Effect of Imidazolium-Based Surface-Active Ionic Liquids on the

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Effect of Imidazolium-Based Surface Active Ionic Liquids on the Orientation of Liquid Crystals at Various Fluid-Liquid Crystal Interfaces Tongtong Tian, Qiongzheng Hu, Yi Wang, Yanan Gao, and Li Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02756 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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Effect of Imidazolium-Based Surface Active Ionic Liquids on the Orientation of Liquid Crystals at Various Fluid-Liquid Crystal Interfaces Tongtong Tian,a, b Qiongzheng Hu,c Yi Wang, a Yan’an Gao,d and Li Yu a, b*

a

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China b

School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China c

Department of Chemistry, University of Houston, Houston, Texas 77204, United States

d

China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.

Corresponding author: Prof. Dr. Li Yu Phone number: +86-531-88364807 Fax number: +86-531-88564750 E-mail address: [email protected]

ACS Paragon Plus Environment

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Abstract A series of imidazolium-based surface active ionic liquids (IM-SAILs), viz. single-chained IM-SAILs, 1-alkyl-3-methylimidazolium bromide n=12,

14,

16),

1-dodecyl-3-methylimidazolium

1-dodecyl-3-methylimidazolium 1-dodecyl-3-methylimidazolium

salicylate

3-hydroxy-2-naphthoate cinnamate

([C n mim]Br, ([C 12 mim]Sal),

([C 12 mim]HNC), ([C 12 mim]CA),

1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C 12 mim]PCA); gemini IM-SAIL, 1,2-bis(3-dodecylimidazolium-1-yl) ethane bromide ([C 12 -2-C 12 im]Br 2 ), along with three short-chained ionic liquids (ILs), ethylammonium nitrate (EAN), propylammonium nitrate (PAN) and butylammonium nitrate (BAN), were synthesized and applied to nematic liquid crystal (LC)-fluid interfaces. Firstly, we evaluated the influence of the length and number of the aliphatic chain, as well as the counterion in the IM-SAIL structures on the anchoring of LCs at the aqueous/LC interface. It was observed that the threshold concentration of [C n mim]Br (n=12, 14, 16) reduced with the increase of aliphatic chain length. And double-chained [C 12 -2-C 12 im]Br 2 has a far lower threshold concentration than the single-chained [C 12 mim]Br. But the alteration of counterions (e.g. Br− and aromatic counterions) scarcely affected the anchoring of LCs at the interface. Secondly, we investigated the alignment of LCs at the diverse IL/LC interfaces with the presence of IM-SAILs. It is found that the variations on both aliphatic chain length and number can remarkably change the trigger points of the orientational transition of LCs at the EAN/LC interface. In specific, with a slight increase in the alkyl chain length of short-chained ILs, as the fluid medium, the orientation of LCs varied tremendously at the IL/LC interface. Therefore, the higher threshold concentration of IM-SAILs and the corresponding more stability in the optical appearance of LCs at the EAN/LC interface than aqueous/LC interface can be ascribed to the discrepancy of microstructure of water and IL. Finally, we verified that the volume ratio of H 2 O to EAN could more dramatically affect the alignment of LCs than the change of IM-SAILs concentration in aqueous solution. This work firstly illustrated the impact of SAIL structures on the LCs orientation at the aqueous/LC,


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IL/LC and H 2 O-IL mixture/LC interfaces, which will inspire us to obtain a stabilized molecular alignment of LCs at the IL/LC interfaces and to further design novel functionalized SAIL molecules for various chemical and biological applications.

