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Author: Subject: Why do some precipitates flocculate when agitated?
agent_entropy
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[*] posted on 16-10-2015 at 05:02
Why do some precipitates flocculate when agitated?


Why do some precipitates flocculate when agitated?

For example, silver chloride exhibits this behavior strongly. When a solution of say sodium chloride is added to a silver nitrate soluton, the precipitate initially appears milky and evenly distributed. But if the mixture is agitated, the precipitate flocculates into larger agglomerations of particles that settle out more quickly.

I don't see why such a change should occur since the chemistry of the mixture has not changed. Not even the temperature has changed, so why does it flocculate?

Maybe the answer is obvious, but I can't seem to come up with it.
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AJKOER
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[*] posted on 21-10-2015 at 12:31


Perhaps some insights from sonochemistry. Per Wikipedia (see https://en.m.wikipedia.org/wiki/Sonochemistry ), to quote:

"With liquids containing solids, similar phenomena may occur with exposure to ultrasound. Once cavitation occurs near an extended solid surface, cavity collapse is nonspherical and drives high-speed jets of liquid to the surface.[5] These jets and associated shock waves can damage the now highly heated surface. Liquid-powder suspensions produce high velocity interparticle collisions. These collisions can change the surface morphology, composition, and reactivity.[8]"
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aga
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[*] posted on 21-10-2015 at 12:43


At a Guess, you're observing the differences between Thermodynamics and Kinetics.

Thermodynamics will say Yes , those particles in suspension would happily react (in some way).

Kinetics probably says they will not meet each other much in a way that they can react.

You shake it up (altering the Kinetics) and they collide more often, thereby react to form maybe some sort of aglomerate, e.g a crystal or a complex, making 'clumps'.

Just my notion.

I'd be more than pleased to be entirely wrong and see a Correct answer to that question.

[Edited on 21-10-2015 by aga]




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Sulaiman
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[*] posted on 21-10-2015 at 13:09


silver chloride may be a little complicated as it is quite photo-sensitive.
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Oscilllator
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[*] posted on 21-10-2015 at 15:08


Perhaps the initially formed tiny globules are separated from each other, but when agitated they come into contact and stick together?

Maybe I am just being a bore, but I do not see anything very mysterious here.
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AJKOER
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[*] posted on 22-10-2015 at 03:50


Quote: Originally posted by Oscilllator  
Perhaps the initially formed tiny globules are separated from each other, but when agitated they come into contact and stick together?

Maybe I am just being a bore, but I do not see anything very mysterious here.


Yes, separated by a weak electrostatic repulsion as they all have the same charge. The small particles have little mass, so gravitational forces not much help.

Agitation (like in using a centrifuge) can overcome the weak electrostatic forces.

[Edit] Apparently, such electrostatic forces exist as to quote an example from an abstract:

"The potential of interactions of particles in concentrated solutions of electrolytes in the steady state is calculated taking into account the electrostatic and solvation (hydration) interactions and weak chemical bonding between the hydration shells."

Source: "Thermodynamics of electrostatic forces interaction in concentrated electrolyte solutions" by G. G. Aseev. Link: http://link.springer.com/article/10.1134%2FS1070363210110022

So electrostatic forces is one possible answer along with solvation interactions and the formation (or lack thereof) of any chemical bonds.

Actual, here is a better reference with full text availability. To quote the abstract:

"In atomic force microscopy, the tip experiences electrostatic, van der Waals, and hydration forces when imaging in electrolyte solution above a charged surface. To study the electrostatic interaction force vs distance, curves were recorded at different salt concentrations and pH values. This was done with tips bearing surface charges of different sign and magnitude (silicon nitride, Al2O3, glass, and diamond) on negatively charged surfaces (mica and glass). In addition to the van der Waals attraction, neutral and negatively charged tips experienced a repulsive force. This repulsive force depended on the salt concentration. It decayed exponentially with distance having a decay length similar to the Debye length. Typical forces were about 0.1 nN strong. With positively charged tips, purely attractive forces were observed. Comparing these results with calculations showed the electrostatic origin of this force.
In the presence of high concentrations (> 3 M) of divalent cations, where the electrostatic force can be completely ignored, another repulsive force was observed with silicon nitride tips on mica. This force decayed roughly exponentially with a decay length of 3 nm and was ∼0.07-nN strong. This repulsion is attributed to the hydration force."

Source: "Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope", by Hans-Jürgen Butt. Link to full text: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1260203/

So it appears we should more correctly add Van der Waals forces to the list as an opposing force in our case of suspended particles generally defined as relating to the attraction of intermolecular forces between molecules (and not the term gravitational, I employed above). In particular, two kinds of Van der Waals forces are generally cited, namely weak London Dispersion Forces and stronger dipole-dipole forces.

[Edited on 22-10-2015 by AJKOER]
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