Structure in Confined Room-Temperature Ionic Liquids - The Journal

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J. Phys. Chem. C 2007, 111, 5162-5168

Structure in Confined Room-Temperature Ionic Liquids Rob Atkin* and Gregory G. Warr School of Chemistry, Building F11, The UniVersity of Sydney, NSW 2006, Australia ReceiVed: NoVember 9, 2006; In Final Form: January 30, 2007

Solvation force profiles for three room-temperature ionic liquids (ILs), ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and 1-ethyl-3-methylimidazolium acetate (C2mimAc) confined between Si3N4 tips and mica, silica, and graphite have been measured using an atomic force microscope. The measurements reveal oscillatory behavior in all cases, with the size of the oscillations corresponding to the physical dimension of the ion pair. The surface charge and roughness and the orientation of cations at the interface are critical determinants of solvation layer formation in ILs. The greatest number of solvation layers is observed for EAN on highly charged, atomically smooth mica. Fewer and more compressible layers are observed for PAN due to its increased molecular flexibility. The lower surface charge and increased roughness of silica produces fewer solvation layers for both EAN and PAN compared to mica. For the EAN- and PAN-graphite systems, any attractive interaction with the substrate is due to the alkyl groups of the amine. This attraction is greater for PAN due to the increased size of the alkyl moiety, leading to stronger solvation forces. Six or seven solvation layers are present in the C2mimAc-graphite system. The C2mim+ ion adopts a flat orientation relative to this the substrate, which is more favorable for solvent layer formation for this IL, due to favorable interactions between the alkyl backbone of the cation and the substrate. Fewer layers are detected on mica and silica because cations in the interfacial layer are orientated with the ethyl group facing the bulk due to electrostatic interactions with these substrates.

Introduction Much scientific interest is currently focused on roomtemperature ionic liquids (ILs). This is due the “designer” properties of some ILs, particularly those based on the imidazolium cation where the physical properties of the liquid may be controlled by incorporation of appropriate functional groups, along with the environmentally friendly properties of ILs, most notably negligible vapor pressure, making ILs attractive candidates as replacements for volatile organic solvents.1-3 These features, combined with the ability of ILs to solubilize unusual combinations of reactants, have produced a vast number of research papers concerning the use of ILs for chemical synthesis.4 More recently, reports concerning the physical characteristics of ILs have emerged, focusing on molecular organization5 and bulk properties of the liquid,6,7 interfacial structure,8-11 dissolved polymers12 including aggregation of block copolymers,13 and the self-assembly of surfactants into micelles,14-22 liquid crystal phases,23 microemulsions.24,25 and surface aggregates.26 In this article, we report near surface oscillating solvation forces in several ILs using an atomic force microscope (AFM). An understanding of solvation forces in IL media is of great importance for their application in lubrication, adhesion, and dye sensitized solar cells,27 as the mobility of confined ions will play a key role in determining electric current flow.28 Oscillating solvation forces result from the arrangement of molecules or ions into discrete liquid layers adjacent to a smooth solid surface.29 This ordering leads to variation in the molecular density profile such that the measured force normal to the interface oscillates with a period approximately equal to the size of the solvent molecule and amplitude that decreases with * E-mail: [email protected].

separation.30,31 Oscillatory forces in a liquid were first demonstrated by Horn and Israelachvili,32 who measured the force between mica surfaces as a function of separation in octamethylcyclotetrasiloxane (OMCTS) using the surface force apparatus (SFA). Subsequent experiments with other nonpolar liquids revealed that the number of measurable oscillations decreases with increasing molecular flexibility, as flexible molecules are able to pack (fill space) effectively without layering.33 Polar solvents produce oscillatory forces superimposed onto an electrostatic double layer force, but the period is still determined only by the molecular size.34 Similarly, the ability of the solvent to form hydrogen bonds had no measurable effect on the oscillation period.30,35 In contrast, surface roughness destroys the ordering in the liquid and the oscillatory solvation forces are replaced by a purely monotonic solvation force.29 The invention of the AFM has allowed near surface molecular ordering to be studied on a wider variety of substrates. However, the contact area between the surface and the AFM tip is approximately 106 times smaller and less well defined than that for SFA, producing reduced signal-to-noise ratios. In AFM, the absolute separation between the tip and the substrate is also not known. To compensate, AFM experiments have frequently been conducted in frequency modulation or tapping mode using home-built instruments or instruments modified by the addition of a lock-in amplifier, which allows the deflection and tipsample interaction stiffness to be monitored simultaneously.36-42 This paved the way for studies on graphite using OMCTS,38,40,43 squalene,39 and their mixture40 and on mica using water,37,44 dodecanol,41 hexadecane,40 OMCTS,40,41 and mixtures of OMCTS and hexadecane.40 Two findings from these studies of mixed solvents have particular relevance to the current work. First, the large repulsive force associated with the first solvent layer for the squalene-OMCTS-graphite system was inter-

