Coassembly of Poly(ethylene glycol)-block-Poly(glutamate sodium

Jul 8, 2013 - Coassembly of Gemini Surfactants with Double Hydrophilic Block Polyelectrolytes Leading to ... RSC Advances 2015 5 (66), 53289-53298 ...
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Coassembly of Poly(ethylene glycol)-block-Poly(glutamate sodium) and Gemini Surfactants with Different Spacer Lengths Yuchun Han, Wentao Wang, Yongqiang Tang, Shusheng Zhang, Zhibo Li,* and Yilin Wang* Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: The coassembly of poly(ethylene glycol)-b-poly(glutamate sodium) copolymer (PEG113-PGlu100) with cationic gemini surfactants alkanediyl-α,ω-bis-(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (designated as C12CSC12Br2, S = 3, 6, and 12) have been studied by isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, small-angle X-ray scattering, zeta potential, and size measurement. It has been shown that the electrostatic interaction of C12CSC12Br2 with the anionic carboxylate groups of PEG113-PGlu100 leads to complexation, and the C12CSC12Br2/PEG113-PGlu100 complexes are soluble even at the electroneutral point. The complexes display the feature of superamphiphiles and assemble into ordered nanosheets with a sandwich-like packing. The gemini molecules which were already bound with PGlu chains associate through hydrophobic interaction and constitute the middle part of the nanosheets, whereas the top and bottom of the nanosheets are hydrophilic PEG chains. The size and morphology of the nanosheets are affected by the spacer length of the gemini surfactants. The average sizes of the aggregates at the electroneutral point are 81, 68, and 90 nm for C12C3C12Br2/PEG113-PGlu100, C12C6C12Br2/PEG113PGlu100, and C12C12C12Br2/PEG113-PGlu100, respectively. Both C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/PEG113-PGlu100 mainly generate hexagonal nanosheets, while the C12C6C12Br2/PEG113-PGlu100 system only induces round nanosheets.

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INTRODUCTION Surfactant/polymer mixtures can gain improved properties or novel functions that surfactant or polymer alone cannot achieve, and consequently have much practical applications, such as paint and coating products, food processing, personal care formulations, enhanced oil recovery, pharmaceutical formulations, and so on.1,2 Understanding the association behavior and association mechanism of surfactants with polymers can offer effective guidance to design surfactant/ polymer mixtures with desired properties and functions. Therefore, the interactions between surfactants and polymers in aqueous solutions have attracted considerable attention and have been extensively studied. Some books and reviews covering different aspects about surfactant/polymer interactions have been published.1−11 Particular attention has been paid to the complexation between ionic surfactants and oppositely charged polyelectrolytes due to their strong interaction and rich phase behavior. However, phase separation from aqueous solution due to the charge neutralization of polyelectrolytes with surfactants largely limits their applications. A novel family of polymer−surfactant complexes formed by double hydrophilic block copolymers (DHBCs) and oppositely charged surfactants have been reported by Kabanov and co-workers.12−17 In their studies, poly(ethylene oxide)-b-poly(sodium methacrylate) (PEO© 2013 American Chemical Society

PMA) was used as a double hydrophilic block copolymer, whereas various cationic surfactants were selected to complex with this block copolymer. They found that the soluble complexes could be formed for all of the studied systems including the electroneutral complexes. Other groups also studied the complexes formed by different double hydrophilic block copolymers and surfactants. For example, Berret and coworkers18,19 studied the complexes of surfactants with oppositely charged DHBCs, including poly(sodium acrylate)− poly(acrylamide) (PANa-PAM), poly(styrene sodium sulfonate)−poly(acrylamide) (PSS-PAM), and poly(trimethylammoniumethylacrylate methyl sulfate)−poly(acrylamide) (PTEA-PAM). The results showed that the complexes exhibit a core−shell structure, in which the core was a dense microphase of surfactant micelles connected by the polyelectrolyte blocks, whereas the corona was a diffuse brush made of the neutral chains. A study from Pispas20 reported the self-assembled nanostructures formed in the mixed solution of poly[(2-sulfamate-3-carboxylate)isoprene-b-ethylene oxide] (SCIEO) with a vesicle-forming surfactant didodecyldimethylammonium bromide (DDAB). It was found that the DDAB Received: May 23, 2013 Revised: July 1, 2013 Published: July 8, 2013 9316

