2 - Surface tension contact angles of ionic liquids - Leandro

(1)

Surface tension, interfacial tension and contact angles of ionic liquids

Rossen Sedev

⁎

Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 2 November 2010

Received in revised form 28 January 2011 Accepted 31 January 2011

Available online 5 February 2011 Keywords:

Ionic liquid Surface tension Interfacial tension Contact angle

Ionic liquids combine the properties of molten salts (they are liquids composed predominantly of ions) and organic liquids (a variety of chemical bonds and interactions are relevant). Their unique properties have attracted significant attention over the last decade. Their interfacial properties are now coming under scrutiny because (i) they are important for their performance in specific applications and also because (ii) they provide an opportunity to study diverse chemical systems under vacuum. Recent examples from the literature are used to illustrate what is currently known. The surface tension, interfacial tension and wettability of ionic liquids resemble these of polar molecular organic liquids. It is physically clear there is a relation between surface tension and molecular structure but only general trends have been identified so far. Structural studies of the free surface of ionic liquids (e.g. with surface spectroscopy) are possible and reveal unique information about the interfacial molecular orientation. The amount of empirical data available is growing rapidly and elements of systematisation are beginning to appear.

1. Introduction

Ionic liquids are organic salts with low melting points[1](often below 100 °C). They were discovered long ago but the synthesis of air and moisture stable ionic liquids gave them a new life. Touted as green solvents they become a fashionable topic and the interest in their properties and applications exploded[2–5]. In recent years, the excitement has gradually morphed into a search for niche applications where ionic liquids could be useful on an industrial scale [1,6]. Naturally, as key properties such as density, viscosity, conductivity, and vapour pressure were being discussed, surface tension was somewhat neglected (interfacial tension and contact angle— even more so). As the researchﬁeld matures, more systematic work is being done on the surface properties of ionic liquids.

From a physical point of view, ionic liquids are complex (and more difﬁcult to understand) because they combine properties from two vastly different types of materials: molten salts (e.g. NaCl above 801 °C) and organic liquids (e.g. molecular solvents like benzene or cyclohexane). The peculiar properties of ionic liquids include: extremely low vapour pressure, high concentration of ions (hence electrical conductivity), good thermal stability and wide electro-chemical window. Their electro-chemical diversity offers unique opportuni-ties to develop solvents with tailored properopportuni-ties. On the negative side: ionic liquids are rather viscous (typically 50–1000 times more viscous than water), impurities such as halides or water may affect their performance, andﬁnally, though essentially non-evaporating

liquids (a major advantage over common solvents), their toxicity may not be negligible[7].

Surface tension is the main property of any liquid–gas interface [8–11]. It is a crucial parameter when dealing with relatively large surface areas and/or curved interfaces, when capillary pressure must be taken into account. In the most common macroscopic model of a liquid-vapour system at equilibrium (the Gibbs model), the total free energy of the system is the sum of the free energies of the two bulk phases and an excess free energy, Fαβ. This correction makes the total

free energy of the model system identical to that of the real system. The surface tension (or surface free energy) is (A being the area of the interface)[8]:

γ =Fα β

A : ð1Þ

There is an equivalent mechanical interpretation, in which the thickness of the transition zone between the two phases is not zero (as in the Gibbs model). Because of the different forces acting in the two phases, the pressure changes inside the transition zone. The surface tension is given by[8]

γ = ∫

∞

−∞ ðPN−PTÞdz: ð2Þ

Where PN is the normal component of the pressure tensor

(identical in both phases at equilibrium) and PT is the tangential

component (which varies throughout the interfacial zone), and z is the coordinate normal to the interface. Thus, the dimensionality of surface tension is energy/area or, equivalently, force/length. For

Current Opinion in Colloid & Interface Science 16 (2011) 310–316

⁎ Tel.: +61 8 8302 3225.

E-mail address:rossen.sedev@unisa.edu.au.

Contents lists available atScienceDirect

Current Opinion in Colloid & Interface Science

(2)

liquids, stretching the interface does not affect the surface tension and it can be measured experimentally[8].

From a microscopic point of view, the molecules near the surface of the liquid experience more attraction to the bulk than to the gas and, therefore, possess a higher energy if residing there[11]. A naïve yet straightforward estimation relates surface tension to the energy of interaction between two molecules, E, the coordination number, N, and the molecular volume, VM:

γ ≅ NE

V_M2=3: ð3Þ

More sophisticated approaches, such as the nearest neighbour model[12] or the scaled particle theory [13], develop further this formula but the dependence on VMremains the same.

It follows from the thermodynamic deﬁnition of free energy (F=U− TS) that γ = uαβ− Tsαβ. Therefore the surface entropy, sαβ, is given

by[8]

sαβ=−∂γ_∂T: ð4Þ

Surface tension decreases linearly or almost linearly with tem-perature. Often the dependence is described by the empirical Eötvös equation[8] (TC being the critical temperature and k an empirical

constant): γ =k TðC−TÞ

VM2=3

: ð5Þ

Interfacial tension is treated in exactly the same way, except that the second phase is an immiscible liquid. Because the interactions of any molecule at the interface with a condensed phase are stronger (than with a gas phase), interfacial tension has lower values when compared with surface tension.