10.1021/jp067420g CCC: $37.00 © 2007 American Chemical Society Published on Web 03/15/2007

Structure in Confined Ionic Liquids preted as meaning that the first solvent layer (squalene) was strongly bound to the substrate.40 Second, the dimensions of the subsequent layer suggest that it was comprised of OMCTS, followed by discrete, alternating layers of squalene and OMCTS extended into the bulk. Molecular dynamics have been used to simulate solvation forces acting between an AFM tip and the surface. The fluid is modelled using interacting particles while the tip and substrate are represented by a large and infinitely large sphere (respectively)45,46 or as rigid arrays of atoms.47 Similar results are reported for both model types. Local variations in the average fluid density show that liquid particles in the vicinity of the tip and surface are highly ordered compared to the bulk. Increasing the radius of the AFM tip was found to affect the amplitude of the force oscillations but not their period. Changing the interaction between particles from a Lennard-Jones potential to a hard sphere interaction resulted in the force between the tip and substrate becoming attractive, but direct comparison between these theoretical studies and experiments is hampered, the models failing to account for tip-substrate interactions, such as dispersion forces. The properties of 1,3-dimethylimidazolium chloride confined between two flat silica surfaces have recently been simulated.28 The cation was modeled as a 10 site rigid body with partial charges assigned to appropriate sites, and particle-particle and particle-substrate interactions were modeled using LennardJones potentials. The density profile perpendicular to the wall suggested an interfacial layer slightly enriched in the cation, followed by oscillations that decay toward the center of the cell. Away from the walls, oscillations in the density profile and electrostatic potential are primarily due to the distribution of anions. An increased number of cations are present in the interfacial layer due to short range attractions between the atoms and the walls. These cations are predominately oriented with their rings perpendicular to the surface, which allows tight packing. As a result, the density of this layer is twice that of the bulk. There is no orientational order beyond the first layer. The authors suggest that the strength of the interaction between the ion and the substrate will influence the density of cations in the interfacial layer. The experiments conducted here allow this statement to be examined. Only one experimental paper has previously reported the presence of surface oscillatory forces in an IL. In 1988, Horn and co-workers used SFA to measure48 four to five oscillations for EAN before the repulsion became so strong that it prevented closer approach. On the basis of the absolute separations, the authors suggested that eight or nine surface layers were probably present and that the similarity between the results obtained in EAN and nonpolar liquids suggested that neither EAN’s ionic nature nor its ability to form a hydrogen-bonding network significantly affected the force profile. As with molecular liquids, the size of the constituent ions and their ability to pack into layers determined the characteristics of the oscillating force. The measured oscillation period of 0.5-0.6 nm is in good agreement with the dimensions of the EAN molecule determined from the density assuming a cubic packing geometry, cf. Figure 1. This suggests that the cation and anion are present in approximately equal numbers in the layers. Although force measurements alone could not determine whether alternating sublayers of positive and negative ions were present, the fact that EAN is autophobic on mica suggests that the first layer is orientated with the ammonium group facing the substrate with ethyl groups facing the bulk.

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Figure 1. Molecular structure, molecular weight (MW), density (F), and ion pair diameter (D) of the ionic liquids used in this study. D is determined from (F) assuming a cubic packing geometry according to the method described by Horn et al.48 Carbon atoms are shaded gray, nitrogen, blue, oxygen, red, and hydrogen, white.

