Tail unsaturation tailors the thermodynamics and rheology of a self-assembled sugar-based surfactant

Abstract

HypothesisThe self-assembly of long-tail surfactants results in the formation of nanoscale structures, e.g. worm-like micelles, with the ability to modify the rheology of the system. However, micelle formation, and thus the alteration of the rheology, is subject to the high Krafft temperature of saturated long-tail surfactants. Hexadecylmaltosides are sustainable surfactants that, in solution, form tailorable viscoelastic fluids. The preparation of monounsaturated sugar-based surfactants is hypothesised to reduce the Krafft point compared to the saturated analogues, therefore increasing the temperature range where the surfactant remains in the micellar form. ExperimentsHere we report the synthesis and characterisation of a novel sugar-based surfactant with an unsaturated C16-tail, namely palmitoleyl-β-d-maltoside (β-C16-1G2). Differential scanning calorimetry was used to probe the temperature stability of the system. The rheology of β-C16-1G2 solutions was investigated by means of rotational and oscillatory rheology, and these results were connected to the mesoscopic structure of the system as shown by small-angle neutron and X-ray scattering, and dynamic light scattering. FindingsThe presence of a double bond on the alkyl chain moiety leads to a depression in the Krafft point, allowing the surfactant to form a thermodynamically stable micellar solution over a wide range of temperatures, i.e. 5–95 °C. The surfactant self-assembles into worm-like micelles which, upon entanglement in the semi-dilute regime, result in the formation of a non-Newtonian, viscoelastic fluid. These observations have important implications in the development of new sustainable formulated products, enabling the preparation of surfactant phases with remarkable thermal resilience.