Breakdown of Lamellar ... - ACS Publications

Nov 5, 2015 - Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjong Malim, Perak, Malaysia. ∥. Equipe Surfaces et Interfaces, Centre Nation...
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Periodic Formation/Breakdown of Lamellar Aggregates with Anionic Cyanobiphenyl Surfactants Masanobu Sagisaka,*,† Yayoi Fujita,† Yusuke Nakanishi,† Hisayuki Takahashi,† Narumi Tsuyoshi,† Craig James,† Atsushi Yoshizawa,† Azmi Mohamed,‡,§ Frédéric Guittard,∥ and Julian Eastoe⊥ †

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan ‡ Department of Chemistry, Faculty of Science and Mathematics, and §Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjong Malim, Perak, Malaysia ∥ Equipe Surfaces et Interfaces, Centre National de la Recherche Scientifique (CNRS), Université Nice Sophia Antipolis, Parc Valrose, 06100 Nice, France ⊥ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom S Supporting Information *

ABSTRACT: This study reports unusual behavior of aqueous-phase lamellar aggregates with a new class of hybrid surfactant, CB-B2ES, having mesogenic units {6-[4-(4-cyanophenyl)phenyloxy]hexyl} and temperature-sensitive oxyethylated (butoxyethoxyethyl) tails. These tails are poorly miscible and likely to microsegregate if the surfactant molecules assemble. Lamellar aggregates appear at CB-B2ES concentrations higher than 5 wt % and were found to undergo repeat formation/breakdown periodically at 30 °C, with an average domain lifetime of ∼10 s. To investigate effects of the temperature-sensitive oxyethylene units on the hydrophilic/lipophilic balance (HLB) of the CB-B2ES bilayers, a fluorescence probe 1-pyrene-carboxaldehide was solubilized in the mixtures to sense the micro-environmental polarities. Fluorimetric measurements suggested that the polarity of CB-B2ES bilayers is very similar to that of the nonethoxylated CB-B2ES analogue at high temperatures (≥65 °C). However, for CB-B2ES, polarity increased with a decreasing temperature, in contrast with the small decrease in polarity observed for analogous non-ethoxylated bilayers. This is consistent with increased hydration of the oxyethylene units in CB-B2ES bilayers at low temperatures. The periodic formation/breakdown and cooling-induced hydrophilicity of the CB-B2ES lamellar aggregates did not appear in the non-hybrid and/or non-ethoxylated surfactant systems. Therefore, the combination of two unsymmetrical tails, one containing oxyethylene units and the other containing cyanobiphenyl terminal tips, must play an important role promoting this unusual behavior.

1. INTRODUCTION

can then be controlled by changes in the temperature around the phase transition. In contrast with solute release induced by a Lβ−Lα phase transition, if upon cooling, the rigidity and surfactant-packing density can be lowered drastically, new carriers capable of triggered release at only low temperatures could be developed (for example, capsules to release antifreezing agents for road surface freezing or to avoid frostbite at a low temperature). To obtain effective and easy control over the material release rate by the temperature, many papers have focused on thermosensitive polymers {e.g., block copolymers with poly(N-isoprpylacrylamide) or poly[2-(2-ethyloxy)ethoxyethyl vinyl ether]};7−11 these polymers are added to typical vesicle dispersions or used themselves as amphiphiles to form vesicles.

Since the late 1970s, many varieties of single- and double-tail surfactants have been investigated for mono- and bilayer formation,1,2 with an aim to mimic biological-like selfassembled superstructures. In the case of double-tail surfactants, the aggregate morphologies usually comprise of vesicles and lamellar bilayers.3 Similar structures have been reported for bilayer-forming single-tail surfactants.4−6 Many studies of vesicles have aimed at controlling the release rate of encapsulated materials (drug or active ingredients) to develop drug delivery systems (DDSs).7−12 One method to control material release is to trigger a lamellar gel (Lβ) to melted lamellar (Lα) phase transition in liquid crystalline (LC) systems.13−16 At the transition temperature, the rigidity and surfactant-packing density of the bilayers drastically decreases and the encapsulated and solubilized compounds tend to be released from the vesicle interiors.12 The material release rate © XXXX American Chemical Society

