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Solvation and Association

For solutions that exhibit strong intermolecular forces, fundamental molecular theory cannot be applied.
For solutions that exhibit strong intermolecular forces such as dipole moments, hydrogen bonds, or complex formation, fundamental molecular theory cannot be applied. Since multi-species chemical mixtures are often in chemical equilibrium, it is frequently useful to describe the mixture as an association or soluble mixture. There is extensive experimental evidence for the formation of dimers when two monomers of acetic acid combine by hydrogen bonding into new molecules, called dimers. Based on the total concentration of acetic acid in the solution, acetic acid dimerizes at various temperatures and various concentrations.

Monomers escape at higher pressures (vapor pressures), and the variation of benzene activity coefficient with acetic acid composition could be because acetic acid under these conditions is mostly monomeric, while pure acetic acid is nearly completely dimerized. Acetic acid molecules associate with benzene molecules in the acetic acid–benzene system, causing Raoult's law to deviate positively.

An interaction that occurs between a solvent and a solute molecule that uses weak bonds is known as solvation. In the acetone-chloroform system, hydrogen bonds form when the atoms of hydrogen in chloroform and acetone combine. As a result of hydrogen bonding, both components are less inclined to escape. In other words, Raoult's Law does not apply in this system.

Hydrogen bonds are frequently found in solutions, but there are also many examples of weak chemical bonds between molecules of dissimilar composition. Creating weak chemical bonds with each other is known as complex formation - the formation of a new biochemical species held together by weak chemical forces rather than physical ones. Generally, these complexes can only exist in solution, and they are inherently unstable, so they cannot always be isolated. Solution behavior is determined by the behavior of molecules, which can form complexes.

For example, silver chloride in water is incredibly insoluble since it can only dissociate slightly to silver ions and chloride ions except for slight dissociation into silver ions. However, adding a small amount of ammonia dramatically increases solubility because of the formation of the complex ion Ag(NH3)6+ from the reaction between six molecules of ammonia and one silver ion. The ammonia binds up silver ions and forces extensive dissociation of silver chloride molecularly, causing silver chloride to dissolve in water.

It has become much more popular to compute the activity coefficient of solutions theoretically using computers in recent years. Often, the calculations have been limited to model systems, namely mixtures of hard-sphere molecules - i.e., small molecules without forces of attraction, which are idealized molecules of finite size. We now know more about the characteristics of simple liquid solutions due to these calculations because their shape and size determine how nonpolar molecules and molecules with hydrogen bonds arrange themselves in space rather than their intermolecular forces.

Using hard-sphere molecules as an example, the results can be transferred to real molecules by changing them for softness, attraction forces, or their ability to overlap (interpenetrate) at higher temperatures. The present state of practical results is still severely limited and the amount of computer calculation required even for very simple binary systems is significant, but there is good reason to believe that future developments in the theory of solution will be increasingly driven by computerized models rather than analytical ones.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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