Effects of a Spacer on the Phase Behavior of Gemini Surfactants in

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Effects of Spacer on the Phase Behaviors of Gemini Surfactants in Ethanolammonium Nitrate Qintang Li, Meihuan Yao, Xiu Yue, and Xiao Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00927 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Effects of Spacer on the Phase Behaviors of Gemini Surfactants in Ethanolammonium Nitrate Qintang Li,a

Meihuan Yao,b

Xiu Yue,c

Xiao Chena*

a, Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China b, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, China c, Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China

*Corresponding author: Xiao Chen Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China [email protected] Tel: +86-531-88365420 Fax: +86-531-88564464

1

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The aggregation behaviors of the quaternary ammonium Gemini surfactants (12-s-12) in a protic ionic liquid, ethanolammonium nitrate (EOAN), were investigated by small angle X-ray scattering, freeze-fracture transmission electron microscopy, polarized optical microscopy and rheological measurements. The rarely reported nonaqueous two-phases in the ionic liquid were observed at lower 12-s-12 concentrations. The up phase was composed of micelles, while only the surfactant unimers or multimers were detected in the low phase. At higher 12-s-12 concentrations, different aggregates were formed. The lamellar phase was observed in the 12-2-12/EOAN system, while the normal hexagonal phases in 12-s-12/EOAN (s = 3, 4, 5, 6, 8) systems and the micellar phase in the 12-10-12/EOAN system. Such a dramatic phase transition induced by the spacer chain length was due to the unique solvent characteristics of EOAN, compared to water and its counterpart ethylammonium nitrate.

2


Page 2 of 34

Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1. Introduction Ionic liquids (ILs) are salts with the melting temperature below 100 oC.1, 2 They are generally composed of large cations such as alkylammonium, 1-alkylimidazolium, 1-alkyl-2-alkylimidazolium, and small anions, such as nitrates, formates, carboxylates and trifluoroacetates.1 As tailorable solvents, ILs could be designed by selecting ions for prospective properties and are regarded as solvents of the future.3 Despite the great diversity, ILs share unique characteristics of low melting temperature, negligible vapor pressure, wide electrochemical window, high thermal stability, and ionic conductivity. Thus, they have attracted much attention in a wide range of applications, such as organic and inorganic synthesis,4-7 catalysis,8-10 chromatographic separation,11-13 electrochemistry14 and biological system.15, 16 It is recognized that ILs are good solvent candidates for self-assembly of surfactants. Depending on the ability of accepting the proton or not, ILs could be clarified as the protic ionic liquids (PILs) and the aprotic ones (APILs). Because of the capability of forming the hydrogen bonding network, the PILs, like water, possess stronger ability as solvent media to promote self-assembling than APILs. Ethylammonium nitrate (EAN), as the most studied PIL, has been widely used as the solvent for the self-assembly of various surfactants.1,

2, 17-19

Despite the single chain cationic and nonionic

surfactants, such as hexadecyltrimethylammonium bromide,20, 21 alkyl oligoethyleneoxide (CiEj),22 Pluronic block copolymers19, 23 and so on, other surfactants with unique structures like silicone and fluorinated zwitterionic surfactants, have been observed to self-assemble into micelles in EAN as well, similar to their behaviors in water.24,

25

The cationic surfactant with double chains,

dimethyldidodecylammonium bromide (DDAB), was also reported to aggregate in EAN and form a stable sponge phase in a wide DDAB concentration and temperature range.26, 27 For another novel 3


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

cationic surfactants, the quaternary ammonium Gemini ones (m-s-m), we have carried out systematic investigations on their phase behaviors in EAN and found the formation of a reverse hexagonal (H2) phase due to the charge screening effect of EAN.28,

29

Meanwhile, the phytosterol ethoxylate

surfactant (BPS-10) was noted to exhibit richer lyotropic liquid crystal (LLC) phase behaviors in EAN compared to APILs like 1-butyl-3-methylimidazolium tetrafluoroborate.30, 31 Though much attention has been paid to aggregates formed in EAN, there were still limited studies related to effects of PIL structure, like changes of the cation alkyl chain length. Atkin et al. reported that the nonionic surfactants CiEj would self-assemble into micelles and LLCs in propylammonium nitrate (PAN) at higher concentrations compared to those in EAN.32 The critical micellization concentration (CMC) values of alkyltrimethylammonium and pyridinium surfactants in PAN were reported to be larger than those in EAN.33 We have also studied the aggregation behaviors of 12-2-12 in PAN and butylammonium nitrate (BAN).28, 34 Despite of the changes of CMC, a dramatic phase transition was observed from the H2 phase in EAN to the normal hexagonal (H1) one in PAN and BAN. With the increase of the alkyl chain in cations, PILs had better hydrocarbon solubility and were easier to penetrate into the solvophobic domains, resulting in the formation of normal structure aggregates.35 Considering the structure-function relationship of the solvent, we turned our attention to another PIL, ethanolammonium nitrate (EOAN). Compared to EAN, the extra hydroxyl group was expected to bring EOAN a stronger ability forming hydrogen bonded network and a worse hydrocarbon solubility.1,17 Though it was the first discovered ionic liquid, EOAN attracted rather little attention as the aggregation medium for surfactants.17 Drummond el al. have investigated the phase behaviors of hexadecyltrimethylammonium chloride, hexadecylpyridinium bromide and polyoxyethylene (10) 4


