Lyotropic Lamellar Structures of a Long-Chain Imidazolium and Their

Article pubs.acs.org/JPCB

Lyotropic Lamellar Structures of a Long-Chain Imidazolium and Their Application as Nanoreactors for X‑ray-Initiated Polymerization Cesar L. Usma, Isabel E. Pacios, and Carmen S. Renamayor* Departamento de Ciencias y Técnicas Fisicoquímicas, Facultad de Ciencias, UNED, Paseo Senda del Rey, 9, 28040 Madrid, Spain ABSTRACT: The lyotropic behavior of the ternary system formed by 1-tetradecyl-3-methylimidazolium chloride, 1decanol, and water is investigated. A lamellar mesophase is formed for a wide range of compositions and is characterized by polarized optical microscopy, low-temperature scanning electron microscopy, small- and wide-angle X-ray scattering with synchrotron radiation, and differential scanning calorimetry. This phase presents onionlike structures. Two lamellar structures are formed: an Lα mesophase between 25 and 50 °C, with an isobaric thermal expansivity of the bilayer thickness of −3.2 × 10−3 K−1, and a lamellar gel phase, when the temperature decreases below 25 °C. This new medium is employed to perform in situ X-ray-initiated polymerization of N-isopropylacrylamide. When the monomer is incorporated in the lamellar structure, it is distributed between the water layer and bilayer interface and its polymerization can be followed by variations in the diffractograms with time.

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INTRODUCTION Ionic liquids (ILs) are organic salts with a low melting point and extraordinary physicochemical properties, for example, high ionic conductivity, nonvolatility, nonflammability, and high stability. A huge number of combinations between cations and anions are possible (1018 ILs are theoretically achievable).1 In fact, ILs are often called “designer solvents” because they have a tailorable nature, which results in tunable properties by selecting an appropriate cation−anion pair. Additionally, they have been proposed as green chemicals. Therefore, they can be employed in several fields, for example, as environmentally friendly solvents in chemical reactions,2 as electrolytes in batteries,3 in energy production,4 in pharmaceuticals,5 and as lubricants.6 Among the different ILs, 1-alkyl-3-methylimidazoliums ([Cnmim]+, where n refers to the number of carbon atoms in the alkyl chain) are extensively investigated for their potential use in several applications. As an example, they have been recently used to generate highly recyclable and easy handling catalytic heterogeneous systems.7−10 No aggregates are formed when n = 2−4,11 but long alkyl [Cnmim]+ (n > 6) possess an inherent amphiphilic nature and combine the unique characteristics of ILs with surfactant properties. Krafft temperatures, critical micelle concentrations, and aggregation numbers for [Cnmim]+ are smaller than their values characteristic for conventional cationic surfactants, such as alkyltrimethylammoniums, having the same alkyl chain length.12−14 These surfactant ILs are considered promising molecules in a wide range of applications, such as15 dispersing agents for carbon nanotubes, catalytic processes, corrosion inhibition, gene and drug delivery agents, templates for inorganic materials, and © 2017 American Chemical Society

antimicrobial and antifungal agents. They form micelles in dilute water solutions,16 and their aggregation in nonaqueous solvents has also been reported.17,18 Furthermore, [Cnmim]+ with n ≥ 8 form lyotropic mesophases at high surfactant concentrations11,19 and longer-chained [Cnmim]+ (n ≥ 12) display thermotropic liquid-crystalline behavior as a result of the microphase separation of ionic imidazolium groups from the flexible lipophilic alkyl chains.20−22 The incorporation of oils23 and long-chained alcohols24,25 has also been investigated, analyzing the lyotropic behavior in the presence of a third component. 1-Tetradecyl-3-methylimidazolium chloride (TMIC) has been used as a template to prepare monolithic supermicroporous silica with a lamellar order by nanocasting.26 The micellar aggregation and micellization thermodynamics in aqueous solutions have also been extensively investigated.16,27−30 These micelles have been explored as drug carriers because drugs such as lidocaine hydrochloride can get adsorbed on the surface of the TMIC micelles, helping in the drug delivery processes.31 The lyotropic behavior of TMIC in a room-temperature IL (ethylammonium nitrate) has also been investigated. Depending on the composition, hexagonal, lamellar, and reverse bicontinuous cubic phases are obtained. The hexagonal mesophase is used to disperse multiwalled carbon nanotubes, with the aim of obtaining composites with high electrical conductivity.32 Received: December 1, 2016 Revised: February 20, 2017 Published: February 27, 2017 2502

DOI: 10.1021/acs.jpcb.6b12101 J. Phys. Chem. B 2017, 121, 2502−2510

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The Journal of Physical Chemistry B

Figure 1. Micrographs of the sample W72R2.0. Left and center: optical microscopy without and with crossed polarizers, respectively (bar = 100 μm). Right: CryoSEM (bar = 10 μm).

