Aggregation Behavior and Total Miscibility of Fluorinated Ionic Liquids

Article pubs.acs.org/Langmuir

Aggregation Behavior and Total Miscibility of Fluorinated Ionic Liquids in Water Ana B. Pereiro,*,† Joaõ M. M. Araújo,† Fabiana S. Teixeira,† Isabel M. Marrucho,† Manuel M. Piñeiro,‡ and Luis Paulo N. Rebelo*,† †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Apartado 127, 2780-157 Oeiras, Portugal Departamento de Física Aplicada, Facultade de Ciencias, Universidade de Vigo, E36310 Vigo, Spain

‡

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S Supporting Information *

ABSTRACT: In this work, novel and nontoxic fluorinated ionic liquids (FILs) that are totally miscible in water and could be used in biological applications, where fluorocarbon compounds present a handicap because their aqueous solubility (water and biological fluids) is in most cases too low, have been investigated. The self-aggregation behavior of perfluorosulfonate-functionalized ionic liquids in aqueous solutions has been characterized using conductometric titration, isothermal titration calorimetry (ITC), surface tension measurements, dynamic light scattering (DLS), viscosity and density measurements, and transmission electron microscopy (TEM). Aggregation and interfacial parameters have been computed by conductimetry, calorimetry, and surface tension measurements in order to study various thermodynamic and surface properties that demonstrate that the aggregation process is entropy-driven and that the aggregation process is less spontaneous than the adsorption process. The novel perfluorosulfonate-functionalized ILs studied in this work show improved surface activity and aggregation behavior, forming distinct self-assembled structures.

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emulsions). All fluorosurfactants have a polar head and a hydrophobic tail. For long perfluorinated tails in addition to water, fluorosurfactants also repel fats such as oil and grease. Fluorinated ionic liquids (FILs) are very appealing in relevant applications of fluorinated surfactants. The intrinsic properties of ionic liquids such as their negligible volatility under atmospheric conditions,9,10 their easy recovery and recyclability, and their tunable toxicity fully justify the use of FILs as new cleaner alternative compounds to replace the perfluorinated surfactants used nowadays in industry. The interest in these novel FILs is that these combine the best properties of fluorocarbon compounds with those of ionic liquids. The potential biological applications of fluorocarbon compounds present a handicap because their aqueous solubility (water and biological fluids) is commonly too low.11 The increment of aqueous solubility could open new frontiers in applications of fluorinated ionic liquids; for example, they could be used to solubilize some proteins12 and enzymes.13 Ionic liquids based on bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, or tetrafluoroborate anions have been frequently characterized and designated as fluorinated ionic liquids. The FIL family, defined as ionic liquids with fluorinated alkyl chain lengths equal to or greater than four carbon atoms,4 has been assessed in a few works.14−18 Therefore, herein the behavior of

INTRODUCTION Fluorocarbon compounds are present in our daily lives and are used for the development of novel compounds with uncommon and unrivaled properties such as fluoropolymers, surfactants, refrigerants, and components of pharmaceuticals, insecticides, and so forth.1,2 The diversity of applications arises from the unique properties of fluorinated carbons, particularly their low surface tension and high capacity for dissolving gases, as well as their low-intensity interactions with standard organic compounds and superior chemical and biological inertness.3,4 The very strong C−F bonds induce an increase in the rigidity and a decrease in the polarity of fluorinated compounds, dictating their molecular structures.4,5 In biological applications, fluorocarbon compounds have been developed as in vivo gas carriers, such as liquid ventilation and artificial blood substitute formulations, as well as drug delivery systems.6 From a more fundamental viewpoint, fluorinated compounds are being investigated to understand the impact of fluorination on the formation, stability, and structure of colloidal systems in comparison to their hydrogenated counterparts.7,8 Previous papers show that the critical aggregation concentration (CAC) values of fluorinated surfactants are analogous to those of hydrogenated surfactants with chain lengths approximately 1.5 times longer. 7,8 Fluorinated amphiphiles are engaged in distinct colloidal systems, including a diversity of self-assemblies (e.g., tubules, vesicles, ribbons, helices) and different types of emulsions (e.g., microemulsions, direct, reverse or multiple emulsions, gel © 2015 American Chemical Society

Received: October 6, 2014 Revised: January 12, 2015 Published: January 12, 2015 1283

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Langmuir Scheme 1. Structures and Abbreviations of Fluorinated Ionic Liquids

