Aggregation behavior of zwitterionic surface active ionic liquids with different counterions, cations, and alkyl chains

Abstract

A group of zwitterionic surface active ionic liquids (SAILs) with different counterions, cations and alkyl chains, 3-(1-alkyl-3-imidazolio)propanesulfonate β-naphthalene sulfonate, (C_nIPS-Nsa, n = 12, 14), 3-(1-dodecyl-3-imidazolio)propanesulfonate benzenesulfonate (C₁₂IPS-Bsa), and dodecyl-N,N-dimethylammonio-3-propane sulfonate β-naphthalene sulfonate (SB-12-Nsa), were synthesized. Their aggregation behaviors in aqueous solutions were systematically investigated by surface tension, dynamic light scattering (DLS) and ¹H NMR spectroscopy. Surface tension and DLS results illustrated that the surface properties, micelle size, and micellization behavior of zwitterionic SAILs in aqueous solutions are significantly affected by the anion type, anionic structure and the hydrophobicity of the alkyl chain. The SAILs with more hydrophobic anions and long alkyl chains are expected to favor the micellization. The steric hindrance and hydrophobicity of the cations, as well as the binding strength of the cations with the anions, also play important roles in the aggregation of zwitterionic SAILs. Additionally, the micelle formation mechanism was acquired by detailed analysis of the ¹H NMR spectra. The existence of π–π stacking between imidazolium and counterions was proved. The enhanced π–π stacking and hydrophobic effect of Nsa⁻ can promote the aggregation of zwitterionic SAILs. Density functional theory (DFT) calculations illustrated that the negative interaction energy of the complexes were C₁₂IPS-Bsa/H₂O > SB-12-Nsa/H₂O > C₁₂IPS-Nsa/H₂O > C₁₄IPS-Nsa/H₂O. It is more difficult to form micelles in complexes with more negative interaction energy. The counterion electronegativity of Nsa⁻ is smaller than that of Bsa⁻, which favors the formation of micelles.

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Introduction

Ionic liquids (ILs) are a class of organic electrolytes whose melting points are generally below 373 K.¹ They have been exploited in many fields, including novel nanomaterials synthesis, catalysis, organic synthesis, and electrochemistry,^2–6 due to their extraordinary physicochemical properties, such as no significant vapor pressures, non-flammability, high ionic conductivity, high thermal stability, and easy recyclability.^7–9 An interesting class of ILs, which consists of long hydrophobic chains, can self-assemble to form various aggregates in aqueous solutions. These ILs have been generally named as surface active ionic liquids (SAILs).^10–20

Recently, SAILs have been extensively studied and widely used in the field of colloid and interface science due to their unique chemical and physical properties. Many new SAILs can be created by the combination of different cation and anion. Such as, numerous SAILs with different headgroups like imidazolium, pyridinium, pyrrolidinium cations and different anions (e.g. Cl⁻, Br⁻, NO₃⁻, I⁻, BF₄⁻, CF₃SO₃⁻, CH₃COO⁻, ClO₄⁻) were investigated. The aggregation behaviors of imidazolium-based SAILs in aqueous solutions have been intensively explored. Watson et al. synthesized a series of closely related ionic liquids [C_nmim][X] (n = 4, 8, 12).¹⁰ They found that only [C₈mim][X] and [C₁₂mim][X] can form micelles, revealing the effect of alkyl chain length on the micellization of ILs. The role of anionic types in aggregation behaviors of 1-decyl-3-methlimidazolium chloride ([C₁₀mim]Cl), 1-decyl-3-methlimidazolium hexafluorophosphate ([C₁₀mim]PF₆) and 1-decyl-3-methlimidazolium bis(trifluoromethane) sulfonamide ([C₁₀mim]NTF₂) in aqueous solutions has been explored by Rebelo and Lopes et al.¹¹ It was observed that no aggregates was detected when Cl⁻ was replaced by BF₄⁻or NTF₂⁻. The aggregation behaviors of [C₈mim]Br, 1-octyl-4-methylpyridinium bromide (4-m-[C₈pyr]-Br), and 1-methyl-1-octylpyrrolidinium ([C₈mpyrr]Br) has been investigated by Wang et al.¹² They reported that the hydrophobicity and steric hindrance of the cations play important roles in the aggregation of these SAILs. Above the studies indicated that the aggregation behaviors of SAILs can be regulated by changing the structures of SAILs, such as alkyl chain lengths, cationic structures and anionic types. However, the ion exchange, which is incomplete and time-consuming, is one of the biggest troubles of modulating SAIL structures.

