Phase Transition of a Quaternary Ammonium Gemini Surfactant

Jan 24, 2014 - Utkarsh U. More , Zuber S. Vaid , Sargam M Rajput , Naved I Malek , Omar A. El Seoud. Colloid and Polymer Science 2017 280, ...
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Phase Transition of a Quaternary Ammonium Gemini Surfactant Induced by Minor Structural Changes of Protic Ionic Liquids Qintang Li, Xudong Wang, Xiu Yue, and Xiao Chen* Key Laboratory of Colloid and Interface Chemistry, Shandong University Ministry of Education, Jinan, 250100, China S Supporting Information *

ABSTRACT: The aggregation behaviors of a Gemini surfactant [C12H25(CH3)2N+(CH2)2N+(CH3)2C12H25]Br2− (12-2-12) in two protic ionic liquids (PILs), propylammonium nitrate (PAN) and butylammonium nitrate (BAN), were investigated by means of several experimental techniques including small and wide-angle Xray scattering, the polarized optical microscopy and the rheological measurement. Compared to those in ethylammonium nitrate (EAN), the minor structural changes with only one or two methylene units (−CH2−) increase in cationic chain length of PIL, result in a dramatic phase transition of formed aggregates. The critical micellization concentration was increased in PAN, while no micelle formation was detected in BAN. A normal hexagonal phase was observed in the 12-2-12/PAN system, while the normal hexagonal, bicontinuous cubic, and lamellar phases were mapped in the 12-2-12/BAN system. Such aggregation behavior changes can be ascribed to the weaker solvophobic interactions of 12-2-12 in PAN and BAN. The unique molecular structure of 12-2-12 is also an important factor to highlight such a dramatic phase transition due to the PIL structure change.

1. INTRODUCTION Ionic liquids (ILs), composed solely of ions, are usually liquids under 100 °C.1−4 As a novel type of solvents, they have attracted much interest in a wide range of applications, such as synthesis,5 catalysis,6−8 electrochemistry,9,10 and other areas due to their unique physicochemical properties. Specifically, their abilities to support the self-assembly of amphiphiles are more and more recognized recently.2,4 Compared to the aprotic ionic liquids (APILs), the protic ionic liquids (PILs) can form a hydrogen-bonded network in the bulk because of the presence of proton donor and acceptor sites, which may help to design lyotropic liquid crystals (LLCs) of high thermo-stability.11−13 Self-assemblies in PILs could be dated back to the early works by Evans in the 1980s,14−16 where ethylammonium nitrate (EAN), the first truly studied PIL, was used as the media of self-assembly, showing more analogous properties to water than other PILs.17,18 They found that in EAN, the micelles could be formed from n-alkyltrimethylammonium bromides, while LLCs formed from lipids. From then on, various aggregation behaviors of surfactants in EAN have been reported. The nonionic polyoxyethylene alkyl ether surfactants were noted to form LLCs by Warr’s group.19 Drummond et al. observed the existence of LLCs from hexadecyltrimethylammonium bromide (CTAB),20 a monoolein and a phytantriol.21 A more detailed study on the aggregation behaviors of CTAB from Wanger’s group has indicated a reversible and temperature-dependent phase transition from the micellar (L1) to the hexagonal phase (H1).22 The double chain cationic molecule, didodecyldimethylammonium bromide (DDAB), however, aggregates in EAN to form vesicles coexisting with a sponge © 2014 American Chemical Society

phase (L3) in the dilute regime and a lamellar phase (Lα) at higher concentrations.23,24 We have also investigated the aggregation behaviors of the nonionic block copolymer EO20PO70EO20 (P123),25 the oleyl polyoxyethylene (10) ether (Brij 97),26 the phytosterol ethoxylate (BPS-n, n = 10, 20, 30)11,27 and a cationic 1-hexadecyl-3-methylimidazoliium chloride (C16mimCl)28 in EAN. Similar or novel LLC phases that could be rarely formed in APILs or water have been confirmed. Although much attention has been paid to the aggregates in EAN, limited studies could been found in its analogues with increased alkyl chain length of cations. Similar to that in EAN, the alkyl oligoethyleneoxide (CiEj) surfactants can also selfassemble into micelles and LLCs in propylammonium nitrate (PAN).29,30 Varela et al. reported the formation of micelles by surfactants from alkyltrimethylammonium and pyridinium families in EAN and PAN.31 It was indicated that PAN has better hydrocarbon solubility due to a longer alkyl chain. The formation of micelles would be therefore less favorable in PAN than in EAN. Compared to PAN, much less work has been done in another analogue of EAN, butylammonium nitrate (BAN), perhaps due to even worse possibility for micelle formation.30 As a continuation of our recent studies in EAN on the aggregation behaviors of the cationic quaternary ammonium Gemini surfactants with a general formula Received: December 17, 2013 Revised: January 24, 2014 Published: January 24, 2014 1522