1-decanol/water/NIPA system, three sets of samples were prepared; two of them with R = 2.0: one with a fixed surfactant content (TMIC + 1-decanol), WIL+OH = 27.5 wt %, a variable NIPA content, and WNIPA in the range of 1−5 wt % and the other with 4 wt % NIPA, a variable water content, and WH2O in the range of 49−78 wt %. The third set corresponds to samples containing 4 wt % NIPA, 68.3 wt % of water, and a variable decanol/TMIC molar ratio (R = 1−2.3). In situ polymerization was performed on one sample with R = 2.0, 3 wt % NIPA, and 50 wt % water. A sample with a similar composition but containing PNIPA instead of the monomer was also prepared. Microscopy. A Nikon Labophot-2 microscope, with and without crossed polarizers, was employed. The samples were placed between a glass slide and cover slip. DSC. The DSC equipment was a Mettler Toledo calorimeter provided with a DSC822 oven and a subambient cooling unit. Two runs at 5 °C min−1 were performed, in a nitrogen atmosphere, using an empty aluminum cell as the reference. The first and second runs were scanned in the ranges 30−5 and 5−50 °C, respectively. Results from the second run are used for calculations. Low-Temperature Scanning Electron Microscopy (CryoSEM). Freeze fracture scanning electron microscopy was performed to visualize the morphology of selected samples. A portion of sample was mounted on the sample holder and immersed in nitrogen slush for freezing. The samples were then transferred to an Oxford CT1500 cryotransfer chamber station unit, in which they were fracturing, etched at −89.6 °C for 20− 30 min and coated with Au for 1.5 min. Thereafter, the freezefractured samples were transferred into a JEOL JSM-5410 SEM, in which the images were recorded at magnifications of up to 15 000×. X-ray Scattering. The experiments were performed at the ALBA Synchrotron radiation facility on beamline BL11-NCD. The samples were loaded in 1 mm ID capillaries and thermostated with a sample-stage Linkam HFSX350-CAP. The samples were irradiated with a wavelength (λ) of 1.00 Å. For SAXS experiments, the sample−detector distance was 2571 mm and the two-dimensional (2D) images were recorded using a pixel detector, imXPAD-s70, with a resolution of 1200 × 1120 pixels and a pixel size of 127 μm. For WAXS experiments, the sample−detector distance was 141 mm and the 2D images were recorded with a Rayonix LX255-HS detector of 1920 × 5760 pixels and a pixel size of 44 μm. The exposure time per frame was 1 s, and the experiments were performed at 15, 20, 25, 30, 35, and 40 °C. Two-dimensional images were integrated to obtain the scattered intensity (I) as a function of the modulus of the scattering vector, q = (4π/λ) sin θ, where 2θ is the

The lyotropic organizations can be used as nanoreactors for polymerization reactions, where characteristics such as molecular weight33 or polymerization kinetics34 can be modified by the structure of the mesophase. On the other hand, template polymerization, in the presence of a cross-linker, can produce nanoparticles, capsules,35 or hydrogels with porosities that can be tuned by the structure of the mesophase.36 In the initial part of this work, the lamellar mesophase of the ternary TMIC/decanol/water system is studied in detail. In the ternary system, decanol is a cosurfactant that modifies the hydrophilic−lipophilic balance, helping to stabilize the lamellar structure over a large region in the phase diagram.37 The effect of temperature and composition on the structure and stability of the lamellar mesophase is analyzed by combining polarized optical microscopy, low-temperature scanning electron microscopy (CryoSEM), small- and wide-angle X-ray scattering with synchrotron radiation (SAXS and WAXS), and differential scanning calorimetry (DSC), with the aim of obtaining a system suitable for the confinement of functional molecules. In the second part of the work, these new lamellar aggregates are employed as nanoreactors to polymerize N-isopropylacrylamide (NIPA). Poly(N-isopropylacrylamide) (PNIPA) belongs to a family of thermoresponsive water-soluble polymers with a strong projection in the production of new smart materials.38 Here, the monomer (NIPA) is confined and its influence on the lamellar structure is analyzed. Finally, polymerization is induced by X-rays in the absence of a chemical initiator. Additionally, the high photon flux of the synchrotron radiation provides a method to monitor the in situ polymerization, via the changes in the SAXS diffractograms, from the initial mixture to the postpolymerized state.36,39

