Association structure and solubility dynamic of methyl red and methylene blue in the system of water, sodium dodecyl benzene sulfonate and pentanol

The purpose of this study was to determine the association structure, solubility, and
homogeneity of methyl red and methylene blue in the system of water, sodium dodecylbenzene
sulfonate (SDBS), and pentanol system, and to analyze the morphology of the association
structure before and after dissolved in methyl red and methylene blue. The mass ratio
composition of water/SDBS/pentanol system at pH 4.5, 7, and 9.5 selected from created Terner
phase diagram to obtain the association structures. From the obtained association structures,
the solubility of methyl red at pH 4.5 and methylene blue at pH 7 and 9.5 was analyzed. The
homogeneity of methyl red and methylene blue was determined by the refractive index. The
results showed that two association structures namely microemulsion (water in oil (w/o) and oil
in water (o/w)) and lamellar liquid crystal phase was obtained. The maximum solubility of
methyl red was found in w/o microemulsion. Meanwhile, the maximum solubility of methylene
blue was found in o/w microemulsion. Refractive index results showed that mostly methyl red
and methylene blue dissolved in the sample homogeneously. The structure of the liquid crystal
microemulsion changed after the addition of methyl red and methylene blue observed by the
Optical Polarizing Microscope.