Usually, Van't-Hoff plot is used in order to establish enthalpy of adsorption process. In this way, we can know if the process in endothermic or exothermic. But, what happen when this plot is not linear? I would like to know your opinion about a two-stage Van't-Hoff plot, with an inflection point. I do not find references related to adsorption which deals with this behaviour. I obtain this trend for the adsorption of a fungicide onto different activated carbons. The tested temperatures are: 10, 25 and 40 ºC. And 25ºC correspond with the inflection point. Thanks in advance!
In Vanthoff equation it is considered that the enthalpy is Temperature independent. However, if the range of T is large enthalpy variation is not constant.
I suggest you choose a smaller range for T.
Thanks for your reply :-)
I have read in some papers, not related with adsorption, that a second order polynomial equation can be applied in order to obtain the value of adsorption enthalpy for each temperature.
Yes right. In engineering thermodynamic textbooks such as smith & Van ness ( chapter 4) you can find these equations. But they are usually applied for chemical reaction not physical interactions. I think the adsorption behavior on your adsorbent may change in two different temperature ranges. So it's better to choose a range with the best linear behavior.
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