Article pubs.acs.org/Langmuir
Controlling the Morphology of Membranes by Excess Surface Charge in Cat−Anionic Fluorinated Surfactant Mixtures Yuwen Shen,†,‡ Zhong-can Ou-Yang,§ Yufeng Zhang,† Jingcheng Hao,*,‡ and Zhaohui Liu*,† †
Institute of Agricultural Resources and Environment, Shandong Academy of Agricultural Sciences, Jinan 250100, PR China Key Laboratory of Colloid and Interface Chemistry of Ministry of Jinan, Shandong University, Jinan 250100, PR China § Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100190, PR China ‡
S Supporting Information *
ABSTRACT: The segregation and phase sequence of semifluorinated cat−anionic surfactant membranes at different excess surface charges was investigated by freezefracture transmission electron microscope (FF-TEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). The thermal behavior of the membranes was evaluated by conductivity, rheology, and deuterium nuclear magnetic resonance (2H NMR). The experimental results show that the cat−anionic fluorinated surfactant mixtures can form faceted vesicles and punctured lamellar phase when there is excess surface charge. The cationic and anionic fluorinated surfactants are stiff in the membranes, like phospholipids in the frozen “crystalline” or “gel” phase. For the system with excess cationic surface charge, the gel-like faceted vesicles and punctured lamellae can transform into smooth-shaped vesicles at 65 °C. However, for the system with no excess charge or with excess anionic surface charge, no phase transformation occurs even at 90 °C. A model was established to demonstrate the mechanism of the formation and transition of the aggregates with different morphologies. The segregation−crystallization mechanism works well with other cosmotropic counterions from the Hofmeister series. The observations provide a better understanding of how to control the membrane morphology of the aqueous solutions of cat−anionic surfactant mixtures.
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surfactants is exactly 1:1.3 Hoffmann et al. first reported the salt-free cat−anionic surfactant system,4,5 which contains strongly repulsive charged cylinders and vesicles. Hao et al. also investigated the vesicles of salt-free cat−anionic hydrocarbon/fluorocarbon surfactant mixed systems.6−12 Zemb et al. observed fascinating aggregates including flat nanodiscs13 and icosahedra14 in salt-free cat−anionic surfactant systems. These aggregates could be formed with only one of the ionic surfactants in excess. Zemb et al. also explained how the mixing ratios control the morphology of the aggregates.15 During cocrystallization, the excess (nonstoichiometric) surfactants accumulate on the edges or pores rather than being incorporated into the crystalline bilayer. Molecular segregation then produces a sequence of shapes controlled only by the initial molar ratio.16−23 In this report, we mixed cationic fluorinated surfactant [CF3(CF2)6CFCHCH2N⊕(CH3)3]I− with anionic perfluorosurfactant CF3(CF2)8CO2Li in water to obtain the gel-like cat−anionic surfactant system. Various aggregates with different surface charge could be observed by changing the original molar ratio of these two fluorinated surfactants, even though the dissociated salt formed by the counterions of the surfactants could screen some surface charge. The molar ratio is given by R
INTRODUCTION For mixed systems consisting of small particles and surfactants, it is a tremendous challenge to control the morphology of the colloid aggregates, especially for bilayer membranes. Understanding the formation mechanism of the mixture as well as its phase behavior at equilibrium is essential to controlling the size and structure of colloid particles. Fluorinated surfactants have unique properties such as the stiffness of their chains and their low miscibility with hydrocarbons. The present study has investigated how to control the morphology of membranes by excess surface charge in cat−anionic fluorinated surfactant mixtures. In addition, a model is presented to demonstrate the mechanism of the formation and transition of aggregates with different