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Structural and thermal behavior of Meglumine-based supra-amphiphiles in bulk and assembled in water Leonardo Miziara Barboza Ferreira, Suzy Sayuri Sassamoto Kurokawa, Jovan Duran Alonso, Douglas Lopes Cassimiro, Ana Luiza Ribeiro de Souza, Mariana Fonseca, Victor Hugo Vitorino Sarmento, Luis Octavio Regasini, and Clóvis Augusto Ribeiro Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03176 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016
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Structural and thermal behavior of Meglumine-based supra-amphiphiles in bulk and assembled in water Leonardo M. B. Ferreira 1, 2‡, Suzy S. Kurokawa 2‡, Jovan D. Alonso 1, Douglas Lopes Cassimiro 2
, Ana Luiza Ribeiro de Souza 2, Mariana Fonseca 2, Victor Hugo V. Sarmento 3, Luis Octávio Regasini 4, and Clóvis Augusto Ribeiro 2*
1
School of Pharmaceutical Sciences, São Paulo State University, Rodovia Araraquara-Jau Km 1, Araraquara, SP 14801-902, Brazil;
2
Chemistry Institute, São Paulo State University, R. Prof. Francisco Degni, s/n, Araraquara, SP 14800-060, Brazil; 3
Department of Chemistry, Federal University of Sergipe, UFS, Vereador Olimpio Grande Avenue, Itabaiana, SE 49500-000, Brazil;
4
Department of Chemistry and Environmental Sciences, IBILCE, São Paulo State University, R. Cristóvão Colombo, 2265, São José do Rio Preto, SP 15054-000, Brazil.
Corresponding author: Tel.: +551633019607
Keywords: supramolecular amphiphiles; meglumine; oleochemicals; castor oil; liquid crystals; water behavior.
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ABSTRACT
Supra-amphiphiles are a new class of building blocks that are fabricated by means of noncovalent forces. In this work, we studied the formation of supra-amphiphiles by combining hydrophilic meglumine (MEG) with hydrophobic maleated castor oils (MACO). Spectroscopic analysis demonstrated that ionic interactions are the main driving force in the fabrication of these materials. Subsequently, supra-amphiphile/water systems were examined for their structure and water behavior by polarized optical microscopy (POM), small angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC). Micellar and lamellar liquid crystalline phases were observed. Finally, we observed that the supra-amphiphiles produced using an excess of MEG retain a large amount of water. As bound water plays an important role in biointerfacial interactions, we anticipate that these materials will display a pronounced potential for biomedical applications.
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1.
INTRODUCTION Self-assembly processes can generate several types of architectures with differing degrees of
complexity. Films, micelles, vesicles, and gels are some of the constructs that have been created from the manipulation of non-covalent interactions.1,
2, 3
By understanding the engineering
principles of these systems, new functional materials with tunable characteristics can be fabricated. In this context, amphiphilic systems have been used for the construction of supramolecular assemblies for years. The best example in this area are liposomes, which are largely utilized as drug carriers and biomembrane models.4, 5 However, conventional amphiphilic systems may be difficult to construct due the complex organic synthesis involved in the creation of new molecules. Thus, supra-amphiphiles have been proposed to circumvent this problem.6, 7, 8, 9, 10
Supra-amphiphiles are dynamic structures based on a non-covalent construction approach.11 Micro- and macromolecules can be used as building blocks to synthesize these systems to generate hydrophobic and hydrophilic moieties. The construction of these assemblies requires the design of suitable associative forces such as hydrogen bonding, ion pairing, hydrophobic interactions and so forth. For example, the utilization of the acid-base interaction as a driving force has been reported in several recent studies. The combination of stearic acid with di- or oligomeric amines produced hydrogels stabilized by salt-bridging interactions. The resulting materials were characterized by a tridimensional packing of fatty acid molecules in a lamellar arrangement, which was further exploited by the authors as a template for the synthesis of silver nanoparticles.12 In another study, the same group reported the production of two-component hydrogels using a combination of lithocholic acid and organic amines. By adjusting the acid-
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amino ratio, the rheological properties of the gels could be modulated.13 Therefore, non-covalent synthesis has become very attractive in efforts to obtain a library of compounds that can be used to create biomaterials with controlled properties. A core component of biomaterials is water. This simple molecule has been shown to have different dynamics and physicochemical properties simply by differences in the arrangement of its supramolecular system.14,