FILs with longer fluorous chains, distinct from conventional fluorinated ionic liquids with fewer than four carbons in the fluorinated alkyl chain, have been detailed. To the best of our knowledge, the scientific research related to FILs with longer fluorous chains has been centered essentially on synthesis and characterization as well as their application as reaction media and as materials.14−18 Also, no accurate characterization of the self-aggregation behavior of these FILs is available in the open literature. Furthermore, the FILs analyzed in this study have been selected from previous works4,6,19,20 because they are totally miscible in water and present a low cytotoxicity in two different human cell culture types.19 Only the perfluorooctanesulfonate anion ([(PFOc)SO3]¯) shows some cytotoxicity in one of the human cell cultures at high FIL concentration (Caco-2 cell with log EC50 = 3.65, 4467 μM but HepG2 with log EC50 >4, >10 000 μM).19 This value is negligible when compared to the corresponding acid, perfluorooctanesulfonic acid (log EC50 = 1.48, 30 μM in PC12 cells),21 which is a new class of environmental contaminant that is globally distributed, persistent, bioaccumulative, and toxic to various species.22 It is important to become aware of the expansion of the production and use of perfluorinated surfactants in forthcoming years because of their utility, economic value, and industrial application. 23 The FILs studied herein based on the perfluorobutanesulfonate anion could be used as neoteric alternative surfactants, which are more environmentally friendly and have tunable physicochemical properties as compared to other fluorosurfactans. The FIL based on the perfluorooctanesulfonate anion has been studied only for comparison and not for application purposes. The objective of this study is to understand the selfaggregation in water of these novel, totally water-miscible fluorinated surfactants as well as quantitatively determine the aggregation and surface properties. The self-aggregation of FIL aqueous solutions has been characterized by conductometry titration, isothermal titration calorimetry, surface tension, dynamic light scattering, transmission electron microscopy, viscosity, and density at 298.15 K. Parameters such as the critical aggregation concentration, degree of ionization of the aggregates and of counterion binding, micellar size, maximum surface excess concentration, effectiveness of the surface tension reduction, and minimum area occupied per fluorosurfactant molecule have been determined and fundamentally examined.

Various thermodynamic parameters such as the enthalpy, entropy, and standard Gibbs free energy of aggregation, and standard free energies of adsorption have been also determined. Furthermore, the self-assembled structures of FILs in aqueous solutions, which are essential to their application, have been evaluated using transmission electron microscopy, dynamic light scattering, and the critical packing parameter determined from surface tension measurements. The integrated analysis of the experimental data obtained by the above-mentioned techniques and parameters calculated allows an accurate assessment of the self-aggregation behavior of the aqueous solutions of these novel surfactant fluorinated ionic liquids.

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EXPERIMENTAL SECTION

Materials. 1-Ethyl-3-methylimidazolium perfluorooctanesulfonate, [EtMeIm][(PFOc)SO3] [>98% mass fraction purity, halides (ion chromatography) 99.8%, anion (ion chromatography) >99.5%)]; 1-ethyl-3-methylimidazolium perfluorobutanesulfonate, [EtMeIm][(PFBu)SO3], [>98% mass fraction purity, halides (ion chromatography) 99.8%, anion (ion chromatography) >99.9%]; 1ethyl-3-methylpyridinium perfluorobutanesulfonate, [EtMepy][(PFBu)SO3], [>99% mass fraction purity, halides (ion chromatography) 99.9%, anion (ion chromatography) >99.9%]; and (2-hydroxyethyl)trimethylammonium perfluorobutanesulfonate, [N1112OH][(PFBu)SO3] [>97% mass fraction purity, halides (ion chromatography) 97%, anion (ion chromatography) >97%] were provided by IoLiTec GmbH. The FIL structures and abbreviations are detailed in Scheme 1. The FILs were dried under vacuum (3 × 10−2 Torr) and stirred at 323.15 K for 2 days preceding their use. The water content for all FILs was ≤100 ppm (coulometric Karl Fischer titration analysis). The final purity of the FILs was verified by elemental analysis and 1H, 13C, and 19F NMR. Prior to use, each FIL was removed from the respective Schlenk flask with a syringe under an inert nitrogen flow. Milli-Q water (Milli-Q Integral water purification system) was used in all experiments throughout the work. Ionic Conductivity Measurements. The ionic conductivities were measured using a CDM210 Radiometer Analytical conductimeter, with a CDC749 electrode, in a glass cell containing a magnetic stirrer at 298.15 K. The glass cell was thermostatted using a water bath controlled to ±0.01 K, and the cell temperature was measured by implementing a platinum resistance thermometer coupled to a Keithley 199 system DMM/scanner, which was calibrated using high-accuracy mercury thermometers (0.01 K). Each FIL aqueous solution was added to the thermostatic cell and stirred for the ionic 1284

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Figure 1. Conductivity profile of FILs in aqueous solution at 298.15 K.

Figure 2. Concentration dependence of the ionic conductivity for [EtMeIm][(PFBu)SO3] in aqueous solution at 298.15 K at (a) the first critical aggregation concentration, (b) the second critical aggregation concentration, and (c) the third critical aggregation concentration. Dashed lines show the maximum change in the gradient of ionic conductivity as a function of FIL concentration, second derivative. conductivity measurements. The conductimeter was calibrated using certified 0.01 and 0.1 D KCl standard solutions supplied by Radiometer Analytical at each temperature. Every conductivity value was measured at least three times, and the uncertainty was estimated to be 1%. Isothermal Titration Calorimetry (ITC). Calorimetric titration was accomplished with a MicroCal ITC200 (GE Healthcare BioSciences AB, Sweden). The reference and sample cells were filled with Milli-Q water and Milli-Q water or FIL solution, respectively. The temperature was stabilized at 298.15 K. The instrument-controlled Hamilton syringe, with a 40 μL volume capacity, was filled with the FIL stock solution prepared in Milli-Q water, and 1 μL aliquots of FIL were added to the sample cell with continuous stirring (500−1000 rpm). The instrument software controlled both the time of addition

and the duration between each addition. The enthalpy change at each injection was measured and depicted against concentration using MicroCal Origin software. Each titration was performed three times, and the data analysis was performed using Microcal Origin software. The principles and basic thermodynamic protocol of ITC were addressed in an excellent review on the subject.24 Surface Tension Measurements. The surface tension was determined at 298.15 K with a Lauda TVT2 tensiometer by the hanging drop tensiometer method. The tank, where the measurements were carried out, was thermostatted in a Polyscience temperature controller with a temperature stability of ±0.01 K. This temperature controller was regulated with a D20KP Lauda thermostat with a resolution of ±0.01 K. The equipment has a control unit and a mechanical unit that are connected to a PC-controlled instrument for 1285

