Effect of the aniline hydrochloride hydrotrope on the microstructure of SDS/water system: Linear rheological behavior

In this work, the linear viscoelastic properties of the sodium dodecylsulfate/aniline hydrochloride/water system were examined in detail as a function of aniline hydrochloride (AHC) to sodium dodecyl sulfate (SDS) molar ratio (R = [AHC]/[SDS]), SDS concentration and temperature. SDS aqueous solutions undergo a structural transition from sphere to rodlike micelles and subsequently to a viscoelastic network of wormlike micelles induced by the hydrotrope AHC at low and intermediate R molar ratios (i.e., R = 0.1-0.6 for 5 wt.% SDS), which screens the repulsions between the neighboring charged head groups. The rheological response indicates that at low R molar ratios and constant surfactant concentration, the rheological behavior of the system is predominantly viscous (G' < G"). However, upon increasing R, the elastic behavior predominates (G' > G") and a maximum of the absolute value of the zero frequency complex viscosity (|eta(0)*|) is observed, which suggest that the micellar length also goes through a maximum. The region of the elastic behavior diminishes at even greater R molar ratios and a predominantly viscous behavior appears again. This drop in |eta(0)*| can be attributed to the transition from linear to branched wormlike micelles that conform the tridimensional viscoelastic network. The dynamic behavior of the system was analyzed in terms of the Granek-Cates theory, which indicates that the viscoelastic network is in the slow-breaking limit. Under these conditions, reptation is found to be the controlling relaxation mechanism for this system.