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Figure 1. Molecular structures of double hydrophilic block copolymer and gemini surfactants studied. mixing the copolymer and surfactant solutions at room temperature, then the solutions were mixed for several minutes using a vortex. No additional mechanical agitation was applied. The charge ratio of C12CSC12Br2 to PEG113-PGlu100 (Z) was defined as the ratio of twice the surfactant concentration to the concentration of carboxylate groups of PEG113-PGlu100. On the basis of the pKa of PEG113-PGlu100, all the carboxylate groups were charged at the pH used here. The concentration of carboxylate groups of PEG113-PGlu100 was kept at 2.8 mM in all experiments. The concentrations of C12CSC12Br2 were varied to obtain different Z values. Isothermal Titration Microcalorimetry (ITC). The calorimetric measurements were conducted using a TAM 2277−201 microcalorimetric system (Thermometric AB, Järfälla, Sweden) with a stainless steel sample cell of 1 mL. The cell was initially loaded with 0.7 mL of buffer or PEG113-PGlu100 buffer solution. The concentrated surfactant solution of C12CSC12Br2 was injected into the sample cell via a 500 μL Hamilton syringe controlled by a 612 Thermometric Lund pump. A series of injections were made until the desired range of concentration had been covered. The system was stirred at 60 rpm with a gold propeller. All of the measurements were conducted at 25.00 ± 0.01 °C. Each ITC curve was repeated at least twice with deviation within 5%. Zeta Potential and Particle Size Measurements. The zetapotential and particle size measurements were performed at 25 °C, using a Malvern Zetasizer Nano-ZS instrument (ZEN3600, Malvern Instruments, Worcestershire, UK) equipped with a 4 mW He−Ne laser at a wavelength of 633 nm. All the measurements were performed at θ = 173°. A clear disposable capillary cell (DTS1060C) was used for both measurements. The zeta potential was calculated using the Helmholtz-Smoluchowski relationship from the mobility measured in an electrophoretic light-scattering (ELS) experiment. Size distribution was derived from a deconvolution of the measured intensity autocorrelation function of the sample accomplished by a nonnegatively constrained least-squares fitting algorithm. The calculation of size distribution from light scattering measurements was based on the assumption that the particles were spherical. Because the selfassemblies obtained in this study are not spherical, the size measurements present only relative values rather than absolute size. Each sample was measured at least twice with a deviation of less than 8%. Cryogenic Transmission Electron Microscopy (cryoTEM). CryoTEM samples were prepared in a controlled environment vitrification system (CEVS) at 28 °C.27 A micropipet was used to load 5 μL of the mixture of copolymer and surfactant onto a lacey support TEM grid, which was held by tweezers. The excess solution was blotted with a piece of filter paper, resulting in the formation of thin film suspended the mesh holes. After waiting for about 5 s to relax any stresses induced during the blotting, the samples were quickly plunged into a reservoir of liquid ethane (cooled by liquid nitrogen) at its melting temperature. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined using a JEM 2200FS TEM (200 keV) at −174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with Digital Micrograph. Circular Dichroism (CD). Far-UV CD spectra were recorded on a JASCO J-815 spectrophotometer at 25 °C using 1 mm path length cell. Scans were obtained in a range between 190 and 260 nm by

vesicles were adsorbed on the block copolymer chains at low copolymer concentration and the nanoassemblies transform to a core−shell, micelle-like structure as the block copolymer concentration increases. Very recently, we synthesized a biocompatible DHBC, poly(ethylene glycol)-b-poly(glutamate sodium) (PEG-PGlu), and studied its coassembly with singlechain surfactant dodecyltrimethylammonium bromide (DTAB). Vesicles, spherical and rod aggregates were generated by changing the length of PGlu block.21 Therefore, the complexation of DHBCs with low-molecular weight surfactants offers a medium to construct tunable and functional superamphiphile assemblies for potential practical applications. In the past 20 years, gemini surfactants have been well developed and have become the star molecules of surfactant family. Gemini surfactants, consisting of two hydrophobic side chains and two polar head groups covalently linked by a spacer at the headgroup level, have lower critical micelle concentrations and much more enriched self-assembling structures than their single-chain counterparts.22−24 Spacer group controls the distance of two head groups of a gemini surfactant and in turn affects the charge density of the aggregate surface of the gemini surfactant. Moreover, the spacer group can be involved in the hydrophobic interactions of the hydrophobic side chains. Thus, gemini surfactants are expected to exhibit quite different coassembling behavior with the DHBC copolymer. In this study, three cationic gemini surfactants with different spacer lengths, i.e., alkanediyl-α,ω-bis-(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (designated as C12CSC12Br2, S = 3, 6, and 12), are chosen to coassemble with PEG113-PGlu100. The molecular structures of PEG113-PGlu100 and C12CSC12Br2 are shown in Figure 1. The complexation and coassembling behaviors have been investigated by varying the charge ratios of the surfactants to the carboxylate groups of copolymer and the unique coassembling structures have been revealed.

