CO2/N2-switchable sol–gel transition based on NaDC/NaCl solution: Experiments and molecular dynamics simulations

Abstract

The CO2/N2-triggered switchable sol–gel transition based on sodium deoxycholate (NaDC) and sodium chloride (NaCl) aqueous solutions has been investigated using both experiments and molecular dynamics (MD) simulations. The effects of CO2 on the phase behavior, viscosity, pH, electrical conductivity, and microstructure of the NaDC/NaCl mixed system were explored. The mixed system exhibits a recycled CO2/N2 response, and switching between solution state and gel state with six orders of magnitude variation in shear viscosity was achieved within a narrow pH range of 7.82–7.20. The FT-IR and 1H NMR spectra of the mixed system before and after CO2 bubbling were measured. The experimental results indicate that NaDC is not directly protonated to deoxycholic acid by the small amount of CO2 present; instead, an acid salt structure with a strong hydrogen bond, that is, [RCOOH⋯OOCR]−, is formed. MD simulation results reveal that, in the aqueous mixed NaDC/NaCl/CO2 system, three types of bridging hydrogen bonds between two deoxycholate (DC−) anions and one H3O+ or HCO3− ion are key factors for the formation of hydrogels. These bridging hydrogen bonds promote the formation of DC− pairs bridged by H3O+ or HCO3− ions (including the acid salt structure) and are beneficial to the growth of micelles and formation of hydrogels. It is worth noting that the bridging hydrogen bond connected by HCO3− ions is an aspect of CO2-induced hydrogels that distinguishes them from acid-induced hydrogels. In addition, the results indicate that CO2 and NaCl have a synergistic effect when inducing hydrogel formation. Based on experiments and MD simulations, a CO2/N2-switchable sol–gel transition mechanism for an aqueous mixed NaDC/NaCl system has been proposed.