Interaction of Nonionic Surfactants and Hydrophilic ... - ACS Publications

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Interaction of Nonionic Surfactants and Hydrophilic Ionic Liquids in Aqueous Solutions: Can Short Ionic Liquids Be More Than a Solvent? Francesc Comelles,* Isabel Ribosa, Juan José González, and Maria Teresa Garcia Department of Chemical and Surfactants Technology, Institute for Advanced Chemistry of Catalonia (IQAC−CSIC), Barcelona, Spain S Supporting Information *

ABSTRACT: The interaction between an ethoxylated nonionic surfactant (C12−14EO8) and three conventional hydrophilic imidazolium-based ionic liquids (bmim-octyl SO4, bmim-methyl SO4, and bmim-BF4) in aqueous solution has been investigated. In most of the reported studies where a surfactant is dissolved in an ionic liquid aqueous solution, conventional ionic liquids are merely considered to be solvents. Consequently, the resulting critical micelle concentration (cmc) is considered to be that of the surfactant. However, given that the three ionic liquids selected showed the typical shape of a surfaceactive compound when the surface tension was plotted against concentration, the role of these compounds as secondary surfactants and consequently the possibility of mixed-micelle formation have been investigated. Different series of experiments where a surfactant and an ionic liquid were combined in a wide range of mole ratios have been performed and treated as typical binary surfactant systems in aqueous solution. It has been found for the three surfactant/ionic liquid systems that depending on the surfactant mole fraction, α1, attractive or repulsive interactions in mixed-micelle formation are produced. Therefore, when we select the appropriate α1 these systems can be adjusted to a given application, depending on whether monomers or micelles are mainly required. ionic liquid solutions.10−16 That is an interesting case involving three components: surfactant, ionic liquid, and water. In the reported studies, the attention is focused on the surfactant, considering that the new cmc obtained is a consequence of the IL effect on the surfactant micellization. The cmc dependence on the ionic liquid concentration in aqueous solution has been described. For very low IL concentrations, the cmc decreases whereas the opposite occurs when the IL concentration increases.11,12 Therefore, the presence of water in systems simultaneously containing surfactant and an ionic liquid seems to be critical, probably as a result of the behavior of each component in water. The ionic liquid deserves special attention beyond its expected function as a cosolvent. It has been reported that for certain common short-alkyl-chain ionic liquids the process of aggregation in aqueous solution can also take place.17−26 For this reason, when a surfactant is dissolved in an aqueous solution containing some of these ILs its possible role as a secondary surfactant should be considered. We claim that these systems should be treated as typical binary surfactant mixtures in aqueous solution, and the obtained cmc values for these systems would not correspond solely to the value of the surfactant (modified by the presence of the IL) but to the mixed micelles of surfactant and ionic liquid. Bearing in mind

1. INTRODUCTION In the last few years, the chemical and physical properties of ionic liquids (ILs) have attracted interest in these compounds with respect to many applications such as organic synthesis, catalysis, chemical separation, electrochemistry, photochemistry, and nanomaterials. One of the most outstanding properties of these compounds is their very low volatility that prevents environmental air pollution and consequently makes them candidates to substitute for volatile organic solvents in different application fields. Accordingly, with their claimed role of green solvents, a wide series of studies where different kinds of conventional surfactants are dissolved in ionic liquids have been reported.1−16 In some of these studies, the surfactant is dissolved in the ionic liquid as a unique solvent.1−9 In that case, it has been found that surfactants aggregate in IL media in a way analogous to that in water, but with two important differences. First, the surface tension of pure ILs (∼35−48 mN/m) is clearly lower than the surface tension of water (∼72 mN/m); consequently, the diminution of surface tension from the neat ionic liquid to the equilibrium value (γcmc) is lower than if the surfactant is dissolved in neat water. The other difference is that the cmc values in ionic liquid are clearly higher than those in water. This fact is attributed to the lower solvophobic interaction between the surfactant alkyl chains in the IL with respect to the hydrophobic interactions between these surfactant alkyl chains in pure water.4−6 In other kind of studies, conventional surfactants were dissolved in aqueous © 2012 American Chemical Society

