Dear Aakash,

Since the surface tension is pH dependent, as a result the viscosity will be pH dependent.

Dependence of Surface Tension of pH The variation in surface tension of 0.1% solutions of the sarcosinates from Table 1, with pH, is presented in Figure 1. In each case, the surface tension is highest above pH 9 and decreases rapidly as the pH is decreased towards 7. For each surfactant the surface tension reaches a minimum between pH 7 and 5.5, with the cocoyl sarcosinate showing a deep minimum around pH 7. Below pH 5, the insoluble acid form of the surfactant begins to predominate and can be seen to separate out as a second phase. 

Figure 1: N-acyl Sarcosinates; pH Dependence of Surface Tension

For viewing Figure 1 and more details, please see the attached file.

Hoping this will be helpful,

Rafik

Please read the following article:

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Surface Tension-Viscosity Relationship for Liquids.

Harold Schonhorn

J. Chem. Eng. Data, 1967, 12 (4), pp 524–525

DOI: 10.1021/je60035a016

Publication Date: October 1967

ACS Legacy Archive

http://pubs.acs.org/doi/abs/10.1021/je60035a016?journalCode=jceaax

Rafik

Hi Rafik,

Many many Thanks for your answer!

Actually, My shampoo is in the pH range 5.5-6.5. If I see the surface tension Vs pH graph, there is not much change in the surface tension in this pH range. While my viscosity changes from 1700 cps to 3600 cps when I change pH 6.5 to 6.1

There are other ingredients too in the shampoo - Sodium cocoyl isethoinate, CAPB etc.

Would you please explain why is it happening.

Dear Aakash,

NaCl is a compound that can change the viscosity. If you wish to increase the viscosity you should add some NaCl up to a certain level.

Rafik

That I know Rafik. For all our shampoos, we use NaCl for gaining viscosity. This sulfate free system is behaving very differently. That's why I wanted to know that why the viscosity is pH dependent.

We have not measured the surface tension. We measured the viscosity at pH 6, 6.2 & 6.5 The viscosity is lowest at 6.5

No dilution done before taking pH or viscosity.