morphologies. Single-chain ionic surfactants can form spherelike micelles1 in solutions above the critical micelle concentration (cmc). Adding counterion can reduce the relative area of the hydrophilic headgroup and the spontaneous curvature of the monolayer, which generates cylindrical micelles and eventually planar bilayers. In 1989, Kaler et al. reported that spontaneous vesicles could be formed by mixing cationic and anionic surfactants at low concentration.2 Counterions produced by the dissociation of surfactant salts can increase the ionic strength of the solutions. Surface charges of the aggregates increase the electrostatic repulsion and prevent aggregates from coming into contact with one another. Normally, only a precipitate could be formed when the ratio between the cationic and the anionic © 2014 American Chemical Society
Received: August 23, 2013 Revised: January 19, 2014 Published: February 21, 2014 2632
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= [CF3(CF2)6CFCHCH2N⊕(CH3)3]I−/CF3(CF2)8CO2Li. The total surfactant concentration of all samples was fixed at 50 mmol·L−1. Different aggregates were incubated with varying excess charge (e.g., from R = 4:6 to 7:3). The excess surface charge not only decreased the average diameter of the vesicles but also made the bilayers irregular and punctured. It was shown that crystalline bilayers resulted from close-packed amphiphilic chains and the edges were enriched in excess charge.16,24,25 When the bilayer was completely frozen, a noticeable feature of the faceted crystallized aggregates and the presence of pores could be clearly seen by FF-TEM. The structure and thermal behavior of the aggregates were evaluated in detail by XRD, rheology, and NMR measurements. Compared to other systems that have similar phase behavior, the excess salt formed by the mixed cationic and anionic surfactants is interesting. The salts would screen the excess charge and inevitably weaken the crystallization and segregation process. Nevertheless, the properties of fluorinated surfactants can make such a process possible. On the basis of the general geometrical considerations of the packing of molecules into distinct aggregate shapes, fluorocarbon surfactants can more easily form bilayers than hydrocarbon surfactants because of the larger cross-section and higher rigidity of the fluorocarbon chain.26−29 Consequently, the conformational freedom is strongly reduced and the gauche defects occur at equilibrium, which facilitates stacking and ordering.30−33
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Conductivity Measurements. The conductivity of the sample at different temperatures was measured using a DDSJ-308A conductivity meter and a DJS-1C glass electrode (China, Shanghai Jingke Company). The ivory gel-like phase was first melted at 65 °C. The heated glass electrode was dipped into the fluid and kept static until the temperature dropped to room temperature. The change in the conductivity with temperature was recorded. Rheological Measurements. Rheological measurements were performed by using a Haake RS6000 rheometer with a cone plate sensor. Temperature was controlled to ±0.1 °C by the thermal controller (Haake TC81) on the RS6000. The rheological properties of the gel-like phase samples (4:6 < R < 7:3) with different mixing ratios of cationic and anionic surfactants are shown in Figures S1 and S2 (Supporting Information). DSC Measurements. Differential scanning calorimetry (DSC) was used to determine the exact temperature of the sol−gel phase transition. A rheometric scientific DSC SP system was employed. The sample was added to an aluminum pan and sealed in the experimental process. DSC thermograms were recorded over the temperature range of 15−100 °C at a heating rate of 0.5 °C/min, which was controlled by liquid N2. NMR Measurements. 1H NMR and 2H NMR spectra were recorded on a Bruker Avance 400 spectrometer equipped with a pulse field gradient module (z axis). The surfactant mixed samples were prepared in deuterium oxide and measured with a 5 mm BBO probe. The temperature was controlled to ±0.1 °C with a thermal controller.