Received: September 23, 2015 Revised: November 5, 2015

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Langmuir Some of these studies have demonstrated controllable drugrelease rates by changing the temperature below the block copolymer lower critical solution temperature (LCST).7−11 Earlier studies17−19 synthesized fluorinated double-tail surfactants with oxyethylene spacers and a sulfosuccinate group and examined lyotropic LC behavior with water. Some of the single surfactant solutions spontaneously form multilamellar vesicles (MLVs) at room temperature, even without the use of a specific preparation protocol. This is an interesting observation, because vesicle formation with common surfactants or lipids usually requires external input, such as ultrasonic irradiation, addition of a co-surfactant (or co-solvent), or heating.17−19 It was concluded that the oxyethylene spacers between the fluorocarbon chains and the sulfosuccinate groups allow for the more rigid fluorocarbon chains to orientate favorably in the curved vesicle bilayers, enhancing the vesicle stability. These results imply that aggregate morphology of double-tail surfactants can be controlled by appropriately modifying the surfactant chain chemical types and structures. With the aim of exploring new functions, such as a quasi-ion channel and cooling-induced material release, a new anionic hybrid surfactant (8F-B2ES) was designed and synthesized having 1H,1H,2H,2H-perfluorodecyl (8F) and 2-[2-(butyloxy)ethyloxy]ethyl (B2E) tails (Figure 1, where X1 = 8F and X2 =

Figure 2. Schematic models for bilayers of the hybrid surfactants having ethoxylated chains (8F-B2ES and CB-B2ES) in water at high and low temperatures. X1 and X2 represent non-ethoxylated and ethoxylated tails, respectively.

Most ionic fluorocarbon−hydrocarbon hybrid surfactants reported previously employed stimuli-insensitive groups (e.g., n-alkyl chains) in the hybrid tails.22−38 In the literature, only two ionic hybrid surfactants can be found with stimulussensitive groups; one of these has redox-responsive ferrocene groups at the n-alkyl tail termini.39 The other, from an earlier study of 8F-B2ES, led to new molecular designs able to generate temperature-sensitive interfacial properties and liquid crystal behavior, owing to the presence of oxyethylene units. This class of thermosensitive hybrid surfactants could broaden the applications of lamellar aggregates. Similar to the use of fluorocarbon chains, a thermotropic mesogen can also help promote lyotropic LC formation and cause microsegregation from saturated hydrocarbon chains.40 In an earlier study, two surfactants sodium 1,2-bis{6-[4-(4cyanophenyl)phenyloxy]hexyloxycarbonyl}ethanesulfonate (SBCPHS; Figure 1) and sodium 1,2-bis(tetradecyloxycarbonyl)ethanesulfonate (SBTDS; Figure 1) were synthesized to examine the effects of cyanobiphenyl (CB) terminal groups on the aqueous LC phase behavior.41,42 Lamellar and columnar phases were formed in aqueous SBCPHS mixtures, and the temperature stability range for SBCPHS liquid crystals was wider than that for SBTDS. This clearly shows that LC phases can be stabilized by intermolecular interactions between the cyanobiphenyl terminal groups, when in the antiparallel arrangement of the SBCPHS molecules. As mentioned above, the introduction of oxyethylene, fluorocarbon, and/or mesogens in the hybrid tails, could be used as approaches for stabilizing curved bilayers to generate new LC morphologies and aggregate nanostructures. In this study, a new hybrid surfactant CB-B2ES with 6-[4-(4cyanophenyl)phenyloxy]hexyl (CB) and butoxyethoxyethyl (B2E) tails (Figure 1) has been synthesized and characterized. Because the two hybrid tails of CB-B2ES are incompatible and almost immiscible,40 CB-B2ES molecules are expected to form bilayers generating quasi-ion channels in water and display new LC morphologies, such as found with 8F-B2ES. This study has examined liquid crystal behavior and hydrophilic/lipophilic balance (HLB) of lamellar aggregates with CB-B2ES and its analogues, as functions of the temperature and composition. Furthermore, the generation of quasi-ion channels was investigated.

Figure 1. Molecular structures of four hybrid surfactants CB-8FS, CBB2ES, 8F-B2ES, and 8F-DeS and two symmetric double-tail surfactants 8FS(EO)2 and SBCPHS. All hybrid surfactants have two isomers with different C−S bond locations.

