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Stabilization of self-assembled lipids in exfoliated organo-nanosheets
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Research paper
Stabilization of self-assembled lipids in exfoliated organo-nanosheets Tomonari Tanakaa, Yoshiyuki Sugaharac,a, Régis Guéganb,c, a b c
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Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan
H I GH L IG H T S
acts as soft confinement system on a lipid. • Organo-nanosheets is confined at either a microscopic and mesoscopic scales. • AA lipid lamellar phase is preserved within the organo-nanosheets. • Thecrystalline • confined lipid lamellar phase exists on a wide range of temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Layered materials Nanosheets Clays Lipids Confinement Preservation
Layered materials represent interesting 2D confinement matrix for both the preservation of organic compounds and were suggested as carriers for biomolecules. An organoclay was prepared with the use of a clay mineral modified by a cationic surfactant and used as a confinement matrix for assemblies formed by 1-monoolein lipid. Monoolein self assembles in water in various lyotropic and thermotropic liquid crystalline phases which were associated to organo-nanosheets resulting from a prior delamination procedure of the organoclay. The lamellar organization confined within the nanosheets displays a different phase sequence than the bulk state with its existence on wide range of temperature.
1. Introduction The use of inorganic layered materials as confinement matrixes has attracted considerable attention due to their high internal surface areas and their abilities to confine guest species within regularly organized interlayer space and to undergo self-orientation at a macroscopic scale [1–6]. In such2 dimensional (2D) geometry confinement; it drives to several advantages as: (i) the absence of aggregation of the encapsulated molecules; (ii) a synergetic effect between the low dimensional confinement and the confined species with preferential both orientation and organization leading to an enhancement of their functions; (iii) a protection or preservation of biomolecules such as DNA, protein, enzyme towards degradation [1,2,7–10]. As exemplified of the latter aspect, lysozymes could be successfully intercalated and preserved in a clay mineral modified by a nonionic surfactant (hybrid nonionic organoclay) through cation exchange showing, within the interlayer space of the layered hybrid material, a self-orientation different from that in bulk solution [1], whereas some of the biomolecules exhibit some structural changes on the external surface. In such extreme
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conditions, the confinement affects both the structure and the dynamics [11–13], with the observation of glassy or amorphous phases showing the interest for drugs or other pharmaceutical derivatives to be much more reactive and efficient for potential therapeutic purposes [4,14]. Thus, the use of layered materials appear as an interesting and powerful way to preserve various biomolecules and drugs within their interlayer spaces for further applications such as drug delivery system (DDS) since both the controlled release of drugs and storage or formulation without the degradation are required in DDS [2,7,15]. 1-Monoolein (1-(cis-9-octadecenoyl)-rac-glycerol) (MO) is an amphiphilic lipid that self-assembles in various lyotropic liquid crystalline (LLCs) phases in water [16]. This lipid can adsorb a certain amount of water and then spontaneously form gel-like phases with unique internal bicontinuous structures, into which both hydrophilic and hydrophobic drugs or other pharmaceutical compounds can be incorporated [17–20]. Their non-toxic, biodegradable and bioadhesive properties also contribute to their applications as drug delivery systems (DDS). However, the susceptible structures of LLCs formed by such lipid can be easily disrupted by external changes, such as temperature or pH
Corresponding author at: Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. E-mail address: [email protected] (R. Guégan).
https://doi.org/10.1016/j.cplett.2019.136954 Received 1 July 2019; Received in revised form 7 November 2019; Accepted 7 November 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tomonari Tanaka, Yoshiyuki Sugahara and Régis Guégan, Chemical Physics Letters, https://doi.org/10.1016/j.cplett.2019.136954
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[16,21].One way to preserve and keep the fragile structure of a lipid consists to associate it with inorganic particles or to confine it within porous materials such as layered materials [1]. In this work, 1-monoolein was associated with organo-nanosheets resulting from the delamination of a clay mineral modified by a cationic surfactant. The MO assemblies confined and stabilized within the spaced created by the organization of the organo-nanosheets were studied by a set of complementary techniques including: small angle Xray scattering (SAXS), X-ray diffraction (XRD), infrared (IR) spectroscopy and thermal gravimetry (TG) reveal interesting features that may find some echo’s in some possible applications such as DDS. 2. Experimental procedure
ν (C-H)
2.1. Materials
ν (O-H)
Na+-montmorillonite (MMT, Tsukinuno, The Clay Science Society of Japan) and hexadecyltrimethylammonium (C16) bromide (CH3(CH2)15N(CH3)3Br, > 98.0%, Wako) were used as received. The modification of a montmorillonite clay mineral (Kunipia F) was conducted following a well-known procedure reported elsewhere [4–6]. Briefly, the clay was fractioned to < 2 μm by gravity sedimentation and purified by well-established procedures in clay science. Sodium exchanges were prepared by immersing the clay into 1 M solution of sodium chloride. Cation exchange was completed by washing and centrifuging four times with dilute aqueous NaCl. Samples were finally washed with distilled deionized water and transferred into dialysis tubes to obtain chloride-free clays and then dried at room temperature. The cation exchange capacity (CEC) of the resulting Na-smectite was estimated at about 100–110 mequiv/100 g, coherent to previous works. 1-monoolein (MO) (C21H40O4, > 99.0%, Sigma-Aldrich), ethanol (C2H5OH, > 99.5%, FUJIFILM Wako) and chloroform (CHCl3, > 99.0%, Wako) were used without further purification in order to prepare a cationic organoclay: C16_MMT.
