Viscoelasticity of anionic wormlike micelles: effects of ionic strength and small hydrophobic molecules

The impact of NaCl and small hydrophobic molecules, used in perfumery, on the viscoelastic properties of aqueous solutions of sodium lauryl ether sulphate is studied. As the salt concentration increases, the viscosity passes through a maximum. Empirically, this behaviour is well known and is referred to as the 'salt curve'. Dynamic rheological measurements revealed a detailed picture of how salt affects the different length scales in the entangled wormlike micelles. Adding hydrophobic molecules does not change the shape of the salt curve, but they can shift the maximum to lower salt concentrations and alter the peak viscosity. The shift in the salt curve can generally be understood as due to either flattening of the interface, caused by insertion of amphiphilic molecules into the surfactant layer, or micellar swelling, caused by solubilization of very hydrophobic molecules in the micellar core. The viscosity decrease at the salt curve maximum is inversely proportional to the hydrophobicity of the added molecules, so less hydrophobic compounds cause larger decreases. It is hypothesized that these molecules interact with the surfactant headgroups and thereby soften the interfacial film. Our analysis suggests that the addition of hydrophobic additives reduces the persistence length and results in an effective shortening of the micelles.