When a liquid is in contact with a solid, both embedded in a third ﬂuid phase, the parameter characterising the amount of adhesion between the liquid and solid is the contact angle,θ. The contact angle is determined by the intermolecular interaction between any two of the three phases as realised by Young[8–10]:

cos θ = γSV−γSL

γ : ð6Þ

Out of the three interfacial tensions only γ can be measured directly (stretching solid surfaces also affects their state) and often empirical relations are invoked in order to close Eq.(6). The most celebrated one was suggested by Zisman[14]and describes a linear decrease of cosθ with γ. By using several liquids, the critical surface tension of wetting, γC, is obtained. It is (almost) independent of

the liquids used and is a characteristic of the solid surface. More elaborated models attempting to account for various interfacial interactions have been developed[10,15].

There is a very strong relation between molecular structure and surface tension. The parachor approach[11]estimates surface tension from the molecular composition of the liquid (based on either individual atoms or functional groups) but it is empirical and incomplete. The general relation between the liquid structure in the bulk and at the surface remains a challenging problem.

2. Surface tension

Known ionic liquids have surface tension values located in between these for alkanes and water (Table 1, abbreviations are listed in theAppendix). In this respect, ionic liquids look more like molecular organic liquids rather than molten salts. This also implies

that, as far as surface tension is concerned, ion pairs should be considered as the relevant elementary entity of the liquid, e.g. when estimating the molecular volume in Eq.(5).

The number of publications reporting surface tension values for various ionic liquids is growing rapidly and it is increasingly difﬁcult to keep up with the latest developments. The protocols used for purifying and drying ionic liquids vary signiﬁcantly between authors and laboratories. Characterisation is often only partial and it is practically impossible to decide on the validity of many results.

Galán Sánchez et al. [23] studied ionic liquids made up of imidazolium, pyridinium, or pyrrolidinium cations paired with dicyanamide (DCA), tetraﬂuoroborate (BF4), thiocyanate (SCN),

methylsulfate (MS), or triﬂuoroacetate (TFA) anions. They reported values of the surface tension and its temperature gradient over the range 293–363 K. Restolho et al.[20]compared the surface tension of C8mim.BF4and C2OHmim.BF4at temperatures between 298 and

470 K. The second tetraﬂuoroborate has a much higher surface ten-sion (Table 1) because the cation, though smaller than C8mim,

con-tains alcohol as a functional group and the importance of hydrogen bonding is enhanced. Torrecilla et al. [24] studied the inﬂuence of the alkyl chain length in 1-alkyl-3-methylimidazolium alkylsulfates at temperatures between 298 and 313 K. The surface tension, at room temperature, dropped by about 50% for methylsulfate (MS) and 40% for ethylsulfate (ES) when the number of carbon atoms in the side chain increased from one to eight. Klomfar et al.[25]reported the surface tension of 1-alkyl-3-methylimidazolium hexaﬂuorophosphate (C_nmim.PF6, n=3, 4, 6, 8) at temperatures from 287 to 353 K. They

confirmed that surface tension values reported for one and the same ionic liquid may differ by up to several mJ/m², i.e. discrepancies are often an order of magnitude larger than the quoted uncertainties. They suggested that the amount of water contained in the ionic liquid may be the reason. At low water content, all water molecules are hydrogen bonded but, at large concentrations, free water molecules are also present[25]. The surface tension of the hexafluorophosphates decreased by about 30% when the alkyl chain length increased from 3 to 8[25]. Zang et al.[26]reported the surface tension of N-hexylpyridine rheniumate over the temperature range 293–343 K. Carrera et al. [18]carried out a detailed study of over 20 ionic liquids containing imidazolium, ammonium, phosphonium and guanidinium cations com-bined with chloride, tetrafluoroborate, bis(trifluoromethanesulfonyl) imide, dicyanamide, p-toluenesulfonate or ethylsulfate anions. Surface tension values at room temperature were reported. Fletcher et al.[27] reported the surface tension (at 293 K) of several ionic liquids (C2mim.

DCA, C3mmim.NTf2, bmpyr.FAP, C2mim.BF4) that could be used as

electrolyte in industrial applications. Ahosseini et al.[19]measured the surface tension of binary mixtures of 1-hexyl-3-methylimidazolium bis (triﬂuoromethylsulfonyl)imide (C6mim.NTf2) and 1-octene at 10, 25, 50

and 75 °C. Carvalho et al.[22]studied several imidazolium, pyridinium,

Table 1

Surface tension of various liquids.