δ (C-H) δ (O-H)
Fig. 1. FTIR spectra of (a) MMT, the raw montmorillonite and (b) C16_MMT, the cationic organoclay.
Plus spectrometer (JASCO) using the KBr disk method. Thermogravimetry (TG) analyses were determined by using a TG8120 thermobalance (Rigaku) at heating rate of 10 °C/min under air flow. Small angle X-ray scattering (SAXS) profiles were obtained with Rigaku Nano viewer. 3. Results and discussion 3.1. Preparation of MMT modified with C16 (C16_MMT) and swelling in solvent
2.2. Preparation of a cationic organoclay: C16_MMT In a round-bottom flask, MMT (1 g) was swollen in pure water (100 mL). C16 bromide (0.547 g) was dissolved in the dispersion of MMT, and stirred at room temperature for 1 day. Thus, with that experimental conditions, the concentration of the tensioactive corresponded to 1.5 times the value of the CEC, ensuring a total covering of the phyllosilicate surface by an organic hydrophilic coating. The precipitation was separated by centrifugation and washed with water until any foam was observed. After vacuum drying at room temperature, MMT modified with C16 cations (C16_MMT) was obtained.
Fig. 1 shows infrared (IR) spectra of both MMT and C16_MMT. The MMT spectrum revealed a broad and intense band at 1030 cm−1 and the shoulder at 1117 cm−1 correspond to the SiO stretching mode. The OH angular deformation is observed at 1635 cm−1, whereas the OH stretching of water spreads out on a wide range of wavenumbers between 3200 and 3600 cm−1. The last broad band at 3626 cm−1 is assigned to the OH stretching of structural hydroxyl groups. In addition to the assignment of the bands of the Na-montmorillonite, the spectrum reveals bands characterizing another crystalline or amorphous phases [4–6,22]. Indeed, the weak band at 800 cm−1 is assigned to the SiO stretching of quartz and silica. After intercalation of C16 surfactant, the new absorption bands at 2920–2850 cm−1 (ν(CeH)) and 1489–1472 cm−1 (δ(CeH)) were observed, whereas the absorption bands at 3475–3420 cm−1 (ν(OeH)) and 1652–1637 cm−1 (δ(OeH)) were vanished. The absorption bands assignable to MMT basal layers at 1041 cm−1 (ν(SieO)) and 3627 cm−1 (ν(OeH)) were observed in both MMT and C16_MMT spectra. It supports that the intercalation did not drive to any disruption of MMT sheets. In the C16_MMTIR spectrum, new absorption bands assignable to CeH bonds and disappearance of OeH bonds indicated the existence of alkyl chains and the exclusion of water molecules adsorbed onto the Na+ (solvation of the exchangeable cations). It was obvious that the inorganic Na cations located within the interlayer space of the starting MMT+ were substituted with C16 via cation exchange due to electrostatic interaction as reported in numerous studies [4–6,23]. The sharp and intense absorption bands at 2920 and 2850 cm−1 corresponding to the antisymmetric and symmetric CH2 stretching modes of the hydrocarbon chains, respectively are recognized to strongly depend on the
2.3. Association of the monoolein with a cationic organoclay In vial, C16_MMT was swollen in chloroform. To this dispersion, MO in a lamellar phase was added at several weight ratio of C16_MMT to MO. After the stirring at room temperature for 1 day and evaporation of chloroform, a hybrid material C16_MMT_MO including both a cationic organoclay and a lipid was obtained. 2.4. Analyses X-ray diffraction (XRD) patterns were obtained with a RINT-1100 diffractometer (Rigaku, Mn-filtered Fe Kα radiation, λ = 0.174 nm) for the characterization of the samples at room temperature while a RINT2500 diffractometer (Rigaku, Ni-filtered CuKα radiation, λ = 0.15406 nm) was used for the temperature evolution of the structure of the samples. XRD patterns for several temperatures: from room temperature to 70 °C were obtained with Rigaku SmartLab. At each temperature, the sample was held for 10 min to reach equilibrium. Fourier transform infrared (IR) spectra were obtained with a FT/IR-460 2
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thermotropic and lyotopic LC, this lipid self-organizes in two different lamellar phases: a thermotropic crystalline LC phase similar as a smectic A phase where the molecules are ordered in layers into a fixed position, and a true swollen liquid Lα phase with a stacking of the molecules in layers without any strong organization of both headgroups and alkyl chains [16]. Thus, these two phases exhibit different feedbacks by diffraction techniques such as XRD for instance with the characterization of many sharp Bragg diffraction peaks and broad diffuse peaks for LC (Fig. 3b) and Lα respectively. 3.3. Characterization of the mixtures of C16_MMT cationic organoclay with 1-monoolein Fig. 4 shows the XRD patterns of the samples resulting from the association of the cationic organoclay with MO for different weight ratio from 0.061 up to 2.56. At low MO ratio regime (i.e. 0.061–0.207), the X-ray diffraction patterns reveal 00 l reflections of the layered organization sequence of the cationic organoclay which, after being exfoliated in chloroform, recuperates its original structure with a restacking of the layers showing an unchanged d001 spacing value of 2.19 nm. This stresses out the no alteration, nor release or discharge of the C16 molecules once mixed with chloroform. However, other diffraction peaks can be observed at lower angle value at 3.69° and even a shoulder at 2.72° for both samples (samples e and f) with an expansion of the gallery openings of about 2.7 nm and 3.6 nm respectively. These latter 00 l reflections point out the possible confinement or intercalation within the interlayer space of a cationic organoclay of the monoolein