Keywords: Thermotropic liquid crystal, fluid interface, surface active ionic liquid, orientation


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1. Introduction During the past decades, sensors based on thermotropic liquid crystals (LCs), a type of materials switching between a crystalline state and an isotropic liquid,1 have drawn widespread attention, and on this occasion, a variety of chemical and biological events have been amplified and transduced into macroscopic optical signals that are visible with the naked eyes under crossed polarizers.2-8 Since the anchoring transition of LCs at the aqueous/LC interface is controlled by the energy in the order of 10-2-10-3 mJ·m-2, the orientation change of LCs is able to be coupled with surfactants,8-9 proteins,10 lipids,11-12 and synthetic polymers13 adsorbed at the aqueous/LC interfaces, inspiring the design of LC-based sensors for the respective processes. These sensors are usually simple, label-free, real-time, and provide good spatial resolution14-18. Therefore, they have become expedient tools to study the reversible adsorption of amphiphiles and quantification of the orientations of LCs at aqueous/LC interface. 5CB (4-cyano-4'-pentylbiphenyl) is famous for its high chemical stability and room-temperature nematic phase state (temperature ranging from 23.5 ℃ to 35 ℃). Therefore, this molecule was usually chosen as an imaging platform to give insight into the configuration of the ‘‘alignment monolayer’’.19 To date, the alignments of LCs at the aqueous/LC interface with surfactants decoration for sensing key molecular and microscopic events have been widely exploited.20-22 For example, it is reported that the gradual addition of sodium dodecyl sulfate (SDS) to the aqueous phase in contact with the LCs could result in a spontaneous bright-to-dark shift in the optical appearance, suggesting the anchoring of LCs was affected by surfactant concentration.9 Subsequently, the orientational behaviors of LCs at aqueous/LC interface were studied and characterized by employing

several

surfactants

with

different

structures.22

For

instance,

ferrocene-containing and bolaform surfactants, which have two hydrophilic headgroups linked by a hydrophobic chain, were confirmed to adopt looped conformations and to keep 5CB molecules planar alignment at aqueous/LC interface in certain concentration regimes. While linear surfactants with a single hydrophilic


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headgroup could perform planar to vertical anchoring at corresponding threshold concentrations.22 More specifically, when in contact with anionic surfactant SDS, cationic surfactant dodecyltrimethylammonium bromide (DTAB) or non-ionic surfactant tetra-(ethylene glycol) monododecyl ether (C 12 E 4 ), the anchoring transition of 5CB appeared from planar to homeotropic state, indicating that these headgroups did not measurably influence the orientation of LCs.22 In addition, a lower surfactant concentration which triggered planar-to-homeotropic anchoring transition of LCs was coupled with increasing aliphatic chain length of alkyltrimethylammonium bromides (C n TABs, n = 8, 12, 16), indicating the intensity of interaction between the surfactant tail and 5CB depended on the length of surfactant aliphatic chain.22 After that, four surfactants with difference in tail branching were additionally reported, viz. linear SDS,

sodium

dodecylsulfonate

(LDS),

isomerically

pure

linear

sodium

dodecylbenzenesulfonate (LDBS), and branched sodium dodecylbenzenesulfonate (BR-DBS). It is found that linear surfactants caused LCs to assume from parallel to homeotropic anchoring shift above their threshold concentrations whereas in the case of branched BR-DBS, the LC adopted a planar anchoring state at all experimental concentrations.23 Moreover, the influence of surfactant tail organization on the orientations of LCs has been probed using LDS (homeotropic anchoring of LCs) and LDBS which comprises a mixture of ortho and para isomers (predominantly planar and tilted orientations of LCs).23 However, in the previous studies so far, the surfactants used to decorate fluid-LC interfaces are limited to traditional surfactants. And due to the good solubility of LCs in most of the common nonaqueous solvents (such as ethanol, methanol and methylene chloride), there is still no specific report in regard to the effect of surfactant structure on the orientational behaviors of LCs at nonaqueous/LC interfaces. In the recent decade, with the increasing challenges of environmentally benign chemical processing, ionic liquids (ILs) have attracted considerable interest as functional isotropic liquid salts.24-27 Due to their specific physicochemical properties, e.g. negligible vapor pressure, high ionic conductivity, and convenient designability, ILs have become one family of the most promising candidates in the fields of


