Article pubs.acs.org/Langmuir
Spontaneous Transformation of Lamellar Structures from Simple to More Complex States Yingying Dou, Panfeng Long, Shuli Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry and Key Laboratory of Special Aggregated Materials, Ministry of Education, Shandong University, Jinan, Shandong 250100, People’s Republic of China S Supporting Information *
ABSTRACT: Spontaneous transformation of lamellar structures, such as multilamellar vesicles from micelles or unilamellar vesicles, is an important challenge in the field of amphiphile molecules, which may serve as models to understand biologically relevant bilayer membranes. Herein, we report a progressive self-assembly progress of Ntetradecyllactobionamide (C14G2) and tetraethylene glycol monododecyl ether (C12EO4) mixtures in aqueous solution. Increasing temperature or surfactant compositions causes spontaneous transformation from simple to high-level aggregates, i.e., from unilamellar vesicles, to coexisting multilamellar vesicles, terraced planar bilayers, and finally terraced planar bilayers. Deuterium nuclear magnetic resonance (2H NMR), freezefracture transmission electron microscopy (FF-TEM), and small-angle X-ray scattering (SAXS) measurements clearly demonstrate the spontaneously progressive self-assembly process. The interlamellar spacing (d) of the bilayers decreases from unilamellar vesicles to the terraced planar bilayers with an increase of the temperature or surfactant compositions. Lamellar samples consisting of terraced planar bilayers at higher temperature still show viscoelastic properties, being Bingham fluids, and both the viscoelasticity and yield stress increase with the composition and decrease with the temperature. The spontaneous transformation of the progressive self-assembly progress of C14G2 and C12EO4 aqueous mixtures is due to a balance of three driving forces, hydrophobic interactions, hydrogen bonding, and steric effects.
1. INTRODUCTION Amphiphiles, especially surfactants, in water can self-assemble into various structures, for example, the self-assembly of alkyl polyglycoside (APG) into micelle, hexagonal phase, cubic phase, lamellar phase, etc.1−5 When mixing another non-ionic surfactant, polyether, with APG, their phase behaviors will be more complex. For example, for the mixing system of ndodecyl-β-D-maltoside (β-C12G2) and hexaoxyethylene dodecyl ether (C12EO6),2,3 their phase behaviors have a middle dependence upon the temperature rather than no dependence as the β-C12G2/water system or a large dependence as the C12EO6/water system. Alkylmaltobionamides (CnG2), considered as a type of surfactant, belonging to the “sugar−lipid hybrid” class,6,7 can be obtained from the combination of alkyl amines and oxidized glycosides in anhydrous ethanol.6−8 CnG2 can also be obtained from lactobionic acid, giving rise to alkyllactobionamide (ALBA)9−12 or lactonoalkylamide (LacCn) nomenclature.13,14 With an increase of the alkyl carbon (n), the critical micelle concentration (cmc) and the solubility of CnG2 in water at room temperature decrease.7 CnG2 with n = 8−12 have been suitably studied at room temperature;6−11,15−17 when n is larger than 12, they are insoluble in water.15 A number of CnG2 with n > 12, e.g., C16G2,6,7 have been studied at 40 and 50 °C, but mixtures of CnG2 with n > 12 and commercial surfactants have not been reported. Tetraethylene glycol monododecyl ether (C12EO4), a kind of polyether non-ionic surfactant, has © 2013 American Chemical Society
abundant phase behaviors in water with concentration and temperature, and there is a large Lα area of its phase diagram.18 When mixed with other surfactants, such as anionic surfactant dialkyl anionic sodium bis(2-ethyl hexyl)sulfosuccinate (AOT), their phase behaviors are changed but also abundant.19 In this paper, we mixed a CnG2 with n > 12, C14G2, with C12EO4 to study the spontaneous transformation of lamellar structures from simple to high level. The detailed data of C14G2, which include electrospray ionization mass spectrometry (ESI−MS), 1 H nuclear magnetic resonance (1H NMR), and Fourier transform infared spectroscopy (FTIR), were shown in Figure S1 of the Supporting Information. The chemical structures of C14G2 and C12EO4 are shown in Figure 1. C14G2 is slightly soluble in water at room temperature but easily soluble in C12EO4 micelle solution. Both C14G2 and C12EO4 are nonionic, biocompatible, biodegradable, nontoxic, and nonirritative surfactants,20 widely used in medical device, cosmetic, food, and pharmaceutical fields.21−26 Phase transition from simple structures to complex structures achieved through a simple temperature change or compositional variation of the non-ionic surfactant system is useful for understanding biological compartments, cells, organisms, and perhaps, even a biogenesis, for example, the structure of the molecular bilayers of the vesicle as a good mimic of the cell membrane,27 the Received: May 2, 2013 Published: September 26, 2013 12901
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Figure 1. Chemical structures and CPK models of (a) tetraethylene glycol monododecyl ether (C12EO4) and (b) N-tetradecyllactobionamide (C14G2). Gray globe, carbon atom; red globe, oxygen atom; blue globe, nitrogen atom; and white globe, hydrogen atom. The CPK models are obtained from MaterialStudio software.