lipid or some parts of the amphiphilic molecule but excludes any complete intercalation of a MO bilayer in regards to its thickness estimated at about 3.4 nm [16]. Indeed, the organophilic surface of C16_MMT may favor the possible adsorption of MO through weak molecular hydrophobic interaction as similar cationic organoclay did for organic pollutants. At a weight of 0.283 (sample b), the proper intercalation of MO was confirmed with the shift of the first 00 l reflection at low angle value, leading to an interlayer distance at about 4.4 nm for the hybrid MOcationic organoclay. Here, the expansion of the interlayer is not enough to ensure a proper confinement of a MO bilayer in regards to the values of the whole materials. However, the surfactant assemblies display a pseudo trimolecular or paraffin structure with a certain flexibility and mobility of the alkyl chains of C16 [5,25] that may re-organize to incorporate a MO bilayer of which thickness is lower than the increase of the interlayer space of the hybrid materials. Thus, the confinement of a bilayer of the monoolein is likely to occur here as it could be observed for other membrane phases such as nonionic surfactant self-organized in a lamellar phase driving to the aggregation of a bilayer within the internal structure of a raw montmorillonite [5,25]. Infrared spectroscopy (not shown) supports the presence of MO, however it is quite difficult to determine the conformation of both organic species due to the overlapping of several absorption bands of the whole components in a sample. In addition, thermal gravimetry (TG) measurements validates the confinement of MO. Fig. 5 shows the TG curves of C16_MMT and C16_MMT_MO at a ratio of 1:0.283. When the temperature raised from room temperature to 800 °C, the mass losses were 30.1% and 62.1%, respectively. TG curves have four steps of weight loss in C16_MMT. One from room temperature to 100 °C corresponded to desorption of water. Second and third step, which occurred at 280 °C and 380 °C respectively, resulted from the degradation of surfactants from the interlayer space of MMT. These two steps of weight loss represent a total of about 17% of organic content which is coherent to a cation exchange of about 150% of the starting kunipia clay. The fourth one corresponded to the structural hydroxyl groups. These results were consistent with XRD analysis, about the proper intercalation of the surfactants within the interlayer space of MMT. In contrast, C16_MMT_MO only showed two steps: one starting at 200 °C attributed to desorption of organic moieties while the second to the hydroxyl groups of the phyllosilicate sheets
Fig. 2. XRD patterns of (a) MMT, the raw montmorillonite and (b) C16_MMT, the cationic organoclay.
density of confined surfactant within the interlayer space but also give some important information about the conformational order of surfactant which may adopt an all-trans conformation here in regards to the value of the wavenumbers of these two bands [4–6,22–24]. Fig. 2 shows the X-ray diffraction (XRD) patterns of MMT and the C16_MMT cationic organoclay. After the intercalation of the cationic surfactant through a cation exchange, the diffraction peaks of the 00l reflections pointed out the layered organization of the materials was shifted to lower angle region (d = 2.19 nm). In contrast to the 00l reflections related to the layered structure along a c axis, the in-plane (a and b axis) diffraction peaks remained at the same angle position (d = 0.45 nm), indicating that the intercalation occurred without any disruption of MMT sheets. Moreover, XRD data at large angle values (not shown) did not exhibit any particular diffraction other than those of MMT suggesting the absence of any crystallization of the organic salts onto the external surface of the phyllosilicate sheets. The interlayer distance (d001 = 2.19 nm) was consistent with previous reported value for the preparation of organoclays with conventional alkylammonium salts. A thickness of a phyllosilicate platelet is estimated at 0.96 nm and for a monolayer arrangement within the interlayer space, the gallery height of a flat-lying alkyl chain can reach 0.4 or 0.45 nm, depending on the orientation of a surfactant [4,6,22,23]. Here, the interlayer distance is expended at larger values indicating a different organization than only lateral arrangement of the surfactants. In correlation to the extension of the interlayer space distance determined by XRD data, the wavenumbers of the CH2 stretching bands of hydrocarbon chains of the surfactant can be used as a probe for the organization of the C16 which was assumed to be pseudo trimolecular layers or adopt a paraffin arrangement. 3.2. Monoolein phase diagram and characterization by SAXS Fig. 3a shows the rich phase diagram in temperature and concentration in water of the monoolein obtained by both polarizing optical microscopy and small angle X-ray scattering techniques (Fig. 3b). This amphiphilic lipid can self-assemble in both lyotropic and thermotropic liquid crystalline phases with assembly in different morphologies including layers, tubes, cubosomes and other complex bicontinuous systems. As exemplified of the dual formation of both 3
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Fig. 3. (a) Phase diagram of the monoolein (MO) in water and (b) SAXS profiles of MO_water 0% in crystalline lamellar (LC) phase and MO_water 40% in the bicontinous cubic (D) phase.
Fig. 4. XRD patterns of (a) C16_MMT and (b)–(g) C16_MMT_MO for different weight MO/C16_MMT ratio: (b) = 1:0.283, (c) = 1:0.75, (d) = 1:1.84, (e) = 1:0.061, (f) = 1:0.207 and (g) = 1:2.56.
