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

The CO 2 /N 2 -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 CO 2 on the phase behavior, viscosity, pH, electrical conductivity, and microstructure of the NaDC/NaCl mixed system were explored. The mixed system exhibits a recycled CO 2 /N 2 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 1 H NMR spectra of the mixed system before and after CO 2 bubbling were measured. The experimental results indicate that NaDC is not directly protonated to deoxycholic acid by the small amount of CO 2 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/CO 2 system, three types of bridging hydrogen bonds between two deoxycholate (DC − ) anions and one H 3 O + or HCO 3 − ion are key factors for the formation of hydrogels. These bridging hydrogen bonds promote the formation of DC − pairs bridged by H 3 O + or HCO 3 − 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 HCO 3 − ions is an aspect of CO 2 -induced hydrogels that distinguishes them from acid-induced hydrogels. In addition, the results indicate that CO 2 and NaCl have a synergistic effect when inducing hydrogel formation. Based on experiments and MD simulations, a CO 2 /N 2 -switchable sol–gel transition mechanism for an aqueous mixed NaDC/NaCl system has been proposed.