Choline based ionic liquids: Interfacial properties of RTILs with strong hydrogen bonding

Fluid Phase Equilibria 322–323 (2012) 142–147

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Choline based ionic liquids: Interfacial properties of RTILs with strong hydrogen bonding José Restolho a,b , José L. Mata a,c , Benilde Saramago a,∗ a b c

Centro de Química Estrutural, Instituto Superior Técnico, T U Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Research Institute for Medicines and Pharmaceutical Sciences, Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-019 Lisbon, Portugal Academia Militar, Pac¸o da Rainha, 29, 1150-244 Lisbon, Portugal

a r t i c l e

i n f o

Article history: Received 23 January 2012 Received in revised form 1 March 2012 Accepted 9 March 2012 Available online 21 March 2012 Keywords: Choline-based ionic liquids Strong hydrogen bonding properties Surface tension Contact angles Polarity

a b s t r a c t A large number of room-temperature ionic liquids (RTILs) have been prepared and characterized to date, but the majority of them are based on halogenated counter anions which limits their application due to toxicological, ecological and economic reasons. To overcome these problems, a new class of ILs has been recently introduced: Bio-ILs composed entirely of biomaterials. Choline based ILs are promising examples of this type of compounds. A series of ILs was recently synthesized using the choline cation and various naturally derived anions. In this work, we present data on surface tension, between 293.15 K and 393.15 K, and wettability of three choline based ILs: choline propionate, choline tiglate, and choline H-maleate. From the contact angle values observed on the hydrophobic substrate and the temperature dependence of the surface tension, the polarity of these compounds was estimated using the Fowkes approach and the Eötvös equation, respectively. Although these ILs present strong hydrogen bonding, low surface tension values were obtained in comparison with other ILs with the same characteristic (e.g. [C2 OHmim][BF4 ]). In contrast, their polarity is very high when compared with that of water which confirms their solvent ability. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, a dramatic growth in the research of RTILs was observed, not only due to their unique physical properties (low melting point, low vapor pressure, high polarity range, high thermal stability, good solvent properties, etc.) but also because of their supposed low toxicity. However, recent toxicological studies came to prove the contrary [1–4]. The majority of the produced and used RTILs have halogen based anions (e.g. [Cl], [PF6 ], [BF4 ], [Ntf2 ]). Although halogens are effective in reducing negative charge density, due to their electron withdrawing effect, they are very harmful for both environment and biological systems. The solutions to overcome this problem should involve the use of ions with well characterized biodegradable and toxicological properties. Ions based on natural derived products are good candidates to form biocompatible ionic liquids. One example is the choline cation that leads to relatively benign ionic liquids with protein stabilization characteristics [5]. The first RTILs based on choline were synthesized by Abott and his collaborators who published a series of studies where they demonstrated that choline cation was suitable to form RTILs [6–8].

∗ Corresponding author. E-mail address: [email protected] (B. Saramago). 0378-3812/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2012.03.016

On the other hand, quaternary imidazolium, ammonium, and phosphonium salts, when combined with some halogen free natural organic anions (e.g. carboxylic acids, amino-acids, etc.) showed low melting points [9–13]. Based on these works, Fukaya et al. [3], produced a series of choline based RTILs, made exclusively of biomaterials. Although these authors have made a systematic study of some physical properties (thermal properties, Kamlet–Taft parameters and viscosity) of these so-called “bio ionic liquids”, data on wettability and surface tension are, to our knowledge, nonexistent. These ILs exhibit higher hydrogen bonding ability than conventional ILs and, thus, should be able to dissolve virtually insoluble materials. In the present work, the surface properties of three cholinebased ILs: choline propionate, [Ch][Propionate], choline tiglate, [Ch][Tiglate], and choline H-maleate, [Ch][H-Maleate], were measured. The structures of these ionic liquids and their Kamlet–Taft parameters, (˛: hydrogen bond acidity, ˇ: hydrogen bond basicity) are presented in Table 1. Strong intermolecular hydrogen bonding may be achieved between the carboxylic acid groups present in the anions and the terminal hydroxyl group of the choline cation. In the H-maleate anion there is the possibility of intra-molecular hydrogen bonding which leads to delocalization of the negative charge and the consequent weakening of the electrostatic interaction with the choline cation. However, the presence of two extra carbonyl groups

