ARTICLE pubs.acs.org/JPCC
Wetting Films of Two Ionic Liquids: [C8mim][BF4] and [C2OHmim][BF4] Jose Restolho,†,‡ Jose L. Mata,†,§ Karina Shimizu,† Jose N. Canongia Lopes,† and Benilde Saramago*,† †
Centro de Química Estrutural, Instituto Superior Tecnico, T U Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Research Institute for Medicines and Pharmaceutical Sciences, Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-019 Lisbon, Portugal § Academia Militar, Pac-o da Rainha, 29, 1150-244 Lisbon, Portugal ‡
ABSTRACT: The stability of wetting films of two ionic liquids, 1-octyl-3methylimidazolium tetrafluoroborate, [C8mim][BF4], and 1-ethanol-3methylimidazolium tetrafluoroborate, [C2OHmim][BF4], on alumina was assessed through the measurement of disjoining pressure isotherms, which represent the dependence of the disjoining pressure on the film thickness. Two experimental techniques were used at low and high disjoining pressure, respectively: captive bubble and modified thin film pressure balance. Theoretical predictions of the disjoining pressure isotherms were made using the microscopic approach of London and Hamaker based on the van der Waals contribution to the disjoining pressure. Good agreement was found between the experimental and the theoretical isotherms for [C8mim][BF4], whereas in the case of [C2OHmim][BF4], the experimental data are slightly shifted toward larger thickness, especially at higher disjoining pressures. An interpretation of these results is given in terms of a corresponding-states principle argument applied to the surface tension and the use of auxiliary molecular dynamics data. The conclusion is that although both Coulomb and dispersion interactions contribute to determine the bulk properties of these ionic liquids the surface properties are mainly related to the dispersive forces.
’ INTRODUCTION Thin liquid wetting films have been under investigation during the past decades because of their importance from both practical and theoretical points of view.1,2 A number of authors have studied films of aqueous solutions of electrolytes and surfactants with the objective of understanding the nature of forces within the film. Liquid films obtained from solutions of biomolecules deserved special attention because of their relevance in biomedical and pharmacological applications. The problem of thin film stability involving liquid crystals and lubricants has also been addressed. Wetting films can provide information on the equilibrium surface forces in the liquid film bounded by two different bulk phases, a solid substrate and a gas phase. The force of interaction of film interfaces equals the disjoining pressure acting within the film. The disjoining pressure derives from several contributions, namely, London dispersion forces, electrostatic double layer forces, and structural forces. The relative importance of each component depends on the nature of the system. Most investigations have been carried out with aqueous systems where the dispersion forces are negligible in comparison with the electrical double layer forces. However, for nonaqueous systems, the dispersion forces are known to control the stability of the liquid film. The confinement of a liquid between two walls induces a molecular layering that is responsible for oscillatory forces designated by structural forces. In a recent review, Boinovich3 summarized different theoretical approaches to explain the nature of hydration, solvation, or structural repulsive forces as r 2011 American Chemical Society
well as hydrophobic attractive forces as a result of deviations of various structural parameters of the thin interlayer from the corresponding values in the bulk. Room-temperature ionic liquids (RTILs) present a rare opportunity to study the interplay among a wide range of molecular interactions such as Coulombic, van der Waals, dipole dipole, hydrogen-bonding, and solvation forces. Furthermore, they are strongly promising materials in applied science because of the possibility of tuning their physicochemical properties according to the pretended application. Although the vast majority of processes in applications of RTILs involve solidliquid interfaces, the amount of research in this area is still scarce. Previous studies were based on sum-frequency vibrational spectroscopy (SFVS),4 X-ray reflectivity,57 atomic force microscopy (AFM),812 and also molecular simulations.1315 These studies had as a common objective the determination of the structure of the ionic liquid near the solid surface. To our knowledge, there are no studies in the literature concerning the interactions in thin ionic liquid films and the kinetics of thinning and rupture. The objective of the present work was to investigate the stability of wetting films of two well-known ionic liquids, 1-octyl-3-methylimidazolium tetrafluoroborate, [C8mim][BF4], and 1-ethanol-3methylimidazolium tetrafluoroborate, [C2OHmim][BF4], in a wide Received: May 26, 2011 Revised: July 4, 2011 Published: July 08, 2011 16116
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Table 1. Parameters a and b of the Linear Equation (n = a + b 3 T (K)) for the Temperature Dependence of the Refractive Index, n, of Alumina, [C8mim][BF4], and [C2OHmim][BF4] at Several Wavelengths, λ aluminaa a
b
[C2OHmim][BF4]
λ = 546.1 nm
1.7669
1.5323
1.5385
589.3 nm 650.0 nm
1.7643 1.7613
1.5228 1.4923
1.5375 1.5331
λ = 546.1 nm
1.3 105
0.0003
0.0002
589.3 nm
1.3 105
0.0003
0.0003
650.0 nm
1.3 105
0.0002
0.0002
(298348) K
(298328) K
(328348) K
temperature range a
[C8mim][BF4]
From ref 23, where the reported values are ordinary refractive indices.