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

oleyl ether in water, EAN, EOAN and diethanolammonium formate.36 Warr el al. carried out the surface tension measurements for C14EO4 in water, EAN and EOAN and found that the CMC values in water were a little smaller than those in EOAN. But both values were much smaller than that in EAN, confirming that EOAN got a stronger driving force for self-assembly than EAN.37 In this paper, we would investigate the phase behaviors of Gemini surfactants (12-s-12) in EOAN and explore the effect of spacer chain length. Our results would be an important supplement to aggregation behaviors of surfactants in EOAN and shed light on the structure effect of spacer in Gemini surfactants.

2. Experimental section 2.1. Materials Gemini surfactants, 12-s-12, were prepared according to the procedures reported previously.38 Their purities were ascertained by 1H NMR in CDCl3 (supporting information). EOAN was synthesized based on the method as described by Drummond et al.36 The purity was ascertained by 1

H NMR in DMSO (supporting information).

2.2. Sample preparation and phase diagram mapping The process for mapping the phase diagram has been described elsewhere.28 All samples were prepared by mixing the Gemini surfactant and EOAN at 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 type and structure 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 phase mapping and then 2 % for the determination of the phase boundaries. 5


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

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 block collimation system and an image plate 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 paste cell. The dilute samples were transferred into the standard quartz capillary with a diameter of 1 mm, where both small and wide angle X-ray scattering (SWAXS) profiles can be simultaneously recorded. The scattering factor q is defined as = (4π/λ)sinθ

(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: () = n()()

(2)

where n is the total number of particles, 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:

()

() = 4 ()

dr

(3)

The structure of micelles was then analyzed with a separation of inter- and intra-particle effects by the generalized indirect Fourier transformation (GIFT).39,40 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 6


Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

WV-CP460). 2.3.3. Freeze-fracture transmission electron microscopy (FF-TEM). 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 oC. Pt/carbon was deposited at an angle of 45 o to shadow the replicas and carbon was deposited at an angle of 90 o 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. 2.3.4. Rheological measurement. The rheological measurements were carried out with a Haake RS 75 rheometer using a Rotor C35/1 system at 55 oC. Frequency sweep measurements were performed under certain shear stress, which was determined from the linear viscoelastic region in the strain sweep measurement. The steady shear viscosity was measured in the range from 0.1 to 1000 s-1.

3. Results and discussion 3.1 Phase behaviors of 12-2-12 in EOAN As shown by the temperature-composition phase diagram in Figure 1, the phase behaviors of 12-2-12 in EOAN were rather simple. At low 12-2-12 concentrations (C12-2-12) below 45 %, the two-phase (L1/L) coexisted, which was rarely observed in the ionic surfactant/IL binary systems.27, 41 With the increase of C12-2-12, the two-phase merged into one single micellar phase (L1). Further increase of C12-2-12 to 70 % led to the formation of a lamellar LLC phase (Lα) here. Details about these phase behaviors were discussed below.

7


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Phase diagram of 12-2-12 in EOAN. 3.1.1 The two-phase In wide ranges of C12-2-12 (5~40 %) and temperature (55~120 oC), the coexistence of two-phase was confirmed by a clear interface observed with the naked eyes. Both phases were transparent and showed no birefringence even under shaking under crossed polarizers. With the increase of C12-2-12, the up phase (UP) volume increased and finally only one phase was observed (Figure S1). The sample at C12-2-12 of 20 % was taken as an example for further characterizations. The 1H NMR was employed to determine the composition of two phases with results shown in Figure S2.41 It was found that the amount of 12-2-12 in UP was around 41 % of the total UP weight while there was little in LP. According to the steady shear results (Figure S3), the solutions of UP and LP both behaved as Newtown fluids. The viscosity of LP was close to EOAN, while that of UP was two orders larger. Based on these results, it was assumed that UP was a surfactant-rich phase consisting of a large quantity of aggregates, while LP was a solvent-rich phase with much fewer or even none aggregates. Further evidence was provided by the SAXS and FF-TEM characterization. As shown in Figure 2a, the SAXS curves of two phases were totally different. The scattering intensity I(q) of the UP curve was basically constant at small q. A broad interaction peak appeared at medium q and I(q) decreased with the oscillation at high q. These characteristics indicated the 8


Page 8 of 34

Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

formation of micelles.41 On the other hand, the I(q) curve of LP was rather weak, indicating no sign of micelles. With the GIFT method, the structure of micelles in the UP could be obtained. According to the PDDF curve shown in Figure 2b, the p(r) went up to the maximum and then decreased to zero slowly. The decay was not linear, indicating the micelles were either ellipsoid or short rod. The short axis of the micelles was around 1.65 nm, which was consistent with the chain length of 12-2-12 (1.67 nm). The long axis was around 6.8 nm, corresponding to the maximum dimension of micelles. Thus, the SAXS results disclosed the existence of micelles in the UP and surfactant unimers or multimers in the LP. The FF-TEM observation results shown in Figure S4 provided further confirmation that aggregates in the UP could be observed while nothing in the LP.

