You should have traces of water in ethanol (since water is wholly miscible with ethanol). These water molecules considered as impurities may play an important role in the peptisation, salvation and stability of the silica particles in ethanol. The role of water traces in organic liquid colloidal dispersion was investigated by several authors (Please see the references cited in the papers that I sent to you).

Regards

Amane

Hi Farooq,

I am assuming that you are talking about a silica gel suspension.  A gel is a colloid suspension of a liquid within a solid, rather than a solid within a liquid.  Typically the solid is cross-linked in a gel, so you would not see Brownian motion.

I hope that helps,

Cheers,

John

Hello Dr. Hansen, I agree that the chromatographic silica is not a true colloid because of the size (2-3 micron) but I guess it is not a true gel. Modern silica particles probably have a monolayer of water at best and I used ultra-dry particles. What do you think why such particles are stable in non-aqueous systems? The typical stability is explained on the basis of electrical charges due to ions. It seems that silica does have charge on the surface even in organic solvents.

Hi Farooq,

A sol is a colloidal dispersion of a solid in a liquid continuous phase.  A gel is a liquid dispersed in a solid continuous phase.  Silica gel would be classified as a nonelastic gel. Some gels liquify readily when shaken to form a sol and then revert back to a gel, this reversible sol-gel transformation is referred to as thixotropy. 

What stabilizes a sol will not necessarily stabilize a gel.  Typically one views the colloids in a sol (for instance from precipitation) as being composed of an electric double  layer that stabilizes the suspension due to coulombic repulsion.  The surface of silica would probably be charged much like the surface of window glass (however surface area to volume would be much different).  The counter-ion, I would imagine to be buried within the silicate network (much like alkali cations within glass). 

In the gel, liquid is trapped as a colloid within a continuous solid phase.  Following agitation, the solvated dispersed particles of silica lose stability, agglomerate into larger masses that mat together forming a semirigid structure that entraps solvent. 

Best regards,

John

Dear Farooq,

unfortunately you did not described the dispersions you speak about in detail.

Depending on the actual dispersion reasons are different.

Some examples.

Aggregate free silica particles in ethanol and similar solvents are stabilized by electrostatics. In ethanol double layer is much more extended.

When you are speaking about products like Aerosil, these are stabilized against sedimentation by interparticle bonding (Si-O-Si) and hydrogen bonding.

I do not agree with the answer given by Mr John E. Hansen. I do not understand why Mr John E. Hansen is talking about silica gel since the question of M. Farooq Wahab concerns the dispersion of silica particles in non-aqueous medium. That is to say a dispersion of small amount of silica particles in a large continue organic liquid medium, so the system described by Mr Farooq Wahab is not a dispersion of a liquid in a solid matrix as wrongly argued by C but a solid dispersed in an organic liquid medium. Mr John E. Hansen should read papers in the literature dealing with the stabilization colloidal particles in non-aqueous liquid media. In this media stabilization can occur via steric or electrical interactions (Electrical surface charges can occur even if the colloidal particles are dispersed in non-aqueous liquids, (see at least references below)

1) Location of the shear plane in the electric double layer in organic medium

B. SIFFERT, A. JADA and J. ELELI-LETSANGO.

J. COLL. AND INTERF. SCI, 163, (1994), pp 324-327.

2) Stability calculations of TiO2 in non-aqueous suspensions. Thickness of the electrical double layer

B. SIFFERT, A. JADA and J. ELELI-LETSANGO.

J. COLL. AND INTERF. SCI. 167, (1994), pp 281-286.

3) Experimental determination of the electron donor and acceptor numbers of oxides by zetametry in organic media

B. SIFFERT, A. JADA, J. ELELI-LETSANGO and E. PAPIRER.

Colloids and Surfaces, A, vol.92, (1994), pp 107-111.

Dear Dr. Sobisch, The dispersions of interest are 2-3 micron porous silica particles which are surface modified with non-ionizable functional groups. If one makes a suspension of such particles in solvents lets say alcohols mixed with chlorinated solvents, the particles are usually appear dispersed under a microscope. Not all alcohols are equally dispersive e.g. methanol and propanol are agglomerating in one case but ethanol is not. So what is the source of surface charges when there are no ions/ electrolytes in the alcohols? I would eliminate steric stabilitization since the surface modification is perhaps a monolayer (there is no polymer coating etc).

You should have traces of water in ethanol (since water is wholly miscible with ethanol). These water molecules considered as impurities may play an important role in the peptisation, salvation and stability of the silica particles in ethanol. The role of water traces in organic liquid colloidal dispersion was investigated by several authors (Please see the references cited in the papers that I sent to you).

Regards

Amane

Stabilization in mixed solvents is another issue. Silica particles in alkanol / hydrophobic solvent mixtures are stabilized by adsorbed layers of the alkanols. At least this is what other found and was experienced in own work in mixtures of alkanols /cyclohexane mixtures.

see

https://drive.google.com/file/d/0B-eBFrtlEwlhZGQ2MWZhZWItZDQ1Zi00ODQxLTg5ZjctNzBkNGZlNDVmNzFm/view?usp=sharing

Thanks for your references, it is very interesting that you noted that traces of water can cause stabilization. Ethanol was of highest quality (but not anhydrous). However, in my observation, if we intentionally add water by 1%, agglomeration can occur. Is there is a general theory (like the colloidal DLVO for aqueous systems) for the forces which cause the stabilization in non-aqueous systems.

Secondly, if (small length) alkanols adsorb on the silica surface, what forces lead to colloidal stabilization, since they are still uncharged? 

think beyond electrostatics. Steric stabilization is also very efficient. At the end it is all about balance of interaction forces, which is also the basis of DLVO.

If particles prefer to interact with surrounding molecules over interparticle attraction this means in other words they are well stabilized against agglomeration. This works for systems with particles/solvents in the same way as for particle/mixtures of solvents in case there is preferential adsorption of one solvent.

The stabilization of dispersions by adsorption (steric) of alcohol is very interesting. I always thought we needed large surfactant molecules to practically see steric stabilization but your work clearly shows that small molecules can also cause steric stabilization. Your lumifuge instrument is potentially very useful for people studying the packing behaviour of silica based suspension into narrow columns at very pressures (>8000 psi). The performance of the column is dictated how nicely the bed was packed. How much pressure (force) can this instrument simulate? 

Maybe for your application it is too small. Of course it depends on density difference and mass concentration. For silica like material we reached excess pressure at the bottom of the cell in the range of 50 000 N/m².

If Brownian movement is absent, the stabilization is due to changes in the rheological properties of the solvent as such.  A sensitive rheometer might be helpful.

Could you please elaborate more as to what is implied by a dispersed suspension in which Brownian motion is absent in a non-aqueous medium? The particles are large (2-3 microns) so I do not expect frantic motion like that of true colloids.

If I understand correctly, Stigs answer refers to stabilization against settling. My comment referred to stabilization against agglomeration.

Dear  Mr. Wahab, I would advise you to find the book  'Surface Forces'  by prof. N.V.Churaev and collegues, who worked at acad. B.V Derjuguin's Deparment of the Institute Of Physical Chemistry (USSR, now Russian Federation).  Boris V. Derjaguin is one of the authors of  DLVO theory. The issues you are interested are discussed in detailes in this book. See, for instance, AMAZON books (2013)

There are several mechanisms to achieve stability of colloidal dispersions in nonpolar media. Have you considered depletion forces? I can recommend the book "Polymeric stabilization of colloidal dispersions" by Donald H. Napper (published in 1983).