In this study, near surface solvation layers in EAN, PAN, and C2mimAc on mica, silica, and graphite are presented. Comparison of results for EAN and PAN allows the effect of increasing the length of the hydrocarbon moiety of the IL on the force profile to be studied, while C2mimAc has a quite different molecular geometry. Materials and Methods C2mimAc was purchased from Aldrich and used as received. Its water content was measured, using Karl Fischer titration, to be 0.35 wt %. EAN and PAN were prepared by reacting equimolar amounts of the appropriate amine and nitric acid to produce an aqueous solution, as described by Evans et al.14 and Poole et al.,49 respectively. Excess water was removed by rotary evaporation followed by purging the solution with nitrogen then heating at 110-120 °C for several hours under a nitrogen atmosphere. This leads to water contents undetectable by Karl Fischer titration and prevents the formation of nitrous oxide impurities that, if present, produce a yellow discolouration. Graphite (monochromators grade ZYH from Advanced Ceramics, OH) and mica (Brown Co., Sydney) were prepared by using adhesive tape to cleave along the basal plane. Silicon wafers were baked at 1000 °C for 100 min in an oxygen atmosphere to produce an oxide layer (SiO2). Hydroxylated silica was prepared by soaking pyrogenic silica in water for 48 h, followed by treatment with 10 wt % NaOH for 30 s and rinsing in water and then ethanol before drying under a nitrogen stream. Force curves were collected using a Digital Instruments NanoScope IIIa Multimode AFM in contact mode. Scan sizes between 10 and 30 nm and a scan rate of 0.1 Hz was used in all experiments presented here. Cantilevers were standard Si3N4 with sharpened tips (Digital Instruments, CA). These were irradiated with ultraviolet light for 30 min prior to use. ILs were held in a fluid cell sealed with a silicone O-ring, which were cleaned by sonication for 20 min in 20 mM dodecyltrimethylammonium bromide solution, rinsed copiously in ethanol and deionized water, and dried using filtered nitrogen prior to use. All force curves were acquired continuously at room temperature, approximately 22 °C. All the data presented was obtained within 2 h of passing the ionic liquid into the AFM cell and the features of the force curves did not alter over 48 h. Typical start distances for force scans were 30-50 nm from the region where the oscillations were observed. The maximum applied

5164 J. Phys. Chem. C, Vol. 111, No. 13, 2007

Figure 2. (A) Force versus distance profile for an AFM tip approaching a mica surface in EAN. At least six steps in the force curve can be seen, extending to a separation of 3 nm. (B) Shows the same approach data along with that recorded as the AFM tip is retracted from the substrate.

force in contact was between 30 and 500 nN. However, no evidence for solvation layers was detected at forces greater than 15 nN in any system. The AFM tip geometry,50,51 spring constant,52 and surface chemistry53 can vary between tips. Over the course of this investigation several different tips from the same batch were used. The spring constants were measured to be 0.07 N/m ( 0.005 using the thermal noise method.54 Repeat experiments revealed that the number and period of the steps (oscillations) were constant unless stated otherwise in the text. There was some variability in the magnitude of the force required to rupture a given layer between tips and as a function of scan rate (effected by changing the scan size; a constant scan rate of 0.1 Hz was used in all experiments presented). As an indication of this variability, the push-through force for the layer closest to the surface was (2 nN for EAN and PAN on mica and graphite and (1 nN for C2mimAc. The roughness of the silica substrate increased the variability in the final push-through force to (2 nN for EAN and PAN, while this value was always between 1 and 3 nN for C2mimAc. Every system was studied over three or more separate experiments. Results and Discussion Force versus separation data for the EAN-mica system is shown in Figure 2. A series of “push-throughs” at discrete separations due to the AFM tip rupturing successive near surface layers produces a series of steps in the force profile. This data is different in form to the wavelike results obtained previously using the SFA48 due to differences in the experimental methods employed for the SFA31 and AFM52 relating to the finer control permitted by the SFA at low forces and small separations. However, the period of the near surface oscillations suggested by both studies is 0.5 nm, in good agreement with the diameter of EAN determined from the bulk density (Figure 1). Horn and co-workers suggested that as many as eight or nine surface layers