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RESULTS AND DISCUSSION Phase Behavior of Cationic and Anionic Surfactant Mixtures at Different Molar Ratios. The phase behavior photographs of [CF 3 (CF 2 ) 6 CFCHCH 2 N ⊕ (CH 3 ) 3 ]I − / CF3(CF2)8CO2Li aqueous solutions with varying molar ratios R = 9:1 to 1:9 at 25.0 ± 0.1 °C are shown in Figure 1a. The total surfactant concentration of all samples was 50 mmol·L−1. The system initially forms a turbid solution for the precipitate/ L1 phase (R = 9:1 to 8:2) and then forms a turbid gel phase (R = 7:3 to 6:4). When R = 5:5 to 3:7, a transparent birefringent gel phase is observed. Afterward, the system enters the precipitate/L1 phase (R = 2:8 to 1:9). The samples with the precipitate/L1 phase (R = 9:1 to 8:2 and 2:8 to 1:9) have no phase transition at high temperature, so only the photographs of the system with molar ratios of R = 7:3 to 3:7 at 90.0 ± 0.1 °C are shown in Figure 1b. At high temperature, the turbid gel phase (R = 7:3 to 6:4) transforms into a transparent fluid with birefringence. However, the transparent gel phase (R = 5:5 to 3:7) has no phase transition even at high temperature. Microstructures of the Gel-like Phase with Different Surface Charges. A series of [CF 3 (CF 2 ) 6 CF CHCH2N⊕(CH3)3]I− and CF3(CF2)8CO2Li aqueous solutions with varying surfactant molar ratios were thermostated at 25.0 ± 0.1 °C for at least 4 weeks. Our investigations focused on the gel-like phase of the cationic and anionic surfactants. Various aggregates with different surface charge could be observed by changing the original molar ratio of the cationic and anionic fluorinated surfactants, as shown in Figure 2a−d. A large number of faceted vesicles can be observed when excess cationic surface charge exists (molar ratio R = 7:3), as shown in Figure 2a. The diameter of the vesicles is around 200−800 nm. When the amount of anionic surfactants is increased (molar ratio R = 6:4), similar aggregates can still be observed in Figure 2b. When the molar ratio of the surfactants reaches R = 5:5, the faceted vesicles transform into smooth-shaped spherical vesicles with a much smaller diameter of around 80−200 nm (Figure 2c). By further increasing the amount of anionic surfactants
MATERIALS AND METHODS
Materials. Cationic fluorinated surfactant [CF3(CF2)6CF CHCH2N⊕(CH3)3]I−(>96%) was a gift from Hoechst, Aktiengesellschaft Werk, Gendorf (Frankfurt, Germany) and was used without further purification. Anionic fluorinated surfactant CF3(CF2)8CO2Li was synthesized by reacting lithium hydroxide (>99%, Shanghai, China) with perfluorodecanonic acid (96%, Sigma-Aldrich) (i.e., CF3(CF2)8CO2H + LiOH → CF3(CF2)8CO2Li + H2O). The product, CF3(CF2)8CO2Li, was purified three times in ethanol. Triply distilled deionized water was used in all experiments. Phase Behavior Observations. The cat−anionic surfactant mixed solutions were first heated to 70 °C in order to dissolve the surfactants efficiently and were then cooled to room temperature with stirring. The ivory gel-like phase could be formed upon cooling when the cationic surfactant was present in excess (5:5 < R < 7:3). The samples became increasingly transparent when the amount of excess cationic surfactant was reduced. When the anionic surfactant was present in excess (3:7 < R < 5:5), the gel-like phase was completely transparent. In contrast to the ivory gel-like phase with excess cationic surfactant, the transparent phase with excess anionic surfactant could not become fluid even at 90 °C. FF-TEM Observations. The structure of the Lα-phase sample solutions was characterized by FF-TEM observations. A small amount of the sample was placed on a 0.1-mm-thick copper disk covered with a second copper disk. The copper sandwich with the samples was immersed rapidly in liquid ethane cooled by liquid nitrogen for several seconds and then stored in liquid nitrogen before fracturing. The frozen sample was transferred into the chamber of the freeze-etching apparatus (Balzers BAF-400D) and fractured at −150 °C and 10−7 Pa. After being etched for 1 min, Pt−C was sprayed onto the fracture face at 45°, and then C was sprayed at 90°. The replicas were examined on a JEOL JEM-1400 TEM operating at an acceleration voltage of 120 kV. X-ray Diffraction (XRD) Measurements. The XRD diffraction patterns were recorded at room temperature using a Rigaku (Japan) diffractometer. The diffraction angle θ was varied from 1 to 10°. Thereby, the magnitude of the diffraction vector h can be obtained (h = 2π sin θ/λ), where λ = 0.154184 nm is the wavelength of the incident X-ray. 2633
