Static and dynamic wetting behaviour of ionic liquids

CIS-01457; No of Pages 10 Advances in Colloid and Interface Science xxx (2014) xxx–xxx

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Static and dynamic wetting behaviour of ionic liquids Iliana Delcheva, John Ralston, David A. Beattie, Marta Krasowska ⁎ Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Adelaide, Australia

a r t i c l e

i n f o

Available online xxxx Keywords: Ionic liquids Wetting Contact angle Line tension Droplets Precursor film

a b s t r a c t Ionic liquids (ILs) are a unique family of molecular liquids (‘molten salts’) that consist of a combination of bulky organic cations coupled to inorganic or organic anions. The net result of steric hindrance and strong hydrogen bonding between components results in a material that is liquid at room temperature. One can alter the properties of ionic liquids through chemical modification of anion and cation, thus tailoring the IL for a given application. One such property that can be controlled or selected is the wettability of an IL on a particular solid substrate. However, the study of wetting of ionic liquids is complicated by the care required for accurate and reproducible measurement, due to both the susceptibility of the IL properties to water content, as well as to the sensitivity of wettability measurements to the state of the solid surface. This review deals with wetting studies of ILs to date, including both static and dynamic wetting, as well as issues concerning line tension and the formation of precursor and wetting films. © 2014 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . 2. Theory 1: static contact angles . . . 3. Theory 2: dynamic contact angles . . 4. Macroscopic contact angles of ionic 5. Small droplets and line tension . . . 6. Precursor films . . . . . . . . . . 7. Dynamic wetting experiments . . . 8. Conclusions and future directions . . Acknowledgements . . . . . . . . . . . Appendix A. . . . . . . . . . . . . . References . . . . . . . . . . . . . . .

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1. Introduction In contrast to aqueous solutions of common salts, ionic liquids (ILs) consist of ions in the absence of solvent molecules. These substances are usually composed of bulky organic cations coupled to organic or inorganic anions. Due to the delocalised charge in the organic components of these substances, as well as the steric mismatch between the IL ions, the formation of a stable crystal lattice is hindered. The presence of large asymmetric cations and strong hydrogen bonds can greatly decrease the temperature of the solid–liquid transition [1]. As a result,

⁎ Corresponding author. Tel.: +61 8 8302 6861. E-mail address: [email protected] (M. Krasowska).

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many of these organic salts are liquids at room temperature. The melting point of these materials is typically around or just below 100 °C. ILs have gained considerable academic and industrial interest due to their unique properties, such as high thermal stability, nonflammability, and negligible vapour pressure. ILs are sometimes called ‘designer’ liquids because their properties (such as viscosity and refractive index) may be easily tailored by small variations of the chemical composition. Their intrinsic characteristics, together with the results obtained from tribological studies [2,3], suggest that these substances are among the potential candidates to replace conventional mineral oils as lubricants. In addition, ILs are being considered as electrolytes in energy applications (Li-ion batteries, electro-optical imaging, supercapacitors, etc.), and for this application, as well as many others, the wetting characteristics of the IL in contact with a solid substrate are critically important.

http://dx.doi.org/10.1016/j.cis.2014.07.003 0001-8686/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Delcheva I, et al, Static and dynamic wetting behaviour of ionic liquids, Adv Colloid Interface Sci (2014), http:// dx.doi.org/10.1016/j.cis.2014.07.003

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I. Delcheva et al. / Advances in Colloid and Interface Science xxx (2014) xxx–xxx

As with any other liquid (molecular or solution), upon contact of an IL droplet with a solid substrate, the drop will evolve towards its thermodynamic equilibrium state: it either spreads completely on the solid, forming a thin (wetting) liquid film, or creates a three-phase (liquid, solid and the surrounding vapour) system, in which the three interfaces intersect at a three-phase contact line [4]. These two types of wetting regimes are referred to as complete and partial wetting, respectively. In the latter regime, the drop wets a certain area of the solid substrate, which is enclosed by the three-phase contact line. The contact angle that is formed at the contact line between the liquid– vapour and the solid–liquid interfaces when the system reaches its state of minimal free energy is called the equilibrium contact angle, θeq. This contact angle is a quantitative measure of the wetting properties. 2. Theory 1: static contact angles The equilibrium contact angle is connected to the three interfacial tensions γSV, γSL, and γLV of the solid–vapour, solid–liquid, and liquid– vapour interfaces. This follows from the requirement of mechanical balance at every point of the contact line. Free energy minimisation leads to Young's equation1 cosθeq ¼

γSV −γ SL : γ LV

ð1Þ

Young's equation can be rewritten in terms of the free energy of cohesion, WC, and adhesion, WA [6]. By definition, WC is the work (free energy) per unit area required to isothermally split in two a column of condensed matter. During this process two new flat liquid–vapour interfaces are created; thus the cohesion free energy per unit area has the form: W C ¼ 2γ LV :

ð2Þ

Analogously, WA is defined as the work (free energy) per unit area required to isothermally split in two parts a column of two condensed phases in contact. In this case, a flat liquid–vapour and a flat solid– vapour interface, respectively, are created, while the initial solid–liquid interface is destroyed. WA is thus given by: W A ¼ γ LV þ γSV −γSL :

ð3Þ

Consequently, Eq. (1) can be re-written as cosθeq ¼

2W A −1: WC

ð4Þ

Young's equation applies to ideally rigid, chemically inert and homogeneous solid surfaces that are smooth at the atomic level; however, real solid substrates rarely possess all these characteristics. In real systems, in which surfaces usually are rough and/or chemically heterogeneous, the contact line is immobile not only when the local contact angle has reached its equilibrium value, but also when it falls in a more or less wide range of contact angle values. These local contact angles correspond to metastable states, i.e. to local minima of the free energy of the system. Among all of these states, only the ones corresponding to the lowest and highest value of the contact angle can be meaningfully characterised. These are known as the receding, θrec, and the advancing, θadv, contact angles. The equilibrium contact angle falls in-between, θrec ≤ θeq ≤ θadv [7], and the difference, H = θadv − θrec, is called contact angle hysteresis. 1 Named after the famous English scientist Thomas Young, who was the first to suggest in his paper from 1805 that “for each combination of a solid and a fluid, there is an appropriate angle of contact, between the surface of the fluid, exposed to the air, and to the solid” [5].