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Table 1 Chemical structure of the bio ionic liquids. Ionic liquid

Kamlet–Taft parametersa (at 25 ◦ C)

Chemical structure

˛

ˇ

[Ch][Propionate]

0.52

0.98

[Ch][Tiglate]

0.59

0.95

[Ch][H-Maleate]

0.75

0.58

a

Data taken from [3].

may still allow for intermolecular hydrogen bonding. According to Fukaya et al. [3], the strong intermolecular hydrogen bonds are responsible for the high viscosity displayed by these choline-based ILs at room temperature, being [Ch][H-Maleate] the less viscous. The surface tension and the contact angles on polytetrafluoroethylene (PTFE) and BK7 glass were measured. The polarity of these ILs was estimated both from the contact angle values observed on PTFE using the approach of Fowkes [14], and from the Eötvös equation [15,16]. The surface tension results were further analyzed at the light of a simplified version of the corresponding states principle proposed by Weiss et al. [17]. The first objective of this study was to contribute to the database of interfacial properties of ionic liquids with results relative to a new family of bio-ionic liquids. The second objective was to profit from the unusual characteristics of these ions, capable of strong hydrogen bonding, to investigate the correlation between the surface tension and intermolecular interactions and liquid nanostructure. 2. Experimental The ionic liquids: choline propionate, [Ch][Propionate], choline tiglate, [Ch][Tiglate], and choline H-maleate, [Ch][H-Maleate], were purchased from Solchemar (Caparica, Portugal) and have a purity > 98%. They were all dried at vacuum (20 Pa) and at 353 K ([Ch][Propionate] and [Ch][Tiglate]), or 333 K ([Ch][HMaleate], due to decomposition problems), for at least 3 days prior to the experiments. The final water mass fraction, checked by Karl–Fischer (coulometric) was ∼705 ppm for [Ch][Propionate], ∼1835 ppm for [Ch][Tiglate] and ∼1650 ppm for [Ch][H-Maleate]. We should point out here that the high water content obtained after drying which could not be further reduced seems to be an intrinsic characteristic of these hygroscopic ionic liquids. The substrates were 1 mm thick PTFE sheet (Goodfellow) and optical quality BK7 glass (Melles Griot). They were carefully cleaned in a detergent solution, rinsed with distilled and deionized water and dried with nitrogen and finally dried during 2 h inside a vacuum oven at room temperature. After drying, the BK7 glass was submitted to argon plasma cleaning for 5 min followed by 10 min under a stream of argon. The surface tension and the contact angle measurements were carried out by the pendant and the sessile drop methods, respectively, using the ADSA-P software (Axisymetric Drop Shape Analysis, Applied Surface Thermodynamics Research Associates, Toronto, Canada) for image analysis. The ambient chamber Ramé–Hart was modified in order to allow for temperature control in the range (298–473) K. During the experiments, the chamber

