Triple-stimuli-responsive hydrogels based on an aqueous mixed sodium stearate and cetyltrimethylammonium bromide system

Abstract

Triple-responsive supramolecular hydrogels were prepared using sodium stearate (SS) and cetyltrimethylammonium bromide (CTAB) in water and physiological saline. Hydrogels can respond to stimuli such as temperature, pH, and CO2 and exhibit tunable rheological behaviors. The high Krafft point and low solubility of SS have prevented its use in many applications. The mixing of SS with the cationic surfactant CTAB decreases the Krafft point and increases the solubility of SS owing to the synergistic self-assembly of the two materials. The prepared hydrogels exhibited good thermosensitivity within the physiological temperature range. For CTAB/SS/H2O mixed systems with a constant molality of CTAB or SS, the gel-to-sol transition temperature increased as the molar ratio of SS to CTAB increased. Under certain conditions, the prepared hydrogels exhibited pH-responsive rheological behavior. The gel–sol transition could be repeated for three or more cycles by changing the pH. The hydrogels also exhibited CO2/N2 switchable behavior, and the gel–sol transition was realized for at least three cycles by alternately bubbling CO2 and N2 into the sample. Steady-state and dynamic rheological measurements,transmissionandreflection polarizingmicroscopy, transmission electron microscopy, dynamic light scattering, differential scanning calorimetry, Fourier-transform infrared spectroscopy, and molecular dynamics simulations were carried out to explore the multiple-stimuli-responsive mechanism of the prepared hydrogels. The molecular insights on the formation of hydrogels and their stimuli responses gained in this work may be helpful for the theoretical development of smart hydrogel systems. We hope that the multiple-stimuli-responsive hydrogels may have potential applications in fields such as sensors, controlled drug delivery, and cosmetics.