Promoting dissolution behaviors of low-carbon alcohol polyoxyethylene polyoxypropylene ether in CO2 by molecular structure optimization and adding cosolvent

The low-carbon alcohol polyoxyethylene (EO) polyoxypropylene (PO) ether surfactants have significant application potential in CO2 flooding for enhanced oil recovery (EOR) in water-bearing oil reservoirs because of their hydrophilic, lipophilic and CO2-philic properties. The dissolution of surfactants in CO2 is the important precondition for the application of the CO2 + surfactant systems injected in oil reservoir. This work combines experiments and molecular dynamics simulations to reveal the dissolution mechanism of low-carbon alcohol polyethers in CO2 at the molecular level in order to promote the dissolution behaviors of these surfactants. The results show that the appropriate adjustment of the molecular structure can promote the dissolution of surfactants in CO2 by the interactions of dispersion forces, Lewis acid–Lewis base (LA-LB) interactions, and potential hydrogen bonding. The increase of the numbers of EO groups, PO groups, and the length of the alkyl chain all lead to the increase of the cloud point pressure due to the weaken of the dispersion forces and LA-LB interactions between the surfactants and CO2 with the increase of the molecular weight of the surfactants. Under the premise of ensuring sufficient hydrophobic, lipophilic and CO2-philic properties of the surfactant molecules, decreasing the molecular weight as much as possible can increase the dissolution properties. Moreover, the distribution adjustment of EO and PO groups in surfactants has a significant impact on the dissolution behaviors. The molecules, in which EO and PO groups cross-dispersed and the terminal hydroxyl group connected to the EO group, have a better CO2-philicity than those with long EO chains and PO chains. Furthermore, ethanol can reduce the cloud point pressure of surfactants in CO2, because that the presence of ethanol can break the stronger van der Waals forces between CO2 molecules and enhance the LA-LB interactions and hydrogen bonding between the hydroxyl groups and CO2.