Liquid Surface tension [mJ/m²]

Perﬂuorohexane (20 °C) 11.9[16] Octane (25 °C) 21.1[17] Tetradecane (20 °C) 26.6[16] N8881.NTf2(20 °C) 22.8[18] C6mim.NTf2(25 °C) 30.8[19] C8mim.BF4(25 °C) 32.7[20] C2mim.PF3(CF2CF3)3 34.8[21] C3mpy.NTf2(20 °C) 36.0[22] C4mim.BF4(20 °C) 44.3[18] C4mpy.SCN (30 °C) 47.7[23] C4mpyr.DCA (20 °C) 56.2[23] C2OHmim.BF4(26 °C) 64.8[20] Water (25 °C) 72.0[17] NaCl (801 °C) 116[17] Mercury (25 °C) 485.5[17]

(3)

pyrrolidinium, and phosphonium ionic liquids with a common NTf2

anion and their surface tension at temperatures between 293 and 353 K. Guan et al.[28]reported the surface tension of 1-propyl-3-methylimi-dazolium glutamate with various amounts of water (from 900 to 22,000 ppm) at temperatures between 318 and 338 K. The surface tension, extrapolated to zero moisture, varied between 48.7 and 54.7 mJ/m² and increased with water content. Liu et al.[21]determined the density and surface tension of 1-ethyl-3-methylimizazolium tris (pentaﬂuoroethyl) triﬂuorophosphate (C2mim.FAP) over the

range 283–338 K. They used the parachor approach to estimate the surface tension of Cnmim.FAP (n=1, 3–6). Liu et al.[29]reported the

surface tension of N-alkylpyridinium bis(tri ﬂuoromethylsulfonyl)-imides (C_npy.NTf2, n = 2, 4, 5), containing very low amounts of

water (600–900 ppm), over the temperature range 283–338 K. Hasse et al.[30]studied the properties of several ionic liquids (1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-(1-ethyl-3-methylimidazolium methyl-phosphonate, 1-ethyl-3-methyl-imidazolium octylsulfate, and 1-butyl-3-butylimidazolium bis(triﬂuoromethylsulfonyl)imide). They reported values of the surface tension of these liquids but referred to it as interfacial tension in most of the text. Strictly speaking, this is not incorrect— the surface tension is the interfacial tension between a liquid and a gas[31]. However, terminology that might confuse readers (e.g. from a different discipline) should be avoided.

The above list contains only papers published in 2009 and 2010. It is clear that the empirical accumulation of data is in full swing and will probably intensify in the near future. In an attempt to capture the current situation, we present surface tension values selected from recent articles in the next twoﬁgures. As a characteristic parameter identifying each ionic liquid we have chosen the molecular volume, VM. This parameter is derived from the two most basic quantities

(molecular weight, M, and density, ρ), is easily obtained (VM= M/

ρNA), has a clear physical meaning, and also appears in many

equations used to interpret surface tension, e.g. the Eötvös Eq.(5). Recent data for ionic liquids combining four different cations [18,22,32]: C4mim (1-butyl-3-methylimidazolium), C8mim

(1-octyl-3-methylimidazolium), N8881(trioctylmethyl-ammonium), and P(14)666

(trihexyltetradecylphosphonium) with a variety of anions (generic symbol X) are shown inFig. 1. The data are almost segregated into four groups reﬂecting the increasing size of the cation in the above order. The data as a whole follow a downward trend.

In an attempt to provide an empirical description the whole set was approximated with the equationγ=a+bVM− 4, shown with a

solid line (this particular equation was selected through statistical analysis of empirical equations containing various mathematical transformations of VM). Due to the very large scatter, this choice

cannot be justiﬁed and it is really a guide to the eye. Values for ionic liquids with different anions appear to be randomly scattered. Thus the size of the cation determines the trend but only for C4mim and

C8mim ionic liquids (considered either separately or together). If

taken on their own, ammonium and phosphonium ionic liquids show no clear correlation between surface tension and molecular volume. The average surface tension values for N8881.X and P(14)666.X

are essentially identical to the surface tension of tetradecane[16] (C14H30, 26.6 mJ/m²) and eicosane[16](C20H42, 28.9 mJ/m²),

respec-tively. From a chemical point of view, this makes sense given the relatively long alkyl chains of both cations. The average surface tension values for C4mim.X and C8mim.X are spaced by few mJ/m² and

the gradient, ∂γ/∂nC, is similar to that found for linear alcohols

(≈0.7 mJ/m²) as an example of polar molecules with variable alkyl chain length. Note that for C4mim.NTf2 (VM= 0.48 nm³) two very

different values of the surface tension (at 25 °C) have been included: (32.8 ± 0.1)[32]and (38.37 ± 0.01)[18]mJ/m², respectively. A similar discrepancy was also found for N8881.NTf2(VM= 0.97 nm³).

Two conclusions can be drawn after examiningFig. 1: (i) surface tension decreases with the molecular volume but the tendency can be very different in magnitude for different cations; and (ii) large discrepancies between different reports for one and the same ionic liquid are not uncommon. For instance, Wandschneider et al.[32] reported 32.8 mJ/m² for the surface tension of C4mim.NTf2, which was

puriﬁed and dried to 220 ppm of water. Carrera et al.[18]reported a much higher value (38.4 mJ/m²) but the liquid was used as received from the supplier, and one could assume that impurities and/or water are responsible for that. However, Freire et al.[33]reported the value 33.6 mJ/m² for a liquid which contained some halide (40 ppm) and less water (120 ppm). In that situation, it is nearly impossible to decide which value is better. It is hoped that improved protocols and independent measurements will gradually resolve that problem.

