What is the difference between salting in and salting out electrolytes?

[thearny]

Recently I read an article describing NaOH as a salting in electrolyte, and I have long held the belief that Na2SO4, Na2CO3 and NaCl etc.. are salting out electrolytes. I cannot see any reason why NaOH should be different. What is the difference between salting in and salting out electrolytes, and what affect do they have? - Any thoughts much appreciated.

 

[chemisttree]

Do you know what salting in means?

 

[thearny]

Not really. I can't quite envisage it. Salting out (I am applying this to surfactants) means that the surfactant is dragged/forced out of solution. So salting in presumably strengthens the solutions stability, but I can see no justification (experimentally) or mechanism for that.

 

[chemisttree]

Like dissolves like. Very polar solutions are better at solubilizing a polar substrate than less polar ones. So, adding salt to an aqueous solution of something that is only moderately polar (like a surfactant) will make it less soluble. Adding salt to something that is very polar can actually increase its solubility provided there is no 'common ion' effect.

 

[thearny]

I read a paper yesterday suggesting that salting out is due to an imblance in the chemical potential caused by salt being less soluble by the headgroups. The chemical potential explanation I can accept, but why would the salt be less soluble around the headgroup? - It just about fits... Many thanks.

 

[chemisttree]

Discussion about headgroups implies a surfactant is discussed.

The structure of water near the headgroup is somewhat organized. This somewhat organized water can potentially have less interaction with another ion in solution... the hydrogen bonds that 'organize' this water are somewhat less available to interact with the soluble ion from the salt. It does sound a bit like handwaving though.