Association Behaviors of Dodecyltrimethylammonium Bromide with

Oct 11, 2012 - The association behaviors of single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with double hydrophilic block co-polymers ...
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Association Behaviors of Dodecyltrimethylammonium Bromide with Double Hydrophilic Block Co-polymer Poly(ethylene glycol)-blockPoly(glutamate sodium) Yuchun Han, Lin Xia, Linyi Zhu, Shusheng Zhang, Zhibo Li,* and Yilin Wang* Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: The association behaviors of single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with double hydrophilic block co-polymers poly(ethylene glycol)-b-poly(sodium glutamate) (PEG 113 −PGlu 50 or PEG113−PGlu100) were investigated using isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, ζ potential, and particle size measurements. The electrostatic interaction between DTAB and the oppositely charged carboxylate groups of PEG−PGlu induces the formation of super-amphiphiles, which further selfassemble into ordered aggregates. Dependent upon the charge ratios between DTAB and the glutamic acid residue of the co-polymer, the mixture solutions can change from transparent to opalescent without precipitation. Dependent upon the chain length of the PGlu block, the mixture of DTAB and PEG−PGlu diblocks can form two different aggregates at their corresponding electroneutral point. Spherical and rod-like aggregates are formed in the PEG113−PGlu50/DTAB mixture, while the vesicular aggregates are observed in the PEG113−PGlu100/DTAB mixture solution. Because the PEG113−PGlu100/DTAB super-amphiphile has more hydrophobic components than that of the PEG113−PGlu50/DTAB super-amphiphile, the former prefers forming the ordered aggregates with higher curvature, such as spherical and rod aggregates, but the latter prefers forming vesicular aggregates with lower curvature.

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INTRODUCTION Self-assembly of amphiphiles in aqueous medium receives extensive research interests mainly from their potential applications in biomedical areas, such as drug and gene delivery systems.1 Amphiphilic block co-polymers, in analogy to classical low-molecular-weight surfactants, can self-assemble into ordered nanostructures in aqueous solution to minimize unfavorable hydrophobe−water interactions.2−4 In contrast, double-hydrophilic block co-polymers (DHBCs) emerge as a new class of block co-polymers with unique and tunable properties to construct hierarchical-ordered supermolecules via self-assembly with various additives.5,6 A typical DHBC consists of two water-soluble blocks but different chemical nature. Usually, one block is ionic, while the other block is non-ionic. One prominent uniqueness is that DHBCs themselves behave like normal polyelectrolytes without showing amphiphilic characteristics, but their amphiphilicity can be modulated by the variation of the temperature, ionic strength, pH value, and addition of oppositely charged substances, e.g., multivalent ions,7−9 surfactants,10−12 and polymers,13−15 to form hierarchical super-amphiphiles. Using such a strategy, several unique super-amphiphiles with stimuli−responsive properties have been reported very recently.16−18 Application of complexing between DHBCs and surfactants is a convenient approach to construct different ordered © 2012 American Chemical Society

structures via self-assembly because of the widely available choices of well-studied surfactants. A big advantage of such strategy is that it is unnecessary to use organic solvents and synthesize new DHBCs. Some pioneering work on the complexation between DHBCs and surfactants was reported by Kabanov and co-workers a decade ago.19−24 They used different cationic surfactants to interact with poly(ethylene glycol)-b-poly(sodium methacrylate) (PEG−PMA) diblocks and found that the resulting solutions were either optically transparent or slightly opalescent. These results were in sharp contrast to the traditional mixtures of homo-polyelectrolytes and oppositely charged surfactants, in which phase separation generally occurred. Using small-angle neutron scattering and small-angle X-ray scattering, Berret and co-workers studied the self-assembly of several charge-neutral diblock co-polymers with conventional single-chain surfactants and demonstrated that the DHBCs/surfactants formed a core−shell structure.25,26 The core was described as a dense microphase of surfactant complexed with the polyelectrolyte blocks of DHBC, while the corona was a diffuse brush composed of the neutral chains of DHBC. More recently, Wang et al. reported a pH-responsive Received: March 23, 2012 Revised: October 10, 2012 Published: October 11, 2012 15134