3. Theory 2: dynamic contact angles Wetting phenomena find many applications in industrial and engineering processes such as flotation, production of high quality coatings, paints and lubricants, and printing. Most of these processes rely on dynamic wetting, i.e. the motion of a liquid, advancing or receding over a surface due to either the unbalanced interfacial tensions at the contact line or the application of an external force on the system [4,8]. The contact angle θ, measured when the contact line is in motion, is the dynamic value. In general, for a Newtonian liquid, two types of mechanisms influence the dynamic wetting process: viscous hydrodynamic flow in the bulk of the liquid and friction at the moving contact line. Two models, each emphasising one of the mechanisms, are usually employed to describe dynamic wetting. The first model, the so-called hydrodynamic approach, is based on the assumption that the excess free energy, which is due to deviations of the shape and the contact angle of the droplet from those at equilibrium, is dissipated by the viscous flow in the moving liquid. For low capillary numbers, Ca = Vη / γLV , for the fluid phase in motion, the relation between the dynamic contact angle and the velocity of the contact line, V, was determined by Cox [9] and Voinov [10]. In a simplified form (for small contact angles and low viscosity surrounding fluid) it reads:   3 3 γLV θ −θeq   ; V ¼ L 9η ln LS

ð5Þ

where η is the viscosity of the liquid, L stands for a characteristic length, such as the droplet size, and LS is a slip length (accounting for a slip boundary condition at the contact line). The positive sign of the velocity denotes an advancing contact line, while the negative: a receding one. More complex expressions for solid-liquid-liquid systems, accounting for the viscosities of both of the fluids, also exist [9]. Although derived for small contact angles, θ, and a ‘liquid wedge’ geometry, this equation was shown to describe well the process of dynamic wetting also for spreading drops and for contact angles up to θC ≈ 13 / 18π (130°) [11]. The other model usually used to describe the dynamics of wetting attributes the dissipation of the excess free energy to the interactions between the molecules of the liquid and the solid phase at the contact line. Blake [12] suggested that the spreading of the liquid is determined by the processes of adsorption and desorption of molecules at the contact line; the movement of the wetting line in a given direction is attributed to a disturbance of the adsorption/desorption equilibrium, characterised by frequency k0, due to the out of balance free energy per unit area γLV(cosθeq − cosθ). According to the molecular-kinetic (MK) model, the macroscopic result of this disturbance is the motion of the contact line with a velocity, V, dependent on the dynamic contact angle, θ:

V ¼ 2k0 λ sinh

  2 γLV cosθeq − cosθ λ 2kB T

:

ð6Þ

Here λ denotes a microscopic characteristic length scale related to an average distance between ‘adsorption sites’, kB is the Boltzmann constant, and T is the absolute temperature. When the argument of the hyperbolic sine function is very small, Eq. (6) can be reduced to the linear form: V¼

 γLV  cosθeq − cosθ ; ζ

ð7Þ

where ζ = kBT / k0λ3 denotes the ‘contact line friction’ coefficient and has the physical dimensions of a shear viscosity.

Please cite this article as: Delcheva I, et al, Static and dynamic wetting behaviour of ionic liquids, Adv Colloid Interface Sci (2014), http:// dx.doi.org/10.1016/j.cis.2014.07.003

I. Delcheva et al. / Advances in Colloid and Interface Science xxx (2014) xxx–xxx

According to the equation above, there seems to be no dependence of the velocity on the viscosity of the liquid; however, it is experimentally known that this property has a significant impact on the dynamic wetting phenomenon, especially for fluids with large viscosities [13]. Blake et al. (e.g. [14–16]) have suggested that the characteristic frequency k0 and consequently the velocity of the wetting line should be proportional to the reciprocal value of the fluid viscosity. Furthermore, an exponential dependence on the work of adhesion, WA, is anticipated [16]: k0 ∝

kB T −W A e ; ηvm

3

directions [27,29–32]. For systems of sessile drop type geometry, k0,⊥ b k0,∥, therefore the rate-determining step in the spreading process was displacements in normal to the substrate direction. Thus, k0,⊥ compared well with the values of k0, extracted from the data fitted with the MK model [27,29,30,32]. Furthermore, the hypothesised exponential dependence of k0 on WA, Eq. (8), and experimentally confirmed in Refs. [20,33] was also manifested via MD simulations [31,32]. These findings suggest that the MK model describes the dynamic wetting phenomenon in a reliable manner.

ð8Þ

where vm is the unit flow volume. This relationship has been verified experimentally for a number of systems [17–21]. During the last two decades, efforts were made to merge the two approaches rather than strictly attributing the energy dissipation in the process of spreading either to viscous friction in the bulk, or to friction at the triple line [22,23]. For example, in 1992 Brochard-Wyart and de Gennes proposed a combined model which accounts for both the hydrodynamic, FHD, and the molecular forces, FMK, that contribute to the total dissipation, Dtot, in the system [22]: 2

Dtot ¼ ð F HD þ F MK ÞV ¼ ðμ þ ζ ÞV ;

ð9Þ

where μ = 3ηln(L / LS) / θ is the friction coefficient in the bulk according to the hydrodynamic model. Since FHD is proportional to the reciprocal value of the dynamic contact angle, one can infer that at small dynamic contact angles the viscous friction in the bulk is significant, while at large contact angles the dissipation due to molecular displacements in the triple line zone is predominant. This seems to be supported by studies on spontaneous and electrically induced dynamic wetting of ILs [24,25], spreading of poly(dimethylsiloxane)s on bare silicon wafers [26], as well as experiments on dewetting induced by bubbles colliding with wet substrates [11]. Molecular dynamic (MD) simulations can reveal both the mechanism of the wetting process and validate the models used to describe wetting [27,28]. Systems typically modelled in large-scale MD simulations are considerably smaller (up to around 106 atoms) compared to the real ones (normally, about 1023 atoms) [29,30]. The systems consist of liquid composed of chain-like molecules of n number of atoms, in order to ensure higher, more realistic viscosity values, and minimise the evaporation in the vacuum; and solid substrate with face-centred cubic crystal lattice. The intramolecular interactions are defined via a pair-wise Lennard-Jones potential [29,30]. Despite the simplifications, MD simulations provide insight into wetting dynamics both at the micro- and macro-levels [27]. Numerous MD simulation studies showed that the MK model compared very well with the computed dynamic wetting data [29,31,32]. In addition, parameters such as the characteristic frequency of molecular displacement, k0, and the average length of molecular displacement, λ, were accessible both directly from the simulations and from fitting the computed results. The agreement between the values derived by these two methods was found to be very good [31,32]. Due to the solid substrate present, layering of the liquid in close proximity to it was evident. As a consequence, molecular displacements in two directions, with two characteristic frequencies, respectively, were distinguished: parallel to the substrate, from the first liquid layer in contact with the solid phase, characterised with frequency k0,∥, and perpendicular to the substrate, from second to first liquid layer, with frequency k0,⊥ [27,29–32]. Anisotropy in the frequencies was manifested, as k0,∥ was found to be higher than k0,⊥. The displacements both in parallel and normal direction at the vicinity of the wetting line were times more obstructed compared to the ones in the bulk. The MD simulations disclosed a possible dissipative mechanism of dynamic wetting: the increase in the strength of solid–liquid interactions leads to lower frequencies, and respectively, to more hindered displacements in both

