Surfactant-based photorheological fluids: effect of the surfactant structure

The effect of the surfactant structure on the mechanical and structural properties of surfactant based photorheological fluids are presented in this paper. Cetyltrimethylammonium bromide (CTABr) mixed with trans-o-methoxycinnamic acid in a basic environment can form photosensitive systems. The driving force is the ability of surfactant molecules to form wormlike micelles in the presence of the anionic photosensitive additive. Taking into account that slight changes in the surfactant monomer's structure can induce drastic modifications of the micellar aggregate features, the role the of the nature of the counterion (in the CTAX type surfactants) or the headgroup size (CTRABr type surfactants) and its influence on the mechanical properties of surfactant based photorheological fluids using trans-o-methoxycinnamic acid (trans-OMCA) as additive were investigated. Rheological studies reported in this paper show that the viscosity of these systems drastically varies only by changing the nature of the surfactant counterion. Moreover, by increasing the bulk simply by replacing the three methyl groups with three ethyl groups in the surfactant headgroup moiety, the viscosity drastically decreases. Highly photosensitive PR fluids can be further obtained using cetyltrimethylammonium trans-o-methoxycinnamate (CTAOMC) as surfactant at neutral pH. In addition to the complete rheological characterization carried out by means of the application of both a steady shear and a dynamic shear stress, a 1H NMR and NOESY study was also performed.