Dear all,
I would like to know how the increase/decrease of the pH or ionic strength of the environment in which two negatively charged bodies are immersed will affect the "thickness" of the electrostatic double layer (EDL)? I have heuristically witnessed that low pH (e.g. 2) makes two negatively charged particles agglomerate, but I have not conducted experiments with high pH, and I have not played with solutions of different ionic strength. Some references would be helpful.
If the groups on the surface of the particles are pH-sensitive you can expect changes in the EDL. In general an increase in ionic strength will decrease the magnitude of the associated zeta potential.
Hi Zoran M. Cenev ,
A good approximation you can use to relate the pH level and ionic strength of the dispersing medium with the extent (or thickness) of the EDL is to calculate the Debye screening length.
Whenever the Debye-Hückel approximation is valid (i.e., whenever the surface potential of particles is low), a simple analytical expression can be used to evaluate the Debye screening length as a direct function of the concentration and valence of the electrolyte(s) in your solution.
Any standard book on colloidal dispersions explains this topic. For example, have a look at Section (14.14) of Israelachvili's (3E), or Section (11.5a) of Hiemenz's (3E).
I hope this helps!
Hi Zoran,
for example in the case of particles that can be deprotonated (or have the possibility of polarize), in a acid medium your "bodies" are use to be protonated and hence less charged which cause their aglomeration and coagulation. On the other hands, at higher pH your bodies are deprotonated and hence the EDL should increase. Nevertheless the final results is up to the nature of the surface of the bodies as well as of the environment.
In general the thickness of this EDL will depend on the bulk electrolyte concentration (cel ), through the Debye parameter, κ ;
k2 = (2e2NA celZ2)/ (ee0 KT)
where
- cel = bulk electrolyte concentration
- k= Debye parameter
- NA = Avogadro number
-z is the valency of the counter-ions
-e= is the dielectric ratio of the continous phase (e.g = 80 for water)
-e0 = is the permitivity of the free space
You can find more information in the open litarature for this topic.
Best
Stefano
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