Effects of Structure Dissymmetry on Aggregation Behaviors of

Article pubs.acs.org/Langmuir

Effects of Structure Dissymmetry on Aggregation Behaviors of Quaternary Ammonium Gemini Surfactants in a Protic Ionic Liquid EAN Xudong Wang,†,§ Qintang Li,†,§ Xiao Chen,*,† and Zhihong Li‡ †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

‡

S Supporting Information *

ABSTRACT: The aggregation behaviors of a series of dissymmetric cationic Gemini surfactants, [CmH2m+1(CH3)2N(CH2)2N(CH3)2CnH2n+1]Br2, designated as m-2-n (with a fixed m + n = 24, m = 16, 14, 12) have been investigated in a protic ionic liquid, ethylammonium nitrate (EAN). Surface tension, polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheological measurements are adopted to investigate the micellization and lyotropic liquid crystal (LLC) formation. The obtained results indicate that the structure dissymmetry plays an important role in aggregation process of m-2-n. With increasing degree of dissymmetry, the critical micellization concentration, the maximum reduction of solvent surface tension, and the minimum area occupied per surfactant molecule at the air/EAN interface all become smaller. The thermostability of formed LLCs is therefore improved because of the more compact molecules. These characteristics can be explained by the enhancement of solvophobic effect due to the increased structure dissymmetry of Gemini surfactants. structures, and a reverse hexagonal phase.10,11 Sikiric's group investigated the influence of spacer length (s = 2, 4, 6) of 12-s14 on their physicochemical and thermal properties.12 The effects of structural dissymmetry on thermodynamic properties and the micellization behavior of m-6-n (m + n = 24, m = 12, 13, 14, 16, and 18) were studied by Wang and co-workers. The enthalpy of micellization was found to decrease greatly with increasing m/n because of a larger hydrophobic contribution.13,14 Then, the micellization progress of m-6-6 (m = 12, 14, 16) and their interactions with the dimyristoylphosphatidylcholine (DMPC) vesicles were explored to get a conclusion that the dissymmetric m-6-6 series are more effective than the symmetric 12-6-12 and the single-chain surfactant DTAB for solubilization of DMPC vesicles, due to the existence of short C6 chains and the larger mismatch extent of hydrophobic chains between DMPC and m-6-6.15 Feng et al. measured the adsorption of the 12-2-n series at the silica/water interface and found an effectively enhanced adsorption ability with increasing n from 8 to 16.16 All these researches have provided valuable information for m-s-n on self-assembly in aqueous environment. But, to the best of our knowledge, little attention has been paid to their aggregation behavior in nonaqueous solvents. It is, however, not only important for better

1. INTRODUCTION Gemini surfactants are constructed of two amphiphilic moieties connected at or very close to the site of head groups by a spacer group.1,2 Their aggregation behavior in aqueous solutions can be greatly modified by changing the length and nature of both the alkyl chains and the spacer groups.3−5 They have many unique properties that are superior to those of their monomeric counterparts, such as the lower critical micelle concentration, higher surface activity, lower Krafft temperature, better wetting ability, and richer aggregate structures.3 Because of their ability to self-assemble and modulate the interfacial properties, Gemini surfactants have widespread applications in food industry,3 gene and drug delivery,6 synthesis of nanostructured materials,7 phase transfer catalysts,8 and oil recovery.9 In recent years, a new class of Gemini analogues, the dissymmetric Gemini surfactants (with two hydrophobic chains of different lengths), have gained much attention. Their aggregation behavior can be modulated by varying the chain lengths. The most investigated type of such surfactants is still the cationic quaternary ammonium salts with a general formula [CmH2m+1(CH3)2N(CH2)sN(CH3)2CnH2n+1]Br2, abbreviated as m-s-n, where s represents the number of carbon atoms in the spacer, and m and n refer to the numbers of carbon atoms in two alkyl chains, respectively.10 Oda et al. pioneered the synthesis of such dissymmetric Gemini surfactants (s = 2) and studied their aggregation behavior in water. With increasing surfactant concentration, certain elongated aggregates are observed, including long worm-like micelles, multilayered © XXXX American Chemical Society