was flushed with pure, dry nitrogen. Further experimental details may be found in Ref. [18]. The contact angles measurements were made at room temperature for the glass substrates, and within the temperature range of (298–343) K for PTFE substrates (to assess the influence of the temperature on the value of the contact angle). The surface tensions were measured in the temperature range of (293–393) K. For each temperature, the surface tension and the contact angle were measured as a function of time until stable values were achieved. The surface tension measurements required 2 h, while for the contact angle determinations, measurements lasted from 4 to 16 h in order to reach a plateau value, the so-called static contact angle. Although dry atmosphere was used during the experiments, we were concerned with an eventual increase in the sample humidity. To assess this we measured the evolution of the water content, in half an hour intervals, of a [Ch][H-Maleate] sample at ambient temperature. First, the Karl Fischer apparatus was checked against a 1000 ppm reference liquid and the standard deviation of the measurements was 81 ppm. The ionic liquid sample suffered an initial increase in the water content of 150 ppm, which remained constant, within the Karl Fischer resolution, until the end of the experiment. From these results, we may conclude that our experimental procedure, after the initial manipulations, preserves the water content of the sample from further increase. The density values, which are necessary for the experimental determination of the surface tension, were measured with an Anton-Paar DMA 5000 vibrating-tube densimeter in the temperature range of (298–363) K. All reported density data were corrected for the effect of viscosity using the internal calibration of the densimeter. To prevent the effect of air bubbles, each measurement was repeated twice. A first measurement was done after inserting the liquid sample inside the vibrating tube and a second measurement after replacing this liquid by another sample. 3. Results The densities of the ionic liquids vary linearly with the temperature according to the following equations: /(g cm−3 ) = 1.2325 − 5.0 × 10−4 T/(K) for [Ch][Propionate], /(g cm−3 ) = 1.2349 − 6.0 × 10−4 T/(K) for [Ch][Tiglate], and /(g cm−3 ) = 1.3785 − 6.0 × 10−4 T/(K) for [Ch][H-Maleate], with uncertainties of ±0.00031 g cm−3 , ±0.00028 g cm−3 and ±0.00019 g cm−3 , respectively. The surface tensions of the ionic liquids are plotted against the temperature in Fig. 1. The uncertainties (95% confidence interval) vary between ±0.01 and ±0.5 mJ m−2 . The first comment to the data of Fig. 1 is the small temperature range of the measurements for [Ch][H-Maleate]. The reason

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Fig. 1. Temperature dependence of the surface tension  of [Ch][Propionate] (), [Ch][Tiglate] (), and [Ch][H-Maleate] (♦). The lines are linear fits to Eq. (1).

for the reduced data has to do with the decomposition suffered by this ionic liquid at temperatures above 340 K which led to absurd values of the surface tension measurements. This decomposition was confirmed by NMR analysis of the sample after being heated to this temperature (data not shown). We should stress that the temperature of 340 K lies well below the value reported by Fukaya et al. [3] for the temperature of 5% weight loss (496 K) determined by thermogravimetric analysis. The surface tension decreases linearly with increasing temperature according to the equation:  = a − bT

(1)

where intercept, a, can be identified with the surface excess energy, ES , and the slope, b, can be identified with the surface excess entropy, SS , which is assumed to be temperature independent [16]. The temperature dependence of the surface tension may also be described by the Eötvös equation: L V 2/3 = k(Tc − T )

(2)

Fig. 2. Temperature dependence of the contact angle, , on PTFE substrates of [Ch][Propionate] (), [Ch][Tiglate] (), and [Ch][H-Maleate] (♦).

practical reasons it is common to include all the non-dispersive interactions in a single term so that: i = id + ind

(4)

where subscript i = S or L indicates, respectively the solid or the liquid, while superscripts d and nd stand for the dispersive and the non-dispersive terms of the surface tension. Assuming the geometric mean rule to describe the dispersive forces between pairs of unlike molecules, the following relation may be deduced [20]: L (1 + cos ) = 2(Sd · Ld )

1/2

(5)

Eq. (5) allows the calculation of the dispersive component of the surface tension of a liquid from its contact angle, , on a purely dispersive solid, once  L and Sd = S are known. The non-dispersive component is then obtained using Eq. (4). This assumption is valid when both phases are non-polar and may be a good estimate when only one of them is polar. From the contact angles measured on PTFE, the dispersive, Ld , and the non-dispersive, Lnd , components of the surface tension of the liquids were calculated using Eqs. (4) and (5), at temperature T = 298 K. The solid was assumed to be purely dispersive with the

where V is the molar volume of the liquid, Tc is the critical temperature and the constant k is inversely proportional to the liquid polarity [15,19]. (For most organic liquids k is approximately 2.1 × 10−7 J K−1 but for a strongly polar liquid, like NaCl in the fused state, it decreases to 0.4 × 10−7 J K−1 ). An alternative way to express the surface tension as a function of temperature is the Guggenheim equation:  = Const