Figure 2. SAXS curves (a) of UP and LP samples from 20 % 12-2-12/EOAN system at 55 oC and corresponding PDDF profile (b) of the UP. As well known, the phase separation of a colloid solution, also known as coacervation, was widely observed in aqueous solutions of surfactants with additives or mixed surfactants.42-44 Generally, the formation of aqueous surfactant two-phase (ASTP) system results from the synergistic effects of weak electrostatic interactions between headgroups and strong hydrophobic interactions of alkyl chains.44 Therefore, the ASTP systems from surfactants without additives or opposite charged surfactants were mainly observed in nonionic and Gemini zwitterionic surfactant aqueous solutions.45,46 However, such a phase separation phenomenon was rarely 9


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reported in surfactant/IL systems. One reason was the weaker solvophobic interactions of surfactants in ILs, which made the aggregation more difficult. On the other hand, the charge screening effects of ILs would weaken the electrostatic interactions between headgroups of ionic surfactants. Therefore, if there was certain factor to make the solvophobic interactions stronger, the two-phase separation might be observed in the surfactant/IL system. As an example, DDAB, with two alkyl chains, had stronger solvophobic interactions in EAN and spontaneously formed two separated lamellar phases.27 It was also suggested that the introduction of some weak interactions, such as hydrogen bonding and π-π interaction, should contribute to such phase separation.47,48 Then, it was not surprising that the phase separation took place in the 12-3OH-12/EAN system instead of in the 12-3-12/EAN system.41 Compared to EAN, EOAN would provide similar charge screening effect but a stronger driving force for self-assembly. Thus, the solvophobic interactions of Gemini surfactants in it were stronger. That was why 12-2-12 could form two-phase in EOAN while only one micellar phase in EAN. Therefore, the obtained results here might provide a simple way to fabricate the nonaqueous surfactant two-phase systems. 3.1.2 The L1 phase When C12-2-12 was above 45 %, only a single phase was observed, which was regarded as the micellar phase. As shown in Figure 3 for the SAXS results of the micellar solution at C12-2-12 of 50 %, both SAXS and PDDF curves were similar to those observed in UP at C12-2-12 of 20 %, indicating that the micelle morphology should be resembled. This further reflected the fact that the micellar phase would gradually replace the two-phase with the increase of C12-2-12.

10


Page 10 of 34

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. The SAXS curve (a) of the 50 % 12-2-12/EOAN sample at 55 oC and its corresponding PDDF profile (b). 3.1.3 The Lα phase Further increase of C12-2-12 led to the formation of LLC phase. Detailed information could be obtained by the POM observations and SAXS measurements. As shown in the POM images from Figure 4a to 4c, the Maltese cross and oil steak texture were observed, confirming the formation of the Lα phase.30 The SAXS curves, however, exhibited only one obvious Bragg peak, corresponding to the 1st scattering peak of these Lα phases.

Figure 4. POM images (a-c) and SAXS curves (d) for 12-2-12/EOAN samples at 55 oC and different concentrations. From a to c, C12-2-12 = 70 %, 75 %, 80 %. As can be seen from the corresponding SAXS curves shown in Figure 4d, the Bragg peaks were shifted to high q values with the increase of C12-2-12, reflecting the reduction of repeat distances (d) of

11


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the Lα phase. Then, the structure parameters of the Lα phase could be obtained according to the method reported previously.30 The changes of d with the volume fraction of solvophobic components (Φa) were shown in Figure 5a, while other structure parameters with the volume fraction of EOAN (ΦIL) were shown in Figure 5b. The linear relationship between log d and log Φa indicated a typical one dimension swelling of the Lα phase. With the increase of C12-2-12, the solvophobic domain thickness (da) kept almost constant and smaller than the chain length of 12-2-12 (1.67 nm), suggesting an interlaced packing of 12-2-12 molecules. The solvophilic domain thickness (dIL) was proportional to IL contents, while the effective cross-sectional area per surfactant molecule (S) kept decreasing, suggesting more dense packing of 12-2-12 molecules.

Figure 5. The plot of log d vs -log Φa (a) and changes of structure parameters with ΦIL (b) for the Lα phase. d, the lamellae distance; da, the solvophobic domain thickness; dIL, the solvophilic thickness; S, effective cross-sectional area per surfactant molecule; Φa, the volume fraction of solvophobic components; ΦIL, volume fraction of IL. The temperature influence on the Lα phase was also explored. The Lα phase of 80 % 12-2-12 was stable in wide temperature range from 55 to 115 oC, as confirmed by results from POM observations and SAXS measurements shown in Figure 6. With the increase of temperature, the oil steak texture was deteriorating and Maltese cross appeared. Further increase of temperature would lead the Lα phase to transform into the L1 phase. Sharp Bragg peaks of SAXS curves were observed in this temperature range and shifted to high q values, indicating decrease of the lamellar repeat distance. 12


Page 12 of 34

Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

This was due to the conformational fluctuation of the hydrophobic chains induced by temperature.49

Figure 6. POM images (a-d) and SAXS curves (e) for the 80 % 12-2-12/EOAN sample at different temperatures. From a to d, 70 oC, 85 oC, 100 oC,115 oC. 3.1.4 Effect of EOAN To disclose the unique effect of EOAN, the aggregation behaviors of 12-2-12 in water and EAN were recalled and compared here. According to the previous researches, except for the L1 phase, the normal hexagonal (H1) and Lα phases were formed in water,50 while the L1 phase and the reverse hexagonal (H2) phase in EAN.28 The Gordon parameter (G) is a good reference to compare the ability for self-assembling among these three solvents. G was defined as "

G = γ⁄ V!