To overcome this problem, zwitterions with a positive and negative charge chemically bonded intramolecularly are the good candidates. The molecular structures of zwitterionic SAILs can be manipulated by the simple mixing of zwitterions and ionic salts.^21–40 The self-assemblies formed by zwitterionic SAILs consisting of imidazolium cations and anionic sites such as sulfonate, sulfonamide, or carboxylic groups have been widely studied. Faruk et al. firstly synthesized the zwitterionic surfactant containing an imidazolium cation and evaluated the chameleon-like behaviors of zwitterionic micelles.²¹ It was found that the molecular structure of zwitterionic surfactant headgroup play important roles in anion binding and control of the micellar zeta potential. Afterwards, they also investigated the effect of cations binding on the chameleon-like nature of zwitterionic micelles.²³ They reported that the micellar surface potential are strongly dependence on the cation valence. Sarkar et al. studied the addition of ionic liquids (dimethylethanol ammonium hexanoate-DAH and dimethylethanol ammonium formate-DAF) induced the changes of zwitterionic micelles.²⁵ It was observed that the alkyl chain present on the anion of DAH leads to its partitioning into the CTAB–DAH interface and behavior as a cosurfactant. The DAF having no alkyl chain on the formate anion shows an almost pure salt effect. Additionally, Yu et al. investigated the surface adsorption and micelle formation of imidazolium-based zwitterionic surface active ionic liquids in aqueous solution.²⁶ These results showed that the alkyl chain length affects the micellar properties remarkably and while temperature, pH, and low salt concentration have a slight effect. However, the effect of structure changes on the zwitterionic SAILs self-assembly have not been systematic studied. The intermolecular interactions among cations, counterions, and solvent molecules during micellization are still unclear.

Herein, we synthesized a group of imidazolium-based zwitterionic SAILs containing sulfonate group, N-alkyl-N′-propanesulfonate imidazolium inner salts (Scheme 1). The effect of counterions, cations and alkyl chains on their aggregation behaviors was systematically investigated by surface tension, ¹H NMR, and DLS. The intermolecular interactions during aggregation are revealed and discussed.

Scheme 1 Chemical structures of the zwitterionic surface active ILs used in this study.

Experimental section

Materials and synthesis

1-Bromododecane (98%), 1-bromotetradecane (98%), sodium benzenesulfonate (98%), 1,3-propanesultone (99%), and imidazole (99%) were purchased from J&K Scientific Ltd. β-Naphthalenesulfonic acid sodium salt (Nsa) (98%) and dodecyl-N,N-dimethylammonio-3-propane sulfonate were the products of TCI. CDCl₃ (99.96%) and D₂O (99.96%) were obtained from Sigma-Aldrich. Acetone, NaOH and tetrahydrofuran (THF) were obtained from Shanghai Chemical Co. All the materials above were used without further purification. Triply distilled water was used throughout the whole experiments.

Synthesis of 3-(1-alkyl-3-imidazolio)propanesulfonate β-naphthalene sulfonate (C₁₄IPS-Nsa)^21,34. 1-Bromotetradecane (10 g, 36.1 mmol) in THF was slowly added to the solution of imidazole (2.95 g, 43.2 mmol) in NaOH (50%) aqueous solution (4.33 g, 54.1 mmol). The reaction mixture was refluxed for 3 days. After the reaction, THF was removed by rotary evaporation. The residue was extracted with CH₂Cl₂/water for three times. The produced 1-tetradecylimidazole was dried under a vacuum at room temperature to acquire clear yellow oil (9.20 g, 93.8% yield). 1-Dodecylimidazole was synthesized following the synthetic procedure for 1-tetradecylimidazole.

