Improved methylene blue two-phase titration method for determining cationic surfactant concentration in high-salinity...

Area of Science:

• Analytical Chemistry
• Surfactant Science

Background:

• The methylene blue (MB) two-phase titration is effective for anionic surfactants but fails for cationic surfactants in high-salinity brine due to inorganic anion interference.
• Epton's end point, where phases appear equally blue, is unreliable in complex matrices.

Purpose of the Study:

• To develop an enhanced MB two-phase titration method for determining cationic surfactant concentrations.
• To adapt the method for use in both deionized water and high-salinity brine environments.

Main Methods:

• A novel "colorless end point" was established, signifying complete MB transfer to the chloroform phase.
• Spectrophotometry measured light absorbance at MB's characteristic wavelength to detect the end point (absorbance < 0.04).

Main Results:

• The improved method demonstrated low overall error (1.274% in DI water, 1.322% in brine) for permanent cationic surfactants.
• It achieved low limits of detection (LOD) of 0.149 mM (DI water) and 0.215 mM (brine).
• For switchable cationic surfactants, errors were 2.22% (DI water) and 0.106% (brine) with LODs of 0.369 mM and 0.439 mM, respectively.

Conclusions:

• The developed spectrophotometric method reliably quantifies cationic surfactants in diverse aqueous matrices, including challenging high-salinity brines.
• This advancement overcomes the limitations of traditional MB titration, offering a more robust analytical solution.