Both phases of vesicles (Lαv) and planar lamellar phase (Lαl) are collectively known as Lα phases, which are commonly used in the literature. In this paper, we present a comprehensive study on the phase behavior of C14G2 and C12EO4 mixtures in water. The phase diagram of the mixtures in water has been constructed on the basis of the extensive crossed polarizer, NMR, and small-angle X-ray scattering (SAXS) experiments. Freeze-fracture transmission electron microscopy (FF-TEM) observations have also been employed to visualize the different phase structures.
application of non-ionic surfactant-based vesicles in drug delivery,28 the study of glycoside in the targeting of cell membrane surfaces, etc.29,30 Deuterium (2H) NMR measurements were performed to determine the structures, including vesicles, terraced planar bilayers, etc., in each phase during the progressive self-assembly process. The degree of deuterium nuclei (spin I = 1) quadrupole splitting reflects the hydration of surfactant aggregates, which is the fraction of water molecules oriented by the aggregation surfaces and their average degree of orientation.31,32 The relation between the degree of deuteron quadrupole splitting, Δ(2H), and hydration of surfactant aggregates is obtained by eq 1 below33 Δ(2 H) = pvQ S = nvQ S(XS/XW )
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Tetraethylene glycol monododecyl ether (C12EO4, >99%) was purchased from Acros Organics (Morris Plains, NJ). N-tetradecyllactobionamide (C14G2, high purity) was a gift provided by Heinz Hoffmann’s lab, Bayreuth University, Germany, and it was proven to be high-purity by ESI−MS (QTOF6510), 1H NMR [Bruker Avance 400 spectrometer, 400 MHz, [D6]DMSO, 25 °C, tetramethylsilane (TMS)] and FTIR (VERTEX 70/70v). Heavy water (D2O, ≥99.9%) was purchased from Aldrich. These chemicals above were all directly used without further purification. The water used in the experiments was prepared by a UPHW-III-90T-type apparatus, with a resistivity of 18.25 MΩ cm. 2.2. Phase Behaviors. The binary phase diagram was obtained by varying the C14G2 concentration at different C12EO4 concentrations with more than 200 sample solutions (2.0 mL). The samples were all prepared by centrifugation for 3 days at 40 °C and then being stirred everyday for about 2 weeks. Then, the samples were stored at the 25 °C incubator for another 2 weeks. The phase identification was accomplished by two polarizers. 2.3. 2H NMR Measurements. 2H NMR measurements were carried on a Bruker Avance 400 spectrometer equipped with a pulsedfield gradient module (z axis). Representative 30 sample solutions (1.0 mL) in heavy water (D2O) according to the phase diagram were chosen to be prepared and transferred to NMR sample tubes. For measurements higher than 25 °C, samples were kept in a water bath with an increase of 5 °C at 2 week intervals. 2.4. FF-TEM Observations. Approximately 4.0 μL aliquots of the viscous sample solutions were dropped onto the specimen carrier and frozen quickly in liquid ethane at −175 °C. After frozen, the sample was fractured and replicated by Leica EM BAF 060 equipment at −175 °C. For the replication, the Pt/C (45°) film was sprayed for 2 nm and the C (90°) film was sprayed for 20 nm. The replica was loaded on a copper grid and observed using a JEOL JEM-1400 electron microscope operating at 120 kV. 2.5. SAXS Measurements. Lamellar samples were characterized by a HMBG-SAX SAXS system (Hecus, Austria), with a tube voltage of 50 kV and a tube current of 40 mA. All samples were swept by SAXS for 600 s with Ni-filtered Cu Kα radiation (0.154 nm). The sample−detector distance was 27.8 cm. The channel width was 54.0 μm, and the center of the primary beam was at channel number 234 of 1024. A KPR system was used to keep samples at an elevated temperature (40 and 55 °C).