3.4. Enhancement of the stability of MO LC phase with the presence of C16_MMT
respectively [5]. For the other samples that appear as gel state, once the ratio of C16_MMT to MO is above 1:0.755 (i.e. samples c, d and g in Fig. 4), the XRD patterns show several diffraction peaks related to 00 l reflections including several orders of diffraction attesting a crystalline layered structure. One may think that the intercalation of MO was successfully achieved at these stages of MO concentration and contributes to better organize the phyllosilicate sheets since the diffraction peaks appeared sharp and thin. Nevertheless, as the XRD patterns of the bulk MO crystalline lamellar phase (Lc) displayed in Fig. 6a, the factor of structure exclusively resulted from the contribution of the scattering of the well-organized structure of the thermotropic MO phase. No peaks assignable to the restacking of a cationic organoclay C16_MMT including or not MO in its internal structure were observed. Therefore, C16_MMT remains in the mixture as exfoliated nanosheets without however any possible identification from XRD analysis but could be easily identified by both TG and FTIR data (not shown).
Fig. 6 shows the XRD patterns of pure 1-monoolein while Fig. 7 displays those of the mixtures of monoolein associated with organonanosheets of a cationic organoclay (once the weight ratio is above 0.75). Here, the diffraction peaks were exclusively those related to the assemblies of monoolein, excluding any restacking of C16_MMT that are still exfoliated in the mixtures. As expected for the pure monoolein and the determination of its phase diagram, in anhydrous condition, monoolein self-organizes in a thermotropic crystalline lamellar phase (Lc), analogous to a smectic A phase for themotropic liquid crystal. According to its phase diagram, the layered structure is lost above a temperature of 40 °C and was confirmed with the disappearance of the 00 l reflections (Fig. 6). In contrast, the temperature evolution the 00l reflections of the monoolein associated with exfoliated organo-nanosheets considerably differs from the bulk MO with an enhancement of the thermal stability and the preservation of a LC phase at temperature up to 70 °C (Fig. 7). While a temperature seems to not affect the layered
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Fig. 5. TG curves of (a) C16_MMT and (b) C16_MMT_MO. Fig. 7. Temperature XRD patterns of C16_MMT_MO_T T = (a) RT, (b) 35°C, (c) 40 °C, (d) 45 °C, (e) 50 °C, (f) 55 °C, (g) 60 °C, (h) 65 °C and (i) 70 °C.
locate MO in a gel sample and to confirm any possible visual confinement of MO in the organization formed by C16_MMT except than those deduced from the thermal behavior of the XRD patterns of the hybrid materials (Fig. 7). Thus, additional experiments such as cryo-TEM or SEM may be useful to appreciate the particular organization of the mixtures and will be performed in a next future. However, layered materials such clay minerals are well known to form particles of which assemblies leave some spaces also called mesopores where confinement occurs. The organo-nanosheets of which layered structure is nevertheless lost may form a disordered confinement system with spaces at a mesoscopic scale where monoolein is confined. In these mesoscopic capsules formed by the phyllosilicate sheets, enough molecules are confined to still get some collective effects driving to phase transitions while the surface, geometry and topological constraints imposed by the nanosheets affect both the dynamics and the phase transitions of a lipid of which elastic properties are extremely sensitive to any weak stimuli or changes [11–14]. Here, the difference with the bulk and preservation on wide range of temperature of a thermotropic lamellar phase formed by MO may result from confinement effects as it could be observed for rigid or hard porous materials. Based on these primary results, it appeared that the hybrid materials formed by lipids and organoclay minerals were obtained via the association of C16_MMT and 1-monoolein but the interests or possible use of the hybrid materials will be studied later.
Fig. 6. Temperature XRD patterns of (a) MO_RT, (b) MO_30 °C and (c) MO_39 °C.
structure of a lipid, the repeating distance of the bilayers, which was identical to that of the bulk-one at room temperature (d001≈5.3 nm), shrinks continuously, by reaching a distance of about 4.6 nm at high temperature (Fig. 7), resulting from an increase of the fluidity and mobility of the alkyl chains of monoolein at those temperatures and the introduction of gauche disorder, and thus decreasing the thickness of a bilayer. Thus, the presence of C16_MMT sheets in the system considerably affects the phase transition here and stabilize the MO in a layered arrangement on a wide range of temperature. Such organization of the clay platelets prevent any phase transition and may act as a cage or confinement systems, limiting the disruption of the lamellar phase through thermal effect [11–14]. Nevertheless, it was rather difficult to
4. Conclusions The association of 1-monoolein lamellar assemblies and modified layered materials to obtain hybrid materials was studied. The association of a cationic organoclay C16_MMT sheets preserve MO lipids by showing crystalline lamellar structures even in high temperature region (40–70 °C), where pristine MO lost its lamellar organization in accordance with its bulk phase diagram. This hybrid material also exhibited lamellar structure regardless the amount of water. Pure MO causes phase transition depending on the amount of water, therefore it seems that the addition of sheets prevented any phase transition. Although precise localization of the different components in the resulting materials needed further characterization by microscopy 5
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measurements, these results stressed out the successful combination of layered materials and lipids leading to a potential candidate for host material in drug delivery system especially for the carrier of hydrophobic drugs.
[9] [10] [11]
Author contributions
[12]
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
[13]
Declaration of Competing Interest [14]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[15] [16]
Acknowledgements [17]
The authors would like to thank the Région Centre (Project MONITOPOL 2017 - 00117247) for their financial support.