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Table 1. Critical Aggregation Concentrations, CAC, Ionization Degree, α, and Gibbs Free Energy of Aggregation, ΔG0agg, Determined by Conductometry at 298.15 K for Fluorinated Ionic Liquids in Aqueous Solutions first CAC

second CAC

third CAC

maximum a

wFIL mmol·kg−1 α ΔG0agg (kJ·mol−1) wFIL mmol·kg−1 α ΔG0agg (kJ·mol−1) wFIL mmol·kg−1 α ΔG0agg (kJ·mol−1) wFIL mmol·kg−1

[EtMeIm][(PFOc)SO3]a

[EtMeIm][(PFBu)SO3]

[EtMepy][(PFBu)SO3]

[N1112OH] [(PFBu)SO3]

0.0008 1.333 0.20 −47.4

0.0060 14.55 0.79 −24.7 0.0158 38.54 0.84 −20.8 0.0332 81.03 0.29 −27.5 0.6836 1667

0.0058 13.82 0.83 −24.0 0.0119 28.25 0.84 −21.8 0.0325 77.25 0.26 −28.2 0.6838 1623

0.0065 16.07 0.85 −23.3 0.0142 35.27 0.83 −21.4 0.0751 186.2 0.45 −21.6 0.5449 1351

Data of the second CAC, third CAC, and maximum are not included because their determination was impossible as a result of their high viscosity.

the precise measurement of liquid with an uncertainty of ±0.07 mN/ m. The radiuses of the needles were calibrated and credited by the supplier. The calibration was verified with pure liquids with known surface tensions. Each sample was measured three times, and the reported surface tension is the average value. Light Scattering Measurements. The dynamic light scattering (DLS) measurements were performed with a Zetasiser Nano Series ZEN3600 (Malvern, U.K.) with a 633 nm laser. A noninvasive backscattering technique (173°) was used for detection. All of the FIL aqueous solutions were prepared with Milli-Q water and filtered immediately prior to measurement using Millex PVDF filters with a pore diameter of 0.22 μm. All samples were mildly shaken during a period of 24 h for equilibration. The experimental conditions were optimized with the aim to maximize the acquisition signal for each sample, and at least 20 acquisitions were taken for each solution condition. The cells temperature was kept constant at 298.15 K. Each sample was measured three times; the reported result is the average value, and the uncertainty was estimated to be 4%. Transmission Electron Microscopy (TEM). A small amount of FIL aqueous solution was placed on a 200 mesh copper grid (3 mm diameter) with Formvar film. The excess sample was removed using filter paper. Samples were dried and TEM was performed using a Hitachi electron microscopy, H8100 model with LaB6 filament, at a working voltage of 200 kV. Viscosity and Density Measurements. The dynamic viscosity and density were measured using an automated SVM 3000 Anton Paar rotational Stabinger viscometer−densimeter in the temperature range between 298.15 and 318.15 K at atmospheric pressure. This equipment uses Peltier elements for fast and efficient thermostability. The uncertainty of the temperature is ±0.02 K, and that of the density is ±0.0005 g·cm−3. The precision of the dynamic viscosity measurements was ±0.5%. Each sample was measured in triplicate, and the reported result is the average value with a maximum relative standard deviation of 0.51%.

behavior can be divided into two distinct segments with the increment of FIL concentration, a conductivity increase followed by a decrease in conductivity. The increase in conductivity in dilute solutions is directly related to the increasing number of free ions in solutions. At higher FIL concentrations, a deviation from the expected linear behavior is observed because of the decrease in fluidity and the consequent reduction in the mobility of the charge carriers. In addition, there is a decrease in the number of charge carriers as a result of the increase in aggregate formation. The critical aggregation concentrations (CACs) can be determined using the first part of the plot (Figure 2 and Figures S1−S3 in the Supporting Information) depicting the conventional behavior and the steep change in the slope attributed to a CAC. The first CAC of the ionic liquids is analogous to the critical micelle concentration of surfactants. All of the perfluorobutanesulfonate-based FIL aqueous solutions present four linear segments in the conventional curves as depicted in Figure 2 and Figures S1−S3 of the Supporting Information, and the transitions are attributed to three critical aggregation concentrations. A variety of examples of surfactants with multiple critical aggregation concentrations can be found in the literature.27 Recently, the self-aggregation of ionic-liquid-based surfactants in water has been studied and has also presented three different critical aggregation concentrations.28,29 Precise values of CACs have been calculated using the Phillips definition, where the CAC is the value corresponding to the maximum change in the gradient of the ionic conductivity versus concentration curve.30 CACs have been calculated using the second derivative of the local polynomial fit. The CAC values determined with the ionic conductivity data together with the maximum in the conductivity plot are given in Table 1. Only in the case of [EtMeIm][(PFOc)SO3] was the experimental determination of the second CAC, third CAC, and maximum impossible because of the high viscosity of the aqueous solution (∼640 mPa·s at a fluorinated ionic liquid mass fraction (wFIL) of 0.058). This fact hindered reproducible experimental results for solutions with wFIL ≥ 0.058. From the analysis of the results, it can be concluded that a length increase in the fluorinated alkyl chain decreases the value of CAC as expected. Furthermore, it is clear that the ammonium cation presents higher CAC values than imidazolium or pyridinium cations and accordingly less surfactant power. Nonetheless, the

