Molecularly-Thin Precursor Films of Imidazolium-Based Ionic Liquids

Article pubs.acs.org/JPCC

Molecularly-Thin Precursor Films of Imidazolium-Based Ionic Liquids on Mica David A. Beattie,† Rosa M. Espinosa-Marzal,‡ Tracey T. M. Ho,† Mihail N. Popescu,† John Ralston,† Céline J. E. Richard,† Pasindu M. F. Sellapperumage,† and Marta Krasowska*,† †

The Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095 Australia Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland

‡

S Supporting Information *

ABSTRACT: Tapping mode atomic force microscopy has been used to investigate the spreading of molecularly thin (up to a few nanometers) precursor films emerging from drops of ionic liquids that partially wet smooth mica surfaces. The lateral extent of the film increases with time and reaches values as large as few millimeters within 12 h. From the observations of the precursor film at several positions and times, its extent l(t) was estimated and used to determine bounds for the coefficient D1 (defined by l(t) = √D1t) that characterizes the rate of spreading. The spreading rate and the film morphology (at micrometer scale) for three different ionic liquids of varying cation molecular structures are compared.

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INTRODUCTION

The mode of relaxation toward equilibrium depends significantly on the volatility of the liquid under ambient conditions.7−10,12,13 If the liquid is volatile, the equilibrium with the ambient gas and the substrate can be established relatively fast via evaporation−condensation onto the substrate. However, if the liquid is nonvolatile under ambient conditions, after an eventual fast spreading toward smaller contact angles upon touching the substrate, the evolution of the drop shape toward equilibrium must proceed via two-dimensional evaporation, i.e., molecules detaching from the drop near the contact with the substrate and moving along the surface of the substrate. For a number of systems it has been shown that the late stages of droplet spreading are accompanied by the formation of a mesoscopically or microscopically thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter.14−35 In respect to the formation and dynamics of such films, the macroscopic drop does play a rather secondary role, solely as a “mass reservoir” for the spreading film.7,10,12,36,37 Such films have been usually associated with liquid-on-solid systems, but in the past decade similar phenomenology was reported for certain solid-on-solid systems.38−45 These precursor films, which are of interest not only for basic science (e.g., their small thickness makes them ideal candidates for investigating the interplay between molecular liquid−liquid, liquid−substrate, and liquid−vapor interactions) but also for

Wetting phenomena are of crucial importance for many technological processes employed by various industries, such as the protective spin coating of surfaces (DVDs, glass lenses, car mirrors, and tinted windows) or inkjet printing.1 One of the typically encountered examples of wetting is that of a droplet deposited on an initially dry substrate. Upon contact with the substrate, the drop will evolve toward its thermodynamic equilibrium state. For a liquid which partially wets the solid, i.e., has a nonzero contact angle (as defined via the Young−Laplace equation), the statistical mechanics picture of the equilibrium state is that of a macroscopic drop (which far from the substrate has a spherical-cap shape, if the effects of gravity are negligible) connecting smoothly with a mesoscopically (tens to thousands of nanometers) or microscopically (molecular size) thin liquidlike film.2−6 Far from the region of contact between the macroscopic drop and the substrate (“foot”), the thickness of this equilibrium film is uniform and temperature dependent.3 For a completely wetting liquid (vanishing contact angle), the equilibrium state is either that of a film of uniform thickness or a flat “pancake” shape, depending on the volume of the drop and the volatility of the liquid.7−10 The thickness heq of the equilibrium film corresponds to the minimum of the so-called effective interface potential2 ω(h), which is the cost in free energy per unit area to maintain a wetting film of prescribed mean thickness h and is related to the disjoining pressure Π(h) = −dω(h)/dh.6,7 heq is related to the equilibrium contact angle θeq and the liquid−vapor surface tension σ via cos θeq = 1 + ω(heq)/σ (see, e.g., refs 3 and 11). © 2013 American Chemical Society

Received: June 25, 2013 Revised: October 13, 2013 Published: October 14, 2013 23676

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Figure 1. Chemical structure of the ions composing the three ionic liquids employed in this study.