 1 − T 11/9 Tc

(3)

The values of ES , SS , k and Tc obtained from the fitting of Eqs. (1)–(3) to our data are given in Table 2. The contact angles of the ionic liquids on PTFE substrates are plotted against the temperature in Fig. 2. The uncertainties (95% confidence interval) vary between ±1◦ and ±4◦ with a tendency for increased scatter at higher temperatures in the case of [Ch][Propionate] and [Ch][Tiglate]. The contact angles on PTFE and on glass, at room temperature, are compared in Fig. 3. According to Fowkes, the surface tension,  L , of a solid or a liquid can be described as a sum of independent contributions each arising from the intermolecular interactions present in such phases. For

Fig. 3. Contact angle, , on PTFE ( temperature.

) and glass () substrates of the ILs, at room

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Table 2 Parameters obtained from the fitting or the temperature dependence of the surface tension to Eqs. (1)–(3). Ionic liquid

[Ch][Propionate] [Ch][Tiglate] [Ch][H-Maleate]

SS /mJ m−2 K−1

0.056 ± 0.003 0.058 ± 0.005 0.069 ± 0.007

ES /mJ m−2

k × 107 /J K−1

50 ± 1 49 ± 1 62 ± 2

1.69 1.91 2.23

value 17.5 mJ m−2 for its surface tension [21]. The polarity fraction defined as the ratio between the non-dispersive component, Lnd , (that is often called “polar” component) and the total surface tension, L = Ld + Lnd was also calculated. The values obtained for each liquid are given in Table 3. The values of the surface tension and the contact angle measurements are given as supplementary material. 4. Discussion We begin with the analysis of the surface tension data presented in Fig. 1 trying to correlate this property with the molecular characteristics of the substances. The values of surface tension of [Ch][Propionate] and [Ch][Tiglate] are close which is expected from the similarity between the structure of both anions. [Ch][HMaleate] has the highest surface tension with the most accentuated temperature dependence. The values of the surface excess energies for [Ch][Propionate] and [Ch][Tiglate] given in Table 2 are low when compared with those reported for other ionic liquids (see e.g. [22]) which is somehow unexpected considering the great ability for intermolecular hydrogen bonding between anions and cations. [Ch][H-Maleate] has the highest surface excess energy and its surface excess entropy is in the upper end of the usual range of surface entropies of ionic liquids and is similar to that of other organic compounds (e.g. 0.072 mJ m−2 K−1 for imidazole) [18,22] which may indicate a low degree of surface ordering. However, we must point out that the determination of the slope b made, in this case, from surface tension data in a relatively narrow temperature range may have a higher inherent error. The rationale behind these numbers is hard to find because there is no apparent correlation between the observed surface tensions and the strength of hydrogen bonding or the size of the anions. [Ch][H-Maleate] which is the IL with the highest surface tension is probably the liquid with weakest intermolecular interactions due to the possibility of intra-molecular hydrogen bonding. The fact that it presents the lowest viscosity among the three ILs investigated confirms this statement. Furthermore, the anions which differ more in size ([Propionate] and [Tiglate]) are those with similar surface tension values. We could also attribute the high surface tension of [Ch][H-Maleate] to the presence of water in the dried sample (∼1650 ppm) but an even higher amount exists in [Ch][Tiglate] (∼1835 ppm) and its surface tension is the lowest. We should stress at this point that, although in some cases it seems possible to establish correlations between the surface tension and the intermolecular interactions, the surface tension is a complex property that depends on the molecular arrangements at the interface. By now, it is well established that ionic liquids may be considered as three dimensional polar networks permeated by nonpolar regions [23]. The presence of an interface with air implies the

Tc /K Eq. (2)

Eq. (3)