range of disjoining pressures. The choice of these ionic liquids was based on the fact that the former has a long side chain in the imidazolium ring, whereas the latter has the alcohol (CH2)2OH as a functional group. [C2OHmim][BF4] has high surface tension and contact angle on hydrophilic substrates compared with the values of [C8mim][BF4].16,17 The stability of the liquid films may be inferred from the shape of the disjoining pressure isotherms that represent the dependence of the disjoining pressure on the film thickness. Different experimental techniques have been used to measure the disjoining pressure and the film thickness.18 In this study, two methods were chosen to assess different ranges of disjoining pressures. The method of the captive bubble was adopted to obtain the disjoining pressure isotherm in the range of low pressures. With this method, an air bubble is pressed against a solid surface immersed in a liquid until a liquid film with equilibrium thickness is attained. The equilibrium situation occurs when the pressure in the film equals the pressure of the gas inside the bubble. The disjoining pressure Π, which opposes film thinning, is equal to ΔP, the pressure difference across the bubble/liquid interface, which is given by the Laplace equation ΔP = 2γ/rb (where γ is the surface tension of the liquid and rb is the radius of curvature of the bubble). Thus, the range of disjoining pressures available for the measurements is determined by the radius of the bubble holder. The determination of film thickness by interferometry is based on the observation of the interference pattern upon reflection of light by the film. Further details of this method may be found in our previous publications.19,20 A modification of the thin film pressure balance (TFPB) using the porous-plate technique was used to extend the disjoining pressure range to higher values. This method that has been applied by several authors21,22 was recently implemented in our laboratory. Details of our apparatus and the experimental technique are given in the Materials and Methods. The experimental isotherms were compared with those calculated using a simple model based on the microscopic approach of London and Hamaker, and the results of this comparison were interpreted at the light of the corresponding states principle applied to the surface tension. To our knowledge, this is the first time that disjoining pressure isotherms for films of RTILs were measured.
manipulated inside a glovebox filled with dried nitrogen. The water content, checked by Karl Fischer, is ∼340 ppm for [C8mim][BF4] and ∼188 ppm for [C2OHmim][BF4]. The water used for cleaning was distilled and deionized. The temperature dependence of the density and the surface tension was determined in a previous work16 in the temperature range of 298 to 470 K: F (g 3 cm3) = 1.32447.0 104T (K) and γ (mJ 3 m2) = 84.30.065T (K) for [C2OHmim][BF4]; F (g 3 cm3) = 1.11717.0 104T (K) and γ (mJ 3 m2) = 49.30.055T (K) for [C8mim][BF4]. The solid substrates used in the captive bubble and TFPB methods were R-alumina plates (Melles Griot, 1 mm thickness and 10 mm diameter). The contact angles of [C2OHmim][BF4] and [C8mim][BF4] on alumina, at room temperature, measured according to the procedure previously explained,17 were 53 and 30°, respectively. The refractive indexes of the ionic liquids were measured with an ABBE 60 refractometer from Bellingham Stanley Limited as a function of the temperature. To get values at three different wavelengths, λ = 546.1, 589.3, and 650.0 nm, we used a 100 W halogen lamp with three interchangeable 10 nm band-pass interference filters (Melles Griot). The temperature dependence of the refractive indexes of the ionic liquids and alumina, measured at different wavelengths, was fitted to a linear equation (n = a + b 3 T (K)), whose parameters are given in Table 1. The refractive index of nitrogen was taken to be 1.0. Experimental Methods. The disjoining pressure isotherms were determined using two methods: the captive bubble in the low-pressure range and a modified thin film pressure balance in the high-pressure range. In both techniques, the film thickness is determined interferometrically by measuring the reflection of monochromatic light. The intensity of the reflected light may be measured with a photosensor (TFPB) or calculated through the analysis of the interference pattern (captive bubble). The film thickness is related with the reflectivity, defined as the ratio of the intensity of the light reflected by the film, If, and the intensity of the incident light, I0, assuming the incident beam to be perpendicular to the film through the so-called Rayleigh equation If ðr1 þ r2 Þ2 4r1 r2 sin2 δ ¼ I0 ð1 þ r1 r2 Þ2 4r1 r2 sin2 δ