[18]
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Research paper
Stabilization of self-assembled lipids in exfoliated organo-nanosheets Tomonari Tanakaa, Yoshiyuki Sugaharac,a, Régis Guéganb,c, a b c
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Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan
H I GH L IG H T S
acts as soft confinement system on a lipid. • Organo-nanosheets is confined at either a microscopic and mesoscopic scales. • AA lipid lamellar phase is preserved within the organo-nanosheets. • Thecrystalline • confined lipid lamellar phase exists on a wide range of temperature.
A R T I C LE I N FO
A B S T R A C T
Keywords: Layered materials Nanosheets Clays Lipids Confinement Preservation
Layered materials represent interesting 2D confinement matrix for both the preservation of organic compounds and were suggested as carriers for biomolecules. An organoclay was prepared with the use of a clay mineral modified by a cationic surfactant and used as a confinement matrix for assemblies formed by 1-monoolein lipid. Monoolein self assembles in water in various lyotropic and thermotropic liquid crystalline phases which were associated to organo-nanosheets resulting from a prior delamination procedure of the organoclay. The lamellar organization confined within the nanosheets displays a different phase sequence than the bulk state with its existence on wide range of temperature.
1. Introduction The use of inorganic layered materials as confinement matrixes has attracted considerable attention due to their high internal surface areas and their abilities to confine guest species within regularly organized interlayer space and to undergo self-orientation at a macroscopic scale [1–6]. In such2 dimensional (2D) geometry confinement; it drives to several advantages as: (i) the absence of aggregation of the encapsulated molecules; (ii) a synergetic effect between the low dimensional confinement and the confined species with preferential both orientation and organization leading to an enhancement of their functions; (iii) a protection or preservation of biomolecules such as DNA, protein, enzyme towards degradation [1,2,7–10]. As exemplified of the latter aspect, lysozymes could be successfully intercalated and preserved in a clay mineral modified by a nonionic surfactant (hybrid nonionic organoclay) through cation exchange showing, within the interlayer space of the layered hybrid material, a self-orientation different from that in bulk solution [1], whereas some of the biomolecules exhibit some structural changes on the external surface. In such extreme
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conditions, the confinement affects both the structure and the dynamics [11–13], with the observation of glassy or amorphous phases showing the interest for drugs or other pharmaceutical derivatives to be much more reactive and efficient for potential therapeutic purposes [4,14]. Thus, the use of layered materials appear as an interesting and powerful way to preserve various biomolecules and drugs within their interlayer spaces for further applications such as drug delivery system (DDS) since both the controlled release of drugs and storage or formulation without the degradation are required in DDS [2,7,15]. 1-Monoolein (1-(cis-9-octadecenoyl)-rac-glycerol) (MO) is an amphiphilic lipid that self-assembles in various lyotropic liquid crystalline (LLCs) phases in water [16]. This lipid can adsorb a certain amount of water and then spontaneously form gel-like phases with unique internal bicontinuous structures, into which both hydrophilic and hydrophobic drugs or other pharmaceutical compounds can be incorporated [17–20]. Their non-toxic, biodegradable and bioadhesive properties also contribute to their applications as drug delivery systems (DDS). However, the susceptible structures of LLCs formed by such lipid can be easily disrupted by external changes, such as temperature or pH
Corresponding author at: Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. E-mail address: [email protected] (R. Guégan).
https://doi.org/10.1016/j.cplett.2019.136954 Received 1 July 2019; Received in revised form 7 November 2019; Accepted 7 November 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tomonari Tanaka, Yoshiyuki Sugahara and Régis Guégan, Chemical Physics Letters, https://doi.org/10.1016/j.cplett.2019.136954
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[16,21].One way to preserve and keep the fragile structure of a lipid consists to associate it with inorganic particles or to confine it within porous materials such as layered materials [1]. In this work, 1-monoolein was associated with organo-nanosheets resulting from the delamination of a clay mineral modified by a cationic surfactant. The MO assemblies confined and stabilized within the spaced created by the organization of the organo-nanosheets were studied by a set of complementary techniques including: small angle Xray scattering (SAXS), X-ray diffraction (XRD), infrared (IR) spectroscopy and thermal gravimetry (TG) reveal interesting features that may find some echo’s in some possible applications such as DDS. 2. Experimental procedure
ν (C-H)
2.1. Materials
ν (O-H)
Na+-montmorillonite (MMT, Tsukinuno, The Clay Science Society of Japan) and hexadecyltrimethylammonium (C16) bromide (CH3(CH2)15N(CH3)3Br, > 98.0%, Wako) were used as received. The modification of a montmorillonite clay mineral (Kunipia F) was conducted following a well-known procedure reported elsewhere [4–6]. Briefly, the clay was fractioned to < 2 μm by gravity sedimentation and purified by well-established procedures in clay science. Sodium exchanges were prepared by immersing the clay into 1 M solution of sodium chloride. Cation exchange was completed by washing and centrifuging four times with dilute aqueous NaCl. Samples were finally washed with distilled deionized water and transferred into dialysis tubes to obtain chloride-free clays and then dried at room temperature. The cation exchange capacity (CEC) of the resulting Na-smectite was estimated at about 100–110 mequiv/100 g, coherent to previous works. 1-monoolein (MO) (C21H40O4, > 99.0%, Sigma-Aldrich), ethanol (C2H5OH, > 99.5%, FUJIFILM Wako) and chloroform (CHCl3, > 99.0%, Wako) were used without further purification in order to prepare a cationic organoclay: C16_MMT.