4. Macroscopic contact angles of ionic liquids in solid–liquid– air systems Despite the fact, that many potential applications of ILs strongly depend on wetting behaviour at the interface between ILs and solid surfaces, there have been very few systematic studies on wetting by ILs under well-controlled conditions (e.g. water and halide content minimised and precisely determined, temperature (T) and relative humidity (RH) well-controlled, and the contact angles measured as a function of those variables), nor with exact information concerning the time from droplet deposition onto the solid surface until the point at which the measured contact angle is constant. In the recent literature, there are several studies dedicated to the measurement of contact angles formed by various ionic liquids on solid substrates. Table 1 lists the contact angle values of different ILs on these (not always smooth, rigid and non-reactive) solids. Since the purity of each IL,2 as well as the experimental conditions, is different for all these studies it is impossible to make any quantitative comparison between them. Contact angles formed by a group of three ILs with a common anion, [NTf2], but different ([Bmpy], [Bmim] and [P6,6,6,14]) cations were determined by Page et al. [36]. The authors synthesised all ILs and they took extensive care of both halide and water removal (and determination) prior to contact angle measurements. In order to avoid any organic contaminants, the authors used a flamed platinum wire to place a droplet of IL onto a poly(tetrafluoroethylene) surface. The static contact angles were measured at 20 °C and RH = 58%, within 1 min from the IL drop deposition. Three subsequent droplets were placed on the top of the previous droplet. The contact angles for the three ILs are given in Table 1 (entry for Ref. [36]) and are practically the same (within experimental error) for [Bmpy][NTf2] and [Bmim][NTf2], while [P6,6,6,14][NTf2] is lower by about 10°. Gao and McCarthy [37] measured advancing and receding contact angles for four (structurally unrelated) ILs and seven solids, four of which were superhydrophobic (as determined by water contact angles) and thus rough. The contact angles of all ILs on rough perfluoroalkyl surfaces were very high (and comparable to those formed by water). The hysteresis in some cases was very low (≤ 2°). This is quite surprising as the interfacial tension of these ILs differs significantly from the interfacial tension of water, and one would expect much lower contact angles. In his recent review, Sedev [38] replotted the experimental contact angle results obtained by Restolho et al. [39] and Carrera et al. [40] in the form of Zisman plots (see Figs. 4 and 5 in Ref. [38], respectively). The cosθ as a function of γLV plot for [Omim] [BF4], [C2OHmim][BF4], [Emim][EtSO4], [Bmim][BF4] and [Empy] [EtSO4] did not follow the expected trend irrespective of whether the solid surface was polar (glass) or apolar (poly(tetrafluoroethylene) and polyethylene). In contrast, the Zisman plot for contact angles measured by Carrera et al. [40] on poly(tetrafluoroethylene), despite the scatter, follows the correct trend [38], whereas, contact angles 2 Water, as an impurity, has significant effect on the physical properties of ILs including interfacial tension (e.g. increasing amount of water in water-miscible ILs increases their interfacial tension [34,35]) and cohesion forces (water in IL promotes formation of hydrogen bonds which results in screening of the electrostatic interactions between ions constituting the IL, and decrease in the cohesion forces [35]). Hence, water has a significant effect on the contact angle and the wetting properties of ILs.

Please cite this article as: Delcheva I, et al, Static and dynamic wetting behaviour of ionic liquids, Adv Colloid Interface Sci (2014), http:// dx.doi.org/10.1016/j.cis.2014.07.003

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Table 1 Static contact angles (unless indicated θadv for advancing contact angles, and θrec for receding contact angles) for IL-solid combinations found in the recent literature. Ionic liquid

Substrate

θ [°]

Ref.

[C4mpy][NTf2] [Bmim][NTf2] [P6,6,6,14][NTf2] [DMetim][MetSO4]

Poly(tetrafluoroethylene); smooth surface.

73.3 ± 1.7 73.3 ± 1.7 63.5 ± 0.6 θadv = 175 θrec = 175 θadv N 175 θrec N 175 θadv = 177 θrec = 175 θadv = 175 θrec = 175 θadv = 170 θrec = 148 θadv N 174 θrec N 150 θadv = 172 θrec = 149 θadv = 174 θrec = 153 θadv = 175 θrec ~ 10 θadv N 126 θrec N 9 θadv = 122 θrec = 9 θadv = 174 θrec b 8 θadv = 170 θrec ~ 10 θadv N 126 θrec ~ 10 θadv = 130 θrec = 17 θadv = 118 θrec = 8 θadv = 101 θrec = 91 θadv = 97 θrec = 86 θadv = 95 θrec = 83 θadv = 100 θrec = 85 θadv = 95 θrec ~ 91 θadv N 91 θrec ~ 80 θadv = 83 θrec = 82 θadv = 95 θrec = 93 θadv = 65 θrec = 34 θadv = 61 θrec = 32 θadv = 59 θrec = 32 θadv = 69 θrec = 42 80 101 93 95 95 20 47 21 23 25 41 79 70 72 71

[36]a

[Emim][EtSO4]

Compressed sample of commercial lubricant [67]. The substrate surface is not smooth.

[Emim][BF4] [DDAM][MetSO3] [DMetim][MetSO4] [Emim][EtSO4] [Emim][BF4] [DDAM][MetSO3] [DMetim][MetSO4] [Emim][EtSO4] [Emim][BF4]

Patterned (staggered rhombus posts) silicon wafer chemically treated with heptadecafluoro(1,1,2,2tetrahydro) decyldimethylchlorosilane. The substrate surface is not smooth. Silicon wafer treated with MeSiCl3 in toluene in the presence of humidified air. The substrate surface is not smooth.

[DDAM][MetSO3] [DMetim][MetSO4] [Emim][EtSO4] [Emim][BF4] [DDAM][MetSO3] [DMetim][MetSO4] [Emim][EtSO4] [Emim][BF4]

Silicon wafer that was treated with an azeotropic mixture of Me3SiCl and SiCl4 in the gas phase at room temperature and 45% relative humidity. The substrate surface is not smooth. Silicon wafer with a layer of covalently attached heptadecafluoro(1,1,2,2tetrahydro) decyldimethylchlorosilane; smooth surface.

[DDAM][MetSO3] [DMetim][MetSO4] [Emim][EtSO4]

Silicon wafer with a covalently attached dimethylsiloxane oligolayer of ~2.5 nm thickness; smooth surface.

[Emim][BF4] [DDAM][MetSO3] [DMetim][MetSO4]

Polyester film; smooth surface.

