Menu

Discussion Forum

New Discussion
Filter

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

How can I ensure Discrete locations for ions in a nanoscale ion-transport system ?

Posted Apr 14, 2015, 9:36 p.m. EDT General, Computational Fluid Dynamics (CFD), Chemical Reaction Engineering, Electrochemistry, Geometry, Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization 7 Replies

Kumar Krishnamoorthy
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

I'm using the Transport of dilute species module, and have coupled this with the Poission-Boltzmann equation (with Steric accomodations) to model transport of Sodium ions and Cholride ions in a nanotube with a charged wall. I'm getting a rise in concentration of oppositely charged ions at the wall surface, and a corresponding dip for likely charged ions, but this doesn't go to 0 mol/m^3 . Considering the dimensions of the system, I believe I should expect the Likely charged ions to not be present at the wall's surface, as it's physically impossible for both kinds of ions to be present at the same location. How can I ensure this ? Or is my assumption inherently wrong ? I have attached an image that has concentration plots for the ionic species. Please note that I have used an Axisymmetric 2D geometric model.
7 Replies Last Post Apr 18, 2015, 12:12 p.m. EDT
Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 15, 2015, 1:33 a.m. EDT
I understand that the nanochannel resides between two spaces on the bottom and top of your plots. In those volumes electroneutrality does not hold, the conc. of anions is ca. 50 mM and that of cations ca. 35 mM on the bottom and ca. 80 mM on the top. Electroneutrality does not hold in the electrical double layer but that is only a few nanometers thick in those concentrations; you have neither given the geometric dimensions.

To me it looks that the surface is positively charged as anions are accumulating on the wall and cations are repelled - provided that I understood correct where your channel is in the pictures. But whatever is the model, electroneutrality must hold in the bulk of the solution.

br
Lasse
I understand that the nanochannel resides between two spaces on the bottom and top of your plots. In those volumes electroneutrality does not hold, the conc. of anions is ca. 50 mM and that of cations ca. 35 mM on the bottom and ca. 80 mM on the top. Electroneutrality does not hold in the electrical double layer but that is only a few nanometers thick in those concentrations; you have neither given the geometric dimensions. To me it looks that the surface is positively charged as anions are accumulating on the wall and cations are repelled - provided that I understood correct where your channel is in the pictures. But whatever is the model, electroneutrality must hold in the bulk of the solution. br Lasse
Kumar Krishnamoorthy
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 15, 2015, 9:51 a.m. EDT
Thank you for your reply ! First, you are right, the surface has a positive charge of 0.05 C/m^2, The diameter of the channel is 4 nm. The Domain is as follows = There is a large bulk concentration of ions in the 'Inlet container', and no concentration in the channel and the other container. The time-dependent behaviour of this is studied. I have attached another picture that has details about the geometry. Again, thank you !
Thank you for your reply ! First, you are right, the surface has a positive charge of 0.05 C/m^2, The diameter of the channel is 4 nm. The Domain is as follows = There is a large bulk concentration of ions in the 'Inlet container', and no concentration in the channel and the other container. The time-dependent behaviour of this is studied. I have attached another picture that has details about the geometry. Again, thank you !

Attachments:

Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 16, 2015, 5:19 a.m. EDT
Hi

In 0.118 M concentration the Debye length is only ca. 1 nm. Hence I do not believe that your model in entirely correct (neither entirely incorrect :)). Charge density is proportional to the difference of cation and anion concentrations in Poisson-Boltzmann equation. Have you looked at that or the potential distribution? Charge relaxations take place is nanoseconds, i.e. electroneutrality should prevail beyond 1 nm from the surface at all times. And in the inlet and outlet compartment there is no surface charge, hence there is no charge separation. How long does it take to exchange the entire liquid volume? Your plot was at t = 1000 ns.

br
Lasse
Hi In 0.118 M concentration the Debye length is only ca. 1 nm. Hence I do not believe that your model in entirely correct (neither entirely incorrect :)). Charge density is proportional to the difference of cation and anion concentrations in Poisson-Boltzmann equation. Have you looked at that or the potential distribution? Charge relaxations take place is nanoseconds, i.e. electroneutrality should prevail beyond 1 nm from the surface at all times. And in the inlet and outlet compartment there is no surface charge, hence there is no charge separation. How long does it take to exchange the entire liquid volume? Your plot was at t = 1000 ns. br Lasse
Kumar Krishnamoorthy
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 16, 2015, 3:32 p.m. EDT
Haha, yeah. This would be better modeled as discrete ions, but what I'm essentially trying to do is make a continuum approximation. Actually, I only ran the model for 1000 ns as, there seems to be no change in concentration distribution than the images that I've plotted after about 800 ns or so. Would you like to take a look at my model ? I will attach it as soon as I get to the computer where I've saved it.
Haha, yeah. This would be better modeled as discrete ions, but what I'm essentially trying to do is make a continuum approximation. Actually, I only ran the model for 1000 ns as, there seems to be no change in concentration distribution than the images that I've plotted after about 800 ns or so. Would you like to take a look at my model ? I will attach it as soon as I get to the computer where I've saved it.
Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 17, 2015, 1:25 a.m. EDT
I can have look at it if you send it.

br
Lasse
I can have look at it if you send it. br Lasse
Kumar Krishnamoorthy
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 17, 2015, 12:14 p.m. EDT
My file size was too big, and I had to clear my solutions to make it small enough to attach here. You would be able to see that there is also a Laminar Flow physics module used in the model, but it is disabled on purpose. The Poisson Boltzmann's equation is a Steric-modified equation set based on this: www.academia.edu/3220012/Steric_Effects_in_Electrolytes_A_Modified_Poisson-Boltzmann_Equation

Thank you !
My file size was too big, and I had to clear my solutions to make it small enough to attach here. You would be able to see that there is also a Laminar Flow physics module used in the model, but it is disabled on purpose. The Poisson Boltzmann's equation is a Steric-modified equation set based on this: http://www.academia.edu/3220012/Steric_Effects_in_Electrolytes_A_Modified_Poisson-Boltzmann_Equation Thank you !
Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago Apr 18, 2015, 12:12 p.m. EDT
You have plenty of issues with the units. Also, I would not define variables but global parameters. I suspect that because your units are not matching, the exponential functions are overshooting seriously, giving physically meaningless results.

br
Lasse
You have plenty of issues with the units. Also, I would not define variables but global parameters. I suspect that because your units are not matching, the exponential functions are overshooting seriously, giving physically meaningless results. br Lasse

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content