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EXPERIMENTAL SECTION Chemicals. The IL TMIC (>98%) was obtained from IoLiTec. 1-Decanol (99%); PNIPA, with a viscous average molecular weight of 1.5 × 104; and NIPA (>97%) were from Aldrich. Deionized water (Milli-Q) was used as a solvent. Sample Preparation. Samples of the TMIC/1-decanol/ water system were prepared by weighing precise amounts of the three components and centrifuging them back and forth several times until complete mixing. They were subsequently allowed to equilibrate for at least 2 weeks at 25 °C. Seven sets of samples were prepared, each of them having the same molar ratio of decanol/TMIC, R = 0.9, 1.0, 1.3, 1.5, 1.7, 2.0, or 2.3, and a variable water content, between 53 and 82 wt %. These samples were named W#R#, where W# is the content of water (wt %) and R# the decanol/TMIC molar ratio. For the TMIC/ 2503


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Figure 2. SAXS diffractograms of the set of samples with 72 wt % water and variable bilayer composition, at 15 and 30 °C. The dotted line corresponds to the upper phase. The intensity, in arbitrary units, is depicted on the logarithmic scale. The diffractograms are shifted in the Y scale for better visualization.

range R = 2.3−1.3. For the samples with macroscopic phase separation (R = 0.9), the peaks corresponding to the upper phase are narrower and shift to higher q-values. The samples with R = 1.0 exhibit an intermediate behavior: at 15 and 20 °C, the peaks shift to higher q-values, indicating that there is a phase separation (see Figure 2, left). However, at higher temperatures the peaks follow the trend of the set of samples, denoting that there is only one phase (see, as an example, Figure 2, right). For the monophasic samples, the figure also shows a small shift in the peaks to higher q-values when the temperature increases from 15 to 30 °C. This suggests a change in the bilayer structure as detailed below. WAXS measurements provide information about the structure of the bilayer. Typical patterns detected in the samples without phase separation are shown in Figure 3, left. At temperatures above 20 °C, the diffractograms correspond to the Lα phase, with a broad band located at 14.2 nm−1, characteristic of the fluid liquid state of the aliphatic chains,43 and two additional bands at 19.5 and 30 nm−1, related to the

scattering angle. In the experiment of X-ray-induced polymerization, the scattering diffractograms were recorded at time intervals of 3.6 s, with irradiation times of 5 s.

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RESULTS AND DISCUSSION Lamellar Structure of the TMIC/1-Decanol/Water System. At room temperature, the samples with the lowest 1-decanol/IL molar ratio (R = 0.9) and the sample with R = 1.0 and 82 wt % water are biphasic. The upper phase is a turbid viscous liquid, and the bottom phase is a transparent viscous liquid. The other samples are gel-like, monophasic, and turbid, although the turbidity depends on the 1-decanol content: samples with R = 1 are slightly turbid, and for those with R > 1, the turbidity increases. All of the samples appear birefringent between crossed polarizers, with the exception of the bottom phase of the biphasic samples. For R > 1, micrometric spherical domains can be observed by optical microscopy (Figure 1, left). These domains, which are responsible for the elevated turbidity of the samples, are detected as spherulites with crossed polarizers (Figure 1, center). In some of the spherical domains it can be appreciated multilayers by optical microscopy, and CryoSEM reveals that these are onionlike structures (Figure 1, right). For R = 1, the micrographs show only oily streaks, characteristic of the lamellar structure; however, the spherical domains are not detected, according to the lower turbidity of these samples. The spontaneous formation of onionlike lamellar structures has been reported previously,40,41 and our results suggest that transformation from planar to spherical structures can be tuned by the 1-decanol content. Nevertheless, the conditions for formation of the onionlike structures have not yet been elucidated despite many reports studying this aspect.42 To obtain quantitative information about the structure of the samples, small angle X-ray scattering (SAXS) measurements were performed. The diffractograms exhibit the characteristic pattern of a lamellar structure, showing two to four peaks at relative positions 1:2:3:4 (see Figure 2 as an example). In the case of samples with phase separation, the upper phase presents the same pattern, whereas the lower phase is isotropic. For a given temperature and water content, the peaks shift to lower q-values when the 1-decanol content diminishes in the

