EAN Binary System

Dec 4, 2015 - The hydroxyl group in the spacer of a cationic Gemini surfactant (12-3OH-12) caused dramatic changes of the phase behaviors in a protic ...
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Unique Phase Behaviors in the Gemini Surfactant/EAN Binary System: The Role of the Hydroxyl Group Qintang Li,† Xudong Wang,‡ Xiu Yue,§,∥ and Xiao Chen*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China Drilling Engineering and Technology Company, Shengli Petroleum Engineering Corporation Limited of SINOPEC, Dongying, 257064, China § Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics and Chemistry and ∥Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi, 830011, China ‡

S Supporting Information *

ABSTRACT: The hydroxyl group in the spacer of a cationic Gemini surfactant (12-3OH-12) caused dramatic changes of the phase behaviors in a protic ionic liquid (EAN). Here, the effects of the hydroxyl group on micellization and lyotropic liquid crystal formation were investigated through the surface tension, small-angle X-ray scattering, polarized optical microscopy, and rheological measurements. With the hydroxyl group in the spacer, the critical micellization concentration of 12-3OH12 was found to be lower than that of the homologue without hydroxyl (12-3-12) and the 12-3OH-12 molecules packed more densely at the air/ EAN interface. It was then interesting to observe a coexistence of two separated phases at wide concentration and temperature ranges in this 12-3OH-12/EAN system. Such a micellar phase separation was rarely observed in the ionic surfactant binary system. With the increase of surfactant concentration, the reverse hexagonal and bicontinuous cubic phases appeared in sequence, whereas only a reverse hexagonal phase was found in 12-3-12/EAN system. But, the hexagonal phases formed with 123OH-12 exhibited lower viscoelasticity and thermostability than those observed in 12-3-12/EAN system. Such unique changes in phase behaviors of 12-3OH-12 were ascribed to their enhanced solvophilic interactions of 12-3OH-12 and relatively weak solvophobic interactions in EAN.

1. INTRODUCTION The Gemini surfactant is a kind of molecule composed of two single chain surfactant moieties connected by a spacer group at the site of head groups or very close to them.1−3 Compared to their monomers, Gemini surfactants have many superior properties, such as the lower critical micelle concentration, higher surface activity, lower Krafft temperature, better wetting ability, and richer aggregate structures.2 Because of these advantages, Gemini surfactants have received much attention and have been widely used in various areas, such as food industry, gene and drug delivery, synthesis of nanostructured materials, phase transfer catalysts, and oil recovery.4−7 For Gemini surfactants with the given structure, their aggregation behavior could be greatly modified by changing the length and nature of both the alkyl chains and the spacer groups and even the couterions. For examples, Oda et al. have carried out systematic researches on aggregation behavior of the dissymmetric Gemini surfactant with the structure [CmH2m+1(CH3)2N(CH2)2N(CH3)2CnH2n+1]Br2 (abbreviated as m-2-n), in aqueous solutions.8,9 Aggregates such as wormlike micelles, multilayered lamellae, and a reverse hexagonal phase were formed with increasing surfactant concentration. The chain length, dissymmetry, and odd/even effects of m-6-n were found by Wang’s group to have influences on the aggregation thermodynamics and therefore the micellization behavior.10−13 © 2015 American Chemical Society

The spacer group, however, also plays an important role in aggregation as reported by Zana’s group on the aggregates formed by 12-s-12 in aqueous solutions. Various aggregates induced by spacer length were observed, like wormlike micelles by 12-2-12 and 12-3-12, spherical micelles by 12-4-12, 12-8-12, and 12-12-12, and vesicles by 12-16-12 and 12-20-12.14 In addition, different counterions might result in different aggregates. As is reported by Oda et al., the counterions with chiral centers would guide the Gemini surfactant to selfassemble into twisted ribbons.15−18 Except for these modulations, the incorporation of the hydroxyl group in Gemini surfactants has been reported. As presented by Devi et al., the hydroxyl in headgroups could increase their polarity and lead micelles to be more hydrated and to grow longer.19−22 With the hydroxyl group in the spacer, it was found by Zhao that the wormlike micelles could be formed and the intermolecular hydrogen bonding was concluded as the driving force to promote the micelles growth greatly.23,24 Similar results were also observed in other Gemini surfactant systems.25,26 Received: October 12, 2015 Revised: November 26, 2015 Published: December 4, 2015 13511