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RESULTS AND DISCUSSION Conductometric Titration. Measurements of ionic conductivity are generally implemented in the study of ionic micellar solutions. These measurements at 298.15 K for four different fluorinated ionic liquids are illustrated in Figure 1, where the experimental ionic conductivity is depicted against FIL concentration for the complete range of composition (from neat water to neat FIL). The behavior observed in Figure 1 is a result of distinct effects that bias ionic conductivity measurements. Similar behaviors have been obtained in the literature by adding an organic solvent25 or water26 to an ionic liquid. This 1286

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Figure 3. Calorimetric titration profiles of [EtMeIm][(PFBu)SO3] in aqueous solution at 298.15 K: (a, b) differential power (dp) against time and (c, d) enthalpy of dilution (dH) against FIL concentration. The first-order differential curves of the dilution enthalpic curve are depicted.

associated with counterions. The values of α are presented in Table 1, and a lower value indicates a better-packed micelle. The degree of counterion binding or fraction of counterions condensed on the aggregate interface, β, can be calculated from the degree of ionization as follows

change in the conductivity behavior, the maximum, happens at a lower FIL concentration. Similar results have been found for imidazolium and pyridinium cations. The CAC values for these FILs are similar to those of perfluorinated anionic surfactants having around seven to eight carbon atoms.7,8 The solubility in water of many of these fully fluorinated anionic surfactants, studied in the referred papers,7,8 is poor, and their CAC values have been determined at higher temperatures. However, the FILs studied in this work are completely miscible in water, and their CAC values have been determined at room temperature (298.15 K). The mutual solubilities of (perfluorobutanesulfonate-based FILs + water) binary systems are driven by the self-aggregation properties of the FILs, as demonstrated in a recent study.31 This behavior is in corroboration with the solubility in water of the potassium salts of perfluorobutanesulfonate and perfluorooctanesulfonate, 52.6−56.6 g/L (295.65−297.15 K)32 and 680 g/L (297.15−298.15 K),33 respectively, where the solubility increases with the increment of the self-aggregation properties. Taking advantage of the ionic liquid building blocks and their intrinsic properties, established on imidazolium, pyridinium and ammonium cations, the development of completely miscible fluorosurfactants based on the perfluorobutanesulfonate and perfluorooctanesulfonate anions has been attained. Following the conventional techniques used in the literature, the ratio of the slopes of the linear segments above and below the CAC has been used to determine the degree of ionization of the aggregates, α, that is related to the fraction of charges of surfactant ions in the micelle counterbalanced by micelle-bound counterions. Monomers of ionic surfactants are completely dissociated in solution, but in their aggregates they are partially

β=1−α

(1)

The values of β are related to the charge density at the aggregate surface, the size of the aggregate, and the hydrophobic nature of the counterion. Ionic liquids with long chains ([EtMeIm][(PFOc)SO3]) favor bulkier aggregate structures, compared to shorter-chained ionic liquids ([EtMeIm][(PFBu)SO3]). In the case of long chains, [EtMeIm][(PFOc)SO3], the number of ions per aggregate and the volume per surfactant ion are higher. Thus, the polar headgroups are packed more closely and are neutralized by a larger fraction of counterions. The outcome is a higher β and smaller α values. However, when the ionic liquid concentration increases, from the second CAC to the third CAC in [EtMeIm][(PFBu)SO3], [EtMepy][(PFBu)SO3], and [N1112OH][(PFBu)SO3], the packing of the polar headgroups becomes tighter, triggering a change in the shape of the aggregate and also higher β and smaller α values (Table 1). On the basis of the pseudophase model of micellization,34 the standard Gibbs free energy of aggregation, ΔG0agg, has been determined for ionic surfactants using the equation 0 ΔGagg = RT (1 + β)ln xCAC

(2)

where R and T are the universal gas constant and absolute temperature, respectively, and xCAC is the critical aggregation concentration defined in mole fraction. All of the calculated 1287

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Table 2. Critical Aggregation Concentration, CAC, Thermodynamic Properties of Aggregation, Surface Properties, and Micellization Parameters of [EtMeIm][(PFBu)SO3] in Aqueous Solution Determined by ITC and Surface Tension at 298.15 K Isothermal Titration Calorimetry (ITC) CAC (mmol·kg−1)

ΔH0agg (kJ·mol−1)

−TΔS0agg (kJ·mol−1)a

14.44 34.58 76.76 106.4

0.385 0.765 −0.332 −0.760

−25.1 −21.6 −27.2

first CAC second CAC third CAC fourth CAC

Surface Tension CAC (mmol·kg−1) γCAC (mN·m−1) 106 Γmax (mol·m−2) Amim (nm2) ΠCAC (mN·m−1) ΔG0ad (kJ·mol−1) a

91.39 24.5 2.60 0.64 44.4 −44.5

Data calculated using ΔG0agg from Table 1 (conductometric titration).

