2.4 MD simulation details
The potential parameters and models of the concentrated H2SO4 and C4 reactants for MD simulations were from the same as our previous work.6,7 The C4 reactants model includes equivalent 2-butene and isobutane with size of 6.0×6.0×4.0 nm3. The size of H2SO4 model is 6.0×6.0×8.0 nm3, containing 2848 H2SO4 molecules, 84 hydronium ions, and 84 bisulphate ions. For the interfacial model, the sandwich-like models were constructed with pure H2SO4 model in the middle and pure C4 reactants model in the both sides. 2 nm gap between pure H2SO4 model and pure C4 reactants model was used to place DESs. The number of ChCl-Pho (1:2) in the gap are 6, 15, and 24, corresponding to the concentration of 1.0 wt%, 3.0 wt%, and 5.0 wt%, respectively. For ChCl-TsOH (1:1), the number is 6, 16, and 26, respectively. For ChCl-BOA (1:2), the number is 6, 12, and 21, respectively. Initially, the energy minimization of 5000 steps was carried out, followed by 8 ns canonical (NVT ) simulations at 298.2 K with nose-hoover thermostat. The isothermal-isobaric (NPT ) ensemble was further conducted for 10 ns to fully equilibrate configures. During the simulation, the number density distribution of each moiety was calculated every 2 ns, and the configures were fully equilibrated after there are no apparent deviations for the density. At last, 6 ns NVT was carried out for data collection of interest with the time steps of 2.0 fs. The semi-isotropic Parrinello Rahman barostat method on z axis was used with 2.0 ps relaxation time at 1 bar for all the NPTensemble to keep the interface unchanged15.
For all the MD simulations, GROMACS software was employed with periodic boundary conditions (PBC) in three dimensions36. The Lennard-Jones interaction and Coulombic interaction were cut off at 1.2 nm. All the covalent bonds related to hydrogen atoms were treated using the LINCS algorithm. Maxwell distribution was employed to obtain the initial atomic velocity. The particle grid Ewald method was used to treat long-range electrostatic interactions. The OPLS-AA was used to treat the interaction between the DESs, H2SO4 and C4 hydrocarbons6,7,29. At the same time, the charges of the DESs were refitted based on the ChelpG method at the Basis set of 6-311++G(d,p) level with Gaussian 09 program37.