precursor films. Many of these ILs have sufficiently large viscosity (for the liquids employed in our study, 30 to 70 times that of water74−77), which makes the dynamics of spreading sufficiently slow as to accommodate the typical scan-time (around 20 min) of a commercial AFM. Finally, one can expect that a very rich structure and dynamics in the spreading (quasi) two-dimensional (in the sense of a thickness that is orders of magnitude smaller than the lateral spatial extent) precursor films of ionic liquids will be promoted by strong spatial correlations imposed both by the bulkiness of the constituent ions and the overall electric neutrality of the ionic liquid.

novel applications (such as mediating thermo-capillary actuation46 or “pumping” of ionic liquid drops along metallic nanowires47), have been the subject of several thorough reviews.7,10,12,36,37 For microscopically thin precursor films, which are the focus of this work, the combination of experiment (see the references above) with theoretical analysis and numerical simulations48−66 led to a good understanding of the dynamics on homogeneous, planar, smooth substrates. One conclusion emerging from these studies is that on smooth, chemically homogeneous surfaces the lateral extent l(t) of the microscopically thin precursor films grows with time as l(t) ≃ √D1t, all of the details about the liquid−solid system being encoded in the coefficient D1. (We note here that there is still a certain degree of debate on the √t dependence; for example, the recent work in ref 67 which re-examined the spreading of PDMS on silicon wafers, reports a power-law dependence slower than √t for l(t)). However, it remains unclear what are the necessary conditions (for example, the balance required between the liquid−liquid and liquid−solid molecules pair interactions) for a precursor film to emerge from a nonvolatile drop (see, e.g., ref 68 for a system in which a precursor does not emerge), or what is the exact dependence of the lateral61−66 and vertical (“terraced spreading”15,69,70) structures in the precursor film on these interactions. Such open questions emphasize the necessity for additional systematic studies and in particular for the less explored situations of partial-wetting that employ a variety of liquids and substrates as well as a variety of methods to characterize the lateral structure in the precursor films at both macroscopic and microscopic length-scales. By using the tapping mode atomic force microscopy (AFM) technique, in this paper we report observations of microscopically thin (up to few nanometers) precursor films emerging out of drops of imidazolium-based ionic liquids that partially wet flat, atomically smooth mica surfaces. The tapping mode AFM technique, which has been previously used to study the spreading of drops31−33 as well as to detect spreading precursor films,34,35 allows us to observe the structure of the film with a lateral resolution of a few nm, while preserving a vertical spatial resolution of less than one nm. The choice of the ionic liquid− mica system71 is motivated by the type of open questions mentioned above. Mica is used as the substrate because of its naturally flat, (nearly) atomically smooth surface, which enables unambiguous AFM imaging. Room temperature ionic liquids (i.e., salt-like materials composed of ion pairs in a liquid state at ambient temperature) can be chemically synthesized from a wide variety of cation−anion combinations. This facilitates an almost “on-demand” design of their physical and chemical properties and therefore allows systematic investigations (as pursued previously for polymers with different molecular weights72) by varying, e.g., the size and chemistry of one of the ions while keeping the other fixed. Moreover, these liquids generically have very low volatility,73 which therefore rules out any role of evaporation−condensation in the formation of