(1)
where p is the fraction of “bound” water molecule, vQ is the quadrupole splitting constant, n is the average hydration number of the surfactant, XS and XW are the mole fractions of surfactant and water, respectively, and S is the order parameter of the bound water molecule. Equation 1 applies to the conditions where (i) there is only “free” water or “bound” water, (ii) exchange between free and bound water is very fast with respect to the 2H NMR relation time, and (iii) the ordering of free water molecules is negligible. When the surfactant aggregates are an anisotropic phase, e.g., terraced planar bilayers, the deuteron quadrupole splitting average is not zero and 2H NMR peaks are divided into two peaks.34,35 If the surfactant aggregates are in an isotropic state, such as micellar solution, the deuteron quadrupole splitting average is zero, giving a single 2H NMR peak.36 Further, apart from the coexistence of different isotropic aggregates, the peaks of different aggregates are superimposed. For two different anisotropic aggregates, one can observe four peaks, i.e., two groups of split signals.37 If anisotropic and isotropic aggregates exist, three peaks are observed, i.e., a single peak for the isotropic phase and a pair of peaks for the anisotropic phase.38 However, if two different isotropic aggregates coexist, the result is one single peak, which does not provide evidence of two aggregates.33 For self-assembled structures of surfactants in water, surfactant lamellar structures, i.e., Lα phases, in solutions could be considered as the typical biomembrane mimetic systems. Many results on the formation of lamellar phases have been reported each year in typical model systems of cationic and anionic (catanionic) mixtures in solutions. However, no reports on the mixtures of C12EO4 with a alkyllactobionamide C14G2 have studied the phase behavior, the self-assembled structures, and properties, which could be due to the difficult preparation and little solubility of C14G2 in water. Basically, 12902
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2.6. Rheological Measurements. The oscillatory shear and steady shear of samples with high viscosity were determined on a HAAKE RheoStress 6000 rheometer with a cone−plate system (C35/ 1° Ti L07 116). Before oscillatory shear measurements ( f = 0.01−10 Hz), a stress sweep was used to obtain a linear viscoelastic region at f = 1 Hz. For steady shear, the viscosity was obtained using the CR mode and the stress yield was obtained from the CS mode, with a check gradient of 0.5 (Δτ/τ)/Δt % and a maximum waiting time of 30.00 s. A constant temperature (T = 25.0 ± 0.1 °C) was kept using a circulator HAAKE DC10 cyclic water bath (Karlsruhe, Germany).
change in phase structures from a micelle phase (L1) to a vesicle phase (Lαv)39,40 and finally to coexisting multilamellar vesicles and terraced planar bilayers (Lαv/Lαl) (Figure 3a). In contrast, at constant C12EO4 (20 wt %), the increase of the C14G2 concentration shows no change in the phase structure (Figure 3b). At a higher C12EO4 concentration (30 wt %), an increase of the C14G2 concentration leads to 2H NMR quadrupole splitting, demonstrating a phase structural transition from vesicles (Lαv) to coexisting vesicles and terraced planar bilayers (Lαv/Lαl) (Figure 3c). These observations indicate a progressive phase structural transition of aqueous C14G2/C12EO4 mixtures dependent upon the C12EO4 concentration but hardly affected by an increasing C14G2 concentration. FF-TEM observations clearly demonstrate the spontaneous transformation of progressive self-assembly of aqueous C14G2 and C12EO4 mixtures from simple to high-level aggregates achieved via composition. At 25 °C, unilamellar vesicles with diameters of about 50 nm are observed in the sample of 20 wt % C12EO4 and 1.0 wt % C14G2 (Figure 4a). Increasing the composition to 30 wt % C12EO4 results in the formation of multilamellar vesicles (Figure 4b). Finally, at 40 wt % C12EO4, coexisting multilamellar vesicles (inset of Figure 4c) and terraced planar bilayers are observed (Figure 4c). 3.2. Phase Diagrams of C14G2/C12EO4/H2O Mixtures with an Increase of the Temperature. With an increase of the temperature, C14G2/C12EO4/H2O mixtures present remarkable phase behaviors. 2H NMR spectra show the complex phase transitions from isotropic phases to vesicle, to the mixture of vesicle and terraced planar bilayers, to terraced planar bilayers, and then to isotropic phases (1.0 wt % C14G2/ 10 wt % C12EO4; Figure 5b). When the C12EO4 concentration is too low or too high, the phase transition becomes simple, for example, from micelle to the mixture of micelle and terraced planar bilayers (1.0 wt % C14G2/5.0 wt % C12EO4; Figure 5a) or from the mixture of vesicle and terraced planar bilayers to terraced planar bilayers (1.0 wt % C14G2/50 wt % C12EO4; Figure 5e). This means that, only at a proper concentration range, there are abundant phase behaviors with an increase of the temperature. A phase diagram for the system (Figure 6a) was constructed using 2H NMR measurements at cC14G2 = 1.0 wt % with varied C12EO4 concentrations and temperatures (Figure 5 and Figure S2 of the Supporting Information). The resulting phase diagram indicates features of the structural changes in the