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[CmH2m+1(CH3)2N+(CH2)sN+(CH3)2-CnH2n+1]Br−2 (abbreviated as m-s-n, where s represents the number of carbon atoms in the spacer, while m and n refer to the numbers of carbon atoms in two alkyl chains),12,13 the effect of PIL alkyl chain length on the phase transition has been noted. Therefore in this paper, the phase behaviors of 12-2-12 in PAN and BAN over a whole concentration range have been presented. Except for the unique properties and tailorable structure of Gemini surfactants,32 the minor structural change of PILs does cause a dramatic transition of the aggregate phase state. Especially, the phase structure reversion from the reversed hexagonal phase to the normal one in such homologue solvents has not yet been seen, which is just because the delicate synergetic effects both from solvent and also from the unique structure of Gemini. The results discussed herein should prove to be a useful complement to the growing body of literature regarding the self-assembled structure of Gemini surfactants in PILs and the effects of solvents.

alcoholic flame until it glowed before each measurement. All measurements were repeated at least twice until the values were reproducible. 2.3.4. Freeze-Fracture Transmission Electron Microscopy (FFTEM). A small amount of solution was mounted on a specimen holder. The sample was frozen by quickly plunging the holder into the liquid ethane cooled by liquid nitrogen. Fracturing and replication were carried out on a freeze−fracture apparatus (EM BAF060, Leica, Germany) at a temperature of −150 °C. Pt/C was deposited at an angle of 45° to shadow the replicas and C was deposited at an angle of 90° to consolidate the replicas. The resulting replicas were transferred onto copper grids and then observed using a JEOL JEM-1400 TEM operated at 120 kV. 2.3.5. Rheological Measurement. The rheological measurements were carried out in the same way as we described before,12,13 with a Haake Rheostress 6000 rheometer using a Rotor C35/1 system at desired temperatures. Frequency sweep measurements were performed in the linear viscoelastic region, which was determined from the strain sweep measurement with the stress varying at a constant frequency of 1.0 Hz.

3. RESULTS AND DISCUSSION In our previous reports,12,13 the 12-2-12 molecules have been observed to self-assemble into micelles and a reverse hexagonal phase (H2) in EAN. To explore the effect of the chain length in protic ionic liquids, the phase behaviors of 12-2-12 in PAN and BAN were investigated using POM and SAXS techniques. 3.1. Phase Behavior of 12-2-12 in PAN. The temperature−composition (T-wt%) phase diagram of the 12-2-12/ PAN system has been mapped and shown in Figure 1. As can

2. EXPERIMENTAL SECTION 2.1. Materials. The Gemini surfactant 12-2-12 was prepared according to the procedures reported previously.32,33 Its purity was ascertained first by 1H NMR (Nuclear Magnetic Resonance) in CDCl3 and then by the elementary analysis (Supporting Information). The ionic liquids EAN, PAN, and BAN were synthesized based on methods as described by Evans et al.15 In a typical preparation, 150 mL of ∼3 M nitric acid was slowly added to the ethylamine solution (11.56 M, 58.4 mL) while stirring and cooling in an ice bath. The water in the product solution was first removed with a rotary evaporator and then with a lyophilizer (MartinchristerALPHA1−2). The residual water content of the final product was determined by Karl Fischer titration to be 0.5 wt %, and its melting point was about 12 °C. The purity was ascertained by 1H NMR in D2O (Supporting Information). The other two ionic liquids were prepared in a similar way. 2.2. Sample Preparation and Phase Diagram Mapping. The process for mapping the phase diagram has been described elsewhere.28,34 All samples were prepared by mixing the 12-2-12 surfactant and PILs with designed compositions (in weight percent, wt %, hereinafter). These mixtures were homogenized by repeating mixing and centrifugation. Then they were equilibrated for at least 3 months before further investigations. The phase change was detected by ocular observation and visual inspection through the crossed polarizers. The liquid crystal types were determined by the polarized optical microscopy and small-angle X-ray scattering techniques. The composition interval was first selected as 5 wt % for a rough mapping and then 2 wt % for the determination of the phase boundaries. 2.3. Characterization. 2.3.1. Small-Angle X-ray Scattering. The X-ray scattering measurements were performed by a SAXSess MC2 highflux small-angle X-ray scattering (SAXS) instrument (Anton Paar, Austria, Cu−Kα, λ = 0.154 nm), equipped with a Kratky blockcollimation system and an image plate (IP) as the detector. The X-ray generator was operated at 40 kV and 50 mA. A standard temperature control unit (Anton-Paar TCS 120) connected with the SAXSess was used to control the temperature at a desired value. The obtained LLC phases were characterized by SAXS, while the dilute samples were transferred into the standard quartz capillary of a diameter of 1 mm, where both small and wide-angle X-ray scattering (SWAXS) profiles can be simultaneously recorded. 2.3.2. Polarized Optical Microscopy. Photographs of samples with birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WVCP460). 2.3.3. Surface Tension Measurement. A tensiometer QBZY-2 from Fangrui (China) was used with a Wilhelmy platinum plate to measure the critical micelle concentration (CMC) at a constant temperature of 25.0 ± 0.1 °C. The plate was cleaned well and heated briefly in an