where r1 and r2 are the normal incidence Fresnel coefficients
’ MATERIALS AND METHODS
r1 ¼
Materials. The ionic liquids were purchased from Solchemar
and have a purity >98%. Before the experiments, the liquids were vacuum-dried at 80 °C for at least 3 days; after that, they were
ð1Þ
ns nf nf ng r ¼ 2 ns þ nf nf þ ng
ð2Þ
where ns, nf, and ng are the refractive indices of the substrate, the liquid film, and the gas, respectively. The phase difference δ is 16117
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Figure 1. (1) Removable cover, (2) cell body, (3) alumina substrate, (4) porous glass plate, (5) 1 mm hole, (6) capillary glass, (7) tilted BK7 glass window, (8) pressure inlet, and (9) outlet to pressure transducer.
defined as δ¼
2πnf l λ
ð3Þ
where l is the film thickness and λ is the wavelength of the monochromatic light. Introduction of the quantity Δ, defined as Δ¼
If Imin Imax Imin
ð4Þ
where Imax and Imin are the maximum and minimum intensities of reflected light that correspond to optical thicknesses multiples, respectively, of half wavelengths and quarter wavelengths, leads to the following equation for the relation between Δ and the thickness ! f 1Δ 2 2πn l ¼ sin ð5Þ 4r1 r2 λ 1 Δ ð1 þ r1 r2 Þ2 In the captive bubble method, the liquid film is formed through the introduction of a nitrogen bubble inside a bubble holder using a gas syringe. This procedure, which is based on the work of Blake and Kitchener,24 has been intensively used in our laboratory. The apparatus and the operating procedure were described in detail in a previous work.19 The disjoining pressure, Π, which is equal to the capillary pressure at equilibrium is given by Π = 2γ/rB, where γ is the surface tension of the liquid and rB is the radius of curvature of the bubble, assuming that its base is spherical, and it may be calculated from the interferometric pattern obtained when the bubble touches the solid. We present here a brief description of the cleaning method. All parts of the cell were submitted to the following procedure: (1) washing and sonication for 15 min in Extran aqueous solution (2% v/v); (2) rinsing and sonication with water for 3 10 min; and (3) drying with nitrogen, followed by drying overnight inside an oven at 60 °C. When dried, all components were transferred into a glovebox filled with nitrogen, where the bottom and the cover of the cell were assembled separately. Both parts were plasmacleaned for 5 min. The modified TFPB was recently implemented in our laboratory, and it is based on the interferometric method of Mysels later extended by Exerowa and Scheludko.25 The liquid film is formed in a hole (diameter of 1 mm) drilled through a porous glass plate (pore size 3 μm) connected to the atmosphere by a capillary glass tube fused laterally on the plate. The plate, enclosed inside a hermetically sealed AISI 316 stainless steel cell, supports an alumina substrate, and the film is formed on this substrate by
adjusting the gas pressure in the cell using a homemade compressor. The cell is enclosed in a double-walled aluminum chamber that allows circulation of water through a temperaturecontrolled bath. A scheme of the cell is shown in Figure 1. The film is illuminated from below with white light through an optical window of BK7 glass (Melles Griot) located at the bottom of the cell. This window is tilted so that the reflected light from its surfaces does not impinge on the measuring system. The reflected light is collected by a fiber optic cable and measured with a photosensor module (H5784 Hamamatsu). Before entering the photosensor, the light beam crosses a 10 nm bandpass interference filter (Melles Griot), and the working wavelength (λ = 578.28 nm) is isolated. The maximum and minimum intensities of reflected light are determined, respectively, when the light is reflected by the alumina plate (film of null thickness) and by the bulk liquid filling the hole, previous to the film formation. The disjoining pressure is given by the following expression Π ¼ Pg Pr ΔFghc þ 2γ=rc
ð6Þ