(4)

where γ is the surface tension at the air/liquid interface, and Vm is the solvent molar volume. The higher the G value is, the stronger the solvent driving force is 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 value (2.8 J·m-3).2 For the two studied PILs here, with a hydroxyl group replacing a hydrogen atom in the methyl group, EOAN had stronger hydrogen bonding network compared to EAN, giving rise to a stronger driving force. Then, EOAN had a little larger Gordon parameter (1.097 J·m-3) than that of EAN (1.060 J·m-3), though both much smaller than that of water.2 It has 13


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

been concluded that solvents with higher Gordon values tend to be linked to larger LLC phase diversity and better thermal stability. It was no wonder that richer LLC phases were observed in water while only one LLC phase was observed in EAN and EOAN. As for the thermal stability of the Lα phase in EOAN, the transition temperature between the Lα and L1 phase was 73 oC at a 75 % 12-2-12 concentration. However, the Lα phase in water was stable under the solvent boiling point. The dynamic rheological results (Figure S5) of the Lα phases showed that the modulus in water were much larger than those in EOAN. This was due to the loose packing of surfactant molecules for their weaker solvophobic interactions in EOAN. As reported in our previous studies, the H2 phase was observed in the 12-2-12/EAN system. Then, there is a question why a Lα phase was observed in the 12-2-12/EOAN system. To understand the aggregates formed in these three systems, the critical packing parameter (CPP) was introduced here. CPP is defined as CPP = & ⁄' ()

(5)

where V is the solvophobic volume of the amphiphile, a0 is the effective headgroup area and lc is the effective chain length of the surfactant in its molten state. In water, 12-2-12 would have the smallest CPP value due to the electrostatic interaction between headgroups. However, the charge screening effect and participation of self-assembly by EAN led 12-2-12 molecule to have a smaller a0 and a bigger V, and therefore a larger CPP value. As a result, the H2 phase was formed. When a hydroxyl group was introduced, there was no tendency for EOAN molecules to disperse into the 12-2-12 domains. What’s more, it was interesting to find that the dielectric constant of EOAN was 60.9 ± 2.0 F/m, smaller than that of water (78.36 F/m) but much larger than that of EAN (26.3 ± 0.5 F/m). This suggested that the charge screening effect in EOAN should be worse than that in EAN. Then, it was 14


Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

reasonable that the area occupied by the surfactant at the solvophilic/solvophobic interface at a 75 % surfactant concentration in EOAN (0.74 nm2) was slightly smaller than that in water (0.84 nm2) but larger in EAN (0.67 nm2), which could be calculated from SAXS results.29 It was concluded that the CPP values of 12-2-12 in these solvents would follow a sequence as EAN > EOAN > water. Therefore, the Lα phase of a smaller CPP was observed in EOAN. In fact, the CPP values of 12-2-12 in EAN and EOAN differed little. The Lα and H2 phases would compete over the high 12-2-12 concentration region and not appear in the surfactant/PIL system simultaneously.2 That was why only one single LLC phase was observed in 12-2-12/EAN or 12-2-12/EOAN systems.

3.2 Spacer effects on phase behaviors Given the different phase behaviors of 12-2-12 in EOAN compared to those in water and EAN, it would be interesting to explore the effect of spacer chain length on 12-s-12/EOAN phase behaviors. For this reason, the systematic study was carried out in the 12-s-12/EOAN systems (s = 3, 4, 5, 6, 8, 10) and the phase behavior in 12-3-12/EOAN was described in detail as an illustration. 3.2.1 Phase behaviors of 12-3-12 in EOAN As shown by the phase diagram in Figure 7a, with the increase of C12-3-12, the two-phase and L1 phase were also observed successively, similar to those observed in 12-2-12/EOAN. When C12-3-12 was increased to 60 %, a new anisotropic phase was observed. However, unlike the Lα phase formed in the 12-2-12/EOAN system, the POM and SAXS results here confirmed the formation of a hexagonal phase. Further increase of C12-3-12 led to no other new phase but only the undissolved solid surfactants. As shown by the POM images in Figure 8a-8d, more and more regular fanlike patterns were observed with the increase of C12-3-12 from 65 % to 80 %. The SAXS curves shown in Figure 8e displayed three peaks with their q values following a 1:√3:2 ratio. With the increase of C12-3-12, the 15


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Bragg peaks moved to higher q values, corresponding to smaller lattice parameters, similar to the situation in water but contrary to that in EAN.28, 50

Figure 7. Phase diagrams of 12-s-12 in EOAN. s = 3, 4, 5, 6, 8, 10.

