Hi John, Thanks for the clear and insightful webinar series! Could you please explain how the optical properties affect attractive forces between particles? I could see how the size/shape affect the relative strength of attractive VDW forces due to differences in contacting surface area between particles, but I'm unsure how optical properties come into play here.
Hi Leshy, Please see the Wikipedia page about London dispersion forces (en.wikipedia.org/wiki/London_dispersion_force) It explains that the forces arise from fluctuations of electron density. Because electromagnetic radiation propagates through a medium through interaction of the radiation's electric field and electrons, there is a connection between the optical properties and the London dispersion forces. This can be exploited. By dispersing particles in a liquid with identical refractive index, the net attraction almost disappears.
In your schematic, the Debye length decreases as electrolyte concentration increases, but why does a decreased Debye length lead to decreased Zeta potential. Since the stern layer does not change in position presumably. From the four figures you have, the stern potential seems to be the same value, so I guess that's why I am a bit confused. Thanks, Dr. Miller
Hi, the purpose of the schematic is to show how the double layer becomes thinner with increasing electrolyte concentration. The way I have drawn them certainly over simplifies what happens near to the particle. For example, in reality, the increased concentration could lead to more adsorbed counterions which would further screen the charge and reduce the zeta potential.
The diffuse layer is part of the continuous phase that includes the bulk. The slipping plane is an imaginary plane that delineates the particle (and any adsorbed material) from the continous phase. Imaginary is the key word here - it's purely a theoretical construct, especially since few dispersions truly meet all the criteria required for the definition of the diffuse electrical double layer.
Dear Mr Miller, I understand that electrolytes play a role in decreasing the EDL thickness but does this theory also apply to when adding acids or bases? Both acid and base disassociate to positive and negative ions, so theoretically it should perform similarly except one increases H+ or OH- ions. Thanks in advance.
Hi Abraham - yes, the addition of any ionic material will decrease the double layer thickness. When calculating ionic strength, you should include the contribution from acid/base, too.
Dear Mr. Miller, I have a curiosity: if generally zeta potential is expressed as the potential difference between the slipping plane and the bulk solution, how this value would compare to the potential difference between the actual particle surface and the slipping plane?
There isn't necessarily any relationship between the two although, for most systems, the potential at the slipping plane is likely slightly lower than the surface potential. However, because the slipping plane occurs at the boundary between the bulk liquid and the particle + any adsorbed species, the presences of adsorbed counterions, surfactants, polymers etc may significantly change the potential between the surface and the slipping plane. A classic example is an ionic surfactant.
That's an interesting question. Ionic strength is, in effect, the sum of the molar concentrations of each ion species in solution (with allowance for their valencies). An ionized surfactant behaves like a high molecular weight salt with one small ion (e.g., Na+, Cl-) etc and a much larger ion (the surfactant itself). The screening ability of the large ion is much lower than for the small ion (for the same reason that a monovalent simple ion is less effective at screening than a polyvalent one). Achieving, say, 1mM ionic strength with SDS (mol wt 288) requires about 4 times the mass to get the same ionic strength as KCl (mol wt 74.5). In addition, a typical surfactant concentration encountered (0.1% w/w) equates to about 0.3mM, which is low enough to have little influence. Of course, the above ignores the fact that the surfactant itself will either adsorb (and change the overall particle charge), form micelles and exist as free molecules below the critical micelle concentration.
Hi~ Mr. Miller Your illustration is really helpful! I would like to translate your video to mandarin. Could you please activate the function of the viewer's subtitle submission in all of your videos? Thanks!
thank you Mr.Miller. your videos are really helpful.
Hi John,
Thanks for the clear and insightful webinar series! Could you please explain how the optical properties affect attractive forces between particles?
I could see how the size/shape affect the relative strength of attractive VDW forces due to differences in contacting surface area between particles, but I'm unsure how optical properties come into play here.
Hi Leshy,
Please see the Wikipedia page about London dispersion forces (en.wikipedia.org/wiki/London_dispersion_force)
It explains that the forces arise from fluctuations of electron density. Because electromagnetic radiation propagates through a medium through interaction of the radiation's electric field and electrons, there is a connection between the optical properties and the London dispersion forces. This can be exploited. By dispersing particles in a liquid with identical refractive index, the net attraction almost disappears.
Thanks thanks thanks....this helped a lot
In your schematic, the Debye length decreases as electrolyte concentration increases, but why does a decreased Debye length lead to decreased Zeta potential. Since the stern layer does not change in position presumably. From the four figures you have, the stern potential seems to be the same value, so I guess that's why I am a bit confused. Thanks, Dr. Miller
Hi, the purpose of the schematic is to show how the double layer becomes thinner with increasing electrolyte concentration. The way I have drawn them certainly over simplifies what happens near to the particle. For example, in reality, the increased concentration could lead to more adsorbed counterions which would further screen the charge and reduce the zeta potential.
@@johnmiller0000 Thank you Dr. Miller.
I think the slipping plane should be between the diffusion layer and the bulk and not between stern and diffusion.
The diffuse layer is part of the continuous phase that includes the bulk. The slipping plane is an imaginary plane that delineates the particle (and any adsorbed material) from the continous phase. Imaginary is the key word here - it's purely a theoretical construct, especially since few dispersions truly meet all the criteria required for the definition of the diffuse electrical double layer.
saved my life!
Dear John, do you have any videos regarding to contact angle calculation? Thanks a lot
I do not, sorry.
Dear Mr Miller,
I understand that electrolytes play a role in decreasing the EDL thickness but does this theory also apply to when adding acids or bases? Both acid and base disassociate to positive and negative ions, so theoretically it should perform similarly except one increases H+ or OH- ions. Thanks in advance.
Hi Abraham - yes, the addition of any ionic material will decrease the double layer thickness. When calculating ionic strength, you should include the contribution from acid/base, too.
Dear Mr. Miller, I have a curiosity:
if generally zeta potential is expressed as the potential difference between the slipping plane and the bulk solution, how this value would compare to the potential difference between the actual particle surface and the slipping plane?
There isn't necessarily any relationship between the two although, for most systems, the potential at the slipping plane is likely slightly lower than the surface potential. However, because the slipping plane occurs at the boundary between the bulk liquid and the particle + any adsorbed species, the presences of adsorbed counterions, surfactants, polymers etc may significantly change the potential between the surface and the slipping plane. A classic example is an ionic surfactant.
@@johnmiller0000 Thank You very much for your answer!
How does the effect of ionic sufactants compare to ion concentration?
That's an interesting question. Ionic strength is, in effect, the sum of the molar concentrations of each ion species in solution (with allowance for their valencies). An ionized surfactant behaves like a high molecular weight salt with one small ion (e.g., Na+, Cl-) etc and a much larger ion (the surfactant itself). The screening ability of the large ion is much lower than for the small ion (for the same reason that a monovalent simple ion is less effective at screening than a polyvalent one). Achieving, say, 1mM ionic strength with SDS (mol wt 288) requires about 4 times the mass to get the same ionic strength as KCl (mol wt 74.5). In addition, a typical surfactant concentration encountered (0.1% w/w) equates to about 0.3mM, which is low enough to have little influence. Of course, the above ignores the fact that the surfactant itself will either adsorb (and change the overall particle charge), form micelles and exist as free molecules below the critical micelle concentration.
Hi~ Mr. Miller
Your illustration is really helpful! I would like to translate your video to mandarin. Could you please activate the function of the viewer's subtitle submission in all of your videos? Thanks!
Hi - thank you for offering to do this - I had no idea. I have enabled the option for all the videos.