Figure 3. Left: WAXS diffractograms of the sample W53R2.0 at different temperatures. The inset corresponds to the second-order peak of the hexagonal arrangement of the alkyl chains in the bilayer (intensity in arbitrary units). Right: optical micrographs with crossed polarizers of the same sample in the Lβ and Lα phases (20 and 25 °C, respectively). 2504


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Figure 4. Bilayer thickness, do, as a function of the 1-decanol/IL molar ratio at 15 and 25 °C (left) and as a function of the temperature for R = 1.3 and 2.3 (right).

water organization.44 At 15 and 20 °C, the pattern is different, showing a sharp peak at 14.84 and 14.77 nm−1, respectively.43 The peak corresponds to the Lβ gel phase, where the aliphatic chains are stacked parallel to the bilayer normal in a hexagonal arrangement; in fact, a second-order peak at a relative position of √3 is also detected. The transition between the Lα and Lβ phases is also observed by optical microscopy, as the texture appears more clearly defined in the gel phase (see Figure 3, right). The characteristic structural parameter of a lamellar mesophase is the lamellar spacing, d, which is the sum of the bilayer spacing, do, and the thickness of the water layer, dw. It is obtained by SAXS from the position of the first diffraction peak, q1, as d = 2π/q1. In binary surfactant/water systems, d follows the dilution law 1/d = (1/do)ΦS, where ΦS is the surfactant volume fraction. Assuming that 1-decanol is located in the bilayer, the dilution law can be expressed as

1/d = (1/do)ΦIL + OH

structures, given that for R = 1.0 the onionlike structures do not appear and do remains constant with water content. Zang et al. studied the lamellar mesophase of ternary systems involving [C8mim]+Cl, water, and alcohols with different alkyl chains (n = 6, 8, 10, 12) and R varying from 0.5 to 2.5. They observed that the thickness of the hydrophobic domain increases with R when the alkyl chain length of the alcohol is equal or greater than that of the IL (i.e., n ≥ 8). To explain this, they proposed that the alcohol is located in two environments: the palisade and interior of the hydrophobic domain. Nevertheless, for n = 6 the thickness of the hydrophobic domain does not change with R, and they argued that a part of the hexanol is incorporated into the palisade and the other part fills the “gap” created as a consequence of the different chain lengths of the molecules that conform the bilayer.24 In another work of the same group, ternary system 1-hexadecyl-3methylimidazolium chloride/1-hexanol/water was investigated, analyzing the variation of lamellar spacing with R (in the range R = 0−1).25 In this case, the alkyl chain of the alcohol is shorter than the hydrophobic chain of the IL, and it is observed that the lamellar spacing increases until a maximum value for R = 0.7 and diminishes thereafter. In accordance with this, it is observed that do diminishes with decanol content for R ≥ 1 in the system studied here, with this effect being greater in the gel phase (Figure 4, left). This variation can be attributed to the smaller length of the 1-decanol alkyl chain and to the increase in the polar head area as a consequence of the interactions among imidazolium cations, hydroxyl groups of 1-decanol, counterions, and water.25 In the gel phase, the rigidity of the alkyl chains, which are in a crystalline organization, enhances these effects in comparison to those in the Lα structure, where the mobility of the chains contributes to dissipating the differences in bilayer spacing with the composition of the samples. The variation of the bilayer spacing with temperature is depicted in Figure 4, right. When the system adopts an Lα structure, the bilayer thickness diminishes linearly with temperature. Additionally, the slope of this variation is similar for the different sets of samples analyzed (0.010 ± 0.001 nm/

(1)

where ΦIL+OH is the bilayer volume fraction that, considering the additivity of volumes, is calculated as ΦIL+OH = ΦIL + ΦOH. When do remains constant, the lamellar spacings of the monophasic samples corresponding to dilution lines follow this law. In this system, the correlations of the linear fits are very good and the intercept is negligible for the samples corresponding to the dilution line with R = 1.0. However, a small intercept is found when R > 1, which indicates variation of the bilayer thickness with water content. As an example, selected values obtained from eq 1 are depicted in Figure 4 as a function of R and temperature. It can be observed that, with the exception of that for samples with R = 1, do decreases with water content, and this effect is greater in the gel phase, that is, at 15 and 20 °C (see Figure 4, right). A similar variation has been reported for the Lα mesophase of N-alkyl-N-methylpyrrolidinium bromide/1-decanol/water,45 and it is explained as a consequence of compression of the bilayers by the expanding water channels. We propose that, at least in this system, it could also be related to the presence of onionlike 2505