Figure 8. POM images (a-d) and SAXS curves (e) for 12-3-12/EOAN samples at different concentrations and 55 oC. From a to d, C12-3-12 = 65 %, 70 %, 75 %, 80 %. As no other LLC phase was observed, it was a little difficult to identify this hexagonal phase as a normal or reverse one. According to our systematic studies on 12-s-12 in these solvents28, 29, 34, 41, this 16


Page 16 of 34

Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

hexagonal phase was believed to be a normal one based on the following two reasons. First, for 12-s-12 (2 < s < 6), with the increase of spacer chain length, the area occupied at the solvophilic/solvophobic interface (S) should be increased, similar to what has been observed in water and EAN systems.38, 50, 51 To illustrate this point more clearly, the structure parameters of the LLC phase at C12-s-12 = 75 % were calculated and analyzed based on the SAXS results. The S value for the Lα phase in the 12-2-12/EOAN system was 0.74 nm2. The values for the assumed H1 phase (S1) and H2 phase (S2) in the 12-3-12/EOAN system were 1.15 and 0.58 nm2 respectively. Obviously, the H1 phase made more sense. In addition, according to the parameters calculated by the assumed H2 phase model, the solvent radius (R2) was 0.89 nm, larger than that (0.84 nm) in the 12-3-12/EAN system. Considering the higher density of EOAN, this was really not reasonable. Therefore, the H1 phase was formed in the 12-3-12/EOAN system. 3.2.2 Phase behaviors of 12-s-12 in EOAN With s ranging from 4 to 8, the two-phase regions were greatly changed (Figure 7b-7e). The L1/L two-phases were transformed into the H1/L two-phases at medium concentrations. It seemed that with the increase of s, 12-s-12 got stronger solvophobic interactions and would form the H1 phases at lower concentration than 12-3-12. Further increase in concentrations led to the complete transformation of the H1 phase. This was confirmed by SAXS and POM results shown in Figure 8.

Figure 9. POM images (insets) and SAXS curves of 12-s-12/EOAN samples. (a) 12-4-12, 75 %，55 oC; (b) 12-5-12, 65 %，55 oC；(c) 12-6-12, 65 %，80 oC. 17


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

To illustrate the effect of spacer, the SAXS and dynamic rheological measurements were carried out in the 12-s-12/EOAN (s = 3, 4, 5) systems at a 70 % concentration. According to the structure parameters calculated by SAXS results (Table S1), the 12-4-12 and 12-5-12 had similar molecular structure and their headgroup areas (S1) were larger than that of 12-3-12. This suggested that with the increase of spacer chain length, 12-s-12 would have a smaller CPP, resulting in the formation of the H1 phase. As shown in Figure S6, the moduli of the H1 phases were much larger than those of the Lα phase in the 12-2-12/EOAN system, confirming the phase transition from Lα to H1 induced by the spacer chain length. These three samples behaved as Maxwell fluids. The elastic moduli (G') increased with frequency (f) and kept constant at high f , while the viscous moduli (G'') increased at low f and decreased at high f. Then, the G' and G'' intersected at a certain frequency, the reciprocal of which was defined as the relaxation time τ. It was noticed that the longer spacer would contribute stronger solvophobic interations. Herein, the moduli of H1 phases were enhanced and the exchange of surfactant molecules between H1 and bulk phase would become more difficult, correspongding to larger τ values (Table S1). When it came to 12-8-12, the increase of s would have some influence. The H1/L two-phase would appear at a higher concentration around 40%. What's more, no single H1 phase was formed in the detected concentration region.

This indicated that the

increase of the area at the

solvophilic/solvophobic interface became more obvious and 12-8-12 got a much smaller CPP value than 12-s-12 (s = 3, 4, 5, 6). With further increase of the spacer length to 10, the area occupied by 12-10-12 at the interface became even larger, leading to a further smaller CPP. As a result, no trace of the H1 phase could be observed. The L1/L two-phase dominated almost the whole concentration and the L1 phase was 18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

formed at higher surfactant concentrations. For such a long spacer, 12-10-12 behaved more like a single chain surfactant rather than the Gemini one. Such phase behaviors of 12-s-12 in EOAN was different from those in water. According to the surface tension results previously reported, the area occupied by 12-s-12 at the air/water interface kept increasing.51 Due to the electrostatic interaction between headgroups in water, these changes might be not big enough to influence CPP of 12-s-12, for such a dramatic phase transition was not observed with changing spacer length.52 Neither did the phase transition happen in EAN, perhaps due to the much larger CPP for the reverse hexagonal phase. 4. Conclusions The aggregation behaviors of quaternary ammonium Gemini surfactants 12-s-12 (s = 2, 3, 4, 5, 6, 8, 10) in a protic ionic liquid, EOAN, have been investigated. The nonaqueous two-phase (L1/L) system was observed at 12-s-12/EOAN systems. The charge screening effect and poor solubility for hydrocarbon chains by EOAN may account for this phenomenon. Such nonaqueous two-phase system might be used in separation and purification. At higher 12-s-12 concentrations, different LLC phases were observed depending on the spacer chain length. In the 12-2-12/EOAN system, only the Lα phase was formed, unlike the observed H1 and Lα phases in water or the H2 phase in EAN. This was because that the 12-2-12 got an intermediate CPP value in EOAN. With the increase of spacer chain length, the Lα phase was transformed into the H1 phase (3 ≤ s ≤ 8), which was originated from the larger headgroup area at the solvophilic/solvophobic interface. As for 12-10-12, the nonaqueous two-phase (L1/L) dominated almost the whole concentration range. Such a phase transition was not so obvious in water and not even observed in EAN. Therefore, By the combination of Gemini surfactant and EOAN, the tailorable coexisting phases or LLC phases were obtained. Our results 19