[Emim][EtSO4] [Emim][BF4] [DDAM][MetSO3] [Omim][BF4] [C2OHmim][BF4] [Emim][EtSO4] [Bmim][BF4] [Empy][EtSO4] [Omim][BF4] [C2OHmim][BF4] [Emim][EtSO4] [Bmim][BF4] [Empy][EtSO4] [Omim][BF4] [C2OHmim][BF4] [Emim][EtSO4] [Bmim][BF4] [Empy][EtSO4]

10 μm thick film of poly(tetrafluoroethylene); smooth surface.

Optical BK7 glass; smooth surface.

1 mm thick film of ultra-high molecular weight polyethylene; smooth surface.

Table 1 (continued) Ionic liquid

Substrate

θ [°]

Ref.

[Emim][HSO4]

Si-supported model surface with surface functionality Si(CH3)2(CH2)2C6F13; smooth surface.

θadv = 103 θrec = 87 θadv = 99 θrec = 84 θadv = 88 θrec = 72 θadv = 86 θrec = 70 θadv = 83 θrec = 72 θadv = 79 θrec = 67 θadv = 76 θrec = 66 θadv = 66 θrec = 54 θadv = 96 θrec = 88 θadv = 88 θrec = 82 θadv = 83 θrec = 78 θadv = 75 θrec = 68 θadv = 77 θrec = 70 θadv = 75 θrec = 65 θadv = 65 θrec = 57 θadv = 51 θrec = 44 θadv = 86 θrec = 70 θadv = 87 θrec = 75 θadv = 70 θrec = 61 θadv = 76 θrec = 64 θadv = 68 θrec = 59 θadv = 67 θrec = 57 θadv = 54 θrec = 43 θadv = 42 θrec = 30 θadv = 81 θrec = 67 θadv = 75 θrec = 59 θadv = 66 θrec = 54 θadv = 63 θrec = 51 θadv = 63 θrec = 51 θadv = 59 θrec = 47 θadv = 54 θrec = 40 θadv = 42 θrec = 30 θadv = 63 θrec = 40 θadv = 59 θrec = 21 θadv = 35 θrec = 15 θadv = 26 θrec = 14 θadv = 26 θrec = 14 θadv = 53 θrec = 22

[68]d

[Dmim][MetSO4] [Emim][MetSO4]

[37]b [Bmim][PF6] [Emim][EtSO4] [Bmim][BF4] [Hmim][PF6] [Omim][PF6] [Emim][HSO4] [Dmim][MetSO4]

Si-supported model surface with surface functionality [OSi(CH3)2]n; smooth surface.

[Emim][MetSO4] [Bmim][PF6] [Emim][EtSO4] [Bmim][BF4] [Hmim][PF6] [Omim][PF6] [Emim][HSO4] [Dmim][MetSO4]

Si-supported model surface with surface functionality Si(CH3)2C18H37; smooth surface.

[Emim][MetSO4] [Bmim][PF6] [Emim][EtSO4] [Bmim][BF4] [Hmim][PF6] [Omim][PF6] [Emim][HSO4] [Dmim][MetSO4]

Si-supported model surface with surface functionality Si(CH3)3; smooth surface.

[Emim][MetSO4] [Bmim][PF6] [Emim][EtSO4] [Bmim][BF4] [Hmim][PF6] [39]c [Omim][PF6] [Emim][HSO4] [Dmim][MetSO4] [Bmim][PF6] [Emim][EtSO4] [Hmim][PF6] [Emim][HSO4]

Si-supported model surface with surface functionality Si(CH2)4CHO; smooth surface.

Please cite this article as: Delcheva I, et al, Static and dynamic wetting behaviour of ionic liquids, Adv Colloid Interface Sci (2014), http:// dx.doi.org/10.1016/j.cis.2014.07.003

I. Delcheva et al. / Advances in Colloid and Interface Science xxx (2014) xxx–xxx Table 1 (continued) Ionic liquid

Substrate

θ [°]

[Dmim][MetSO4]

Si-supported model surface with surface functionality Si(CH2)3NH2; smooth surface.

θadv = 51 θrec = 20 θadv = 39 θrec = 14 θadv = 31 θrec = 15 θadv = 15 θrec b 10 θadv = 40 θrec = 15 θadv = 37 θrec = 13 θadv = 24 θrec = 11 θadv = 22 θrec b 10 b10 θadv = 66 θrec b 58 θadv = 64 θrec = 60 θadv = 69 θrec = 64 θadv = 17

[Bmim][PF6] [Emim][EtSO4] [Hmim][PF6] [Emim][HSO4] [Dmim][MetSO4]

Si-supported model surface with surface functionality Si(CH2)3N(CH3)+ 3 ; smooth surface.

[Bmim][PF6] [Emim][EtSO4] [Hmim][PF6] [Bmim][OTf]

[Bmim][BF4] [Bmim][NTf2] [Bmim][DCA] [Omim][Cl] [Omim][NTf2] [C3Omim][BF4] [C4Dmim][NTf2] [Aliquat][Cl] [Aliquat][NTf2] [Aliquat][DCA] [P6,6,6,14][Cl] [P6,6,6,14][DCA] [P6,6,6,14][p-TsO] [(di-h)2dmg][DCA] [(di-h)2dmg][Cl] [Emim][EtSO4] [Bmim][BF4] [Omim][BF4] [Omim][NTf2] [C3Omim][BF4] [C2Omim][DCA] [(di-h)2dmg][Cl] [Hmim][Cl] [Omim][Cl] [Dmim][Cl] [Emim][NTf2] [Bmim][NTf2] [Hmim][NTf2] [Bmim][I]

Gold with a monolayer of C8SHe; smooth surface. Gold with a monolayer of C12SH; smooth surface. Gold with a monolayer of C18SH; smooth surface. Gold with a monolayer of 11mercapto-1-undecanol; smooth surface Silica with a monolayer of C8SiCl3; smooth surface. Silica with a monolayer of C12SiCl3; smooth surface. Silica with a monolayer of C18SiCl3; smooth surface. Silica with a monolayer of F3C(CF2)5(CH2)2SiCl3; smooth surface. Silica with a monolayer of H2C = C5H9SiCl3; smooth surface. Silica with a monolayer of (HB-O) H2C = C5H9SiCl3; smooth surface. Poly(tetrafluoroethylene); smooth surface; smooth surface.

Glass; smooth surface.

Microscope cover glass slide; smooth surface. Mica; atomically smooth surface.

Silicon wafer; smooth surface

Ref.

