Impact of Nanoscale Surface Heterogeneity on Precursor Film Growth and Macroscopic Spreading of [Rmim][NTf₂] Ionic Liquids on Mica

Abstract

The connection between the interfacial properties of ionic liquids and their wetting behavior has been studied very little to date and not at all on heterogeneous surfaces. Therefore, we have investigated the static and dynamic wetting for a family of ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], on mica, where R represents an ethyl, butyl, or hexyl alkyl chain on the imidazolium ring. Spreading is impacted greatly by a precursor film that forms on both homogeneous and heterogeneous mica surfaces. Macroscopically, the initial viscous spreading of the ionic liquid droplet on bare mica occurs within seconds but is then followed by a very slow relaxation that can be closely correlated with the typical time-scales of the precursor film growth. The contact angle for [emim][NTf2] and [bmim][NTf2] relaxes from about 40° to 23° over 30 and 90 min, respectively. For [hmim][NTf2], the process takes approximately 24 h and approaches complete wetting. The thickness of the precursor films for [emim][NTf2], [bmim][NTf2], and [hmim][NTf2] were 0.53, 0.65, and 1.0 nm, respectively, according to atomic force microscopy (AFM). These values are consistent with a monolayer of ionic liquid cations on mica, rather than ion pairs. A monolayer of octadecylphosphonic acid (OPA) on mica prevents both the formation of a precursor film and the relaxation of the contact angle. However, only a partial surface coverage of ~60% OPA is required to have the same effect. Quenching of precursor film formation (and associated contact angle relaxation) is due to an increasingly connected network of OPA regions that closes the nanoscale paths of bare mica on which the precursor film can develop via surface diffusion.

Electrotunable lubricity with ionic liquids: the influence of nanoscale roughness

“…This enable us to model both partial wetting and full wetting states. We recall that experiments with imidazolium salts and mica have reported both states [22][23][24] . Increasing the IL-surface interaction to ε ILS,2 induces 1) a shift in the maximum of the "volcano" curve towards lower surface charges, and 2) a significant increase in the friction at low surface charge density (see Figure 8).…”

Squeezout phenomena and boundary layer formation of a model ionic liquid under confinement and charging

“…Second, it improves the plate wetting behavior, which without tails is unrealistically strong, while the wetting by real IL of surfaces such as mica is only partial. 18 Partial wetting is plausibly associated, at least in some cases, with a first monolayer of cations whose outward pointing tails may provide a "phobic" gap against further layerby-layer film growth. That is realized in our "tailed" model (TM) but not in the simpler charged LJ "salt-like" (SM) model.…”

“…18,23 Squeezout phenomena and layering necessarily involve intimate features of the liquidsolid plate interface, not disconnected with those determining the plate wettability. Thus, it is important to monitor the plate wetting habit of our IL models.…”

Morphology of Imidazolium‐Based Ionic Liquids as Deposited by Vapor Deposition: Micro‐/Nanodroplets and Thin Films

“…[27,[30][31][32][33] Due to their low vapor pressure, ILs are candidates for precursor film studies and the first study of precursor films spreading ahead of macroscopic droplets of imidazolium-based ILs (on smooth mica surfaces) was carriedo ut by Beattie et al [34] Additionally,i nr ecent literature reports there are several publications dedicated to interfacial tension studies, including the measuremento fc ontact anglesf ormed by various ILs on different solid substrates. [27,[30][31][32][33] Due to their low vapor pressure, ILs are candidates for precursor film studies and the first study of precursor films spreading ahead of macroscopic droplets of imidazolium-based ILs (on smooth mica surfaces) was carriedo ut by Beattie et al [34] Additionally,i nr ecent literature reports there are several publications dedicated to interfacial tension studies, including the measuremento fc ontact anglesf ormed by various ILs on different solid substrates.…”