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

would expand studies of an important PIL, EOAN, as self-assembling solvent and help understand the aggregation mechanism of Gemini surfactants in PILs. Associated content Supporting information 1

H NMR results of 12-s-12, EOAN and two phases; UP volume percentage of two coexisting

phases; steady and dynamic rheological results; FF-TEM images; phase parameters. Acknowledgements We are thankful for the financial supports from the National Natural Science Foundation of China (21373127 and 21673129) and the Specialized Research Fund for the Doctoral Program of Higher Education (20130131110010). References 1.

Greaves, T. L.; Drummond, C. J. Protic ionic liquids: properties and applications. Chem. Rev.

2008, 108, 206-237. 2.

Greaves, T. L.; Drummond, C. J. Ionic liquids as amphiphile self-assembly media. Chem. Soc.

Rev. 2008, 37, 1709-1726. 3.

Rogers, R. D.; Seddon, K. R. Ionic liquids - solvents of the future? Science 2003, 302, 792-793.

4.

Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev.

1999, 99, 2071-2083. 5.

Lagrost, C.; Carrie, D.; Vaultier, M.; Hapiot, P. Reactivities of some electrogenerated organic

cation radicals in room-temperature ionic liquids: toward an alternative to volatile organic solvents? J. Phys. Chem. A 2003, 107, 745-752. 6.

Yong, Z.; Schattka, J. H.; Antonietti, M. Room-temperature ionic liquids as template to

monolithic mesoporous silica with wormlike pores via a sol-gel nanocasting technique. Nano Lett. 2004, 4, 477-481. 7.

Wang, Y.; Yang, H. Synthesis of CoPt nanorods in ionic liquids. J. Am. Chem. Soc. 2005, 127,

5316-5317. 20


Page 20 of 34

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

8.

Wasserscheid, P.; Keim, W. Ionic liquids - new "solutions" for transition metal catalysis. Angew.

Chem. Int. Ed. 2000, 74, 157-189. 9.

Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Ionic liquid (molten salt) phase organometallic

catalysis. Chem. Rev. 2002, 102, 3667-3692. 10. Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Z. Ionic liquids: application in catalysis. Catal. Today 2002, 74, 157-189. 11. Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Solvation of 1-butyl-3-methylimidazolium hexafluorophosphate in aqueous ethanol-a green solution for dissolving 'hydrophobic' ionic liquids. Chem. Commun. 2001, 2070-2071. 12. Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. Carbon dioxide induced separation of ionic liquids and water. Chem. Commun. 2003, 572-573. 13. Liu, X. X.; Yang, Q. W.; Bao, Z. B.; Su, B. G.; Zhang, Z. G.; Ren, Q. L.; Yang, Y. W.; Xing, H. B. Nonaqueous lyotropic ionic liquid crystals: preparation, characterization, and application in extraction. Chem. Eur. J. 2015, 21, 9150-9156. 14. Buzzeo, M. C.; Evans, R. G.; Compton, R. G. Non-haloaluminate room-temperature ionic liquids in electrochemistry - a review. ChemPhysChem 2004, 5, 1106-1120. 15. Kumar, A.; Venkatesu, P. Overview of the stability of α-chymotrypsin in different solvent media. Chem. Rev. 2012, 112, 4283-4307. 16. Mood, S. H.; Golfeshan, A. H.; Tabatabaei, M.; Jouzani, G. S.; Najafi, G.; Gholami, M.; Ardjmand, M. Lignocellulosic biomass to bioethanol, a comprehensive review with a focus on pretreatment. Renew. Sust. Energ. Rev. 2013, 27, 77-93. 17. Greaves, T. L.; Drummond, C. J. Protic ionic liquids: evolving structure-property relationships and expanding applications. Chem. Rev. 2015, 115, 11379-11448. 18. Hao, J. C.; Zemb, T. Self-assembled structures and chemical reactions in room-temperature ionic liquids. Curr. Opin. Colloid Interface Sci. 2007, 12, 129-137. 19. Zhang, G. D.; Chen, X.; Zhao, Y. R.; Ma, F. M.; Jing, B.; Qiu, H. Y. Lyotropic liquid-crystalline phases formed by Pluronic P123 in ethylammonium nitrate. J. Phys. Chem. B 2008, 112, 6578-6584. 20. Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Many protic ionic liquids mediate hydrocarbon-solvent interactions and promote amphiphile self-assembly. Langmuir 2007, 23, 402-404. 21