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stainless-steel sample cell of 1 mL. The cell was initially loaded with 0.7 mL of buffer or PEG−PGlu buffer solution. The concentrated surfactant solution 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. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpy of concentrated sucrose solution. The experiments were repeated at least twice with deviation within 5%. ζ Potential and Particle Size Measurements. The ζ potential and particle size measurements were performed at 25 °C, using a Malvern Zetasizer Nano-ZS instrument (ZEN3600, Malvern Instruments, Worcestershire, U.K.) equipped with a 4 mW He−Ne laser at a wavelength of 633 nm. A clear disposable capillary cell (DTS1060C) was used for both measurements. The ζ potential was calculated from the mobility measured during an electrophoretic light-scattering (ELS) experiment using the Helmholtz−Smoluchowski relationship. The particle size results are given in both size distribution and Z average. Size distribution is derived from a deconvolution of the measured intensity autocorrelation function of the sample accomplished using a non-negatively constrained least-squares fitting algorithm. Z average is the intensity weighted mean hydrodynamic size of the ensemble collection of particles measured by dynamic light scattering (DLS). It is derived from a Cumulants analysis of the measured correlation curve. All of the measurements were performed at θ = 173°. The experiments were repeated 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 the co-polymer 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 a thin film suspended by 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 about −174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan chargecoupled device (CCD) and processed with DigitalMicrograph. Circular Dichroism (CD). Far-ultraviolet (UV) CD spectra were recorded on a JASCO J-815 spectrophotometer at room temperature using a 1 mm path length cell. Scans were obtained in a range between 190 and 260 nm by taking points at 0.5 nm, with an integration time of 0.5 s, and five scans were averaged and smoothed to improve the signal-to-noise ratio.

super-amphiphile based on a benzoic imine bond between double-hydrophilic block co-polymer methoxy-poly(ethylene glycol)114-b-poly(L-lysine hydrochloride)200 and 4-(decyloxy)benzaldehyde.17 Such a super-amphiphile self-assembled into spherical micelles in water under physiological conditions, and the micelles disassembled at pH 6.5, i.e., near the extracellular pH of tumor cells, thus providing a new carrier for loading and releasing guest molecules. In brief, the complexation of DHBCs with low-molecularweight surfactants offers a platform to construct the tunable and functional super-amphiphile aggregates. Herein, this strategy is expanded to biodegradable and biocompatible poly(ethylene glycol)-b-poly(glutamate sodium), from which functional aggregates can be constructed with the aid of surfactants for potential biomedical applications. Figure 1 shows the structures

Figure 1. Molecular structures of double hydrophilic block copolymers.

of poly(ethylene glycol)-b-poly(glutamate sodium) (PEG− PGlu). Two PEG−PGlu co-polymers, i.e., PEG113−PGlu50 and PEG113−PGlu100, in which the subscripts represent the degrees of polymerization (DP) for respective segments, are explored. DTAB is chosen as a cationic surfactant. The complexation and association behaviors of PEG−PGlu/DTAB are explored by varying the molar ratio of DTAB and carboxylate groups.

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EXPERIMENTAL SECTION

Materials. PEG113−PGlu50 and PEG113−PGlu100 were synthesized by ring-opening polymerization of L-benzylglutamate N-carboyxanhydride (NCA).13 The number-averaged molecular weight and molecular weight distribution were characterized using gel permeation chromatography (GPC)/laser light scattering (LLS) with N,Ndimethylformamide (DMF) containing 0.2 M LiBr as the eluent. The Mn values are 15 000 and 33 000 Da, with the polydispersity index (PDI) value of about 1.2, for PEG113−PGlu50 and PEG113−PGlu100. The DP of PGlu was also determined from 1H nuclear magnetic resonance (NMR) using PEG113 as the reference, which was consistent with GPC/LLS results. 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 (DI) water. The solution was then lyophilized to give the product as a white solid. DTAB was purchased from Aldrich and used as received without further purification. All of the experiments were carried out in phosphate buffer of pH 7.4 with an ionic strength of 10 mM. All chemicals were analytical-grade. DI water was obtained from Milli-Q equipment. The co-polymer/surfactant complexes were prepared by mixing the co-polymer and surfactant solutions at room temperature, and then the mixtures were mixed for several minutes using vortex. No additional mechanical agitation was applied. The charge ratio (Z) was defined as the molar ratio of the surfactant concentration (Ct) to the concentration of carboxylate groups of PEG−PGlu (Ci). The concentration of carboxylate groups of PEG−PGlu (Ci) was kept at 2.8 mM in all experiments. The concentrations of surfactant DTAB 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