changes upon aggregation, ΔH0agg, for [EtMeIm][(PFBu)SO3] at 298.15 K. This ionic liquid has been chosen as an example because FILs based on the perfluorobutanesulfonate anion present similar behavior with the increment in FIL concentration (Figure 1 and Figures S1−S3 in the Supporting Information). Figure 3a,b;c,d depicts the variation of the differential power (dP) and differential enthalpy (dH) against the concentration of [EtMeIm][(PFBu)SO3], respectively. Differential power data with time display exothermic changes before and after the first CAC (the last injections of Figure 3a start to show endothermic changes), whereas only endothermic changes are displayed for the determination of the other CACs (Figure 3b) throughout the dilution process. After aggregation, the formation of aggregates takes place alongside the dilution of aggregates. These exothermic changes (Figure 3a) have been ascribed to the preeminence of enthalpy changes occurring due to the dilution of the formed aggregates as compared to the enthalpy changes occurring due to the aggregation. Calorimetric titration plots (Figure 3c,d enthalpograms) have been classified into three types, A−C, taking into account the shape of the plot of the enthalpy of dilution against surfactant concentration at a particular temperature.43 The calorimetric titration plots corresponding to the two first aggregations (first and second CACs) of [EtMeIm][(PFBu)SO3] at 298.15 K exhibit type B enthalpograms, wherein positive slope changes in dH data are observed before each CAC and thereafter dH acquires a slight plateau region (Figure 3c,d). The calorimetric profile for the fourth CAC also follows a type B enthalpogram, albeit here a distinct plateau region before aggregation followed by a continuous exponential decay in enthalpy values is observed against concentration (Figure 3d). Despite the enthalpogram exhibited in the third CAC ending in a marked plateau region, it is analogous to a type C enthalpogram, wherein an exponential growth followed by an exponential decay in dH is displayed, and the maximum corresponds to CAC. Table 2 details the CAC values obtained by ITC for [EtMeIm][(PFBu)SO3], which are in corroboration with the values determined from conductivity and surface tension measurements (Tables 1 and 2, respectively) within the limits of the sensitivity of each implemented technique. Each CAC is determined according to the Philips criterion,30 i.e., zero of the second derivative of the differential enthalpy against total FIL concentration. The first-order differential curves of the ITC enthalpograms are depicted in Figure 3c,d to indicate each

ΔG0agg values are shown in Table 1. This thermodynamic parameter accounts for the free energy implicated in transferring one mole of surfactant from the aqueous solution to the micellar pseudophase34 and indicates the free-energy difference per mole between FIL monomers in aqueous solution and in aggregates. Negative free energy indicates the spontaneous aggregation of surfactants. As expected, ΔG0agg is more negative for the FILs having larger counterions (higher β and smaller α values). Furthermore, the values of ΔG0agg become larger with the growth of the hydrophobic tail (fluorinated chain) length. The increase in the fluorinated chain length favors the formation of aggregates resulting from the increment of the interaction between the fluorinated alkyl chains. Isothermal Titration Calorimetry. Isothermal titration calorimetry (ITC) is a very useful technique because it provides quantitative thermodynamic information in a single experiment. Furthermore, ITC is becoming a powerful technique for understanding the self-organization of surfactants into aggregates because it is label-free and relies only on measuring the heat evolved during the molecular interaction.35 ITC allows the determination of the CAC and the enthalpy of aggregation (ΔH0agg) of a surfactant in a single experiment without the use of any probe. Several techniques have been used in the literature to study the self-aggregation phenomena of ionic liquid aqueous solutions comprising conductivity, surface tension, fluorescence and solvatochromic probe methods, 1H NMR, small-angle neutron scattering, isothermal titration calorimetry, and density.36 Tariq et al. determined by ITC the critical micellar concentration and the enthalpy of micellization of dialkylpyrrolidinium bromide ILs in water at three temperatures (288.15, 298.15, and 323.15 K).36 Bai et al.37 reported microcalorimetric results for alkylimidazolium surfactant ionic liquids at 308.15 K. Geng et al.38 also studied the self-aggregation of long-chained ionic liquids using ITC at 298.15 K. Heintz et al.39 and Galgano and El Seoud40 developed a calorimetric study of the self-aggregation behavior of ionic liquid aqueous solutions with diverse cations/anions. Besides self-aggregation monitoring, ITC has also been implemented in the study of interactions of ionic liquids with distinct materials, such as the interactions between imidazolium-based ILs and neutral polymer agarose chains in various concentration regimes41 and between β-casein micelles and imidazolium-based ILs with different hydrophobicities.42 Isothermal calorimetric titration has been performed to measure the critical aggregation concentration and the enthalpy 1288

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Langmuir CAC. The observed ΔH0agg is found to be positive for the first and second CACs, whereas it is negative for the third and fourth CACs. The calculated values of ΔG0agg (conductometry, Table 1) and ΔHagg0 (ITC, Table 2) have been used to 0 determine the standard entropy of aggregation (ΔSagg ), reported in Table 2, from the Gibbs−Helmholtz equation: 0 0 0 ΔGagg = ΔHagg − T ΔSagg

minimum around the breakpoint. The breakpoint of the two parts is denoted as the CAC point, and the corresponding surface tension is denoted as γCAC. The CAC and γCAC values are reported in Table 2. The value of γCAC can be considered to be the measure of efficiency of the FIL to occupy the air/water interface. The value of surface tension at the CAC is similar to those measured in studies of fluorinated sulfonic salt surfactants.7,8 The surface method is a versatile tool because it simultaneously estimates the CAC value and gives crucial information about the adsorption characteristics of solutes at the air/water interface.46 For the adsorption of [EtMeIm][(PFBu)SO3] at the air/water interface, the maximum surface excess concentration, Γmax, can be determined by taking into account the Gibbs adsorption isotherm47 as follows

