I am wondering whether a neutral oxygen atom/molecule (so a particle without charge) can stick to a negatively charged surface. I would say yes because oxygen can form negative ions but can it be attached to the surface without binding chemically to the surface?
Hi Marek,
yes that might indeed be a possibility but can VdW binding occur when a neutral particle hits a negatively charged surface? I always thought of VdW as inter atomic or molecular binding but not as 'sticking' onto surfaces...
Well, Wikipedia says that VdW bounds are not of chemical nature. Moreover, neutral atom/molecule does not feel Coulomb (electrostatic) interaction just because the nearby substrate is negatively charged. But, in fact, your question is outside of area of my interest - I'm not a chemist nor material science specialist. This is why I ended my first response with question mark.
One must be careful to use words meanngfully: van der Waals forces are forces between electric charges, as are ``chemical'' forces. So the statement that vdW forces aren't chemical forces is, just, wrong. That was believed in the time before the nature of electric charge and its relevance for chemistry was understood, perhaps; but it's not the case in the last century or so.
The question doesn't change if one replaces oxygen atom/moelcule by an electric dipole. So the problem becomes the study of the force between an electric dipole and a charged surface.
That's a standard exercise, whose outcome is the calculation of the potential energy of the dipole/surface system.
The next issue to be clarified is what is meant by an electric dipole ``sticking'' to a charged surface. That can be addressed by calculating the potential energy of the system as a function of the distance between the dipole and the surface and finding, whether this energy does have a minimum at a special value of the distance that's smaller than the size of the dipole, which sets the length scale. It would seem reasonable to claim that, if such a special value of the distance exists, this is what defines ``sticking''.
It can be that no such distance exists in the static limit, but the dipole can oscillate about an average distance, of course. If the average distance and the amplitude of the oscillations remain bounded, this is what ``sticking'' can be understood to mean.
Thanks, Stam for our excellent answer, I will have to think about this
Approximately one and a half years ago, the problem of a shortage of oxygen concentrators for a huge number of new patients with the COVID-induced pneumonia and the emergence of a corresponding huge new market for such oxygen concentrators has greatly interested the medical industry around the world. Naturally, I and many other researchers became interested in the question of how the productivity of such oxygen concentrators could be significantly increased. That is, the question boiled down to the question of how can we temporarily intensify the physical adsorption of oxygen or nitrogen molecules on adsorbents commonly used in oxygen concentrators or, alternatively, how can we temporarily modify the surface of these adsorbents to increase the adsorption rate of either oxygen or nitrogen molecules? It is very likely that it was this major physical and technical problem associated with the current COVID-19 pandemic that attracted the interest of the author of the initial question of this discussion. Considering that some of my old papers were devoted to the problem of the electric charge-dipole attraction and the significant acceleration of polar gas molecules, for example, water vapor molecules, towards an electrically charged surface sites and / or towards the electrically charged aerosol nanoparticles, my interest in the aforementioned problem of the temporal possible enhancement of the physical adsorption rates of non-polar molecular oxygen or non-polar molecular nitrogen on multiple artificially electrically charged surface sites of the standard oxygen or nitrogen sorbents used in the standard oxygen concentrators, for example, using a pulsing high-voltage corona discharge, was a natural interest a few months ago. Under standard conditions, the atmospheric oxygen molecule competes with the atmospheric nitrogen molecule for a good (spontaneously electrically charged) spot /s on the surface of the adsorbent. In fact, it is generally accepted that this competition is a competition between the quadrupole electric moments of the oxygen molecule and the nitrogen molecule, wherein nitrogen's higher quadrupole electric moment gives the nitrogen molecule a relatively more thermally stable state on the adsorbent surface as compared to the oxygen molecule. If we try to simply separate oxygen molecules from the air due to their negative charging in corona or glow discharge plasma, then it is clear. that the efficiency of such a process may not be very high.
However, if we try to periodically electrically charge and next recharge areas of the standard adsorbent surface using a discharge plasma in an oxygen concentrator, this could indeed increase the efficiency and rate of oxygen separation in such modified oxygen concentrators, due, however, to a more intense charge-quadrupole attraction of the nitrogen molecule towards temporarily highly charged spots induced on the adsorbent surface. It is likely that the investment made in such plasma-modified oxygen concentrators can be justified in the context of the ongoing pandemic.
Information on the competition between oxygen and nitrogen quadrupole electric moments at spontaneously electrically charged sites of standard adsorbents in oxygen concentrators can be found, for example, here:
https://core.ac.uk/download/pdf/48655747.pdf
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