Received: June 20, 2012 Revised: August 10, 2012 Published: September 21, 2012 14522

dx.doi.org/10.1021/la302501j | Langmuir 2012, 28, 14522−14530

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3. RESULTS AND DISCUSSION 3.1. Surface-Active Properties of the Individual Components. Whereas for long-alkyl-chain ionic liquids the surfactant behavior has been widely reported,34−42 conventional short-chain ILs are mainly considered to be solvents. Consequently, there are several published data about the physical and chemical parameters of these compounds, such as the viscosity, density, electrical conductivity, flash point, and refractive index.43−47 In some of these publications, the influence of a certain water content on different properties such as the density, viscosity, and surface tension is considered, but in general without taking into account the possible surfactant behavior. To evaluate the potential surfactant activity of the ionic liquids selected in this study, the variation of the surface tension against the logarithm of the concentration was determined. In Figure 1, the obtained graphs for all of the

this possibility, in the present work binary systems consisting of a conventional ethoxylated nonionic surfactant such as C12−14EO8 and a common hydrophilic ionic liquid (bmimoctyl SO4, bmim-methyl SO4, or bmim-BF4) in aqueous solution have been investigated. In this study, performed by means of surface tension measurements, binary aqueous solutions containing surfactant and an ionic liquid in different mole ratios have been considered as typical binary surfactant systems in which mixed micelle formation is expected. This approach can contribute to a better understanding of the interaction between surfactants and ionic liquids in aqueous solution and to a new interpretation of the results reported in the literature when only the role of solvent is considered for the ionic liquid.

2. EXPERIMENTAL SECTION 2.1. Materials. Nonionic surfactant Dehydol PLS 8 corresponding to C12−14EO8 was obtained from Cognis. Ionic liquids bmim-octyl SO4, bmim-methyl SO4, and bmim-BF4 were purchased from Fluka and used as received without further purification. Deionized water was obtained from a Milli-Q device. 2.2. Sample Preparation. In the first series of experiments, aqueous ionic liquid solutions at constant concentrations were prepared and used as surfactant dilution media. From an initial surfactant concentration and through successive dilutions with this ionic liquid solution, decreasing concentrations of surfactant were obtained whereas the ionic liquid concentration remained constant. Consequently, the C12−14EO8 mole fraction α1 in the mixture decreases progressively through dilution. In the second series of experiments, aqueous C12−14EO8 solutions at constant concentrations were prepared and used as ionic liquid dilution media. In this case, α1 increases when the ionic liquid is progressively diluted. In the third series of aqueous binary systems, the nonionic surfactant and ionic liquid were combined together in different mole ratios. Each initial concentrated solution (total concentration including the two components) was diluted progressively with water. Obviously in these cases, the C12−14EO8 mole fraction α1 is maintained constant through the dilution. 2.3. Surface Tension Measurements. The surface tension of each solution was measured with a Krüss K12 tensiometer by the Wilhelmy plate method at 25 °C. The glass containers and the plate were cleaned in a sulfuric acid−chromic acid mixture and rinsed thoroughly with distilled water. The plate was flame dried before each measurement. The surface tension was considered to be at equilibrium when the standard deviation of five consecutive determinations did not exceed 0.10 mN/m. From the plots of surface tension versus the logarithm of the compound concentration, the intersection between the line corresponding to the linear decrease in the surface tension and the line of stabilized surface tension was taken as the critical micelle concentration (cmc). The effectiveness of the surface tension reduction (γcmc) and the surfactant efficiency given by the parameter pC20 (the negative of the logarithm of the necessary concentration to decrease the surface tension of pure water by 20 units, i.e., 52 mN/m) were also determined from the graph. However, although at present the validity of the Gibbs analysis is under discussion27−33 this widely used equation was applied to determine the maximum surface excess concentration: Γm = −(Δγ/Δ log C)/2.303nRT, with Δγ/Δ log C being the slope of the straight line of the graph before the cmc and n being the number of molecular species in solution (n = 1 for nonionic surfactants and n = 2 for ionic surfactants and n ranging from 1 to 2 in the mixtures). The minimum area per molecule adsorbed at the saturated water/air interface (Am) expressed in Å2 was obtained from Am = 1016/NAΓm, where NA is Avogadro’s number and Γm is the maximum surface excess concentration expressed in mol·cm−2.