(3)

The dynamic nature of the aggregation process and the counterion binding of aggregates are considered to influence the ΔH0agg of the process. Neither of these effects is rigorously accounted for in the conductivity data treatment. However, the direct determination of ΔH0agg by calorimetry includes the consequences of the above effects in the measurements. ITC and conductometric titration measurements reveal that, as for the imidazolium- and pyrrolidiunium-based surfactant ILs36−38 and conventional surfactants,44 the aggregation process for the studied FIL, [EtMeIm][(PFBu)SO3], is controlled by entropy at 298.15 K (as illustrated in Figure S4 of the Supporting Information). Surface Properties and Critical Packing Parameter. Surface tension has been measured to evaluate the surface activities of the aqueous solutions of FILs. The increment in FIL concentration originates in the accumulation of molecules at the air/water interface avoiding unfavorable contact between fluorinated chains and water. The free energy of an amphiphilic molecule solubilized in the bulk phase is greater than that of a molecule located at the interface.45 Then, adsorption at the interface prompts the diminishing of the surface energy. When the air/water interface becomes saturated, dissolution of the FIL ions in water takes place with the increment in FIL concentration leading to the increment in the number of unfavorable fluorinated alkyl chains in water and consequently an increase in free energy of the system. After the CAC, FIL ions assemble to form aggregates with the fluorinated alkyl chains oriented toward the aggregates’ interior. These aggregates are not surface-active, maintaining the solution surface tension at an approximately constant value after the CAC.45 Figure 4 depicts the surface tension measurements as a function of [EtMeIm][(PFBu)SO3] concentration at 298.15 K. Discontinuities in the previous first and second CACs are not observed; only one discontinuity at an FIL concentration similar to those of the third CAC (conductometric titration) and the third to fourth CAC (ITC) is detected. The high purity of the studied FILs is demonstrated by the absence of a

Γmax = −

⎛ dγ ⎞ 1 ⎜ ⎟ 2.303nRT ⎝ d log C ⎠

T

(4)

where R is the universal gas constant, T is the absolute temperature, C is the concentration of FIL, and n is the number of species in solution. The concentration at the interface depends on the surfactant concentration. The n parameter is taken to be n = 2 − α (degree of ionization of the aggregates and taken to be the same for the surface layer) for the ionic surfactants.7,47 The minimum area occupied per surfactant molecule at the air/water interface, Amim, can be determined as follows A min =

1018 NA Γmax

(5)

where NA is Avogadro’s number. Both Γmax and Amim values are listed in Table 2. As reported,48 larger Γmax or smaller Amin signifies a denser arrangement of surfactant molecules at the solution surface. Furthermore, the structure of the hydrophilic headgroup of surfactant molecules is a primary factor in the determination of Γmax and Amim values. The effectiveness of the surface tension reduction, ΠCAC, is directly related to the capacity of a surfactant to decrease the surface tension of solutions. Furthermore, this parameter expresses the maximum reduction in surface tension that occurs by the dissolution of surfactant molecules. This parameter can be determined as follows

ΠCAC = γ0 − γCAC

(6)

where γ0 and γCAC are the surface tension of water and the surface tension of the solution at the CAC, respectively. Besides, the standard free energy of adsorption, ΔG0ad, has been calculated using46,49 0 0 ΔGad = ΔGagg −

ΠCAC Γmax

(7)

The results of this analysis are reported in Table 2. The ΔG0ad value is greater than the ΔG0agg value, indicating that the process of adsorption of surfactant at the air/water interface is more spontaneous than the process of aggregation. Its ratio is 1.6, suggesting a 1.6-fold efficacy of the adsorption process over that of the aggregation process. The critical packing parameter, P, is an important factor in understanding the self-aggregation process, shape of aggregates, for a surfactant. According to Israelachvili et al.,50 the structure

Figure 4. Surface tension isotherm at 298.15 K vs [EtMeIm][(PFBu)SO3] aqueous solution concentration. 1289

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Figure 5. Effect of the variation of FIL concentration on the size distribution of aggregates of (a) [EtMeIm][(PFBu)SO3] and (b) [EtMepy][(PFBu)SO3] in aqueous media from DLS measurements of micellar solutions at 298.15 K. V0(nm 3) = 0.0424 + 0.0416(nc − 1)

of the aggregate can be predicted from the packing parameter, defined as follows

P=

(10)

where nc is the number of carbon atoms in the fluorocarbon alkyl chain. Taking into account these equations, the P values are 0.439 (from eq 9) and 0.442 (from eq 10). However, another approximation has been considered in the literature with the aim of obtaining the packing parameter for fluorinated compounds.55 The fluorinated atom is significantly larger than the hydrogen atom. Then, the perfluorinated chains are bulkier than hydrogenated chains with cross sections (V0/lc) of ∼30 and ∼20 Å, respectively. Taking into account this approximation, the P value is 0.471. The P values for [EtMeIm][(PFBu)SO3] in the transition determined by surface tension are close to 0.5, which is characteristic of cylindrical or lamellar micelles (Figure 2c). It has been observed by TEM images, as discussed in the following section, that the possible mechanism of this phase transition corresponds to a micellar shape change from globular micelles to cylindrical or lamellar micelles. These structures are stable in fluorinated surfactants because the fluorinated chains are bulkier than the hydrogenated ones. To form stable spherical aggregates, fluorosurfactants need larger headgroups in order to balance the effect of the bulky perfluoroalkylated tail.52