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The Journal of Physical Chemistry B °C), indicating that it is associated with the lipophilic part of the bilayers. This contraction is characteristic of the hydrocarbon chains in a liquidlike structure.46 From the variation of do with temperature, the isobaric thermal expansivity of the bilayer thickness, α, can be obtained as follows

⎛ ∂d ⎞ α = ⎜ o ⎟ /do ⎝ ∂T ⎠

(2)

In this system, α = −3.2 × 10−3 K−1. A value of the same order was obtained for 1-dodecyl-3-methylimidazolium 3-hydroxy-2naphthoate in water (α = −4.8 × 10−3 K−1).47 However, for mixed bilayers of dioleoylphosphatidylserine with long-chain alcohols48 and for 1,3-didecyl-2-methylimidazolium chloride with decanol49 the values of α are −1 × 10−3 and −1.3 × 10−3 K−1, respectively. These data suggest that there are important differences in the thermal expansivity of bilayers formed by single- and double-tailed surfactants. The bilayer thickness is greater in the gel phase (15 and 20 °C), in which the aliphatic chains are in an all-trans conformation. The aliphatic chain length can be estimated from the Tanford equation: l (nm) = 0.15 + 0.1265nC, where nC is the number of carbon atoms in the aliphatic chain.50 Moreover, the contribution of the imidazolium polar head should be added, which has been estimated to be 0.14 nm.51 Thus, the theoretical maximum value of the bilayer thickness in this system is 4.12 nm. The experimental do values are lower, suggesting that the gel phase is partially interdigitated (LIβ) due to the presence of 1-decanol, which has a shorter chain length. As expected, interdigitation of the chains of apposing monolayers is favored at high concentrations of 1-decanol, that is, at high R-values. In most of the samples, do diminishes when the temperature rises from 15 to 20 °C. However, when R ≥ 1.7 and WH2O ≥ 72 wt %, do increases (see Figure 4, right). These features would indicate a change in the extension of the interdigitation in the LIβ phase, although this could also be promoted by changes in the area of the polar head. DSC measurements are sensitive for detecting phase transitions and are useful to quantify the thermodynamic parameters related to these transitions.52,53 The thermograms exhibit an endothermic peak on heating and an exothermic peak on cooling that, according to the previous SAXS and microscopy results, can be attributed to the lamellar gel to Lα transition. This is a reversible thermotropic transition, with a hysteresis of 2−3 °C between the heating and cooling scans. The gel-to-fluid phase transition temperatures decrease slightly with the water content (see Figure 5, up). The influence of bilayer composition is depicted in the bottom part of Figure 5. The transition temperature increases with the 1-decanol content, indicating that the presence of alcohol stabilizes the gel phase, reaching a plateau for R = 2.0. For the upper phase of the samples with R = 0.9, the transition temperature occurs at 19.5 ± 0.5 °C, which is a value significantly lower than that corresponding to the monophasic samples. At this temperature, the transition for the samples with R = 1.0 is also detected, indicating that in these samples there is a phase separation at temperatures below 20 °C, in agreement with the SAXS results. The transition enthalpy is rather similar for the samples corresponding to the dilution lines (fixed R and variable water contents), and the averaged values are depicted in Figure 6 as a function of the bilayer composition. As expected, a clear decrease in ΔH with R can be observed because 1-decanol has a

Figure 5. Temperature of the gel−liquid crystal transition for the monophasic samples, on heating scans, as a function of the water content (up) and decanol/IL molar ratio (down).

