Molecular architecture modulates self-assembly and micellar rheology of model ionic surfactant systems

Understanding and predicting the rheology of micellar systems is key in formulation design with wide-reaching implications for the development of products such as shampoos and detergents. In micellar systems comprising ionic surfactants, predictive models are uniquely challenging to construct as a result of the combined effects of salt screening and surfactant polydispersity on micelle self-assembly. In this work, we provide critical insights into how the amphiphilic nature of ionic surfactants controls self-assembly and rheological behaviour. For pure sodium lauryl ether sulphate surfactants, we demonstrate that the properties of micellar solutions can be described from the average properties of the constituent ingredients. Furthermore, we show that there are three distinct viscosity regimes with varying salt concentrations, and that formulation/property relationships can be systematically controlled by three key aspects of the surfactant molecular geometry in relation to micelle self-assembly: (1) the size of the hydrophilic headgroup (degree of ethoxylation), (2) the length of the hydrocarbon tail, and (3) the polydispersity of the surfactant solutions. In systems with multiple headgroup lengths, the salt concentration required to reach peak viscosity depends exclusively on the average number of ethoxy linkers, while the peak viscosity varies with the relative proportions of the surfactant components. The observed Gaussian symmetry in viscosity trends underscores the intricate relationship between molecular structure and macroscopic behaviour in these systems. These findings have implications for improvements in rheological, thermodynamics, molecular, and predictive models and the design and development of novel formulations.