B2E).20 This hybrid chain surfactant forms lamellar bilayers rich in X2 microdomains as a result of poor affinity with the fluorocarbon X1 moieties.21 The microdomains are expected to act as quasi-ion channels by changing the degree of oxyethylene hydration, which can be tuned by the temperature (as shown in Figure 2). As predicted, the 8F-B2ES lamellar aggregates gave hydrophobic bilayers at higher temperatures and then also hydrophilic layers at lower temperatures. Interestingly, upon cooling 8F-B2ES bilayers, the interiors became hydrophilic, similar to typical spherical micelles and the lamellar aggregates started to distort. The lamellar aggregates of an 8F-B2ES analogue bearing no oxyethylene units did not display this switchable LC behavior, and therefore, it appears that hydrated oxyethylene microdomains within the 8F-B2ES bilayers could trigger distortion of lamellae. B

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2. EXPERIMENTAL SECTION 2.1. Materials. Three anionic hybrid surfactants and one anionic symmetric double-tail surfactant were used. The hybrid surfactants were CB-B2ES (a mixture of two isomers: sodium 1-{6-[4-(4cyanophenyl)phenyloxy]hexyloxycarbonyl}-2-{2-[2-(buthyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2(buthyloxy)ethyloxy]ethyloxycarbonyl}-2-{6-[4-(4-cyanophenyl)phenyloxy]hexyloxycarbonyl}ethanesulfonate), CB-8FS (a mixture of two isomers: sodium 1-{6-[4-(4-cyanophenyl)phenyloxy]hexyloxycarbonyl}-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate and 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2{6-[4-(4-cyanophenyl)phenyloxy]hexyloxycarbonyl}ethanesulfonate), and 8F-B2ES (a mixture of two isomers: sodium 1-(1H,1H,2H,2Hper fluorodecy loxyc arbonyl)-2-{2 -[2-(butylox y)ethylo xy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H,1H,2H,2Hperfluorodecyloxycarbonyl)ethanesulfonate). These hybrid surfactants were synthesized and purified as described in SI-1 of the Supporting Information and a previous report.20 The symmetric double-tail surfactants SBCPHS (sodium 1,2-bis{6-[4-(4-cyanophenyl)phenyloxy]hexyloxycarbonyl}ethanesulfonate) and 8FS(EO)2 (sodium 1,2-bis{1H,1H,2H,2H-perfluorodecyloxycarbonyl}ethanesulfonate) were synthesized in a previous study.17−19,41,42 The structures of the final products were elucidated by infrared (IR) spectroscopy (FTS-30, Bio-Rad Laboratories, Berkeley, CA) and proton nuclear magnetic resonance (1H NMR) spectroscopy (JNM-GX270, JEOL, Tokyo, Japan). The purity of the final compounds was confirmed by elemental analysis (EA 1110, CE Instruments, Ltd., Wigan, U.K.) and normalphase high-performance liquid chromatography (HPLC, SIL 150A-5 column, Intersil) with an ultraviolet (UV) detector (λ = 254 nm). Chloroform was used as the eluent in the HPLC analysis. Ultrapure water with a resistivity of 18.2 MΩ cm was obtained from a Millipore Milli-Q Plus system (Millipore, Billerica, MA) and used for the experiments. 2.2. LC Properties. The initial phase assignments for surfactant/ water mixtures were determined by polarizing microscopy (BX-51, Olympus) equipped with a thermal stage (TS62, INSTEC) and a temperature controller (STC200, INSTEC). To avoid evaporation of water from the mixtures, a polymer gel was used to seal a cover glass onto the glass slide on which the mixture was placed. The heating and cooling rates were both 2 °C min−1. To examine changes in micro-environmental polarity in the hydrophobic region of the liquid crystals, fluorescence measurements were conducted spectrometerically (RF-5300, Shimadzu, Kyoto, Japan), with pyrene 1-carboxaldehyde (PyCHO) as the probe. The micro-environmental polarities around the probe molecules were found from the wavelength of maximum fluorescence, λmax, for PyCHO.17,43 Mixtures at 5 wt % surfactant/water with 4 μM PyCHO were prepared, and then used for measuring fluorescence spectra between 370 and 450 nm at an excitation wavelength of 366 nm. Measurements were also made on 5 wt % surfactant/water mixtures without PyCHO to determine the background.

Figure 3. Polarizing optical micrographs of a 30 wt % CB-8FS/water mixture at 30 °C.

