Balance of Coordination and Hydrophobic Interaction in the Formation

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Balance of Coordination and Hydrophobic Interaction in the Formation of Bilayers in Metal-Coordinated Surfactant Mixtures Hongshan Tian, Dong Wang, Wenlong Xu, Aixin Song,* and Jingcheng Hao Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: Metal−ligand coordination and hydrophobic interaction are two significant driving forces in the aggregation of mixtures of Mn+ surfactants and alkyldimethylamine oxide (CnDMAO) in aqueous solutions. The coordinated systems exhibit rich aggregation behavior. This study investigated the effect of Mn+ ions (Zn2+, Ca2+, Ba2+, Al3+, Fe3+, La3+, Eu3+, and Tb3+) and hydrophobic chains (hydrocarbon and fluorocarbon) on the formation of metal-coordinated bilayers. We found that fluorocarbon chains and branched hydrocarbon chains are preferable to the corresponding linear hydrocarbon chains for the formation of an Lα phase. Moreover, Lα phases formed by fluorocarbon chains exhibited higher viscoelasticity than ones formed by the hydrocarbons, and the bilayers formed by branched chains were rather flexible, revealing obvious undulation. The construction of bilayers was also strongly affected by metal ions due to their variable coordination ability with CnDMAO. Our results contribute to the understanding of the formation of metal-coordinated bilayers, which is driven by the interplay of noncovalent forces.

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(CnDMAO)5−8 to form diverse aggregates, especially vesicles. In these metal-coordinated surfactant mixtures, Mn+ ions are tightly associated with the headgroups of surfactants, constructing monolayer or bilayer membranes of aggregates. These systems have been used as templates for preparing nanoor micromaterials.5−8 A series of metal−surfactants participated systems, using different metal ions (Zn2+, Ca2+, Ba2+, Al3+, etc.) as counterions have been studied and reviewed.5−8 In a previous report, we studied the effect of the hydrophobic chains of CnDMAO and the hydrophilic headgroups of the anionic surfactants on the phase behavior of coordinated systems.7 The results showed that the aggregation behavior of the metal− ligand complexes was strongly affected by the polar groups and the chain length of the surfactants. The noncovalent interactions of Mn+ surfactants/tetradecyldimethylamine oxide (C 14DMAO) systems in aqueous solutions are shown in Scheme 1; zinc carboxylate [(RCOO)2Zn]/C14DMAO mixtures were used as an example. This study investigated two aspects of the effect of Mn+ surfactant structures on the aggregation behavior of the metal-coordinated surfactant mixtures. First, to test the hydrophobic interaction, various (RCOO)2Zn compounds with different hydrophobic chains were mixed with C 14DMAO; second, to detect the metal coordination, perfluoropelargonate salts with different metal ions as counterions, (C8F17COO)nM (Mn+ = Ca2+, Ni2+, Al3+, Tb3+, etc.), were

INTRODUCTION The self-assembly structures of surfactants in solutions that use noncovalent interactions (e.g., hydrogen bonding, electrostatic interaction, hydrophobic interaction, metal-coordination, steric effect, π−π stacking, etc.) have attracted significant research attention.1−10 Multilevel surfactant self-assemblies can be obtained by choosing or designing surfactants with different hydrophobic and hydrophilic structures that will construct or modulate the noncovalent interactions.11,12 Metal−ligand coordinated surfactant mixtures5−8 have many advantages for processes that use the noncovalent forces to form the aggregates in solutions.13,14 A number of metal−ligand coordinated systems with diverse self-assembly structures and novel functions have been constructed.15,16 The metal-coordinated complexes needed to fabricate self-assembly structures can be obtained by mixing metal ions with copolymers and peptides to form supramolecules of linear, branched, and starlike structures.17−25 Another general route is to use surfactants with Mn+ ions (n = 2 or 3) as counterions. Anion surfactants with Mn+ ions (n = 2 or 3) as counterions usually have high Krafft points and are hardly soluble in water, exhibiting very simple aggregation behavior in aqueous solutions at room temperature.26 To improve the solubility, one can modulate the ratio of monovalent to divalent or multivalent metal ions, as the size of the aggregates usually changes with the mixing ratios.27−30 Moreover, adding cosurfactants or nonionic surfactants can also induce the solubility of multivalent metal surfactants, along with the formation of different aggregates.26 As a typical example, these surfactants can easily coordinate with alkyldimethylamine oxide © 2013 American Chemical Society