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RESULTS AND DISCUSSION ITC is a direct and sensitive method to measure the heat change during interaction processes and has been extensively applied to study the association behaviors of surfactants with macromolecules.28−31 For direct comparison, the control experiment was performed by titrating the concentrated DTAB solution of 200 mM into the buffer. Then, ITC experiments were performed by titrating the same DTAB solution into the PEG113−PGlu50 or PEG113−PGlu100 solution. The PEG113−PGlu50 and PEG113−PGlu100 solutions were prepared using the buffer solution. Panels a and b of Figure 2 present the raw calorimetric curves of titrating the DTAB solution into the buffer and into the PEG113−PGlu100 solution, respectively. These data show the variation of heat flow P as a function of time. The observed enthalpies (ΔHobs) are obtained by integrating the peak area of each injection and plotted against the charge ratio Z (Figure 2c). Apparently, the 15135

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anionic unit, the differences in the interaction magnitude for these two systems should result from the different DP values for these two co-polymers. In addition, the corrected calorimetric data, where the enthalpies of DTAB titrated into buffer have been subtracted from the enthalpies of DTAB titrated into co-polymer, are shown in Figure S1 of the Supporting Information. We found that the corrected calorimetric curves show almost the same change trend as the uncorrected curves and the electroneutral complexes are still obtained in the vicinity of Z = 7. In the following text, we focus on the PEG113−PGlu100/DTAB system under different conditions. With the addition of DTAB, the PEG113−PGlu100/DTAB interaction curve initially shows a high endothermic enthalpy change and the solution at this stage is transparent with a slightly bluish color. At Z = 2, the observed enthalpy change shows an abrupt decrease and continues to decrease in a tardy manner until reaching an apparent minimum at Z = 7. At Z = 7, the solution becomes cloudy, as shown in the photo of Figure 2c. Further increasing Z values causes the opposite effect, and the observed enthalpy change starts to slightly increase until merging with the dilution curve of DTAB. The Glu unit of PEG113−PGlu100 carries negative charges at pH 7.4, and thus, it will interact with the positively charged DTAB by a strong electrostatic attraction force. The calorimetric curve presents a large endothermic enthalpy change and a significant decreasing tendency upon the addition of DTAB. Similar calorimetric curves have been reported in many complex systems, where the two interacting molecules carry the opposite charges.32−35 The reason is that binding of one DTAB molecule with a Glu unit will release two counterions, which will increase total system entropy. It was known that such an entropically driven mechanism is also known to govern many electrostatic interactions.32 The minimum point at Z = 7 implies that the electrostatic binding of the DTAB molecules with the co-polymer attains a saturation state, beyond which no more DTAB molecules can bind to the co-polymers. Because the average diameters of the aggregates of the PEG113−PGlu100/DTAB complexes are 73 nm at Z = 1 and 90 nm at Z = 7 from DLS measurement, it means that the complex aggregates are already formed before Z = 2. As a result, the aggregate size increases with the Z value increasing. This point is also supported by the change of the solution visual state from transparent to cloudy. To further know the interaction information at low Z values, PEG113−PGlu100/DTAB was selected as a representative to perform the ITC experiments by titrating 20 mM DTAB solution (just above the critical micellar concentration of DTAB) into the buffer and the PEG113−PGlu100 solution. Figure 3 shows the observed enthalpy changes against the charge ratio Z. The observed enthalpies are very small at the initial two points, then abruptly start to increase, and reach a maximum at Z = 0.5, followed by a steep decrease until a minimum value is attained at Z = 1. At Z = 1, the mixture solution is transparent. After Z = 1, the observed enthalpies increase slightly again, attain the second endothermic maximum at about Z = 1.1 (the mixture solution is still transparent), and then decrease continuously until coinciding with the dilution curve of DTAB into buffer after Z = 2. The mixture solution at the final point (Z = 2.4) is slightly turbid with blue light, as shown in Figure 3 (the bottle), which indicates the formation of some relatively large aggregates. Two endothermic peaks have been obviously observed from the interaction calorimetric

Figure 2. Raw calorimetric titration curves of titrating DTAB solution into the (a) buffer and (b) PEG113−PGlu100 solution, which show the variations of heat flow P as a function of time. (c) Observed enthalpy changes against the charge ratio Z for the titration of the DTAB solution into the (●) PEG113−PGlu50 and (▲) PEG113−PGlu100 solutions. The solid line is the results of the control experiment.