where (Pg Pr) is the difference in pressure inside and outside the cell measured with a differential pressure transducer (Paroscientific model 5306D-101, 06 psid), ΔFghc is the hydrostatic pressure of the liquid column in the glass tube (ΔF is the difference in density between the liquid and the gas, g is the gravitation acceleration, and hc is the height of the liquid column), and 2γ/rc is the capillary pressure in the glass tube of radius rc. The height of the liquid column was measured with a cathetometer as a function of the applied pressure. Before filling the cell with the ionic liquid, all parts were carefully cleaned. Both stainless steel cell and alumina plate were submitted to the following cleaning procedure: (1) 15 min of sonication in Extran aqueous solution (2% v/v), (2) 10 min of sonication in water (repeated twice), (3) rinsing with distilled and deonized water, and (4) drying under nitrogen flow, followed by drying at 80 °C for at least 2 h. Immediately before use, the alumina plate was plasmacleaned for 5 min. The porous glass plate was boiled 30 min in ethanol, followed by 30 min of boiling in Extran aqueous solution (2% v/v) and 3 30 min boiling in water and, finally, dried at 80 °C for at least 2 h. Periodically, the porous plate was heated in the oven at 600 °C during 12 h to remove any trace of contaminants. The cell was filled with the sample through the capillary with a constant volume of liquid (∼0.2 mL). When equilibrium was achieved, the experiment started by increasing the pressure stepwise and allowing the film to equilibrate for at least 1 h at each chosen pressure until the reflected intensity remained constant. After equilibration, 200 intensity points were recorded during 400 s, where the If values were obtained from the average of all data points. The measurements were carried out at 55 °C for [C8mim][BF4] and at 65 °C for [C2OHmim][BF4]. These temperatures were chosen to ensure that the viscosities of the ionic liquids (86 mPa 3 s for [C8mim][BF4] and 85 mPa 3 s for [C2OHmim][BF4]16) were compatible with not very long equilibration times. Temperature had no effect on the disjoining pressure isotherms, as demonstrated by the similarity of Π versus l measurements at different temperatures, in the range (3575) °C (data not shown). Simulation Methods. All simulations used to complement the interpretation of the experimental data were performed using molecular dynamics algorithms, implemented using the DL_POLY program.26 The molecular force field used in the simulations of the 16118
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Figure 2. Disjoining pressure, Π, versus thickness, l, for films of [C8mim][BF4] at 55 °C. The open symbols represent data obtained with the captive bubble method, whereas the closed symbols refer to the values obtained with the modified TFPB. The line represents the theoretical predictions of LondonHamaker.
two ionic liquids studied in this work is based on the CLaP forcefield.27 For both ionic liquids [C8mim][BF4] and [C2OHmim] [BF4], we started from low-density initial configurations composed of 150 ion pairs. The boxes were equilibrated in isothermalisobaric ensemble conditions for 400 ps at 300 K using the NoseHoover thermostat and isotropic barostat with time constants of 0.5 and 2 ps, respectively. Electrostatic interactions were treated using the Ewald summation method considering six reciprocal-space vectors, and repulsivedispersive interactions were explicitly calculated below a cutoff distance of 1.6 nm. (All long-range corrections were applied, assuming the system has an uniform density beyond that cutoff radius.) Details concerning this type of simulation can be found elsewhere.27 Furthermore, simulations of the vapor phase were carried out, considering it to be formed by isolated neutral ion pairs. These were equilibrated in canonical ensemble (NVT) conditions for 40 ns at 300 K using the NoseHoover thermostat with a time constant of 1.0 ps. Electrostatic interactions were treated using the usual Ewald summation method, and a cutoff distance of 500 Å was applied here. Because the statistics are poor as a result of the small number of atoms, each MD production run took 40 ns, and 32 such runs were used to calculate the average gas-phase properties. The cohesive energy of the bulk liquid (which can be related to the enthalpy of vaporization) was calculated as the difference between the configurational energies of the simulation boxes representing the liquid and gas phase. The van der Waals and Coulomb contributions to the total cohesive energy are calculated based on the interaction energies emerging from the implemented short-range (van der Waals) and long-range (Coulomb) potential functions, respectively. The former is based on the Lennard-Jones 126 potential, with repulsive and dispersive terms.
