Rheology and Microstructure Studies
of SDS/CTAB/H₂O System¹

S. Zhang^a and H. N. Teng^b

^a College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao, Shandong, 266042 China

^b College of Chemistry and Environmental Engineering, Shangdong University of Science and Technology,
Qingdao, Shandong, 266510 China

Received December 12, 2006

Abstract—Phase behavior of mixed sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide
(CTAB) aqueous solution was studied. The rheological properties and microstructure were investigated using
a rheostat and freeze-fracture technique and are shown to be closely related to the phase behavior. Experimental
investigations reveal two symmetrical aqueous two-phase systems (ATPS) in the ternary phase diagram of
SDS/CTAB/H₂O system. In the surfactant rich phase of ATPS or in the adjacent stoichiometric state of ATPS,
the system has high viscosity because of its long range ordered structure. Lamellar phase was found in the high
viscosity samples in which the cationic and anionic surfactant are in 1 : 3 or 3 : 1 stoichiometry. In addition, the
viscosity has a tendency to increase when salt was added to the solution. The viscosity increase is due to the
salt can screen the repulsion between different charged headgroups and thus reduces the effective size of sur-
factants and facilitates the spherical or rod likes micelles to be transformed to worm-like micelles which can
form hexagonal or liquid crystal phases. Large-size salt ions like sodium sulfate (especially organic salt ions)
have more significant effect on the surfactant solution viscosity.

DOI: 10.1134/S1061933X0801016X

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