calorimetric curves for the titrations of DTAB into the PEG113− PGlu50 and PEG113−PGlu100 solutions show significant differences from the calorimetric curve of the control experiment. The differences are most likely arising from the interactions of PEG113−PGlu50 or PEG113−PGlu100 with DTAB. From Figure 2c, we find that both co-polymer/DTAB mixtures display a similar change trend, but the interaction magnitude between PEG113−PGlu100 and DTAB is much stronger than that of PEG113−PGlu50 and DTAB. Because the structures of PEG113− PGlu50 and PEG113−PGlu100 are the same except the DP of the 15136

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complexes of DTAB, with the comb-type co-polymers consisting of an anionic backbone and hydrophilic non-ionic poly(N,N-dimethylacrylamide) side chains. To explain why the electroneutrality occurs above Z = 1 and verify if the electroneutral point has dependence upon the surfactant structure, we use C10TAB and C14TAB to complex with PEG113−PGlu50. The results show that the Z values at the electroneutral point have strong dependence upon the hydrophobic chain length of the surfactant. For example, the Z value is 18 for the PEG113−PGlu50/C10TAB complex and 1 for the PEG113−PGlu50/C14TAB complex. The Z value of 7 for PEG113−PGlu50/DTAB (C12TAB) just falls between them. The reason is probably that not all surfactant cations added to the system form ionic bonds with the carboxylate groups of the PEG−PGlu molecules because of the dynamic equilibrium among the free surfactant monomers, the surfactant micelles, and the surfactant−co-polymer complexes. Besides participating in the forming PEG−PGlu/DTAB complex, a certain amount of DTAB molecules form distinct DTAB micelles. The surfactant−co-polymer complexes would coexist with the “free” surfactant micelles and surfactant monomers. As a result, the measured ζ potential value is averaged for all species within the complex systems. Therefore, the neutralized complexes can only be obtained at Z > 1 because the extra DTAB molecules are required to stabilize surfactant−co-polymer complexes. Taking the PEG 113−PGlu50/DTAB complexes as the example, we also measured the normalized Rayleigh ratios at q0 = 2 × 10−3 (θ = 90°) and the hydrodynamic diameter at different Z values. The results are shown in Figure S2 of the Supporting Information. Both the normalized Rayleigh ratio and hydrodynamic diameter start to increase from Z = 1, indicating that the relatively large co-polymer/surfactant aggregates start to form at Z = 1. The normalized Rayleigh ratio attains a high value at around Z = 7, whereas the hydrodynamic diameter is also kept stable at Z = 7. Therefore, we mainly focus on the sizes and morphologies of PEG−PGlu/ DTAB complexes at the electroneutral point of Z = 7, to compare the effect of the ionic block length on resulting assemblies. As shown in Figure 4, the apparent aggregate size distributions are polydispersed with the average diameters of 52 and 90 nm for PEG113−PGlu50/DTAB and PEG113−PGlu100/ DTAB, respectively. Obviously, the aggregate size of the PEG113−PGlu50/DTAB complex is smaller than that of the

Figure 3. Observed enthalpy changes (ΔHobs) against the charge ratio Z for the titration of 20 mM DTAB solution into () buffer and (▲) PEG113−PGlu100. (Inset) Photo of the mixture solution at the final point (Z = 2.4).

curve. Similar results were reported by different groups. For example, Berret et al.35 also found the existence of two endothermic peaks at the calorimetric curve of the titration of DTAB solution into poly(sodium acrylate)-b-poly(acrylamide) (PANa6500−PAM37 000) co-polymer solution and showed that the secondary endothermic peak strongly depends upon the absolute concentration of DTAB. Tam et al.33,34 concluded that the first endothermic peak characterizes the electrostatic binding of the surfactant to co-polymer chains, whereas the second peak corresponds to the micellization of polymer-bound surfactant molecules. Because ζ potential can reflect the charge state of aggregates in solution, we thus performed ζ potential measurements on the PEG113−PGlu50/DTAB and PEG113−PGlu100/DTAB complexes at different Z values. The results are summarized in Table 1. The ζ potential values change from negative to Table 1. ζ Potential Values of the PEG113−PGlu50/DTAB and PEG113−PGlu100/DTAB Complex Aggregates at the Different Charge Ratios (Z) ζ potential (mV) Z = Ct/Ci