DOI: 10.1021/acs.langmuir.5b03809 Langmuir 2015, 31, 13511−13518

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2.3.2. 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/Carbon was deposited at an angle of 45° to shadow the replicas and carbon 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 JEM1400 TEM operated at 120 kV. 2.3.3. Polarized Optical Microscopy. Photographs of samples with birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a charge-coupled device camera (Panasonic Super Dynamic II WV-CP460). 2.3.4. 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 block-collimation 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 with the dilute samples transferred into the standard quartz capillary of a diameter of 1 mm, where both small and wide-angle Xray scattering profiles can be simultaneously recorded. The scattering factor q is defined as

Solvents also play an important role on self-assembly of surfactants. Specifically, the ionic liquids (ILs), either protic (PILs) or aprotic (APILs), depending on their ability of forming hydrogen bonding network, have been paid much attention.27,28 Among them, the protic ethylammonium nitrate (EAN) was the most popular due to its easiness of preparation and high self-assembling capability. Many researches have been done using EAN as the solvent on the micellization behavior29,30 and self-assembled aggregates, ranging from micelles, 31−33 vesicles 34 and lyotropic liquid crystals (LLC).35−38 In our previous studies, we have also focused on the phase behaviors of normal and asymmetric cationic Gemini surfactant in EAN and the delicate structural effect of this protic IL.30,37,39 To explore further the structural effect of Gemini in EAN, we paid our attention in this paper to the phase behavior of a cationic Gemini surfactant with a hydroxyl group in the spacer, 2-hydroxyl-propanediyl-α, ω-bis(dimethyldodecylammonium bromide) (12-3OH-12), in EAN. The molecular structures of Gemini surfactants were shown in Figure 1. Because of the extra hydrogen bonding

q= Figure 1. Molecular structures of Gemini surfactants.

⎛ 4π ⎞ ⎜ ⎟sin θ ⎝ λ ⎠

(1)

in which λ is the wavelength of the X-ray and 2θ is the scattering angle. The scattering intensity I(q) for monodisperse, homogeneous, and spherical particles is generally described by

between the spacer and EAN, coexistence of two separated micellar phases was observed and a quite interesting dramatic change in the phase behavior took place, compared to those in 12-3-12/EAN system. To the best of our knowledge, such a unique phase behavior of Gemini surfactant in ionic liquid has not been reported in the past. Our results demonstrated that the novel aggregation behavior could be induced by molecular structure modification of surfactants and the solvent choice.

I(q) = nP(q)S(q)

(2)

where n is the total number of particles, and P(q) and S(q) are the form and structure factors. By the Fourier transformation of pair distance distribution function (PDDF), p(r), P(q) is defined as P(q) = 4

2. EXPERIMENTAL SECTION

∫0

∞

p(r )

sin(qr ) dr qr

(3)

The structure of micelles was analyzed with a separation of inter- and intraparticle effects by the generalized indirect Fourier transformation (GIFT).44−46 2.3.5. Rheological Measurement. The rheological measurements were carried out with a Haake Rheostress 6000 rheometer using a Rotor C35/1 system at desired temperatures. In the viscosity measurement, the shear rate was increased from 0.1 to 1000 s−1 in 42 steps. Frequency sweep measurements were performed in the linear viscoelastic region that was determined from the strain sweep measurement with the stress varying at a constant frequency of 1.0 Hz. For the H2 phase here, the shear stress was set to 100 Pa. 2.3.6. Fourier Transformed Infrared (FTIR) Spectroscopy Measurement. FTIR spectra were recorded from 400 to 4000 cm−1 with a resolution of 4 cm−1 using an Alpha-T spectrometer (Bruker). The samples were coated on the KBr plate.

