It would be interesting to know how much SDS and SDBS you are using. But as Zinc stereat is hydrophobic I guess it will be quite a lot. So what I think is happening is that you get lytropic crystalline phases(lammelar most likely) in you dispersion because of the high concentration, these crysatlline phases take sometime to grow and will increase your viscosity. You could try to add an alcohol(butanol for instance) as a co surfactant which can lead to a microemulsion or you decrease the concentration of zinc stereat.

Hi Martin, thank you for the answer. Will the formation of a microemulsion system in this case lead to a much less thixotropic behavior? If so, how?

What is the purpose? When microemulsion is formed zinc stearate will be dissolved.

I am reading some articles and book chapters, which suggest that a hydrotrope, such as a sodium salt of xylene sulfonic acid, may possibly be used to achieve flow at relatively low viscosity under similar conditions, by prevent the formation of micelles/liquid crystal structures .  Is it a feasible approach in my case? Does butanol mentioned above have the hydrotroping function?  And any suggestions regarding the choice of hydrotrope candidates?

https://books.google.com/books?id=f2bJVtfN0t8C&pg=PA30&lpg=PA30&dq=cosurfactant+inhibition+micelle+formation&source=bl&ots=CFCcPj3BYx&sig=5g91hR32dLOXHvPB86vSVyupNGc&hl=en&sa=X&ved=0ahUKEwi1sIzmjdPLAhWLGR4KHY_7BYsQ6AEIMDAC#v=onepage&q=cosurfactant%20inhibition%20micelle%20formation&f=false

it seems to me the situation you refer to is somewhat different from yours - you have a problem with solids not a micellar solution. Butanol is more considered a cosolvent.

Nevertheless similar approaches may change your system too by getting the stearate dissolved. Please keep in mind that especially hydrotropes need high concentration to work. Also with cosolvents you will need appreciable amounts. Likely you will not end up with a dispersion but with a solution.

Dear all,

Yes. I have a problem with zinc stearate particles at relatively high solid loading >=45wt% (s.g. of zinc stearate=1.1) dispersed in water, with the use of moderate amount of surfactant mixtures (30wt%, see wikipedia link). In other words, due to the presence of large amount of zinc stearate particles, much less total amount of surfactants was needed to form lyotropic liquid crystal structures interconnecting the zinc stearate particles absorbed with the same surfactant molecules. Less surfactant was due to the much larger zinc stearate particles size and correspondingly much less specific surface area than micelles in a micellar solution without zinc stearate particles.

I must admit this part of knowledge is new to me, so maybe I am not thinking straight. You are welcome to correct me.

I am going to try hydrotropes, e.g. sodium salt of cumene sulfonic acid. Probably I would not try cosolvent because the objective is to achieve a dispersion. Dissolved stearate does not work for the purpose in this specific application.

You should be aware that by dissolving larger amounts of surfactant, cosolvent or hydrotrope you will change structure of continuous phase. Do not know if this might be a problem for your application.

Maybe surfactant concentration is already to high, with probably high micellar concentration leading to depletion flocculation.

I would check whether there is an optimum in surfactant concentration using either alkylbenzene sulfonate or alkane sulfonate, alternatively a nonionic surfactant with high EO content 25 or larger.

Another approach would be to match refractive index of stearate with continuous phase.

Do not know whether this would be possible by adding glycerol.

Will the addition of hydrotrope like sodium salt of cumene sulfonic acid change the HLB of SDS/SDBS mixture? How does the hydrotrope molecules interact with the zinc stearate particles in this case? Will they be absorbed onto the zinc stearate particle surface like SDS/SDBS molecules, or instead stay in the continuous phase mostly?

As far as I know hydrotrope action is through changing water structure. Is this a problem for your application. HLB does not make much sense for anionic surfactants.

It seemed that SDS + SDBS + SSCSA (sodium salt of cumene sulfonic acid) would not work, as the ionic strength was too high in this case due to the large amount of salts involved.

So, making an microemulsion looks more feasible now.  What amount of alcohol should I add to a micelle solution e.g. 60g water/1g SDS/2g SDBS to make such a microemulsion of surfactants? Since zinc stearate is not exactly an oil, but rather hydrophobic particles, do I need to ADD AN EXTRA AMOUNT OF OIL to complete the microemulsion composition?

Difficult to say a priori. Would first do trial on possibility to dissolve the stearate maybe first in a cosurfactant like octanol. Alternatively to try to dissolve in rapeseed methylester.

Then going further by adding surfactant to obtain full solubility in micro'emulsion'.