Figure 4. Phase diagrams of 1, 5, 10, and 30 wt % surfactant/water mixtures as a function of the temperature. The symbols M, L, and SQ mean optically isotropic micellar phase, lamellar phase, and sponge (or cubic) phase, respectively.

mixtures over concentrations of 10−30 wt % and temperatures higher than 40 °C; these may be sponge or bicontinuous cubic phases composed of bilayers. From the microscopic observations, phase diagrams for the surfactants [except 8FS(EO)2] were prepared as functions of the temperature and concentration. In the case of 8FS(EO)2, the Krafft temperature (TK) was raised to 73 °C as a result of the very hydrophobic double perfluorooctyl tails and the lamella LC formed at above TK or by ultrasonication for longer than 15 min at room temperature.17−19 In comparison between these phase diagrams in Figure 4, lamella LCs tend to be stabilized against dilution by increasing the number of cyanobiphenyl mesogens in the molecules (NCB), which started to appear from 10 wt % for 8FB2ES (NCB = 0), 5 wt % for CB-B2ES (NCB = 1), and 1 wt % for SBCPHS (NCB = 2). The stabilization effect of NCB on lamellar LCs arises from the intermolecular cyanobiphenyl− cyanobiphenyl interactions in an antiparallel arrangement,41,42

3. RESULTS AND DISCUSSION 3.1. Unique LC Behavior of the Ethoxylated Hybrid Surfactant CB-B2ES in Water. Polarizing microscope observations were carried out for the aqueous surfactant mixtures at 20−80 °C. Figure 3 shows the optical textures of CB-8FS/water mixtures at 30 wt % at 25 °C. A cross-nicol observation found maltase cross and oily streak textures in the CB-8FS/water mixture, suggesting the formation of lamellar liquid crystals.20,44,45 These textures often appeared at the other concentrations of CB-8FS as well as with other surfactant systems, as denoted by the symbol “L” in Figure 4. Figure 4 shows phase diagrams of 1−30 wt % surfactant/water mixtures as a function of the temperature. As previously reported,20 optically isotropic systems were observed in 8F-B2ES/water C

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(1) a higher hydrophobicity of the CB-8FS bilayer center as a result of a closer packing of surfactant molecules preventing water penetration13−16 and/or (2) enhanced partitioning of PyCHO molecules from bulk water into the layer centers. In contrast with the trend for CB-8FS, λmax for CB-B2ES gradually increased with a decreasing temperature, reaching 425−427 nm below 25 °C. The λmax values of 425−427 nm were still lower than in ethanol (>443 nm), suggesting that most PyCHO molecules remain in the bilayers. Elevated λmax upon cooling for CB-B2ES suggests the bilayer interior to be less hydrophobic. These links between polarity and temperature for CB-8FS and CB-B2ES are very similar to those for 8FS-DeS and 8F−B2ES, respectively. Namely, the pyrene-sensed micro-environmental polarity (I1/I3) for 8FS-DeS was almost independent of the temperature, but for 8F-B2ES, it increased with a decreasing temperature.20 The difference in the polarity−temperature trend is clear between non-ethoxylated (8F-DeS and CB-8FS) and ethoxylated (8F-B2ES and CB-B2ES) hybrid surfactants. The observed I1/I3 and λmax behaviors, seen with the ethoxylated hybrid surfactant lamellar LCs, do not appear for the ionic surfactants in the absence of oxyethylene units.20 These behaviors appear to be typical of ethylene oxide (EO)containing non-ionic surfactants (e.g., Pluronic surfactants P65, P85, and P105);46 hence, it could be due to enhanced hydration of the oxyethylene units at lower temperatures. This implies that oxyethylene domains generate pseudo-ion channels within the ethoxylated hybrid surfactant bilayers at low temperatures, as depicted in Figure 2. 3.3. Periodic Formation/Breakdown Behavior of CBB2ES Lamellar Aggregates in Water. Typical lamellar LCs are usually stable and do not show any morphology changes, over at least several hours, without external forces or changes in experimental conditions.1−6,47 However, CB-B2ES lamellar LCs were found to exhibit rather intriguing spontaneous behavior over time. Figure 6 shows changes in lamellar LC maltase cross textures in the 30 wt % CB-B2ES/water mixture at 30 °C as a function of elapsed time, observed under a polarizing microscope (see details of changes in textures in the movie at http://www.st.hirosaki-u.ac.jp/~lclab/sagisaka/gyouseki/ movie.html). Interestingly, these textures were found to repeatedly disappear and reappear, suggesting periodic formation/breakdown of the lamellar LCs. These micrographs suggested a typical lifetime for lamellar domains to be ∼10 s. For example, the maltase cross texture (see white arrow) appeared about 1 s after commencing observations but disappeared at around 14 s. Because this periodical formation/breakdown behavior was not found in the other aqueous lamellar LC mixtures of CB-8FS, 8F-B2ES, and 4FS(EO)4, it could be driven by incompatibility of the hybrid tail moieties, being oxyethylene units and cyanobiphenyl terminal groups. In previous work, the hybrid surfactant, 8F-B2ES, having a fluorocarbon tail and a hydrocarbon tail containing oxyethylene units, also displayed unique LC behavior. For example, clear lamellar LC textures of 8F-B2ES at temperatures above 40 °C became notably distorted upon cooling to 40 °C.20 Distortion of the 8F-B2ES lamellar aggregates was suggested to result from changes in hydration of oxyethylene domains upon cooling, as shown in Figure 2, which can reduce molecular packing in the 8F-B2ES bilayers. The hydration of oxyethylene units in CBB2ES bilayers could also be promoted with a decreasing temperature and trigger the unique formation/breakdown behavior of the lamellar domains reported above.