Figure 1. Phase diagram of 12-2-12/PAN binary system. L1, H1, and V1 denote the normal micelle solution, the hexagonal phase, and the bicontinuous cubic phase, respectively.

be seen, the LLC phase behavior of 12-2-12 here is not the same as and even “contrary” to that in EAN, where only a reverse hexagonal phase was observed.12,13 At a low 12-2-12 concentration (below 53 wt %), an isotropic solution is formed with only one weak and broad peak in its SAXS curve, indicating the formation of micelles. A narrow two-phase coexisting region appears at concentrations between 53 and 56 wt %, as illustrated by the visual images of a 55.0 wt % sample (Supporting Information Figure S1). At a concentration higher than 56 wt %, a normal hexagonal phase (H1) is formed. Then an isotropic phase appears when the concentration goes up to 80 wt %, which can be identified as a bicontinuous cubic phase (V1). These LLC phases were determined based on both the general phase sequence,35 and the relatively lower Gordon parameter of PAN than that of EAN,20,21 which should therefore result in a higher threshold concentration than in EAN to form the reverse hexagonal phase. More detailed 1523

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Figure 2. (a) SWAXS curves for 12-2-12 at various concentrations in PAN; (b) FF-TEM image of micelles formed in 15.0 wt % 12-2-12/PAN sample.

presentation and discussion on aggregate structure will be depicted as follows. The Micellar Phase (L 1 ). According to the basic physicochemical properties of PILs, the studied EAN, PAN and BAN all have an intermediate range order depending on the chain length of cations.36 To demonstrate the formation of micelles in the 12-2-12/PAN system, the SAXS combined with WAXS measurements were adopted and the obtained scattering curves with 0, 5.0, 10.0, and 15.0 wt % 12-2-12 in PAN at 25 °C are shown in Figure 2a. Two wide peaks can be clearly seen in the curve of PAN with the first one at low scattering factor (q) due to nonpolar domains of PILs and the second one at high q corresponding to the correlation distance of alkyl chains.30,37,38 With 5.0 wt % 12-2-12 in PAN, the first peak moves left of a lower q due to the dispersion of 12-2-12 among the PAN molecules, indicating small changes in the PAN bulk structure. Increasing 12-2-12 content more, however, this peak becomes broad and is attributed to the overlapping of scatterings from both the micelles and the PAN.30 The CMC of 12-2-12 is thus regarded as a value around 10.0 wt %, in accordance with that (10.8 wt %) obtained by the surface tension measurement (see below). When the 12-2-12 concentration comes to 15.0 wt %, the scattering from micelles becomes so strong that the first peak from PAN is nearly submerged.30 Unlike the rodlike and even wormlike micelles formed with increasing surfactant concentration in water,39,40 those formed in PILs become much smaller in size due to the decreased solvophobicity. With the solvent changed from EAN to PAN, the weaker solvophobic interactions would result in smaller aggregates. The micelles will thus become less ellipsoidal then, which has been observed in single chain cationic and nonionic surfactants in PILs.29,31 The evidence on micelle formation is also manifested by FFTEM observation with the result shown in Figure 2b. Many spherical micelles with a diameter about 20 nm could be observed in the sample of 15.0 wt % 12-2-12 in PAN. Therefore, the CMC value of 12-2-12 in PAN is much larger than that in EAN (lower than 5 wt %, see detailed results shown in Figure S2 in Supporting Information). To obtain the thermodynamic parameters on 12-2-12 micellization behaviors in PAN, the surface tension (γ) measurement was also carried out with the obtained result shown in Figure 3. For comparison, the profile measured in EAN before was also shown.13 Similar to those of 12-2-12 in EAN and water,13,41 a typical surfactant-like curve of γ on log of 12-2-12 concentration can be observed, indicating the formation of micelles. The corresponding CMC value in PAN (198.8 mmol·L−1 or 10.8 wt %) is much larger than that in EAN, which is a common result as observed from n-