15
Calorimetric studies have classified the water as bound,
interfacial and free water according to the proximity to hydrophilic groups.16 Recent studies demonstrate that these different categories of water impact a diverse range of biological responses that are dependent on interfacial interactions.17, 18, 19 Accordingly, by recognizing that the type of water is an important structural feature of biomaterials, gaining basic knowledge of how to manipulate them is pivotal to modulating the functionality of these types of systems. Recently, our group has identified the formation of supramolecular polymers from Flunixin-meglumine (MEG) adducts. Spectroscopic analyses showed that MEG is directly involved in the polymerization. Light scattering measurements showed that the material had a molecular weight of approximately 290 ± 88 MDa. The resulting supramolecular polymer underwent a reversible reaction and was stable for up to 50-60 days depending on the heat treatment.20 Similar studies in which MEG is able to induce the formation of supramolecular assemblies are not common in the literature. Only a few studies have reported the existence of oligomeric species in MEG-based compounds.21, 22 The basic characteristics of MEG makes it suitable for interacting with lipids, owing to its high acidic functionality. As MEG has a strong hydrophilic nature due to its high hydroxyl content, we hypothesized that, by controlling the proportion of MEG in a supra-amphiphile, we could manipulate the amount of bound water. To examine this rationale, we designed a synthetic approach for MEG-based supramolecular
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amphiphiles with the combined hydrophilic characteristics of amino carbohydrates and hydrophobic characteristics of modified castor oil (MACO), which has carboxylic functionalities. This approach was achieved by the insertion of additional MEG to the supraamphiphile through increasing the acidic functionalities of MACO. Furthermore, a stoichiometric excess of MEG in the MEG to MACO ratio was also evaluated.
2. EXPERIMENTAL SECTION 2.1. Materials Castor oil (CO), maleic anhydride (MA) and MEG were purchased from Sigma-Aldrich and used as received. Organic solvents were purified according to standard procedures. Ultrapure water (Millipore® system) was utilized in all experiments. All other reagents were analytical grade and obtained from Sigma-Aldrich.
2.2. Synthetic procedures for obtaining maleated castor oils Two different maleated castor oils were prepared using literature procedures with minor modifications.23 The synthetic procedures were based on two different molar ratios of MA and CO, which are denoted MACO1 (MA/CO 1:1) and MACO2 (MA/CO 2:1). For MACO1 synthesis, 1 mol of CO (938 g mol-1) was added to 1 mol of MA (98.0 g mol-1) in a three-neck round-bottom flask. The flask was fitted with a heating mantle/agitator, cold-water condenser, and nitrogen inlet tube. The reaction proceeded for six hours at 100°C under continuous stirring in a nitrogen atmosphere. The temperature was reduced to room temperature, and the reaction was maintained overnight. For MACO2 synthesis, 2 mol of MA was used.
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2.3. Acidity of CO, MACO1, and MACO2 Acid-base titrations with KOH were carried out to characterize the degree of acidity of CO and the maleated derivatives (MACO1 and MACO2). The acid value was defined as the amount (mg) of KOH needed to neutralize 1 g of oil. For this test, a suitable amount of sample was weighed and solubilized in an ethanol-diethyl ether solution (1:2 v/v). Two aqueous solutions of KOH (0.01 mol L-1 and 0.1 mol L-1) were used for the titrations of CO, MACO1, and MACO2. Titrations were performed in triplicate, and the result is an average of the obtained values.
2.4. Synthetic procedures for obtaining MEG-based supra-amphiphiles Four types of supra-amphiphiles were prepared with different amounts of MEG. Each previously synthesized maleated castor oil produced two types of supra-amphiphiles. The procedure utilized an acid-base reaction between the precursors (MACO1 or MACO2) and MEG. Based on the acid number of the precursors, the amount of MEG (195.2 g mol-1) was determined, and the stoichiometric ratio was calculated. MACO1MEG1 and MACO2MEG1 correspond to the reaction between MACO1 or MACO2, respectively, and the exact amount of MEG necessary to neutralize the maleated oil. A doubled amount of MEG was used for supraamphiphiles MACO1MEG2 and MACO2MEG2. The maleated castor oils (1.00 g) were solubilized in ethyl acetate using magnetic stirring. MEG was suspended in warm methanol and added in a dropwise manner to the maleated castor oil solution. The reaction was maintained for 12 hours at room temperature, and the product was dried under nitrogen flux.
