Micellar solution with pH responsive viscoelasticity and colour switching property

Abstract

Macroscopic properties of amphiphilic systems can be reversibly controlled by tailoring micellar morphology via appropriate choice of additive and external stimulus. In this work we report an aqueous micellar system showing pH responsive viscoelasticity and optical properties. pH sensitive behaviour of a phenolic acid, namely, ortho-coumaric acid (OCA) is effectively utilized to tune self-assembly in cetyltrimethylammonium bromide (CTAB) solution. Reversible switching between colourless, gel-like state and fluorescent green colour, liquid-like state can be attained by pH adjustment. pH dependent changes in bulk properties and the microstructures responsible for the behaviour were studied by means of rheology, UV-vis and fluorescence spectroscopic techniques, small angle neutron scattering (SANS) and cryogenic transmission electron microscopy (cryo-TEM). Rheological studies suggested transition between viscoelastic fluid and Newtonian liquid or vice versa, with specific changes in pH. Viscoelasticity of the system is attributed to the presence of entangled threadlike micelles. pH sensitive interactions between surfactant micelles and phenolic additive is regarded as the key factor regulating the morphological transitions and related flow behaviour in CTAB–OCA solution.