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(molar ratio R = 4:6), the smooth-shaped vesicles are again replaced by faceted polyhedron-like vesicles (Figure 2d), which have a diameter of around 100−400 nm. The results imply that the cat−anionic surfactant systems with excess surface charge favored faceted polyhedron-like aggregates. 1 H NMR Spectra of the Gel-like Phase. 1H NMR spectra were acquired on a Bruker DRX 400 spectrometer at 25 and 35 °C. The typical 1 H spectra of the 100 mmol·L − 1 [CF3(CF2)6CFCHCH2N⊕(CH3)3]I− micelle phase at 35 °C (slightly above its Krafft point) and the 1H spectra of the gel-like phases of cat−anionic surfactant mixtures with different mixing ratios at 25 °C are shown in Figure 3. The variation in
Figure 1. (a) Photographs of the phase transition of the [CF3(CF2)6CFCHCH2N⊕(CH3)3]I−/CF3(CF2)8CO2Li system without (top) and with (bottom) crossed polarizers at 25.0 ± 0.1 °C. R was (a) 9:1 (turbid solution), (b) 8:2 (precipitate/L1 phase), (c) 7:3 (turbid gel phase), (d) 6:4 (turbid gel phase), (e) 5:5 (transparent gel phase), (f) 4:6 (transparent gel phase), (g) 3:7 (transparent gel phase), (h) 2:8 (precipitate/L1 phase), and (i) 1:9 (precipitate/L1 phase). (b) Photographs of the phase transition of the [CF3(CF2)6CFCHCH2N⊕(CH3)3]I−/CF3(CF2)8CO2Li system without (top) and with (bottom) crossed polarizers at 90.0 ± 0.1 °C. R was (c) 7:3 (transparent Lα phase), (d) 6:4 (transparent Lα phase), (e) 5:5 (transparent gel phase), (f) 4:6 (transparent gel phase), and (g) 3:7 (transparent gel phase).
Figure 3. 1H NMR spectra of cat−anionic surfactant systems with different mixing ratios. (a) 100 mmol·L−1 cationic surfactant micelle phase at 35 °C, (b) R = 7:3, (c) R = 6:4, and (d) R = 5:5. T = 25 °C.
the chemical shift originated from the changing shielding effect of the electron clouds around the nucleus. A strongly electronegative absorbing molecular environment can shift the resonance frequency to higher magnetic field. Compared to the spectrum of the micelle phase (Figure 3a), all of the signals of segments with hydrogen (Figure 3b−d) shifted to higher magnetic field. Considering that the fluorocarbon chains are strongly electronegative, the hydrophobic tails of the surfactants are aligned much more closely in the gel-like phase than in the cationic micelle phase. Microstructures of the Gel-like Phase with Excess Cationic Surface Charge. Freeze−fracture transmission electron microscopy was used to further characterize the systems with excess cationic surface charge. It is noteworthy that besides the faceted vesicles a large number of grid holes exist when there is excess cationic surface charge (R = 6:4, Figure 4a−c). Figure 4a,c shows different areas of the same sample at R = 6:4. At high magnification, the average diameter of the holes is determined to be around 10−30 nm, which implies that the presence of holes on the bilayer can release the mechanical tension at the edges. Diffraction peak position q1/ q2/q3 in Figure 4d is strictly 1:2:3, which is a typical Bragg scattering pattern corresponding to the 001, 002, and 003 planes of a 1D long-range-ordered lamellar structure. The interlayer distance calculated by d = 2π/qmax is around 1.65 nm. The chain lengths of the cationic and anionic surfactants are 1.54 and 1.38 nm (simulated by Accelrys Material Studio 4.0 software), respectively. The interlayer distance was much
Figure 2. FF-TEM images of aggregates for systems with different surfactant molar ratios: (a) R = 7:3, (b) R = 6:4, (c) R = 5:5, and (d) R = 4:6.
