How Dimethyl Sulfoxide Disrupts the Hydrogen Bond Network in Hydroxy-Functionalized Ionic Liquids

We use molecular dynamics simulations to investigate the influence of the hydrogen bond (HB) catching agent dimethyl sulfoxide (DMSO) on the HB network in the hydroxy-functionalized ionic liquid (IL) 1-(4-hydroxybutyl)pyridinium bis(trifluoromethanesulfonyl)imide [HOC₄Py][NTf₂]. Three characteristic HBs are observed: between cations and anions (c-a), between two cations (c-c), and between cations and DMSO molecules (c-m). We quantify the thermodynamic stability of all HB species using a van 't Hoff analysis, observing that the IL HB network is significantly disrupted by the addition of DMSO. At low DMSO concentrations, stable (c-m) HBs tether DMSO molecules to the cations, leading to their molecular-level dispersion within the IL phase. Generally, the weakest HBs are found between cations and anions, while (c-m) HBs are stronger than (c-c) HBs. Adding DMSO, however, is also affecting the equilibria involving (c-m) HBs due to a competition of hydrogen bonded DMSO with energetically favorable DMSO-DMSO contacts. HB population correlation functions are used to study the HB kinetics, which reflect the relative thermodynamic stability of HBs and scale excellently with the viscosity. Adding DMSO leads to a strong decrease in the viscosity of the simulated mixture, which is in reasonable agreement with available experimental data.