1-Tetradecylimidazole (9.0 g, 34.1 mmol) was dissolved in 80 mL acetone. Then an equimolar amount of 1,3-propanesultone (4.16 g, 34.1 mmol) in acetone (40 mL) was added dropwise into a solution at 0 °C under a nitrogen atmosphere. Then reaction mixture was stirred for 5 days at room temperature. After the reaction, a white powder was obtained following purification by acetone twice. The 3-(1-alkyl-3-imidazolio)propanesulfonate (C₁₄IPS) was dried under vacuum at room temperature and a white powder was acquired (7.63 g, 58% yield). The C₁₄IPS-Nsa was obtained from the mixture of C₁₄IPS and Nsa with equal mole. The C₁₂IPS-Nsa was synthesized following the synthetic procedure for C₁₄IPS-Nsa.

¹H NMR spectrum (CDCl₃, 400 MHz), δ (relative to TMS), (C₁₄IPS) ppm: 9.67 (s 1H), 7.52 (s 1H), 7.19 (s 1H), 4.57 (t 2H), 4.25 (t 2H), 2.85 (t 2H), 2.39 (t 2H), 1.99 (m 2H), 1.31 (m 22H), 0.89 (t 3H).

¹H NMR spectrum (CDCl₃, 400 MHz), δ (relative to TMS), (C₁₂IPS) ppm: 9.71 (s 1H), 7.57 (s 1H), 7.21 (s 1H), 4.58 (t 2H), 4.25 (t 2H), 2.85 (t 2H), 2.38 (t 2H), 2.05 (m 2H), 1.32 (m 22H), 0.88 (t 3H).

Sample preparation

Samples were prepared by weighing the designed compositions of ionic liquid in stoppered glass vials, and then the H₂O was added to the ionic liquid. The samples were mixed using a vortex mixer and they were stored in a thermostat for at least 1 week for equilibration before further investigations.

Characterization

Surface tension measurements were carried out on a model JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., accuracy ±0.1 mN m⁻¹) using the ring method. The temperature was controlled by a thermostatic bath with an accuracy of 25 ± 0.1 °C. Each sample was equilibrated for 15 min and all measurements were repeated at least twice until the values were reproducible.

The size distributions of the micelles were determined by dynamic light scattering (DLS) using a Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtracFLEX application software program. All measurements were made with a laser diode (780 nm wavelength, 3 m Wnominal, Class IIIB at the scattering angle of 180). The temperature was controlled with a thermostat (F31C, Julabo) with an accuracy of 25 ± 0.1 °C.

¹H NMR spectra were operated on a Bruker Advance 400 spectrometer equipped with a pulse field gradient module (Z axis) using a 5 mm BBO probe. The instrument was run at a frequency of 400.13 MHz at 25 ± 0.1 °C. The obtained chemical shifts (δ_obs) of the discrete protons were examined as a function of concentration below and above CMC.

Results and discussion

Surface properties and micellization parameters

Surface tension measurements were carried out to evaluate the aggregation behavior of SAILs in aqueous solution. Fig. 1 demonstrates the surface tension (γ) versus concentration (C) plots for the solutions of zwitterionic SAILs at 25 °C. The surface tension gradually decreases with increasing IL concentrations until to a plateau region, indicating the formation of micelle. The critical micelle concentration (CMC) can be obtained from the distinct break point and the corresponding γ_cmc (surface tension at CMC) are listed in the Table 1.

Fig. 1 Surface tension as a function of concentration of (a) C₁₂IPS-Nsa, SB-12-Nsa; (b) C₁₂IPS-Nsa, C₁₂IPS–Bsa, and (c) C₁₂IPS-Nsa, C₁₄IPS-Nsa in aqueous solutions at 25 °C.

