Hello.
I was measuring interfacial tension between few samples for analysis of the results of an experiment.
The interfacial tension between 4% PVA (in DI water) and TPGDA oil is 7.2 mN/m, 4% PVA + 10% Calcium Chloride (in DI water) and TPGDA oil is 2.2 mN/m and 4% PVA + 20% Calcium Chloride (in DI water) and TPGDA oil is 3.4 mN/m so on.
There is a sudden fall in the IFT upon addition of 10% salt and thereafter it increases with salt concentration. Why is this so? What makes the trend to fall initially?
One possible explanation could be that up to an 'optimum' concentration, salts accelerate the diffusion of surface active components in crude oil that bind oil and water phases thereby reducing IFT. However, as concentration increases, an effect known as 'salting' takes precedence where excess of salt will bind with water and ionize resulting in lower interaction between oil and water. Refer to Lashkarboloki et al (2014), Bai et al (2010) as well as my paper SPE 180274-MS for similar findings.
Hope this helps.
Hi Manoj. The increase in IFT with salt concentration below the saturation limit of the salt is obvious but, this didn't really answer my question. The baseline solution in my case is 4% PVA in DI water (PVA is the surfactant) and when I add 10% salt to this baseline solution, I see a sudden fall in the value of the IFT. I am not sure about the surface chemistry in this regime.
Thanks!
IFT should decrease below the saturation limit of the salt as indicated by the 10%case and increase thereafter as indicated by the 20% case. My suggestion would be to do a couple more tests and fine tune the results to see where the optimum lies and how IFT behaves to understand the trend and the underlying surface chemistry.
Cheers
Hi Manoj. Thanks for your reply. I had repeated the experiments many times and got the same trend. PFA, the figure. The red line in the plot (with triangles) is IFT. The dashed vertical lines represent the saturation range of the salt. The question really is what is that making the trend to fall upon addition of the salt (initially). Can you comment based on the figure attached?
Thanks.
Since you're saying surfactant is involved, my guess is the CMC of the surfactant is shifting and causing this behavior. What do the other lines represent?
The blue one represents the viscosity and the black one the fluid velocity.
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