Received: October 9, 2012 Revised: November 8, 2012

A

dx.doi.org/10.1021/la304004u | Langmuir XXXX, XXX, XXX−XXX

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was obtained. This compound (20 mmol) and the second type alkyl bromide (4 equiv) were refluxed in ethyl acetate (250 mL) for 2 days. After evaporation, the residue was recrystallized three times from chloroform−acetone mixed solvent to yield the m-2-n surfactant. The purities of intermediates and final products were ascertained by 1H NMR spectra (see Figure S1−S5 in Supporting Information) in CDCl3 and the elementary analysis. Gemini surfactant 16-2-8, 1H NMR, δ 0.88 (t, 6H), 1.25−1.39 (m, 36H), 1.83 (s, 4H), 3.52 (s, 12H), 3.68 (m, 4H), 4.77 (s, 4H); C, H, N analysis found (calculated) %, C, 58.69 (58.62), H, 4.555 (4.558), N, 10.80 (10.82). Gemini surfactant 14-2-10, 1H NMR, δ 0.88 (t, 6H), 1.25−1.39 (m, 36H), 1.83 (s, 4H), 3.53 (s, 12H), 3.71 (m, 4H), 4.77 (s, 4H); C, H, N analysis found (calculated) %, C, 58.35 (58.62), H, 4.535 (4.558), N, 10.67 (10.82). Gemini surfactant 12-2-12, 1H NMR, δ 0.88 (t, 6H), 1.25−1.38 (m, 36H), 1.81 (s, 4H), 3.52 (s, 12H), 3.71 (m, 4H), 4.72 (s, 4H); C, H, N analysis found (calculated) %, C, 58.57 (58.62), H, 4.481 (4.558), N, 10.57 (10.82). The ionic liquid EAN was synthesized as described by Evans et al.38 In a typical synthesis, a portion of ∼3 M nitric acid was slowly added to the ethylamine solution while stirring and cooling in an ice bath. The water in 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 of EAN was ascertained by the 1H NMR spectrum in D2O. 2.2. Sample Preparation and Phase Diagram Mapping. The process for mapping the phase diagram has been described elsewhere.36,39 All samples were prepared by mixing the m-2-n surfactants and EAN with designed compositions (in weight percent, wt %, thereinafter). These mixtures were homogenized by repeating mixing and centrifugation. Then, they were equilibrated for at least 3 months before further investigation. 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% for a rough mapping and then 2% for the determination of the phase boundaries. 2.3. Characterization. 2.3.1. Small-Angle X-ray Scattering. The obtained LC phases were characterized by an HMBG-SAX X-ray small-angle scattering system (Austria) with a Ni-filtered Cu Kα radiation (0.154 nm) operating at 50 kV and 40 mA. The distance between the sample and detector was 27.8 cm. For comparison, one sample exhibiting the two-phase coexistence was determined with the SAXS beamline (1W2A) of the Beijing Synchrotron Radiation Facility (BSRF) in China under the experimental conditions of 0.154 nm incident X-ray wavelength, 1850 mm distance between the sample chamber and the detector, and a 400 s data accumulation time. The S series programs (http://www.ihep.cas.cn/dkxzz/bsrf/ yonghudaohang/ziyuanxiazai/) are used for SAXS data transformation and processing. Except for the enhanced intensity, no major difference in the scattering profiles could be found between these two X-ray sources. 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. Rheological Measurement. The rheological measurements were carried out with a Haake Rheostress 6000 rheometer using a Rotor C35/1 system at 40.0 ± 0.1 °C. 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. 2.3.4. Surface Tension Measurement. A tensiometer K100 from Krüss (Germany) 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 alcoholic flame until it glowed before each measurement. Twenty milliliters EAN was used as the starting solution, and then a small