Table 1 Surface properties and micellization parameters of C_nIPS-Nsa (n = 12, 14), C₁₂IPS-Bsa and SB-12-Nsa in aqueous solutions at 25 °C

	CMC (mmol L^−1)	γcmc (mN m^−1)	Γmax (μmol m^−2)	Amin (Å^2)
SB-12-Nsa	1.6	40.6	2.75	60.4
C12IPS-Bsa	1.8	40.5	2.90	57.3
C12IPS-Nsa	1.0	39.7	3.09	53.8
C14IPS-Nsa	0.2	38.9	3.13	53.1

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Surface activity expresses that a compound has the capacity to reduce the surface tension of solution. As can be seen from Fig. 1a, it is obvious that C₁₂IPS-Nsa can form micelles easily at lower concentration than SB-12-Nsa. The γ_cmc of C₁₂IPS-Nsa is smaller than SB-12-Nsa, indicating the C₁₂IPS-Nsa possesses a greater ability to reduce the surface tension of water. It is probably due to the difference in their cation structures. The delocalized positive charge and steric effect generating by rigid and bulky imidazolium rings are benefit to the micellization.²⁶ The (γ–C) plots for the zwitterionic SAILs with different counterions, C₁₂IPS-Nsa and C₁₂IPS-Bsa in aqueous solution at 25 °C are shown in Fig. 1b. Obviously, compared with C₁₂IPS-Bsa, the smaller value of CMC for C₁₂IPS-Nsa is obtained. The counterions of Nsa⁻ can promote the formation of micelles than that of Bsa⁻. Meanwhile, a lower γ_cmc value for C₁₂IPS-Nsa was also obtained as compared with C₁₂IPS-Bsa, which suggests that C₁₂IPS-Nsa possesses a greater ability to reduce the surface tension of water. It is possible that naphthalene ring can dramatically increase the hydrophobicity of zwitterionic SAILs. Other possibility was proposed that the existence of π–π stacking among Nsa⁻ moieties with imidazolium facilitated the micellization. The (γ–C) plots for zwitterionic SAILs (C_nIPS-Nsa n = 12, 14) with different alkyl chain length are shown in Fig. 1c. It can be seen that with the increasing length of the hydrocarbon chain, the CMC values decrease due to the enhanced hydrophobicity.

Applying the Gibbs adsorption isotherm to the surface tension plots, the maximum excess surface concentration (Γ_max) and the area occupied by a single amphiphile molecule at the air–liquid interface (A_min) were estimated.⁴¹ Γ_max and A_min both reflect the arrangement situation of amphiphile molecules at the air–liquid interface.²⁰ A greater value of Γ_max or a smaller value of A_min means a denser arrangement of surfactant molecules at the surface of solution. As shown in Table 1, the Γ_max value of C₁₄IPS-Nsa is lower than C₁₂IPS-Nsa, indicating that C₁₄IPS-Nsa molecules at the air–water interface are arranged more densely than C₁₂IPS-Nsa. It is also clearly seen that C₁₂IPS-Nsa has a smaller Γ_max value than C₁₂IPS-Bsa and SB-12-Nsa, although C₁₂IPS-Nsa have a much larger headgroup. Except the enhanced hydrophobicity of C₁₂IPS-Nsa, it is may be likely that the enhanced π–π stacking between the cations and anions of C₁₂IPS-Nsa could attract the molecules to pack more compactly at the air–water interface.

The micelle sizes and size distributions of zwitterionic SAILs

DLS was utilized to further investigate the size distributions of the aggregates formed by zwitterionic SAILs. The micelle sizes of 10 mM zwitterionic SAILs aqueous solution are shown in Fig. 2. When the counterions varies from benzene sulfonic anions to naphthalene sulfonic anions, the hydrodynamic radii (R) of the micelles increases from 1.8 to 2.3 nm. It can be explained by the more hydrophobic and steric effect of naphthalene ring than benzene ring.⁴² As shown in Fig. 2b, compared with the SB-12-Nsa, the imidazole ring of C₁₂IPS-Nsa induced the size of aggregate from 1.7 to 2.3 nm. This is probably due to the higher hydrophobic zwitterionic group of C₁₂IPS-Nsa, which has less reorganizing structure effects of water than ammonium cations.²⁶ Compared with C₁₂IPS-Nsa (R = 2.3 nm), the larger values of R for C₁₄IPS-Nsa (R = 2.5 nm) is ascribed to the increased hydrophobicity resulting from the extension of the hydrocarbon chain.