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separation techniques,28 carbohydrate chemistry,29 organic synthesis and catalysis,30 enzymatic reactions,31 energy environment25 and electrochemistry.32 In addition, due to negligible vapor pressure of ionic liquids (ILs), forming and keeping a long-time stabilized and even persistent molecular alignment of LC at the ILs/LC interfaces is expected. The inherent amphiphilic character of some ILs bearing long alkyl chain usually exhibits superior surface-active properties to traditional surfactants. Therefore, they have emerged as a class of novel surfactants and been named for surface active ionic liquids (SAILs).33-37 Besides, the structures of SAIL molecules with various functional groups can be designed and synthesized based on their potential applications in some chemical and biological events. Such researches will not only enrich the construction methods of LC-based biosensors, but also expand the application fields of ILs. To the best of our knowledge, the orientational behavior of 5CB involving SAILs and ILs has been scarcely examined. In our previous work, we firstly put forward a noninvasive approach to attain persistent and reversible photoresponsive alignment of LCs based on the stable self-assembled monolayer formed

by

a

photoswitchable

azobenzene-containing

surfactant,

viz.

4-ethylazobenzene-4'-(oxy-hexyl) trimethyl ammonium bromide (azoTAB) at the nitrate ethylamine (EAN)/LC interface.38 In view of the inherent ionic nature of ILs and the amphiphilicity of SAILs, this work aims to use diverse structural SAILs and ILs as modifiers and media, respectively, and to trigger the orientational transition of LCs at various fluid (including aqueous, IL and H 2 O-IL mixture)/LC interfaces. It provides a fundamental insight into the impact of SAILs’ chemical structures (e.g. length and number of aliphatic chain, and the nature of counterion) on the orientation of LCs at various fluid/LC interfaces.

2. Experimental section 2.1 Materials Gold specimen grids (75 mesh, pitch=340 μm, bar=55 μm, hole=285 μm) were bought from Beijing GILDER. 1-Methylimidazole (99%) was purchased from Acros


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Organics and distilled prior to use. Nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB), octadecyltrichlorosilane (OTS), 1-bromododecane (98%), 1-bromotetradecane (98%), 1-bromohexadecane (98%), sodium salicylate (NaSal) (99%), imidazole (99%), 1,2-dibromoethane (99%), 3-hydroxy-2-naphthoic acid (98%), trans-cinnamic acid (99%), trans-4-hydroxycinnamic acid (98%), and heptane were all the products of J&K Scientific Co., Ltd. of China. Sulfuric acid was bought from Beijing Chemical Works. Nitric acid (65-68 wt%) was obtained from Kangde Chemical Reagent Factory of China. HCl, NaOH, ethanol, methanol, chloroform, hydrogen peroxide (30% w/v) and ethylamine aqueous solution (65-70 wt%) were all obtained from Sinopharm Chemical Reagent Co., Ltd. Both n-propylamine (99%) and n-butylamine (≥99%) were purchased from Shanghai Aladdin Reagent Company of China. Dichloromethane, ethyl acetate, ethyl alcohol, and acetone were all obtained from Tianjin Fuyu Chemical Reagent Company of China. Acrylonitrile was purchased from Shandong Xiya Chemical Industry Co., Ltd. of China. Isopropanol was bought from Tianjin Kemiou Chemical Reagent Company of China. Triply distilled water was used to prepare aqueous solutions. 2.2 Synthesis of IM-SAILs and ILs The IM-SAILs and short-chained ionic liquid (ILs) were synthesized according to the procedures reported in the previous works.35-41 Their chemical structures were ascertained by 1H NMR spectroscopy with a Bruker Advance 400 spectrometer and demonstrated in Figure 1 (1H NMR details and synthetic procedures presented in the Supporting Information).


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Figure 1. Chemical structures of ionic liquids consisting of [C 12 mim]+ and several aromatic counterions (Sal-, HNC-, trans-PCA-, cis-PCA-, trans-CA- and cis-CA-) (a); [C n mim]Br (n=12, 14, 16) (b); [C 12 -2-C 12 im]Br 2 (c); EAN, PAN and BAN (d).