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EXPERIMENTAL SECTION

Materials. PEG113-PGlu100 was synthesized by ring-opening polymerization of L-benzylglutamate N-carboxyanhydride.25 The number averaged molecular weight and molecular weight distribution were characterized using gel permeation chromatography/laser light scattering with N,N-dimethylformamide containing 0.2 M LiBr as eluent. The Mn value is 33 000 Da with a polydispersity index (PDI) value of about 1.2. The DP of PGlu is about 100 determined from 1H nuclear magnetic resonance using PEG113 as reference. The benzyl group was deprotected using HBr/acetic acid in the trifluoroacetic acid solvent at 0 °C. The product was then neutralized using 1 M NaOH solution and exhaustively dialyzed against deionized water. The solution was then lyophilized to give product as white solid. Gemini surfactants C12CSC12Br2 were synthesized and purified according to the method of Zana et al.26 All the solutions were prepared in phosphate buffer of pH 7.4 with ionic strength of 10 mM. Pure water was obtained from Milli-Q equipment. The copolymer/surfactant complexes were prepared by 9317

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taking points at 0.5 nm with an integration time of 0.5 s, and five scans were averaged and smoothed to improve signal-to-noise ratio. Small Angle X-ray Scattering (SAXS). SAXS measurements were performed at the Brookhaven National Laboratory (Brookhaven, Changchun) on the X21 beamline. The C12CSC12Br2/PEG113- PGlu100 mixed solutions were placed in quartz capillary with a diameter of 1 mm. A wavelength of 1.76 Å was used for the incoming beam and the sample−detector distance was 1 m. With the detector in the off-center position, the accessible q range was 0.01 − 0.4 Å−1, with a resolution Δqfwhm/q of 1.25%. Δqfwhm denotes here the full width at the halfmaximum of diffraction peaks characterizing ordered structures.

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RESULTS AND DISCUSSION ITC is a direct method to measure the energy change during interaction processes and has been extensively applied to study the interactions of surfactants with macromolecules.28−31 So, ITC experiments were first performed to obtain the interaction information of the three gemini surfactants with PEG113PGlu100. Figure 2 presents representative raw calorimetric

Figure 3. The observed enthalpy changes against the charge ratio Z for the titration of 10 mM C12C3C12Br2 (a), C12C6C12Br2 (b), and C12C12C12Br2 (c) into 28 μM PEG113-PGlu100 (solid circles). The concentration of PEG113 -PGlu100 is expressed by the molar concentration of carboxylate groups of PEG113-PGlu100. Solid lines correspond to the curves for the surfactants into buffer against the corresponding surfactant concentrations without copolymer.

results from the breakup of the added micelles into the surfactant monomers, and the further dilution of the monomers. When the final surfactant concentrations are above the CMC, the added micelles are only diluted, and finally ΔHobs drops toward zero. For titrating the C12CSC12Br2 solutions into the PEG113PGlu100 solution, the ΔHobs curves for the three gemini surfactants display similar varying trends. ΔHobs initially shows very high endothermic values and the solutions at this stage are transparent or slightly bluish. Then, ΔHobs starts to decrease sharply from Z = 0.7 until attaining the minimum value at Z = 1, and the solutions become cloudy with light blue. After Z = 1, ΔHobs starts to increase until coinciding with the dilution enthalpy curve of the corresponding surfactants without PEG113-PGlu100, and the solutions still remain cloudy. Obviously, with the increase of the spacer length of the surfactants, the changing slope becomes steeper and steeper in the ΔHobs curve beyond Z = 1 and before merging with the dilution curve. This means that the surfactant amount required for attaining the final interaction saturation reduces with the increase of the spacer length. To understand the resultant structures upon the interaction C12CSC12Br2 with PEG113-PGlu100, the C12C6C12Br2/PEG113PGlu100 system was selected as a representative to perform zeta potential and aggregate size measurements. Figure 4 presents the zeta potential and size values of the C12C6C12Br2/PEG113-

