After calculating and graphing the evolution of internal energy with varying nitrobenzene concentrations across different temperatures, I noticed that the Eint values were initially positive at low concentrations and then became negative.
I should also mention that I used an activated carbon with an acidic surface and a high surface area for this experiment.
Could someone please provide an explanation for this observed behaviour?
Just to be sure: you are using the correct convention "higher internal energy = less stable" and "lower internal energy = more stable", right?
I've seen the opposite case, i.e. the early adsorption stages being highly favorable while the later stages were more and more unstable (e.g.Cs on a different metal). The reason was that at first, the Cs could get rid of electron density while the later arriving Cs can no longer do that and is actually repelled because surface dipoles are coming closer to each other and thus becoming repulsive.
You are describing nitrobenzene on an unspecified surface, if it's unstable in the beginning, that would mean that [whatever your surface is] and nitrobenzene don't really like each other. If it becomes favorable later, that would mean it's actually mostly binding to itself and not the surface.
I’m sorry, I forgot to mention that I used an activated carbon with pHpzc ~ 4 and high specific surface area ~1700m2/g.
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