Dear Akhilesh,

When the interaction potential between the particles is purely attractive, the aggregation process is solely limited by mutual diffusion (or Brownian motion) of the particles, one refers to fast, rapid or diffusion limited aggregation (DLA). When the interaction potential shows an intermediate barrier, the aggregation is slowed down by the fact that numerous attempts will be necessary to overcome this barrier, and one refers to slow or reaction limited aggregation (RLA). The aggregation can be tuned from fast to slow by varying the concentration of salt, pH, or an other additive. Since the transition from fast to slow aggregation occurs in a narrow concentration range, and one refers to this range as the critical coagulation concentration (CCC).

In your case, you have raised the pH from 2 to 5; this is a very big change which might resulted in a vast shift from CCC towards the instability of the sol (see attached figure).

Generally, change in pH and concentration of electrolytes has a significant effect on the stability of sol.

Hoping this will be helpful,

Rafik

There is likely a problem in comparing concentrated and dilute beyond particle concentration itself, e.g. change in dissolution equilibria, concentration of dissolved ions.

In present case data obtained in dilute regime are meaningless for understanding of situation in concentrated regime.

Yes dear

The stability of sol is highly affected by concentration change, pH, aggregation and /or agglomeration.

At pH 2-3 highly dispersion stability leads to stable particles as polynuclear cataions, reflecting the presence of enough positive surface charges on these  particles to maintain the high stability degree. These highly dispersible cataions due to the high positive surface charge embody themselves in polymeric chains and as the fluid's pH increased to 5, as a result of dilution by base fluid, a basic catalyst acts over middle of these chains allowing the formation of highly interconnected polymer clusters structure with less positive charge until reaching the case of null particles surface charge (iso-electric pH) for all clusters within the sol matrix. At this stage the sol loss their stability and settles down.  These clusters are in some cases non-continuous and non- uniform in the matrix of solution. The characterization of the fluid then changed from solution at pH 2-3  to colloidal dispersion then to suspension at pH>5 with observable effect on the suspension stability and accordingly the particles movement within the sol are also changed from kinetic to Brownian to Convection movement according to the common solutions characterizations.

Please go through my full text on Research gate titled  

"Experimental Stability and Thermal Characteristics Enhancement Analysis of Water Based Boehmite Nanofluid"

thank you

First you need to define "stability". Do you observe flocculation (reduction in he number of individual particles)?

Do you use packing-density as an indirect value to describe your "stability"?

It is also not clear what you mean by 12% solid content at 1000C. You would not have a "sol" at 1000C!

Herbert Giesche

Sir by stability of sol I mean to say remain in sol form without undergoing gelation. For lower pH it remains stable for a longer period of time, for higher pH above 5, it undergoes gelation.

And by 12% solid content means 100 g of sol when fired at 1000 C gives 12% solid content, just given that information to describe about the particle concentration of sol .   .

I want to know that whether the Zeta potential values (related to sol stability) will be different or same for concentrated and dilute sol at same pH.

The zeta-potential is not really being affected by the concentration of your solid particles. However, you stability will be affected by the concentration, since you will have a higher chance of collisions between your particles. Another side-effect could be the concentration of dissolved ions in your solution, which could be higher at the higher particle concentration. Higher ion concentrations will lead to lower zeta potential.

However, some of your description is still "confusing". You mention that the sol becomes stable at a pH of 5 but you then mention that the zeta-potential is highest at pH 5.

One or the other statement seems to be wrong.

Why higher concentrations of chitosan produced nanoparticles with lower values of zeta potential?