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2.5. Preparation of the supra-amphiphile/water systems A series of systems consisting of a supra-amphiphile and water in different proportions were prepared. The compounds were accurately weighed in glass vials, and then water was added to the vials. An ultrasound bath was used to aid homogenization. The systems were kept at room temperature for 24 hours before measurement to ensure equilibration.
2.6. Characterizations 2.6.1. Infrared and NMR spectroscopy Infrared absorption spectra (Perkin-Elmer Spectrum 2000 spectrometer) were obtained from 4,000 to 400 cm-1 with 1 cm-1 spectral resolution using 32 scans and KBr pellets. 1H and
13
C
NMR spectra (Varian Inova 500 NMR spectrometer) were obtained by dissolving the samples in deuterated dimethyl sulfoxide (DMSO-d6).
2.6.2. Thermogravimetric analysis Thermal degradation was followed by thermogravimetric analysis (TA Instruments SDT 2960) over a 30-550°C temperature range using an open alumina pan as a reference and alumina sample pans under a dynamic nitrogen atmosphere (flow rate: 50 mL min-1) at a heating rate of 10°C min-1.
2.6.3. Differential scanning calorimetry
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Differential scanning calorimetry (Mettler Toledo DSC1 Stare System) equipped with a cooling apparatus was employed. DSC curves were obtained using approximately 10-12 mg samples in open aluminum pans under nitrogen atmosphere (50 mL min-1) at a heating rate of 10°C min-1. For CO, MACO1 and MACO2 analysis, the method consisted of heating from -80°C to 25°C. For analysis of supra-amphiphiles MACO1MEG1, MACO1MEG2, MACO2MEG1 and MACO2MEG2, the method consisted of a heating scan from -80°C to 140°C, followed by a cooling scan back to -80°C and then reheating until 140°C. To assess the thermo-oxidative stability of the compounds, the oxidation onset temperature method (ASTM E2009-08) was employed using approximately 3 mg samples under an oxygen atmosphere (50 mL min-1) at a heating rate of 10°C min-1 until detection of the exothermic inflection point.24 Supraamphiphile/water systems were characterized by subzero temperature differential scanning calorimetry (SZT-DSC). Approximately 5 mg samples were weighed and sealed in aluminum pans under nitrogen atmosphere (50 mL min-1). The cooling and heating rates were 5°C min-1. Phase transition temperatures in the DSC curves were determined from the measurements using the Stare Software. The crystallization temperature (Tc), melting temperature (Tm) and glass transition temperature (Tg) were defined based on the onset temperature of the event. The content of unfreezable water was obtained from the difference between the mass of the absorbed water and that of the total mass of freezable water.
2.6.4. Polarized optical microscopy Samples were prepared by placing a drop of each supra-amphiphile/water system between a coverslip and glass slide, which were then examined under polarized light. An optical Leica Microscope equipped with a digital camera was used to analyze various fields of each sample at
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room temperature. The isotropic and anisotropic behavior of the samples were recorded. Pictures were taken at a magnification of 20,000.
2.6.5. Small angle X-ray scattering The structural arrangement was analyzed by SAXS. The experiments were carried out on the D1B-SAXS1 beamline at the Laboratório Brasileiro de Luz Síncrotron (LNLS, Campinas, Brazil). A silicon-W/B4C toroidal multilayer mirror was used to monochromatize the X-ray beam (wavelength λ = 1,499 Å) and was collimated by a set of slits defining a pinhole geometry and detected on a Pilatus 300k detector. The sample-to-detector distance was 815 mm, covering a scattering vector q (q = (4π/λ)sinθ) ranging from 0.14 to 4.0 nm−1, where 2θ was the scattering angle. Measurements were performed at room temperature, and silver behenate powder, as a standard, was measured under the same conditions to calibrate the sample-to-detector distance. Transmission, dark current and mica sheet corrections were performed. The parasitic scattering produced by slits was subtracted from the total scattering intensity.
3. RESULTS AND DISCUSSION
3.1. Controlling the carboxylic functionality of maleated castor oil by stoichiometry CO is mostly composed of ricinoleic acid esters and therefore is a natural polyol. This characteristic can be explored in certain types of chemical modifications such as the maleation reaction.25, 26 So-called maleated castor oil (MACO) has numerous carboxyl functionalities and can be employed to produce several derivatives. In this work, MACO was used as the hydrophobic species in the acid-base reaction with MEG.