understanding of the effect of solvents on self-assembly, but also necessary for expanding applications in nonaqueous or water-sensitive cases. As a novel nonaqueous media, the ionic liquids (ILs) have currently attracted much interest for their properties characterized by negligible vapor pressure, wide electrochemical window, nonflammability, good catalytic properties, high thermal stability, and ionic conductivity compared to the organic solvents.17,18 They have found wide applications in the areas of organic synthesis and catalysis, biochemical engineering, materials science, electrochemistry, carbohydrate chemistry, nanotechnology, and separation techniques.17,19−23 Besides, some ILs have been used as good nonaqueous solvents to promote the self-assembly of amphiphiles.24 As a truly protic ionic liquid (PIL) showing more analogous properties than other PILs to water, ethylammonium nitrate (EAN)25 was first used as the solvent to construct self-organized structures in the early 1980s.26,27 Evans and co-workers reported the formation of lamellar liquid crystals by lipids in EAN.28,29 A series of nonionic surfactants, CnEm, were found by Warr et al. to form micelles and lyotropic liquid crystals (LLCs) easily in EAN due to hydrogen bonding.30 Other surfactants, like hexadecyltrimethylammonium bromide (CTAB),31 myverol 18-99K,32 phytantriol,32 and N-hexadecyl-N-methylpyrrolidinium bromide,33 were all observed to form the lyotropic liquid crystals in EAN. We have also explored the phase behaviors of the nonionic block copolymer EO20PO70EO20 (P123) in EAN, and observed several aggregate structures like the normal micellar cubic, hexagonal, lamellar, and reverse bicontinuous cubic phases.34 Only a normal hexagonal phase was found in the binary system of a nonionic oleyl polyoxyethylene (10) ether (Brij 97) and EAN,35 while a more complex lyotropic behavior could be observed for the cationic 1-hexadecyl-3-methyl-imidazolium chloride (C16mimCl) in EAN.36 Recently, we moved our interest to the symmetric Gemini surfactants (m-2-m, m = 10, 12, 14) in EAN, and a reverse hexagonal phase has been identified in these systems, which is different from those observed in aqueous media.37 Herein, as a part of the continuing research on such systems, a series of dissymmetric Gemini surfactants, m-2-n (m + n = 24, m = 16, 14, 12), are designed to investigate micellization and lyotropic aggregate formation in EAN. Changing the chain length difference (m − n) provides a method for tuning the packing parameter of molecules and is therefore the driving force for their aggregation. The obtained results should extend both the knowledge on the effect of dissymmetry and the application areas to nonaqueous conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. The m-2-n type Gemini surfactants with the structure shown in Figure 1 were prepared according to the procedures reported previously.10,12,13 Typically, the N,N,N′,N′tetramethylethylenediamine (0.1 mol) and the first type alkyl bromide (0.8 equiv) were heated in acetonitrile (250 mL) at 40 °C for 3 days. After evaporation and crystallization from ether, the corresponding pure alkyl dimethyl[1-(2-dimethylamino)ethyl]ammonium bromide

Figure 1. Structures of the studied m-2-n type Gemini surfactants (m + n = 24, m = 12, 14, 16). B


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amount of surfactant solution was dropped and stirred, from which the concentration can be calculated.40 The time interval between each measurement was 15 min. Each measurement was repeated at least 3 times with drops of size >10 μL, and the average was then calculated and the precision in the measurements was ±0.001 mN/m.

minimized due to the equal number of hydrophobic units for both intermolecular and intramolecular interactions. However, the ratio of hydrophobic units interacting intermolecularly to those intramolecularly will increase as the dissymmetry is enhanced, and therefore, the solvophobic interactions will be magnified.13−15 For clearer comparison, the effectiveness of the surface tension reduction, ΠCMC is determined as follows:

3. RESULTS AND DISCUSSION Like the symmetric Gemini molecules, the dissymmetric surfactants exhibit similar aggregation behaviors in EAN, where molecules pack more densely than in water due to charge screening between headgroups, which leads to a higher critical packing parameter (CPP) in EAN, and thus, a new reverse hexagonal phase (HII) can be observed.37 The solvophobic effect plays an important role in the selfaggregation of Gemini molecules in EAN, which is dominated by the entropic contribution.31,37,41 However, the dissymmetry of molecular structures really causes some discrepancy in the micellization process and phase behaviors of concentrated systems in EAN. 3.1. Micellization of m-2-n in EAN. The basic aggregation properties of m-2-n in EAN as reflected by their critical micelle concentration (CMC) were collected by surface tension (γ) measurements at 25 °C. The curves of γ versus surfactant concentration (C) for three dissymmetric Gemini surfactants are shown in Figure 2. Similar to those in aqueous solutions, γ

ΠCMC = γ0 − γCMC

(1)

where γ0 and γCMC are the surface tensions of EAN and the solution at CMC respectively. ΠCMC represents the maximum reduction of solvent surface tension caused by the dissolution of surfactant molecules. The measured values of ΠCMC are also listed in Table 1. It is obvious that the ΠCMC increases in a sequence 16-2-8, 14-2-10, 12-2-12. The higher the dissymmetry, the lower the surface activity of m-2-n in EAN. This might result from the fact that the increased dissymmetry makes the molecule behave more similarly to its monomeric counterpart. For better understanding of m-2-n aggregation behaviors in EAN, other parameters of micelle formation are calculated. According to the well-known mass action model, the standard Gibbs free energy of micellization can be calculated from the equation below:42,43 ΔGm = 2RT (1.5 − β)ln XCMC