Fig. 2 Size and size distributions of the aggregations formed by 10 mM zwitterionic SAILs at 25 °C.

Mechanism

NMR analysis is a sensitive and accurate technique to investigate the intermolecular interaction of classical surfactants.^43–50 The protons of zwitterionic SAILs have different chemical shifts (δ_obs) below and above the CMC, and the observed chemical shifts (ESI Table S1†) can reflect the microenvironment of protons in the aggregates. Before the formation of micelle, no significant change in chemical shifts is observed, since the zwitterionic SAILs molecules are only dissolved in D₂O as monomer. However, upon micellization, the chemical shifts of protons change obviously. The change of the chemical shift, Δδ (=δ_obs − δ_mon), as a function of concentration for various zwitterionic SAILs, is shown in Fig. 3.

Fig. 3 Variation in chemical shifts for protons of zwitterionic SAILs at different concentration at 25 °C; (a) C₁₂IPS-Nsa, (b) C₁₄IPS-Nsa, (c) C₁₂IPS-Bsa, and (d) SB-12-Nsa.

In general, the chemical shifts of protons on the surfactants move downfield during aggregation in the absence of any other specific interaction.^37,42 As can be seen from Fig. 3a, b and d, the protons on hydrocarbon chains of zwitterionic SAILs show an upfield shift upon aggregation except the terminal methyl group protons (H₇) of C₁₄IPS-Nsa, C₁₂IPS-Nsa and SB-12-Nsa. This observed trend can be deduced that the naphthalene sulfonic anion would penetrate into the hydrophobic regions. The insertion of naphthalene ring brings a more significant shielding effect to the protons on the alkyl chain due to the circular current effect. The upfield shifts of terminal methyl group protons can be attributed to the shielding effect. Meanwhile, the insertion of naphthalene ring restrains the conformation change of alkyl chain from trans to gauche conformation, leading to the upfield shift of the protons on terminal methyl group. It is also interesting to find that the terminal methyl group protons (H₇) of C₁₂IPS-Bsa shift downfield and the protons (H₉) on the C-9 between imidazolium ring and sulfonic acid are upfield shift in the Fig. 3c. The decreasing polarity of microenvironment lead the terminal methyl group protons (H₇) downfield shift. The shielding effect may be contributed to the upfield shift of H₉. We proposed that the benzene sulfonic anion is adsorpted at the surface of the micelles. This suppose of conformation is confirmed by the optimized the structures from DFT calculations.

The protons at C-3 of the imidazolium ring have no signals due to the vicinity of electronegative nitrogen, and are available for hydrogen bonding with water.⁴¹ When the counterion is naphthalene sulfonic acid anions (see Fig. 3a, b and d), the protons of naphthalene ring and headgroup are upfield shift. This is mainly because of the existence of π–π stacking among adjacent naphthalene moieties and the π–π stacking between naphthalene ring and imidazolium ring. The resulting circular current effect makes the protons around the headgroup shift upfield due to the shielding effect.^13,46,48,49 Additionally, the shifts of the protons on the head group of C₁₂IPS-Nsa are more obvious than SB-12-Nsa, as shown in Fig. 4a and d. This result is possibly due to the enhanced shielding effect by the π–π stacking of imidazolium ring with naphthalene ring. With the results shown in Fig. 4c and a conclusion is that the protons of aromatic ring near the sulfonate (H₁₁, H₁₅) shift upfield, and other protons on aromatic ring (H₁₂, H₁₃, H₁₄) shift downfield. The imidazolium ring and phenyl were combined together through the cooperative effect of π–π stacking and electrostatic attraction. The circular current effect leads the protons (H₁₁, H₁₅) to shift a higher field. In addition, this circular current effect has quite rigorous geometrical restriction; only the protons localized in the field of the ring are substantially shifted.^48,49 Other protons, H₁₂, H₁₃, H₁₄, were adsorpted at the surface of the micelle and cannot be affected by the shielding effect, and thus the protons shift downfield. According to the analysis above, the possible packing structures of molecules in the micelle are depicted and shown in Scheme 2.