2.3 Treatment of glass microscope slides The OTS-coated glass slides were prepared on the basis of our previous reports.7-8 In brief, the glass microscope slides were first immersed in “piranha solution” for 30 min at 80 ℃. (“piranha solution” is 70% H 2 SO 4 /30% H 2 O 2; Caution: “piranha solution” reacts violently with organic materials and should be operated with extreme caution; do not store the solution in closed containers.) The slides were, in turn, rinsed thoroughly with water, ethanol, and methanol, and then dried under gaseous N 2 , followed by heating to 120 ℃ overnight prior to OTS deposition. The “piranha-cleaned” glass slides were immersed in the OTS/heptane solution for 30 min. Finally, they were rinsed with methylene chloride and dried under a stream of N 2 . 2.4 Preparation of the optical cells Gold specimen grids were first put onto the OTS-treated glass slides. Then, 5CB was heated to the isotropic phase (>35 ℃) and subsequently ≈1 μL heated 5CB was dispensed onto each grid. The superfluous 5CB confined in the grid was removed by contacting a 20 μL capillary tube. Then, 110 μL of aqueous or nonaqueous solution was added into optical cell at room temperature. All the results were repeated at least three times.


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2.5 Examination of polarized optical microscopy (POM) images The optical images of 5CB were acquired at room temperature on a polarized optical microscopy (XPF-800C, Tianxing, Shanghai, China), appending with a 2.5× objective lens and a digital camera (TK-9301EC, JVC, Japan). 2.6 Photoisomerization of trans-[C 12 mim]PCA and trans-[C 12 mim]CA For light-triggered trans-cis isomerization, the samples of the trans-[C 12 mim]PCA and trans-[C 12 mim]CA were irradiated with a CHF-XM35-500W ultrahigh-pressure short arc mercury lamp with optical filters (365 nm). The temperature was kept at 25 ℃ for avoiding overheating to use a thermostat water bath, and the distance between the aqueous solutions of trans-[C 12 mim]PCA and trans-[C 12 mim]CA with light source was fixed at 10 cm.

3. Results and Discussion 3.1 Effect of Aliphatic Chain Length and Number, and Counterion of IM-SAILs in Aqueous Solution on the Orientation of 5CB. In other studies elsewhere, traditional surfactants with various structures were demonstrated to introduce the orientational ordering transistion of the LCs at the aqueous/LC interface.9,22,23 In this study, several IM-SAILs (Figure 1) with different aliphatic chain length and number, and counterion were used to decorate the aqueous/LC and IL/LC interfaces, respectively. The LC-based sensor (shown in Figure S1) was developed by confining doped 5CB in a transmission electron microscopy (TEM) grid to prevent the dewetting of 5CB, and the grid was supported on an OTS-coated glass.9 The influence by IM-SAILs with various aliphatic chain length on the orientation of LCs at the aqueous/LC interface was firstly explored. [C n mim]Br (n=12, 14, 16) was dissolved in water and then added onto the TEM grids. As a result, a spontaneous bright-to-dark transition was observed in the optical appearance of LCs (Figure 2a) under the POM. According to the previous reports9,42, it has been well evidenced that bright-to-dark optical response of nematic liquid crystals is caused by the in-plane orientation to the homeotropic anchoring of LCs (Figure 2b). Figure 2a displays that a