(2) −1

−1

where R is the gas constant (8.314 J mol K ), T is absolute temperature, XCMC is CMC in molar fraction, and β is the ionization degree of the micelle, which can be taken as 1 due to the exclusively ionic shell surrounding the aggregates in ILs.31,32 Moreover, in the case of m-2-n adsorption at the air/EAN interface, the maximum surface excess concentration Γmax and the minimum area Amin occupied per surfactant molecule at the air/EAN interface can be calculated according to the Gibbs adsorption isotherm eq 3:44 Figure 2. Surface tension isotherms at 25 °C as a function of m-2-n concentration for EAN solutions of 16-2-8 (■); 14-2-10 (*); 12-2-12 (•).

Γmax = −

1 ⎛ dγ ⎞ ⎜ ⎟ nRT ⎝ d ln C ⎠

(3)

where n is the number of solute species whose concentration at the interface changes with the surfactant concentration C, and dγ/d ln C is the slope of γ vs ln C profile when the concentration is near CMC. As an ionic liquid, the ionic nature of EAN causes a large degree of charge screening, which results in the value of n being taken as 1.31,32 Amin can then be obtained from eq 4:44

decreases with increasing m-2-n concentration, indicating that the formation of an amphiphilic monolayer at the free air/EAN interface reduces the surface tension of pure EAN. At higher m2-n concentrations, γ remains nearly constant. From the breakpoint of the two parts in the profile, the CMC is assigned and listed in Table 1, which suggests the formation of micelles in EAN solutions.13−15 Except for different lengths in two hydrocarbon chains, three m-2-n Gemini molecules have the same headgroups, spacer units, and total number of hydrophobic units. It is interesting to see from Figure 2 that their CMCs decrease with the dissymmetry extent. In other words, the addition of a methylene unit to the long alkyl chain is more effective for micellization than to the short one. For the symmetric Gemini surfactant, 12-2-12, the hydrophobic interaction will be

A min =

1 ( × 1023) NA Γmax

(4) −1

where NA is Avogadro’s number (6.022 × 10 mol ). The calculated values of ΔGm, Γmax, and Amin are also listed in Table 1. All negative values of ΔGm imply the spontaneous formation of thermodynamically stable micelles. It is indicated that ΔGm is greatly influenced by m/n values. The higher the degree of dissymmetry, the more negative the ΔGm values, which reflects an enhanced solvophobic interaction between the alkyl chains 23

Table 1. Surface Properties of Three m-2-n Molecules in EAN at 25 °C m-2-n

CMC (mmol L−1)

γCMC (mN m−1)

ΠCMC (mN m−1)

Γmax (μmol m−2)

Amin (Å2)

ΔGm (kJ mol−1)

16-2-8 14-2-10 12-2-12

2.63 11.20 25.32

37.17 34.20 32.42

12.93 15.90 17.68

2.83 2.47 2.03

58.7 67.3 81.8

−20.68 −17.09 −15.07

C


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as presented by CMC results.13,14 Furthermore, the decreasing values of Amin with increasing dissymmetry suggests more dense packing between Gemini molecules, which further confirms an increased solvophobic effect.45 It is interesting to compare the difference between EAN and water as solvents on the micellization of m-2-n. As a control experiment, the surface tension was also measured in aqueous solutions at 25 °C for two m-2-n (16-2-8, 14-2-10) to study their aggregation behavior and CMCs, with data shown in Figure 3. The parameters related to aggregation were calculated