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MATERIALS AND METHODS

Ionic Liquids and Mica Substrates. The following three imidazolium-based ionic liquids (minimum 99% purity, purchased from Iolitec), with same anion and different size cations (see Figure 1), have been used in this study: 1-ethyl-3methylimidazolium bis(trifluoroethyl sulfonyl)imide, [EMIM] [TFSI], 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide, [BMIM] [TFSI], and 1-hexyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMIM] [TFSI]. These ILs are hygrophobic and thus less prone to absorb water (we note, however, that water absorption by ILs cannot be completely prevented under ambient conditions). Mica (a silicate mineral with the chemical composition KAl2(AlSi3O10)(F,OH)2) was used as the substrate. Discs of mica, 15 mm in diameter, of the highest grade (V1) were purchased from TedPella. These can be easily cleaved, directly prior the experiment, to expose a nearly atomically smooth “fresh” surface. Experimental Sample Preparation. The surface of the mica disc was cleaved and placed on the stage of an optical microscope (Olympus BH2-UMA) just prior to the deposition of ionic liquid drops (which minimizes the time of exposure to the atmosphere). A cleaned thin glass capillary was used to deposit drops (few hundred μm in diameter) of ionic liquid onto the mica surface. Such samples were kept in closed glass Petri dishes in a laminar flow cabinet for up to 12 h and were moved only at certain times to perform AFM imaging. Tapping Mode AFM Imaging. All AFM experiments were conducted inside a clean room (Class 1000) at a temperature of 22 ± 1 °C and relative humidity of 39 ± 2%. The AFM (Multimode 8 with a Nanoscope V controller, Bruker), placed on an active antivibration table (Vision IsoStation, Newport), was equipped with a vertical engagement scanner “E” (maximum scan range 10 μm in the in-plane x and y directions, and nominal 2.5 μm in the normal to the surface z direction). Rectangular antimony (n) doped Si cantilevers (either TAP150A (Bruker) of nominal resonance frequency 150 kHz, arm length of 125 μm, and arm width of 35 μm, or FESP (Bruker) of nominal resonance frequency 50−100 kHz, arm length of 200−250 μm, and arm width of 23−33 μm) were 23677

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Figure 2. (a) Optical microscope image of an ionic liquid drop on mica. The three directions (“top”, “right”, and “bottom”), along which scans are performed at predefined positions, are shown by the gray arrows. (b) Schematic picture of a macroscopic drop in contact with, and feeding, a precursor film. The advancing edge of the film, which determines the lateral extent l, may be defined, e.g., as the iso-coverage curve corresponding to a certain low, threshold value. (c) AFM topography image of mica surface before the drop of ionic liquid is deposited. (d and e) AFM topography and phase images of a film on mica at a point situated 10 μm away from the macroscopic contact line of an [EMIM][TFSI] drop.

Figure 3. Equilibrium shapes and contact angles of macroscopic drops of the selected ionic liquids (and their reflection) on mica surface.

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used. To allow scans of the mica surface at distances (measured from the macroscopic contact line of the ionic liquid drop) varying from micro- to millimeters, the AFM was combined with an optical microscope with 10× objective (Nikon) and CCD color camera module (XC 999, Sony). The AFM images were processed and analyzed using the WSxM 4.0 SPMAGE 09 Edition (Nanotec)78 and NanoScope Analysis v 1.3 (Bruker) software packages. The surface scans were performed in tapping mode, which allows simultaneous acquisition of topographical (height) and phase (elastic properties) images without mechanical contact between the AFM tip and the surface.

RESULTS AND DISCUSSION

Before depositing ionic liquid drops, the mica surface (substrate) was imaged in air at a few points on the surface. (We note that mica surfaces exposed to air are known to dramatically reduce their charged state and become quasineutral, which is attributed to adsorption of various components from the surrounding atmosphere.79 In particular, it is known that water has a fast adsorption kinetics on mica surfaces;80 at room temperature and relative humidity level of 39%, corresponding to the conditions in our laboratory, a submonolayer coverage saturates within tens of seconds.81). Figure 2a shows a representative example; we conclude that the 23678

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Figure 4. AFM topography images of precursor film on mica surface around a 390 μm base diameter drop of [EMIM][TFSI]. The scans are performed after 2 (top) and 12 (bottom) h at the locations (from the edge of the drop) noted at the bottom of each column.

light blue in panel e). Consequently, we infer that these topographic and phase images evidence an ultrathin (below 3 nm thick) film, the nature of which yet remains to be established (see below), on top of the mica surface. (The nomenclature of “film”, in the sense of a coarse-grained coverage of the surface by the liquid, which is implicit when discussing molecularly thin films of submonolayer coverage, is used in order to connect with the measurements employing techniques with lower lateral resolution, in the range of tens of micrometers, such as ellipsometry or scanning Auger microscopy. On the microscopic level, allowed by the high lateral resolution of the AFM, the structure is shown to be inhomogeneous, formed by domains of small patches, and “film” refers to the average (either radial or over many independent experiments) coverage at that particular spatial location and moment of time.) We further note that the observation of phase image showing a two-phases structure correlated almost perfectly with the topographic features holds for all of the AFM scans collected and discussed below. In the rest of the manuscript, we therefore present only topographic AFM images. In Figures 4−6 we present representative results of the AFM topographic images of the mica surfaces on which [EMIM][TFSI], [BMIM][TFSI], and [HMIM][TFSI] drops have been deposited, respectively. As discussed in the context of Figure 2c, the topographic features evidenced in Figures 4−6 represent a film which can be, in principle, either an IL precursor film or solely the result of adsorption of some atmospheric component. The latter scenario is ruled out by the fact that to be consistent with our experimental observations such adsorption would have to be dependent both on location (compare, e.g., the first and