Atkin and Warr were present in the EAN-mica system but that only four or five layers were able to be measured before the repulsion became too great for the SFA to overcome, at a separation of about 2.5 nm.48 Steps due to the first six solvent layers (the last at 3 nm) are clearly visible (Figure 2A). There may be further steps at 3.5 and 4 nm, consistent with oscillations measured using the SFA, but it is difficult to be certain due to the level of background noise. The magnitude of the force required to rupture each layer increases as the tip moves closer to the surface. This suggests that ordering into layers decays further from the substrate as the molecular structure approaches that of bulk EAN. The autophobic behavior of EAN on mica suggests that EA+ ions are adsorbed onto the substrate with the ammonium groups associated with a surface charged site and the ethylene moiety facing the bulk liquid. This should give rise to thinner layers, so it is surprising that the width of the step closest to the substrate is 0.5 nm, consistent with all other steps. Data presented below for silica shows that the most likely explanation is a layer of strongly adsorbed EA+ ions electrostatically bound to the mica that cannot be displaced by the AFM tip. As EA+ occupies an area greater than the that of the charged site on mica (one site per 0.48 nm2),55 the degree of surface charge neutralized cannot exceed 87%.48 Thus, even at saturation coverage, there is always more than one surface charge site for each EA+ ion. This could account for the strength of the adsorption of the first layer. Horn et al.48 raised the possibility of alternating sublayers of positive and negative ions within solvation layers, and molecular modeling of molten KCl at a charged interface suggests that ionic sublayers form when the surface charge is sufficiently high, of the order of 32 µC cm-2.56 As the surface charge of fully ionized mica is comparable with this value, sublayers may be present for IL-mica systems, but further experiments are required to test this theory. While there is no evidence of significant attraction between the tip and the substrate, there is a substantial adhesion (6 nN) as the tip is retracted (Figure 2B). This is a consequence of attractions between EA+ adsorbed layers on the tip and on mica. Zero force is reached at a separation of 2 nm, which corresponds to the four solvent layers of EAN. Increasing the size of the hydrocarbon moiety of the IL from C2 in EAN to C3 for PAN only slightly increases the molecular volume (Figure 1) but does significantly alter the data, cf. Figure 3. Most notably, the steps are not vertical in PAN suggesting increased compressibility of the layers. Steps due to solvent layers are present at separations of 0.5, 1, and 2 nm, although it should be noted that the innermost layer at 0.5 nm was not always observed. Combined with the fact that the expected third step at 1.5 nm was never observed, this may suggest that PAN adopts a morphology where strongly and weakly formed molecular layers alternate, raising the possibility of alternating sublayers of propylammonium and nitrate. It is likely that the increased compressibility of PAN smears the sharpness of the features in the force profile. As the measured force beyond 2 nm is zero, the number of AFM detectable near surface layers is reduced to four in PAN as compared to six or seven in EAN. Previous SFA experiments comparing the behavior of cyclohexane and n-octane concluded that the more flexible n-octane molecules were able to pack more efficiently, resulting in fewer solvation layers.57 The increased length of the alkyl length group of PAN compared to EAN produces a similar effect. The adhesion observed on retraction is increased for PAN compared to EAN, due to stronger interactions between PA+ ions associated with mica and on the tip. On retraction, the AFM

Structure in Confined Ionic Liquids

Figure 3. (A) Force versus distance profile for an AFM tip approaching a mica surface in PAN. At least six steps in the force curve can be seen, extending to a separation of 3 nm. (B) Shows the same approach data along with that recorded as the AFM tip is retracted from the substrate.