V0 A min lc

(8)

where V0 is the effective volume occupied by hydrophobic chains in the aggregate core, considered to be an incompressible fluid, lc is the maximum effective length, also denoted as the critical chain length, and Amin is taken as the effective headgroup surface area occupied by the hydrophilic group at the aggregate/solution interface.11,29,46,47 With the increment in P values, the structure of aggregates can be spherical when P ≤ 0.33, cylindrical when P ≤ 0.5, lamellar when P ≤ 1, or inverted when P > 1. In this work, this parameter has been used with the aim of obtaining the structure of the transition ascertained by surface tension in the [EtMeIm][(PFBu)SO3] aqueous system and predicts the possible mechanism of this phase transition. The volume V0 and the length lc have been obtained from a modification of Tanford equations51 to fluorinated compounds that can be calculated from the following equations52−54 V0(nm 3) = 0.0545 + 0.0380(nc − 1)

lc(nm) = 0.204 + 0.130(nc − 1)

lc(nm) = 0.200 + 0.134(nc − 1)

(9)

or 1290

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Figure 6. TEM images of [EtMeIm][(PFBu)SO3] aggregates in aqueous solution at FIL concentrations of (a, b) 2.5 times that of the first CAC and (c, d) 2 times to that of the second CAC.

Characterization of Self-Assembled Structures. Dynamic light scattering (DLS) is useful for studying the properties of colloids, macromolecules, and polymers because it is an absolute, noninvasive, and nondestructive technique. Actually, this technique can determine surfactant mutual diffusion coefficients, including contributions from both aggregates and relatively mobile free surfactant molecules.56 The aggregate size is directly related to rheological parameters such as the viscosity of micellar solutions, which are relevant to many surfactant applications.57 Self-assembled structures of aqueous solutions of FILs have been analyzed at a concentration 2 times that of first and second CACs, 3 times that of the third CAC (CAC values determined by conductometry, reported in Table 1), and before and after the maximum and at the maximum (as the conductivity profile depicted in Figure 1 and reported in Table 1). The hydrodynamic diameter, Dh, of the aggregated structures measured from DLS for [EtMeIm][(PFBu)SO3] and [EtMepy][(PFBu)SO3] is shown in Figure 5. DLS results, detailed in Table S1 of the Supporting Information, point out the formation of micelles in water of [EtMeIm][(PFBu)SO3], [EtMepy][(PFBu)SO3], and [N1112OH][(PFBu)SO3] at a concentration 3 times that of the third CAC and at higher concentrations (before and after the maximum and at the maximum in conductivity measurements), with Dh in the range of 2.3−7.5 nm. Aqueous solutions of all FILs at concentrations 2 times that of the first and second CACs form aggregates of

micelles with Dh in the range of 110−198 nm, as detailed in Table S1 of the Supporting Information. The Dh values for [N1112OH][(PFBu)SO3] are always smaller than the ones for [EtMeIm][(PFBu)SO3] and [EtMepy][(PFBu)SO3] at concentrations 3 times that of the third CAC and higher. The difference in terms of size between the perfluorobutanesulfonate-based ILs can be attributed to the higher sphericity of the [N1112OH]+ cation compared to that of both [EtMeIm]+ and [EtMepy]+ cations, allowing a higher compaction of the counterions at the micellar interface. The width reported in Table S1 in the Supporting Information is related to the absolute polydispersity, and its increment corresponds to monomodal solutions (a single peak) that are very polydisperse on account of different aggregate sizes. Additionally, the Dh values at concentrations 2 times that of the first and second CAC are larger than those usually obtained for micelles of ionic liquids (ranging from 4 to 20 nm),56 supporting the hypothesis of the formation of aggregates of micelles. Furthermore, TEM images (Figure 6) also corroborate the formation of micelles and aggregates of micelles. To characterize the molecular aggregates of FIL in solutions, TEM measurements have also been performed. [EtMeIm][(PFBu)SO3] has been chosen as an example of the characterization of the different types of aggregates because all FILs present similar behavior with the increment of FIL concentration. Figure 6 displays the self-assembled structures of [EtMeIm][(PFBu)SO3] that have been imaged through this 1291

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Figure 7. Walden plots for binary systems ([EtMeIm][(PFBu)SO3] (red symbols) + water and ([EtMepy][(PFBu)SO3] (blue symbols) + water) against FIL concentration at 298.15 K: (a) classification based on the classical Walden concept for ionic liquids and (b) Walden plots for the binary systems.

