Effect of Additives on Surfactant Micelle Shape Transformation: Rheology and Molecular Dynamics Studies

Abstract

Several additives (NaSal, 1SHNC, and 2SHNC) were added into aqueous solutions of a cationic surfactant, 3-tetradecyloxy-2-hydroxy-propyl trimethyl ammonium bromide (R14HTAB). The mixed solutions were investigated using rheological measurements, molecular dynamics (MD) simulations, cryogenic transmission electron microscopy (cryo-TEM), 1H NMR, and Fourier transform infrared (FTIR) spectroscopy. The results demonstrated that the zero-shear viscosity (η0) increased with increasing salt concentrations until reaching a maximum and then decreased. The variations in viscosity were related to transformation of the micellar configuration and were confirmed by cryo-TEM and MD simulations. The ability of the three organic salts to enhance the viscoelasticity of R14HTAB solutions is in the order of NaSal < 1SHNC < 2SHNC. The synergistic interaction between counterions and surfactant molecules was analyzed using 1H NMR spectrograms, FTIR spectroscopy, and MD simulations. These findings provide new insight into the self-assembly and bulk properties of ionic surfactant/organic salt mixtures.