Figure 4. Force versus distance profile for an AFM tip approaching and retracting from a mica surface in C2mimAc. The separation distances for each layer prior to push-through are given on the plot.

tip jumps from contact to a separation of 2 nm, the distance associated four solvent layers, as observed for EAN. Three steps approximately corresponding to the physical size (Figure 1) of the IL are noted in Figure 4 for the C2mimAcmica system. The steps for are not vertical like those for EAN but are closer to vertical than for PAN, showing that the compressibility of the molecular layers is between that of EAN and PAN. The number of layers and the force require to push through each layer are lower than values for both EAN and PAN. It may be that the physical shape of C2mimAc hinders layer formation or that increased flexibility is imparted by the large size of both the cation and the anion. The layer closest to the substrate is the same size as those at greater separations; however, as for EAN and PAN, results presented below may suggest that there is a layer of C2mimAc+ strongly bound to the mica that cannot be displaced by the AFM tip. Alternatively, it is possible that the sterically hindered charged site of C2mimAc+ reduces the level of interaction of the cations with the mica which allows the layer to be displaced by the tip. As molecular modeling suggests that the surface layer is only

J. Phys. Chem. C, Vol. 111, No. 13, 2007 5165 slightly enriched in the cation,28 this would account for the similar size of the surface layer and those further from the substrate. The adhesion of the tip to mica on retraction is much less than that for EAN and PAN. This may be a consequence of the absence of a hydrogen bond network in this IL, reduced levels of cation adsorption on the substrate and on the tip due to the greater molecular size of C2mim+, lower cohesive energy interactions within the liquid, or some combination of these factors. For the purposes of this work, the primary differences between the silica and mica substrates are the reduced surface charge and increased roughness of silica. The density of charged sites on mica is high (one site per 0.48 nm2),55 whereas for silica one site per 20 nm2 at neutral pH in water is typical.58 However, as the surface charge for amorphous silica is due to protonation or deprotonation of surface hydroxyl groups, which is determined by the relative population of charged species at the surface, the charge density of silica is expected to be significantly higher in ILs, but still less than mica. The root mean square (rms) roughness of the silica used in this study was found by AFM imaging to be 1.3 nm for a 5 × 5 µm region and 0.2 nm for a 300 × 300 nm region, which is sufficient to broaden otherwise distinct solvation layers.29 Roughness greater than the radius of the molecule can smear out solvation layers completely.32 Figure 5A shows a typical force profile for the EAN-silica system. The increased roughness of the silica substrate compared to atomically smooth mica smears the repulsive peaks considerably, but clear steps in the data are still present. The fact that steps are observed at all may be due to the small interaction area between the substrate and the AFM tip, which has a radius of only ∼20 nm. On this scale, the rms roughness is of the order of only 0.05 nm (determined by taking the average rms roughness of twenty 20 × 20 nm areas of a silica substrate). On approach, a repulsion beginning at 2.7 nm separation is measured, with small, irregularly spaced steps accompanying a marginal increase in force. Clear steps in the data, with the width approximately equal to the diameter of EAN, occur at 1.8, 1.3, 0.8, and 0.25 nm as the repulsive force increases to 10 nN. This suggests that at least five solvation layers are present. The steps are not vertical like those on mica due to surface roughness. Of critical importance is the fact that the layer nearest the surface has a thickness of only 0.25 nm. This suggests that this layer is comprised primarily of EA+ ions with the ammonium group next to a surface charged site like that suggested (but never observed) for mica. That is, favorable interactions between the ammonium and the substrate results in a surface layer rich in the cation, as suggested by theory28 and experiment.59 The lower surface charge of silica means that this layer is less strongly bound than for mica, so at sufficiently high force the AFM tip can displace it and move into direct contact with the substrate. On retraction, there is negligible adhesion of the AFM tip to the surface due to the reduced level of electrostatic adsorption of EA+ to silica compared to mica. As a result, the force profile for the retraction is quite similar to that recorded for the approach. The force curve for the PAN-silica system presented in Figure 5B is quite similar to that for EAN. Jumps in the force curve from 1.65 to 1.1 nm and from 1.1 to 0.4 nm accompany an increase in the measured repulsive force. The separation then remains almost constant until the repulsive force reaches 6 nN, at which point the AFM moves into contact with the substrate. As for EAN, the thickness of the layer next to the substrate is less than those further out and the diameter of PAN, which

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Atkin and Warr

Figure 5. Force versus distance profile for an AFM tip approaching and retracting from a silica surface in (A) EAN, (B) PAN, and (C) C2mimAc.