form hydrogen-bonded network structures in bulk,26,48−50 which puts them as good candidates as self-assembling media. Thus, the driving forces for m-2-n aggregation should both originate from the entropy gain due to the transfer of the hydrocarbon out of the solvent.46 However, there are two main differences between EAN and water as solvents. One is the higher solubility of hydrocarbons in EAN than in water due to the existence of short alkyl chain in cations. The other one is the H-bond network structure which is tetrahedral in H2O, while being only three-dimensional in EAN.26,48−50 Both factors induce a weaker driving force to form micelles in EAN than in water, and thus higher CMC values in EAN. Therefore, it becomes easy to understand the little influence of dissymmetry on CMC in aqueous systems, because such enhanced hydrophobic interaction is not dominant compared to the strong driving force from water. However, the solvophobic interaction in EAN system behaves compatibly with the weak driving force from solvent and hence exhibits a large dissymmetry effect on micellization. In addition, as can be seen from Tables 1 and 2, Amin of m-2-n is smaller in EAN than in water. Such a reduction is due to the ionic nature of EAN, which causes a larger degree of charge screening and therefore more dense packing of the Gemini head groups.31,32 3.2. LLC Behaviors of m-2-n in EAN. To further explore the difference of weak molecular interactions caused by structure dissymmetry, the lyotropic liquid crystalline phase formed in m-2-n/EAN systems has been investigated. 3.2.1. LLC Phase Behaviors. With the same procedures described previously,37 the temperature−composition (T-X) phase diagrams of three m-2-n/EAN binary systems have been mapped and shown in Figure 4. Similar aggregation behavior could be found. At lower m-2-n concentrations (below 60%), the systems form an isotropic solution (L1) with only one weak and broad scattering peak observed in their SAXS curves. A narrow two-phase coexisting region appears with increasing concentration (about 60−70%). At concentrations higher than 70%, a hexagonal phase is formed and confirmed as a reverse one (HII) according to our previous research.37 Except for these apparent similarities, however, a little contracted L1 region from 64% (12-2-12) to 62% (16-2-8) with increasing dissymmetry can still be noted. The molecular structure dissymmetry also induces a packing order discrepancy in their formed LLCs. During the mapping of the phase diagrams, the optical birefringence textures observed by POM could provide certain subtle difference caused by dissymmetry. In L1 phase, at 30 wt % concentration for three systems, samples behave as homogeneous, transparent, and flowable solutions with no

Figure 3. Surface tension isotherms at 25 °C as a function of m-2-n concentration for aqueous solutions of 16-2-8 (•); 14-2-10 (▲).

Table 2. Surface Properties of 16-2-8, 14-2-10, and 12-2-12a in Water at 25 °C

a

m-2-n

CMC/(mmol L−1)

γCMC/ (mN m−1)

Γmax/(μmol m−2)

Amin/(Å2)

16-2-8 14-2-10 12-2-12

0.19 0.30 0.41

37.2 32.3 32

2.46 2.06 1.62

67.6 80.5 102

Data for 12-2-12 is from ref 47.

and listed in Table 2. Obviously, the obtained CMC values of m-2-n in water are about ten times lower than those in EAN. This observation is in keeping with the results of previous studies,46 due to the generally weaker solvophobic effect in EAN than in water.38 From Table 2, we can also see the similar aggregation trends of m-2-n in two solvents: the values of CMC, ΠCMC, and Amin all decrease with increasing dissymmetry. However, by detailed comparison, the dissymmetry effect of m-2-n on CMC values in both solvents is found different: the CMC ratio of 12-2-12 to 16-2-8 is about 2 in water, but becomes 10 in EAN. As we know, both water and EAN can

Figure 4. Phase diagrams for m-2-n/EAN binary mixtures, L1 and HII denote, respectively, the normal micelle phase and reverse hexagonal phase. (a) 12-2-12; (b) 14-2-10; (c) 16-2-8. D


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optical texture as shown in Figure 5a1−c1. In HII phase, at 70 wt % concentration, however, the clear fanlike patterns are

Figure 6. SAXS curves for the reversed hexagonal phases of m-2-n/ EAN systems at 40 °C.

Table 3. Structural Parameters of HII Phases in m-2-n/EAN system at 40 °Ca m-2-n

concn (wt %)

D (Å)

RII (Å)

DII (Å)

SII (Å2)

12-2-12 14-2-10 16-2-8

75.0 75.0 75.0

33.5 34.5 36.6

8.5 8.6 8.7

16.5 17.3 18.2

66.5 65.7 64.9

a RII is the radius of the reverse cylinder-like aggregates; dII is the thickness of hydrocarbon domains; SII is the area per molecule of amphiphile at the hydrophilic/hydrophobic interface.