3. RESULTS AND DISCUSSION 3.1. Determination of the Phase Transition Induced by the Surfactant Concentration at Room Temperature. When the C12EO4 concentration is low, its micelle could dissolve little C14G2; therefore, only micelles or their mixture is formed (Figure 2). For the limited ability of the C12EO4 micelle
Figure 2. Phase diagram with varied C12EO4 and C14G2 concentrations at room temperature (T = 25.0 ± 0.1 °C). L1, micelle; L1/P, mixture of the micelle and precipitate; L1/L1′, mixture of two micelles; and Lα, lamellar phase.
to dissolve C14G2 molecules, a mixture of micelle and precipitate is obtained at a high C14G2 concentration and a low C12EO4 concentration. With an increase of the C12EO4 concentration, lamellar phases are observed by crossed polarizers with birefringent textures. To distinguish the lamellar phases clearly, 2H NMR, FF-TEM, SAXS, and rheological measurements were performed. From 2H NMR, at room temperature and constant C14G2 (1.0 wt %), an increase of the C12EO4 concentration leads to a
Figure 3. 2H NMR spectra of C14G2/C12EO4 mixtures with different surfactant concentrations; T = 25.0 ± 0.1 °C: (a) C14G2 at 1.0 wt % with an increase in the C12EO4 concentration and (b and c) constant C12EO4 at 20 or 30 wt %, respectively, with an increase in the C14G2 concentration. The observed shoulders (arrows) are small splitting peaks. 12903
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Figure 4. FF-TEM images of the 1.0 wt % C14G2/C12EO4 system at 25 °C with varied C12EO4: (a) 20 wt %, (b) 30 wt %, and (c) 40 wt % C12EO4. The observed structures are (a) unilamellar vesicles, (b) multilamellar vesicles, and (c) coexisting (inset) multilamellar vesicles and terraced planar bilayers.
concentration of C12EO4 (5.0−40 wt %; see Figure S2 of the Supporting Information). At T > 60 °C, the terraced planar bilayers are not observed. The phase diagram can be corrected to the 2H NMR spectra via the observed peaks for sample solutions, their broadening and splitting patterns. There are four peaks for sample solutions of 50 wt % C12EO4 and 1.0 wt % C14G2 mixtures at 55 and 60 °C (arrows in Figure 5e). The high surfactant concentration of the C12EO4 and C14G2 mixtures may possess a high enough viscosity to slow the movement of surfactant molecules. As we have well-known, the normal angles of the planar bilayer lamellae and the magnetic field cover 0−90°. In the magnetic field, the terraced planar bilayers tend to be parallel to the magnetic field. However, because of the high enough viscosity, the surfactant bilayers could not be parallel to the magnetic field totally. As a consequence, the spare splitting peaks of surfactant bilayers are perpendicular to the magnetic field. The spare splitting peaks should be inconspicuous because of mainly parallel orientation of the surfactant bilayers to the magnetic field. Phase transition temperatures may be obtained from the phase diagram. With an increase of the C12EO4 concentration, the phase transition temperature from Lαv phase to Lαv/Lαl phase decreases at a low C12EO4 concentration, but from the Lαv/Lαl phase to Lαl phase, the phase transition temperature first decreases and then increases after 30 wt % (Figure 6a). That is because, at high C12EO4 concentrations (>30 wt %), the sample viscosity increases and the slower movement of the surfactant molecules results in higher phase transition temperatures. However, C12EO4 consists of a flexible component that favors the formation of the terraced planar bilayers at a high C12EO4 concentration. As a consequence, their competition results in the variation of the phase transition temperature. The phase diagram at constant C12EO4 (20 wt %), shown in Figure 6b, obtained from Figure S3 of the Supporting Information, is simpler with an increase of the temperature, resulting in the conversion of the Lαv phase into a Lαv/Lαl phase, followed by a Lαl phase, and finally an isotropic phase. The phase transition temperatures increase with the C14G2 concentration, indicating that C14G2 is detrimental to the phase structural transitions. This is due to the stiffness of the C14G2 molecule and its larger headgroup. As a result, increasing C14G2 concentration is not favorable for the formation of the Lαl phase and can be explained by the packing parameter, P (P = v/al, where v is the volume of the hydrocarbon part of the surfactant, a is the mean cross-sectional headgroup surfactant area, and l is the alkyl chain length). With an increase of C14G2, P decreases, thereby reducing the propensity of the phase structural