substrates we employed (mica plus the eventual neutralizing atmospheric adsorbate) are indeed atomically smooth. Upon depositing a drop of ionic liquid on the freshly cleaved mica substrate (time t = 0), after an initial fast spreading, the drop comes to rest and its macroscopic shape becomes quasi time-independent; we then measured the contact angle. As shown in Figure 3, for each of the ionic liquids on the mica surface, the corresponding contact angle is significantly greater than 0°, confirming a partial wetting state in each case. It can also be seen that the contact angle value is the same at the left and the right of the shape, as expected for an atomically smooth, homogeneous substrate; no contact angle hysteresis manifestations have been observed in any of the experiments performed. Tapping mode AFM scans of the areas centered at a few predefined locations: 10 μm, 100 μm, 1 mm, and 5 mm from the edge of the drop (which is the circular shape at the left of Figure 2a; note that the AFM cantilever is also visible in the image) along each of the three directions shown in Figure 2a are performed at several time intervals. Each such scan produces a “topography” (the height of surface in respect to the flat base plane) and a “phase” (a relative measure of the local “elasticity”) image. Figure 2d,e shows a typical example of such images (in this case [EMIM][TFSI] after 2 h at a location 10 μm from the edge of the drop). Panel e, which is basically binary colored, light and dark blue (the phase difference between them being around 37°), indicates that only two types of phases (two different elastic properties) are present; by visual inspection of the feature shapes in the two panels, one concludes that the topographic features (the lighter blue in panel d) correlate perfectly with one of the two phases (the 23679

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Figure 5. AFM topography images of precursor film on mica surface around a 420 μm base diameter drop of [BMIM][TFSI]. The scans are performed after 2 (top) and 12 (bottom) h at the locations (from the edge of the drop) noted at the bottom of each column.

the lateral extent of the precursor film is given by l(t) ≈ √D1t, the images in the last columns in Figures 4−6 allow us to set bounds on the effective rate of spreading coefficient D1. (We note that, owing to the lack of knowledge of the complex interactions between the ions and ions with mica, a connection with a specific spreading model, i.e., a specific expression for D1, is not possible at this stage of our studies.) This is so because for both [EMIM][TFSI] and [BMIM][TFSI] the film did reach the location at 5 mm, at a time between the observations at 2 and 12 h, while for [HMIM][TFSI] the spreading dynamics is slower and the location at 1 mm is reached at a time between these two observations. Thus for both [EMIM][TFSI] and [BMIM][TFSI] it should hold that 6 × 10−10 m2/s < D1 < 3.4 × 10−9 m2/s, while for [HMIM][TFSI] one obtains 2 × 10−11 m2/s < D1 < 6 × 10−10 m2/s. These values are of the same order of magnitude (at the lower range for [HMIM][TFSI]) as the ones reported in the literature for PDMS precursor films on various substrates23 (but, in contrast to our system, for a complete wetting situation) or metal-on-metal systems.41,42 Within the model of precursor film spreading via surface diffusion of IL molecules (ion pairs) with pair interactions in addition to the steric repulsion,61−64,66 the slower spreading kinetics of [HMIM][TFSI] could be due to both stronger binding to the surface for [HMIM][TFSI] as well as stronger pair interactions than in the case of the other two ILs. The first would imply a larger affinity of [HMIM][TFSI] for the mica surface, a scenario which seems unlikely (see, e.g., the molecular dynamics simulations studies reported in refs 82 and 83). The latter is consistent with a larger bulk viscosity of [HMIM][TFSI] (≃68 Pa × s, compared to ≃50 Pa × s for