aggregates, RII, changes slightly from 8.5 to 8.7 Å. However, the thickness of hydrocarbon domains, dII, increases from 16.5 to 18.2 Å. Because the total number of carbons of two alkyl chains in three Gemini molecules are the same, such LLC structure parameter changes should only be caused by the relative length of alkyl chains. In the aggregation process, the longer alkyl chain extends more, which would enlarge the solvophobic regions. Besides, as mentioned above, the larger dissymmetry will strengthen the intermolecular interactions between Gemini molecules. Both effects would enhance the solvophobic effect to promote the LLC formation more easily as confirmed by the contracted L1 region. In addition, the reduced area per molecule of amphiphile at the hydrophilic/hydrophobic interface, SII, with dissymmetry also provides evidence of more dense packing of Gemini molecules. 3.2.2. Rheological Properties of the HII Phase. The dissymmetry effect might also be reflected from the viscoelasticity properties of the HII phase. As shown in Figure 7 on the typical steady-shear rheograms (apparent viscosity as a function of shear rate) in the HII phase for three m-2-n/EAN samples, they all exhibit large viscosities at low shear rate and a shear-thinning phenomenon which is usually observed in gels at higher shear rate. From the data of frequency sweep measurement shown in Figure 8, however, it can be asserted

Figure 5. POM images of m-2-n/EAN systems for 12-2-12 (a), 14-210 (b), and 16-2-8 (c) at two surfactant concentrations, respectively, of 30% (isotropic region, series 1) and 75% (LLC region, series 2 to 4) and temperatures (°C) at 25 (a1−c1), 40 (a2−c2), and 98 (a3), 100 (a4, b3), 105 (b4), 107 (c3), 112 (c4).

observed as shown in Figure 5a2−c2. Besides, it is noted from these images that such a regular fanlike texture becomes a little deteriorated with increasing molecular dissymmetry, which should reflect the reduced molecular packing order in EAN. The reason might be that, with the dissymmetry enhancement, the intermolecular and intramolecular interactions become different. The longer chains or shorter chains prefer packing together respectively by themselves, and thus the molecular arrangement becomes more orderless.51,52 At higher temperatures, the formed HII phase could be gradually destroyed (Figure 5a3−c3) and the fanlike patterns would be lost (Figure 5a4−c4). The transition temperatures in m-2-n/EAN systems were found at about 100, 103, and 109 °C, respectively, for 122-12, 14-2-10, and 16-2-8 systems. A little improved thermostability of the HII phase with Gemini structure dissymmetry is thus concluded, which should originate from the enhancement of main driving force, the solvophobic effect, in LLC formation. The observed structure changes from POM texture in HII phase have been further disclosed by SAXS measurements. As shown in Figure 6, three Bragg scattering peaks can be seen with the relative scattering factor (q) positions of 1:√3:2, corresponding to a reversed hexagonal structure.53 At the same concentration (75%), the q of the first scattering peak (q1) becomes smaller for three m-2-n molecules with increasing dissymmetry. Therefore, the distance between two centers of the neighboring cylinders, D, becomes larger according to the formula D = 4π/√3q1. Based on D values, other parameters are calculated according to equations S1−S2 (Supporting Information) and listed in Table 3. It can be seen from Table 3 that the D value increases from 33.5 to 36.6 Å, and the radius of the reverse cylinder-like

Figure 7. Steady-shear rheological data collected at 40 ± 0.1 °C for m2-n/EAN systems. E


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is smaller than that in EAN, indicating more densely packed Gemini molecules in formed LLC phase, which is in accordance with the aggregation behaviors of single chain surfactants in water and EAN.33,55 As is discussed in the micellization of Gemini molecules above, it is concluded that the Amin value of 16-2-8 is smaller in EAN than in H2O at the interface, showing a reverse trend compared with the SII in more concentrated HII phase. This can be explained from the relative strength of the solvent effect at different concentrations. In dilute solutions, the charge repulsion of Gemini headgroups in water is stronger than that in EAN due to the latter charge screening effect, while at higher concentrations, the Gemini molecules have stronger solvophobic effects in water than in EAN,26,48−50 which becomes obvious and exceeds the charge screening effect of EAN, and thus causes a more dense packing of Gemini molecules in water. Besides, the relatively contracted solvent domains of the HII phase in EAN will also allow Gemini molecules to be arranged with the larger area per molecule at the interface.33,55

Figure 8. Changes of storage modulus G′ (solid) and loss modulus G″ (empty) versus frequency for the HII phase of three m-2-n/EAN systems at 40 °C.