Figure 5. Temperature-dependent 2H NMR spectra of samples with constant cC14G2 = 1.0 wt % and varied C12EO4: cC12EO4 = (a) 5 wt %, (b) 10 wt %, (c) 20 wt %, (d) 30 wt %, and (e) 50 wt %. The indicated shoulders (arrows) are the splitting peaks because of perpendicular orientation to the magnetic field41 or appearance of a new phase.
phase sequence with an increase of the temperature. First, when the C12EO4 concentration is low (45 wt %), only a Lαv/Lαl to Lαl phase transition was observed. More significant, phase separation induced by a temperature above 60 °C was only observed at an intermediate 12904
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Figure 6. Phase diagrams for the C14G2/C12EO4 system as a function of the temperature determined by 2H NMR data: (a) at cC14G2 = 1.0 wt % with varied C12EO4 and (b) at cC12EO4 = 20 wt % with varied C14G2.
3.3. SAXS and Rheological Measurements of Lamellar Phases. The interlamellar spacing (d) of the bilayers in the lamellar phases (Lαv and Lαl) was determined from SAXS spectra (panels a, c, and e of Figure 8) and FF-TEM images (see Figure S4 of the Supporting Information). The profile of SAXS data for 30 wt % C12EO4 and 1.0 wt % C14G2 in water at 25 °C is shown in Figure 8a; a ratio of q1/q2 = 1:2 indicates that the sample is the lamellar phase containing large vesicles with diameters of about 500 nm (Figure 4b). A calculated interlayer distance (d = 10.50 nm) from FF-TEM observations (see Figure S4a of the Supporting Information) is in agreement with the SAXS data, where d (=2π/qmax) was determined to be around 10.54 nm (Figure 8a). The d values of two further samples, 40 wt % C12EO4/1.0 wt % C14G2 (25 °C; Figure 8c and Figure S4b of the Supporting Information) and 30 wt % C12EO4/1.0 wt % C14G2 (55 °C; Figure 8e and Figure S4c of the Supporting Information), were determined to be 7.56 and 6.56 nm, respectively. The obtained d values suggest that, with an increase of the surfactant concentration or temperature, the interlamellar spacing of surfactant bilayers is reduced. An an increase of temperature could cause flexibility in the surfactant molecule chains, allowing them to be more easily intertwined, thus reducing the d value. The rheograms of the 1.0 wt % C14G2/30 wt % C12EO4 sample at 25 °C are shown in Figure 8b, displaying typical characteristics for vesicle systems.37,38,42 These vesicle systems are similar to viscoelastic fluids behaving like a Bingham fluid with comparable yield stress values (see Figure S5a of the Supporting Information). The storage modulus (G′) of the mixtures remains more or less constant at 250 Pa over the total investigated frequency range. The loss modulus (G″) decreases somewhat with increasing frequency but keeps a constant value around 45 Pa. The complex viscosities (|η*|) linearly decrease over the whole frequency range with a slope of −1. It is a gel sample of 1.0 wt % C14G2/30 wt % C12EO4 at 25 °C, and its yield stress is 5.90 Pa (see Figure S5a of the Supporting Information). In comparison to other oscillatory shears and yield stresses of 1.0 wt % C14G2/40 wt % C12EO4 (25 °C; Figure 8d and Figure S5b of the Supporting Information) and 1.0 wt % C14G2/30 wt % C12EO4 (55 °C; Figure 8f and Figure S5c of the Supporting Information), one could observe that, with an increase of the C12EO4 concentration, the storage modulus (G′), the loss modulus (G″), and the yield stress (τy) all increase, while when the temperature increases, they all decrease. The reason may be that the increase of surfactant molecules is beneficial for the higher viscosity and tighter
transition. In addition, at a low C14G2 concentration and high temperature, the sample phase separated, resulting in a single sharp NMR peak (panels b−d of Figure 5). Under these conditions, the temperature is high enough to break the hydrogen bonds among the C12EO4 and C14G2 molecules; thus, the structures disaggregate, and phase separation occurs. At a high C14G2 concentration, the additional hydrogen bonds require more energy for the destruction, requiring a higher temperature for the phase structural transition. The progressive self-assembly from simple to high-level aggregates can also be achieved by increasing the temperature of the 1.0 wt % C14G2/20 wt % C12EO4 sample observed by FFTEM micrographs (panels a and b of Figure 7). In this mixture,