Figure 2. The raw calorimetric titration curves of titrating 10 mM C12C6C12Br2 solution into the buffer (a) and 28 μM PEG113-PGlu100 solution (b), respectively, which show the variations of heat flow P as a function of time.

curves of titrating 10 mM C12C6C12Br2 solution into the buffer and into the PEG113-PGlu100 solution, respectively. These two raw calorimetric curves have significant differences, indicating strong interaction between the copolymer and the gemini surfactant. The varying tendencies of the raw calorimetric curves for the other two surfactants are similar to the case of C12C6C12Br2. The observed enthalpies (ΔHobs) were obtained by integrating the peak areas of the raw calorimetric curves, and are plotted against the apparent charge ratio of C12CSC12Br2 to PEG113-PGlu100 (Z), as shown in Figure 3. For the three gemini surfactants, the ΔHobs curves of interacting with copolymer are obviously different from the blank experiments. Because the initial surfactant concentrations are larger than their critical micelle concentrations (CMCs), the surfactants exist as micelles before titrations. For titrating the C12CSC12Br2 solutions into the buffer, ΔHobs shows high endothermic value followed by an abrupt decrease at a threshold concentration, corresponding to CMC. When the final surfactant concentrations are below CMC, ΔHobs mainly 9318

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PGlu100 system at Z ranging from 0 to 2.3, which covers the region of the whole calorimetric curve.

Summarizing the above results from the calorimetric, zeta potential, and size measurements, the interaction process between PEG113-PGlu100 and C12CSC12Br2 can be outlined. The Glu unit of PEG113-PGlu100 carries negative charges at pH 7.4. Upon the addition of C12CSC12Br2, PEG113-PGlu100 binds with the positively charged C12CSC12Br2 molecules by strong electrostatic attraction, approved by the large endothermic ΔHobs value and its decreasing tendency as well as the decrease of the negative zeta potential. The large endothermic ΔHobs value suggests that the electrostatic binding is accompanied by strong dehydration and counterion release of the charged groups, which is similar to the previous report.32−35 With the increase of the C12CSC12Br2 concentration, the reduced zeta potential of the mixture promotes the coassembly of C12CSC12Br2 with PEG113-PGlu100, as shown by the size increase. Beyond Z = 1, both the size and the zeta potential of the mixed aggregates nearly attain constant values. The critical point at Z = 1 implies that the binding of C12CSC12Br2 with PEG113-PGlu100 reaches an electrostatic saturation state. Afterward, the C12CSC12Br2 molecules cannot electrostatically bind to PEG113-PGlu100 anymore, however, the further added C12CSC12Br2 molecules still interact with the C12CSC12Br2 molecules which have already been bound to copolymer via hydrophobic interaction, until the final interaction saturation point is attained. The hydrophobicity of C12CSC12Br2 increases with the increase of the spacer length, so the surfactant amounts required for attaining the saturation point of the final interaction decrease with the increase of the spacer length. On the basis of the above results, it is concluded that the coassembled aggregates of the three C12CSC12Br2/PEG113PGlu100 mixtures at or beyond Z = 1 show great stability. Moreover, even storing the mixtures for more than 2 weeks, the changes in zeta potential and average size were negligible. Therefore, the mixtures at Z = 1 are chosen to investigate the assembly structures of the C 12 C S C 12 Br 2 /PEG 113 -PGlu 100 systems in the following text. The average sizes of the C12CSC12Br2/PEG113-PGlu100 aggregates at Z = 1 are shown in Table 1. The C12C6C12Br2/

Figure 4. ζ potential values (top) and average sizes (bottom) of the C12C6C12Br2/PEG113-PGlu100 aggregates at different C12C6C12Br2/ PEG113-PGlu100 charge ratios (Z).