Figure 3. Surface tension curves of 12-2-12 in PAN and EAN at 25 °C.

alkyltrimethylammonium or n-alkylpyridinium chlorides and bromides in PILs.31 Such a fact suggests a much lower solvophobic effect of 12-2-12 in PAN than in EAN, due to the longer cation chain length of PAN. Other surface parameters, such as the effectiveness of γ reduction (ΠCMC), the surface excess at the air/IL interface (Γmax), the minimum area per surfactant molecule adsorbed at the air/IL interface (Amin), and the standard Gibbs free energy of micellization (ΔGm), can be calculated according to eqs 1−4 below: ΠCMC = γ0 − γCMC Γmax = −

A min =

dγ RT d ln CMC

1 NA Γmax

(1) (2)

(3)

ΔGm = 2RT (1.5 − β) ln XCMC

(4)

where γ0 and γCMC are surface tensions of the pure solvent and the solution at CMC, respectively, T is the absolute temperature, NA is Avogadro’s number, and R is the gas constant. The obtained parameters and those in EAN13 are listed in Table 1 for comparison. The Amin in PAN is obviously larger than that in EAN due to its better hydrocarbon solubility and more penetration between 12-2-12 molecules. The interactions between Gemini surfactants are hence weakened and the micellization process becomes less favorable, as is reflected by the smaller ΔGm value in PAN. The Hexagonal Phase (H1). The samples of the H1 phase are stiff and transparent in appearance with clear birefringent POM textures as shown in Figure 4a−c. With increasing 12-2-12 1524

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Table 1. Surface Properties of 12-2-12 in PAN and EAN at 25 °C

a

solvent

CMC (mmol·L−1)

γCMC (mN·m−1)

ΠCMC (mN·m−1)

Γmax (μmol·m−2)

Amin (Å2)

ΔGm (kJ·mol−1)

PAN EANa

198.8 25.32

33.23 32.42

6.85 17.68

0.643 2.03

258 81.8

−9.29 −15.07

Data of 12-2-12 in EAN was taken from ref 13.

Figure 4. POM images (a, b, c) and SAXS curves (d) for samples of a 12-2-12/PAN system observed at 40 °C and different concentrations (wt %). From a to c, C12‑2‑12 = 64.9, 69.6, and 74.7.

concentration from 64.9 wt % to 69.6 wt % and then to 74.7 wt %, the typical fanlike texture becomes more and more regular and perfect, indicating a more densely packing of Gemini molecules.12,13 The SAXS measurements provide further structural details of the H1 phase. As can be seen from the SAXS patterns shown in Figure 4d, three sets of Bragg peaks could be identified with their relative positions (take 64.9 wt % sample as an example, q = 2.34, 4.05, and 4.68 nm−1) corresponding to a 1:√3:2 relationship, indicating the hexagonal phase structure. The second scattering peak looks relatively weak, which has been observed in other systems and ascribed to a cancellation between the factors of form and structure for the SAXS intensity.25,42−44 The structure parameters could therefore be calculated and analyzed based on the H1 phase model. The lattice parameter, D (i.e., the distance between two centers of the neighboring cylinders), can be calculated from the scattering factor corresponding to the first Bragg peak (q1) according to the formula D = 4π/√3q1. Other parameters can be derived then from D12,45 with results listed in Table 2. From Table 2, the D values are observed to expand from 31.1 to 32.9 Å with 12-2-12 concentration increased from 64.9 to 74.7 wt %. The R1 values increase correspondingly from 13.2 to 15.0 Å, while the d1 values decrease from 4.7 to 2.9 Å. All these results suggest that the column micellar core becomes more expanded and the solvent layer becomes thinner with increasing 12-2-12 concentration. Such a phenomenon is also observed in Gemini surfactant systems with EAN as solvent where a reverse hexagonal phase is formed, which was attributed to the expansion of hydrocarbon layer of the cylinder.12 For a normal