[41]f

θadv = 67 θrec = 56 θadv = 72 θrec = 64 θadv = 69 θrec = 65 θadv = 91 θrec = 57 θadv = 50 θrec = 33 θadv = 19 80.51 ± 0.79g 66.25 ± 0.09 81.71 ± 0.26 47.11 ± 0.26 49.58 ± 0.21 88.19 ± 0.21 70.62 ± 0.34 56.53 ± 0.42 46.20 ± 0.48 63.28 ± 0.39 64.18 ± 0.49 68.95 ± 0.20 39.77 ± 0.14 58.96 ± 0.06 39.66 ± 0.71 29.38 ± 0.65 38.27 ± 0.22 13.08 ± 0.14 22.84 ± 0.23 42.38 ± 0.27 26.45 ± 0.29 18.84 ± 0.10 14.592 ± 0.539i 10.815 ± 0.301 105.951 ± 1.295 42 ± 2 31 ± 2 34 ± 1 θadv = 40 ± 4 θrec = 24 ± 2

[40]h

[59]j

[42]k

[53]l

measured by the same authors on a glass surface are scattered. The different behaviour most probably reflects the characteristics of the solid surface. The surface of poly(tetrafluoroethylene) is well-

5

defined and inert, while the surface of glass is reactive and, due to its higher intrinsic specific surface energy, when exposed to air is known to adsorb various components from the surrounding atmosphere (either organic contaminants, or water vapour). The layer of adsorbate may not only affect the surface energy but also reduce its surface charge. Hence, unless the glass surface is carefully pre-treated with an organic monolayer, or the experiments are conducted under inert and dry gas atmosphere, it is difficult to get reliable and representative data. Cione et al. [41] studied the wettability of [Bmim][OTf] on model surfaces prepared by the assembly of thiol- and silane-based monolayers. Both advancing and receding contact angles of [Bmim][OTf] were found to be significantly larger than those of dicyclohexyl, a model molecular liquid chosen by the authors due to its very similar (± 1 mN/m) interfacial tension. The differences were more pronounced on the methyl-terminated silane monolayers. Furthermore, the contact angles by [Bmim][OTf] did not exhibit any trend with either thiol or silane chain length. The authors speculated that this could be due to strong electrostatic interactions between ions within the IL drop and an increase in the cohesion forces. There have been two independent studies of contact angles of [Emim][NTf2], [Bmim][NTf2] and [Hmim][NTf2] on mica surfaces. The first study by Beattie et al. [42] reported contact angles of 42 ± 2°,

Notes to Table 1: a T = 20 °C, relative humidity (RH) = 58%. ILs were synthesised by the authors. Prior to contact angle measurements, ILs were handled inside a dry glovebox of RH ≤ 5%. b Room temperature, RH = 45%. No effort was made to purify ILs. ILs were handled in air. No experimental error in contact angle measurements was reported. c Room temperature, low RH — the experimental chamber was either dried with silica gel, or in case of [C2OHmim][BF4], purged with dry nitrogen. ILs of purity ≥ 98%. ILs were dried at 80 °C for three days (or more), the water content was assessed by Karl Fischer titration and was ~690 ppm for [Empy][EtSO4], ~287 ppm for [Emim][EtSO4], ~384 ppm for [Bmim][BF4], ~344 ppm for [Omim][BF4], and ~800 ppm for [C2OHmim][BF4]. Prior contact angle measurements ILs were handled inside a dry glovebox under dry nitrogen atmosphere. d T and RH were not known for contact angle measurements. ILs of purity 95–98%. In order to determine water and volatile compound content in ILs, the authors carried out thermal gravimetric analysis (TGA). Since the weight loss below 200 °C was 0.3–0.5%, the authors concluded that ‘ILs were essentially water free’. e Monolayers of n-alkanethiols — CH3(CH2)n − 1SH, where n = 8, 12 or 18 (first three entries for Ref. [41]), monolayers of silanes — CnSiCl3, where n = 8, 12 or 18 (entries 5–7 for Ref. [41]). f IL was synthesised by the authors. T and RH were not known for contact angle measurements. g The authors of Ref. [40] claim that the precision of contact angle measurements is ‘of the order of 0.01°’; however we have some concerns whether that is possible. It is very difficult to get such (or even 0.1°) precision as: (i) Poly(tetrafluoroethylene) surface is not atomically smooth (despite enormous efforts including precise T and RH controlled cell, active anti-vibration isolation, constant light intensity during the measurement and working with atomically smooth mica, we have never reached precision better than 0.5°), (ii) contact angle measurements are sensitive to the light adjustment, from our experience slight variations in light intensity can cause contact angle difference of 2–5° (depending whether Young–Laplace or tangent fittings are used) and (iii) static contact angle of ILs on any surface will depend on T and RH and may evolve with time, none of these parameters are mentioned by the authors of Ref. [40]. h T and RH were not known for contact angle measurements. Purity of [Emim][EtSO4], [Bmim][BF4], [Bmim][NTf2], [Bmim][DCA], [Omim][Cl], [Omim][BF4], [Omim][NTf2], [Omim][DCA], [C10mim][BF4], [C10mim][DCA], [C3Omim][BF4], [C4Dmim][NTf2], [C2OHmim][DCA] and [Aliquat][Cl] is not known. Purity of [P6,6,6,14][Cl] and [P6,6,6,14][DCA] was 98%. [Aliquat][NTf2], [Aliquat][DCA], [(di-h)2dmg][DCA] and [(di-h)2dmg][Cl] were synthesised by the authors. [P6,6,6,14][p-TsO] was prepared by the authors from [P6,6,6,14] [Cl] by anion exchange. All ILs were kept under vacuum at 80 °C until the water and halogen content was below 150 and 50 ppm, respectively. Static contact angle is an average of five independent measurements. i We think such precision in contact angle measurements is not possible (see comment ‘g’ above); decimal points are insignificant. j Purity of ILs was ≥98%. The ILs were degassed and dried at 70 °C under vacuum prior the contact angle measurements. T, RH and water content are unknown. k T = 22 ± 1 °C, RH = 39 ± 2%. Purity of ILs was ≥99%. Static contact angles were measured after 2 h. l T and RH were not known for contact angle measurements. Purity of IL was not known.