Figure 7. FF-TEM images of the (a and b) 1.0 wt % C14G2/20 wt % C12EO4 sample and (c and d) 1.0 wt % C14G2/30 wt % C12EO4 sample at different temperatures, with multilamellar vesicles and terraced planar bilayers at 50 °C (a and c) but only the latter form at 55 °C (b and d).
unilamellar vesicles (Figure 4a) are converted to coexisting multilamellar vesicles and terraced planar bilayers at 50 °C (Figure 7a). A further temperature increase resulted in only the terraced planar bilayers at 55 °C (Figure 7b). Comparable FFTEM images of 1.0 wt % C14G2/30 wt % C12EO4 mixtures at 25, 50, and 55 °C also show the same spontaneous transformation of the progressive self-assembly progress (Figure 4b and panels c and d of Figure 7). The above FFTEM observations coincide with the 2H NMR data in panels c and d of Figure 5. 12905
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Figure 8. (a, c, and e) SAXS profile and (b, d, and f) rheogram of the oscillatory shear for samples of 1.0 wt % C14G2/30 wt % C12EO4 at 25 °C (a and b) and 55 °C (e and f) and 1.0 wt % C14G2/40 wt % C12EO4 at 25 °C (c and d).
related to the smaller headgroup and more flexible tail of C12EO4 molecules compared to those of C14G2 molecules. As a result, at an appropriate C12EO4 concentration, the bilayer membranes easily bend into vesicles because of the large steric hindrance of the C14G2 headgroups, while the packing parameter is in the region of 0.5−1. With an increase of the C12EO4 concentration, this steric hindrance decreases and the bilayer membranes become flexible in the system. When the concentration of C12EO4, which has a smaller headgroup, is large enough, the mean cross-sectional headgroup surfactant area (a) is much smaller to cause the packing parameter (P) to be nearly 1. As a consequence, multilamellar vesicles transfer into terraced planar bilayers. The formation of the driving forces of vesicles or terraced planar bilayers is a balance of the hydrogen bonds between C12EO4 molecules and C14G2 molecules, the hydrogen bonds of C12EO4 molecules or C14G2 molecules with water molecules, the hydrophobic effect
packing of aggregates, which leads to the arising of G′, G″, and τy, while the temperature breaks the hydrations of surfactant molecules with water molecules and makes molecules move fast, resulting in the decrease of viscosity; thus, the values of G′, G″, and τy decrease. 3.4. Scheme and Mechanism of the Phase Transitions Induced by the Surfactant Concentration or Temperature. The spontaneous phase structural transitions from unilamellar vesicles to multilamellar vesicles to coexisting multilamellar vesicles and terraced planar bilayers and finally to terraced planar bilayers are illustrated in Scheme 1. The transitions are controlled by an increasing surfactant concentration or temperature, and the scheme satisfies all of the experimental observations for the progressive self-assembly from simple to high-level aggregates. With an increase of the C12EO4 concentration at room temperature (25 °C), a spontaneous phase structural transition occurs, which may be 12906
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4. CONCLUSION In conclusion, in the C14G2/C12EO4/H2O system, the spontaneous progressive self-assembly of various structures may be controlled by increasing either the surfactant concentration or temperature. The phases were observed to be transited from simple aggregates, such as micelle and unilamellar vesicle, to complex aggregates, such as multilamellar vesicle, the mixture of vesicle and terraced planar bilayers, and terraced planar bilayers with an increase of the surfactant concentration or temperature. The interlamellar spacing of the vesicles and the terraced planar bilayers can be rationalized in the observed structures of the spontaneous self-assembly process, which decreases with composition or temperature. Phases with vesicles (1.0 wt % C14G2/30 wt % C12EO4 at 25 °C), terraced planar bilayers (1.0 wt % C14G2/30 wt % C12EO4 at 55 °C), or their mixtures (1.0 wt % C14G2/40 wt % C12EO4 at 25 °C) exhibit viscoelastic properties, which can be explained by a Bingham fluid model. The storage modulus (G′), the loss modulus (G″), and the yield stress (τy) increase with an increase of the surfactant concentration and decrease as the temperature increases. The driving forces of the progressive self-assembly are a balance of hydrogen-bonding, hydrophobic, and steric effects. Spontaneous self-assembly processes and the present identification of vesicles to terraced planar bilayers with non-ionic surfactant mixtures may be helpful in understanding analogous phenomena in biological compartments or cells.
