A series of unexpected thermo-responsive phenomena were discovered in an aqueous solution of the cationic gemini surfactant, 2-hydroxypropyl-1,3-bis(alkyldimethylammonium chloride) (n-3(OH)-n(2Cl), n = 14, 16), in the presence of an inorganic salt. The viscosity change trend for the 14-3(OH)-14(2Cl) system was investigated in the 20-40 °C temperature range. As the temperature increased, the viscosity of the solution first decreased to a minimum point corresponding to 27 °C, and then increased until a maximum was reached, after which the viscosity decreased again. In the 16-3(OH)-16(2Cl) system, the gelling temperature (T(gel)) and viscosity changes upon heating were similar to those in the 14-3(OH)-14(2Cl) system above 27 °C. The reversible conversion of elastic hydrogel to wormlike micelles in the aqueous solution of the 16-3(OH)-16(2Cl) system in the presence of an inorganic salt was observed at relatively low temperatures. Various techniques were used to study and verify the phase-transition processes in these systems, including rheological measurements, cryogenic transmission electron microscopy (cryo-TEM), electric conductivity, and differential scanning calorimetry. The abovementioned phenomena were explained by the formation and destruction of intermolecular hydrogen bonds, and the transition mechanisms of the aggregates were analyzed accordingly.

Viscoelastic solutions formed in the mixed aqueous solutions of 3tetradecyloxy-2-hydroxypropyltrimethylammonium bromide (R 14 HTAB) and aromatic salts such as sodium salicylate (NaSal), sodium 1-hydroxynaphthalene-2-carboxylate (1SHNC), and sodium 2-hydroxynaphthalene-3-carboxylate (2SHNC) were systematically studied by steady and dynamic shear rheology in terms of concentration and temperature. In the absence of a salt, R 14 HTAB only produced spherical or short cylindrical micelles within a range of concentrations of 100−400 mmol kg −1 . The addition of aromatic salts induced one-dimensional growth of micelles generating wormlike micelles. Zero-shear viscosity of the solutions shows viscosity maxima behavior in the examined range of salt concentration, where the strongest and the most stable network structures were formed. The changes in the viscoelastic behavior are a result of variation of the structural relaxation time, indicating that the flow behavior is primarily controlled by micellar kinetics. The microstructure of wormlike micelles and the reason for the variation in the microstructure with an increase in the additive concentration has been analyzed by infrared and 1 H NMR spectra measurements. The zero-shear viscosity reduces exponentially with increasing temperature.

The rheological properties of aqueous systems composed of each of the four homologous cationic surfactants (3-alkoxy-2-hydroxypropyl trimethyl ammonium bromides, C n HTAB, n = 12, 14, 16 and 18) in the presence of an anionic surfactant, sodium octanoate (SO), have been studied by using steady state and frequency sweep rheological measurements. The effects of surfactant concentration, hydrophobic chain length and temperature were investigated. In C 14 HTAB solution, the viscosity shows shear thinning in the concentration range of C C14HTAB [320 mmol/kg. Addition of SO promotes the micellar growth and results in the generation of wormlike micelles. Zero-shear viscosity (g 0 ) of the binary surfactant system exhibits a maximum point in the investigated concentration range, suggesting the interaction between C 14 HTAB and SO molecules is strongest at the optimal ratio of C 14 HTAB with SO. The decrease in viscosity was attributed to be the transition from entangled wormlike micelles to branching micelles after the maximum point, cryo-TEM images revealed the changes in the structure of the wormlike micelles.