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The acid number for CO was 0.72 mg KOH g-1 oil, indicating low acidity, which is correlated to the absence of free fatty acids. In CO, these fatty acids mainly form triglycerides. After the maleation reaction, there was a considerable increase in the acid number to 51.46 and 99.86 mg KOH g-1 oil for MACO1 and MACO2, respectively. These data confirmed the insertion of maleic anhydride residues in the fatty acid chains. The MA/CO 1:2 stoichiometric ratio produced a larger acid number compared with the MA/CO 1:1 stoichiometric ratio due to the formation of a large number of carboxylic acid groups. To gain a structural description of the process of maleation, we conducted spectroscopic analysis (Table 1). The main structural properties of the maleated products include the presence of carboxyl functionalities, as observed in the IR spectra, and the formation of new double bonds (1H and
13
C NMR data). The increased number of carboxyl functionalities in MACO2 can be
observed by the increase in coalescing bands at 3447 cm-1 in the CH absorption region (Figure S1). The newly formed double bond is confirmed by NMR chemical shifts (Figures S2 and S3). Therefore, MACO1 and MACO2 share the structural feature of carboxyl functionalities that have been esterified to the hydroxyl units of castor oil. The difference between these two precursors is the number of carboxyl groups that can be inserted on the triglyceride molecule.
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Table 1. Structural features of the maleated products Spectroscopic Technique
Structural features of maleation reaction
IR
Band at 3447 cm-1 (hydroxyl stretch) coalescing with the CH absorption region is indicative of carboxyl group formation.
1
H NMR
Signals at δ 6.36 (H-2’) and δ 6.23 (H-3’) are assigned to the newly formed double bond.
Signals at δ 166.3 and 164.7 belong to the acid and ester 13
C NMR
carbonyl groups, respectively. Signals at δ 128.7 (C-2’) and 124.2 (C-3’) are assigned to the newly formed double bond.
3.2. Formation of the supra-amphiphiles The visual appearance of the four synthesized supra-amphiphiles was dependent on the amount of MEG used during preparation (Figure S4). As the proportion of MEG increased, the waxy material acquired a resin-like appearance. Information regarding the formation of these systems can be derived by comparing the NMR spectra of the supra-amphiphiles with those of MEG and the maleated precursors (Figure 1). The displacement of methylene hydrogen signals (NCH2CHOH) in the low-field region indicated the formation of ionic interactions between MEG and maleated precursors. The MEG 1H NMR spectrum showed these hydrogens as a doublet at δ 2.55. On the other hand, the 1H NMR spectra of the supra-amphiphiles showed
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several low-intensity multiplets in the δ 2.55-2.95 region instead, which could be related to the presence of methylenes. The ionic interaction between MACO1 or MACO2 and MEG was also observed by variations in the methynic proton chemical shifts related to the newly formed double bond. For maleated precursors, these signals were observed in the region of δ 6.23-6.36. For supra-amphiphiles, these signals were observed in the region of δ 6.23-6.70. The separation of the signals confirmed the ionization of the carboxylic acid group, in which the resonance effect deshields the methynic hydrogen in the β-position of the carboxylate group. Salt formation was, therefore, a basic step in the formation of supramolecular amphiphiles from maleated precursors and MEG.
Figure 1. The ionic interaction as observed by NMR spectroscopy. Comparison of 1H NMR spectra of (A) MEG and (B) maleated oils with the supra-amphiphiles.
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We then sought to understand how the stoichiometry could interfere with the formation of these supra-amphiphiles. A rough observation allowed us to sort the products into two distinct groups based on similarities in the
1
H chemical shifts (Figure 2). MACO1MEG1 and
MACO2MEG1 are in one group, while MACO1MEG2 and MACO2MEG2 are in another group. In the 1H NMR spectrum of MEG, a singlet at δ 2.27 was assigned to the methyl hydrogens NCH3, while in the MACO1MEG1 and MACO2MEG1 spectra, the same signal was shifted to δ 2.54 and 2.52 by ∆δ = +0.27 and ∆δ = +0.25, respectively. The same methyl protons showed a chemical shift of δ 2.45 in the spectra of supra-amphiphiles MACO1MEG2 and MACO2MEG2, giving ∆δ = +0.19. This variation is indicative of the secondary amine group ionization of MEG by ionic interactions with the carboxylic acid groups of the maleated precursors. The magnitude of this change appears to be related to the amount of MEG, which was used in the formation of the supra-amphiphiles. A reduced amount of MEG led to a greater variation, indicating more pronounced ionic interactions in this scenario. This likely occurs as a result of the disruption of the MEG-MACO ionic bond by the formation of MEG-MEG H-bonds in the excess of MEG.
