Effect of tail branching on the phase behavior and the rheological properties of amine oxide/ethoxysulfate surfactant mixtures

The formulation of highly concentrated products with low viscosity is a key issue of the surfactants industry, for both economic and ecological reasons. The rational design of surfactant tails bearing a limited number of short side chains has been proposed as a suitable strategy to meet this demand, with negligible effects on biodegradability. In this work, we investigate a mixed surfactant system, in which the branched surfactant N,N-dimethyl-2-propylheptan-1-amine oxide (C10DAO-branched) is combined with a linear alkyl ethoxysulfate one (AES). For comparison, we also study the mixtures in which the branched amine oxide is replaced by its linear isomer (C10DAO-linear). The phase behavior of this surfactant mixture in water is investigated across the entire composition range by polarized optical microscopy and small angle X-ray scattering. Moreover, the shear viscosities and viscoelastic moduli of representative samples are determined by rheological measurements. C10DAO-branched/AES aqueous mixtures form isotropic micellar solution and lamellar structures. Low viscosity was found for all these mixtures, including the most concentrated ones. In contrast, in C10DAO-linear/AES mixtures an extended hexagonal phase is detected, which presents a high viscosity. These results demonstrate tail branching and mixed aggregation to synergistically contribute to the design of high-concentration low-viscosity surfactant mixtures.