Table 2. Structural Parameters of H1 Phases in the 12-2-12/ PAN System concentration (wt %)

temperature (°C)

D (Å)

R1 (Å)a

d1 (Å)a

S1 (Å2)a

64.9 69.6 74.7 69.6 69.6

40.0 40.0 40.0 55 70

31.1 32.1 32.9 31.7 31.4

13.2 14.1 15.0 13.9 13.8

4.7 3.9 2.9 3.9 3.8

139.2 130.4 122.5 131.9 133.0

a

R1 is the radius of cylinder-like aggregates; d1 is the thickness of solvent layer between cylinders; S1 is the area per molecule of surfactants at the solvophilic/solvophobic interface. (D = 2R1 + d1)

hexagonal phase in PAN here, this should be explained from more densely packing of surfactants and the substitution of PAN molecules by 12-2-12 in the palisade layer of cylinder with the reduction of PAN amount. In other words, with increasing the 12-2-12 concentration from 64.9 to 74.7 wt %, the PAN molecules involved in the cylinder palisade become less due to the stronger hydrocarbon interactions between Gemini molecules than those between Gemini and PAN molecules. Such a change can also be reflected by the diminishing S1 values with increasing 12-2-12 amount, indicating a more densely packing of Gemini molecules, which is in accordance with the POM observations in Figure 4a−c. To illustrate the effect of temperature on the H1 phase, the sample with 69.6 wt % 12-2-12 is taken as an example here. Although the H1 phase region covers a temperature range from 25 to 85 °C, the typical fanlike texture are rather clear below 75 °C. The texture begins to deteriorate at 80 °C and extinguish at 1525

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85 °C, when the sample turns into an isotropic phase. The corresponding SAXS curves also prove such a phase transition (Figure S3) and the obtained structure parameters at two other temperatures (55 and 70 °C) are also listed in Table 2. The first SAXS peak moves to a higher q position with the increase of temperature, indicating a smaller repeat lattice spacing. This can be considered to originate from the temperature-inducing conformational fluctuation of the hydrophobic chains, which may shorten the average length of alkyl chains,12 as reflected by the shrinkage of R1 values. The viscoelastic properties of this hexagonal phase were characterized by the rheological measurement. A typical frequency sweep result for the H1 phase formed at 69.6 wt % 12-2-12 in PAN is shown in Figure S4. The storage (G′) and loss (G″) moduli values increase with the frequency (f) and overlap at a certain point, after which the G′ keeps increasing while G″ decreasing with f. This behavior is typical for the hexagonal phase and follows a Maxwell model: liquid-like (G″ > G′) at low frequencies and solid-like (G′ > G″) at high frequencies.46 The Bicontinuous Cubic Phase (V1). The V1 phase appears closely behind the H1 phase, forming a two-phase coexisting region at the 12-2-12 concentration around 77.8 wt %. The phase structures at this concentration is characterized by SAXS at different temperatures with results shown in Figure S5. At low temperatures (below 45 °C), two sets of Bragg scattering peaks with their relative positions corresponding respectively to a H1 phase (1:√3:2) (q = 2.16, 3.74, and 4.32 nm−1) and a V1 phase (√3:√4) (q = 2.26, and 2.60 nm−1) are observed. With the increase of temperature, the peaks from the H1 phase become weak, while those from V1 become more and more obvious. Thus, a phase transition occurs from the coexisting phase (H1 + V1) to the V1 phase, which has no birefringent textures observable by POM. Such a V1 phase was also detected in the 12-2-12/BAN system and its detailed structural characters will be described there. 3.2. Phase Behavior of 12-2-12 in BAN. The mapped temperature−composition (T−wt%) phase diagram of the 122-12/BAN system is shown in Figure 5. Except for the

Unlike the micellar phase observed in the 12-2-12/PAN system, the aggregation becomes different in the 12-2-12/BAN system. This conclusion was obtained from the SWAXS measurements in BAN at different 12-2-12 concentrations with the results shown in Figure 6. As can be seen from these

Figure 6. SWAXS curves for 12-2-12 at various concentrations in BAN.