’ RESULTS AND DISCUSSION Figures 2 and 3 report the disjoining pressure isotherms (disjoining pressure, Π, vs thickness, l) measured in a wide range of pressures for [C8mim][BF4] and [C2OHmim][BF4],
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Figure 3. Disjoining pressure, Π, versus thickness, l, for films of [C2OHmim][BF4] at 65 °C. The open symbols represent data obtained with the captive bubble method, whereas the closed symbols refer to the values obtained with the modified TFPB. The line represents the theoretical predictions of LondonHamaker.
Figure 4. Images of films of [C8mim][BF4]: (a) stable film with thickness = 18 nm and (b) unstable film before rupture at Π ≈ 500 Pa.
respectively. The open symbols represent data obtained at low disjoining pressures with the captive bubble method, whereas the closed symbols refer to the values obtained at higher disjoining pressures with the porous plate technique. The lines on these graphs correspond to a theory discussed later. The first observation is the very good agreement exhibited by the two sets of data points. The disjoining pressure isotherms obtained with the modified TFPB prove to be reversible because the same data were measured by increasing or decreasing the pressure. The films were stable in the whole range of pressures being the maximum pressure limited by the features of the compressor. Film rupture was rarely observed and was attributed to the presence of impurities in the porous glass. Figure 4a,b shows images of a stable film and of unstable film, a few seconds before rupture, of [C8mim][BF4]. The colors in Figure 4b indicate the instabilities that precede the film rupture. Comparison of the experimental disjoining pressure isotherms with the theoretical predictions allows the assessment of the relative importance of the various contributions for the disjoining pressure. According to the well-known theory of colloidal stability, 16119
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DLVO theory after Derjaguin and Landau,28 both van der Waals and double-layer contributions have to be considered in the calculation of the disjoining pressure. Later, a contribution from structural forces was also considered.29 The van der Waals contribution may be either positive or negative depending on the dielectric properties of the three phases. A positive contribution exists when the dielectric properties of the liquid are intermediate between those of the two limiting media. This is the case for the ionic liquid between alumina and air. The electrostatic contribution derives from the electrostatic repulsion between the diffuse parts of the electric double layers of both film surfaces. The origin of the steric and structural forces is less clear. However, for films of polyelectrolyte solutions,30 aqueous foam films31 and emulsion films,32 oscillatory forces have been measured between the film surfaces and interpreted as deriving from structural effects. The van der Waals disjoining pressure may be calculated through the following equation33 ΠvdW ðlÞ ¼
Asl 3 f All 3 f 0 6πl3
ð7Þ
where Asl and All are the Hamaker constants for the solid/liquid and the liquid/liquid interactions and f and f 0 are retardation correction functions.34 These Hamaker constants may be calculated from the characteristic frequencies, υic, and the limiting values of the dielectric constants of the liquid and the solid, εi0, according to the procedure described in a previous work,35 where the index i stands for the solid substrate (s) or the liquid (l) 27 hνlc νsc εl0 1 εs0 1 Asl ¼ 32 ðνlc þ νsc Þ εl0 þ 2 εs0 þ 2 ð8Þ 27 εl0 1 2 hυlc All ¼ 64 εl0 þ 2 whereas the retardation correction functions are l < 3λc =2π
f ¼ 1:01 0:28psl þ 0:0143ðpsl Þ3 0:00193ðpsl Þ4 f 0 ¼ 1:01 0:28pll þ 0:0143ðpll Þ3 0:00193ðpll Þ4
forces in [C2OHmim][BF4], whereas this percentage increased to 43% in the case of [C8mim][BF4]. Important contributions from dispersion forces are the norm for ionic liquids, with some ionic liquid families showing contributions above 50%.27 In a recent work,17 we determined the polarity of ionic liquids defined as the ratio between the nondispersive component (that is often called “polar” component) and the total surface tension using the Fowkes approach. Polarity fractions of 0.38 and 0.32, respectively, for [C2OHmim][BF4] and [C8mim][BF4] were calculated. It is interesting to point out that whereas calculations based on the surface behavior (polarity fractions) indicated a predominance of dispersive interactions, the