Figure 1. Surface tension against the logarithm of concentration for the nonionic surfactant and the three ionic liquids.

compounds, including the nonionic surfactant, are represented. In addition to the surfactant behavior expected for C12−14EO8, similar curves were observed for the ionic liquids denoting that a certain aggregation is produced. In Table 1, the surfactant parameters obtained for each of these compounds are shown. Table 1. Surfactant Parameters Obtained Experimentally for the Considered Compounds compound C12−14EO 8 bmim-octyl SO4 bmim-methyl SO4 bmim-BF4

cmc (mM)

γcmc (mN/m)

pC20

Γm × 10−10 (mol·cm−2)

Am (Å2)

0.023 30

28.4 31.5

6.15 2.56

3.282 1.870

51 89

350

45.0

0.86

1.600

104

850

35.0

0.87

1.903

87

For bmim-octyl SO4, the surfactant activity has also been reported in the literature with cmc values of around 30 mM17−19,22 that can be understood from the relatively long alkyl chain in the anion. For bmim-BF4 and bmim-methyl SO4, the explanation of their surfactant-like behavior is not very clear considering their molecular structures. For bmim-BF4, among the numerous published data19−26 some studies seem to confirm our results, reporting cmc values of about 800−1000 mM.19−22 However, for bmim-methyl SO4 there are discrep14523


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Table 2. Values of cmc for C12−14EO8 Dissolved in Ionic Liquid Aqueous Solutions and Its Mole Fractions α1 at the cmc nonionic C12−14EO8 dissolved in aqueous solutions of ionic liquid dilution media

bmim-octyl SO4

bmim-methyl SO4

bmim-BF4

IL conc (mM)

γ (mN/m)

cmc (mM)

C12−14EO8 α1

cmc (mM)

C12−14EO8 α1

cmc (mM)

C12−14EO8 α1

0 (water) 0.70 1.70 40 50 80 100

72 60 55 60 60 55 55

0.023 0.029 0.033

1 3.98 × 10−2 1.85 × 10−2

0.023

1

0.023

1

0.060

1.50 × 10−3 0.036

7.19 × 10−4

0.073

7.29 × 10−4

0.080

ancies in the literature concerning the formation or absence of aggregates and in the corresponding cmc values.21,22 Our cmc of about 350 mM was also confirmed by conductivity measurements. (See conductivity vs concentration graphs for the three ILs in the Supporting Information.) This relatively high value seems to be logical in view of the well-established value of around 30 mM for bmim-octyl SO4. 3.2. Systems with C12−14EO8 Dissolved in Aqueous Solutions of Ionic Liquids. In a first series of experiments, we reproduced the conventional case of a surfactant dissolved in aqueous media containing an ionic liquid at a constant concentration. The ionic liquid aqueous solutions used as dilution media were those with the appropriate IL concentrations to give surface tensions of 55 and 60 mN/m. These relatively high values of the surface tension were selected to have a wide enough range to detect changes from the minimum γcmc of C12−14EO8 (about 28 mN/m) up to the limiting γ of the dilution media. Although in these cases ionic liquids are considered to be only aqueous cosolvents, it would be interesting to consider the ratios between surfactant and ionic liquid in the aqueous solutions. It must be kept in mind that when the ionic liquid concentration is constant the surfactant mole fraction α1 decreases continuously through dilution. Table 2 shows the cmc values for C12−14EO8 in pure water and in the different ionic liquid solutions together with the corresponding α1 mole fractions calculated at the cmc. The very low α1 values obtained are a consequence of the predominant ionic liquid concentrations in these binary solutions with respect to those of C12−14EO8 around the cmc. In Table 2, it can be seen that the presence of an ionic liquid leads to higher cmc values for C12−14EO8 with respect to those in pure water. These values increase with the ionic liquid concentration in the aqueous phase, confirming the behavior reported in the literature. The usual explanation is that the solvophobic interaction between the surfactant alkyl chains in the ionic liquid is lower than the hydrophobic interaction between the surfactant alkyl chains in water. In Figure 2, to avoid overlapping, only the case of C12−14EO8 dissolved in water or in a 0.70 mM bmim-octyl SO4 aqueous solution (γ = 60 mN/m) is shown as an example of the data of Table 2. (The other systems are reported in the Supporting Information.) The C12−14EO8 cmc increase with respect to the cmc in water is attributed to the presence of an ionic liquid in the aqueous medium. In these systems, it is generally assumed that the modified cmc values correspond only to the surfactant. However, taking into account the surfactant-like behavior of the ionic liquids displayed in Figure 1, we should revise this assumption. For this purpose, a second set of experiments were designed where the ionic liquid was diluted with an aqueous

