Molecular design of switchable nanochannels modified by zwitterion polymer chains with dissipative particle dynamics simulation

Conventional artificial nanochannels have obvious shortcomings in permeability regulation and environmental friendliness, which challenge their practical applications. Polymer-modified switchable nanochannels are considered as a potential solution owing to their high chemoselectivity and controllability. Herein, systematic dissipative particle dynamics (DPD) simulations are performed to investigate the conformational transition and stretching behavior of polysulfobetaine in the presence of an explicit solvent. Polysulfobetaine is one kind of zwitterion polymer with a responsive property to the change of salt ion concentration, which is grafted onto the internal surface of the nanochannel to achieve the "open-close-open" switch. The simulation results indicate that the grafting ratio of 1.52 chains/rc2 (rc is a reduced unit of length, 1 rc = 0.857 nm) and salt ion concentration of 5 wt% NaCl are optimal parameters to achieve the "open-close-open" functionality. Specifically, salt ions promote the closure of nanochannels via regulating the electrostatic attraction of the polymer chain in z-direction and the excluded volume effect caused by conservative forces. On the other hand, the competition mechanism between electrostatic force and conservative force is revealed, i.e. at low salt concentration, electrostatic force dominates and close the nanochannel; With salt ions concentration increasing, conservative force gradually exceeds electrostatic force, and becomes the dominant force and reopens the nanochannel. This study provides some molecular insights for the design of stimulus-responsive artificial nanochannels, which may find application in the exploitation of natural gas hydrates, such as temporary plugging agents in well drilling for gas hydrate reservoirs.