last panels in the first rows of Figures 4−6), as well as, at a given location, on time (compare, e.g., the last panels in the two rows of Figures 4−6), respectively. These aspects are incompatible with the spatial translational invariance of the system, i.e., a macroscopically large planar disk exposed to a uniform atmosphere. On the other hand, the spatially decreasing surface coverage with increasing distance from the edge of the drop at a given time, in correlation with the increase in time of the local coverage at a given location, as observed in all three cases, is indicative of mass transport from the macroscopic drop along the surface. Further support for the IL precursor film formation is provided by the results of complementary X-ray photoelectron spectroscopy (XPS) elemental surface analysis (see also the Supporting Information) which we have performed for [HMIM][TFSI]. The results show the presence of S (which is solely due to the [TFSI] ion) on the surface at millimeters away from the macroscopic drop. Also, the XPS elemental surface analysis (which has a penetration depth of 7−10 nm) detects, at the same off drop locations, the chemical signature of the mica, which further support our assertions that the dark blue areas in the AFM scans do show the mica surface and that the precursor film thickness must be of microscopic (a few nanometers) range. We therefore conclude that our experiments unequivocally prove the formation and spreading of IL precursor films that are microscopically thin (thickness below few nanometers) but have a macroscopic lateral extent (few millimeters, thus 6 orders of magnitude larger than the thickness). Under the assumption, which is motivated by the observations reported for various other systems [see refs 7, 10, 12, 16−45, 48, and 61−66], that the time dependence of 23680

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Figure 6. AFM topography images of precursor film on mica surface around a 470 μm base diameter drop of [HMIM][TFSI]. The scans are performed after 2 (top) and 12 (bottom) h at the locations (from the edge of the drop) noted at the bottom of each column.

that the precursor film is compressible in two-dimensions while the surface coverage by IL exhibits a smooth decrease with the distance from the edge of the drop. The structure within the slower spreading [HMIM][TFSI] (Figure 6) is somewhat in between the two other cases. A rather compact structure in two-dimensions, although not as dense as the one for [EMIM][TFSI], is observed closer to the drop; but the surface coverage (and the average size) of the patches, while decreasing monotonically with the distance from the edge of the drop, yet maintains significant values also at locations distant from the edge (up to 1 mm away from the edge of the drop, in this case). These results for [BMIM][TFSI] and [HMIM][TFSI] resemble the features in the spatial dependence of the average coverage exhibited by models assuming transport of mass from the drop to the advancing edge by surface diffusion of molecules (IL pairs of ions) through the spreading film in the presence of moderate attractive pair interactions.61−64,66 Finally, we note that in order to preserve charge-neutrality for the drop the mass transport by surface diffusion is expected to involve the ionic liquid molecule (cation−anion pair) as an electrical dipole rather than isolated ions. If the mica surface exposed to the surrounding atmosphere remains even weakly charged, these cation−anion IL dipoles would (partially) orient normal to the surface within the first layer, which is in direct contact with the surface, and likely in the second layer, too (if three-dimensional clusters/structures do form). We indeed noticed that for most of the topographical features observed in the AFM images the changes in height are in steps of around 1 nm or slightly larger (see also the Supporting Information); this is close to the expectations for the size of an ion pair whose dipole is oriented normal to the surface (and, for the first layer,

[BMIM][TFSI] and ≃32 Pa × s for [EMIM][TFSI]74−77) at room temperature. The use of AFM imaging allows unprecedented insight into the local structure and morphology of the precursor film. (Yet, we remind the reader that for an in-depth understanding of the precursor film dynamics such detailed information about the local structure should be combined with complementary measurements covering larger spatial scales and reporting average, over tens to hundreds of μm2, surface coverage as function of the distance from the edge, using techniques such as ellipsometry15−28,31−33,36 or Auger scanning microscopy.39−45) In all cases, the local structure, at microscopic scales, is that of quasi two-dimensional patches of ILs, their size and number density on the surface being dependent on the IL, on the location from the edge of the drop, and on the time at which the observation was made. For [EMIM][TFSI] (Figure 4), the images at locations close to the drop indicate a region of high surface coverage, i.e., a quasi two-dimensional liquid-like, and this “bulk” region of the precursor seems to be preceded by one where the patches are smaller and have a lower surface density, i.e., a two-dimensional gas-like phase ahead of it. This is compatible both with the scenario of a compact film and mass transport to the advancing edge via diffusion of IL molecules on top of the film65 and with that of transport through the film in the presence of significant attractive interactions between the ILs molecules (pairs of ions).61−64,66 In contrast, Figure 5 indicates that for [BMIM][TFSI] the local structure is that of both large and small patches of ionic liquid occurring at both high and low local coverages; the general trend of decreasing surface coverage from the edge of the drop toward the advancing edge is preserved. For this case it seems very likely 23681

