Why doesn't enthalpy of water-ethanol mixtures show symmetry?

[Jackymoon13]

Hello,

I was doing some research on enthalpy of water-ethanol mixtures at different ratios, and when looking at all the information graphed, I don't understand why it produces the trend line it does. Why doesn't it produce a 'quadratic' trend line? Shouldn't having a 10:90 water-ethanol exert the same amount of energy that a 90:10 ratio would?

The graph in question: https://i.stack.imgur.com/lgSbi.png

Thanks.

 

[Lord Jestocost]

I suppose that you are thinking of a description of the mixing enthalpy in terms of a regular solution model which gives a 'quadratic' dependence as a function of concentration. Regular solution models assume that the short-range environment around a given molecule can be expressed in terms of a random distribution of the constituents, viz. in terms of overall concentration values. In reality, however, one has to think about clusters (central molecule + environment) with a certain short-range ordering. Assume, as a simple example, that one has mixture of small molecules A and much larger molecules B (with attractive interactions between A and B). The mixing enthalpy will be different whether one dilutes a few small molecules in a sea of large molecules or vice versa.