profiles, the first scattering peak position, which is closely related to the formation of micelles in PIL, moves left to lower q values with the increase of 12-2-12 concentration. It was suggested, however, that the first peak position would keep constant when micelles were formed.30 Then, such a scattering peak could not result from micelle formation, but still from the nonpolar domains of BAN. This point can be further verified by FF-TEM observation (Figure S6) where no sign of micelles was identified in the 50.0 wt % 12-2-12/BAN sample. It has been indicated from the above discussion that a more unfavorable micellization process occurs when the solvent changes from EAN to PAN. For BAN, the increased cation chain length will further improve the hydrocarbon solubility of Gemini molecules and therefore weaken the solvophobic interaction. Similar observation for undetectable micelle has also been reported in nonionic surfactants/BAN system due to weak solvophobic interactions.30 Therefore, the driving forces are insufficient for 12-2-12 molecules to aggregate into micelles in BAN. To exhibit the rich phase behaviors in the BAN system, three typical concentrations of 12-2-12 corresponding respectively to the formation of hexagonal, bicontinuous cubic, and lamellar phases were chosen for discussion with their SAXS and POM data shown in Figure 7. The Hexagonal Phase (H1). In BAN medium, the hexagonal phase could be still formed at a 12-2-12 concentration range between 57 and 66 wt %. The typical SAXS curve and POM image of the 63.6 wt % sample are shown in Figure 7a. The fanlike texture and 1:√3:2 relative q positions corresponding to three Bragg peaks (q = 2.36, 4.10, and 4.71 nm−1) clearly indicate the existence of a hexagonal structure. It is indentified as a normal hexagonal morphology based on its occurrence position in the phase diagram, which is usually located before that of the lamella phase.25,35 The phase parameters are calculated with R1 = 12.9 Å, d = 5.0 Å, and S1 = 142.9 Å2. It can be noted from the phase diagram that the H1 phase formation in BAN starts at a higher 12-2-12 concentration and presents lower phase transition temperatures compared to those in PAN. This can be explained directly by the reduced Gordon parameter due to the longer cation chain in BAN, and thus the weak driving force to form well-organized aggregates.4

Figure 5. Phase diagram of 12-2-12/BAN binary system.

occurrence of a lamellar phase at high 12-2-12 concentrations, the similar phases seem also to appear in BAN compared to those in PAN. The samples appear like clear transparent solutions at a wide 12-2-12 concentration region below 55 wt %. With increasing concentration, three lyotropic liquid crystalline phases are present in a sequence from hexagonal to bicontinuous cubic and to lamellar ones. However, the longer cation alkyl chain in BAN really causes certain different aggregation behaviors. 1526

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Figure 7. SAXS curves for the lyotropic phase at different 12-2-12 concentrations (wt%) in BAN at 40 °C with the corresponding POM images inset. From a to c, C12−2−12 = 63.6, 67.6, and 81.9.

The Bicontinuous Cubic Phase (V1). Similar to the situation in PAN, the V1 phase follows closely to the H1 phase at the 122-12 concentration ranging from 66 to 69 wt %. Compared to the H1 phase, the samples from V1 phase behave a little more rigid. Take the sample at 67.6 wt % 12-2-12 as an example for analysis. Its SAXS curve and corresponding POM image are shown in Figure 7b. Although the sample looks transparent, its POM image only exhibits a dark background to indicate an isotropic structure for poor influence on the polarization of the light. The SAXS pattern is obviously different from that of the H1 phase and displays clearly two Bragg peaks (q = 2.42 and 2.9 nm−1) with their positions of a√3:√4 relationship, corresponding to probably the first (211) and second (220) reflections from the phase structure of the Ia3d bicontinuous cubic symmetry. Meanwhile, such two scattering peaks may also correspond to the second (111) and third (200) reflections from the Pn3m cubic structure, or the third (211) and fourth (220) reflections from the Im3m cubic structure. However, such possibilities of the latter two structures could be excluded because their first reflections, which usually should be the most intense, are not found in the SAXS curve. In addition, the possible micellar cubic phase for such a scattering profile is also excluded because it is usually located between the micellar and the H1 phases. Therefore, this phase is ascribed to the V1 phase of Ia3d symmetry. Such a V1 phase is not stable at high temperatures and liable to transform into the Lα phase. The Lamellar Phase (Lα). Not the same as in EAN and PAN systems, a new birefringent phase (Lα) region is found at high 12-2-12 concentrations (above 70 wt %), coexisting with the V1 phase. The samples are transparent and softer than those of the V1 phase. Figure 7c shows the SAXS curve and POM image for a sample of 81.9 wt % 12-2-12 measured at 40 °C. The typical oily streak texture and the 1:2 q position relationship for two SAXS Bragg peaks (q = 2.36 and 4.72 nm−1) definitely confirm a lamellar structure of this phase, which could be stable over 100 °C. The phase parameters are calculated according to equations reported before.11 The thickness of solvophobic domain, dc, is 10.9 Å, smaller than the hydrocarbon chain length of 12-2-12, due to the interdigitation of alkyl chains. The effective cross-sectional area per surfactant molecule, as, becomes much smaller (84.54 Å2) than that of the H1 phase, perhaps due to the increased solvophobic interactions at higher 12-2-12 concentrations. Rheological Measurements. The rheological properties of three formed LLCs were characterized with their frequency sweep results shown in Figure S7. The H1 phase here exhibits a similar viscoelastic behavior to that in the 12-2-12/PAN system. Both G′ and G″ moduli values are smaller and keep increasing