Figure 6. Force versus distance profile for an AFM tip approaching and retracting from a graphite surface in (a) EAN, (B) PAN, and (C) C2mimAc.

suggests a layer of electrostatically adsorbed PA+ next to the surface. The PA+ layer can be penetrated using the AFM on silica due to the substrate’s low surface charge but not on mica where the PA+ layer is more strongly bound (Figure 3). There is little adhesion between the surface and the tip on retraction, so the retraction data is quite similar to that recorded on approach. The force profile for the C2mimAc-silica system presented in Figure 5C suggests one electrostatically bound layer of C2mim+ that extends to a separation of 0.5 nm. This thickness is consistent with the ethyl group being orientated approximately normal to the substrate as suggested by a recent sum frequency spectroscopy study of similar ILs adsorbed to silica8 and by molecular modeling.28 Beyond this layer, a second solvation layer that exhibits a step at 1.15 nm, consistent with the expected diameter of C2mimAc (Figure 1) and the period observed on mica. The push-through force associated with each layer is considerably less than that for EAN and PAN as C2mimAc packs less effectively into solvation layers. This is also consistent with the fewer layers detected. There is little adhesion of the AFM tip to the substrate on retraction. It is worth noting that the surface charge associated with the Si3N4 AFM tips is lower than a pure silica substrate, so

electrostatic interactions between cations and the tip will be reduced. Furthermore, as the roughness of the AFM tip50 is increased relative to the silica substrates, this will decrease the number of solvation layers. For both of these reasons, we contend that, for all systems, the measured solvation surfaces are primarily due to interactions between the solvent and the substrate and that solvent-tip effects are less important. The force profile for EAN on graphite (Figure 6A) has very different form to that on mica and silica. On approach, the tip experiences an attraction toward the substrate, causing jumps from a separation of 1.7 to 1.1 nm to 0.4 nm while the force is decreased from 0 to -1.5 nN to -3 nN. (The jump distance (Dj) can be used to estimate the Hamaker constant (A) for this system using Dj ) (AR/3k)1/3, where k is the spring constant and R is the tip radius. Using measured values for R (20 nm) and k (0.07 N m-1), A ) 2 × 10-20 is found. This value is higher than for other liquids, but increasing R to 40 nm gives A with the expected order of magnitude. This increase in R is reasonable given the change in tip geometry due to wear that occurs over the course of an AFM experiment.50) This small jump-in distance is consistent with an attraction between the tip and the substrate due to dispersion forces. A 0.4 nm separation is maintained as the force is increased from -3 to