2.1. Materials. Two Gemini surfactants, 12-3OH-12 and 12-3-12, were prepared according to the procedures reported previously.40,41 Their purities were ascertained first by 1H NMR and 13C NMR in CDCl3, then by the elementary analysis and mass spectrometry (Supporting Information). EAN was synthesized based on methods as described by Evans et al.42 The purity was ascertained by 1H NMR in DMSO (Supporting Information). 2.2. Sample Preparation and Phase Diagram Mapping. The process for mapping the phase diagram has been described elsewhere.43 All samples were prepared by mixing the Gemini surfactant and EAN with designed compositions (in weight percentage, %, thereinafter). These mixtures were homogenized by repeatedly mixing and centrifugation. Then they were equilibrated for at least one month before further investigations. The phase change was detected by ocular observation and visual inspection through the crossed polarizers. The lyotropic 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% for a rough mapping and then 2% for the determination of the phase boundaries. 2.3. Characterization. 2.3.1. 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 with an alcohol burner until it glowed red before each measurement. All measurements were repeated at least twice until the values were reproducible.

3. RESULTS AND DISCUSSION 3.1. Micellization Behavior. To obtain the interfacial properties and thermodynamic parameters of Gemini surfactants (12-3OH-12 and 12-3-12) in EAN, the surface tension (γ) measurements were carried out with the results shown in Figure 2. Typical surfactant-like curves of γ with the concentration could be observed, indicating the formation of micelles. These two Gemini surfactants had similar surface tension curves for their minor structure difference. The CMC of 12-3OH-12 (11.4 mM, 0.61%) was slight lower than that of 13512

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driving force to the micellization process as well as solvophobic interactions, resulting in a lower CMC.47 3.2. Phase Behavior of 12-3-12 in EAN. With one CH2 group increase in the spacer, a similar phase behavior was observed in the 12-3-12/EAN system compared to that in 12-212/EAN.37 As shown by the phase diagram in Supporting Information Figure S1, the micellar (L1) and reverse hexagonal (H2) phases were formed successively. With more surfactants, no other new phase but only undissolved solid surfactant was observed. The micelle formation was confirmed by FF-TEM with the result shown in Supporting Information Figure S2. At a surfactant concentration of 55%, ellipsoid and short rod aggregates with the size around 20 nm were observed, which could be regarded as micelles. Some larger or irregular aggregates were due to the distortion between micelles because of the ex situ observation. At higher 12-3-12 concentrations, the hexagonal phase appeared. Detailed information could be obtained by the POM and SAXS measurements. As shown in Supporting Information Figure S3a−c, with the increase of surfactant concentration (C12‑3‑12) from 70 to 80% the POM images showed more and more regular fanlike patterns The SAXS curves in Supporting Information Figure S3d displayed three peaks with their scattering vector q values of a 1:√3:2 ratio. Both results were characteristic for the hexagonal phase. According to our previous studies, this hexagonal phase was assigned as the H2 one.37 3.3. Phase Behavior of 12-3OH-12 in EAN. In comparison to 12-3-12, although only slightly different surface properties in EAN were caused by the hydroxyl group in 123OH-12, a dramatic difference in phase behavior between these two surfactants was observed. As shown by the phase diagram in Figure 3 and Supporting Information Figure S4, the phase

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

12-3-12 (12.9 mM, 0.65%). Both of them were much larger than those in water, due to the weaker solvophobic interactions in EAN.30,37 The effectiveness of γ reduction (ΠCMC), the surface excess of surfactant (Γmax) and the minimum area per surfactant molecule (Amin) adsorbed at the air/IL interface, and the standard Gibbs free energy of micellization (ΔGm) could be calculated according to eqs 4−7 below ΠCMC = γ0 − γCMC Γmax = −