7.99 × 10

−4

Figure 2. Surface tension vs logarithm of the C12−14EO8 concentration in water and in 0.70 mM bmim-octyl SO4 (γ = 60 mN/m).

solution containing the nonionic surfactant at a constant concentration. 3.3. Systems with Ionic Liquids Dissolved in C12−14EO8 Aqueous Solutions. In a new series of experiments, the ionic liquids were diluted with aqueous solutions of C12−14EO8 at constant concentrations. The selected C12−14EO8 concentrations are those giving surface tensions of 45, 50, and 55 mN/m in a water solution. The interest in these experiments lies in the fact that if only the nonionic surfactant is responsible for the surface activity then by keeping this compound at a constant concentration no changes in surface tension would be expected through ionic liquid dilution. However, when Δγ/Δ log C for the ionic liquids was plotted, typical graphs of surface activity appeared. Obviously, these surfactant profiles must be attributed to the ionic liquids. Furthermore, the fact that these cmc values are lower than those of the ionic liquids in pure water suggests the possibility of mixed-micelle formation between the nonionic surfactant and the ionic liquid. In Table 3, the cmc values of ILs in pure water and in C12−14EO8 solutions as well as the α1 C12−14EO8 mole ratios calculated at the cmc are displayed. In these systems, with the surfactant concentration being constant but the ionic liquid being progressively diluted, α1 increases. These α1 values are extremely low, in the range of 1 × 10−5 to 1 × 10−6, because of the low surfactant concentration. Figure 3 shows as an example a plot of Δγ/Δlog C for each ionic liquid dissolved in pure water and in a 4.68 × 10−4 mM C12−14EO8 aqueous solution. The presence of surfactant produces a decrease in the cmc with respect to the cmc value in pure water. In addition, by dilution with surfactant solution, 14524


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Table 3. Values of the cmc for Ionic Liquids Dissolved in C12−14EO8 Aqueous Solutions and C12−14EO8 Mole Fractions α1 at the cmc ionic liquids dissolved in aqueous solutions of C12−14EO8 dilution media C12−14EO8 aqueous solution

bmim-octyl SO4

bmim-methyl SO4

bmim-BF4

conc (mM)

γ (mN/m)

cmc (mM)

C12−14EO8 α1

cmc (mM)

C12−14EO8 α1

cmc (mM)

C12−14EO8 α1

0 4.68 × 10−4 1.00 × 10−3 2.00 × 10−3

72 55 50 45

30 17 21 23

0 2.75 × 10−5 4.76 × 10−5 8.69 × 10−5

350 200 214

0 2.34 × 10−6 4.67 × 10−6

850 500 500 500

0 9.36 × 10−7 2.00 × 10−6 4.00 × 10−6

a

The cmc cannot be obtained because for bmim-methyl SO4 γcmc is already 45 mN/m and no increase in surface tension is produced when diluting with a 2.00 × 10−3 mM solution of C12−14EO8. a

the next series of experiments, mixtures of C12−14EO8 and ionic liquid over a wide range of constant mole fractions including α1 values that were as small as those reported in Tables 2 and 3 were prepared. To determine whether there is an interaction between the surfactant and ionic liquid, the experimental cmc values were compared to the theoretical ones corresponding to ideal behavior according to the Clint equation48 α1 (1 − α1) 1 = + cmccalc cmc1 cmc 2

(1)

where cmccalc is the theoretical value for the mixture without interaction, cmc1 and cmc2 are the individual cmc values of components 1 and 2, respectively, and α1 is the mole fraction of the first component (C12−14EO8) in the solution. When the cmccalc and the cmc experimental (cmcexp) are different, interaction between the two components exists. The mole fraction of compound 1 in the mixed micelle (X1M) can be obtained by iteratively solving eq 2 in Rubingh’s regular solution theory.49

Figure 3. Surface tension vs logarithm of concentration for the ionic liquids in a 4.68 × 10−4 mM C12−14EO8 aqueous solution (γ = 55 mN/ m) and in water (open symbols).

the surface tensions tend logically to 55 mN/m, the surface tension value for the aqueous C12−14EO8 solution. 3.4. Binary Systems C12−14EO8/Ionic Liquid at Constant Mole Ratios. The general procedure for studying binary surfactant systems in aqueous solutions by tensiometry is to plot the surface tension of different solutions in which the two components are mixed in different mole ratios against the logarithm of the total concentration of both compounds. From these graphs, the cmc of the mixed system can be obtained. In

