some of the molecule and will have a zero dipole-moment.

Polar molecules as solvents can solvate polar solutes, including ions, creating a solvation shell around each aspect of the solute. Remember that, while water is best studied, other polar solvents can dissolve polar solutes, especially solvents like methanol, ethanol, acetonitrile, acetone, and DMSO. Polar solvents can easily solvate ions; however, nonpolar solvents cannot solvate these ionic substances. Water is a good solvent for any type of acid or base because it can either donate or accept hydrogen ions.

Solvation is favored only if the Gibbs free energy of the solution is less than the free energy of the separate solvent and solute. The free energy of the system will be a negative value because of the change in enthalpy and entropy of the solution versus the solvent/solute separately.

Solvation involves multiple steps with different energy consequences. There needs to be “space” in the solvent to have the solute enter it—a process that requires energy. Then there needs to be an interaction between the solvent and solute that is favorable in order for the solution to occur. There will be a gain in entropy (or disorder) of the system but this is offset by the favorable interaction between the solute and solvent. Gases tend to be less soluble at high temperatures, which is not the case with solids and liquids in solution.