A min =

(4)

dγ RT d ln CMC

(5)

1 NA Γmax

(6)

ΔGm = 2RT (1.5 − β)ln XCMC

(7)

where γ0 and γCMC are surface tensions of the pure solvent and the solution at CMC, respectively, T is the absolute temperature, NA is the Avogadro’s number, R is the gas constant, and β is the ionization degree of the micelle, which can be taken as 1 due to the charge screening of ILs.30,31 To make it clear, details of these obtained thermodynamic parameters are listed in Table 1. It could be seen that 12-3OH-12 presented a larger ΠCMC, a smaller Amin, and a more negative ΔGm value, which indicated a higher surface activity and more densely packing of 12-3OH-12 and therefore more favorable micellization. Such a trend was in accordance with what was observed in aqueous solutions.47 EAN, like water, could also form three-dimensional hydrogen bonding network. Because of the weaker self-assembling capability of EAN, the parameters like ΠCMC and ΔGm were lower than those in water. On the other hand, the charge screening by EAN reduced the electrostatic repulsion between Gemini headgroups and the electronic repulsion between them could be neglected.31 Therefore, the surfactant molecules would pack more densely at the air/EAN interface and the intermolecular hydrogen bonding played the same role. What’s more, the hydroxyl group in the spacer provided an extra

Figure 3. Phase diagrams of 12-3-12 and 12-3OH-12 in EAN at 25 °C. TPs, two coexisting phases; L1, the micellar phase; H2, the reverse hexagonal phase; V2, the reverse bicontinuous cubic phase.

morphology changed from two coexisting phases (TPs) to L1, H2, and finally the bicontinuous cubic phase (V2). More details about phase behavior difference were described below. 3.3.1. Two Coexisting Phases. Once the concentration of a surfactant reaches CMC, a micellar phase would be formed. However, in our system the micellar phase separated into two coexisting phases, which would not mix and were stable for

Table 1. Surface Properties of Gemini Surfactants in EAN at 25°C Gemini surfactants

CMC (mmol·L−1)

γCMC (mN·m−1)

ΠCMC (mN·m−1)

Γmax (μmol·m−2)

Amin (Å2)

ΔGm (kJ·mol−1)

12-3-12 12-3OH-12

12.9 11.4

33.4 32.8

15.2 15.8

1.89 2.09

87.9 79.5

−16.7 −17.1

13513

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Figure 4. SAXS curves for UP and LP solutions in a 25% 12-3OH-12/EAN sample at 25 °C (a) and corresponding profiles of PDDF (b) and S(q) (c). Inset in (b) is the enlarged PDDF of the LP.

Figure 5. POM images (a−c) and SAXS curves (d) for 12-3OH-12/EAN samples at 25 °C and different 12-3OH-12 concentrations. From panels a−c, C12‑3OH‑12 = 70, 75, and 80%.

the two coexisting phases were turned into a single dense micellar phase. Further investigations were carried out on the sample at C12‑3OH‑12 of 25%. Through comparison of hydrogen signals from the 1H NMR spectra, the content of 12-3OH-12 in two phases were obtained, 40% in the UP and 3.7% in the LP. The surface tension of both the UP and LP solutions was around 32.8 mN·m−1, indicating the formation of micelles in both phases. According to the steady shear rheological results in Supporting Information Figure S6, the LP solution was a Newtonian fluid with a viscosity around 20 mPa·s, close to that of EAN. The UP solution, however, behaved as a Newtonian fluid at low shear rates while showing a little shear thinning behavior at high shear rates. Its viscosity was approximately hundreds of times larger than that of LP. These results confirmed our hypothesis that the UP was a dense micellar phase while the LP was a dilute one. Both micellar solutions were then characterized by the SAXS measurements. As shown in Figure 4a, the SAXS curves of these two phases were obviously different. The I(q) of UP was larger than that of LP, indicating more aggregates formed.