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the cation is lying on the surface82,83). The question of the detailed film structure in the direction normal to the surface as well as that of the long times and large spatial scales dynamics and structure of the coarse-grained (locally averaged) surface and mass coverage distributions as functions of the distance from the macroscopic contact line are the subject of future work and will be reported elsewhere.

(2) Dietrich, S. Wetting Phenomena. In Phase Transitions and Critical Phenomena; Domb, C., Lebowitz, J. L., Eds.; Academic Press: London, 1988; Vol. 12, pp 1−218. (3) Bauer, C.; Dietrich, S. Quantitative Study of Laterally Inhomogeneous Wetting Films. Eur. Phys. J. B 1999, 10, 767−779. (4) Dobbs, H. The Modified Young’s Equation for the Contact Angle of a Small Sessile Drop from an Interface Displacement Model. Int. J. Mod. Phys. 1999, 13, 3255−3259. (5) White, L. R. On Deviations from Young’s Equation. J. Chem. Soc., Faraday Trans. 1977, 73, 390−398. (6) Churaev, N. V.; Starov, V. M.; Derjaguin, B. V. The Shape of the Transition Zone Between a Thin Film and Bulk Liquid and the Line Tension. J. Colloid Interface Sci. 1982, 89, 16−24. (7) De Gennes, P. G. Wetting: Statics and Dynamics. Rev. Mod. Phys. 1985, 57, 827−863. (8) Cazabat, A. M.; Fraysse, N.; Heslot, F.; Levinson, P.; Marsh, J.; Tiberg, F.; Valignat, M. P. Pancakes. Adv. Colloid Interface Sci. 1994, 48, 1−17. (9) Ruckenstein, E. Conditions for Spreading as Single Molecules or as a Thin Planar Drop. J. Colloid Interface Sci. 1982, 86, 573−574. (10) Leger, L.; Joanny, J. F. Liquid Spreading. Rep. Prog. Phys. 1992, 55, 431−486. (11) Rauscher, M.; Dietrich, S. Wetting Phenomena in Nanofluidics. Annu. Rev. Mater. Res. 2008, 38, 143−172. (12) Bonn, D.; Eggers, J.; Indekeu, J.; Meunier, J. Wetting and Spreading. Rev. Mod. Phys. 2009, 81, 739−805. (13) Shikhmurzaev, Y. D. Capillary Flows with Forming Interfaces; Chapman and Hall/CRC: Boca Raton, FL, 2008. (14) Fowkes, F. M.; Harkins, W. D. The State of Monolayers Adsorbed at the Interface Solid-Aqueous Solution. J. Am. Chem. Soc. 1940, 62, 3377−3386. (15) Heslot, F.; Fraysse, N.; Cazabat, A. M. Molecular Layering in the Spreading of Wetting Liquid Drops. Nature 1989, 338, 640−642. (16) Bangham, D. H.; Saweris, Z. The Behaviour of Liquid Drops and Adsorbed Films at Cleavage Surfaces of Mica. Trans. Faraday Soc. 1938, 34, 554−570. (17) Ghiradella, H.; Radigan, W.; Frisch, H. L. Electrical Resistivity Changes in Spreading Liquid Films. J. Colloid Interface Sci. 1975, 51, 522−526. (18) Beaglehole, D. Profiles of the Precursor of Spreading Drops of Siloxane Oil on Glass, Fused Silica, and Mica. J. Phys. Chem. 1989, 93, 893−899. (19) Ausserré, D.; Picard, A. M.; Léger, L. Existence and Role of the Precursor Film in the Spreading of Polymer Liquids. Phys. Rev. Lett. 1986, 57, 2671−2674. (20) Leger, L.; Erman, M.; Guinet-Picard, A. M.; Ausserre, D.; Strazielle, C. Precursor Film Profiles of Spreading Liquid Drops. Phys. Rev. Lett. 1988, 60, 2390−2393. (21) Daillant, J.; Benattar, J. J.; Bosio, L.; Leger, L. Final Stages of Spreading of Polymer Droplets on Smooth Solid Surfaces. Europhys. Lett. 1988, 6, 431−436. (22) Heslot, F.; Cazabat, A. M.; Levinson, P. Dynamics of Wetting of Tiny Drops: Ellipsometric Study of the Late Stages of Spreading. Phys. Rev. Lett. 1989, 62, 1286−1289. (23) Voué, M.; Valignat, M. P.; Oshanin, G.; Cazabat, A. M.; De Coninck, J. Dynamics of Spreading of Liquid Microdroplets on Substrates of Increasing Surface Energies. Langmuir 1998, 14, 5951− 5958. (24) Daillant, J.; Zalczer, G.; Benattar, J. J. Spreading of Smectic-A Droplets: Structure and Dynamics of Terraces. Phys. Rev. A 1992, 46, R6158−R6161. (25) Betelú, S.; Law, B. M.; Huang, C. C. Spreading Dynamics of Terraced Droplets. Phys. Rev. E 1999, 59, 6699−6707. (26) Ma, X.; Gui, J.; Smoliar, L.; Grannen, K.; Marchon, B.; Bauer, C. L.; Jhon, M. S. Complex Terraced Spreading of Perfluoropolyalkylether Films on Carbon Surfaces. Phys. Rev. E 1999, 59, 722−727. (27) Fraysse, N.; Valignat, M. P.; Cazabat, A. M.; Heslot, F.; Levinson, P. The Spreading of Layered Microdroplets. J. Colloid Interface Sci. 1993, 158 (1), 27−32.