conductivity behavior occurs, as discussed in the Conductometric Titration section. Figure 7a illustrates the Walden plot in the classification suggested by Angell et al. for “good” and “poor” ionic liquids. The straight black line represents the “ideal” Walden line that corresponds to the behavior of an ideal electrolyte (aqueous KCl solution; system fully dissociated and with ions of equal mobility).58,59 The experimental results obtained for the binary [EtMeIm][(PFBu)SO3] and [EtMepy][(PFBu)SO3] + water systems are plotted in Figure 7. The Walden plot demonstrates equal ionicity (equal vertical distance to the ideal electrolyte line) with the increment of water concentration in the (FIL + water) binary mixture, for both FILs, from a pure ionic liquid to the maximum value of ionic conductivity shown in Table 1 where the behavior reverses (Figure 7b) and increasing the concentration of water leads to a binary mixture farther from the ideal electrolyte (straight line). These results validate the formation of the above-mentioned distinct aggregates (spherical micelles, rodlike micelles, etc.).

technique. TEM micrograhs show different aggregates that corroborate the size distribution detail from DLS measurements. At a concentration 2.5 times that of the first CAC (CAC values determined by conductometry, reported in Table 1), aggregates of spherical micelles are observed (Figures 6a,b and 2a). The TEM images also demonstrate that the transition between the first CAC and the second CAC corresponds to micelles growing from spherical to globular (Figures 6c,d and 2b). The third CAC shown in Figure 2c and samples with higher FIL concentration were impossible to analyze with the TEM studies. (This phase transition has been characterized using surface tension measurements and the critical packing parameter, as discussed above.) The Walden plot establishes a relationship between the molar conductivity (calculated from density and ionic conductivity values; check Table S2 and Figure S5 in the Supporting Information) and the fluidity (calculated from dynamic viscosity values; check Table S2 and Figure S6 in the Supporting Information) of a solution and is a functional tool for measuring the ionicity of ionic liquids.58,59 The conductivity and fluidity properties are directly dependent on the formation of ion pairs or larger aggregates (charged or noncharged) or the existence of ionic networks. For example, the structural changes occurring in the mixtures of ionic liquids with inorganic salts have been probed using this versatile tool.60 In this work, we have used the Walden plot to study the behavior of concentrated FIL solutions, from neat FIL to the maximum in the conductivity profile (Figure 1), where a change in

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CONCLUSIONS This work investigates the total miscibility and self-aggregation behavior in water of novel anionic surfactant ionic liquids with fluorinated chains. The characterization of the aqueous solutions of these FILs has been performed using conductometric titration, ITC, tensiometry, Walden plot, DLS and TEM techniques that enable the locatation of the phase transitions and the characterization of the self-assembled structures. The 1292

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Langmuir CAC values obtained by the different techniques are in corroboration, allowing an exact identification of the phase transitions of the aqueous solutions of FILs. The increment in the anion fluorinated chain length for imidazolium-based FILs decreases the CAC values, and for the FILs based on the perfluorobutanesulfonate anion, the ammonium-based presents less surfactant power than the imidazolium- or pyridiniumbased FILs. Furthermore, the experimental results also permit us to study various thermodynamic and surface properties. Aggregation parameters have been calculated by conductimetry, calorimetry, and surface tension measurements. From the analysis of the results, the standard Gibbs free energy of aggregation is similar for all of the FILs based on the perfluorobutanesulfonate anion ([EtMeIm]+, [EtMepy]+, and [N1112OH]+ cations) and is more negative with the increment of the anion fluorinated chain length ([(PFBu)SO3]− and [(PFOc)SO3]−) for imidazolium-based FILs. Additionally, the analysis of the standard Gibbs free energy and enthalpy of aggregation for [EtMeIm][(PFBu)SO3] demonstrates that the aggregation process is mainly entropic. Also, the standard free energies of adsorption calculated from surface tension data indicate that the adsorption process is more spontaneous than the aggregation process. The self-assembled structures of FILs in an aqueous medium have been analyzed by DLS, TEM, Walden plot, and the critical packing parameter (calculated from surface properties). Perfluoroanionic amphiphilic ionic liquids form distinct aggregated structures depending on the total concentration in aqueous solution. The possible mechanism of the phase transitions identified in this work corresponds to a micellar shape change from monomers to spherical micelles (first transition), from spherical micelles to globular micelles (second transition), and from globular micelles to cylindrical or lamellar micelles (third transition). The stable self-assembled structures found in the aqueous medium may justify the completely water solubility of these novel fluorinated ionic liquids.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from FCT/MCTES (Portugal), through grants SFRH/BPD/84433/2012 (A.B.P.) and SFRH/BPD/ 65981/2009 (J.M.M.A.) and through projects PTDC/EQUFTT/118800/2010 and PEst-OE/EQB/LA0004/2013, is gratefully acknowledged. The NMR spectrometers are part of The National NMR Facility, supported by FCT/MEC (RECI/ BBB-BQB/0230/2012).

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ASSOCIATED CONTENT

S Supporting Information *

Concentration dependence of the conductivity at 298.15 K for [EtMeIm][(PFOc)SO 3 ], [EtMepy][(PFBu)SO 3 ], and [N1112OH][(PFBu)SO3] in aqueous solution. Thermodynamic properties of aggregation at 298.15 K for [EtMeIm][(PFBu)SO3] in aqueous solutions. Parameters obtained from DLS measurements for all FILs. Viscosity, density, and ionic conductivity for binary systems [EtMeIm][(PFBu)SO3] + water and [EtMepy][(PFBu)SO3] + water at several temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +351 214411277. Tel: +351 214469414. *E-mail: [email protected]. Fax: +351 214411277. Tel: +351 214469441. Author Contributions

A.B.P. and J.M.M.A. contributed equally. Notes

The authors declare no competing financial interest. 1293

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