A similar analysis was conducted for NTf2 ionic liquids

[18,22,27,32,34]and the result is shown inFig. 2. Selecting a single anion did not result in appreciably smaller overall scatter, though some better grouping in the middle is evident. The same empirical equation was used to guide the eye (black line) and the trend is slightly shifted upwards (theﬁt fromFig. 1is shown inFig. 2with a dashed line). Results reported by different authors are shown with different symbols and colours to illustrate, once again, the scatter and how difﬁcult it is to decide which data are more credible. The trends inFigs. 1 and 2are very similar but because the difference between them is comparable or smaller than the scatter we cannot conclude unequivocally that this is a general trend for ionic liquids.

VM [nm³] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 [mJ/m²] 20 25 30 35 40 45 50 C₄mim.X C₈mim.X N₈₈₈₁.X P_(14)666.X γ

Fig. 1. Surface tension of selected ionic liquids at room temperature. The solid line is a guide to the eye.

VM [nm³] 0.2 0.4 0.6 0.8 1.0 1.2 1.4 [mJ/m²] 20 25 30 35 40 45 50 γ

Fig. 2. Surface tension of bis(triﬂuoromethylsulfonyl)imide (NTf2) ionic liquids at room temperature. The solid lines are guides to the eye (the dashed grey line is identical to the black line inFig. 1).

(4)

Nevertheless, the inﬂuence of the cation, through its contribution to the molecular volume of the ion pair, is evident.

In all cases the surface tension was found to decrease linearly with temperature (away from the critical temperature). The values of the gradient,−∂γ/∂T, are usually in the range 0.04–0.20 mJ/(m².K)— Table 2. In this respect, ionic liquids are not markedly different from other liquids.

The strong link between surface structure and surface tension is widely acknowledged and surface sensitive techniques that provide orientational information are most useful. Ionic liquids offer a unique experimental advantage: they do not evaporate appreciably under vacuum and therefore can be studied with vacuum-based spectro-scopic techniques.

Santos and Baldelli [35] used sum frequency generation (SFG) spectroscopy to assess the effect of a systematically longer alkyl chain in 1-dialkylimidazolium-3-alkyl sulphates (Cnmim.CmOSO3, n,

m = 1–4). Longer alkyl chains, whether attached to the cation of the anion, lead to lower surface tension and were detected to point away from the liquid. Santos and Baldelli[36]used the same approach to probe the surface of C4mim.PF6and C4mim.BF4. They concluded that

the imidazolium ring lies parallel to the surface and the butyl chain is preferentially located outside the bulk of the liquid. The addition of benzene affected both liquids and benzene was present at the surface as conﬁrmed with surface tension measurements. Martinez and Baldelli[37]studied further the free surface of C4mim.BF4, C4mim.

DCA, and C4mim.MS in vacuum. They conﬁrmed the presence of both

cations and anions at the surface. The imidazolium ring adopts an orientation essentially parallel to the surface the alkyl chains of the cation protrude in the vacuum. Improvements on the SFG technique allowed them to estimate the tilt angle of the imidazolium ring. Surface tension and surface potential measurements were used to complement the SFG information.

Lockett et al. [38] used angle-resolved X-ray photoelectron spectroscopy (AR-XPS) to probe the surface of puriﬁed Cnmim.BF4

(n = 4, 6, 8) with a variable depth sensitivity. They concluded that the alkyl chain of the imidazolium cation is oriented away from the liquid. Lockett et al.[39]studied the structure of the surface of several ionic liquids in vacuum, with AR-XPS and synchrotron-based XPS. They showed that the surface layer is chemically different from the bulk liquid. It contains more aliphatic carbon atoms from the side chains of the anion and cation and also shows an unequal distribution of cations and anions perpendicularly to the surface. Lovelock et al. [40] demonstrated the enrichment of the surface layer in aliphatic carbon for C41Im.NTf2(n = 2–16) when n N4. The location of chemically

non-equivalent atoms with respect to the interface was estimated. Kolbeck et al.[41] further scrutinised 1-octyl-3-methylimidazolium.X ionic liquids (X = Cl, Br, I, BF4, PF6, Tf, NTf2, NPf2, FAP) and proposed a model

of the ionic liquid-vacuum surface. Maier et al.[42]reviewed AR-XPS results for a variety of ionic liquids and discussed their surface structure in relation to chemical differences.

Because ionic liquids can be studied under vacuum, they provide unprecedented opportunities for examining the structure of the liquid surface.

As ionic liquids are considered for engineering applications the surface tension of binary and ternary mixture is required. Ahoseini

et al.[19]measured the surface tension of (C61Im.NTf2+ 1-octene)

mixtures. The surface tension decreased with 1-octene concentration up to the solubility limit. Domanska and Królikowska[43]measured the surface tension of 1-butyl-3-methylimidazolium thiocyanate (C4mim.SCN) mixed with butanol, pentanol, or hexanol at

tempera-tures between 298 and 328 K. Hydrogen bonding played a key role over the whole range of concentrations. Rilo et al.[44]studied binary mixtures of Cnmim.BF4 (n = 2,4,6,8) ionic liquids with water and

ethanol. Surface tension of mixtures with water or ethanol decreased with the length of the alkyl chain on the cation. However, in water the ionic liquids showed a surfactant-like behaviour.