over one year. Such a phenomenon in IL was only reported in the didodecyldimethyl-ammonium bromide/EAN system in which large vesicles were formed in the lower phase (LP) while giant vesicles were in the upper phase (UP).34 Such a phase separation was a spontaneous process in aqueous solutions, which was first observed in polymer aqueous solutions.48 Recently, it has been widely reported in aqueous two-phase system (ASTP) including an ionic surfactant and opposite charged polyelectrolyte mixture,49,50 anionic/cationic,51,52 and anionic/nonionic surfactant mixture.53 The two phases are separated as the surfactant-rich one and the surfactant-poor one. ASTPs have therefore received much attention for the applications in separation and purification.54,55 In our system, the two coexisting phases started at C12‑3OH‑12 a little larger than CMC and ended at about C12‑3OH‑12 of 55%. Considering the relatively larger density of EAN (1.2 g·cm−3) than 12-3OH-12 (1.1 g·cm−3), the UP was supposed to be a surfactant rich phase and the LP was a solvent rich phase. As shown in Supporting Information Figure S5, with increasing C12‑3OH‑12 the UP volume percentage increased fast and finally 13514

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Langmuir Table 2. Structure Parameters of H2 Phases in the 12-3OH-12/EAN System at 25 °Ca concentration (%)

D (Å)

R2 (Å)

d2 (Å)

S2 (Å2)

70 75 80

33.5 34.1 (33.8)b 34.6

9.1 8.4 (8.3)b 7.6

15.3 17.3 (17.2)b 19.6

82.2 68.8(67.8)b 57.3

a D is the lattice parameter; R2 is the core radius of the reverse cylinder-like aggregates; d2 is the thickness of hydrocarbon domains; and S2 is the area per the surfactant molecule at the solvophilic/solvophobic interface. bValues in the brackets were for 12-3-12.

the less interlace of surfactants with their increasing content. Such extension of the alkyl chains enhanced more densely packing of the surfactant molecules, as reflected by smaller values of area per surfactant molecule at the solvophilic/ solvophobic interface (S2). The orderness of phase structures was thus improved, as supported by the more and more perfect fanlike patterns in POM observation (from Figure 5a−c). Compared to 12-3-12, the increased solvophilicity due to the hydroxyl group could extend the H2 phase bounadry to C12‑3OH‑12 over 85%. It would be interesting to compare the structure and properties of the H2 phase formed by these two Gemini surfactants. The H2 phases at the surfactant concentration of 75% were taken as an example. It could be seen from Table 2 that the structure parameters like R2, d2, and S2 were almost the same, indicating these two molecules were packed in a similar way. Their rheological properties shown in Figure 6 also

