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Nov 2, 2014 - Ethylammonium nitrate (EAN) was synthesized as described by Evans et al.(23) In a typical process, 3 M nitric acid was slowly added to t...
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Comparative Investigations on Mixing Behaviors of Cationic Gemini Surfactant with Surface Active Ionic Liquid in Water and in Ethylammonium Nitrate Shuzhen Wang,†,§ Tianxiang Yin,†,§ and Weiguo Shen*,†,‡ †

School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China

‡

S Supporting Information *

ABSTRACT: The properties of mixed surfactants of 1-tetradecyl-3-methylimidazolium bromide (C14mimBr) and the gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide (12-2-12) in water and in ethylammonium nitrate (EAN) were studied by surface tensiometry. The critical micelle concentration (cmc) values of the mixed surfactants were determined and found to be much larger in EAN than in water. The nonideal mixing behaviors were observed at the air/solvent interface and in the mixed micelle both in water and in EAN. The surface pressure at the cmc (πcmc), the maximum surface excess (Γmax), the minimum surface area per molecule (Amin), the standard Gibbs free energies of micellization (ΔG0m) and adsorption (ΔG0ads), and the excess Gibbs free energy of mixed micelles (ΔGE) were determined. It was found that physicochemical properties of C14mimBr/12-2-12 mixed systems in water and in EAN were significantly different, which was discussed in terms of different properties of the solvents.

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INTRODUCTION

showed a large charge screening effect of the IL on surfactants and hence displayed nearly ideal mixing behaviors. In this work, we investigate the mixed behaviors of the gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide (12-2-12) with 1-tetradecyl-3-methylimidazolium bromide (C14mimBr) at various total surfactant compositions in water and in the IL ethylammonium nitrate (EAN) by surface tension measurements. The cmc values, the interfacial properties, and the surfactant−surfactant interaction on the air/solvent interface and in mixed micelles are studied in water and EAN and compared with each other for better understanding of the influence of solvent properties on mixing behaviors of the surfactants.

Surfactants are widely applied in both industry and daily life, and these applications mostly rely on their mixtures, which is due to their superior activity over the activity of single pure surfactants.1,2 Mixed surfactants can exhibit lower critical micelle concentrations and better surface properties than single surfactants and often produce synergistic effects, which result from the nonideal mixing behavior of the mixed micelles.2−4 Properties of mixed surfactant systems have been extensively studied by various methods in recent decades.5−8 Gemini surfactants become more prevalent than traditional surfactants because of their advantages of unusual physicochemical properties such as higher surface activities, lower critical micelle concentrations (cmc’s), better solubilizing power, low Kraff points, and better viscoelastic properties.9,10 Therefore, the mixed micellization behaviors of gemini surfactants with other conventional surfactants have been paid much more attention.11−17 However, most of the reported investigations on mixed surfactant systems are in aqueous solutions. In recent decades, ionic liquids (ILs) have been presented as good candidates for green solvents because of their unique properties such as the wide liquid state range, the negligible vapor pressure, the favorable solvation behavior, and the high reactivity and selectivity.18 Comprehensive studies concerning the aggregation behaviors of surfactants in ILs have been published.19 However, to the authors’ best knowledge, there are only two studies concerned with the behaviors of mixed surfactants in ILs: the study on nonionic surfactant mixture in [Bmim]PF6 (1-butyl-3-methylimidazolium hexafluorophosphate) by Sakai20 and the mixture of cationic surfactant and anionic surfactant in [Emim][EtSO4] (1-ethyl3-methylimidazolium ethylsulfate) by Bermudez.21 The latter © 2014 American Chemical Society