PEG113−PGlu50/DTAB

PEG113−PGlu100/DTAB

0 0.5 1 2 7 12

−18.9 −10.9 −6.1 −2.3 1.7 5.5

−38.5 −21.1 −12.8 −4.1 0.3 3.7

positive with the increase of Z values from 0 to 12, which is due to the binding of DTAB onto the co-polymer. The electroneutral complexes are obtained in the vicinity of Z = 7. At Z = 1, the ζ potential values are still negative, which are −6.1 and −12.8 mV for the PEG113−PGlu50/DTAB and PEG113− PGlu100/DTAB complexes, respectively. The result is different from the previous reports,19,20,22 where the electroneutral points were obtained at Z = 1. The authors presumed that all of the surfactant ions in the system formed stoichiometric complexes via ionic bonds with oppositely charged groups of polymers. Only the study from Balomenou et al.36 showed that the electroneutral complexes were obtained at Z = 4 in the

Figure 4. Distributions of the aggregate diameters for the (●) PEG113−PGlu50/DTAB and (▲) PEG113−PGlu100/DTAB complexes at 25.0 ± 0.5 °C. 15137

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PEG113−PGlu100/DTAB complex. The reason is that the ionic block in the former is shorter than that of the latter. The morphologies of the PEG−PGlu/DTAB complexes at Z = 7 were characterized by the cryoTEM technique. A great advantage of cryoTEM is that it allows for the direct visualization of nanoscopic objects in hydrated states without drying samples. The cryoTEM images of the PEG−PGlu/ DTAB complexes are shown in Figure 5 and Figure S3 of the

the co-polymer. Moreover, the PEG113−PGlu100/DTAB complex has a more prominent structure peak (at q0 = 0.16 Å−1) compared to the PEG113−PGlu50/DTAB complex, which shows that PEG113−PGlu100/DTAB forms much more ordered core−shell aggregates. This is in accordance with the cryoTEM result that PEG113−PGlu100/DTAB forms highly ordered vesicle aggregates, whereas PEG113−PGlu50/DTAB forms spherical and rod aggregates. In addition, the above complex solutions at Z = 7 show great stability. ζ potential and size measurements indicate that the changes in the charge state and size are negligible after storing for more than 3 weeks, and the longer storage time did not cause any noticeable precipitation. The secondary structure of the poly(glutamate sodium) segment was characterized using CD spectroscopy. Figure 6 shows the CD spectra of the PEG−PGlu co-polymer alone and the PEG−PGlu/DTAB complexes in solutions. The CD spectra of PEG113−PGlu50 and PEG113−PGlu100 themselves exhibit some differences. The CD spectrum of PEG113−PGlu50 has double minima at 208 and 222 nm and a maximum at 196 nm, showing the α-helix conformation. For PEG113−PGlu100, the CD spectrum shows two extreme values, with a maximum centered at ∼217 nm and a minimum centered at ∼198 nm, which is the typical spectrum of the random-coil conformation. The addition of DTAB induces significant variations of CD spectra for both of the co-polymers, and the variations become more significant with the addition of more DTAB molecules. Because of the strong absorbance of the DTAB molecule itself, the precise CD spectra cannot be obtained at higher Z values. We did not observe the ordered secondary from the CD spectra. For PEG113−PGlu50, the addition of DTAB induced the disappearance of the negative band around 208 nm, whereas for PEG113−PGlu100, the addition of DTAB induced the significant ellipticity decrease and obvious red shift at 198 nm. Liu et al.37 also showed that alkyltrimethylammonium ions with bulky headgroups are ineffective at inducing transformation of the secondary structure of polypeptide and thought that the steric hindrance of the surfactant headgroup may inhibit the formation of ordered conformation. The addition of DTAB can destroy the second structure of PEG− PGlu alone, proving that DTAB can interact strongly with PEG−PGlu. According to all of the above experimental results, the mechanism of the PEG−PGlu/DTAB complex formation is proposed, as shown in Figure 7. Upon the addition of DTAB (far below its critical micelle concentration), the DTAB molecules cannot self-aggregate into the micelles and would

Figure 5. CryoTEM images for the PEG−PGlu/DTAB mixture solutions at Z = 7: (a) PEG113−PGlu50/DTAB complex and (b) PEG113−PGlu100/DTAB complex.

