Dear reseachers,
there is the phenomena that clay particles aggregate and flocculate in salt water. But in fresh water they don't aggregate and develop a laminated structure when deposited.
I read that the repulsion forces of the faces of the clay platelets are responsible that they do not aggregate in fresh water. In salt water there are enough positively charged ions to counteract this phenomena so that an aggregation is possible.
Here is my question: Is it possible that the Na and Ca ions in salt water are bound to the edges of the clay platelets that the edges are positively charged that they can built edge-to-face-contacts with other clay platelets in this environment?
Best regards
Dear Christian,
The explanation that you illustrated to the phenomenon ("positively charged that they can built edge-to-face-contacts with other clay platelets") is right, however, the terminology to be used should be flocculation and not aggregation. For more on this topic please see the following text:
Clay is a colloid. Colloidal particles have special properties due to their very small
size. Firstly, their large surface area in relation to their mass makes them very reactive; in clays, this reactivity is shown as an electrostatic attraction of cations.
Secondly, colloids can exist in water as either: suspensions (dispersed) or as gels (flocculated). The tendency of a colloid to flocculate or disperse depends on three things: (1) the nature of the colloidal particles; (2) the total salt concentration; and (3) the nature of the adsorbed ions.
The type and amount of different cations in a clay-water-electrolyte system have a
major influence on double layer interaction. Flocculation to describe particles that are connected edge–to–edge or edge–to–face, Aggregation to describe particles that are connected face–to–face.
TERMINOLOGY
Dispersed: No face-to-face association of clay particles
Aggregated: Face-to-face association (FF) of several clay particles.
Flocculated: Edge-to-Edge (EE) or edge-to-face (EF) association
Deflocculated: No association between aggregates or particles.
For more information, please see the following link:
http://civil.emu.edu.tr/courses/civl553/Lec11%20Fabric%20[Compatibility%20Mode].pdf
Hoping this will be helpful,
Rafik
You may refer to the book 'Van Olphen, H., 1966.An Introduction of Clay Colloid Chemistry. Interscience, New York.' pages 93-95. With best wishes-
Dear Mr. Karaman,
thank you your your explanation. It is very helpful; especially the difference between aggrgation and flocculation.
thank you, Mr. Pal for your literature tip.
Best Regards
as always there is field by field difference in terminology.
Agglomeration is the general term for forming loose particle collectives and also synonym to flocculation when system is solid liquid. Aggregates are strong, more or less fused particle collectives, which is for platelets as clays usually face to face orientation.
Unfortunately, terms agglomeration and aggregation in some fields are used with the opposite meaning.
Towards the original question - clay platelets have negative charges on faces and positive charge centres at the edges. Therefore clays can be flocculated at identical pH value by cationic polyelectrolytes as well as by low concentration of anionic polyelectrolytes .
In water at neutral pH clay platelets negative charges dominate. By adding salt reach of repulsive forces is reduced and charged colloids can be flocculated. In case of clay platelets then they can agglomerate with face to edge orientation.
At certain salt concentrations, the stern layer of negative ions around the clay platelet is partially removed so that the positive charge of the edge surface is exposed to the solution. Yet, the faces of the clay platelets still carry negative ions and thus have a negative charge. This condition is extremely favorable for aggregation since positively-charged edges can easily attach to the negatively-charged faces. This is also called "card-house" aggregation.
Regarding the terminology that has been discussed in the above answers, it highly varies with the context of application. Typically, "aggregation" is the general term that is used for all of its different types: "agglomeration", "coagulation", and "flocculation". In the nanoparticle context "aggregation" term is also used for a specific type of the phenomena which results in stronger aggregates in terms of the association between individual particles. This is in contrast to "agglomeration" which can be easily subject to "disagglomeration" or "fragmentation".
Hope this helps
Related to the above explanations, I've a doubt. Why clay edge is+ve?
Malavika,
you may look into the text books for clays. As mentioned, there is experimental evidence - i.e. flocculation by positively and also by negatively charged species.
The reason behind this phenomenon is of cause chemical composition and structure. Clays are alumosilicates. Negative charges are due to silicates positive charges due to aluminium atoms. Clay specialists may give a more detailed explanation.
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