with frequency, though the G″ can be still seen higher than G′ at low f, while lower than G′ at high f values. Such a behavior fits well with the Maxwell fluid model, where the H1 phase is capable of flowing at low f under shear and behaves like an elastic solid at high f. As for the V1 phase, the G′ and G″ values are significantly larger than those of the H1 phase, as reflected by its stiff and elastic properties. A finite relaxation time is observed in this sample, further indicating that this cubic phase belongs to a bicontinuous structure.47,48 When it comes to the Lα phase, the G′ and G″ values overlap at low f, and G′ dominates in a wide frequency range. Therefore, with the increase of 12-2-12 concentration, the H1, V1, and Lα phases are formed in sequence, and the samples behave more and more elastic. 3.3. Effects of PILs on 12-2-12 Aggregate Structure. How do PILs (EAN, PAN, and BAN) act during the aggregation of 12-2-12? Two aspects of view, that is, the changes of driving forces and the structure of aggregates, could be considered to explain their influences. The driving force of solvent for the surfactant self-assembly is usually evaluated by Gordon parameter (G).4 It can be estimated by eq 5: G=

3

γ Vm

(5)

where γ is the surface tension at the air/liquid interface, and Vm is the solvent molar volume. The G value can be used as a measure of the cohesive energy density of solvents. The higher the Gordon parameter is, the stronger the solvent driving force acts for self-assembly. Water as a solvent has been known to exhibit the strongest driving force for 12-2-12 self-assembly because of its highest Gordon parameter.4 As for the PILs studied here, the increase of chain length of cations would cause a reduction of γ but an increase of Vm, and therefore a decrease in the value of Gordon parameter.49 Due to the similar chemical structures of these three PILs, the driving force differences as reflected by their Gordon parameters would play the decisive role in determining the final phase behaviors of 12-2-12, even with the hydrogen bonding still being considered. As a result, the CMC values in these solvents follow a sequence of CMC(H2O) < CMC(EAN) < CMC(PAN), while no micelle is detected in BAN. The threshold 12-2-12 concentration needed for H1 phase formation increases in a similar solvent sequence of H2O (∼40 wt %) < PAN (∼53 wt %) < BAN (∼56 wt %). The thermostability of H1 phase also becomes worse when the solvent turns from PAN to BAN, where the solvent Gordon parameter decreases. Obviously, the increase of the hydro1527

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phase in DTAB/PIL systems and a H2 phase in 16-2-8/PIL systems could be observed. No phase transition was observed in these two surfactants/PIL systems with the increase of cation chain length.

