Polarity
A popular aphorism used for predicting solubility is "like dissolves like".[9] This statement indicates that a solute will dissolve best in a solvent that has a similar
polarity to itself. This view is rather simplistic, since it ignores many solvent-solute interactions, but it is a useful rule of thumb. For example, a very polar (
hydrophilic) solute such as
urea is very soluble in highly polar water, less soluble in fairly polar
methanol, and practically insoluble in non-polar solvents such as
benzene. In contrast, a non-polar or
lipophilic solute such as
naphthalene is insoluble in water, fairly soluble in methanol, and highly soluble in non-polar benzene.
[10]
Liquid solubilities also generally follow this rule. Lipophilic plant oils, such as olive oil and palm oil, dissolve in non-polar solvents such as alkanes, but are less soluble in polar liquids such as water.
Synthetic chemists often exploit differences in solubilities to separate and purify compounds from reaction mixtures, using the technique of
liquid-liquid extraction.
Insolubility and spontaneous phase separation does not mean that dissolution is disfavored by enthalpy. Quite the contrary, in the case of water and hydrophobic substances,
hydrophobic hydration is reasonably exothermic and enthalpy alone should be favor it. It appears that entropic factors the reduced freedom of movement of water molecules around hydrophobic molecules lead to an overall hydrophobic effect.
this is from wiki
http://en.wikipedia.org/wiki/Solubility#Polarity