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EXPERIMENTAL SECTION

Chemicals. 1-Tetradecyl-3-methylimidazolium bromide (C14mimBr, >99% mass fraction) was purchased from Cheng Jie Chemical Co. Ltd. (Shanghai, China) and dried under vacuum for 48 h before use. The gemini surfactant 12-2-12 was synthesized according to the literature22 from the reaction of 1bromododecane (Sigma-Aldrich, >97% mass fraction) with N,N,N′,N′-tetramethylethylenediamine (Shanghai Linfeng Chemical Reagent Co. Ltd., >99% mass fraction). The purity of the synthesized 12-2-12 was confirmed by 1H NMR (500 MHz, CDCl3; δ = 0.88 (t, 6H), 1.20−1.45 (m, 36H), 1.83 (m, 4H), 3.53 (s, 12H), 3.71 (m, 4H), 4.74 (s, 4H)). Ethylammonium nitrate (EAN) was synthesized as described by Evans et al.23 In a typical process, 3 M nitric acid was slowly Received: Revised: Accepted: Published: 18202

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equilibrium before measurement. The uncertainty in conductivity measurement was estimated to be 0.5%.

added to the ethylamine solution with stirring and cooling in an ice bath. The water in the raw product solution was rotary evaporated and further removed with a lyophilizer (FD-1, Boyikang Ltd. Beijing, China). The residual water content of the final product was determined by Karl Fischer titration to be about 0.3 wt %. The purity of EAN was ascertained by 1H NMR (500 MHz, D2O; δ = 1.20 (t, 3H), 3.04 (m, 2H)). The synthesized 12-2-12 and EAN were dried under vacuum at 333.15 K in the presence of P2O5 for 2−3 days before use. The structures of C14mimBr and 12-2-12 are shown in Scheme 1.

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RESULTS AND DISCUSSION Critical Micelle Concentrations. Surfactants in pure and mixed states self-assemble to form micelles. The threshold concentration of the micelle formation, called the critical micelle concentration (cmc), is an important criterion for understanding the fundamentals of the self-organizing process. In this work, the cmc’s of C14mimBr/12-2-12 mixed surfactant systems with various overall mole fractions α (the mole fraction of 12-2-12 in the total surfactants) in water and in EAN were determined as the break points in the plots of the surface tension (γ) against the natural logarithm of Ctot (total surfactant concentration),24 which are shown as examples in parts a and b of Figure 1 with several α values in water and in EAN, respectively. (The plots of the surface tension (γ) against the natural logarithm of Ctot at all mole fractions α are presented in Figure S1 in the Supporting Information.) Moreover, the cmc values of mixed C14mimBr/12-2-12 surfactant systems in aqueous solutions were also determined by measurements of conductivity, where the plots of conductivity against Ctot are shown in Figure S2 in the Supporting Information. The obtained values of cmc are summarized in Table 1, where the values for pure C14mimBr and 12-2-12 in water and

Scheme 1. Structures of 12-2-12 and C14mimBr

Table 1. Values of Critical Micelle Concentration (cmc) for C14mimBr/12-2-12 Mixed Surfactants in Water and in EAN at 298.15 K

Surface Tension Measurements. The surface tensions of mixed surfactant solutions in water and in EAN were measured using the hanging platinum plate method by a tensionmeter supplied by Shanghai Hengping Instrument Co. (Shanghai, China) with an accuracy of ±0.1 mN·m−1. The temperature was controlled by a water circulating bath with a precision of ±0.1 K. The tensionmeter was calibrated with pure water, and the plate was washed and burned over a flame before each measurement to ensure its cleanness. Typically, it took 0.5−1 h to reach equilibrium before the measurement. Conductivity Measurements. Conductivity measurements were performed with a digital conductivity meter supplied by Leici Co. (Shanghai, China) using a titration method. The conductivity meter was initially calibrated by a standard KCl solution with a concentration of 0.01 mol·L−1. A certain amount of water was transferred into a cell, which was placed in a water bath with temperature being controlled within ±0.1 K. The concentrated surfactant solution was prepared and titrated into the cell by a microsyringe. The cell was shaken after each titration and kept undisturbed to reach thermal