The growing number of surface tension studies generates an interest in rationalising the data. Ghatee et al.[45]have examined numerous data about diverse types of ionic liquids and proposed an empirical relation between surface tension and viscosity,μ (a and b being empirical constants):

lnγ = ln a + bμ−0:3: ð7Þ

Eq.(7)is an extension of the empirical equation suggested by Pelofsky [46] (where the viscosity exponent was −1). The −0.3 exponent was arrived at by linearising viscosity versus temperature dependences [45]. The analysis of the above equation is not particularly insightful but represents a natural trend toward rationalising the rapidly growing body of experimental data.

Larriba et al.[47]hypothesised that the dimensionless parameter γVM2/3/kBT is a function of only the packing fraction ϕ (= Vions/VM). In

their semi-empirical approach, the data for several ionic liquids almost collapsed on a single master curve, which was different from the one found for molten salts. Varela et al.[48]employed a modiﬁed pseudo-lattice theory of concentrated ionic solutions and modelled the short-range interactions with a Lennard–Jones potential. They were able to derive the parachor model (γ~ρ4_{) from their model. The}

structure of mixtures with water was described as cations and water molecules placed in the nodes of the pseudo-lattice and immersed in a continuum structureless distribution of anions. The model predicted correctly the surface pressure isotherm of several ionic liquid-water mixtures over the whole range of ionic liquid concentration. Weiss [49] examined the surface tension of ionic liquids within the framework of the principle of corresponding states as formulated by Guggenheim. He concluded that the behaviour of C_nmim.NTf2ionic

liquids is much closer to that of moderately polar liquids rather than molten inorganic salts. Weiss et al.[50]reached the same conclusion about C41Im.PF6— its ionicity was lower than expected. The degree

of ionicity in ionic liquids has been recently examined by MacFarlane et al.[51].

Fortunato et al.[52]studied various mixtures of a protic solvent and an ionic liquid (C41Im.X, X = BF4, PF6, Cl, Br). They used the

solvatochromic parameters approach to assess polarity and solvent interactions. After obtaining the acidity and basicity of the mixtures they proposed a series of tailored solvents offering speciﬁc solute– solvent and solvent–solvent interactions. While surface tension was not considered in this work it is certainly a suitable system to explore the importance of hydrogen bonding in more quantitative terms. Heggen et al.[53]studied the surface tension of bmim.PF6by molecular

dynamics simulations. The decline with temperature was obtained correctly but the predicted absolute values were underestimated. It seems that the methodology needs further reﬁnements.

The surface tension of ionic liquids attracts the interest of various researchers, though it is often regarded as a tool to obtain other pa-rameters[54]rather than a topic on its own. New data sets are being published at an increasing rate but quality is uneven. Our understanding of the surface tension of ionic liquids is rather incomplete and current interpretations are mostly empirical.

Table 2

Temperature gradient of the surface tension (surface excess entropy) of various liquids.

Liquid −∂γ/∂T [mJ/(m².K)] C4mpyr.TFA 0.04[23] C4mpy.DCA 0.12[23] C4mim.SCN 0.20[23] Water 0.16[11] NaCl 0.07[11] Mercury 0.20[11]

(5)

3. Interfacial tension

The number of studies reporting the interfacial tension of ionic liquids with immiscible liquids is rather small. Ahosseini et al.[19] reported the interfacial tension between C61Im.NTf2 and 1-octene.

The interfacial tension,γαβ, decreases with temperature slightly faster

than the surface tension,γ (Fig. 3).

Zhang et al.[55] reported the interfacial tension of C_nmim.PF6

(n = 5–8) against hexane and heptane at temperatures between 283 and 343.15 K. Gardas et al.[56]carried out a thorough study on the interfacial tension between imidazolium ionic liquids and water or alkanes at temperatures between 293 and 323 K. The interfacial tension decreased linearly with temperature and increased as the alkane changed from hexane to octane and decane. As the length of the alkyl chain length on the imidazolium cation increased, the interfacial tension with alkanes decreased but the one with water increased. Correlations based on parachor or mutual solubility methods were in agreement with the experiment. Rodríguez et al. [57]reported and correlated the interfacial tension in the ternary system (heptane + thiophene + C2mim.NTf2) in a study related to the

desulfurisation of naphtha.

The interfacial tension of ionic liquids is of key importance in many applications and is expected to receive much more attention in the near future.

4. Contact angle

Relatively few ionic liquid studies have dealt with contact angles. Restolho et al. [58] measured the contact angle of several ionic liquids on three surfaces: glass (polar surface) and polyethylene and polytetraﬂuoroethylene (non-polar surfaces). Their results are presented as a Zisman plot inFig. 4. For a homologous series of liquids, one would expect a straight line with a negative slope, becoming slightly concave at larger surface tensions (i.e. deviating from Zisman's line).

The results inFig. 4follow a plausible trend: the contact angle,_θ, increases as the surface tension of the liquid,γ, increases and lower contact angles are found on more polar surfaces. Quantitatively, however, both critical surface tensions and slopes are inconsistent. Similarﬁndings, in both trend and scatter, have been reported earlier by Gao and McCarthy[59]. Restolho et al.[58] also suggested the polarity of the ionic liquids estimated with Fowkes approach cor-relates with the constant k in the Eötvös Eq.(5).

