I work on the adsorption of ions and inhibitors on CNT (armchair structure). Recently, I did a molecular dynamics simulation with Gromacs software with a Charmm 27 force field. But CNT only interacts with water up to a distance of 0-2 nm a lot. Its interaction with other ions and inhibitors at a distance of more than 2 nm is minimal. I slightly modified the CNT charge with the DFT quantum method, but there was very little change in the species adsorption. In my previous laboratory work, the CNT absorbed both the inhibitor and the ion, but absorption does not seem to occur in simulating molecular dynamics. I do not know precisely where the problem is. Here is my Structure file and topology.
Carbon nanotubes (CNT) are good hydrogen gas adsorbents. One way to store hydrogen is by adsorbing the H2 atoms on the surface of the carbon. Since it is an adsorption process (i.e., a physical process), there are no chemical reaction involved and there are no changes to the carbon structure.
The adsorbed hydrogen atoms can be adsorbed by heating the CNTs.
Hope this helps answer your question.
Reza Khiabani
You wrote to me with a request to answer this question. I replied that I was not an expert in calculation methods. Here I will add the following. Calculation methods do not reveal anything new. Usually they confirm what has already been discovered by experimental methods. Adsorption on nanoparticles of CNTs necessarily occurs, but you got that it does not occur by calculation methods. This is exactly the lack of calculation methods. Based on their results, conclusions cannot be drawn about natural phenomena.
Thanks, Professor Yuri Mirgorod, for taking the time to answer my question. A general question that has been on my mind about CNT is that maybe you can help in this regard. Can cation − π in uncharge CNT adsorb ions such as Na and Mg? Is adsorption in CNT because of π?
Reza Khiabani
Yes, benzene, of the rings of which CNTs are composed, has amphoteric properties. Its molecule forms weak intermolecular donor-acceptor bonds with a proton, possibly with a sodium ion and with ammonia. For CNTs, these properties can be enhanced.
Since the intermolecular forces that contribute to the attraction between CNTs and surrounding molecules decay with r-6, it is not that surprising that they get very weak beyond 2nm. For ions you may have some image potential forces which may reach mildly beyond that. A physisorbed species would probably have a distance below 1nm between the closest CNT and adsorbate atoms.
However, physisorption is not a long time stable process, so a room temperature MD would probably show a fast desorption process.
Therefore your results are not that surprising when we want to stick with physisorption.
Now, let's discuss another possibility, and that is chemisorption, i.e. the formation of actual bonds chemical between your adsorbate and the CNTs. That is impossible to simulate with force field methods since these cannot form or break chemical bonds, everything will stay bonded as you defined initially. The minimum level of calculation for chemisorption would be the semiempiric calculations such as PM7 or other xNDO methods. If your ion involves a transition metal, the results with that will probably be very inaccurate, still, so I'm not sure MD is actually the way to go and you may require single-point DFT here.
Thank you very much, Dr. Jürgen Weippert, for answering my question patiently and carefully. As you mentioned and I read earlier in the literature for laboratory work, the CNT adsorb ions by pi-cation, and inhibitor tend to interact with ions (attraction). Eventually, a physical connection occurs between the CNT-ion-inhibitor (physisorption). The problem is that ions do not even come close to CNT during simulation. The distance between CNT-nonWater is always more than 2 nm. As shown in the video I sent, all atoms except water are very far from CNT (the simulation box size is 5 x 5 x 5). But as expected, I saw physisorption in laboratory work.
I just had a coarse look into Charmm and I did not find any hints how (or if) it handles image potential forces which I would expect to be a major part of the physisorbing attraction here. Unfortunately, my knowledge of force field parametrizations is not really up to date, so I can't name you a field that's better here.
Also, if a pi system bonds to a cation, that is actually the formation of a weak ligand field, and that is not completely physisorption but actually weak chemisorption, I am not convinced that this can be properly accounted for with force fields in general (but I may be wrong on that one).
Dr. Jürgen Weippert,
After your message, I also doubted how the pi bond works (physisorption or chemisorption). Finally, after reading a couple of articles near my research, I think pi bond chemisorbed ions, and I also found the following article that simulates CNT via DFT. Thank you very much for devoting the time to finding my research problem.
doi:10.1016/j.diamond.2008.11.021
I wish you the best,
Reza Khaiabani