cmc/mmol·L−1 water α 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

surface tension 2.41 1.86 1.53 1.40 1.28 1.14 1.08 1.05 1.01 0.93 0.89

± ± ± ± ± ± ± ± ± ± ±

0.12, 2.6925 0.07 0.07 0.05 0.05 0.06 0.06 0.05 0.05 0.05 0.05, 0.8422

EAN conductivity 2.59 2.07 1.64 1.44 1.30 1.19 1.12 1.05 1.02 0.95 0.89

± ± ± ± ± ± ± ± ± ± ±

0.07 0.05 0.03 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01

surface tension 37.2 28.2 24.6 22.4 20.7 17.8 17.6 17.4 17.5 17.3 23.1

± ± ± ± ± ± ± ± ± ± ±

2.0, 3527 1.8 1.8 1.5 1.5 1.4 1.4 1.3 1.3 1.3 1.7, 25.326

Figure 1. Plots of surface tension γ against ln Ctot (Ctot refers to the total surfactant concentration) for C14mimBr/12-2-12 mixed surfactants in (a) water and (b) EAN. 18203

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be calculated by n = 3xσ + 2(1 − xσ) in water while it is taken to be 1 in EAN because the large amount of ions in EAN may completely neutralize the charged surfactant head groups.26 The average minimum surface area Amin can be deduced on the basis of Γmax:

in EAN are in good agreement with those reported by other authors22,25−27 and the cmc values in water determined by conductivity measurements and surface tension measurements coincide well with each other. The cmc values of pure components and the mixtures in EAN were significantly larger than those in water as reviewed by Drummond,19 which is due to the fact that the hydrophobic tails of the surfactants are more soluble in EAN as a result of the nonpolar nanostructure and weaker H-bond network in EAN.19,28−31 Surfactant−Surfactant Interactions. Interactions at Mixed Adsorbed Monolayer. The surfactants orient at the air/solvent interface and cause reduction of the surface tension. The composition of the adsorbed mixed monolayer of a mixed surfactant system can be described by the formula proposed by Rosen:32 (1 − x σ )2 ln((1 − α)C12/((1 − x σ )C10)) (x σ )2 ln(αC12/(x σC20))

A min =

πcmc = γ0 − γcmc

(1)

(2)

σ

where β is the interaction parameter of surfactants at air/ solvent interface; xσ refers to the mole fraction of 12-2-12 in the mixed monolayer. C01, C02, and C012 are the concentrations of C14mimBr, 12-2-12, and their mixture at various α’s, respectively, at which the surface tension has a common appropriate value (37 mN·m−1 in this work). The values of C01, C02, and C012 for various α’s are presented in Table S1 of the Supporting Information and used to determine the composition xσ and interaction parameter βσ in the mixed monolayer by fitting these values with eqs 1 and 2, which are summarized in Table 2. The interaction parameters βσ are negative both in water and in EAN, showing the attractive interactions.

1 1−α α = + cmc* cmc1 cmc 2

water α

x

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

0.400 0.505 0.577 0.632 0.675 0.721 0.769 0.820 0.887

EAN σ

σ

β

x

βσ

−1.91

0.353 0.477 0.560 0.628 0.700 0.751 0.813 0.865 0.928

−1.18

The amount of surfactant adsorbed per unit area of the surface can be deduced by the the Gibbs adsorption equation,33 which gives the expression of the maximum surface excess Γmax as Γmax = −

1 ⎛ ∂γ ⎞ ⎜ ⎟ nRT ⎝ ∂ ln C ⎠T

(6)

where cmc1 and cmc2 refer to the cmc values of pure components for C14mimBr and 12-2-12, respectively. The cmc* values are shown as dotted lines and compared with the experimental values of the cmc (solid circles) in Figure 2. It can be seen from Figure 2 that cmc < cmc* for all mixed micelles with various α’s both in water and in EAN, indicating negative deviations from ideality and the attractive interactions in the mixed micelle. Moreover, the deviation from ideality in EAN is more obvious. Rubingh’s equation has been widely applied in describing the nonideality in binary surfactant mixtures. According to Rubingh’s model,38 the mole fraction xm of 12-2-12 for each given α in the mixed micelle and the interaction parameter βm can be obtained by

