Greetings,
I’m trying to mimic the changes in the contact angle when some random sites on the topmost layer of a neutral hydrophobic surface become partially charged. The net charge of the surface would be -48 in that case. In order to keep the system charge-neutral, an equivalent number of counter ions (sodium cations) were included in the (flexible SPC) water cluster, composed of 2000 molecules. The problem is that the extent of decrease in the contact angle, from the zero-charge to the charge-bearing case, is more than what is expected from a theoretical point of view. I believe the issue can be attributed to the high concentration of counter-ions in water (corresponding to the molarity of ~1.2) which result in an intense spreading of the cluster. I was wondering if there is a way to make the whole system neutral without explicitly introducing the counter-ions.
Thank you in advance for your comments.
Best regards,
Monir
Greetings,
I’m trying to mimic the changes in the contact angle when some random sites on the topmost layer of a neutral hydrophobic surface become partially charged. The net charge of the surface would be -48 in that case. In order to keep the system charge-neutral, an equivalent number of counter ions (sodium cations) were included in the (flexible SPC) water cluster, composed of 2000 molecules. The problem is that the extent of decrease in the contact angle, from the zero-charge to the charge-bearing case, is more than what is expected from a theoretical point of view. I believe the issue can be attributed to the high concentration of counter-ions in water (corresponding to the molarity of ~1.2) which result in an intense spreading of the cluster. I was wondering if there is a way to make the whole system neutral without explicitly introducing the counter-ions.
if you want explicit charges to represent the charged state, you need explicit counter charges. i am not aware of any alternate method that is reliable.
What you may want to try is to confine a large part of the counter charges in a region suitably far away from the surface. but keep in mind, that that will introduce a significant potential drop across your interface. you would have to make certain, that one is reasonable.
Beyond that, you may want to try increasing the system size.
please note similar problems exists for people that are studying voltage gated membrane proteins. perhaps you can find some inspiration there.
Axel.
Dear Axel,
Thank you very much for the response.
I have already tried inserting the charge-balancing ions in a region away from the surface, but it only works when the separation distance is too large (around 200 nm), which causes the computation time to be so long. In order to eliminate the interactions among water molecules and these ions at shorter distances, I applied the “neigh_modify exclude” command. However it appears that as long as the kspace_style is being used, those interactions are taken into account. I’d be grateful if you could let me know of any suggestions you may have in this regard.
Best regards,
Monir
Dear Axel,
Thank you very much for the response.
I have already tried inserting the charge-balancing ions in a region away from the surface, but it only works when the separation distance is too large (around 200 nm), which causes the computation time to be so long. In order to eliminate the interactions among water molecules and these ions at shorter distances, I applied the “neigh_modify exclude” command. However it appears that as long as the kspace_style is being used, those interactions are taken into account. I’d be grateful if you could let me know of any suggestions you may have in this regard.
sorry, but if you want to do what you describe you want to do with the level of accuracy that you are looking for, then those calculations are going to be expensive. or to put it differently: there ain’t no escape from the blues. 
the neigh_modify exclude thing is certainly wrong in this case and will result in bogus results apart from not having much effect even without kspace.
your system is still very small compared to what people with those membrane proteins i was mentioning are facing. they have to apply for time on large parallel supercomputers to do their research. …and they need very long trajectories, too.
axel.
Thank you for the clarification; I appreciate it.
Best regards,
Monir
Dear Monir,
Also, from a physical point of view, it seems to me that you are
trying to replace a system that is probably not perfect (indeed force
fields for ions in water might not be very good at high molality) by
another system that is simply incorrect. A "charged" interface between
an aqueous electrolyte and a solid surface is always electrically
neutral, with the charge of the solid being exactly compensated by the
charge of the ions in the electrical double layer.
So I think the solution to your problem lies elsewhere. Don't hesitate
to send a more detailed description of the problem to get more
suggestions.
For instance, surface charge only appears where the electrolyte is in
contact with the substrate. So you would need a kind of reactive
surface with surface groups that could dissociate (into a charged
surface group and a counter-ion) in the presence of the electrolyte,
which seems complex. I think in practice people trying to model such
effects rather compute the interfacial free energy of the different
interfaces involved at the contact line (liquid-solid, liquid-vapor,
solid-vapor), but I'm not an expert of that kind of calculations.
Best regards,
Laurent