With the increase of the Z values, the zeta potential values vary from large negative value around −40 mV to small positive value about 5 mV, and the electroneutral complexes are obtained in the vicinity of Z = 1. This result is different from the DTAB/PEG113-PGlu100 system, where the electroneutral complex is obtained at Z = 7.21 In the previous study,21 it has been proven that the Z value at the electroneutral point of PEG113-PGlu100 with single-chain surfactant strongly depends on the hydrophobic chain length of surfactant, that is, the Z value has a close relationship to the self-assembling ability of surfactant. The reason is that not all surfactant cations added to the system bind with the carboxylate groups of the PEG113PGlu100 molecules through electrostatic interaction because of the dynamic equilibrium among the free surfactant monomers, the surfactant micelles, and the surfactant−copolymer complexes. The surfactant−copolymer complexes would coexist with the free surfactant micelles and surfactant monomers. The measured zeta potential value was averaged for all the species in the systems. A surfactant with stronger self-assembling ability can assist its binding with PEG113-PGlu100. Because of the weaker self-assembling ability of DTAB, more DTAB molecules were required to reach the electrostatic binding equilibrium with PEG113-PGlu100. Herein, C12CSC12Br2 has very strong selfassembling ability and electrostatic binding ability due to its double hydrophobic chains and two charged head groups. Therefore, the binding equilibrium is reached around Z = 1. The sizes of the C12C6C12Br2/PEG113-PGlu100 aggregates are less than 10 nm at low Z values, i.e., with less C12C6C12Br2. From Z = 0.3, the size suddenly increases to above 60 nm, and then increases very slowly. The final size is about 75 nm. The results indicate that C12C6C12Br2 induces PEG113-PGlu100 to aggregate since Z = 0.3, thereafter the aggregate size almost does not change anymore.

Table 1. The Average Sizes of the C12CSC12Br2/PEG113PGlu100 Aggregates at Z = 1 complex

average size (nm)

C12C3C12Br2/PEG113-PGlu100 C12C6C12Br2/PEG113-PGlu100 C12C12C12Br2/PEG113-PGlu100

81 68 90

PEG 113 -PGlu 100 aggregates are the smallest, and the C12C3C12Br2/PEG113-PGlu100 aggregates are smaller than that of C12C12C12Br2/PEG113-PGlu100. Macroscopic observation also indicate that all the C12CSC12Br2/PEG113-PGlu100 mixture solutions are bluish cloudy, while the PEG113-PGlu100 solution alone is very clear (Figure S1 in Supporting Information). This proves that C12CSC12Br2 induces PEG113-PGlu100 to coassemble into larger aggregates in the solutions. Figure 5 shows the CD spectra of the PEG113-PGlu100 copolymer alone and the C12CSC12Br2/PEG113-PGlu100 solutions at Z = 1. The CD spectrum of PEG113-PGlu100 exhibits two extrema, a maximum centered at ∼217 nm and a minimum at ∼198 nm, which is the typical random coil conformation. However, ordered secondary conformation is not observed in the CD spectra of the C12CSC12Br2/PEG113-PGlu100 solutions. C12CSC12Br2 only induces significant ellipticity decrease and 9319

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Specially, the cryoTEM images show that all the three C12CSC12Br2/PEG113-PGlu100 systems display ordered nanosheet structures, and the C12C6C12Br2/PEG113-PGlu100 system has some differences from the other two systems. Both C12C3C12Br2/PEG113-PGlu100 (Figure 6a) and C12C12C12Br2/ PEG113-PGlu100 (Figure 6c) mainly present hexagonal nanosheets accompanied with a very small amount of round and elliptical nanosheets, and the hexagonal nanosheets have welldefined edges with an approximately 120° angel between adjacent two of them. Very similar hexagonal morphologies were observed by Nizri et al. in the systems of poly(diallyldimethylammoniumchloride) with sodium dodecyl sulfate and of sodium polyacrylate with alkyltrimethylammonium bromide.37,38 However, the C12C6C12Br2/PEG113-PGlu100 (Figure 6b) system only forms round nanosheets. Small-angle X-ray scattering (SAXS) technique is also used to study the structures of the C12CSC12Br2/PEG113-PGlu100 aggregates, and the results are shown in Figure 7, where the

Figure 5. CD spectra of the solutions of PEG113-PGlu100 alone and C12CSC12Br2/PEG113-PGlu100 at Z = 1.