901 848 896

1024 962 1024

reorganization of this three-dimensional structure with segregation of the non-polar parts to the surface. According to Tariq et al. [24], a more “exposed” polar network leads to higher values of the surface tension. Thus, while [Ch][Propionate] and [Ch][Tiglate] are able to expose the terminal alkyl groups of the anions to the surface, the presence of two carboxylic acid groups in the H-maleate anion is responsible for the higher  of [Ch][H-Maleate]. The values of k are close to the typical value for organic liquids in agreement with the findings of other authors [19,25], but [Ch][HMaleate] stands out as the least polar one. These k values correlate very well with the Kamlet–Taft parameters shown in Table 1. Propionate and tiglate anions lead to very high ˇ values because they are the conjugate bases of weak acids. [Ch][H-Maleate] has a weak proton accepting ability (lower ˇ value) due to the intra-molecular hydrogen bond of the hydrogen maleate anion that weakens the possibility of establishing intermolecular hydrogen bonds. As a consequence, the choline cation in [Ch][H-Maleate] gives rise to the highest ˛ value. The estimates for the (hypothetical) critical point temperatures presented in Table 2 are strongly dependent on the equation chosen to fit the data. The same problem was detected by Tariq et al. when estimating the critical temperature of the 1-alkyl3-methylimidazolium bis(trifluoromethylsulfonyl) amides family [24]. The uncertainty associated with these estimations derives from the large extrapolations in the surface tension data involved when using Eqs. (2) and (3). However, this is the only way to have predictions of the critical temperatures since they are not experimentally accessible. From the critical temperature, Tc , the boiling temperature may be estimated using the empirical relation Tb = 0.7 Tc [24]. Their values vary between (594–673) K for [Ch][Tiglate] and (631–717) K for [Ch][Propionate]. These boiling temperatures are low when compared with those estimated for other ionic liquids but still higher than their decomposition temperatures [3]. The smaller anion, propionate, seems to exhibit higher normal boiling point temperature reflecting the fact that concentrating the negative charge in a smaller volume enhances the electrostatic interaction with the cation. In order to understand further the nature of the intermolecular forces that are responsible for the surface behavior of the studied ILs, we applied a simple, two-parameter form of the correspondingstates principle to the surface tension data. According to Weiss et al. [17], the reduced surface tension may be defined by: R = c



(6)

2/3 Tc c M −2/3

where c ≈ 1.016 is a constant if  is measured in mN m−1 , M is the molar mass in g mol−1 , Tc is the critical temperature in K, and c is the critical density in g cm−3 . The critical densities were estimated from the average densities, , ¯ in the temperature range of

Table 3 Dispersive, Ld , and non-dispersive, Lnd , components of the surface tension and polarity fractions, Lnd /L , of the three choline-based ILs at 298 K. Ionic liquid

Dispersive component Ld

Non-dispersive component Lnd

Polarity fraction Lnd /L

[Ch][Propionate] [Ch][Tiglate] [Ch][H-Maleate]

12 ± 2 10 ± 2 17 ± 1

22 ± 2 22 ± 2 25 ± 1

0.65 0.69 0.60

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Fig. 4. Reduced surface tension,  R , (y-axis) as a function of the reduced temperature, TR , (x-axis) for low and high temperature molten salts. Data for Ar, NaCl, KCl, [C8 mim][BF4 ] and [C2 OHmim][BF4 ] are taken from [26].

The contact angles vary according to the surface tension, i.e., the liquid with the highest surface tension presents the highest contact angles and liquids with similar surface tensions present the same contact angle. For all liquids, the contact angles on glass are low which may be due to the electrostatic interactions between the surface hydroxyl groups and the ions of the IL, but still higher than that of water on glass ( water ≈ 0◦ ). On PTFE all ionic liquids have contact angles smaller than that of water ( water = 118◦ ). The values of the contact angles measured on PTFE allowed the estimation of a polarity fraction,  nd /L , for the ionic liquids L as referred above. Comparison of polarity fractions (Table 3) with the polarity parameters, k, obtained from the fitting to the Eötvös equation (Table 2) shows good agreement with respect to [Ch][HMaleate] being the least polar, but there is an inversion for the other two liquids. Considering the rather large uncertainty associated with the estimated Tc values that are involved in the use of the Eötvos correlation, the values of polarity parameter, k, deserve less confidence. The polarity fractions are higher than the values obtained in our laboratory for other ionic liquids (all close to 0.4) [26]. When compared with the polarity fractions of the standard liquids considered as “polar” (taken from [28]): water (0.70), glycerol (0.47) and formamide (0.33), the choline-based ILs studied may be considered highly polar. 5. Conclusions