Contact angles reﬂect the interfacial interactions between the three materials meeting at the contact line[8]. The theory of contact angles suggests that several liquids with known parameters can be used to probe the nature of a solid surface[15]but traditionally the choice of liquids has been very limited. Ionic liquids with their diverse ions and tunable chemistry are excellent candidates for probe liquids (as suggested by Gao and McCarthy[59]).

Cione et al.[60] compared the contact angles of C41Im.Tf and

bicyclohexyl on a series of gold and silica surfaces hydrophobized with thiol-based and silane-based monolayers, respectively. In most cases the two liquids, although having very similar surface tensions, showed contact angles that differed by at least 10°. This has not been seen with common organic liquids and one is tempted to suggest an experimental artefact. The rapid accumulation of empirical data will probably resolve this problem.

Carrera et al. [18] reported contact angles of numerous ionic liquids on Teﬂon (PTFE) and silica. The contact angles measured on PTFE are scattered but follow the trend seen with ordinary organic liquids (Fig. 5).

A quadratic extrapolation to contact angle zero (solid line) gives a critical surface tension of wetting of 17.5 mJ/m² which is reasonably close to tabulated values[15]. The magnitude of the slope (0.04 m²/mJ atγb34 mJ/m²) is also typical[8]. [mJ/m²] 25 30 35 [mJ/m²] 0 5 10 15 20 10 °C 25 °C 50 °C 75 °C

γ

αβ

γ

Fig. 3. Interfacial tension,γαβ, and surface tension,γ, of the mixture (C61Im.NTf2+1-octene) at different temperatures[19]. [mJ/m²] 30 40 50 60 70 cos -0.5 0.0 0.5 1.0 PTFE PE Glass

γ

θ

Fig. 4. Static advancing contact angles of ionic liquids (C81Im.BF4, C41Im.BF4, C21Py.ES, C21Im.ES and C2OHC1Im.BF4) on polytetraﬂuoroethylene (PTFE), polyethylene (PE) and glass (room temperature)[58].

10 20 30 40 50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 PTFE Silica [mJ/m²] cos

γ

θ

(6)

Contact angles measured on Teflon are usually reproducible as the surface is well-defined and inert. The situation is quite different with surfaces such as glass or silica. Because of the higher intrinsic surface energy these surfaces are prone to contamination and care is required to obtain a stable and reproducible level of hydrophobisation. The results for silica inFig. 5reflect the better wettability of glass (in comparison to PTFE) but they are scattered and most probably not representative.

The number of studies reporting contact angles of ionic liquids in solid–liquid–liquid systems is very limited. In an electrowetting study of ionic liquids on aﬂuoropolymer coating immersed in hexadecane, Paneru[61]obtained the static contact angle for two series of ionic liquids: (i) common anion (Cn1Im.BF4, n = 2,4,6,8,10) and (ii)

common cation (C41Im.X). These data are shown in Fig. 6 in the

format of a Zisman plot (cosine of the contact angle,θ0, versus ionic

liquid-hexadecane interfacial tension,γ). The static advancing contact angle is shown but contact angle hysteresis, as often found in SLL systems[62], was very low (≤2°) and therefore these contact angles are reasonably close to the equilibrium ones. All results cluster around two straight lines.

For the tetraﬂuoroborate series, a gradual increase in the length of the alkyl chain of the imidazolium cation induces a slight decrease in the contact angle (the contact angles are very large 145–162°). The ionic liquids from the C41Im group are more scattered: I lies on the

same line but PF6, TFA and NTf2sit on a second, much steeper line. The

Zisman plot is a convenient representation of the results but the linear parameters are out of range.

The number of reports on contact angles of ionic liquids will probably follow the trend for surface and interfacial tensions. It must be stressed that the quality of contact angle measurements depends critically not only on the ionic liquid but also on the solid surface[10,14].

As in mostﬁelds of active research, publications are piling up quickly and it becomes more and more difﬁcult to keep track of the latest developments. A newly launched ionic liquid database aims at collecting comprehensive and up-to-date information on ionic liquids. Delph-IL[63]is searchable by compounds or properties, has broad coverage and, importantly, attempts to provide quality indicators. 5. Conclusion

Studies of the interfacial properties of ionic liquids are rapidly multiplying. Discrepancies between different reports can be signiﬁcant and the reasons are often unclear. Surface tension data are available but the correlation with the molecular structure is rather vague. Surface spectroscopy of ionic liquids in vacuum is possible and this is an interesting new ﬁeld of research. Interfacial tension and mixtures

containing ionic liquids have received even less attention. The wetting properties of ionic liquids, much as their surface and interfacial tensions, are similar to those of polar molecular organic liquids.