Table 2. Values of the Mole Fraction (xσ) of 12-2-12 and Interaction Parameter (βσ) in the Mixed Monolayer for C14mimBr/12-2-12 Mixed Surfactants in Water and in EAN at 298.15 K σ

(5)

where γ0 and γcmc refer to the surface tensions of the solvent and the surfactant solution at cmc, respectively. The calculated values of Γmax, Amin, and πcmc in water and in EAN are summarized and compared in Table 3. Much larger values of πcmc in water suggest the stronger effectiveness of the surfactant to reduce the surface tension in the aqueous solution. The values of Γmax and Amin for pure components are in reasonable agreement with those reported in the literature.26,34,35 The values of Γmax are larger in EAN and the corresponding values of Amin are smaller, which may be due to the electrostatic screening of the repulsion between the surfactant’s head groups resulting in closer packing of surfactants at the air/EAN interface.26,36 Interactions in Mixed Micelles. The cmc values of mixed surfactants often show departures from the ideal critical micelle concentrations cmc* predicted by Clint’s equation:37

=1

(x σ )2

(4)

with NA representing Avogadro’s constant. Moreover, the effectiveness of the surfactant tension reduction can be measured by the surface pressure at cmc, πcmc, which is defined as

ln((1 − α)C12/((1 − x σ ))C10)

βσ =

1014 NA Γmax

α ·cmc = x m exp[β m(1 − x m)2 ]cmc 2

(7)

(1 − α)cmc = (1 − x m) exp[β m(x m)2 ]cmc1

(8)

A nonlinear fit of the cmc values for various α’s with eqs 7 and 8 gave the optimized values of βm and xm, which are listed in Table 4. Similar to the situation in the mixed monolayer, the negative values of βm are observed both in water and in EAN, indicating that the interaction between two different surfactants in the mixed micelle is apparently attractive. Moreover, more negative values of βm in EAN indicate that mixed surfactants in EAN experience more attractive interactions due to the screening effect on electrostatic repulsive interaction between head groups of surfactants.

(3)

where T is the absolute temperature and R is the gas constant. (∂γ/(∂ ln C))T may be obtained from the slope of the linear plot of the surface tension against ln Ctot near cmc, and n may 18204

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Table 3. Surface Pressures at the cmc (πcmc), Maximum Surface Excesses (Γmax), and Minimum Surface Areas per Molecule (Amin) for C14mimBr/12-2-12 Mixed Surfactants in Water and in EAN at 298.15 K water πcmc (mN·m−1)

α 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

36.4 40.2 40.7 41.8 42.2 41.4 41.9 41.5 41.3 41.0 41.8

EAN

Γmax (106 mol·m−2)

Amin (nm2)

34

πcmc (mN·m−1)

Γmax (106 mol·m−2)

Amin (nm2)

13.7 13.9 14.1 14.5 15.0 15.4 15.4 15.6 15.8 15.9 17.1

2.10 1.80 1.81 1.74 1.73 1.60 1.65 1.59 1.64 1.63 1.85, 2.0326

0.79 0.92 0.92 0.96 0.96 1.04 1.01 1.05 1.01 1.02 0.90, 0.8226

34

2.07, 2.06 1.64 1.53 1.39 1.51 1.61 1.60 1.49 1.40 1.59 1.68, 1.64,35 1.6235

0.80, 0.81 1.01 1.09 1.19 1.10 1.03 1.03 1.11 1.19 1.04 0.99, 1.00,35 1.0235

Figure 2. Plots of critical micelle concentration (cmc) against α for C14mimBr/12-2-12 mixed surfactants in (a) water and (b) EAN. The solid circles represent the experimental data; the dotted lines refer to the results from Clint’s equation; the solid lines refer to the results from Rubingh’s equation. m Table 4. Values of the Mole Fractions (xm) of 12-2-12 in the Mixed Micelle, Activity Coefficients (γm 1 ) for C14mimBr and (γ2 ) for 12-2-12, and Interaction Parameters (βm) for C14mimBr/12-2-12 Mixed Surfactants in Water and in EAN at 298.15 K

