Review by Bryan Hassell: AP 255 Fall 11
From: Self-assembly in the electrical double layer of ionic liquids, S. Perkin et. al, Chem. Commun., 2011, 47, 6572–6574
Keywords: self assembly, electrical double layer or double layer
Here the structure of two ionic liquids confined between negatively charged mica sheets as studied. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Their results suggest a transition from alternating cation–anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.
Fig. 1 Measured force, FN, between two mica surfaces (normalised by R) as a function of film thickness, D, for (a) (C4C1im)(NTf2) and for (b) (C6C1im)(NTf2). Forces were measured from DB200 nm but only the region between 0–15 nm where non-zero forces were observed is shown. Open diamonds indicate points measured on approach of the surfaces (decreasing D) while open circles indicate data measured on retraction of the surfaces. Filled triangles indicate points measured from the jump-apart of the surfaces from an adhesive minimum. The lines are a guide to the eye to show the oscillatory nature of the forces, with solid lines through measured regions and dashed lines through the regions inaccessible to measurement due to the jump-in from the previous energy barrier. The inset tables show, for each graph, the average values of D for each force wall and its standard deviation as well as the inter-wall separations, DD. The columns of the tables are labelled according to our interpretation of the ion structure in the film at that separation: for (C4C1im) the n-value refers to the structure C(AC)n, and for (C6C1im) the n-value refers to (ACCA)n.
Fig. 2 Suggested ion orientations in (a) (C4C1im)(NTf2) at a film thickness of 2.3 nm showing alternating cation and anion monolayers for C(AC)2; (b) (C6C1im)(NTf2) at 3.4 nm showing a single bilayer, (ACCA)1, and (c) (C6C1im)(NTf2) at a film thickness of 8.3 nm showing two bilayers, (ACCA)2.