δ (C-H) δ (O-H)
Fig. 1. FTIR spectra of (a) MMT, the raw montmorillonite and (b) C16_MMT, the cationic organoclay.
Plus spectrometer (JASCO) using the KBr disk method. Thermogravimetry (TG) analyses were determined by using a TG8120 thermobalance (Rigaku) at heating rate of 10 °C/min under air flow. Small angle X-ray scattering (SAXS) profiles were obtained with Rigaku Nano viewer. 3. Results and discussion 3.1. Preparation of MMT modified with C16 (C16_MMT) and swelling in solvent
2.2. Preparation of a cationic organoclay: C16_MMT In a round-bottom flask, MMT (1 g) was swollen in pure water (100 mL). C16 bromide (0.547 g) was dissolved in the dispersion of MMT, and stirred at room temperature for 1 day. Thus, with that experimental conditions, the concentration of the tensioactive corresponded to 1.5 times the value of the CEC, ensuring a total covering of the phyllosilicate surface by an organic hydrophilic coating. The precipitation was separated by centrifugation and washed with water until any foam was observed. After vacuum drying at room temperature, MMT modified with C16 cations (C16_MMT) was obtained.
Fig. 1 shows infrared (IR) spectra of both MMT and C16_MMT. The MMT spectrum revealed a broad and intense band at 1030 cm−1 and the shoulder at 1117 cm−1 correspond to the SiO stretching mode. The OH angular deformation is observed at 1635 cm−1, whereas the OH stretching of water spreads out on a wide range of wavenumbers between 3200 and 3600 cm−1. The last broad band at 3626 cm−1 is assigned to the OH stretching of structural hydroxyl groups. In addition to the assignment of the bands of the Na-montmorillonite, the spectrum reveals bands characterizing another crystalline or amorphous phases [4–6,22]. Indeed, the weak band at 800 cm−1 is assigned to the SiO stretching of quartz and silica. After intercalation of C16 surfactant, the new absorption bands at 2920–2850 cm−1 (ν(CeH)) and 1489–1472 cm−1 (δ(CeH)) were observed, whereas the absorption bands at 3475–3420 cm−1 (ν(OeH)) and 1652–1637 cm−1 (δ(OeH)) were vanished. The absorption bands assignable to MMT basal layers at 1041 cm−1 (ν(SieO)) and 3627 cm−1 (ν(OeH)) were observed in both MMT and C16_MMT spectra. It supports that the intercalation did not drive to any disruption of MMT sheets. In the C16_MMTIR spectrum, new absorption bands assignable to CeH bonds and disappearance of OeH bonds indicated the existence of alkyl chains and the exclusion of water molecules adsorbed onto the Na+ (solvation of the exchangeable cations). It was obvious that the inorganic Na cations located within the interlayer space of the starting MMT+ were substituted with C16 via cation exchange due to electrostatic interaction as reported in numerous studies [4–6,23]. The sharp and intense absorption bands at 2920 and 2850 cm−1 corresponding to the antisymmetric and symmetric CH2 stretching modes of the hydrocarbon chains, respectively are recognized to strongly depend on the
2.3. Association of the monoolein with a cationic organoclay In vial, C16_MMT was swollen in chloroform. To this dispersion, MO in a lamellar phase was added at several weight ratio of C16_MMT to MO. After the stirring at room temperature for 1 day and evaporation of chloroform, a hybrid material C16_MMT_MO including both a cationic organoclay and a lipid was obtained. 2.4. Analyses X-ray diffraction (XRD) patterns were obtained with a RINT-1100 diffractometer (Rigaku, Mn-filtered Fe Kα radiation, λ = 0.174 nm) for the characterization of the samples at room temperature while a RINT2500 diffractometer (Rigaku, Ni-filtered CuKα radiation, λ = 0.15406 nm) was used for the temperature evolution of the structure of the samples. XRD patterns for several temperatures: from room temperature to 70 °C were obtained with Rigaku SmartLab. At each temperature, the sample was held for 10 min to reach equilibrium. Fourier transform infrared (IR) spectra were obtained with a FT/IR-460 2
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thermotropic and lyotopic LC, this lipid self-organizes in two different lamellar phases: a thermotropic crystalline LC phase similar as a smectic A phase where the molecules are ordered in layers into a fixed position, and a true swollen liquid Lα phase with a stacking of the molecules in layers without any strong organization of both headgroups and alkyl chains [16]. Thus, these two phases exhibit different feedbacks by diffraction techniques such as XRD for instance with the characterization of many sharp Bragg diffraction peaks and broad diffuse peaks for LC (Fig. 3b) and Lα respectively. 3.3. Characterization of the mixtures of C16_MMT cationic organoclay with 1-monoolein Fig. 4 shows the XRD patterns of the samples resulting from the association of the cationic organoclay with MO for different weight ratio from 0.061 up to 2.56. At low MO ratio regime (i.e. 0.061–0.207), the X-ray diffraction patterns reveal 00 l reflections of the layered organization sequence of the cationic organoclay which, after being exfoliated in chloroform, recuperates its original structure with a restacking of the layers showing an unchanged d001 spacing value of 2.19 nm. This stresses out the no alteration, nor release or discharge of the C16 molecules once mixed with chloroform. However, other diffraction peaks can be observed at lower angle value at 3.69° and even a shoulder at 2.72° for both samples (samples e and f) with an expansion of the gallery openings of about 2.7 nm and 3.6 nm respectively. These latter 00 l reflections point out the possible confinement or intercalation within the interlayer space of a cationic organoclay of the monoolein