(X1M)2 ln(α1cmcexp/X1M cmc1) (1 − X1M)2 ln[(1 − α1)cmcexp/(1 − X1M)cmc 2]

=1 (2)

By applying this X1M value to eq 3, the interaction parameter β for the mixed micelles can be obtained: M

Table 4. Surface-Active Parameters for Binary Systems C12−14EO8/IL as a Function of the Surfactant Mole Ratio, α1a

a

α1C12−14EO8

cmccalc(mM)

cmcexp(mM)

0.854 0.483 0.217 1.43 × 10−5

0.028 0.049 0.11 30

0.044 0.080 0.23 22

0.898 0.522 0.236 3.33 × 10−6

0.026 0.044 0.097 333

0.044 0.070 0.15 320

0.889 0.495 0.296 6.46 × 10−5

0.026 0.046 0.078 251

0.035 0.080 0.135 240

γcmc(mN/m)

pC20

{C12−14EO8/bmim-octyl SO4} 29.8 5.52 30.1 5.07 29.9 4.79 30.0 2.49 {C12−14EO8/bmim-methyl SO4} 30.0 5.49 30.5 5.05 31.5 4.82 43.4 1.00 {C12−14EO8/bmim-BF4} 31.0 5.38 30.5 5.34 29.7 5.06 32.5 1.82

Δγ/Δ log C

Γm × 10−10 (mol · cm−2)

−19.30 −21.84 −20.59 −24.99

3.38/n 3.83/n 3.61/n 4.38/n

49 43 46 38

x x x x

n n n n

−20.81 −27.37 −24.42 −16.72

3.65/n 4.80/n 4.27/n 2.93/n

45 35 39 57

x x x x

n n n n

−24.88 −18.53 −19.34 −16.08

4.36/n 3.25/n 3.39/n 2.82/n

38 51 49 59

x x x x

n n n n

Am (Å2)

This is an abbreviated version of the table. The full data set appears in the Supporting Information. 14525


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fractions α1 and negative βM should be found only for extremely low α1. However, in most cases, it was not possible to solve eq 2 to obtain X1M because the iteration process led to values for which the function was not defined. Other authors also reported problems in solving these equations in some cases.59 Another approach to getting X1M and consequently βM was to represent graphically the functions corresponding to the numerator and denominator of eq 2 and determine the intersection of the graphs. This allows us to extend the few data obtained directly by iteration of eq 2. However, even by applying this method a real solution for X1M was not always obtained. When it was possible, the corresponding βM values were reported in Table 5. To fill the gaps among the very low

ln(α1cmcexp/X1M cmc1) (1 − X1M)2

(3)

Negative values of β indicate attractive interactions between the components in the mixed micelle, positive βM indicates repulsive interactions, and βM = zero indicates the ideal case of no interaction. In Table 4 (Supporting Information), the surface-active parameters for the three binary systems composed of C12−14EO8 and bmim-octyl SO4, bmim-methyl SO4, and bmim-BF4, respectively, are displayed as a function of the C12−14EO8 mole fraction, α1. An abbreviated version of Table 4 is included in the article. Although generally in binary systems the considered values of α1 move from 0.1 to 0.9, in our case we extended this range to α1 values that are as low as those previously calculated in Tables 2 and 3. These low values correspond to those usually found in common applications of dissolutions of a nonionic surfactant in an aqueous solution of ionic liquid. The parameters are the calculated cmc (cmccalc), experimental cmc (cmcexp), surface tension at the cmc (γcmc), pC20, slope of the straight line Δγ/Δ log C, maximum adsorption at the saturated interface (Γm), and minimum area occupied per molecule at the saturated interface (Am). Γm and Am are expressed as a function of n, the factor of the Gibbs equation related to the number of species in solution. In the case of ionic/nonionic systems, n should move from 1 (for a single nonionic surfactant) to 2 (for a single ionic surfactant) but its real value is difficult to determine. Rosen50−52 recommends the addition of a swampy amount of electrolyte to each single and mixed solution to ensure a constant ionic strength in the aqueous solutions, in which case n can be taken to be 1. We consider, however, that the effect of incorporating another component would completely alter our systems, so this possibility was not considered. As a consequence, the factor n and indirectly Γm and Am remain undetermined. On the contrary, the slope Δγ/Δ log C as a specific characteristic of each system is reported in Table 4. In general, for all of these systems the experimental cmc values are clearly higher than the calculated ones, denoting repulsive interactions between the components in the mixed micelle; consequently, βM should be positive. This is an unusual behavior that has been reported in limited cases.53−55 However, for C12−14EO8 mole fractions lower than a given value that depends on each system (α1 < 1.04 × 10−3 for bmim-octyl SO4, α1 < 1.46 × 10−4 for bmimmethyl SO4, and α1 < 5.55 × 10−4 with bmim-BF4), a change in the type of interaction between components is observed. The experimental cmc values are lower than the calculated ones, reflecting attractive interactions between surfactant and ionic liquids. As already mentioned, the usual way to quantify the deviation of experimental cmcexp from ideal cmccalc in binary systems of surface-active compounds is using the βM interaction parameter of Rubingh’s equation. But according to Rosen,50−52 in ionic/nonionic systems the presence of a constant electrolyte concentration in solutions of both individual compounds and binary mixtures is needed to ensure reliable βM values because in the derivation of the equations electrical effects were not considered. Given that these are not the conditions in our systems, we assumed that a rough estimation of βM values can be obtained by application of eqs 2 and 3 as other authors do.56−58 Accordingly to the behavior observed for these systems, positive βM should be found for most of mole M