the surface tension measurements using the empirical correlation c ≈ 0.333. ¯ The critical temperatures are those calculated with Guggenheim equation and given in Table 2. The reason to choose these values instead of those obtained with the Eötvös equation is the better estimation of the experimental value of Tc for argon provided by Guggenheim equation. In a recent paper [26], the authors plotted the reduced surface tension as a function of reduced temperature (TR = T/Tc ) for several ionic liquids and for molten salts and compared the data with Guggenheim’s universal curve applied to argon, a purely dispersive fluid. They found that the reduced surface tensions of all ionic liquids deviate strongly from the data of fluids dominated by Coulomb interactions (molten salts) and lie close to the master curve for dispersive liquids (Fig. 5f of [26]). The negative deviation that is observed for [C2 OHmim][BF4 ], is consistent with the small size of the ions and the presence of hydrogen bonds. [C8 mim][BF4 ] lies close to the dispersive master curve. This means that the negative deviations that would be created by the Coulomb interactions present at the surface are compensated by the positive deviations caused by the dispersive interactions between the long alkyl chain of the cation segregated at the surface. We now add to that figure the data relative to the choline based ILs. The results are shown in Fig. 4. While [Ch][Propionate] and [Ch][Tiglate] present negative deviations characteristic of strong hydrogen bonding, [Ch][H-Maleate] is similar to [C8 mim][BF4 ] as a result of weaker hydrogen bonding. Between [Ch][Propionate] and [Ch][Tiglate], the higher negative deviation found for the former liquid is consistent with the smaller size of its anion which reduces the dispersive interactions and enhances the Coulomb ones. The behavior of [Ch][H-Maleate] is in good agreement with our previous comment on the fact that the intra-molecular hydrogen bonding in this compound decreases the strength of intermolecular hydrogen bonding in comparison with [Ch][Propionate] and [Ch][Tiglate]. We analyze now the wettability data presented in Figs. 2 and 3. The contact angle of [Ch][H-Maleate] is practically independent of the temperature, while for the two other liquids there is a decrease in the contact angles with rising temperature. For [Ch][Tiglate] the observed decrease is more pronounced than the usual temperature dependence of 0.1◦ /K reported for many systems at low temperatures (<373 K) [27].

The surface tension and contact angles on hydrophilic and hydrophobic substrates of three new biocompatible ionic liquids based on natural derived products [3] were determined in this work. From these data, it was possible to infer about the polarity of the liquids. The surface tension results were analyzed in terms of a simple, two-parameter form of the corresponding-states principle. The three choline based ionic liquids, [Ch][Propionate], [Ch][Tiglate], and [Ch][H-Maleate], which are characterized by strong hydrogen bonding have very high polarities but relatively low surface tensions. [Ch][H-Maleate], the ionic liquid where intermolecular hydrogen bonding is weakened by the high probability of intra-molecular bonding, has the highest value of surface tension and the lowest polarity. Furthermore, the corresponding-states principle suggests that the surface behavior of this ionic liquid is mainly determined by dispersive interactions, in contrast with [Ch][Propionate] and [Ch][Tiglate], where the electrostatic interactions are dominant. These results on the interfacial properties of RTILs with strong hydrogen bonding contribute to clarify the widely acknowledged, but ill-understood, correlation between surface tension and the molecular structure. Evidence is given to the fact that the surface tension cannot be directly correlated with the strength of the bulk intermolecular interactions but depends on the molecular rearrangements imposed by the presence of an interface. Acknowledgments This study was financially supported by the project PEstOE/QUI/UI0100/2011. José Restolho acknowledges grant SFRH/BD/73228/2010. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fluid.2012.03.016. References [1] X. Wang, C. Ohlin, Q. Lu, Z. Fei, J. Hu, P. Dyson, Green Chem. 9 (2007) 1191–1197. [2] D. Zhao, Y. Liao, Z. Zhang, Clean-Soil Air Water 35 (2007) 42–48.