Acknowledgements

The author gratefully acknowledges the support obtained through the Department of Further Education, Employment and Training (Government of South Australia), the RLDP and TRGGS 2009–10 (University of South Australia). This work was also supported by the Department of Innovation, Industry, Science and Research (Australian Government) through the Australia–India Strategic Research Fund. Appendix

Cations

Abbreviation Name Formula

C21Im or emim 1-ethyl-3-methylimidazolium (C2H5)CH3Im+ C2OHmim 1-ethanol-3-methylimidazolium HO(CH2)2CH3Im+ C31Py or pmpy 1-propyl-3-methylpyridinium (C3H7)CH3Py+ C41Im or bmim 1-butyl-3-methylimidazolium (C4H9)CH3Im+ C41Py or bmpy 1-butyl-3-methylpyridinium (C4H9)CH3Py+ C41Pyr or bmpyr 1-butyl-3-methylpyrrolidinium (C4H9)CH3Pyr+ C61Im or hmim 1-hexyl-3-methylimidazolium (C6H13)CH3Im+ C81Im or omim 1-octyl-3-methylimidazolium (C8H17)CH3Im+ N8881 trioctylmethylammonium (C8H17)3CH3N+ P(14)666 trihexyltetradecylphosphonium C14H29(C6H13)3P+

Anions

Abbreviation Name Formula

BF4 Tetrafluoroborate BF4− Br Bromide Br− Cl chloride Cl− DCA dicyanamide N(CN)2− ES ethylsulfate C2H5OSO3− FAP tris(pentafluoroethyl)trifluorophosphate PF3(C2F5)3− I iodide I−

MPO methylphosphonate CH3OHPO2−

MS methylsulfate CH3OSO3− MSO methanesulfonate CH3SO3− NTf2 bis(trifluoromethanesulfonyl)imide N(SO2CF3)2− OS octylsulfate C8H17OSO3− PF6 hexafluorophosphate PF6− PTSO p-toluenesulfonate CH3C6H4SO3− SCN thiocyanate SCN− Tf trifluoromethanesulfonate CF3SO3−

TFA triﬂuoroacetate CF3COO−

References

[1] Plechkova NV, Seddon KR. Chem Soc Rev 2008;37:123.

[2] Seddon KR, Rogers RD. Ionic liquids: industrial applications for green chemistry. Washington, D.C: American Chemical Society; 2002.

[3] Seddon KR, Rogers RD. Ionic liquids III: fundamentals, progress, challenges, and opportunities. Washington, D.C: American Chemical Society; 2005.

[4] Ohno H. Electrochemical aspects of ionic liquids. Hoboken, N.J: Wiley-Interscience; 2005.

[5] Wasserscheid P, Welton T. Ionic liquids in synthesis. 2nd ed. Weinheim: Wiley-VCH; 2008.

[6] Ritter SK. Chem Eng News 2008;86:36.

[7] Pham TPT, Cho C-W, Yun Y-S. Water Res 2010;44:352.

[8] Adamson AW, Gast AP. Physical chemistry of surfaces. 6th ed. New York: Wiley; 1997.

[9] de Gennes PG, Brochard-Wyard F, Quéré D. Capillarity and wetting phenomena: drops, bubbles, pearls, waves. New York: Springer; 2004.

[10] Neumann AW, Spelt JK. Applied surface thermodynamics. CRC Press; 1996. [11] Birdi KS. Surface tension and interfacial tension of liquids. In: Birdi KS, editor.

Handbook of surface and colloid chemistry. Boca Raton: CRC; 1997. p. 71. [12] Eustathopoulos N, Drevet B, Nicholas MG. Wettability at high temperatures.

Amsterdam: Pergamon; 1999.

[13] Neumann HM. J Solution Chem 1977;6:33. [14] Zisman WA. Adv Chem Ser 1964;43:1.

0 5 10 15 20 25 30 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 Cnmim.BF4 C₄mim.X C10mim C₈mim _C 6mim C2mim BF₄ PF₆ TFA NTf₂ C₄mim I [mJ/m²] cos

γ

θ

Fig. 6. Static contact angle of ionic liquids (Cn1Im.BF4and C41Im.X) on Teﬂon AF1600 immersed in n-hexadecane (room temperature)[61].

(7)

[15] van Oss CJ. Interfacial forces in aqueous media. 2nd ed. Boca Raton, FL: Taylor & Francis; 2006.

[16] Jasper JJ. J Phys Chem Ref Data 1972;1:841.

[17] Lide DR, editor. Handbook of chemistry and physics. 90th ed. Boca Raton: CRC; 2009. [18] Carrera GVSM, Afonso CAM, Branco LC. J Chem Eng Data 2010;55:609. [19] Ahosseini A, Sensenich B, Weatherley LR, Scurto AM. J Chem Eng Data 2010;55:1611. [20] Restolho J, Serro AP, Mata JL, Saramago B. J Chem Eng Data 2009;54:950. [21] Liu Q-S, Tong J, Tan Z-C, Welz-Biermann U, Yang J-Z. J Chem Eng Data 2010;55:2586. [22] Carvalho PJ, Neves CMSS, Coutinho JAP. J Chem Eng Data 2010;55:3807. [23] Galan Sanchez L, Ribe Espel J, Onink F, Meindersma GW, de Haan AB. J Chem Eng

Data 2009;54:2803.