water

EAN

α

xm

γm 1

γm 2

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

0.279 0.423 0.529 0.613 0.689 0.754 0.813 0.873 0.937

0.958 0.906 0.856 0.812 0.768 0.729 0.693 0.655 0.614

0.749 0.831 0.884 0.920 0.948 0.967 0.981 0.991 0.998

βm

xm

γm 1

γm 2

βm

−0.55

0.279 0.381 0.451 0.510 0.568 0.624 0.683 0.752 0.844

0.880 0.788 0.716 0.652 0.589 0.528 0.464 0.395 0.310

0.426 0.533 0.609 0.673 0.735 0.792 0.848 0.903 0.961

−1.64

m The activity coefficients γm 1 for C14mimBr and γ2 for 12-2-12 in the mixed micelle were calculated with the values of βm and xm by eqs 9 and 10:

γ1m = exp[β m(x m)2 ] γ2m = exp[β m(1 − x m)2 ]

According to Rosen,39−41 there are three synergisms in (1) surface tension reduction efficiency, (2) mixed micelle formation, and (3) surface tension reduction effectiveness, corresponding to three conditions: condition 1:

(9) (10)

βσ < 0

and are listed in Table 4. The lower values of the activity coefficients than unity both in water and in EAN indicate a negative deviation from ideality. The active coefficients in EAN show larger negative deviations from unity than those in water. Synergisms for Mixed Surfactants. Synergisms can exist in the mixture of two surfactants and can be determined by the interaction parameters and the properties of pure components.

and

|β σ | > ln

and

|β m| > ln

C10 C20

condition 2: βm < 0

cmc1 cmc 2

and 18205

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Table 5. Standard Gibbs Free Energies of Micellization (ΔG0m), Standard Gibbs Free Energies of Adsorption (ΔG0ads), and Excess Gibbs Free Energies of Mixed Micelles (ΔGE) for C14mimBr/12-2-12 Mixed Surfactants in Water and in EAN at 298.15 K water α

ΔG0m

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1

−1

(kJ·mol ) −14.9 −15.6 −16.1 −16.3 −16.5 −16.8 −16.9 −17.0 −17.1 −17.3 −17.4

ΔG0ads

EAN −1

(kJ·mol )

−32.2 −40.0 −42.7 −46.3 −44.5 −42.5 −43.1 −44.8 −46.6 −43.1 −42.3

−1

ΔG (kJ·mol ) E

ΔG0m

−1

ΔG0ads

(kJ·mol ) −8.2 −8.8 −9.2 −9.4 −9.6 −10.0 −10.0 −10.0 −10.0 −10.1 −9.3

−0.28 −0.34 −0.34 −0.33 −0.29 −0.26 −0.21 −0.15 −0.08

(kJ·mol−1)

−15.0 −16.6 −16.9 −17.8 −18.3 −19.6 −19.4 −19.9 −19.7 −19.8 −18.6

ΔGE (kJ·mol−1) −0.82 −0.96 −1.01 −1.02 −1.00 −0.96 −0.88 −0.76 −0.54

Figure 3. Plots of excess Gibbs free energy ΔGE against the mole fraction xm of 12-2-12 in C14mimBr/12-2-12 mixed micelles in (a) water and (b) EAN. The solid circles are experimental data. The solid lines are calculated from the Redlich−Kister equation.