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

21. Lopez-Barron, C. R.; Wagner, N. J. Structural transitions of CTAB micelles in a protic ionic liquid. Langmuir 2012, 28, 12722-12730. 22. Araos, M. U.; Warr, G. G. Self-assembly of nonionic surfactants into lyotropic liquid crystals in ethylammonium nitrate, a room-temperature ionic liquid. J. Phys. Chem. B 2005, 109, 14275-14277. 23. Lopez-Barron, C. R.; Li, D. C.; Wagner, N. J.; Caplan, J. L. Triblock copolymer self-assembly in ionic liquids: effect of PEO block length on the self-assembly of PEO-PPO-PEO in ethylammonium nitrate. Macromolecules 2014, 47, 7484-7495. 24. Zhang, S. H.; Liu, J.; Li, N.; Yang, X. J.; Zheng, L. Q. Aggregation behavior of silicone surfactants in ethylammonium nitrate ionic liquid. Colloid Polym. Sci. 2012, 290, 1927-1935. 25. Wang, X. L.; Long, P. F.; Dong, S. L.; Hao, J. C. First fluorinated zwitterionic micelle with unusually slow exchange in an ionic liquid. Langmuir 2013, 29, 14380-14385. 26. Lopez-Barron, C. R.; Basavaraj, M. G.; DeRita, L.; Wagner, N. J. Sponge-to-lamellar transition in a double-tail cationic surfactant/protic ionic liquid system: Structural and rheological analysis. J. Phys. Chem. B 2012, 116, 813-822. 27. Lopez-Barron, C. R.; Li, D. C.; DeRita, L.; Basavaraj, M. G.; Wagner, N. J. Spontaneous thermoreversible formation of cationic vesicles in a protic ionic liquid. J. Am. Chem. Soc. 2012, 134, 20728-20732. 28. Wang, X. D.; Chen, X.; Zhao, Y. R.; Yue, X.; Li, Q. H.; Li, Z. H. Nonaqueous lyotropic liquid-crystalline phases formed by Gemini surfactants in a protic ionic liquid. Langmuir 2012, 28, 2476-2484. 29. Wang, X. D.; Li, Q. T.; Chen, X.; Li, Z. H. Effects of structure dissymmetry on aggregation behaviors of quaternary ammonium Gemini surfactants in a protic ionic liquid EAN. Langmuir 2012, 28, 16547-16554. 30. Yue, X.; Chen, X.; Wang, X. D.; Li, Z. H. Lyotropic liquid crystalline phases formed by phyosterol ethoxylates in room-temperature ionic liquids. Colloids Surf. A 2011, 392, 225-232. 31. Yue, X.; Chen, X.; Li, Q. T. Comparison of aggregation behaviors of a phytosterol ethoxylate surfactant in protic and aprotic ionic liquids. J. Phys. Chem. B 2012, 116, 9439-9444. 32. Atkin, R.; Bobillier, S. M. C.; Warr, G. G. Propylammonium nitrate as a solvent for amphiphile self-assembly into micelles, lyotropic liquid crystals, and microemulsions. J. Phys. Chem. B 2010, 114, 1350-1360. 22


Page 22 of 34

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

33. Fernandez-Castro, B.; Mendez-Morales, T.; Carrete, J.; Fazer, E.; Cabeza, O.; Rodriguez, J. R.; Turmine, M.; Varela, L. M. Surfactant self-assembly nanostructures in protic ionic liquids. J. Phys. Chem. B 2011, 115, 8145-8154. 34. Li, Q. T.; Wang, X. D.; Yue, X.; Chen, X. Phase transition of a quaternary ammonium Gemini surfactant induced by minor structural changes of protic ionic liquids. Langmuir 2014, 30, 1522-1530. 35. Greaves, T. L.; Mudie, S. T.; Drummond, C. J. Effect of protic ionic liquids (PILs) on the formation of non-ionic dodecyl poly(ethylene oxide) surfactant self-assembly structures and the effect of these surfactants on the nanostructure of PILs. Phys. Chem. Chem. Phys. 2011, 13, 20441-20452. 36. Chen, Z. F.; Greaves, T. L.; Fong, C.; Caruso, R. A.; Drummond, C. J. Lyotropic liquid crystalline phase behaviour in amphiphile-protic ionic liquid systems. Phys. Chem. Chem. Phys. 2012, 14, 3825-3836. 37. Wakeham, D.; Warr, G. G.; Atkin, R. Surfactant Adsorption at the Surface of Mixed Ionic Liquids and Ionic Liquid Water Mixtures. Langmuir 2012, 28, 13224-13231. 38. Zana, R.; Benrraou, M.; Rueff, R. Alkanediyl-α,ω-bis(dimethyl alkylammonium- bromide) surfactants. 1. Effects of spacer chain length on the critical micelle concentration and micelle ionization degree. Langmuir 1991, 7, 1072-1075. 39. Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Schubert, K. V.; Kaler, E. W.; Glatter, O. Small-angle scattering of interacting particles. III. D2O-C12E5 mixtures and microemulsions with n-octane. J. Chem. Phys. 1999, 112, 10623-10632. 40. Fritz, G.; Bergmann, A.; Glatter, O. Evaluation of small-angle scattering data of charged particles using the generalized indirect Fourier transformation technique. J. Chem. Phys. 2000, 113, 9733-9740. 41. Li, Q. T.; Wang, X. D.; Yue, X.; Chen, X. Unique phase behaviors in the Gemini surfactant/EAN binary system: the role of the hydroxyl group. Langmuir 2015, 31, 13511-13518. 42. Kizilay, E.; Kayitmazer, A. B.; Dubin, P. L. Complexation and coacervation of polyelectrolytes with oppositely charged colloids. Adv. Colloid Interface Sci. 2011, 167, 24-37. 43. Veis, A. A review of the early development of the thermodynamics of the complex coacervation phase separation. Adv. Colloid Interface Sci. 2011, 167, 2-11. 23