[24] Torrecilla JS, Palomar J, Garcia J, Rodriguez F. J Chem Eng Data 2009;54:1297. [25] Klomfar J, Souckova M, Patek J. J Chem Eng Data 2009;54:1389.

[26] Zang S-l, Fang D-W, Li J-x, Zhang Y-Y, Yue S. J Chem Eng Data 2009;54:2498. [27] Fletcher SI, Sillars FB, Hudson NE, Hall PJ. J Chem Eng Data 2010;55:778. [28] Guan W, Tong J, Chen S-P, Liu Q-S, Gao S-L. J Chem Eng Data 2010;55:4075. [29] Liu Q-S, Yang M, Yan P-F, Liu X-M, Tan Z-C, Welz-Biermann U. J Chem Eng Data

2010;55:4928.

[30] Hasse B, Lehmann J, Assenbaum D, Wasserscheid P, Leipertz A, Froba AP. J Chem Eng Data 2009;54:2576.

[31] McNaught AD, Wilkinson A. IUPAC compendium of chemical terminology (gold book). IUPAC; 2009.

[32] Wandschneider A, Lehmann JK, Heintz A. J Chem Eng Data 2008;53:596. [33] Freire MG, Carvalho PJ, Fernandes AM, Marrucho IM, Queimada AJ, Coutinho JAP. J

Colloid Interface Sci 2007;314:621.

[34] Carvalho PJ, Freire MG, Marrucho IM, Queimada AJ, Coutinho JAP. J Chem Eng Data 2008;53:1346.

[35] Santos CS, Baldelli S. J Phys Chem B 2009;113:923. [36] Santos CS, Baldelli S. J Phys Chem C 2008;112:11459. [37] Martinez IS, Baldelli S. J Phys Chem C 2010;114:11564.

[38] Lockett V, Sedev R, Bassell C, Ralston J. Phys Chem Chem Phys 2008;10:1330. [39] Lockett V, Sedev R, Harmer S, Ralston J, Horne M, Rodopoulos T. Phys Chem Chem

Phys 2010;12:13816.

[40] Lovelock KRJ, Kolbeck C, Cremer T, Paape N, Schulz PS, Wasserscheid P, Maier F, Steinrueck HP. J Phys Chem B 2009;113:2854.

[41] Kolbeck C, Cremer T, Lovelock KRJ, Paape N, Schulz PS, Wasserscheid P, Maier F, Steinrueck HP. J Phys Chem B 2009;113:8682.

[42] Maier F, Cremer T, Kolbeck C, Lovelock KRJ, Paape N, Schulz PS, Wasserscheid P, Steinrueck HP. Phys Chem Chem Phys 2010;12:1905.

[43] Domanska U, Krolikowska M. J Colloid Interface Sci 2010;348:661.

[44] Rilo E, Pico J, Garcia-Garabal S, Varela LM, Cabeza O. Fluid Phase Equilib 2009;285:83. [45] Ghatee MH, Zare M, Zolghadr AR, Moosavi F. Fluid Phase Equilib 2010;291:188. [46] Pelofsky AH. J Chem Eng Data 1966;11:394.

[47] Larriba C, Yoshida Y, Fernandez de la Mora J. J Phys Chem B 2008;112:12401. [48] Varela LM, Carrete J, Turmine M, Rilo E, Cabeza O. J Phys Chem B 2009;113:12500. [49] Weiss VC. J Phys Chem B 2010;114:9183.

[50] Weiss VC, Heggen B, Muller-Plathe F. J Phys Chem C 2010;114:3599.

[51] MacFarlane DR, Forsyth M, Izgorodina EI, Abbott AP, Annat G, Fraser K. Phys Chem Chem Phys 2009;11:4962.

[52] Fortunato GG, Mancini PM, Bravo MV, Adam CG. J Phys Chem B 2010;114:11804. [53] Heggen B, Zhao W, Leroy F, Dammers AJ, Muller-Plathe F. J Phys Chem B 2010;114:

6954.

[54] Esperanca JMSS, Canongia Lopes JN, Tariq M, Santos LMNBF, Magee JW, Rebelo LPN. J Chem Eng Data 2010;55:3.

[55] Zhang X-j, Wang J-y, Hu Y-q. J Chem Eng Data 2010;55:4687.

[56] Gardas RL, Ge R, Ab Manan N, Rooney DW, Hardacre C. Fluid Phase Equilib 2010;294:139.

[57] Rodriguez H, Francisco M, Soto A, Arce A. Fluid Phase Equilib 2010;298:240. [58] Restolho J, Mata JL, Saramago B. J Colloid Interface Sci 2009;340:82. [59] Gao L, McCarthy TJ. J Am Chem Soc 2007;129:3804.

[60] Cione AM, Mazyar OA, Booth BD, McCabe C, Jennings GK. J Phys Chem C 2009;113: 2384.

[61] Paneru MN. Electrowetting of ionic liquids in solid/liquid/liquid systems. PhD Thesis, University of South Australia; 2010.

[62] Paneru M, Priest C, Sedev R, Ralston J. J Am Chem Soc 2010;132:8301. [63] Novionic Delph-IL.http://www.delphil.net/web/html/. 2010.