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Figure 2. Expected supra-amphiphiles structures drawed on the basis of NMR results. The degree of carboxyl functionality differentiates the hydrophobic part between MACO1 and MACO2 derivatives. The amount of MEG linked to each carboxyl group differentiates the hydrophilic part between supramolecules prepared in two mol ratios.
3.3. Thermal behavior DSC curves of CO showed a glass transition temperature (Tg) in the region of -60°C to -50°C. MACO1 and MACO2 showed a Tg in the same region, with a slight increase in temperature as the maleation degree increases (Figure S5). This result suggests enhanced interactions between the triglyceride molecules, possibly due to hydrogen bonds between the carboxylic groups inserted in the chains. MEG-based supra-amphiphiles showed a characteristic amorphous profile.
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The Tg occurred between -40°C to 25°C in both the first and second heating scan (Figure 3). For all samples, the Tg shifted to higher temperatures in the second heating scan, which likely occurred as a result of the formation of crosslinks between the triglyceride chains induced by heat treatment.
Figure 3. DSC curves of supra-amphiphiles showing the effect of heating on the glass transition temperature.
The thermo-oxidative stability of CO decreased significantly during the maleation process (Table 2). This result is expected due to the insertion of a new double bond in the triglyceride molecules, which is a focus of oxidation process. Supra-amphiphiles MACO1MEG1 and MACO2MEG1 showed a slight increase in thermo-oxidative stability. However, this stability was lost when a doubled amount of MEG was added to MACO1MEG2 and MACO2MEG2. The reason for these results are unclear but may be due to possible microstructural differences in the supra-amphiphiles playing a role in oxygen diffusion in the matrix, leading to distinct thermooxidation profiles. The TGA of CO, maleated oils and supra-amphiphiles showed three to four main decomposition events (Figure S6). The events occurred in consecutive order and demonstrated a
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complex mechanism. MACO1 and MACO2 were less thermally stable than CO. Maleated castor oils are carboxylated derivatives, which undergo decarboxylation at high temperature. MEGbased supra-amphiphiles have greater thermal stability than their respective maleated precursors. Most likely, this increased stability is related to the formation of carboxylates, which are more thermostable than the original acids. Furthermore, the amount of MEG that was used in the formation of supra-amphiphiles positively affects the thermal stability of the products, as shown by the onset temperature increase for MACO1MEG2 and MACO2MEG2.
Table 2. Thermal profiles of CO, maleated precursors and MEG-based supra-amphiphiles
DSC PROFILE
Sample
CO
MACO1
MACO1MEG1
MACO1MEG2
TG PROFILE
1st Heating
2nd Heating
OOT
Decomposition
Glass
Glass
(°C)
Temperature
Transitions
Transitions
(°C)
(°C)
-
-
-65.6/-29.0
-58.7/-27.6
-
-
-60.2/-17.5
-57.6/-13.1
Start Temperature (°C)/Final temperature (°C) Mass Loss (%)
(°C)
201.2
174.6
200.2
176.6
364.6
203.9
83.2
318.1
1st Step
2nd Step
3rd Step
4th Step
280.0/426.9
426.9/462.3/
462.3/478.1/
-
82.2
15.9
112.0/299.1
299.1/407.5
407.5/477.6
-
8.1
55.4
35.2
64.8/175.0
175.0/255.6
255.6/403.3
403.3/493.6
7.7
5.4
53.7
31.1
254.0/371.8
371.8/410.4
410.4/456.2
456.2/486.7
50.9
23.0
12.3
1.7
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MACO2
MACO2MEG1
MACO2MEG2
-
-52.8/-21.3
-26.0
-
-50.2/-13.6
-13.5
173.6
188.3
172.6
165.8
135.6
230.3
73.8/286.4
286.4/391.6
391.6/483.1
-
13.6
40.5
46.2
166.6/221.5
221.5/392.0
392.0/453.2
453.2/498.2
5.3
58.5
23.3
4.0
96.6/261.0
261.0/353.6
353.6/495.0
-
10.3
40.3
41.5
3.4. Water behavior in supra-amphiphile/water systems The structural organization of water molecules in self-assembled systems has been demonstrated to impact the physicochemical and biological properties of materials.17,
27
The