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bright optical image appeared when [C n mim]Br (n=12, 14, 16) aqueous solutions were within a low concentration range, declaring a strong planar birefringence of 5CB at the aqueous/LC interface. As the concentration of these three IM-SAILs increased, the optical textures of LCs changed from bright to dark appearance. This can be attributed to the self-assembled monolayers (SAMs) formed by IM-SAILs at the aqueous/LC interface. As shown in Figure 2a, the threshold concentrations, at which bright images of LCs exactly transformed into dark ones at the aqueous/LC interface, were 0.01, 0.005 and 0.001 mM for [C 12 mim]Br, [C 14 mim]Br and [C 16 mim]Br, respectively. Such concentration-transition dependence indicates that the aliphatic chain length of IM-SAILs can remarkably impact the alignment of 5CB at the aqueous/LC interface, which is in consistence with the earlier report.22 And the possible reasons for this phenomenon can be ascribed to the following aspects. Firstly, the anchoring of 5CB largely depends on the areal density of surfactant molecules adsorbed at the aqueous/LC interface.9,22 Once surfactant tail at the interface exceeds a critical areal density, 5CB will vary from a planar orientation to vertical anchoring. Thus with the elongation of aliphatic chain length, [C n mim]Br will possess higher limiting areal density and lower threshold concentration. Secondly, stronger Van der Waals interaction between LC and IM-SAIL molecules with longer chains may be favorable for the penetration of 5CB into the self-assembled monolayers of IM-SAIL. Accordingly, this may expedite an extension of the alkyl chain of IM-SAIL and an anchoring of 5CB located closer to the aqueous/LC interface.22, 43


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Figure 2. Cross-polarized optical images of 5CB in the presence of [C n mim]Br (n=12, 14, 16) aqueous solutions with various concentration at the aqueous/LC interface (a); Illustration of the LC alignment from planar-to-homeotropic state at the aqueous/LC interface, owing to addition of [C n mim]Br solutions, which corresponds to transformation of the optical response of LCs from bright to dark appearance (b).

So far, it has been studied by several early reports9,22,23 that the structural characteristics of surfactant, such as headgroups, aliphatic chain length, tail organization and tail branching, have great influence on the orientations of LCs at the aqueous/LC interface. Yet whether some other structural factors, e.g. aliphatic chain number and diverse species of counterion of surfactants, can trigger an anchoring transition of 5CB has barely been discussed. Figure 3a shows the optical micrographs under POM of 5CB in the presence of gemini IM-SAIL ([C 12 -2-C 12 im]Br 2 ) and Figure 3b illustrates the LC alignment from planar-to-homeotropic state at the interface due to addition of [C 12 -2-C 12 im]Br 2 solutions. When the concentration of [C 12 -2-C 12 im]Br 2 was 0.0005 mM, which is


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much lower than the threshold concentration (0.01 mM) of single-chained [C 12 mim]Br, a uniform dark optical appearance corresponding to homeotropic anchoring of 5CB was immediately observed. This indicates that the number of aliphatic chains of IM-SAIL plays an important role in the response of 5CB. Specifically, in the chemical structure of [C 12 -2-C 12 im]Br 2 molecule, two cationic groups ([C 12 mim]+) are linked via a spacer group (-CH 2 CH 2 -) through covalent bond. This structure feature leads to the close connection of the two hydrophobic groups and the sequent enhancement on the intramolecular hydrophobic interaction. Moreover, the electrostatic repulsive interaction between [C 12 mim]+ cations was also weakened due to the existence of spacer group (-CH 2 CH 2 -)44.

Figure 3. Cross-polarized optical images of 5CB in contact with various concentration of gemini [C 12 -2-C 12 im]Br 2 aqueous solution (a); Illustration of the LC alignment from planar-to -homeotropic state at the interface due to addition of [C 12 -2-C 12 im]Br 2 solutions (b).

Next, four IM-SAILs with various aromatic counterions, viz. [C 12 mim]Sal, [C 12 mim]HNC, [C 12 mim]CA and [C 12 mim]PCA were designed and synthesized to decorate the aqueous/LC interface, respectively. The corresponding optical images of 5CB were observed and shown in Figure 4, which demonstrates that all of the investigated IM-SAILs aqueous solutions could induce 5CB to undergo orientational