Table 5. Extremely Low α1 Values with Their Associated cmc's, X1M, and βM for the Three Investigated Systems α1 at cmc (C12−14 EO8) 0.0120a 1.04 × 10−3a 8.69 × 10−5 4.76 × 10−5 2.75 × 10−5 1.43 × 10−5a 0

a

9.99 1.46 4.67 3.33 2.63 2.34 0

× × × × × ×

10−4 10−4a 10−6 10−6a 10−6 10−6

7.19 5.55 6.46 4.00 2.00 9.36 0

× × × × × ×

10−4 10−4a 10−5a 10−6 10−6 10−7

cmccalc (mM)

cmcexp (mM)

{C12−14EO8/[bmim-octyl SO4]} 1.8 2.5 12.7 13 27 23 28 21 29 17 30 22 30 30 {C12−14EO8/[bmim-methyl SO4]} 26.6 80 109 110 327 214 333 210 336 214 338 200 350 350 {C12−14EO8/[bmim-BF4]} 31 50 40 60 251 240 740 500 791 450 822 500 850 850

X1 M

βM

0.579 0.196 0.213 0.250 0.176

+0.085 −1.258 −2.566 −4.461 −3.763

0.694 0.252 0.247 0.238 0.249

+0.065 −3.142 −3.701 −3.910 −4.432

0.586 0.287 0.282 0.242

−1.114 −2.349 −3.831 −4.309

Some data from Table 4 are also included.

α1 values from Table 4 where the change from repulsive to attractive interactions is produced, we take advantage of the studies described above when one of the components was kept at a constant concentration whereas the other was diluted. If we convert these systems to typical binary systems by considering the total concentration (surfactant plus ionic liquid) and the α1 calculated at the cmc, then we can dispose of new complementary data. In Table 5, these α1 data with their associated values of cmccalc and cmcexp, C12−14EO8 mole fraction X1M in the mixed micelle, and interaction parameter βM are reported. When cmcexp values are clearly higher than the cmccalc (adata taken from Table 4), no values of X1M and βM can be obtained. For decreasing α1, cmcexp values became lower than cmccalc and negative βM values were obtained as a measure of the attractive interaction between the surfactant and the ionic liquid. These results seem to indicate a close correspondence between the cmc data and the interaction parameter βM within this range of low α1, in spite of the absence of electrolyte in the solutions. To explain the change in behavior from repulsive to 14526


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Figure 4. Graphs of Δγ/Δ log C for binary mixtures (a) C12−14EO8/bmim-octyl SO4, (b) C12−14EO8/bmim-methyl SO4, and (c) C12−14EO8/bmimBF4. (d−f) Experimental and calculated cmc values for these binary systems as a function of the C12−14EO8 mole ratios.

