Alkyl Tail Aggregations Break Long-Range Ordering of Ionic Liquids Confined in Subnanometer Pores

Manipulation of the lateral structure of electric double layers (EDLs) emerges to become an important method to improve the differential capacitance of supercapacitors because of the correlation between the differential capacitance and lateral structural evolution. Space confinement is one of the methods that can significantly influence the lateral structures of EDLs. In this paper, all-atom molecular dynamics simulations were employed to study the in-plane structure of room-temperature ionic liquids [C_nmim][PF₆] (n = 1, 2, 4, and 6) confined in subnanometer slit pores. Lateral ordering of ions and orientation of imidazole rings are systematically characterized under the influence of the size of slit pores and lengths of alkyl tails linking to imidazole rings. In our simulations, crystalline, partially ordered, and disordered phases of ions are observed. The ordering of ions can be manipulated by the formation of tail aggregations through the influence of both the tail length and pore size. With the increase in alkyl tail and pore size, the number and size of tail aggregation increase, and the ordering of ions in pores decreases. The formation of the alkyl tail aggregation can decrease the columbic ordering of ILs in the slit pore. Besides the lateral ordering, orientation of the imidazole ring is found to be strongly correlated with the pore size but is independent with the alkyl tail length. These results highlight the importance of the geometric structure of ions on ordering and provide new insights in the manipulation of lateral ordering to increase the energy density of supercapacitors.