New surfactants design for CO2 applications: Molecular dynamics simulations of fluorocarbon–hydrocarbon oligomers

Abstract

Novel block co-oligomers are designed as candidate surfactants in near-supercritical CO2 environment, with the CO2–phobic block consisting of ethyl propionate and ten different types of ethylene monomers, flanked on either side by eight repeat unit fluorinated CO2–philic blocks. Single chain molecular dynamics simulations are performed to understand their conformational and dynamic properties. Depending on the side chain type, the CO2–phobic blocks are prone to shrinkage in the CO2 environment, while the CO2–philic blocks preserve their vacuum dimensions. The overall chains form U-shaped planar structures with flapping motion of the fluorinated arms; thus, we expect bilayer micelle formation under these conditions. The origin of the CO2–oligomer interactions is investigated and van der Waals interactions are found to dominate over electrostatic interactions in the CO2 environment. Calculations of the radial distribution function for the solvent molecules around the oligomer backbone show a solvation shell around 5–6 Å, irrespective of the oligomer type; density of the solvent around the oligomer, on the other hand, varies with type of side chain due to the interactions between the CO2 molecules and the oligomer, and the available volume around the side chain. The local chain dynamics is investigated by orientational autocorrelation functions, and the characteristic time of the relaxation of selected C–H and C–F bonds is found to depend on the local friction experienced by the fluctuating atoms and the energy barrier that needs to be surmounted during the relaxation process. The simple exponential decay of the correlation functions for the C–H bond is common for all oligomer types, whereas the stretched exponents take on smaller values depending on the side chain for the C–F bond vector, implying that the fluorinated blocks are exposed to more complicated dynamical processes.