lipid or some parts of the amphiphilic molecule but excludes any complete intercalation of a MO bilayer in regards to its thickness estimated at about 3.4 nm [16]. Indeed, the organophilic surface of C16_MMT may favor the possible adsorption of MO through weak molecular hydrophobic interaction as similar cationic organoclay did for organic pollutants. At a weight of 0.283 (sample b), the proper intercalation of MO was confirmed with the shift of the first 00 l reflection at low angle value, leading to an interlayer distance at about 4.4 nm for the hybrid MOcationic organoclay. Here, the expansion of the interlayer is not enough to ensure a proper confinement of a MO bilayer in regards to the values of the whole materials. However, the surfactant assemblies display a pseudo trimolecular or paraffin structure with a certain flexibility and mobility of the alkyl chains of C16 [5,25] that may re-organize to incorporate a MO bilayer of which thickness is lower than the increase of the interlayer space of the hybrid materials. Thus, the confinement of a bilayer of the monoolein is likely to occur here as it could be observed for other membrane phases such as nonionic surfactant self-organized in a lamellar phase driving to the aggregation of a bilayer within the internal structure of a raw montmorillonite [5,25]. Infrared spectroscopy (not shown) supports the presence of MO, however it is quite difficult to determine the conformation of both organic species due to the overlapping of several absorption bands of the whole components in a sample. In addition, thermal gravimetry (TG) measurements validates the confinement of MO. Fig. 5 shows the TG curves of C16_MMT and C16_MMT_MO at a ratio of 1:0.283. When the temperature raised from room temperature to 800 °C, the mass losses were 30.1% and 62.1%, respectively. TG curves have four steps of weight loss in C16_MMT. One from room temperature to 100 °C corresponded to desorption of water. Second and third step, which occurred at 280 °C and 380 °C respectively, resulted from the degradation of surfactants from the interlayer space of MMT. These two steps of weight loss represent a total of about 17% of organic content which is coherent to a cation exchange of about 150% of the starting kunipia clay. The fourth one corresponded to the structural hydroxyl groups. These results were consistent with XRD analysis, about the proper intercalation of the surfactants within the interlayer space of MMT. In contrast, C16_MMT_MO only showed two steps: one starting at 200 °C attributed to desorption of organic moieties while the second to the hydroxyl groups of the phyllosilicate sheets
Fig. 2. XRD patterns of (a) MMT, the raw montmorillonite and (b) C16_MMT, the cationic organoclay.
density of confined surfactant within the interlayer space but also give some important information about the conformational order of surfactant which may adopt an all-trans conformation here in regards to the value of the wavenumbers of these two bands [4–6,22–24]. Fig. 2 shows the X-ray diffraction (XRD) patterns of MMT and the C16_MMT cationic organoclay. After the intercalation of the cationic surfactant through a cation exchange, the diffraction peaks of the 00l reflections pointed out the layered organization of the materials was shifted to lower angle region (d = 2.19 nm). In contrast to the 00l reflections related to the layered structure along a c axis, the in-plane (a and b axis) diffraction peaks remained at the same angle position (d = 0.45 nm), indicating that the intercalation occurred without any disruption of MMT sheets. Moreover, XRD data at large angle values (not shown) did not exhibit any particular diffraction other than those of MMT suggesting the absence of any crystallization of the organic salts onto the external surface of the phyllosilicate sheets. The interlayer distance (d001 = 2.19 nm) was consistent with previous reported value for the preparation of organoclays with conventional alkylammonium salts. A thickness of a phyllosilicate platelet is estimated at 0.96 nm and for a monolayer arrangement within the interlayer space, the gallery height of a flat-lying alkyl chain can reach 0.4 or 0.45 nm, depending on the orientation of a surfactant [4,6,22,23]. Here, the interlayer distance is expended at larger values indicating a different organization than only lateral arrangement of the surfactants. In correlation to the extension of the interlayer space distance determined by XRD data, the wavenumbers of the CH2 stretching bands of hydrocarbon chains of the surfactant can be used as a probe for the organization of the C16 which was assumed to be pseudo trimolecular layers or adopt a paraffin arrangement. 3.2. Monoolein phase diagram and characterization by SAXS Fig. 3a shows the rich phase diagram in temperature and concentration in water of the monoolein obtained by both polarizing optical microscopy and small angle X-ray scattering techniques (Fig. 3b). This amphiphilic lipid can self-assemble in both lyotropic and thermotropic liquid crystalline phases with assembly in different morphologies including layers, tubes, cubosomes and other complex bicontinuous systems. As exemplified of the dual formation of both 3
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Fig. 3. (a) Phase diagram of the monoolein (MO) in water and (b) SAXS profiles of MO_water 0% in crystalline lamellar (LC) phase and MO_water 40% in the bicontinous cubic (D) phase.
Fig. 4. XRD patterns of (a) C16_MMT and (b)–(g) C16_MMT_MO for different weight MO/C16_MMT ratio: (b) = 1:0.283, (c) = 1:0.75, (d) = 1:1.84, (e) = 1:0.061, (f) = 1:0.207 and (g) = 1:2.56.