which will be one of driving forces for aggregation in bilayers, as seen in earlier papers. 3.2. Low-Temperature-Induced Micro-environmental Polarity in the Ethoxylated Hybrid Surfactant Bilayers. To examine the hydration of the oxyethylene units in the surfactant bilayers, as shown in Figure 2, PyCHO was employed as a fluorescence micro-environmental polarity probe for the bilayer interiors with CB-B2ES and CB-8FS. The wavelength at maximum fluorescence of PyCHO (λmax) is known to be longer when the PyCHO molecules are located in a higher polarity environment.17,43 To calibrate changes in λmax of pure PyCHO as a function medium polarity, the spectra ware measured in various solvents (water, methanol, and ethanol; see Figure SI-1 of the Supporting Information), showing that λmax decreases with solvent polarity (λmax of 466− 471 nm in water, >448−453 nm in methanol, and >443−447 nm in ethanol) but was almost independent of the temperature over the range studied. λmax of PyCHO in the 5 wt % surfactant/water mixtures was measured next as a function of the temperature, and the results were shown in Figure 5.

Figure 5. Changes in fluorescence peak maximum wavelength (λmax) of PyCHO in water containing 5 wt % CB-8FS and CB-B2ES as a function of the temperature. As a comparison, temperature dependencies of I1/I3 for pyrene solubilized in 10 wt % 8F-B2ES and 8F-DeS/ water mixtures from a previous study20 are also shown.

Another measure of polarity uses pyrene as a probe and determines the I1/I3 value, the ratio of first (373 nm) to third (384 nm) vibronic peak intensities in the fluorescence spectra. This ratio is also affected by the micro-environmental polarities: the lower the I1/I3, the lower the polarity.20,29 In addition to λmax for PyCHO, I1/I3 values of pyrene solubilized in 10 wt % 8F-B2ES and 8F-DeS lamellar phases measured in a previous study20 are also plotted to highlight the effect of oxyethylene units. At high temperatures of ≥65 °C, both CB-8FS and CB-B2ES exhibited significantly lower λmax values of ∼410 nm compared to those in typical polar solvents (water, methanol, and ethanol) (Figure SI-1 of the Supporting Information), suggesting that most of the PyCHO molecules are located not in bulk water but rather in the less polar environment (hydrophobic center) of the bilayers. In the case of CB-8FS, λmax slightly decreased upon lowering the temperature and finally settled at 407 nm at 5 °C. This is probably due to one or both of the following two reasons: a lower temperature induces D

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Figure 6. Periodic appearance/disappearance of maltese cross textures for the 30 wt % CB-B2ES/water mixture observed by polarizing optical microscopy.