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Figure 5. FF-TEM images of the temperature-induced phase transition from punctured lamellae and irregular vesicles to solely vesicles (R = 6:4). (a) Room temperature; (b) T ≥ 65 °C.
not only controlled by the molar ratio of the surfactants but also influenced by the structure of the aggregates. The variation of conductivity with temperature is shown in Figure 6b. The conductivity initially stays almost constant at around 1072 μS· cm−1 at 20 °C and then decreases linearly above 37 °C, indicating the formation of a new structure. The conductivity of the 40 mmol·L−1 pure LiI solutions is 3540 μS·cm−1 at 20 °C. The ratio of immobilized counterions is about 2:3. Because this temperature is lower than the phase-transition temperature determined by DSC, it is assumed that a pretransition occurs before the full phase transition. Considering the decrease in conductivity and the observed birefringence through polarizers, this new phase should consist of vesicles that can wrap a considerable number of charges to decrease the conductivity. According to the paper reported by Kaler et al.2,34,35 at above the chain melting temperature, diluted unilamellar vesicles are the most stable state with or without excess salt. A low-frequency, low-amplitude temperature sweep is one of the simplest rheological experiments for studying gels. The gel sample was placed between the plates of the rheometer and sheared with a constant stress of 1 Pa at a constant frequency of 1 Hz while the temperature was raised. From the value of the modulus and its behavior as a function of temperature, an educated guess of the structure could be made. From Figure 6c, it can be observed that as the temperature increases, the storage modulus G′ begins to decrease notably at ∼35 and ∼65 °C, which indicates the beginning of the pretransition and phase transition, respectively. The transformation from the gel-like phase to the birefringent Lα phase with rising temperature was determined by 2H NMR measurements (Figure S5 in Supporting Information). The 2H NMR results show the same phase transformation as the results in Figure 6c. Possible Mechanism of the Sol−Gel Transition. Below the chain melting temperature, the planar bilayer can be formed as a result of the synergistic effect between the cationic and anionic surfactants. It is assumed that the planar lamellae are composed of cationic and anionic surfactants with a 1:1 packing ratio. With excess surface charge, the cationic surfactants are repulsed and form pores to reduce the energy of curvature. Such punctured lamellae exist only when the surfactants are in the insoluble frozen state. It has been reported that36 excess surfactant easily forms pores on its own as a result of the packing properties of unscreened charged surfactant headgroups. The surface charge of our system can be screened by the counterions of the surfactants. The frozen lamellae still exist because of the lower water solubility and higher stiffness of the fluorinated chains.37−41
Figure 4. Microstructure characterization of the gel-like phase with excess positive surface charge (R = 6:4): (a−c) FF-TEM images; (d) XRD curve. T = 25 °C.
shorter than the sum of the chain lengths of these two surfactants, which resulted in the tilted chains of the bilayers and confirmed that the bilayers were completely frozen. Therefore, irregular vesicles and tilted punctured lamellae can be obtained by introducing excess surface charge. It is worth noting that the 002 peak in Figure 4d was more intense than the 001 peak. An alternating intensity distribution would indicate that the reflection from the midplane of the film interferes with that from the headgroups. This was not observed for interdigitated phases, as observed for phospholipid bilayers. In this specific case, one would expect a higher electron density for the hydrophobic regions and a dip in the middle and in the headgroup region. Hence, the two corresponding reflections interfere destructively for odd reflexes and constructively for even reflexes. Phase Transition during the Crystallization and Segregation Process. Above the chain melting temperature, the gel-like phase with excess cationic surface charge can transform from faceted vesicles and punctured lamellae into smooth-shaped flexible vesicles. Figure 1 shows photographs of the phase transition of the sample at different temperatures. For the system with excess cationic surface charge, the ivory gel-like phase becomes a transparent fluid above the chain melting temperature. Nevertheless, for the system with excess anionic surface charge, no phase transition occurs above the chain melting temperature (Figure S4 in the Supporting Information). In this melted state (T ≥ 65 °C), the rigidity of the bilayer was low,15 and the diameter of the vesicles thus decreases to 50−100 nm (Figure 5b). The DSC curve in Figure 6a shows the exact phase-transition temperature. An endothermic peak can be observed at around 67.7 °C, which is much higher than the Krafft temperature of both the cationic fluorinated surfactant (35 °C) and the anionic fluorinated surfactant (