Fig. 4 Geometries optimized at the B3LYP/6-31G (d, p) level: (a) C₁₂IPS-Nsa, (b) C₁₄IPS-Nsa, (c) C₁₂IPS-Bsa, and (d) SB-12-Nsa. Color code for atoms: blue, nitrogen; red, oxygen; dark gray, carbon; light gray, hydrogen.

Scheme 2 Schematic illustration of the proposed structure of micelle formed by zwitterionic SAILs (C₁₂IPS-Bsa, C₁₂IPS-Nsa as examples) in aqueous solution.

DFT calculations

To better understanding the aggregation behavior mechanism of zwitterion SAILs, the interactions of zwitterion SAILs with H₂O were investigated. Density functional theory (DFT) calculations using the Gaussian 09 package with the hybrid B3LYP functional and the 6-31G (d,p) basis set were performed. The structures of the isolated zwitterion SAILs (C₁₂IPS-Nsa, C₁₄IPS-Nsa, C₁₂IPS-Bsa, SB-12-Nsa), H₂O, and the (1 : 1) binary complex of zwitterion SAILs with H₂O were first optimized. The most stable optimized structures are presented in Fig. 4. Their interaction energies (E_int), which is the energy difference between complexes and the monomers, are listed in the Table 2. As shown in Table 2, the order of the calculated negative interaction energy of the complexes were C₁₂IPS-Bsa/H₂O > SB-12-Nsa/H₂O > C₁₂IPS-Nsa/H₂O > C₁₄IPS-Nsa/H₂O. The complexes with more negative interaction energy are more difficult to form micelles,^51–55 resulting in a higher CMC value. This is consistent with the surface tension measurement results (Table 1). Furthermore, the electrostatic potentials at the 0.001 e bohr⁻³ isodensity surfaces of Nsa⁻ and Bsa⁻ counterions were shown in the Fig. 5. It is clearly that both counterions are electronegative and the electronegative of Nsa⁻ is smaller than Bsa⁻. This phenomenon means that the electrostatic repulsion of Nsa⁻ is weaker than that of Bsa⁻ anion, which primarily accounts for the lower CMC value of C₁₂IPS-Nsa than C₁₂IPS-Bsa.

Table 2 Interaction energy (−E_int) of zwitterion SAILs with H₂O

Zwitterion SAILs	C14IPS-Nsa	C12IPS-Nsa	C12IPS-Bsa	SB-12-Nsa
Interaction energy (−Eint/kJ mol^−1)	5.77	8.26	30.10	17.39

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Fig. 5 B3LYP/6-31G (d,p) electrostatic potentials, in hartrees, at the 0.001 e bohr⁻³ isodensity surfaces of Bsa⁻ and Nsa⁻ counterions.

Conclusion

The effects of different counterions, cations and alkyl chain lengths on the aggregation behavior of zwitterionic SAILs were investigated. The existence of π–π stacking in the counterions, the increase of alkyl chain length and the larger volume of cations can reduce the values of CMC and increase the size of micelles. ¹H NMR results illustrate that the imidazolium cation and aromatic anion are combined together through the cooperative effect of electrostatic, π–π stacking and hydrophobic interactions. The protons on imidazolium ring and naphthalene ring shift upfield due to the shielding effect, which is a powerful evidence for the existence of π–π stacking. DFT calculations mainly manifest the negative interaction energy of the complexes were C₁₂IPS-Bsa/H₂O > SB-12-Nsa/H₂O > C₁₂IPS-Nsa/H₂O > C₁₄IPS-Nsa/H₂O, revealing C₁₄IPS-Nsa is easier to form micelles in aqueous solutions. The electronegative of Nsa⁻ is smaller than that of Bsa⁻, which is also favor the micellization. The obtained results can help us to regulate the self-assembly of zwitterionic SAILs and lay a foundation for their application in various fields.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No. 21573132), the National Basic Research Program (2013CB834505), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120131130003) and the National Natural Science Foundation of China (No. 21403151).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02986h

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