Dear Neetish Kumar Maurya,

Apparently with increasing temperature the chemical activity of the surfactant grows and at certain temperatures a chemical reaction could happens between the surfactant and nanoparticles that could be formed a new substance with certain colloidal structure

Dear Hakim,

Thanks for your reply, 

I would like mention the fact that the anionic surfactant and nanoparticle are repulsive in nature due to electrostatic repulsion between them. And at room temperature, the dispersion of nanoparticles in the surfactant solution is quite stable.  However, I will look into your possibility of  increased chemical activity at increased temperature. 

Actually, I was interested in knowing about the stability of the emulsion. As several references have reported that emulsion viscosity will increases when droplets will start to coalescence. On physical appearance emulsion seems to be stable and average droplet diameter has not increased by too much to support this claim. 

Increments of T° increase hydrophobic interactions that can produce different structures between surfactnt molecules or nanoparticles that may change viscosity.

Some hydroxylated fatty acids in presence of a counterion changes structure from micelles to microtubes at different T° and that produces large variations of viscosity.  

Jorge might be right, if there is enough excess surfactant to form giant micelles. However, if there is not a large amount of excess surfactant, the sudden viscosity increase is due to adhesion between the droplets. See link to the classic article in Langmuir about emulsion stabilized by ethoxylated nonionic surfactants. 

Your results are intriguing as you have a stable mix of anionic surfactant and anionic nanoparticles that creates an unstable (flocculated) emulsion when their mixture adsorbs at the oil-water interface. I can't explain why that happens.

http://pubs.acs.org/doi/abs/10.1021/la00036a006

 Thank you, Jorge and Alan for your reply.

Jorge can you give me any reference for  the phenomenon which you have mentioned. I will look into the possibility of it. 

Why there is first a decrease in viscosity seems clear considering that viscosity of continuous phase also decreases with increase in temperature. In case of coalescence viscosity should also decrease because effective dispersed volume also decreases.

There is not enough information to understand viscosity increase, however, flocculation is the likely reason.

To guess, if the droplets are stabilized with metal or metal oxide nanoparticles covered by anionic surfactant, dissolution of metal ions may lead to flocculation.

Further, in extreme case desorption of surfactant due to enhanced solubility may also trigger flocculation.

Dear Titus Sobisch

Thanks for your reply,

Can you direct me towards any research paper which explains this flocculation in such system.

Dear Neetish,

I have no paper for your system, I do not know in detail, however, problem of flocculation goes back to the general basic balance of attractive and repulsive forces, which can be quite complex. If attractive forces dominate structure builds up leading to increased viscosity.