There was a broad peak at high q values in the UP curve, reflecting the interactions between aggregates. Meanwhile, the LP curve decayed in the measured q range, implying weaker interactions between aggregates. With the GIFT method, the PDDF of these two phases were obtained with the profiles shown in Figure 4b. The p(r) intensity of the LP was much lower than that of UP, suggesting presence of only fewer aggregates. Both p(r) curves showed a peak and decayed slowly at high r values, suggesting the formation of nonspherical aggregates. Thus, the ellipsoid micelles were assumed in the two phase, which were also consistent with the observation from FF-TEM in Supporting Information Figure S7. The micelles had a similar axis length a around 1.6 nm, close to the alkyl chain length of 12 methylene groups (1.67 nm). The micelles in the LP had an even larger dimension Dmax of 8.8 nm than that of 6.8 nm in UP. The interaction between micelles might prohibit the growth of micelles in UP. Such interactions between micelles were easy to learn from the S(q) profiles shown in Figure 4c. The S(q) of the LP was around 1, while the S(q) of the UP deviated from 1 in a large scale, suggesting dilute and dense aggregates in the LP and UP respectively, as confirmed in Supporting Information Figure S7 by large amounts of micelles in the UP while only few micelles in the LP. Compared to the situations in the 12-3-12/EAN and 123OH-12/water systems, where no phase separation was observed, the formation of two coexisting phases here could be ascribed to the synergetic effects by the hydroxyl group and the solvent. Because of the electrostatic interactions between headgroups of ionic surfactants in water, their micellar phase separation was rarely observed compared to nonionic surfactants. Only with other addictives, such a phase separation could take place. Herein, EAN behaved as both a salt and a solvent. Its charge screening effects reduced the electrostatic interactions between micelles. Meanwhile, the hydrogen bonding between 12 and 3OH-12 and EAN also enhanced the attractive interaction between micelles, thus leading to the phase separation. Further increase of C12‑3OH‑12 would lead the two coexisting phases into a dense micellar phase as confirmed by FF-TEM in Supporting Information Figure S8. 3.3.2. The H2 Phase. As C12‑3OH‑12 reached 65%, the hexagonal phase was formed. Figure 5 presented characteristic POM images and SAXS profiles. With the increase of C12‑3OH‑12, the scattering vector q corresponding to the first peak decreased, meaning the lattice parameter D was expanding. This phase was identified as the reverse hexagonal phase and its structure parameters could be obtained according to the method reported previously.30,37 As can be seen from the listed values in Table 2, with C12‑3OH‑12 increasing from 70 to 80% the solvent core radius (R2) values decreased from 9.1 to 7.6 Å while the hydrocarbon layer thickness (d2) was increased from 15.3 to 19.6 Å. The solvent core became contracted for the less solvent content and the hydrocarbon layer was expanded for

Figure 6. Dynamic rheological results of the H2 phases at 75% surfactant concentration and 25 °C.

exhibited similar behaviors. At lower frequencies, the loss modulus (G″) was larger than the storage modulus (G′), showing a viscous behavior. At higher frequencies, however, G′ was larger than G″, exhibiting an elastic behavior. Such a trend belonged to a typical rheological change of the hexagonal phase.56 It was noticeable that the moduli of the 12-3-12 sample were one order larger than those of 12-3OH-12 and the relaxation time was larger.57 These results indicated that the H2 phase formed by 12-3-12 was more ordered. Considering the same hydrocarbon chain length for both surfactants and therefore the similar solvophobic interactions, the hydroxyl group should account for such a difference. The cylinders in the H2 phase formed by 12-3OH-12 were more solvophilic in EAN due to the hydroxyl groups at the interface, which was more capable of flowing under shear. The less ordered structure was also reflected by less regular fanlike pattern in POM images of 12-3OH-12/EAN samples. The difference in phase structure ordering was also reflected from the thermal stability investigation. The temperature effects 13515

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Figure 7. SAXS curves for the H2 phases of 75% 12-3OH-12 (a) and 12-3-12 (b) in EAN at different temperatures.

3.3.4. Analysis of 12-3OH-12/EAN Self-Assembling Features. It was interesting to explore why the 12-3OH-12/EAN system could exhibit such a richer phase behavior. For this aim, the critical packing parameter (CPP) was generally used as a powerful tool to understand the possible self-assembled aggregates. CPP is defined as (V/a0lc), where V is the average volume of the surfactant molecule, a0 is the effective headgroup area, and lc is the effective chain length of the surfactant in its molten state.59 The reverse phase aggregates are formed with the CPP > 1. Therefore, the V2 phase formed here should reflect the information that the 12-3OH-12 molecules would have a smaller a0 than that of 12-3-12 at higher concentrations. This was consistent with the result from the surface tension measurements. Except for the electrostatic screening of EAN, the hydroxyl group could form hydrogen bond with EAN molecules, as confirmed by FTIR in Supporting Information Figure S9, making 12-3OH-12 molecules pack more densely. In addition, more solvophilic character of 12-3OH-12 molecules led to a better solubility, making them possible to self-assembly at a higher concentration. The phase behaviors of these two surfactants in water, however, were similar and no difference was observed.60 It seemed that the hydroxyl group had a limited effect in water. This might result from the dominant hydrophobic interactions over the hydrogen bondings. In EAN, the solvophobic interactions also played a leading role but not as strong as in water. The hydrogen bondings would then have a more important effect on the intermolecular interactions, thus resulting in more complicated phase behaviors in EAN.