values obtained for the bulk (MD simulations) suggest the opposite situation. This discrepancy is only apparent because both methods refer to different situations (the bulk in the case of the MD simulations and the liquidvacuum interface in the case of the Fowkes approach) and also to two different ways to tally the contributions (short-range (van der Waals) versus long-range (Coulomb) interactions in the case of the MD simulations and dispersive versus nondispersive in the case of the Fowkes approach). It must be stressed that one of the distinctive characteristics of ionic liquids is their highly complex, nonisotropic structure. This lack of fluid isotropy that is already dominant in the bulk is also present (and enhanced) at the surface.37 The reorganization of the 3D structure of the ionic liquid imposed by the 2D nature of the surface boundary implies a further segregation of the polar and nonpolar domains of the ionic liquid. The latter domains (alkyl side chains attached to the polar heads of the ions) tend to concentrate at the surface.37,38 This fact justifies the dominance of the nonpolar interactions at the surface, estimated by the Fowkes approach. In both situations (bulk and surface), the contribution of Coulombic (nondispersive) forces is higher, as expected, for [C2OHmim][BF4] than for [C8mim][BF4]. To investigate further the role of the various types of intermolecular forces on the surface behavior of the ionic liquids, we decided to apply a simple, two-parameter form of the corresponding-states principle to the surface tension data. According to Weiss et al.,39 the reduced surface tension may be defined by
ð9Þ l > 3λc =2π
sl 1
sl 2
f ¼ 1:47ðp Þ 0:816ðp Þ f 0 ¼ 1:47ðpll Þ1 0:816ðpll Þ2
ð10Þ
where psl = 2πl/λslc and pll = 2πl/λllc , where λslc was calculated from the arithmetic average of the characteristic frequencies of the solid and the liquid. Calculation of the electrostatic component for electrolyte solutions is based on the GouyChapman theory that relies on the dilute-solution approximation, which cannot be applied to an ionic liquid. Modern statistical mechanics of dense Coulomb systems, or density functional theory, would be needed to deal with properties of the interfacial double layer of ionic liquids.36 Application of these theories is beyond the scope of our work. Furthermore, it is now known that although ionic liquids are made of ions, the large size of these ions (some of them with long alkyl side chains) is responsible for important dispersion forces, in particular, if compared with inorganic molten salts.27 MD simulations of the energetics of vaporization of both ionic liquids under study were used to estimate the dispersive and Coulombic contributions to the cohesive energy at room temperature. Dispersive forces were found to account for 31% of the total
γR ¼ c
γ 2=3 Tc Fc M 2=3
ð11Þ
where c is a constant ∼1.016 if γ is measured in mN 3 m1, M is the molar mass in g 3 mol1, Tc is the critical temperature in K, and Fc is the critical density in g 3 cm3. The critical temperatures were obtained from the fitting of Guggenheim equation (γ = Const(1T/TC)11/9) to the surface tension versus temperature data.16 The critical densities were estimated from the average densities, F̅ , in the temperature range of the surface tension measurements using the empirical correlation Fc ≈ 0.333F̅ . This correlation was found from the comparison of experimental values of liquid densities40 and critical densities41 for a series of molecular liquids. Figure 5 shows plots of the reduced surface tension as a function of reduced temperature (TR = T/Tc) for several families of molecular liquids and for molten salts. The experimental values of the surface tension and the critical parameters Tc and Fc for these molecular liquids were compiled from the data given in the two references cited above. The reduced surface tensions of NaCl and KCl were calculated from surface tension data reported by Jaeger,42 and the critical parameters were taken from Kirshenbauml et al.43 The experimental data are compared with Guggenheim’s universal curve applied to argon, a purely dispersive fluid. 16120
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Figure 5. Reduced surface tension, γR, (y axis) as a function of the reduced temperature, TR, (x axis) for several representative families of liquids: noble gases (a), diatomic and triatomic molecules (b), alkanes (c), haloalkanes (d), alcohols (e), and low- (this work) and high-temperature molten salts (f).