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31 ± 2°, and 34 ± 1°, respectively. The authors used ILs of 99 wt.% purity and water content below 200 ppm. The contact angle measurements were carried out at a temperature of 22 ± 1 °C and relative humidity of 39 ± 2%. The authors claim that after an initial (ms scale) fast spreading the drop comes to rest and its macroscopic shape becomes quasi-time-independent. The second study, by Wang and Priest [43], reports significant relaxation of contact angles formed by the same ILs on a mica surface. In the case of [Emim][NTf2] the contact angle relaxation (from 40° down to 23°) occurred over 30 min. The relaxation time increased with the cation alkyl chain: for [Bmim][NTf2] the relaxation occurred over 90 min (during this time the contact angle decreased from 40° to 23°), and for the longest chain [Hmim] [NTf2], the relaxation from 44° to almost complete wetting took ~24 h. Both studies were carried out under the same temperature and relative humidity conditions, but used ILs and mica from different suppliers. ILs used in the second study had a slightly higher water content (510, 370, and 220 ppm for [Emim][NTf2], [Bmim][NTf2] and [Hmim] [NTf2], respectively). In addition, the ILs used by Wang and Priest were stored under controlled conditions (sealed containers with desiccant in a clean room environment) for an extended time prior to the measurements [43], while the ones used by Beattie et al. [42] were freshly opened several days before the experiments. Based on these parallel studies, it appears that minor differences (manufacturer, water content, and storage) can have a major impact on wetting behaviour in what would appear to be identical systems. It is clear from the above summary that there is significant room in the literature for further careful investigation of the static contact angle of ILs on substrates of varying chemistry, polarity, roughness, and heterogeneity. These studies should be performed with structurally related ionic liquids and with extreme care regarding the experimental conditions, to ensure accurate and meaningful results. 5. Small droplets and line tension With decreasing droplet size, the contribution from line tension free energy, i.e. the excess free energy arising from the different spatial arrangements of the molecules at the three-phase contact line, becomes increasingly important [44]. Once the droplet spatial size reaches the sub-micron and nanometre regime, line tension effects can no longer be neglected [45,46], and the size dependence of the wetting behaviour for such submicro-/nano-scopic droplets is described by a modified Young's equation3: cosθ ¼

γSV −γ SL τ τ − ¼ cosθeq − : γLV Rd γ LV Rd γLV

ð10Þ

From Eq. (10) it is clear that when the line tension, τ, is positive, a liquid drop will exhibit a larger contact angle than Young's contact angle, θeq. When the solid surface is not ideal (rough, chemically heterogeneous, reactive, etc.), the base of the droplet may deviate from a circular shape. In such a case Rd should be attributed to the local radius of the curvature of the three-phase contact line, rather than the base droplet radius [47]. The line tension originates from the molecular interactions at the three-phase contact line. Since the inclination of the phases affects the extent of the molecular interactions, the line tension is related to the contact angle. This leads to the conclusion that the assumption of a constant line tension is inappropriate and Eq. (10) should only be used as an approximation [48]. It was postulated that interfacial forces (van der Waals, electrostatic, etc.) acting in the three-phase contact region distort the shape of the interface [49–51]. Solomentsev and White calculated the line tension as the 3

Note that Eq. (10) only holds for spherical cap-shaped droplets.

excess free energy per unit length of the contact line relative to the free energy of droplet-substrate system in the absence of the interactions between the substrate and free liquid interface [52]. The authors calculated the interaction energy, invoking the Derjaguin approximation, for a microscopic droplet profile. They derived an explicit expression for the line tension as a function of the contact angle as a quadrature of the interaction energy over the interaction energy per unit area between planar half-spaces [52]. They also showed that for attractive interactions the line tension is negative, while it can be positive when the interfacial interaction has two components: short-range attractive and long-range repulsive [52]. There is only one study dedicated to the line tension of ILs. Heim and Bonaccurso [53] investigated the dependence of [Bmim][I] droplet size on measured contact angles on clean silicon wafers. The small droplets were generated by evaporation of IL at a temperature close to the boiling point. The authors used tapping mode Atomic Force Microscopy (AFM) to image [Bmim][I] droplets in the nano- and submicronrange4 and then, by extracting droplet cross-sections, measured static contact angles (see Fig. 1a for 3D AFM image of [Bmim][I] droplets) for quite a wide size range (Rd from ~ 20 nm up to ~ 620 nm, i.e. the size range was varied by a factor of 30). They found out that the IL contact angle decreased with the droplet size (e.g. smaller droplets showed enhanced wetting of the silicon surface). The value determined for the line tension was negative and of the order of 10−11 J/m.5 The dependence of the contact angle as a function of droplet radius cannot be described with the concept of line tension and Eq. (10) for the entire droplet size range — see Fig. 1b. This is not an uncommon observation for small droplets of molecular liquids. Non-linear dependence of the contact angle as a function of droplet radius was reported for series of n-alkanes (where 9 ≤ n ≤ 12) by Checco et al. [54] for an equally wide factor of 30 droplet size range. Heim and Bonaccurso [53] noted that the line tension must change in a more complex manner than the one postulated by the modified Young's equation, as originally proposed by others [49–51]. The conclusions of this work rely on the assumption that sample preparation (heating the IL to 250 °C to form the nanodroplets) did not alter the properties of the liquid, i.e. the interfacial tension and macroscopic contact angle, which the authors did not determine. In addition, [Bmim][I] similarly to [Bmim][Br] and [Bmim][Cl] is hygroscopic. The water uptake (from the ambient laboratory atmosphere) will be much larger for smaller droplets (due to the higher surfaceto-volume ratio), affecting IL viscosity and probably, interfacial tension. Note that an increasing amount of water in water-miscible ILs increases the interfacial tension [34], which could result in the same trend (e.g. smaller droplets exhibiting enhanced wetting characteristics).

6. Precursor films The first observation of a thin (in nm range) liquid film surrounding droplets of IL deposited on atomically smooth mica was reported for [Bmim][PF6] — methanol [55] and [Bmim][NTf2] — ethanol [56] solutions. The character and origin of these thin films, i.e. whether they were precursor IL films emanating from the submicron-sized IL droplets, or receding IL films being formed upon evaporation of the solvent and affected by pinning, could not be unambiguously determined. Due to their negligible vapour pressure [57], ILs are perfect candidates for precursor film studies. Any role of evaporation–condensation in the formation of precursor films can be ruled out, and such IL precursor 4 The authors took all the precautions (such as checking the effect of imaging parameters on the height and spatial size of the imaged soft droplets) and estimated the error of the determined line tension in the range of 25%. 5 The authors used only part of their data. Because the influence of line tension is stronger for smaller droplets Heim and Bonaccurso used only [BMIM][I] droplets with radii smaller than 100 nm for the fitting procedure.

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while for [Hmim][NTf2] 2 × 10−11 m2/s b D1 b 6 × 10−10 m2/s. These values are of the same order of magnitude as those reported in the literature for poly(dimethylsiloxane) precursor films on various substrates [58]. The authors also compare the precursor film formed at a similar distance from the edge of one drop or two drops of [Hmim] [NTf2] (see Fig. 2a and b), as well as compare these to the film that is left behind a retracting [Hmim][NTf2] drop (see Fig. 2c). Wang and Priest [43] reported the occurrence of precursor films formed by the same ILs on mica surfaces modified by a sub-monolayer of octadecylphosphonic acid (OPA). The authors took advantage of the fact that OPA, at its partial surface coverage, forms island of welldefined height (2 nm) [43]. They use this height as a reference point to determine the thickness of the precursor film. The authors, using tapping mode AFM, detected that a thin film of IL developed between OPA islands within 90 min. The thickness of this film varied with the cation hydrocarbon chain and was: 0.53, 0.65 and 1.00 nm for [Emim][NTf2], [Bmim][NTf2] and [Hmim][NTf2], respectively. The authors also performed Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) experiments on the mica surface away (up to 2 mm) from the edge of the IL droplet and a signal, attributable to the IL cation, which decreased in intensity with increasing distance from the droplet edge, was detected.