carbon solubility in solvent decreases the solvophobic interactions of the surfactant hydrocarbon chains and results in a weak driving force for self-assembly. Meanwhile, except for the general aggregation behavior differences brought out due to G value variation, the different chain lengths in three PIL cations do cause a dramatic phase morphology change. Generally, the critical packing parameter (CPP) can be used as a simple and powerful tool to interpret and understand the possible self-assembled aggregates. CPP is defined as (v/a0lc), where v is the average volume of the amphiphile, a0 is the effective headgroup area and lc is the effective chain length of the amphiphile in its molten state.50 The general relationships between the organized aggregates and CPP are as follows: the spherical micelles for CPP < 1/3, the rod shaped micelles for 1/3 < CPP < 1/2, the bilayers and vesicles for 1/2< CPP < 1, and the reverse structures for CPP > 1. The PIL molecules can participate in the aggregation process of surfactants by interactions with surfactants’ headgroups at the solvophilic/solvophobic interface, resulting in the charge screening effect. As discussed previously in the 12-2-12/EAN system, both the decrease of a0 due to the charge screening of EAN and the increase of v due to the EAN short alkyl chains squeezing into the 12-2-12 aggregate solvophobic regions would cause an increase of CPP, resulting in the formation of a reverse hexagonal phase.12 With the alkyl chain length increased by one −CH2−, however, such a reverse phase structure is turned back to the normal one. Compared to EAN, the longer hydrocarbon chain of PAN may expand the area occupied by surfactant headgroups at the interface as reported by Warr’s group, and thus leading to the formation of aggregates with small CPP.29 From the results of the surface tension measurement, the Amin value in the 12-2-12/PAN system is much larger than that in 12-2-12/EAN, indicating a better chance for PAN molecules to squeeze into the space between 12-2-12 molecules, which is perhaps due to the increased interaction between them. The partition of PAN molecules can also increase the apparent volume of 12-2-12. These factors would induce the aggregate phase structure to return back from the reverse H2 in EAN to the normal H1 in PAN. Such a change is further evidenced by the absence of micelles and the formation of normal LLC phases in BAN. Therefore, the PIL molecules act here not only as solvents but also as a kind of cosurfactants. Compared with water, the PILs have a greater charge screening effect and thus lead to smaller a0 values of surfactants. On the other hand, although the solvophobic interactions by these PIL molecules are weak, they could still participate in the self-assembly process to cause an increase in a0 and v. The competition between such two opposite factors induces different kinds of CPP, as is reflected by various aggregate morphologies formed in 12-2-12/PILs. However, without the unique molecular structure of 12-2-12, it is impossible to exhibit such dramatic phase changes only due to the minor increase in PIL cation chain length. To illustrate this aspect, two surfactants with the quaternary ammonium headgroups were chosen for aggregation behavior comparison. One is dodecyltrimethylammonium bromide (DTAB) with a single alkyl chain, and the other is a Gemini surfactant with dissymmetric alkyl chains, 16-2-8. The former has a smaller CPP for its relatively larger area at the surface than 12-2-12. On the other hand, the 16-2-8 has a larger CPP as demonstrated by the formation of a H2 phase in water51 and a smaller area at the surface by surface tension measurements.13 Only a lamellar

4. CONCLUSIONS The aggregation behaviors of the Gemini surfactant (12-2-12) in two PILs, PAN and BAN, have been investigated and compared to their counterpart with short alkyl in the cation, EAN. The longer cation alkyl chains in PAN and BAN result in their better hydrocarbon solubility and weaker solvophobic interactions. The larger CMC value in PAN than in EAN and the undetectable micelle formation in BAN are thus observed. The reverse LLC phase structure in EAN is now returned back to normal ones in 12-2-12/PAN or BAN systems, probably due to the partition of solvent molecules in between the hydrocarbon chains of 12-2-12. The small change of cation alkyl chain length in PAN and BAN therefore makes them behave not only as solvents but also as cosurfactants. The unique Gemini structure of 12-2-12 should be also responsible for such a dramatic phase change in between EAN and PAN or BAN. It is the delicate equilibrium between the surfactant solvophobic interactions in solvents and structural factors of both solvents and the surfactant that causes these interesting phase behaviors. The obtained results here should be a useful supplement to the aggregation behavior of cationic surfactants in PILs and reveal the possibility of designing specific selfassembled aggregates.

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ASSOCIATED CONTENT

* Supporting Information S

Chararizations of the Gemini surfactant and PILs; visual images of the 12-2-12/PAN system (Figure S1); SWAXS curves for 122-12/EAN (Figure S2) and 12-2-12/PILs at different temperatures (Figures S3 and S5); rheological measurements (Figures S4 and S7); FF-TEM image for 12-2-12/BAN sample (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Address: Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China. E-mail: [email protected]; Tel: +86-531-88365420; Fax: +86-531-88564464. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support from the National Natural Science Foundation of China (20973104, 21033005, and 21373127) and the Specialized Research Fund for the Doctoral Program of Higher Education (20130131110010).

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REFERENCES

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