obvious red shift of the minimum at 198 nm. This implies that the C12CSC12Br2 surfactants interact strongly with PEG113PGlu100 and in turn destroy the second structure of PEG113PGlu100. Previously, Liu et al.36 showed that alkyltrimethylammonium ions with bulky head groups are ineffective in inducing ordered structure of polypeptide, and thought that the steric hindrance of surfactant headgroup may inhibit the formation of ordered conformation. The present results may be caused by the same reason. Subsequently, the morphologies of the C12CSC12Br2/PEG113PGlu100 aggregates at Z = 1 are characterized by the cryoTEM technique. Figure 6 and Figure S2 (Supporting Information) show the cryoTEM images of the C12CSC12Br2/PEG113-PGlu100 aggregates, and present a wide range of nanoaggregates with a highly ordered structure. The sizes of the C12C3C12Br2/PEG113PGlu100 and C12C6C12Br2/PEG113-PGlu100 aggregates are about 40−70 and 30−40 nm, respectively, whereas the sizes of the C12C12C12Br2/PEG113-PGlu100 aggregates are in the range of 50−130 nm. The cryoTEM images confirm that the size of the mixed aggregates increases in the order of C12C6C12Br2/ PEG 1 1 3 -PGlu 1 0 0 , C 1 2 C 3 C 1 2 Br 2 /PEG 1 1 3 -PGlu 1 0 0 and C12C12C12Br2/PEG113-PGlu100. Although this order is the same as that shown in Table 1, the size values are different, because cryoTEM and dynamic light scattering are different in principle and the sizes obtained from dynamic light scattering are average values on the assumption of spherical aggregates.

Figure 7. X-ray scattering intensities for the C12CSC12Br2/PEG113PGlu100 mixtures in phosphate buffer of pH 7.4. An arrow indicates the position of the structure peak at q0 = 0.17−0.19 Å−1.

scattered intensity is plotted as a function of the scattering vector q = (4π/λ) sinθ, and λ and θ are X-ray wavelength and half of the scattering angle, respectively. These three C12CSC12Br2/PEG113-PGlu100 solutions exhibit similar SAXS patterns with a single reflection at the scattering vector of

Figure 6. CryoTEM images for the C12CSC12Br2/PEG113-PGlu100 solutions at Z = 1. (a) C12C3C12Br2/PEG113-PGlu100; (b) C12C6C12Br2/PEG113PGlu100; (c) C12C12C12Br2/PEG113-PGlu100. 9320

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Figure 8. The proposed coassembling mechanism for the C12CSC12Br2/PEG113-PGlu100 systems.

0.17−0.19 Å−1. The corresponding Bragg spacing values (d = 2π/q) are 3.7, 3.5, and 3.3 nm for C12C3C12Br2/PEG113PGlu100, C12C6C12Br2/PEG113-PGlu100, and C12C12C12Br2/ PEG113-PGlu100, respectively. The SAXS results indicate that the aggregate structures of the three C12CSC12Br2/PEG113PGlu100 systems are similar. Additionally, the spacing values are very similar to the report of Nizri et al.37 In their SAXS results of poly(diallyldimethylammoniumchloride) and sodium dodecyl sulfate in dry state, besides a sharp reflection at 0.187 Å−1 (spacing of 3.36 nm), a very weak secondary reflection can be observed at a scattering vector of 0.322 Å−1 (spacing of 1.95 nm), and the ratio of the peak positions is close to 31/2, characteristic of a hexagonal lattice. In the present study, the absence of the secondary reflection may be due to the low sample concentration in the solutions. Because the hydrophobic chain length of the gemini surfactants used here is about 1.7 nm, the Bragg spacing may correspond to the lengths of the surfactant bilayers. In addition, the maximum intensity at the sharp reflection for the C12C6C12Br2/PEG113-PGlu100 system is obviously smaller than both C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/PEG113-PGlu100 systems. The possible reason is that the C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/ PEG113-PGlu100 systems can form much larger and more ordered aggregates than the C12C6C12Br2/PEG113-PGlu100 system. In brief, the three C12CSC12Br2/PEG113-PGlu100 mixtures form ordered nanosheets, and especially the nanosheets formed by C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/PEG113PGlu100 have well-defined hexagonal nanostructures. In the previous study,21 PEG113-PGlu100 was found to form vesicles with cationic single-chain surfactant DTAB. The reason for the difference may be that the bicharged hydrophilic head groups of C12CSC12Br2 can strongly bind to the copolymer, and the C12CSC12Br2 molecules bound on the copolymer tend to strongly aggregate due to their double hydrophobic chains, thus strengthening interpolymer interaction. According to all the above results, the coassembling mechanism of C12CSC12Br2 with PEG113-PGlu100 is proposed in Figure 8. Similar to the DTAB/PEG-PGlu system,21 the electrostatic interaction between C12 C SC 12 Br 2 and the oppositely charged carboxylate groups of PEG113-PGlu100 generates the complexes. The complexes possess the characteristic of superamphiphiles. The PGlu block chain with the bound C12CSC12Br2 molecules constitutes the hydrophobic part of the superamphiphiles, whereas the PEG block chain constitutes the hydrophilic part of the superamphiphiles. The formed superamphiphiles would assemble into sandwich-like arrangement, where the middle part of “sandwich” is formed by the