Analysis of the Figure 5ae clearly indicates that apart from the exceptions of helium and hydrogen the molecular liquids that deviate from Guggenheim’s curve toward lower values of the reduced surface tension (negative deviations) have small molecules and strong hydrogen bonds. As the size of the molecules increases, the reduced surface tension also increases, and the deviations become positive. A case in point is the series of nalkanols (Figure 5e) that shows reduced surface tensions below the line for methanol, “ideal” behavior for propanol, and positive deviations for longer alkanols. Figure 5f shows the results for the ionic liquids, [C8mim][BF4] and [C2OHmim][BF4], and two inorganic molten salts. The most striking result is that the
reduced surface tensions of both ionic liquids deviate strongly from the data of fluids dominated by Coulomb interactions (molten salts) and lie close to the master curve for dispersive liquids. The negative deviation observed for [C2OHmim][BF4] is consistent with the small size of the ions and the presence of hydrogen bonds. Similar results were reported by Leroy et al.,44 who found that the corresponding states surface-tension data of [bmim][BF4] and [bmim][NTf2] are similar to typical values of liquefied noble gases. Our main point here is that although Coulomb interactions are important in the bulk behavior of [C8mim][BF4] and [C2OHmim][BF4], the application of the corresponding states 16121
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The Journal of Physical Chemistry C principle to reduced surface tension clearly indicates that the behavior of their liquid/vapor interface is mainly determined by the London dispersive interactions. Coincidentally, a very interesting case is that of [C8mim][BF4] that lies closer to the dispersive master curve of Argon than many of the molecular fluids depicted in Figure 5ae. This does not mean that the interactions in this particular ionic liquid are purely dispersive; it means that the negative deviations that would be created by the Coulomb interactions present at the surface (subdued but not negligible) are compensated by the positive deviations caused by the dispersive interactions between the long alkyl chain of the cation segregated at the surface. As in the case of n-alkanols, where the negative deviations caused by hydrogen bonding tend to be canceled-out by positive deviations produced by longer alkyl chains and a breakeven point is achieved around C3, in the case of imidazolium-based ionic liquids, such a break-even point is reached only for longer alkylside chains (C8) (and more important dispersive contributions) due to the presence of the coulomb interactions. However, we must be aware that this is a simplified version of the corresponding states principle where no shape effects were considered. Furthermore, uncertainties in the determination of the critical temperature and critical density from long-range extrapolations may also affect our conclusions. The reasoning presented above led us to try a comparison between the experimental disjoining pressure isotherms and those based only on the van der Waals contribution. The isotherms (Π vs l) calculated according to eq 7 are represented by the lines in Figures 2 and 3. The deviation between theory and experiment is almost negligible for [C8mim][BF4], whereas the data for [C2OHmim][BF4] shifts toward larger values of the film thickness, mainly at higher disjoining pressures. These results confirm that the main forces that are responsible for the stability of thin films of ionic liquids are dispersive interactions. The deviation found for [C2OHmim][BF4] is in agreement with the existence of strong hydrogen bonds. We may also speculate that other repulsive interactions, namely, structural forces, could be responsible for this film thickening. These forces are known to depend on the orientational ordering at the interface between the film and the solid.45 Recent MD simulations of the behavior of [C2OHmim][BF4] and [C8mim][BF4] at solid interfaces12,15 showed that the stratification process is more effective for the former ionic liquid, supporting the above speculation on the role of structural forces. Further systematic investigations on a series of ionic liquids would be necessary for a better understanding of the nature of the interactions within the liquid films.
’ CONCLUSIONS The main outcome of this work is the finding that the behavior of thin wetting films of two ionic liquids, [C8mim][BF4] and [C2OHmim][BF4], is mainly determined by the London dispersion forces. This conclusion is in agreement with the results of the application of the corresponding states principle to these ionic liquids. The reduced surface tension of [C8mim][BF4] is very well described by Guggenheim’s universal curve for simple, purely dispersive fluids. In the case of [C2OHmim][BF4], there is a deviation of the reduced surface tension toward lower values, which is the typical behavior of molecular fluids with small molecules and strong hydrogen bonds. This deviation should be related to the small shift toward larger thickness of the disjoining pressure isotherm of [C2OHmim][BF4] from the theoretical van der Waals contribution.
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