Microphotonics for monitoring the supramolecular thermoresponsive behavior of fatty acid surfactant solutions

Abstract The development and the ability of an optical integrated polymeric resonator, acting as a surface light probe, for monitoring temperature-induced supramolecular phase transitions is presented in this work. The homogeneous detection of the transitions between different self-assembled structures in an aqueous solution of fatty acids (12-hydroxystearic acid, in association with amino-pentanol) was studied by investigating the coupling between the solution and the integrated photonic micro-cavity. Tuning the self-organized assemblies of surfactant is very attractive for many applications, such as cosmetic products, food, drug delivery and medical, and the development of alternative tools – especially those requiring minute amount of solution – to monitor their structural changes are essential. These original studies at temperatures ranging from 17 to 24 °C, based on a statistical treatment of optical resonance spectra, have evidenced the thermoresponsive nature of the optical features, and that different regimes occur with temperature. The optical results were corroborated with the measurement of the solution viscosity as a function of temperature, confirming that we can ascribe the optically-detected regimes to a surfactant assembly shifting reversibly from a tubular shape to a micellar one. The comparison between the optical and the rheological responses showed different accuracies: while the viscosity data exhibited a rather smooth and monotonous transition, the behavior changes were sharper and non-monotonous in terms of optical properties, allowing us to unambiguously identify in intermediate regime between 18.5 and 20 °C. These morphological transition experiments represent a unique opportunity to extend the numbers of available techniques studying these systems through integrated optical techniques with potential opportunities of real time detection and working on low sampling volume.