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I am currently reading a paper titled "Polystyrene Nanoplastics-Enhanced Contaminant Transport: Role of Irreversible Adsorption in Glassy Polymeric Domain" (link here).

In the paper, the authors mention the term 'desorption hysteresis' as the mechanism of the interaction between nanoplastics and contaminants. Here's the relevant excerpt (from the introduction section):

... at low nanoparticle (i.e., “carrier”) concentrations significant facilitated transport of organic contaminants requires not only strong adsorption of contaminants to the carriers, but also significant desorption hysteresis (a collective term referring to both slow desorption kinetics and thermodynamically irreversible adsorption) of contaminants from the carriers. Desorption hysteresis can either be due to the physical entrapment of contaminants in the complex matrices of the carriers, or strong specific adsorptive interactions that lead to irreversible binding of contaminants to the carriers.

I am having some difficulty in understanding what do they mean by adsorption hysteresis. From what I've read, hysteresis is how the system in its current state is related to some past state, but I don't understand this concept fully.

Could someone please explain what is desorption hysteresis? I couldn't even understand the explanation in parentheses (right after the term in bold above). Is it about the rate of desorption of contaminants from nanoparticles?

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Hysteresis occurs when the state of the system depends on its past states---that is, the state of the system is memory-dependent.

The canonical example is a ferromagnet: if you apply and slowly ramp up an external magnetic field to a ferromagnet for some time, and then slowly ramp the magnetic field back down for the same amount of time, you might expect the ferromagnet to return to its original state, but it doesn't. Instead, if you track the magnetization of the magnet as a function of the applied magnetic field, you see these hysteresis loops. Each applied magnetic field corresponds, therefore, to two possible system states, and which state the system exists in will depend on its past history.

Desorption hysteresis is hysteresis applied not to magnetization but to adsorption and desorption, and your excerpt then goes on to describe the physical interactions that drive this phenomenon.

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  • $\begingroup$ Thanks, but I still don't understand how the concept of hysteresis applies to desorption. Does it mean that whether the desorption of contaminants from nanoparticles occurs or not depends on the system's past? If so, what past state is that? $\endgroup$ – Don_S Jul 29 '18 at 12:02
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    $\begingroup$ @Don_S, yes. The article mentions thermodynamically irreversible adsorption. If you repeatedly perform adsorption-desorption cycles, eventually the adsorption sites in your material will be clogged up by these irreversible adsorption events and your material won't be able to adsorb and desorb as much stuff anymore. $\endgroup$ – a-cyclohexane-molecule Jul 29 '18 at 12:08
  • $\begingroup$ In the end of the Materials and Methods section, the authors calculate the Hysteresis Index (HI), and they say that "the higher the HI value, the greater degree of desorption hysteresis". From the HI calculation, it appears that the greater the observed concentration of adsorbed-phase (over the calculated concentration), the higher HI will be, but what does that mean? Why would the degree of desorption hysteresis be greater if the observed concentration will be larger than the calculated concentration? Also, what is the practical meaning of the desorption hysteresis degree being "greater"? $\endgroup$ – Don_S Jul 29 '18 at 12:18
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    $\begingroup$ @Don_S, those questions are unrelated to your original question and to my answer, and I don't feel comfortable answering them. I recommend you post those questions as a separate question. $\endgroup$ – a-cyclohexane-molecule Jul 29 '18 at 12:21
  • $\begingroup$ Are they truly unrelated? It all revolves around the concept of hysteresis which I've never encountered before, and so I have a hard time grasping. I thought that the hysteresis index is part of that concept, and that it would help me understand what they are talking about in the paper. I feel that opening a different post about it would be redundant, but I might be wrong. Let's see if anyone else prefers it that way too. Thanks for your help! $\endgroup$ – Don_S Jul 29 '18 at 12:24

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