Introduction

• Solvation/dissolution refers to the interaction between solvent and solute.
  1. Solvent refers to the liquid that dissolves the substance and form solution with.
  2. Solute refers to the substances that is dissolved in the solution [1].

   

• Solvents can further be divided into Polar and Non Polar
  1. Non Polar : These are formed by molecules do not contain any charge which means they interact mainly through Van der waal’s forces of attraction. Most organic solvents are Non Polar in nature. Non Polar solvents are normally used when we want the whole power of the nucleophile to be used and none should be taken by the attraction in a solvent. For example it is used in the reactions which involves the Grignard’s Reagent(R-Mg-X). Examples of non polar solvents include Benzene, Touluene,Carbon Tetrachloride(CCl₄),etc.
  2. Polar : There are solvents where there is electronegative difference between the molecules and it’s constituent atoms. They are normally intermolecularly bonded by the forces greater than the Van der waal’s force like Dipole-Dipole interaction(aprotic) and sometimes even Hydrogen bonding(protic). These solvents are used when we need to use polar solutes since they will only disassociate in a polar solvent. Examples include water,ammonia , Dimethyl Sulphoxide(DMSO), etc[2]

• Solvation is govern by a general rule of solubility – “like dissolves like”
  1. In order to dissolve in water, the solute has to first disrupt the hydrogen bonding between two water molecule and pushes them apart.(source: http://chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx75/tx75.html)
  2. Non-polar molecules have no net charge hence the hydrogen bonding between the water are strong enough to exclude them. In contrast, the non-polar molecules interacts with non-polar solvants via van der waal interactions [1,2].
• What about molecules that contains both non-polar and polar groups (amphiphilic) eg. alcohol ?
  1. The solubility of such molecule is largely dependent on the size of alkyl group. (In general, alcohols with fewer than 4 carbons are soluble in water while those with more than 4 carbons are insoluble in water.
  2. The solubility of the molecule also depends on the structure. If the contact surface of the non-polar region of the molecule is minimized, the molecule may still be soluble in water even though it contain more than 4 carbons (1 Oxygen atom can drag approximately 3 Carbon molecule into solution in water)[1].
     

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Assembly of micelle

https://www.youtube.com/watch?v=xh-HeDYID3U

Concept of Universal solvent

 The solvent that is closest to being a universal solvent is likely to be water. In year 2009, Professor Song Jianxing shows that it is possible to solubilise proteins that was once thought to be insoluble [3]. Alternatively, there are molecules that are amphiphilic eg. Detergent that can act as a mediator to allow non-polar molecules to be soluble in water.

Cleansing action of Soap/detergent

Types of interactions in dissolution

Polar molecules with polar solvent

• Hydrogen bonding
• Ionic bonding
• Dipole-dipole
• Ion-dipole

Non polar

1. Van der waal
2. Hydrophobic effects (avoiding contact with hydrophillic solvent)

Other factors that will affect solubility

• Temperature & Pressure: Related to energy required to overcome the interaction between two solvent molecule.
• Salt concentration: The amount of solvent molecule available to dissolve the solute.

Reference

1. Bruice, P.Y., Organic chemistry. 5th edition ed.: Pearson International Edition.
2. http://chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx75/tx75.html
3. Song, J., Insight into “insoluble proteins” with pure water. FEBS letters, 2009. 583(6): p. 953.