Figure 6. Enthalpy per mole of the bilayer corresponding to the gel− liquid crystal transition as a function of bilayer composition.

shorter alkyl chain, which hinders the crystalline organization of the bilayer. The intercept of the linear fit provides a theoretical value of the transition enthalpy for the binary system (IL/ water) of 12 kJ/mol. The extrapolation assumes a linear variation in the whole range of R that could be nonrealistic. Despite this, similar values were obtained for dialkyldimethylammonium bromides.54 Monomer Confinement. To investigate the influence of the monomer, NIPA, on the system, three sets of samples were prepared. The first set corresponds to a dilution line, where the monomer content and bilayer composition remain constant (WNIPA = 4 wt % and R = 2). In this set, the turbidity increases when the water content diminishes. Figure 7, left, shows that the diffractograms of these samples exhibit the characteristic lamellar pattern. As expected for a dilution line, the peaks shift to higher values of the q-vector when the surfactant content increases (i.e., when the water content diminishes), indicating that the lamellar spacing becomes shorter (see Figure 7, upperright). Nevertheless, the two samples with the lowest water content (49 and 54 wt %) do not follow the tendency: the peaks are sharper and narrower and the lamellar spacings are higher than expected, meaning that there is a microscopic phase separation. As the monomer is hydrophilic, it can be assumed that it is incorporated in the water layer and consequently the effective concentration of NIPA in the water layer increases as the water content decreases. As a result, the highest NIPA 2506


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Figure 7. Left: Diffractograms at 25 °C corresponding to the set of samples with R = 2, 4 wt % NIPA, and variable water content (intensity in arbitrary units; the diffractograms are shifted in the Y scale for better visualization). Right: Lamellar spacing as a function of surfactant volume fraction (upper), R (center), and monomer volume fraction (lower). Open symbols correspond to the lamellar phase of the samples with two phases.

where do is the bilayer thickness in the absence of NIPA, ANIPA = 0.82 nm2 is the surface area in contact with water of a NIPA molecule included in the bilayer,56 νNIPA is the volume of the molecule, and f is the fraction of NIPA incorporated in the bilayer. Figure 8, center, shows that the composition of the bilayer does not have a large influence on f. On the other hand, f increases with an increase in the surfactant or NIPA contents (see Figure 8, upper and lower) because there is an increase in the effective concentration of the monomer in the water layer, which promotes the interaction of the monomer with the bilayer.

concentration in these samples induces destabilization of the lamellar system. In the second set of samples, the concentration of water and monomer remain constant (WNIPA = 4 wt %, WH2O = 68.3 wt %) and the composition of the bilayer varies, from R = 1 to 2.3. For R = 1, there is a macroscopic phase separation, with the upper phase being lamellar and the bottom phase, isotropic. This indicates that in the presence of monomer a higher amount of alcohol is needed to stabilize the lamellar mesophase, which suggests an interaction of the monomer with the alcohol. The other samples are slightly turbid, monophasic, and lamellar. Figure 7, center-right, illustrates that the lamellar spacing diminishes with R, following the same trend of the system without monomer (see explanation above). In the third set of samples, the composition and bilayer concentration remain constant (R = 2 and WIL+OH = 27.5 wt %) and the effect of monomer concentration is analyzed. The samples are slightly turbid, and the lamellar spacing decreases linearly with monomer content (see Figure 7, lower-right). The simple substitution of water by NIPA molecules would not induce any variation in the lamellar spacing. Therefore, the results reveal the existence of an interaction between the bilayer and monomer that could be produced by hydrogen bonding through the carboxylic group of NIPA and hydroxyl group of 1decanol. In fact, the bilayer thickness diminishes in the presence of monomer for the three sets of samples investigated. As an example, in this set, do diminishes from 3.02 nm for 1 wt % NIPA to 2.76 nm for 5 wt % NIPA. This indicates that the monomer is partially incorporated in the bilayers, thus increasing the lamellar surface area. Assuming that the IL and alcohol are entirely contained in the bilayers and that the NIPA is distributed between the bilayers and water layers, the inverse of the repeat distance is given by the following expression55 fA 1 1 = + NIPA ΦNIPA d do 2vNIPA

Figure 8. Fraction of NIPA molecules incorporated in the bilayer as a function of the surfactant volume fraction (upper), R (center), and monomer volume fraction (lower). Dotted lines are eye guides.

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Figure 9. Left: Selected diffractograms showing the evolution of the diffraction peaks associated with X-ray polymerization, for cumulative irradiation times in the range of 7−97 s, corresponding to the sample with R = 2.0, 3 wt % monomer and 50 wt % water (intensity in arbitrary units and logarithmic scale). Right: diffractograms showing the effect of incorporating NIPA (blue), PNIPA by simple mixture (green), and PNIPA by in situ polymerization after 97 s of irradiation (black). Intensity in arbitrary units.