cycle: the gauche form exists predominantly for a period of ∼10 s, and then, after that, the fraction of the trans form increases up to the composition where lamellar aggregates begin to break down. Then, after an interval of several seconds, the concentration of the gauche form recovers to reform lamellar aggregates. In terms of condition 1, the temperature of 30 °C where periodic behavior was found could be critical to promote reversible gauche/trans conformational changes. As seen in Figure 6, this temperature results in a mid-range λmax value (∼420 nm), meaning a balanced HLB of the ethoxylated tail, as compared to 410 nm at T > 65 °C (the highest hydrophobicity) and 427 nm at T < 25 °C (the highest hydrophilicity). This critical temperature could minimize the free energy difference of ethoxylated tails located in bulk water (trans form) and in the bilayer centers (gauche form) and, hence, allow for reversible conformation changes. If conformation changes are key for driving periodic phase behavior, the reason why the other ethoxylated surfactant 8F-B2ES did not show phase fluctuations is explainable. The strong hydrophobicity and oleophobicity of fluorocarbon tails in 8F-

Why does the periodical formation/breakdown behavior of CB-B2ES lamellar aggregates happen? In general, a lamellar LC is stabilized by a suitable HLB and critical packing parameter (CPP).47 From this viewpoint, such periodical behavior would be possible only if there are changes in the micrometer-scale HLB and/or CPP over typical intervals of ∼10 s. In the case of CB-B2ES, the HLB of the ethoxylated tails and the degree of hydration can be tuned by the temperature. At a low temperature, enhanced oxyethylene hydration is likely to favor the ethoxylated tail orienting toward water (trans form), away from the bilayer center (gauche form), as shown in Figure 7. A typical single-headed di-chain surfactant CB-B2ES will have CPP ∼ 1, and this change in hydration should give rise to a lower CPP, to be more like a double-headed, single-chain surfactant, hence, favoring micelle formation more than lamellar LCs.47 Then, the periodic behavior seen for 8F-B2ES lamellar LCs could arise, given that (1) gauche/trans conformation changes happen frequently, as shown in Figure 7, and (2) the molar ratio [gauche]/[trans] on a micrometer scale switches on a time scale of ∼10 s. The oscillating behavior could be due to a E

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phobic groups will be a good way to stabilize vesicles or any other bilayer-based liquid crystals, which are usually thermodynamically unstable or rarely formed.47 An increased micro-environmental polarity with lowering the temperature was shown based on fluorescence peak shifts of PyCHO solubilized in the lamellar aggregates. The fluorescence peak maximum wavelength was almost independent of the temperature for the non-ethoxylated CB-B2ES analogue surfactant lamellar LCs, whereas (2) it was red-shifted upon cooling for the CB-B2ES lamellar LCs, consistent with an increasing micro-environmental polarity of the bilayer interiors. At 30 °C, where the micro-environmental polarity became intermediate between the highest (at ≥65 °C) and lowest (at ≤15 °C) values, (3) CB-B2ES lamellar aggregates showed unique behavior of spontaneous, periodic formation/breakdown with domain lifetimes of ∼10 s. Such unusual temporal formation/breakdown of lamellar aggregates has not been observed before with the related non-ethoxylated hybrid surfactants (CB-8FS and 8F-DeS) and the ethoxylated CBB2ES analogue surfactant (8F-B2ES). It appears that the hybrid structure of the surfactant CB-B2ES, especially owing to cyanobiphenyl terminal tips and oxyethylene units, is the origin of this oscillatory phase behavior. The reason why the periodic behavior happens may be due to temporal changes in heterogeneity of CB-B2ES chain conformations, namely, gauche and trans forms with different CPPs appropriate for micelles and lamellar LCs, respectively. For typical binary mixtures of non-ethoxylated ionic surfactants and water, increased micro-environmental polarity with lowering the temperature is unusual given that lamellar aggregates are stable over a wide temperature range. In addition, the periodic formation/breakdown and coolinginduced distortion of lamellar aggregates is quite rare in the field of lyotropic LCs. From these standpoints, this new surfactant class, exemplified by CB-B2ES or 8F-B2ES, both with ethoxylated tails as one of the two distinct chains, lead to interesting phenomena, suggesting the formation of temperature-responsive aqueous nanopores and softening of those bilayers upon cooling. In the case of fluorocarbon−hydrocarbon hybrid surfactants with temperature-sensitive ferrocene groups,39 oxidation as well as an increase in the temperature was found to induce interesting changes in interfacial properties (critical micelle concentration and aqueous surface tension) and aggregate morphology transitions (coil-like to vesicle-like aggregates or vesicle-like to unaggregated). Further studies should synthesize novel hybrid surfactants having different numbers of EO units and examine changes in interfacial properties, nanostructures, and material release characteristics as a function of the temperature. To obtain liquid crystals with unusual morphologies/nanostructures, siloxane and hyper-branched alkyl chains would also be interesting groups to consider, because they also have low miscibility with ethoxylated tails.40 These will advance the molecular design of hybrid surfactants to achieve quasi-ion channels, specifically for applications as drug delivery systems (for example, to avoid frostbite and capsules for antifreeze agents). In addition, if dynamic changes in lyotropic liquid crystal morphology (or dissipative structures) can be controlled by manipulating molecular conformational changes, novel applications of lyotropic LC systems may emerge in the future.