confinement of water to interfaces and nanocompartments gives distinct thermal profiles compared with bulk water. To categorize the types of water present in the systems, we analyzed several typical thermal profiles. Free water (or water with bulk-like behavior) has a sharp crystallization peak that occurs below 0°C due to the supercooling effect. During heating, this type of water melts at 0°C. Freezable bound water crystallizes and melts at temperatures below 0°C. The peak of freezable bound water is usually broader than that of free water. Finally, unfreezable bound water does not show a DSC peak. DSC cooling curves of the systems with low water content (Wc = 10% for MACO1MEG1 and MACO2MEG1 and Wc = 10 and 20% for MACO1MEG2 and MACO2MEG2) did not show first-order thermodynamic phase transitions. This is representative of unfreezable bound water. In these compositions, the water molecules interact strongly with the hydrophilic part of the
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supra-amphiphile (Figure 4). As the water content increases, the fraction able to undergo freezing also increases. At Wc = 20 and 30% for MACO1MEG1 and MACO2MEG1 and Wc = 30% for MACO1MEG2 and MACO2MEG2, the systems contained freezing bound water. The presence of this type of water indicates that the hydrophilic part of the supra-amphiphiles was saturated by a first hydration shell, allowing for the crystallization of excess of water molecules. Structurally, the so-called freezable bound water contains a second layer of water molecules that interact with the hydrophilic surface, and thus the freezing process occurs differently than with free water.28 This increased interaction imposes difficulties on the diffusion of water molecules, and, therefore, freezing takes more time to occur. The existence of free water was observed in supra-amphiphiles/water systems of Wc = 40 and 50% and above (data not shown). The sharp peak is indicative of bulk-like behavior. Moreover, it is important to note that the DSC cooling curves showed a supercooling effect for these compositions, which represents the presence of water molecules in the liquid state below 0°C.
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Figure 4. DSC cooling curves of MEG-based supra-amphiphiles/water systems. The DSC heating curves from the previously cooled samples confirm the behavior highlighted in the cooling curves (Figure 5). There were no events recorded on systems with low water content. As the water content increased, a peak related to melting appeared. However, it was not possible to observe individual peaks for each type of water, as was observed in the cooling
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curves. The melting point depression was, therefore, uniquely indicative of the existence of freezable bound water.
Figure 5. DSC heating curves of MEG-based supra-amphiphiles/water systems.
When the melting temperature (Tm) and bound water is plotted against water content, we can better understand the differences among the supra-amphiphiles (Figure 6). MACO1MEG1/water
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and MACO2MEG2/water systems represent two extremes in behavior. The excess MEG in MACO2MEG2 contributed to a higher quantity of water being restrained in the systems. This evidence is clearly demonstrated by the lower Tm and higher percent of bound water observed in MACO2MEG2. MACO1MEG2 and MACO2MEG1 showed intermediate behavior until Wc = 30%. Bound water, both freezable and unfreezable, play important roles in biointerfacial interactions. For instance, the presence of unfreezable bound water in proteins and biomembranes is involved in the functional aspects of biochemical processes. In polymer systems, the presence of freezable bound water is correlated with biocompatibility.14 Therefore, it is necessary to gain basic knowledge for manipulating these categories of water for the design of new biomaterials.
Figure 6. Relation between water content and (a) Tm and (b) bound water.
3.5. Structural characterization of the supra-amphiphile/water systems Supra-amphiphiles/water systems with 10, 20 and 30% water showed a gel-like appearance and an increased content of bound water (freezable and non-freezable). To gain a structural description and characterize the possible lyotropic behavior of these systems, polarized
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optical microscopy (POM) studies were performed. Isotropic and lamellar phases were observed (Figure 7). MACO1MEG1 and MACO2MEG2 presented as dark fields at all water compositions, indicating isotropic behavior. Similar behavior was observed for MACO1MEG2 with Wc = 30%. These results suggest the predominance of micellar phases in the mixture. In contrast, at different water contents, MACO1MEG2 showed characteristic textures of the lamellar phase, as did the supra-amphiphile gels of MACO2MEG1, as shown by the presence of Maltese crosses.