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transitions from planar to perpendicular states. The concentration at which a distinct optical response of 5CB from bright to dark for [C 12 mim]Sal, [C 12 mim]HNC, [C 12 mim]CA and [C 12 mim]PCA is 0.008, 0.007, 0.009 and 0.009 mM, respectively. Obviously, the introduction of aromatic counterion makes the threshold concentration of IM-SAILs slightly lower than the conventional SAIL with bromide anion, [C 12 mim]Br, which corroborates that counterions species of IM-SAILs only slightly affect the anchoring of 5CB. The discrepancy in the orientational behavior of 5CB in contact with aqueous solutions of different IM-SAILs may be a consequence of an equilibrium of the following two competitive processes. On one hand, these larger aromatic counterions are more effective than Br- for the screening the intermolecular electrostatic repulsion among the polar headgroups, thus facilitating the adsorption of [C 12 mim]+ cations at the aqueous/LC interfaces. On the other hand, the bulky aromatic counterions could result in larger steric hindrance for the adsorption of surfactant molecules at the aqueous/LC interfaces. Herein, the slightly lower threshold concentration of IM-SAILs with aromatic counterion may imply a weak dominance of the former process.


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Figure 4. Cross-polarized optical images of 5CB in contact with various concentration of IM-SAILs with aromatic counterions aqueous solutions at the aqueous/LC interface .

It

is

noted

that

under

UV

irradiation,

both

cinnamic

(CA−) and para-hydroxy-cinnamic (PCA−) anions can undergo trans to cis photoinduced isomerization, in which the hydrophilicity deceases and the steric hindrance increases above,

we

for varieties of cinnamate derivatives.45,46 Based on the fact

continue

exploring

whether

the

trans-cis

isomerization

of

CA− and PCA− anions could alter the orientation of 5CB at aqueous/LC interface. To verify this assumption, [C 12 mim]CA or [C 12 mim]PCA aqueous solutions with different concentrations were respectively exposed to UV light for different time interval, and they were then added dropwise into the 5CB LCs in the TEM grids every 15 min. As shown in Figure 5a and 5b, 0.001 mM and 0.01 mM [C 12 mim]CA aqueous solutions caused bright and dark optical images respectively, and they were unchanged with the extension of exposure time to UV light. In addition, no obvious change was found for the optical images of LCs in contact with [C 12 mim]CA aqueous


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solution under 0.001 mM or above 0.01 mM upon UV light irradiation. Meanwhile, similar phenomena were observed for [C 12 mim]PCA aqueous solutions (Figure 5c and 5d). Therefore, these results may imply that the aromatic counterions of IM-SAILs could only slightly induce the arrangement of LCs at the aqueous/LC interface.

Figure 5. Cross-polarized optical images of 5CB in contact with 0.001 mM (a), 0.01 mM (b) [C 12 mim]CA aqueous solutions, and 0.001 mM (c), 0.01 mM (d) [C 12 mim]PCA aqueous solutions at the aqueous/LC interface under UV irradiation (0~60 min).

3.2 Effect of Aliphatic Chain Length and Number, and Counterion of IM-SAILs in short-chained ILs on the Orientation of 5CB. In our previous work, we firstly introduced a short-chained IL, nitrate ethylamine, (EAN), into fluid/LC interfaces, and subsequently developed one noninvasive approach to achieve both persistent and reversible alignment of LCs by forming a stable self-assembled monolayer (SAM).38 Our results revealed the potential application of the IL/LC interface for the monitoring some biochemical events. Here


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we established several other IL/LC interfaces to further explore the orientation of LCs at IL/LC interfaces in the presence of IM-SAILs solution. To probe the association between hydrophobic chain length of IM-SAILs and the anchoring of LCs at the IL/LC interface, we dissolved [C n mim]Br (n=12, 14, 16) in EAN to decorate the LC interfaces. Figure 6 showed the orientational behaviors of LCs coupled with the adsorption of different concentration of IM-SAILs at the EAN/LC interface. Optical appearance of LCs failed to change from bright to dark even though the concentration of [C 12 mim]Br in EAN was higher than its CMC value, 139 mM47. This suggests 5CB adopted a planar anchoring at the EAN/LC interface in the presence of [C 12 mim]Br. Whereas when LCs were incubated with 10 mM (

Effect of Imidazolium-Based Surface-Active Ionic Liquids on the

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