Structure in Confined Ionic Liquids -2 nN, at which time the tip penetrates the layer and moves into contact with the substrate (no additional layer could be detected up to an applied force of 400 nN). Two adhesions are detected on retraction. The first is between the tip and the substrate to a force of -7 nN. The second is between the tip and the adsorbed EA+ layer at a separation of 0.35 nm with a maximum adhesive force equal to -7.5 nN. The tip then jumps out of contact with the substrate, reaching zero force at a separation of 2.4 nm. Interactions between the ethyl group and graphite may produce a slight interfacial excess of this ion, similar to that observed for the squalene-OCMTS-graphite system.59 The role of the surface charge on the Si3N4 tip is not clear at this stage, but other tip chemistries may yield quite different force profiles. In a previous paper,26 we presented a force curve for EAN on graphite that did not show the steps in the data present in Figure 5. This is because the rate of approach (and retraction) of the tip was significantly faster due to the larger scan size used (∼100 nm). The force curve for PAN on graphite is presented in Figure 6B. A small repulsion occurs between 1.9 and 1.2 nm, followed by a jump from 1.2 to 0.75 nm on approach. Here, there is a second, much steeper repulsion, with the force increasing from 0 to 8 nN as the separation decreases from 0.75 to 0.35 nm. The tip then pushes through the intervening layer into contact with the substrate. The greater force required to rupture the interfacial layer for PAN compared to EAN is a consequence of the increased size of the hydrocarbon moiety of the propylammonium ion, which interacts more strongly with the substrate, in line with theoretical predictions.28 If, as may be expected, the Hamaker constant for PAN is smaller than that for EAN, a lower attraction between the tip and the substrate may also contribute to the higher measured force (the absence of a jump to contact for the PAN system prevents the determination of a Hamaker constant). Note that the final pushthrough distance is somewhat lower than expected, which may due to PA+ in the interfacial layer being, on average, orientated flat along the substrate, although greater layer compressibility may also contribute. On retraction, a -4.5 nN adhesive force is measured, after which the tip jumps first to a separation of 0.4 nm at approximately the same force and then to 1.5 nm at zero force. The data for C2mimAc on graphite (Figure 6C) has the most unusual form of any of the systems investigated in this study. On approach, the force becomes slightly attractive at a separation of 4.6 nm, followed by clear steps at 3.95, 3.25, and 2.45 nm, approximately consistent with dimension of C2mimAc. The force then becomes slightly more attractive, producing a jump to 1.7 nm followed by a repulsion at 1.05 nm. The level of data noise increases markedly at this point. A push-through occurs at a separation of 0.4 nm with the maximum repulsive force equal to 2 nN, substantially less than that for PAN (Figure 6B). The final push-through distance of 0.4 nm is consistent with the C2mimAc ring being orientated parallel to the graphite substrate. The form of this data is reminiscent of simulations for a liquid of weakly interacting molecules,46 which may suggest that the molecular cohesion is smaller for C2mimAc than for EAN and PAN. On retraction, an initial adhesion to -2 nN is noted, followed by a second adhesion at a separation of 0.4 nm of magnitude -5 nN. Two further steps at 1.05 and 1.65 nm are seen at this force, after which a large jump to a separation of 3.25 nm occurs as the adhesive force is reduced to -2 nN. This force remains essentially constant until a separation of 5.25 nm is reached, where a second large jump occurs to 6.1 nm and the force reaches zero. The form of this

J. Phys. Chem. C, Vol. 111, No. 13, 2007 5167 data was repeatable between experiments within the error limitations stated in the methods section, except that the three steps in the data at 2.4, 3.25, and 3.95 nm were not always so clear. We suggest that the parallel alignment of the surface C2mimAc layer on graphite favors the formation of six-seven layers, far more than its perpendicular orientation on silica and mica. General Comments The form of the results obtained for the EAN-mica system suggests that solvation layers in this system are particularly well formed. The high surface charge of mica produces a strong interaction between the surface and the interfacial layer and may even be sufficient to produce nitrate and ethylammonium sublayers. This, combined with the atomic smoothness of the substrate, leads to eight or nine solvation layers, the most of any system investigated here. The lower surface charge and increased roughness of silica decreases the number of detectable solvent layers. Graphite is also atomically smooth, but an attraction between the substrate and AFM tip occurs. Only two or three solvation layers are detected against an attractive background. The force required to rupture the interfacial layer is low, which may suggest that any interaction between the ethyl moiety of EA+ and the substrate is also weak. However, the tip-substrate attraction will lower the measured force. For all substrates, the steps in the force curves are less vertical for PAN than EAN, showing that PAN layers are more flexible. This is most likely due to the increased size of the alkyl group, which allows the hydrocarbon sublayers to become interdigitated. The larger alkyl group also increases the attraction to graphite for PAN compared to EAN, where the force required to rupture the interfacial solvent layer is greatly increased. The results for C2mimAc are generally quite different to those for EAN and PAN. On mica and silica, fewer steps are observed, and those that are present are less well defined. This is a consequence of two factors. First, the positive charge of C2mim+ is located farther from the surface than for the primary ammoniums, which decreases the electrostatic attraction between the cation and the surface. Second, the thickness of the interfacial layer of C2mim+ suggests that the ethyl group is orientated normal to the substrate which, as results on graphite show, is relatively unfavorable for the formation of solvation layers for this IL. Only three layers form on mica, and two, on silica. On graphite attractive interactions between the substrate and hydrocarbon backbone of the C2mim+ ion suggest the cation adopting a flat conformation. For this IL, such an orientation is more favorable for the production of solvation layers than with the ethyl group orientated toward the bulk, as six or seven solvation layers can be detected. Conclusions The experiments conducted in this work show that the surface charge, surface roughness, and orientation of the cations in the interfacial layer are of great importance for the formation of solvation layers in ILs. The thickness of the solvent layers, other than the one next to the substrate, is determined purely by the physical size of the IL ion pair. For EAN and PAN, the greatest number of solvation layers are observed on highly charged, atomically smooth mica. Fewer layers and much greater layer compressibility are observed for PAN due to its increased molecular flexibility. Comparison of results obtained on silica and mica show that an electrostatically bound cation layer that cannot be displaced with the AFM tip (within the force range used in these experiments) is present on mica. For EAN and