Supporting Information. The PEG113−PGlu50/DTAB complex solution is slightly bluish, and the cryoTEM image (Figure 5a) displays the irregular spherical and rod aggregates with a diameter of ∼15 nm. For the PEG113−PGlu100/DTAB complex, the solution is turbid and the cryoTEM image (Figure 5b) demonstrates that the vesicles with the diameter ranging from 90 to 170 nm are the major species. The wall thickness of the vesicles is about 15 nm. Here, the vesicle membrane exhibits the rugosity and irregularities, which are due to the structure characteristics of PEG113−PGlu100/DTAB. In contrast to the conventional well-defined diblock co-polymer, PEG113− PGlu100/DTAB is more or less like a toothbrush structure because the DTAB molecules are selectively complexed with the PGlu segment. Therefore, the hydrophobic segments are more rigid and not well-defined. A small-angle X-ray scattering technique is also used to study the PEG−PGlu/DTAB complex structures, and the results are shown in Figure S4 of the Supporting Information. As described by Berret et al. in their studies,25,26 the co-polymers associate with DTAB into colloidal complexes, which have a core−shell microstructure. The core was described as a dense microphase of surfactant complexed with the polyelectrolyte blocks of the co-polymer, while the corona was a diffuse brush composed of the neutral chains of

Figure 6. CD spectra of the PEG−PGlu alone and PEG−PGlu/DTAB complexes: (a) PEG113−PGlu50/DTAB and (b) PEG113−PGlu100/DTAB. 15138

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Obviously, the PEG113−PGlu100/DTAB super-amphiphile has more hydrophobic components than the PEG113−PGlu50/ DTAB super-amphiphile because of its longer Glu block chain. Thus, the PEG113−PGlu100/DTAB super-amphiphile would form the vesicle aggregates with small curvature, whereas the PEG113−PGlu50/DTAB super-amphiphile would form the aggregates with large curvature, including the spherical and rod aggregates.

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CONCLUSION The association behaviors of cationic surfactant DTAB with the double hydrophilic block co-polymers PEG−PGlu have been investigated. Driven by the combination of electrostatic and hydrophobic interactions, the PEG113−PGlu50/DTAB and PEG113−PGlu100/DTAB complexes form the supramolecular aggregates with different morphologies. The PEG113−PGlu50/ DTAB complexes prefer forming the spherical and rod aggregates, while the PEG113−PGlu100/DTAB complexes prefer forming the vesicular aggregates. Our results showed that we can control the assembly morphology by mixing easily available surfactant with DHBCs to obtain different supramolecular aggregates. This method affords an array of parameters to tune resulting aggregate morphology and properties. Moreover, we focused on the relationship between the ionic block length and resulting co-polymer/surfactant complex structure in this study. We demonstrated that the ionic block length of the co-polymer has a significant effect on the formed complex structure. Also, the structure of surfactants is important to determine the interaction magnitute with DHBCs. These studies will help us to further understand the interactions in DHBC/surfactant systems and to enhance structure control over DHBC/ surfactant complexes.

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ASSOCIATED CONTENT

* Supporting Information S

Additional ITC data, light scattering data, small-angle X-ray scattering data, and cryoTEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. Proposed mechanism of the binding and aggregation for the PEG−PGlu/DTAB complexes: (left) PEG113−PGlu50/DTAB and (right) PEG113−PGlu100/DTAB.

AUTHOR INFORMATION

Corresponding Author

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

only bind to the Glu blocks via electrostatic attraction forces. With the increase of the DTAB concentration, the DTAB molecules already bound to the PEG−PGlu co-polymers would form the micelle-like aggregates because of the relatively high local concentration of hydrophobic chains. When the DTAB concentration is further enhanced, more and more DTAB molecules would be bound to the Glu blocks of the copolymers, therefore promoting the hydrophobic association of the DTAB molecules already bound to the different co-polymer chains and finally leading to the ordered and stable assemblies. Both PEG113−PGlu50 and PEG113−PGlu100, after binding with enough DTAB molecules, behave like the super-amphiphiles. However, the associations of DTAB with PEG113−PGlu50 and PEG113−PGlu100 produce different ordered structures. The PEG113−PGlu50/DTAB complex forms the spherical and elongated rod aggregates, while the PEG113−PGlu100/DTAB complex forms the vesicle structure. The Glu block chain with the bound DTAB molecules constitutes the hydrophobic part of the super-amphiphiles, whereas the EO block chain constitutes the hydrophilic part of the super-amphiphiles.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grants 21025313, 50821062, 21003137, and 21021003), and we are thankful to Prof. Yongfeng Men and Dr. Jiaxue Liu, from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, for providing the small-angle X-ray scattering measurements.

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REFERENCES

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dx.doi.org/10.1021/la303646r | Langmuir 2012, 28, 15134−15140