Figure 7. Representative POM optical textures for the isotropic (dark field) and lamellar (Maltese crosses) phases of supra-amphiphiles/water systems.
SAXS studies were performed to confirm the phases observed by POM (Figure 8). The SAXS curves of MACO1MEG1 and MACO2MEG2 presented similar profiles to that of MACO1MEG2 with 30% water content, showing a broad peak characteristic of the isotropic phase, which is typical of micellar systems and correlates with the dark field image observed by POM. It is worth noting that an intense peak was present at a q-value of 1.31 nm-1 for
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MACO2MEG2 with 10% water content, indicating a minor contribution from a structural phase that was not observed by POM.
Figure 8. Scattering intensity versus scattering vector q for supra-amphiphiles/water systems in different proportions. (a) Supra-amphiphiles based on a 1:1 MACO/MEG stoichiometric ratio.
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(b) Supra-amphiphiles based on a 1:2 MACO/MEG stoichiometric ratio. (c) Binary phase diagram of supra-amphiphiles/water systems in the concentrated regime. Of all the compositions, MACO1MEG2 (10 and 20% water content) and MACO2MEG1 (10, 20 and 30% water content) show the presence of lamellar phases by the relative peak positions, which are 1:2:3. As shown in Figure 8, MACO1MEG2 presents in the lamellar phase, as evidenced by the presence of two sharp SAXS peaks with relative positions of 1:2. To calculate the correlation distance between the scattering objects and consequently the lattice parameter, we used the equation d =2π/qmax, where qmax is the scattering vector of the first order peak. From analysis of the SAXS curves, we note that the lattice parameter increased from 5.66 nm to 5.81 nm as the water content increased from 10% to 20%, likely due to the swelling effect of water. Upon increasing the water content to 30%, a broad SAXS peak was observed, indicating that the binary mixture was converted to the isotropic phase due to the formation of micelles. The same behavior was observed for MACO2MEG1, which presented in the lamellar phase, where the lattice parameter increased from 5.41 nm to 7.22 nm as the water content increased from 10% to 30%. A swelling effect was also observed; however, the presence of a broad peak with a q-value centered at 0.85 nm-1 indicated a phase transition. These results confirm those obtained by POM and show that the addition of water leads to liquid crystalline structures depending on the composition. Increasing the water content leads to isotropic systems. The transformation of the supra-amphiphiles into micellar aggregates was also indicated by the turbidity of the solutions.
4. CONCLUSIONS
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Ionic interactions between meglumine (MEG) and maleated castor oils (MACO) were employed to produce supra-amphiphiles. The properties of the compounds were dependent on the amount of MEG used in fabrication, as evidenced by spectroscopic and thermal analysis. The self-assembly of supra-amphiphiles in water led to isotropic and lyotropic liquid crystalline phases with different types of water arrangements depending on the composition and quantity of MEG present in the supra-amphiphiles. An excess of MEG produced systems with larger quantities of bound water. In conclusion, this study demonstrates that we can manipulate the type of water in supra-amphiphile systems by adjusting the proportion of hydrophilic and hydrophobic building blocks. This finding opens a new avenue for examining the role of water behavior on biointerfacial interactions in supra-amphiphile-based gels. Research in this direction is important as materials scientists are aligning efforts to drive the implementation of a new paradigm centered on the functional design of nanoarchitectonics. Therefore, by recognizing that the type of water is an important element in different biomaterials, such as gels, we can pursue methods to control the water type using simple synthetic procedures.
ASSOCIATED CONTENT Supporting Information. Spectroscopic and thermal characterization data of supra-amphiphiles. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(
[email protected], +55 16 33019607).
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Author Contributions The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT This work was financially supported by grant #2013/08411-0 of the São Paulo Research Foundation (FAPESP). We acknowledge the LNLS (Campinas, SP, Brazil) staff for use of their SAXS facilities.
ABBREVIATIONS CO, castor oil; FTIR, Fourier-transform infrared spectroscopy; MA, maleic anhydride; MACO, maleated castor oil; MEG, meglumine; NMR, nuclear magnetic resonance spectroscopy; TGA, thermogravimetric analysis; DSC, differential scanning calorimetry
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Graphical Abstract
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