hydrophobic association of the C12CSC12Br2 molecules on the PGlu chains, whereas the top and bottom parts of “sandwich” are the PEG chains. After the electroneutral point, the outer surface of aggregates is dominantly covered by the ethoxylate groups with almost no net charges. Some of the excess C12CSC12Br2 molecules can bind with the aggregates through hydrophobic interaction and the surfactant charges are arranged at the aggregate surface, so the aggregates can carry weak positive charges. The hexagonal shape of the nanosheets may be related to crystallization property of these superamphiphiles. In addition, as to C12C3C12Br2 and C12C6C12Br2, the shorter spacer has to keep an extended conformation in the coassembly, whereas the spacer of C12C12C12Br2 is long and hydrophobic enough to incorporate into the hydrophobic core of aggregates, thus taking a looplike conformation.39 Moreover, C12C3C12Br2 tends to form closely packed aggregates with a relative large aggregation number due to the shorter spacer length,40 whereas C12C12C12Br2 facilitates the assembly of a large number of surfactant molecules because the spacer with a looplike conformation can join the assembly. Therefore, both C 12 C 3 C 12 Br2 /PEG 113 -PGlu 100 and C 12 C 12 C 12 Br2 /PEG 113 PGlu100 prefer to form larger and more compact aggregates, whereas C12C6C12Br2/PEG113-PGlu100 prefers to form smaller and looser aggregates.

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CONCLUSIONS The coassembly of a hydrophilic copolymer PEG113-PGlu100 with cationic gemini surfactants C12CSC12Br2 have been investigated. Driven by the electrostatic interaction of C12CSC12Br2 with the anionic carboxylate groups of PEG113PGlu100, the C12CSC12Br2/PEG113-PGlu100 superamphiphiles are constructed. The superamphiphiles are soluble even at the electroneutral point and assemble into ordered nanosheets with a sandwich-like packing. The gemini molecules bound on the PGlu chains associate through hydrophobic interaction, constituting the middle part of the nanosheets. The top and bottom of the nanosheets are the hydrophilic PEG chains. The spacer length of the gemini surfactants obviously affects the size and morphology of the resulted aggregates. C12C6C12Br2 with the medium spacer length forms the smallest aggregates with PEG113-PGlu100 and the aggregates are mainly round nanosheets, whereas C12C3C12Br2 or C12C12C12Br2 with a short or a long spacer mainly produces larger and well-defined hexagonal nanosheets with PEG113-PGlu100. These results reveal that novel ordered nanostructures can be constructed through the coassembly of double hydrophophilic block copolymer with gemini surfactants, and the size and morphology of the 9321

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nanostructures can be adjusted by changing the spacer length of the surfactants. The assembling nanostructures combine the merits of both the surfactants and the biologically friendly copolymer, and thus may have potential applications in biological materials.

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ASSOCIATED CONTENT

S Supporting Information *

Solution photos and additional cryoTEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.L.); [email protected] (Y.W.). Fax: 86-10-82615802. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grants. 21025313, 21003137, 21021003). We are thankful to Dr. Gang Ji from the Institute of Biophysics of CAS for his generous help during cryoTEM experiments, and to Prof. Yongfeng Men and Dr. Jiaxue Liu from the Changchun Institute of Applied Chemistry of CAS for providing small-angle X-ray scattering measurements.

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REFERENCES

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