7. Dynamic wetting experiments

Fig. 1. (a) 3D 25 μm × 25 μm AFM height image of [Bmim][I] droplets evaporated onto silicon wafer, z-scale 1.50 μm; (b) cosine of the measured contact angle, θ, as a function of inverse of droplet base radius, Rd. Triangular symbols represent measured contact angles for macroscopic drops. Reprinted from Ref. [53]. Copyright 2014, with permission from American Chemical Society.

films must be formed by a surface diffusion mechanism only. The first direct study of precursor films spreading ahead of the macroscopic droplets of imidazolium-based ILs on a pristine mica surface was carried out by Beattie et al. [42] They used tapping mode AFM to investigate the spreading of molecularly thin (up to a few nanometres) precursor films emerging from drops of [Emim][NTf2], [Bmim][NTf2] and [Hmim][NTf2] on smooth mica surfaces. All three ILs form finite contact angles on mica (see Table 1, entry for Ref. [42]), and three of them form a thin (a few nanometres) precursor film. Further support for the IL precursor film formation was provided through complementary X-ray photoelectron spectroscopy (XPS) elemental surface analysis (see the Supporting information of Ref. [42]) which was performed for [Hmim][NTf2]. The XPS results show the presence of sulfur (which is solely due to the [NTf2] ion) on the mica surface a number of millimetres away from the macroscopic IL drop. The XPS detected, at the same ‘off drop’ locations, the chemical signature of the mica surface, supporting the authors' AFM findings, i.e. that the thickness of the IL precursor film must be in the range of a few nanometres. The lateral extent of the film increases with time and reaches values as large as a few millimetres within 12 h. Based on the AFM images taken at the same distance from the macroscopic drop as a function of time the authors set limits on the effective rate of spreading coefficient, D1 [42]. For both [Emim][NTf2] and [Bmim][NTf2] they obtained 6 × 10−10 m2/s b D1 b 3.4 × 10−9 m2/s,

The use of ionic liquids in dynamic wetting has many actual and potential applications, and significant scope for studying the fundamental models of spreading of liquids on surfaces. The ability to tailor the molecular size of the ionic liquid anion and cation provides a systematic means to explore aspects of the molecular kinetic model, such as the characteristic length scale of the spacing between adsorption sites, or the volume of displacement at the three-phase contact line. Equally, the range of viscosities that exists within a family of ionic liquids (such as the imidazolium ILs with different hydrocarbon chain lengths attached to the cation) permits the hydrodynamic model of wetting and dewetting to be tested. Poleski et al. [59] studied the capillary rise of imidazolium ionic liquids6 within columns of glass beads, 7 which were either hydrophilic, or hydrophobic. It was seen that the length of the imidazolium cation hydrocarbon chain influenced the wetting rate of the ILs on a hydrophobic substrate, with [Omim][Cl] wetting faster than [Hmim] [Cl]. The authors claim that the measurements with the hydrophilic beads were hampered by the presence of water adsorbed at the surface of the beads, thus affecting the viscosity of the ILs during measurement. Thus, care should be taken in assessing the reliability of this study. Stalcup and co-authors [60] used two ionic liquids of very different size and viscosity ([Emim][EtSO4] and ECOENG™ 500) to study modes of energy dissipation in moving contact lines on silicon wafer substrates. The liquids were seen to display mixed behaviour in terms of their dissipation mechanisms: molecular at initial spreading time and hydrodynamic at longer times. The combined model used for the work (formulated by de Ruitjer et al. [23]) gave reasonable fits to contact angle/radius versus time plots. The larger (and more viscous) of the ILs displayed much slower wetting kinetics, reflected in the smaller frequency term in the MK model (k 0). The characteristic length scale, λ, was seen to vary only slightly with the size of the ILs.

6

ILs of a purity ≥98%. ILs were dried at 70 °C under vacuum prior using. Glass beads were cleaned and degreased, with methanol:acetone:chloroform (1:1:1 by volume) and dried at 130 °C. 7

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Fig. 2. (a) Optical image of a [Hmim][NTf2] drop on a mica surface (top), AFM topography image of the [Hmim][NTf2] precursor film at the point ‘a’ 12 h after the drop was deposited (middle), and cross sections (bottom). (b) Similar with (a), but for the case that two drops have been deposited near each other. The point ‘b’ is located between the two drops and at distances from the edges of the drops that are similar to that of the point ‘a’ in panel (a). (c) Topography and cross section at ‘c’ of the IL film formed on the mica surface via the retraction of a drop, as shown in the top images. Reprinted from Ref [42]. Copyright 2014, with permission from American Chemical Society.

Li and co-authors [18] focused on a systematic study of the wetting behaviour of two series of imidazolium-based ILs8 ([BF4] and [NTf2]) on Teflon AF1600 substrates. The authors reported that there is a direct relationship between the contact line friction coefficient, ζ, and the work of adhesion, WA, for all of the studied ILs on Teflon AF1600 solid substrates: ζ increases with WA, which agrees with the theoretical predictions (Eqs. (7) and (8)) of the MK model. It was found that the equilibrium frequency of the molecular jumps, k0, was affected by the physicochemical properties of the liquid: k0 decreases with both increasing viscosity of the IL and WA. In addition, the characteristic jump length scale, λ, was seen to be essentially independent of WA. Fig. 3 shows ln(ζ / η) as a function of the work of adhesion for all studied imidazolium-based ILs on the Teflon AF1600 surface. The authors determined vm from the gradient of the plot and Eq. (8) in Ref. [18] obtaining value 0.19 ± 0.05 nm3, which is of the same magnitude as the molecular volume of ion pairs. This indicates that the contact line moves through the individual displacement of ion pairs rather than clusters of molecules as found for some molecular liquids [19,20]. During the initial stages of spontaneous spreading at high velocity

8 ILs of a purity b98%, further purified (dissolution in either MilliQ water (water-miscible ionic liquids) or 75% isopropanol aqueous solution (water-immiscible ionic liquids)), followed by an adsorption of organic residuals at high-purity charcoal (Sigma-Aldrich), filtration through 0.2 μm Teflon filters (Whatman), and extraction with acetyl acetate or pure water to remove inorganic impurities. The final step was a removal of water and volatile organic compounds by evaporation under moderate vacuum. The water content determined after purification was below 1000 ppm, and the chloride content was lower than 20 ppm.

and large contact angles, molecular dissipation prevails, whereas, at lower speeds and rather small contact angles viscous dissipation is more important.