3.4. Enhancement of the stability of MO LC phase with the presence of C16_MMT
respectively [5]. For the other samples that appear as gel state, once the ratio of C16_MMT to MO is above 1:0.755 (i.e. samples c, d and g in Fig. 4), the XRD patterns show several diffraction peaks related to 00 l reflections including several orders of diffraction attesting a crystalline layered structure. One may think that the intercalation of MO was successfully achieved at these stages of MO concentration and contributes to better organize the phyllosilicate sheets since the diffraction peaks appeared sharp and thin. Nevertheless, as the XRD patterns of the bulk MO crystalline lamellar phase (Lc) displayed in Fig. 6a, the factor of structure exclusively resulted from the contribution of the scattering of the well-organized structure of the thermotropic MO phase. No peaks assignable to the restacking of a cationic organoclay C16_MMT including or not MO in its internal structure were observed. Therefore, C16_MMT remains in the mixture as exfoliated nanosheets without however any possible identification from XRD analysis but could be easily identified by both TG and FTIR data (not shown).
Fig. 6 shows the XRD patterns of pure 1-monoolein while Fig. 7 displays those of the mixtures of monoolein associated with organonanosheets of a cationic organoclay (once the weight ratio is above 0.75). Here, the diffraction peaks were exclusively those related to the assemblies of monoolein, excluding any restacking of C16_MMT that are still exfoliated in the mixtures. As expected for the pure monoolein and the determination of its phase diagram, in anhydrous condition, monoolein self-organizes in a thermotropic crystalline lamellar phase (Lc), analogous to a smectic A phase for themotropic liquid crystal. According to its phase diagram, the layered structure is lost above a temperature of 40 °C and was confirmed with the disappearance of the 00 l reflections (Fig. 6). In contrast, the temperature evolution the 00l reflections of the monoolein associated with exfoliated organo-nanosheets considerably differs from the bulk MO with an enhancement of the thermal stability and the preservation of a LC phase at temperature up to 70 °C (Fig. 7). While a temperature seems to not affect the layered
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Fig. 5. TG curves of (a) C16_MMT and (b) C16_MMT_MO. Fig. 7. Temperature XRD patterns of C16_MMT_MO_T T = (a) RT, (b) 35°C, (c) 40 °C, (d) 45 °C, (e) 50 °C, (f) 55 °C, (g) 60 °C, (h) 65 °C and (i) 70 °C.
locate MO in a gel sample and to confirm any possible visual confinement of MO in the organization formed by C16_MMT except than those deduced from the thermal behavior of the XRD patterns of the hybrid materials (Fig. 7). Thus, additional experiments such as cryo-TEM or SEM may be useful to appreciate the particular organization of the mixtures and will be performed in a next future. However, layered materials such clay minerals are well known to form particles of which assemblies leave some spaces also called mesopores where confinement occurs. The organo-nanosheets of which layered structure is nevertheless lost may form a disordered confinement system with spaces at a mesoscopic scale where monoolein is confined. In these mesoscopic capsules formed by the phyllosilicate sheets, enough molecules are confined to still get some collective effects driving to phase transitions while the surface, geometry and topological constraints imposed by the nanosheets affect both the dynamics and the phase transitions of a lipid of which elastic properties are extremely sensitive to any weak stimuli or changes [11–14]. Here, the difference with the bulk and preservation on wide range of temperature of a thermotropic lamellar phase formed by MO may result from confinement effects as it could be observed for rigid or hard porous materials. Based on these primary results, it appeared that the hybrid materials formed by lipids and organoclay minerals were obtained via the association of C16_MMT and 1-monoolein but the interests or possible use of the hybrid materials will be studied later.
Fig. 6. Temperature XRD patterns of (a) MO_RT, (b) MO_30 °C and (c) MO_39 °C.
structure of a lipid, the repeating distance of the bilayers, which was identical to that of the bulk-one at room temperature (d001≈5.3 nm), shrinks continuously, by reaching a distance of about 4.6 nm at high temperature (Fig. 7), resulting from an increase of the fluidity and mobility of the alkyl chains of monoolein at those temperatures and the introduction of gauche disorder, and thus decreasing the thickness of a bilayer. Thus, the presence of C16_MMT sheets in the system considerably affects the phase transition here and stabilize the MO in a layered arrangement on a wide range of temperature. Such organization of the clay platelets prevent any phase transition and may act as a cage or confinement systems, limiting the disruption of the lamellar phase through thermal effect [11–14]. Nevertheless, it was rather difficult to
4. Conclusions The association of 1-monoolein lamellar assemblies and modified layered materials to obtain hybrid materials was studied. The association of a cationic organoclay C16_MMT sheets preserve MO lipids by showing crystalline lamellar structures even in high temperature region (40–70 °C), where pristine MO lost its lamellar organization in accordance with its bulk phase diagram. This hybrid material also exhibited lamellar structure regardless the amount of water. Pure MO causes phase transition depending on the amount of water, therefore it seems that the addition of sheets prevented any phase transition. Although precise localization of the different components in the resulting materials needed further characterization by microscopy 5
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measurements, these results stressed out the successful combination of layered materials and lipids leading to a potential candidate for host material in drug delivery system especially for the carrier of hydrophobic drugs.
[9] [10] [11]
Author contributions
[12]
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
[13]
Declaration of Competing Interest [14]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[15] [16]
Acknowledgements [17]
The authors would like to thank the Région Centre (Project MONITOPOL 2017 - 00117247) for their financial support.
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