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

44. Wang, M. N.; Wang, Y. L. Development of surfactant coacervation in aqueous solution. Soft Matter 2014, 10, 7909-7919. 45. Peresypkin, A. V.; Menger, F. M. Zwitterionic geminis. Coacervate formation from a single organic compound. Org. Lett. 1999, 1, 1347-1350. 46. Mukherjee, P.; Padhan, S. K.; Dash, S.; Patel, S.; Mishra, B. K. Clouding behaviour in surfactant systems. Adv. Colloid Interface Sci. 2011, 162, 59-79. 47. Huang, X.; Cao, M. W.; Wang, J. B.; Wang, Y. L. Controllable organization of a carboxylic acid type gemini surfactant at different pH values by adding copper(II) ions. J. Phys. Chem. B 2006, 110, 19479-19486. 48. Zhang, Q.; Gao, Z. N.; Xu, F.; Tai, S. X.; Liu, X. G.; Mo, S. B.; Niu, F. Surface tension and aggregation properties of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer. Langmuir 2012, 28, 11979-11987. 49. Gupta, A.; Willis, S. A.; Waddington, L. J.; Stait-Gardner, T.; Campo, L. d.; Hwang, D. W.; Kirby, N.; Price, W. S.; Moghaddam, M. J. Gd-DTPA -dopamine-bisphytanyl amphiphile: synthesis, characterisation and relaxation parameters of the nanoassemblies and their potential as MRI contrast agents. Chem. Eur. J. 2015, 21, 13950-13960. 50. Alami, E.; Levy, H.; Zana, R. Alkanediyl-α,ω-bis(dimethylalky1ammonium bromide) surfactants. 2. Structure of the lyotropic mesophases in the presence of water. Langmuir 1993, 9, 940-944. 51. Alami, E.; Beinert, G; Marie, P.; Zana, R. Alkanediyl-α,ω-bis(dimethylalky1ammonium bromide) surfactants. 3. Behavior at the air-water interface. Langmuir 1993, 9, 1465-1467.

24


Page 24 of 34

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Graphical Abstract 36x16mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Phase diagram of 12-2-12 in EOAN. 60x45mm (300 x 300 DPI)


Page 26 of 34

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. SAXS curves (a) of UP and LP samples from 20 % 12-2-12/EOAN system at 55 oC and corresponding PDDF profile (b) of the UP. 45x16mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. The SAXS curve (a) of the 50 % 12-2-12/EOAN sample at 55 oC and its corresponding PDDF profile (b). 44x16mm (300 x 300 DPI)


Page 28 of 34

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4. POM images (a-c) and SAXS curves (d) for 12-2-12/EOAN samples at 55 oC and different concentrations. From a to c, C12-2-12 = 70 %, 75 %, 80 %. 47x34mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The plot of log d vs -log Φa (a) and changes of structure parameters with ΦIL (b) for the Lα phase. d, the lamellae distance; da, the solvophobic domain thickness; dIL, the solvophilic thickness; S, effective cross-sectional area per surfactant molecule; Φa is the volume fraction of solvophobic components; ΦIL, volume fraction of IL. 42x14mm (300 x 300 DPI)


Page 30 of 34

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 6. POM images (a-d) and SAXS curves (e) for the 80 % 12-2-12/EOAN sample at different temperatures. From a to d, 70 oC, 85 oC, 100 oC,115 oC. 37x13mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Phase diagrams of 12-s-12 in EOAN. s = 3, 4, 5, 6, 8, 10. 90x101mm (300 x 300 DPI)


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 8. POM images (a-d) and SAXS curves (e) for 12-3-12/EOAN samples at different concentrations and 55 oC. From a to d, C12-3-12 = 65 %, 70 %, 75 %, 80 %. 37x13mm (300 x 300 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. POM images (insets) and SAXS curves of 12-s-12/EOAN samples. (a) 12-4-12, 75 %，55 oC; (b) 12-5-12, 65 %，55 oC；(c) 12-6-12, 65 %，80 oC. 30x7mm (300 x 300 DPI)


Page 34 of 34

Effects of a Spacer on the Phase Behavior of Gemini Surfactants in

Recommend Documents