Fig. 3. ln(ζ / η) as a function of WA for Teflon AF1600 surface. Reprinted from Ref. [18]. Copyright 2014, with permission from Royal Society of Chemistry.

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In work specifically related to dynamic wetting and modes of contact line energy dissipation, Li et al. [17] studied the forced wetting (electrowetting) and natural dewetting (relaxation following removal of DC field) for ionic liquids on a fluoropolymer. The ILs studied included comparisons of common cations with varying anion (e.g. [Bmim][BF4] and [Bmim][NTf2]), and common anion and varying cation (e.g. [Omim][BF4] and [Bmim][BF4]). The electrowetting and retraction rate may be described by the MK and hydrodynamic models, with energy being dissipated in the bulk and at the contact line. The former model is more important at small contact angles. In related studies [24,61,62], knowledge about the chemical stability and the degree of cleanliness of the solid substrate, as well as the purity of the IL were shown to play a very important role in electrowetting leading to results that are in profound disagreement. Certain ILs (for example, [Bmim][BF4]) were found to electrowet substrates coated with fluoropolymer both reversibly and irreversibly by different research groups [63,62]. One area of application focus for ionic liquids and dynamic wetting is in the use of ionic liquids as electrolytes in energy storage devices, such as Li-ion batteries and supercapacitors [25]. The importance of wetting of electrode materials is apparent when one considers the fabrication of these devices, and the need to have good imbibition within a porous material. In addition, it is probable that the actual functioning of the devices will rely to some extent on how wettable the electrode material is by the ionic liquid, for this will influence the efficiency of charge transfer at the solid–liquid interface. Kuhnel et al. [64] have published one of the first dynamic wetting studies of ionic liquids on Li-ion battery electrodes, in addition to measurements of mixtures of ILs with more common Li-ion battery electrolytes. The authors used two techniques: tensiometry and impedance spectroscopy, which use gravimetric and electrochemical means, respectively, to probe the penetration of a liquid into a porous medium. As is quite common with studies of IL electrolytes for power applications, the desirable quality of the electrolyte is impeded by the presence of the ionic liquid — in this case, penetration speed. A significant reduction in the wetting time (wetting time of a porous electrode was up to 20 times longer than with electrolytes comprising organic solvents) was attributed to the interplay between higher viscosity and lower interfacial tension. Another study in which IL-solvent mixtures have been investigated as electrolytes was published by Szparaga et al. [65]. Theoretical modelling of IL-solvent mixtures in contact with an electrode surface was performed, indicating wetting transitions for thin films of IL against the electrode, which resulted in advantageous capacitance characteristics. This study indicates that, in the application of IL-solvent mixtures in energy storage systems, it may be possible to combine good bulk electrolyte characteristics (high mobility, low viscosity) with additional

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optimum thin film characteristics that enhance the core function of the device. This theoretical study was limited in scope due to the need to consider a smooth homogeneous electrode surface. Experimental studies on real electrodes – which are often porous – would be required to determine whether such phase separation and subsequent wetting transitions could actually be observed in real systems. 8. Conclusions and future directions The scientific literature on the physicochemical behaviour of ILs has undergone a period of significant growth over the last decade. The static wetting and dynamic behaviour of ILs is still in the early stages of growth, with a significant number of unanswered questions and gaps in the literature. This is surprising, given the importance of wetting phenomena in industrial processes, and the potential for ionic liquids to be used in systematic, fundamental studies of static and dynamic wetting. Aspects such as water and halide content, combined with issues of surface preparation and variability, have resulted in some conflicts in the literature and uncertainty as to the exact role of IL anion and cation chemistry in wetting phenomena. Where the appropriate care and attention has been paid to IL and substrate characterisation and preparation, the key studies have been highlighted in this article. With more widespread adoption of the required procedures to ensure accurate and reproducible experimental outcomes, future studies into theories of wetting with ILs, line tension effects with sub-micron droplets, and precursor film growth mechanisms will have a greater chance to fully reveal the influence of IL chemistry on wetting phenomena. In parallel, static structural studies at solid–liquid and liquid–vapour interfaces will continue to play an informative role. The first glimpses of this are seen in the dynamic wetting studies highlighted in this article. However, even here, there is room for increased scale of investigation, with a need for both systematic and wide-ranging variation in key bulk liquid and molecular parameters (e.g. more studies with series of ILs with greater degree of variation in ion pair size and viscosity). Of course direct, spectroscopic interrogation of the moving contact line would be most useful. Recent micro-scale interfacial tension measurements by passive resonance of capillary waves naturally excited by thermal fluctuations [66] show promise as well, as it is one of very few non-invasive techniques for microfluidic interfacial tensiometry. Acknowledgements The authors acknowledge the financial support from the Swiss National Science Foundation Sinergia scheme, grant no. 136191: ‘Designing Interactions across Interfaces in Ionic Liquids’.

Appendix A

Cations

Anions

Abbreviation

Name

Abbreviation

Name

[Aliquat] [Bmim] [Bmpy] [C2OHmim] [C3Omim] [C4Dmim] [(di-h)2dmg] [DMetim] [Dmim] [Emim] [Empy] [Hmim] [Omim] [P6,6,6,14]

Methyl-trioctylammonium 1-Butyl-3-methylimidazolium 1-Butyl-3-methylpyridinium 1-Ethanol-3-methylimidazolium 1-(2-Methoxyethyl)-3-methyl-imidazolium 1-Butyl-2,3-dimethylimidazolium Tetra-n-hexyl-dimethylguanidinium 1,3-Dimethylimidazolium 1-Dectyl-3-methylimidazolium 1-Ethyl-3-methylimidazolium 1-Ethyl-3-methylpyridinium 1-Hexyl-3-methylimidazolium 1-Octyl-3-methylimidazolium Trihexyltetradecylphosphonium

[BF4] [Br] [Cl] [DCA] [EtSO4] [HSO4] [I] [MetSO3] [MetSO4] [NTf2] [OTf] [PF6] [p-TsO]

Tetrafluoroborate Bromide Chloride Dicyanamide Ethyl sulfate Hydrogen sulfate Iodide Methyl sulfonate Methyl sulfate Bis(trifluoromethylsulfonyl)imide Trifluoromethanesulfonate Hexafluorophosphate p-Toluenesulfonate

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Please cite this article as: Delcheva I, et al, Static and dynamic wetting behaviour of ionic liquids, Adv Colloid Interface Sci (2014), http:// dx.doi.org/10.1016/j.cis.2014.07.003