Scheme 1. Visualization of the Observed Structural Phase Transitions in the Aqueous System of C14G2 and C12EO4 Mixtures with the Increase of the C12EO4 Concentration or Temperaturea
a Bilayer vesicle models (left top, 1.0 wt % C14G2/20 wt % C12EO4 at 25 °C) had a ∼25 nm radius; multilamellar vesicles (left below, 1.0 wt % C14G2/30 wt % C12EO4 at 25 °C) had a ∼250 nm radius; and the interlamellar spacing (d) of bilayers (right, 1.0 wt % C14G2/30 wt % C12EO4 at 55 °C) was ∼6.5 nm. The analysis was based on the experimental data of FF-TEM and SAXS measurements. A sector in each vesicle model for bilayers and multilamellae has been cut out to enhance the visibility. Middle models show the coexisting multilamellar vesicles and terraced planar bilayers.
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of C12EO4 and C14G2 in water, and the steric effect of the C14G2 headgroup. With regard to the observed temperature dependence for phase structural transitions, a higher temperature reduces the hydrogen bonding among C12EO4 and C14G2 molecules or between them with water molecules, resulting in a smaller headgroup hydration shell, which leads to the decrease of the mean cross-sectional headgroup surfactant area (a). Simultaneously, the surfactant chains become more flexible and easily overlap, leading the decrease of the alkyl chain length (l). As a result, the packing parameter (P) increases. When P is up to around 1, vesicles transfer into terraced planar bilayers. On the other hand, the Helfrich curvature model43 can be used to explain the phase transition from unilamellar vesicles to multilamellar vesicles and then to terraced planar bilayers, because they are all molecular bilayer membranes. When the surfactant concentration is low enough, dilute and independent unilamellar vesicles could be formed and stabilized by entropy and the renormalization of the elastic constants, which is beneficial for curvature at large length scales.43 When the C12EO4 concentration increases to be larger than the critical concentration, at which vesicles begin violently interacting and then being closely packed, unilamellar vesicles nested to be multilamellar vesicles rapidly and smoothly. The multilamellar vesicle phase is stabilized by the entropy from three aspects: the spontaneous translational entropy of each species of aggregates, the fluctuation entropy of all of the individual layers within each multilamellar vesicle, and the finite translational entropy from the relative displacements of the centers of each shell within the multilamellar vesicles.44 For its unique structures, in multilamellar vesicles, the undulations of each shell are constrained by its neighbors. For small multilamellar vesicles, the Helfrich repulsion between neighbor shells is weak, but for large multilamellar vesicles with a large C12EO4 concentration or high temperature, the extra bending energy ultimately destabilizes the multilamellar vesicle phase, leading to the formation of terraced planar bilayers.
ASSOCIATED CONTENT
S Supporting Information *
Structural characterization of C14G2 molecules, detailed 2H NMR spectra with an increase of the temperature, interlamellar spacing from FF-TEM observations, and yield stress (Figures S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-531-88363532. Fax: +86-531-88564750. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (21033005 and 21273134), the National Basic Research Program of China (973 Program, 2009CB930103), and the Graduate Innovation Foundation of Shandong University (GIFSDU) (yyx10099).
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