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I am quoting a sentence from Non ideal solutions:

if the interactions (between solute and solvent) repel each other, the activity coefficient is greater than 1.

How can interactions between molecules repel each other? Don't they always line up so that the negative end is in contact with positive (and vice versa)?

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    $\begingroup$ I’m voting to close this question because incomprehensible. $\endgroup$ – Todd Minehardt Sep 23 '20 at 0:48
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The classic case of having activity coefficient greater than 1 is the solution of very hydrophobic organic solvents in water. For example, if you have toluene or benzene in water, these molecules do not prefer to stay in water. This type of work was a popular research topic in the 50-70s.

Note that here we are talking about neutral solution so the concept of ordered lining up does not arise.

There are plenty of papers on this topic. Just search activity coefficient of benzene in aqueous solutions.

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  • $\begingroup$ Note that the saturated solution of benzene in water has the same activity as pure benzene ( or rather as benzene in saturated solution of water in benzene ). OTOH, the same concentration of benzene in toluene would have activity much lower. $\endgroup$ – Poutnik Sep 23 '20 at 16:28

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