In Situ X-ray-Initiated Polymerization. X-rays have been previously used as an initiator for polymerization of acrylamides in lyotropic mesophases providing, additionally, a method to follow the changes in the structure by the variations in the Xray scattering curves.36 Here, the X-ray-initiated polymerization has been followed for a sample with R = 2.0, containing 3 wt % of monomer and 50 wt % of water, during the first 97 s. Higher irradiation times are not considered because the peaks become wider and lower in intensity, indicating that processes of degradation and/or diffusion take place. Figure 9, left, depicts the evolution of the SAXS diffractograms with irradiation time. Initially, the diffractogram shows the two peaks characteristic of the TMIC/1-decanol/water/ NIPA lamellar system. The SAXS pattern is modified along polymerization as follows: the initial peaks corresponding to the lamellar system with monomer disappear, and new peaks are developed at lower q-values. The relative position of these new peaks is also 1:2, indicating that the lamellar order is conserved, although with a higher spacing. Figure 9, right, shows that the incorporation of 3 wt % PNIPA by simple mixture induces the same effect as in situ polymerization, indicanting that X-ray initiated polymerization has been successfully achieved. Nevertheless, there are differences in the peak positions obtained with both methods, which could be explained if there is a low yield of in situ polymerization or there are differences in the molecular weights of both polymers. Unfortunately, the reaction occurs in the region in which the sample is irradiated, that is, a few millimeters, and therefore it is not possible to analyze the composition of the sample and the polymer molecular weight after polymerization. After polymerization, two scenarios can be proposed: (i) PNIPA is a water-soluble polymer that can remain in the water layer. It is expected that the interaction of the polymer with the bilayer will be lower in comparison to that with the monomer. Therefore, polymerization will lead to a decrease in the area of the polar group and, as a consequence, an increase of the bilayer thickness. (ii) If the polymer coils are large enough, as compared with the water layer thickness, a new isotropic polymer-rich phase could be developed. This new phase would be composed mainly of the polymer and part of the water, joined by a small part of 1-decanol and TMIC due to hydrogen bonding through the amide groups. In this case, redistribution of the components in the two phases would explain the final lamellar spacing. Nevertheless, an intermediate picture, with

part of the polymer included in the lamellar mesophase and the other part excluded, cannot be discarded.57

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CONCLUSIONS The lyotropic liquid-crystalline behavior of the ternary TMIC/ 1-decanol/water system is analyzed as a function of the water content, 1-decanol/IL molar ratio, and temperature. 1-Decanol is incorporated into the bilayer as a cosurfactant, and a lamellar mesophase is formed over a wide range of compositions and temperatures. At room temperature, onionlike lamellar structures appear for R > 1. The system presents the following thermotropic behavior: at 15 °C, there is an interdigitated lamellar phase, LIβ; above this temperature, in the range of 20− 25 °C, the transition to the lamellar liquid-crystalline phase, Lα, occurs. This transition temperature shows dependence on the bilayer composition, with the gel phase being stabilized by the alcohol. The structural parameters are characterized as a function of the composition and temperature. The bilayer thickness decreases, either with the water or decanol contents, this effect being greater in the gel phase. Because the alkyl chain of decanol is shorter than the alkyl chain of the IL, this is an expected variation, whereas the diminution with an increase in water content can be associated with the presence of onionlike structures. The bilayer thickness also shows a temperature dependence. The results suggest that the thermal contractivity of the bilayer thickness in the Lα structure is greater for singletailed than double-tailed surfactants. In the LIβ mesophase, the variation of bilayer thickness with temperature depends on the sample composition. When the monomer NIPA is incorporated, it is distributed between the water layer and bilayer. The fraction of NIPA incorporated in the bilayer does not depend on the bilayer composition, but increases with monomer or surfactant (1decanol + IL) concentration. The in situ X-ray-initiated polymerization of NIPA can be followed by the evolution of SAXS diffractograms. Along polymerization, the lamellar order is conserved, although the lamellar spacing becomes higher. Xray-initiated polymerization is a new method to incorporate polymers in complex systems and is a promising procedure to obtain advanced materials.

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AUTHOR INFORMATION

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The Journal of Physical Chemistry B ORCID

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Carmen S. Renamayor: 0000-0003-2225-0513 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SAXS experiments were performed at the BL11-NCD beamline at ALBA Synchrotron, with the collaboration of ALBA staff.

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REFERENCES

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Lyotropic Lamellar Structures of a Long-Chain Imidazolium and Their

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