on the H2 phases were studied by SAXS and POM measurements with SAXS results shown in Figure 7. With the increase of temperature, the Bragg peaks shifted to higher q values, indicating a contraction of lattice distance. This was due to the higher mobility of the hydrophobic chains induced by temperature.58 The fanlike patterns in POM also became deteriorated and finally of no birefringence when the H2 phase transformed to the isotropic micellar phase. For 12-3OH-12, the SAXS curve at 85 °C showed no Bragg peak but a weak and broad band, indicating the formation of micelles. The transition temperature of the 12-3OH-12/EAN sample was about 82 °C, which was lower than that of the 12-3-12/EAN sample (around 118 °C). Below these transformation temperatures, sharp Bragg peaks in SAXS curves and regular patterns in POM images could be recovered. The worst thermal stability of the 12-3OH12/EAN samples was because the more solvophilic hydroxyl group “weakened” the solvophobic interations. 3.3.3. The V2 Phase. With further increase of 12-3OH-12 concentration, the sample became rather stiff and showed no texture under POM (Figure 8), indicating a phase transition

4. CONCLUSIONS In summary, we investigated the effects of the hydroxyl group on phase behaviors of Gemini surfactants in EAN for the first time. With a hydroxyl group in the spacer, 12-3OH-12 was more solvophilic than 12-3-12 and could form hydrogen bonding with the protic solvent. Thus, the 12-3OH-12 exhibited a stronger ability in micelles formation and more densely packing at the air/EAN interface. Comparison on the rheological properties and thermostability of the formed H2 phases also reflected the interaction difference between these two Gemini surfactants. The formation of V2 phase at a high concentration further suggested a better solubility and more densely packing of 12-3OH-12 and thus a larger CPP. It is interesting also that the phenomenon of coexisting two micellar phases was observed in wide concentration and temperature ranges, which was not reported before for Gemini surfactants in ILs. The detailed origin is still under investigation. It is believed that the electrostatic screening effect of EAN and extra

Figure 8. SAXS curve for a 90% 12-3OH-12/EAN sample and the corresponding POM image (inset) at 40 °C.

occurrence. The corresponding SAXS curve of 90% C12‑3OH‑12 at 40 °C was shown in Figure 8. Two obvious Bragg peaks were exhibited at q of 2.11 nm−1 and 2.44 nm−1. Their relative positions as a√3:√4 ratio was consistent with the first two reflections (211 and 220) from the Ia3d structure of a bicontinuous cubic phase. Considering the phase sequence, this phase was assigned as a reverse cubic phase (V2). The V2 phase was observed at the concentration ranging from 87 to 92%. As the temperature increased, the V2 phase would be transformed to a less curved H2 phase. 13516

DOI: 10.1021/acs.langmuir.5b03809 Langmuir 2015, 31, 13511−13518

Article

Langmuir

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hydrogen bonding should play important roles. These two coexisting phases might find applications in separation and purification. Our results would serve as a good illustration of surfactant design in self-assembly and would shed light on solvent effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03809. 1H NMR, 13C NMR, elemental analysis, mass spectrometry of Gemini surfactants, and 1H NMR of EAN. Phase diagrams of the 12-3-12/EAN and 12-3OH12/EAN systems. POM and SAXS of the H2 phase in the 12-3-12/EAN system. FF-TEM images of micellar solutions. The UP volume percentage. The steady shear results of the two phases. FTIR spectra for H2 sample and EAN. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-531-88365420. Fax: +86531-88564464. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b03809 Langmuir 2015, 31, 13511−13518