Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems and the Effect of Added Electrolytes

The formation of viscoelastic wormlike micelles in mixed amino acid surfactant/nonionic surfactant aqueous systems in the presence of different counterions and salts is reported, and the effects of the different electrolytes on the rheological behavior are discussed. N-dodecanoylglutamic acid (LAD) is neutralized with biologically relevant L-lysine and L-arginine to obtain anionic surfactants (LAD-Lys2, LAD-Arg2) which form aqueous micellar solutions at 25 degrees C. Addition of a nonionic surfactant, triethyleneglycol mono n-tetradecyl ether (C14EO3), to the aqueous solutions of both LAD-Lys2 and LAD-Arg2 causes the zero-shear viscosity (eta(0)) to increase with C14EO3 concentration gradually at first, and then sharply, indicating one-dimensional growth of the aggregates and eventual formation of entangled wormlike micelles. Further addition Of C14EO3 ultimately leads to phase separation of liquid crystals. Such a phase separation, which limits the maximum attainable viscosity, takes place at lower C14EO3 concentrations for LAD-Lys2 compared to LAD-Arg2 systems. It was found that the rheological behavior of micellar solutions is significantly affected by the addition of Na+X- salts (X=Cl-, Br, V, NO3-). The maximum viscosities obtained for the systems with added salt are all higher than that of the salt-free system, and the onset of wormlike micelle formation shift towards lower nonionic surfactant concentrations; upon addition of electrolyte. The maximum attainable thickening effect of anions increases in the order NO3->I->Br->Cl-. The effect of temperature was also investigated. Phase separation takes place at certain temperature, which depends on the type of anion in the added salt, and decreases in the order I->NO3->Br-approximate to Cl-, in agreement with Hofmeister's series in terms of amphiphile solubility. The thermoresponsive rheological behavior was also found to be highly dependent on the type of anion, and anomalous trends, i.e. viscosity increase with temperature, were observed for all anions except Br.