ΔGE = RT ((1 − x m) ln γ1m + x m ln γ2m)

condition 3: βσ − βm < 0

and

|β σ − β m| > ln

and are listed in Table 5. The negative values suggest that the mixed micelles are stable. Moreover, the larger values of ΔGE were observed in EAN, showing the mixed micelles were more stable in EAN which may be due to the large electrostatic screening effect. The values of ΔGE can be correlated by the Redlich−Kister equation:45

C10·cmc 2 C20·cmc1

respectively. We used the data reported above to evaluate the conditions of the synergisms for the mixed surfactants in water and in EAN. It was found that there existed synergisms in the surface tension reduction efficiency and in the surface tension reduction effectiveness for the mixed surfactants in water, while the synergism in the mixed micelle formation existed for the mixed surfactants in EAN. Gibbs Free Energies of Adsorption and Micellization for Mixed Surfactants. The standard Gibbs free energy of mixed micellization ΔG0m and standard Gibbs free energy of adsorption ΔG0ads are expressed as42−44 ΔGm0 = RT ln(cmc) 0 ΔGads = ΔGm0 −

πcmc Γmax

(13)

N

ΔGE = x m(x m − 1) ∑ Bi (2x m − 1)i i=0

(14)

The values of ΔG of C14mimBr/12-2-12 mixed micelles in water and in EAN were fitted to eq 14 with a simple quadratic term separately to obtain the optimal values of B0, which were 1.38 in water and 4.08 in EAN. The fitting results are compared with the experimental ones in Figure 3, indicating good agreement with each other. E

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(11)

CONCLUSION In this work, the properties of the mixed surfactants of the surface active IL 1-tetradecyl-3-methylimidazolium bromide (C 1 4 mimBr) and the gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammonium dibromide (12-212) in water and in IL ethylammonium nitrate (EAN) were comparatively studied by surface tensiometry. Some conclusive remarks are as follows: 1. The cmc values determined for the mixed surfactants in EAN were much larger than those in water, showing less ability

(12)

where cmc is expressed in units of moles per liter. The values of ΔG0m and ΔG0ads were calculated and are listed in Table 5. The less negative values of ΔG0m and ΔG0ads in EAN imply that both micellization and adsorption are less favorable than those in water. The excess Gibbs free energy of mixed micelles ΔGE can be calculated by 18206

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of EAN in supporting self-aggregation of surfactants due to the high solubility of hydrophobic tails of the surfactants in EAN as a result of its nonpolar nanostructure and weak H-bond network. 2. The properties at the air/solvent interface such as the surface pressure at the cmc (πcmc), the maximum surface excess (Γmax), and the minimum surface area per molecule (Amin) were found to be different in water and in EAN, which may be attributed to the strong electrostatic screening in EAN. 3. Nonideal mixing behaviors were observed at the air/ solvent interfaces and in the mixed micelles both in water and in EAN. Synergisms exist in surface tension reduction efficiency and in surface tension reduction effectiveness for the mixed surfactants in water, while the synergism in mixed micelle formation exists for the mixed surfactants in EAN. 4. The standard Gibbs free energy of micellization (ΔG0m), the standard Gibbs free energy of adsorption (ΔG0ads), and the excess Gibbs free energy of mixed micelles (ΔGE) for the mixed surfactants were determined. The comparisons of these properties in water and in EAN showed that both adsorption and micellization were less favorable in EAN; however, the mixed micelles were more stable in EAN. Until now, the thermodynamic behaviors of mixed micelles in IL were rather limited as compared to the wide applications of ILs.46,47 Thus, more thermodynamic investigations for more mixed micelles in ILs are highly required, which is essential to extending the potential applications of mixed micelles in ILs in fields of micellar catalysis, separation process, nanomaterial synthesis, etc.19,48,49

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ASSOCIATED CONTENT

* Supporting Information S

Plots of surface tension against total surfactant concentration for various α values of C14mimBr/12-2-12 mixed surfactants in water and in EAN; plots of conductivity against total surfactant concentration for C14mimBr/12-2-12 mixed surfactants in aqueous solutions; surfactant concentrations at given surface tension for various α values. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64250804. Fax: +86 21 64250804. E-mail: [email protected]. Author Contributions §

S.W. and T.Y. contributed equally as the first authors.

Funding

This work was supported by the National Natural Science Foundation of China (Projects 21173080, 21373085 and 21303055). Notes

The authors declare no competing financial interest.

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REFERENCES

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Industrial & Engineering Chemistry Research

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