5168 J. Phys. Chem. C, Vol. 111, No. 13, 2007 PAN on graphite, interactions with the substrate occur through the alkyl groups of the amine. This attraction is greater for PAN, leading to solvation forces of increased magnitude. For C2mimAc, the greatest numbers of solvation layers are observed on atomically smooth graphite. This is because interactions between C2mim+ and the substrate lead to the ring being orientated parallel to the surface, which is more favorable for solvent layer formation for this IL. On mica and silica, interfacial attraction is due to electrostatic interactions between the surface and the sterically hindered charge of C2mim+, which leads to the ion being orientated with its alkyl group facing the bulk. Three layers are detected on atomically smooth mica compared to two layers for silica, due to the increased roughness and lower surface charge of the silica substrate. Acknowledgment. The authors wish to thank Shannon Notley (Australian National University) for measuring the AFM spring constants. This work was supported by an Australian Research Council (ARC) Discovery Project (DP0556126). R.A. wishes to acknowledge the ARC for provision of a postdoctoral fellowship. References and Notes (1) Forsyth, S. A.; Pringle, J. M.; MacFarlane, D. R. Aust. J. Chem. 2004, 57, 113. (2) Rodgers, R. D.; Seddon, K. R. Science 2003, 302, 792. (3) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275. (4) Welton, T. Chem. ReV. 1999, 99, 2071. (5) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.; Padua, A. A. H.; Sheppard, O.; Soper, A. K. J. Phys. Chem. B 2006, 110 (24), 12055-12061. (6) Canongia Lopes, J. N.; CostaGomes, M. F.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 16816. (7) Lachwa, J.; Szydlowski, J.; Najdanovic-Visak, V.; Rebelo, L. P. N.; Seddon, K. R.; Nunesda Ponte, M.; Esperanca, J. M. S. S.; Guedes, H. J. R. J. Am. Chem. Soc. 2005, 127 (18), 6542-6543. (8) Fitchett, B. D.; Conboy, J. C. J. Phys. Chem. B 2004, 108 (52), 20255-20262. (9) Liu, Y.; Zhang, Y.; Wu, G.; Hu, J. J. Am. Chem. Soc. 2006, 128 (23), 7456-7457. (10) Millefiorini, S.; Tkaczyk, A. H.; Sedev, R.; Efthimiadis, J.; Ralston, J. J. Am. Chem. Soc. 2006, 128 (9), 3098-3101. (11) Rivera-Rubero, S.; Baldelli, S. J. Am. Chem. Soc. 2004, 126 (38), 11788-11789. (12) Triolo, A.; Russina, O.; Keiderling, U.; Kohlbrecher, J. J. Phys. Chem. B 2006, 110 (4), 1513-1515. (13) He, Y.; Li, Z.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128 (8), 2745-2750. (14) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89. (15) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (16) Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. Chem. Commun. 1996, 1625. (17) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J. Phys. Chem. 1983, 87, 3537. (18) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. L. Langmuir 2002, 18, 7258. (19) Fletcher, K. A.; Pandey, S. L. Langmuir 2004, 20, 33. (20) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. M. C. J. Mater. Chem. 1998, 8, 2627.

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