Intermolecular forces
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Why are esters insoluble in water?
Ulex writes
Whether or not two substances mix depends on the total entropy change which would occur on mixing. This, in turn, is made up of two parts: the entropy change of the system and the entropy change of the surroundings.
Since a mixture is obviously more randomly arranged than the two components separately, the entropy change of the system is always going to be positive for the formation of any mixture from its components. To put this another way: two liquids will always mix unless there is something preventing it happening.
This something, in the case of esters and water, is the intermolecular forces which have to be broken on mixing and those which would be formed if mixing occurred.
Ester molecules are held together by dipole-dipole attraction and van der Waals forces. Water molecules are held together by all types of intermolecular force, but mostly by strong hydrogen bonding.
Ester molecules are attracted to water molecules by hydrogen bonding mostly but this attraction is weaker than the forces of attraction which exist in the water and the ester separately.
In particular the ester would have to break up the network of hydrogen bonding in water in order to mix with it. The opportunities for hydrogen bonding between ester molecules and water molecules are much less than the extensive opportunities for hydrogen bonding in water alone.
Consequently the enthalpy change of mixing is positive and the entropy change in the surroundings negative. This is enough to make the total entropy change of mixing negative so that very little mixing takes place.
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updated: 27 September 2004
