Dear Fahimeh,

I suppose this is foaming of an aqueous solution, right? Most anti-foam agents for aqueous foams work by making tiny apolar droplets which cause the aqueous foam films to break. To disperse these apolar (oil) droplets ,and to keep them stable in dilute emulsions, is to stabilize them with a suitable surfactant. This surfactant lowers the surface tension.

Aqueous foam stabilization is however caused by surface tension gradients, so in fact you should not lower the surface tension of the solution that is foaming, this will only stimulate additional foaming. The change in surface tension after you added an anti-foam agent is from the surfactants present in that product. So your anti-foam is an oil in water emulsion stabilized by surfactant. When you add it to an aqueous system the surfactant will help disperse the oil, next the surfactant will redistribute because you diluted the anti-foam by putting it in an aqueous foaming solution. The oil droplets now again become hydrophobic and can do their work in breaking aqueous foam-films.

Please also remember, each anti-foam has an optimal working range. Too low it will not work sufficiently, too high brings in too much surfactant so that foaming is enhanced in stead of lowered.

I hope this helps, kind regards, Leo

Dear Leo,

Thank you for answering my question.

Yes, foam is in aqueous medium and Antifoam is an oil-in-water emulsion with emulsifier, but when antifoam is added to foam media, it does not spread and creates a tunnel in foam and goes down.

What factors affect this phenomenon? Can the reduction of surface tension in antifoam improve it? Reduction or increase in HLB emulsifiers can help?

Kindly reply,

Thanks

Dear Fahimeh,

From your description I guess you are adding the anti-foam on top off the foam, correct? It also sounds like once the anti-foam has passed the foam-layer it collects in an aqueous layer which is withdrawn leaving the solution basicly without anti-foam, correct?

Without knowing the specifics of your process, and with respect to foaming every system is different and requires a specific solution, I can only give you some general guidelines.

1. Anti-foam is preferably injected at a point where turbulence is high so that the turbulence helps in mixing the anit-foam.

2. I normally try to inject the anti-foam in the aqueous layer, just below the foam layer. This way the rising foam will carry the anti-foam to where it should work.

3. If your aqueous phase is being replenished or constantly removed, a single dosage of anti-foam may not be sufficient. You may want to consider a continuous dosage. In small columns or vessels I have suceessfully used HPLC-pumps to dosage the anti-foam.

Finally, you may want to look at the possibility to avoid foam at all. For foam one needs a liquid, a gas, something surface active (Impurities? Side products? Proteins?) and mixing. If you elmininate either one of those factors the foaming will stop. Normally liquid cannot be avoided, if gas is not generated by the process you may want to look at that, surfae active impurities could be removed by active carbon as they adsorb quite strongly onto that, finally mixing or input of energy could be avoided by choosing milder mixing, not filling a vessel by 'dropping the liquid from great height into a vessel' etc. A critical review of your process conditions could alleviate the foaming issue.

Reducing surface tension, again, is tricky. If you would want to do this try adding a bit of an alcohol (ethanol will do fine). But please be careful, ethanol may also aggrevate the foaming. In some systems one can use a knock down of foam by spraying ethanol onto the foam. This kills the foam in most cases immediately, but this trick works only once. Adding more ethanol when the foaming returns may give even stronger foam.

I hope this helps, kind regards, Leo

Seems you expect too much, you have to spray it onto the foam

Dear Leo,

Thank you for your complete description

But, my main problem is not to remove foam, I am researching about anti-foams.To check the effect of anti-foam, make a foam in aqueous medium, separately and add a drop of the anti-foam on top off the foam. But anti-foam, an emulsion of oil (silicon) in water with a mixture of emulsifiers, can not spread on the foam surface and remove the foam, but only creates a hole in the foam and goes down. What factors contribute to increasing the strength of anti foam spreading in the foam media? Reducing surface tension, changing the amount of emulsifiers or changing their HLB ?

Kindly reply, Thank so much for devote time.

for testing anti-foam efficiency you should follow Leo's advice - dose the antifoam in the highly turbulent region during formation of foam, not on top after foam has formed.

you can see: Standard Test Method for Effectiveness of Defoaming Agents

ftp://185.72.26.245/Astm/2/01/Section%2011/ASTM1105/PDF/E2407.pdf

Dear Mrs. Kashani

First of all, surface tension depends on the temperature, composition, and pressure. For anti-foam effect, reducing surface tension does not always correspond to ani-foam. As an example, SDS has low values of surface tension (about 28 mN/m at its CMC), but it is a highly foaming agent. A good anti foam is the one helping to reduce amount or fraction of foaming agent at the interface. In order to recognize this, you need experimental measurements and a model. Recently, I have conducted an investigation. I have attached it for you.

Best Regards,

Shahin Khosharay

Dear Fahimeh,

You quote an ASTM-method for a defoamer..... An antifoam and a defoamer are two completely different compounds.

An antifoam should give a sustained foam-breaking action, it is dosed in the highly turbulent region, just below the foam. It is normally an oil with a surfactant to help dispersion.

A defoamer should break foam once and completely. It is sprayed on top of the foam. A defoamer, normally, works only once.... A common defoamer is ethanol. Once it is in the liquid phase it no longer works, nor do new dosages on top of the foam work. For a defoamer a low surface tension is required.

So, make sure you know what compound you want: a defoamer or an antifoam?

I hope this helps, kind regards, Leo

Thanks to all for their explanations

Hello Fahimeh Kashani,

An interesting subject and a contemporary problem.

Surface tensions arise from different mechanical properties of the substrate relative to the coating. For stress reduction, methods are used to create gradient layers to reduce stresses between the layers.

Wishing for success

Emil Yankov

Emil,

this is either a kind of view from a different angle or only confusion?

In classical terms origin of surface tension is an enhanced energetic state of atoms or molecules at the interface. Differently from atoms or molecules in bulk they do not have neighbours of equal energetic state on every side around them. Therefore every molecules energetically prefer to be in the bulk, none to be at the interface. Therefore system tries to minimize interfacial area.