Irreversible structural change of a dry ionic liquid under nanoconfinement

Abstract

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Acceleration of diffusion in ethylammonium nitrate ionic liquid confined between parallel glass plates

“…10 Vibrational sum frequency spectroscopy and X-ray reflectivity studies 11 confirmed the existence in EAN of significant interfacial structures within a ca 3 nm thick layer. Surface structures have also been found in other ILs, [12][13][14] sometimes persisting up to tens of nanometers into the bulk 13,14 with a clear dependence on the molecular nature of the surface. Exceptionally extensive surface-induced structures with a thickness up to 2 µm were recently demonstrated for a number of imidazolium bis(trifluoromethylsulfonyl)imide ILs by Anaredy and Shaw.…”

Fundamental aspects of electric double layer force-distance measurements at liquid-solid interfaces using atomic force microscopy

“…In all of these applications the interfacial structure of ILs at the solid-liquid interface plays a crucial role in determining their performance, and therefore it is essential to have techniques able to characterize the interface and build structure-function relationships. In recent years numerous efforts have gone toward understanding the complex structure of ILs at the solid-liquid interface using both theoretical approaches6789101112 and experimental methods such as scattering techniques131415, sum-frequency generation161718, surface-force apparatus19202122232425, and scanning probe techniques. For the latter, scanning tunneling microscopy (STM)2627282930313233343536 and dynamic3738394041 and static42434445464748495051 atomic force microscopy (AFM) approaches have made large progress towards imaging the ion layers in two and three dimensions at neutral and charged surfaces such as mica, silica, gold, and highly-oriented pyrolytic graphite (HOPG).…”

The study of ionic liquid behavior at solid-liquid interfaces

“…58 Espinosa-Marzal et al depicted long-ranged (~60 nm) liquid to solid transition of an IL under repeated confinement that persists even after the confinement is removed. 59 Formation of lamellar phase of an ILC is also observed under the influence of shear with these structures resembling liquid foams as evidenced from polarized micrographs. 60 These studies help to understand the influence of nanoconfinement and shear on the behavior and application of IL films , not only to facilitate the choice of IL for an application, but also to relate their interfacial behavior to their structure and bulk properties.…”

“…58 Additional studies using emersion 79 shear. 59 In very recent work, over-crowded layers of ions were observed at an electrode surface, accompanied by a significant increase in differential capacitance. 80 At the air- show that there is often strong layering near surfaces, 57 and that even strong (simulated)…”

“…One such study on 1-hexyl-3methyl-imidazolium ethylsulfate under nanoconfinement and significant shear forces show irreversible, solid-like structures which extend up to 60 nm from an adjacent solid surface as confirmed by surface force apparatus, AFM, and WAXS studies. 59 Blanchard et al…”