that the samples are not gels but highly viscoelastic liquid crystals.54 This is because the storage (G′) and loss (G″) moduli change in different ways with frequency. G′ keeps increasing in the measured frequency region, while G″ increases only at low frequency and then decreases a little with some fluctuations. A viscous behavior exists at lower frequencies, where the G″ is larger than the G′, but at higher frequencies, the G′ is larger than the G″ showing elastic behavior.55 Such changes are consistent with the typical rheological properties of the reverse hexagonal phase.56 It is noticeable from Figure 8 on the profiles of moduli versus frequency that the increase of the dissymmetry could elevate the G′ and G″ values, and the same for the apparent viscosity, which should also be attributed to the increased intermolecular interactions. For the HII phase structure here, the cylinders of m-2-n molecules are hexagonally packed with the alkyl chains extruded out. With increasing dissymmetry, the elongated chains will enhance the solvophobic attractive forces and result in more gel-like networks inside and therefore the large moduli. 3.2.3. Solvent Effect. Except for studies above on the dissymmetry effect of Gemini molecules, comparison of such a structural influence on the LLC phases formed, respectively, in EAN and water has been a special concern. To present the difference more effectively, the Gemini molecule with the largest dissymmetry, the 16-2-8 has been chosen. As reported by Oda’s group, with increasing 16-2-8 concentration in H2O, the system exhibits the L1, Lα (lamelar), and HII phases in sequence.11 Unlike its counterpart in EAN, there is no obvious fan-like POM pattern at 75 wt % concentration (see Figure S6 in Supporting Information). Three Bragg peaks are clearly observed in the corresponding SAXS curve with the relative q positions of 1:√3:2 (see Figure S7 in Supporting Information). With the formula mentioned above, the parameters are calculated and listed in Table 4 with those of the 16-2-8/ EAN system for comparison. Some discrepancies can be noted from these data. The radius of the reverse cylinder (RII) in water is larger because of the smaller density of water (1.0 g/ mL) than that of EAN (1.2 g/mL), which should lead to larger solvent cores inside the reverse cylinders. The area per 16-2-8 molecule at the hydrophilic/hydrophobic interface (SII) in H2O

4. CONCLUSIONS The effects of structure dissymmetry on aggregation behaviors of a series of Gemini surfactants in EAN have been investigated. With increasing dissymmetry, the solvophobic effect of m-2-n molecules gets stronger as reflected by the decrease of critical micellization concentration, more densely packed molecular arrangement, and improved thermostability of LLC phase. Compared with the similar m-2-n Gemini systems in water, the molecules pack more compactly in EAN at low concentrations, while at higher concentrations, such packing is denser in H2O. This is a competing result between the charge screening effect and the solvophobic effect. In EAN, owing to the charge screening, the electrostatic repulsive interaction between the headgroups of the Gemini surfactant tends to be weakened. The role of chain−chain solvophobic interaction among the surfactant molecules becomes stronger. This effect becomes more serious with an increase in the degree of dissymmetry. Our results that no lamellar phase is found seem to also support the conclusion by Oda et al. that the symmetry is a critical factor for the formation of the tubular/lamellar phases.10,11 They found that the dissymmetry may cause an increase of the spontaneous curvature. The charge screening role of EAN further promotes such an effect to induce only the HII phase formation. All these data indicate that the aggregation behaviors of m-2-n can be regulated by controlling the molecule structure dissymmetry and solvent, which could be used as two means to tune the packing parameter. The obtained results here should be an important supplement to the self-assembly of Gemini surfactants in nonaqueous solvents.

■

S Supporting Information *

NMR spectra and additional calculations and references. This material is available free of charge via the Internet at http:// pubs.acs.org.

Table 4. Structural Parameters of HII Phases for m-2-n with Different Solvents at 40 °C m-2-n

solvent

concentration (wt %)

D (Å)

RII (Å)

DII (Å)

SII (Å2)

16-2-8 16-2-8

H2O EAN

75.0 75.0

43.1 36.6

11.1 8.7

20.9 18.2

61.3 64.9

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


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These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Zhonghua Wu for the kind help with synchrotron SAXS measurements and the SAXS station with the Beijing Synchrotron Radiation Facility (BSRF) in China are acknowledged. We are thankful for the financial support from the National Natural Science Foundation of China (20773080, 20973104, and 21033005) and Shandong Provincial Science Fund (2009ZRB01147).

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