attractive interaction between the components with decreasing α1, the first consideration to keep in mind is the high surface activity of the nonionic surfactant that even for extremely low mole fractions in the aqueous solution (Table 5) mole fractions of X1M higher than 0.200 are attained in the mixed micelles. At these extremely low α1 values, the insertion of nonionic surfactant monomers between ionic liquid molecules in the micelles produces a decrease in the electrostatic self-repulsion of the ionic liquid leading to mixed micelle formation at lower cmc's than expected, giving rise to negative βM. When α1 increases (Table 5), the C12−14EO8 composition in the mixed micelle becomes predominant (XM1 ≈ 0.600−0.700), and in this case, the insertion of ionic liquid monomers in the

surfactant micelles probably leads to an electrostatic repulsion that increases the cmc as the slightly positive βM indicates. We can expect the same behavior for higher α1 although no values for XM1 can be obtained. In Figure 4a−c to avoid overlapping, only some of the registered Δγ/log C plots (total concentration) of the systems in Table 4 are displayed. (All of the graphs are reported in the Supporting Information.) It can be seen that small amounts of nonionic surfactant (very low α1) are sufficient to decrease the initial cmc of the ionic liquids drastically. Moreover, for the C12−14EO8/bmimmethyl SO4 binary system, a drastic diminution of γcmc from 45.0 to 32.2 mN/m is produced for an α1 value of as low as 14527


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1.46 × 10−4. This abrupt change could be justified by the formation of mixed micelles. It is noteworthy that until now this possibility has been considered only for long-alkyl-chain ionic liquids with surfactant properties39 but not for short-alkylchain ionic liquids, which are merely considered to be cosolvents where the surfactant is dissolved. In Figure 4d−f experimental and calculated cmc values are plotted as a function of α1 for all binary mixtures of the three systems. Given that α1 and the cmc values varied over a range of several orders of magnitude, the graphs were plotted on a double-logarithmic scale. As can be observed, the change from attractive to repulsive interaction depends on the value of α1. In the case of bmim-octyl SO4 (d), attractive interaction occurs when α1 < 1.00 × 10−3 whereas for the systems with bmimmethyl SO4 (e) and bmim-BF4 (f) this occurs when α1 < 1.00 × 10−4. For higher α1 values, repulsive interactions between the components of the binary system are produced given the higher cmcexp values with respect to the cmccalc, although no βM values can be calculated. Accordingly, with these results a decrease or increase in the cmc can be achieved through the adjustment of the mole ratio between the components in the aqueous solution. Therefore, by controlling the experimental conditions of the binary surfactant/ionic liquid systems, the cmc values could be modulated for a specific application.

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ASSOCIATED CONTENT

S Supporting Information *

Graphs of conductivity versus concentration for ILs. Plots of γ versus log C for C12−14EO8 dissolved in water, bmim-methyl SO4, or bmim-BF4 aqueous solutions. Full data set of surfaceactive parameters for binary systems C12−14EO8/ILs (Table 4, complete). All graphs of γ versus log C for binary systems C12−14EO8/IL at different surfactant mole fractions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fcff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Spanish Ministry of Economia and Competitividad (project CTQ2010-17990) and the Generalitat of Catalunya (SGR2009-1331) for financial support.

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REFERENCES

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4. CONCLUSIONS Imidazolium-based ionic liquids bmim-octyl SO4, bmim-methyl SO4, and bmim-BF4 display surface activity in aqueous solution. The differences in such surface activity depend on the nature of the counterion, and the best surfactant properties are found for the most hydrophobic anion (octyl SO4). Because of the mentioned surface activity, when these ionic liquids are in aqueous solution with a nonionic surfactant, a typical case of binary surfactant systems should be considered. The differences between the experimental and the calculated cmc values confirm that interaction between both components occurs. A change from attractive to repulsive interaction has been detected for each system depending on the surfactant mole fraction α1. For extremely low surfactant mole fractions (α1 < 0.001 for system C12−14EO8/bmim-octyl SO4 or α1 ≤ 0.0001 for systems C12−14EO8/bmim-methyl SO4 and C12−14EO8/ bmim-BF4), experimental cmc values are lower than the calculated ones, indicating attractive interactions between surfactant and ionic liquid (i.e., synergism in mixed-micelle formation). When the surfactant mole fractions are higher than those indicated for each system, experimental cmc values are higher than the calculated ones, indicating repulsive interactions in mixed-micelle formation. Through selecting the appropriate surfactant mole fraction α1, a binary nonionic surfactant/ionic liquid system can be tailored for a specific application, depending on whether mainly monomers or micelles are required. The fact that short-alkyl-chain ionic liquids usually used as cosolvents in aqueous solutions can display surfactant activity introduces a new element to be kept in mind for a better knowledge of surfactant and ionic liquid interactions. Under this approach, a new interpretation of some results reported in the literature could be made. 14528


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