Figure 7. Gauche−trans conformational changes for the CB-B2ES molecule (e.g., the isomer sodium 1-{6-[4-(4-cyanophenyl)phenyloxy]hexyloxycarbonyl}-2-{2-[2-(buthyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate) inducing periodical formation/ breakdown of lamellar aggregates.

B2ES destabilizes the gauche form to position the hydrated ethoxylated tail closer and does not permit gauche/trans conformational changes in the low-temperature regions of greater EO hydration. If the formation/breakdown mechanism mentioned above is true, another question arises: “why does the dominant conformational change happen periodically over micrometer scales?”. Generally, periodical changes in the ratio [gauche]/ [trans] in typical molecules will not occur over micrometer scales (one lamellar aggregate) and several seconds, even if it may happen nanometrically (e.g., one micelle) and over a much shorter time window (e.g., nanoseconds). One possibility to give rise to this periodical change is through corporative transitions between the layer-by-layer-type microsegregation and the intralayer-type configuration. A schematic representation of the transition and an extended explanation are displayed in SI-3 of the Supporting Information.

4. CONCLUSION A previous study20 aimed to generate quasi-ion channels within bilayers and introduced the hybrid surfactant 8F-B2ES, having both a C8 fluorocarbon tail and an ethoxylated hydrocarbon tail. The micro-environmental polarity of 8F-B2ES bilayers was investigated and found to increase with a decreasing temperature, being highly hydrophilic at temperatures of ≤40 °C. Distortion of the lamellar aggregates also happened upon cooling. These phenomena suggested the formation of aqueous nanopores with oxyethylene units, namely, quasi-ion channels. This study introduced the new 8F-B2ES analogue hybrid surfactant CB-B2ES having hydrocarbon tails with cyanobiphenyl mesogens as terminal tips and examined liquid crystal behavior of the CB-B2ES/water mixtures as functions of the temperature, concentration, and time; three new findings (1− 3) are revealed. Lamellar aggregates in water appeared at concentrations of >5 wt % for CB-B2ES and >10 wt % for 8F-B2ES, and (1) the difference in concentrations where aggregates start to appear suggests that cyanobiphenyl terminal tips stabilize bilayer structures at lower concentrations than with C8 fluorocarbon tails. Employing layer-oriented thermotropic mesogens (e.g., cyanobiphenyl or phenyl pyrimidine) as part of the hydroF

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03564. Surfactant synthesis for two hybrid surfactants CB-B2ES and CB-8FS and characterization by 1H NMR, Fourier transform infrared (FTIR) spectroscopy, and elemental analysis, micro-environmental polarity of PyCHO in various solvents without any surfactant, and cooperative and periodical conformation change by transition between layer-by-layer and intralayer microsegregations of CB-B2E lamellar aggregates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +81-172-39-3579. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Japan Society for the Promotion of Science (JSPS) [KAKENHI, Grant-in-Aid for Young Scientists (A), 23685034], and leading research organizations [Research Councils of the United Kingdom (RCUK, through EPSRC EP/I018301/1) and Agence Nationale de la Recherche (ANR, 13-G8ME-0003)] under the G8 Research Councils Initiative for Multilateral Research Funding, G8-2012. Craig James thanks the JSPS for an 18 month fellowship (JSPS Postdoctoral Fellowship for Foreign Researchers) and the Engineering and Physical Sciences Research Council (EPSRC, Grants EP/I018301 and EP/ I018212/1).

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