J. Restolho et al. / Fluid Phase Equilibria 322–323 (2012) 142–147 [3] Y. Fukaya, Y. Iizuka, K. Sekikawa, H. Ohno, Green Chem. 9 (2007) 1155–1157. [4] J. Arning, M. Matzke, Curr. Org. Chem. 15 (2011) 1905–1917. [5] R. Vrikkis, K. Fraser, K. Fujita, D. MacFarlane, G. Elliott, J. Biomech. Eng. 131 (2009) 074514. [6] A.P. Abbott, G. Capper, D.L. Davies, H.L. Munro, R.K. Rasheed, V. Tambyrajah, Chem. Commun. (2001) 2010–2011. [7] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun. (2003) 70–71. [8] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, J. Am. Chem. Soc. 126 (2004) 9142–9147. [9] J. Wilkes, M. Zaworotko, J. Chem. Soc. Chem. Commun. (1992) 965–967. [10] R. Lau, M. Sorgedrager, G. Carrea, F. Pantwijk, F. Secundo, R. Sheldon, Green Chem. 6 (2004) 483–487. [11] K. Fukumoto, M. Yoshizawa, H. Ohno, J. Am. Chem. Soc. 127 (2005) 2398–2399. [12] J. Kagimoto, K. Fukumoto, H. Ohno, Chem. Commun. (2006) 2254–2256. [13] G. Ou, M. Zhu, J. She, Y. Yuan, Chem. Commun. (2006) 4626–4628. [14] F.M. Fowkes, Ind. Eng. Chem. 56 (1964) 40–52. [15] J.L. Shereshefsky, J. Phys. Chem. 35 (1931) 1712–1720. [16] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, 6th Ed., Wiley, New York, 1997.

147

[17] V.C. Weiss, B. Heggen, F. Müller-Plathe, J. Phys. Chem. C 114 (2010) 3599– 3608. [18] J. Restolho, A.P. Serro, J.L. Mata, B. Saramago, J. Chem. Eng. Data 54 (2009) 950–955. [19] J. Tong, M. Hong, W. Guan, J.B. Li, J.Z. Yang, J. Chem. Thermodyn. 38 (2006) 1416–1421. [20] N.T. Correia, J.J.M. Ramos, B.J.V. Saramago, J.C.G. Calado, J. Colloid Interface Sci. 189 (1997) 361–369. [21] F. Liu, W. Shen, Colloids Surf. A 316 (2008) 62–69. [22] J. Restolho, J.L. Mata, B. Saramago, J. Colloid Interface Sci. 340 (2009) 82–86. [23] L. Santos, J.N. Canongia Lopes, J.A.P. Coutinho, J. Esperanc¸a, L. Gomes, I.M. Marrucho, L. Rebelo, J. Am. Chem. Soc. 129 (2007) 284–285. [24] M. Tariq, A.P. Serro, J.L. Mata, B. Saramago, J. Esperanc¸a, J.N. Canongia Lopes, L. Rebelo, Fluid Phase Equilibr. 294 (2010) 131–138. [25] J.Z. Yang, X.M. Lu, J.S. Gui, W.G. Xu, Green Chem. 6 (2004) 541–543. [26] J. Restolho, J.L. Mata, K. Shimizu, J.N. Canongia Lopes, B. Saramago, J. Phys. Chem. C 115 (2011) 16116–16123. [27] A.W. Adamson, J. Colloid Interface Sci. 44 (1973) 273–281. [28] R.J. Good, in: K.L. Mittal (Ed.), Contact Angle, Wettability and Adhesion, VSP, Utrecht, The Netherlands, 1993.