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CONCLUSIONS In conclusion, for three types of imidazolium-based, partially wetting ionic liquids on mica surfaces we have evidenced, by using tapping mode AFM and complementary XPS elemental surface analysis, the emergence and spreading of ultrathin precursor films with macroscopically large lateral extent. By analogy with the spreading dynamics l(t) = √D1t observed for other liquid−solid or solid−solid systems, we have estimated bounds for the spreading rate coefficient D1 in each case. The morphology of these films at mesoscopic (micrometer) lateral scales, which is possible to explore by using AFM techniques, reveals a very complex picture of quasi two-dimensional IL patches with sizes dependent on the IL, on the location from the edge of the drop, and on the time at which the observation was made. The local density of patches (their number per unit area) indicate, particularly for [EMIM][TFSI] and [HMIM][TFSI], a high coverage (quasi two-dimensional liquid-like) phase closer to the drop edge preceded by a low coverage (quasi two-dimensional gas-like) phase toward the advancing edge of the precursor. This can be tentatively attributed mainly to attractive interactions between molecules (ion pairs) moving by surface diffusion, in accordance to previously proposed theoretical models.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the technical aspects of the AFM measurements. Results of XPS analysis experiments for the case of [HMIM][TFSI] precursor film on mica. Additional experimental results for precursor film formation between two drops, film formation upon retraction of a liquid drop, and the influence of preexposure of mica to surrounding atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel:(+61) 883026861. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.B. acknowledges the financial support from the Australian Research Council (ARC) Future Fellowship scheme, Grant No. FT 100100393. M.N.P. and J.R. acknowledge the financial support from the ARC Discovery Project scheme, Grant No. DP 1094337. M.K., R.M.E.-M., M.N.P., and J.R. acknowledge the financial support from the Swiss National Science Foundation Sinergia scheme, Grant “Designing interactions across interfaces in ionic liquids”.

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REFERENCES

(1) Padday, J. F. Wetting, Spreading, and Adhesion; Academic Press: New York, 1978. 23682

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