Received: May 29, 2012 Revised: February 20, 2013 Published: February 25, 2013 3538

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Scheme 1. Noncovalent Interactions in Bilayers of Mn+ Coordinated Systems for (R-COO)2Zn/C14DMAO mixtures

for at least 4 weeks at 25.0 ± 0.1 °C, until they were unchanged over an extended period of time. Surface Tension Measurements. Surface tension measurements were performed on a Tensionmeter K100 (Krüss Company), using the plate method at 25.0 ± 0.1 °C. The time interval between every two measurements was about 8 min. The maximum surface excess concentration, Γmax, and the polar-head surface area at the CMC, a0, were calculated from the Gibbs equation: Γmax = −

⎛ dγ ⎞ 1 lim ⎜ ⎟T 2.303nRT c → cmc⎝ dlog c ⎠

a0 = 1/NΓmax

(1) (2)

where c is the surfactant concentration, N is Avogadro’s number, R is the gas constant, and T is the temperature. The value of n is considered to be 1, as the Mn+ surfactants (n ≥ 2) hardly dissociate, and almost no metal ions exist in the solutions, given the property of nonionic species.26 Rheological Measurements. Rheological measurements were carried out on a HAAKE RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). For the oscillatory measurements, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep to ensure that the selected stress was in the linear viscoelastic region. Samples were measured at 25.0 ± 0.1 °C with the help of a cyclic water bath. Cryogenic (Cryo)-TEM Observations. Cryo-TEM samples were prepared in a controlled environment vitrification system (CEVS). A micropipet was used to load about 5 μL of solution onto a TEM copper grid coated with carbon support film, which was then blotted with two pieces of filter paper to form a thin film. After 4 s, the sample was quickly plunged into a reservoir of liquid ethane (cooled with liquid nitrogen) at −165 °C. The vitrified sample was then stored in the liquid nitrogen until it was transferred to a cryogenic sample holder (Gatan 626) and examined on a JEOL JEM-1400 TEM (120 kV) at about −174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with a digital micrograph. The cryo-TEM observations were performed by Dr. Hao’s group.

mixed with C14DMAO in aqueous solutions. We found that the aggregation behavior exhibited remarkable differences when the hydrophobic chains and counterions were varied. On the basis of the results, we suggest that the self-assembly process is driven by the interplay of metal−ligand coordination and hydrophobic interaction.

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EXPERIMENTAL SECTION Chemicals and Materials. The tetradecyldimethylamine oxide (C14DMAO) aqueous solution (30 wt %) was received as a gift from Clariant Company’s affiliate in China. The solution was freeze-dried and then crystallized three times in acetone. The perfluorononanoic acid (C8F17COOH, 98%), 3,5,5trimethylhexanoic acid (branched C8H17COOH, 97%), and nonanoic acid (C8H17COOH, 98%) were purchased from the Fluorochem Company (England), the Tokyo Chemical Industry company, and the Sinopharm Chemical Reagent Company, respectively. They were all used directly without further purification. Other chemicals used were all of P. A. quality. Preparation of Anionic Surfactants with Mn+ as Counterions. To prepare the (C 8 F 17 COO) n M, the C8F17COOH was dissolved in ethanol and then mixed with NaOH aqueous solutions at an equal molar ratio under stirring at room temperature. After filtration, the precipitates (C8F17COONa) were washed with water and ethanol, successively, and then dried at about 50 °C. The (C8F17COO)nM were obtained by mixing the C8F17COONa aqueous solutions with excess salt solutions of CaCl2, ZnCl2, NiCl2, AlCl3, La(NO3)3, Eu(NO3)3, and Tb(NO3)3 at 50 °C. The mixtures were cooled to room temperature, and all of the precipitates were filtered until no Cl− ions were detected. The powders were dried at about 50 °C. To prepare (C8H17COO)2Zn and (b-C8H17COO)2Zn, the C8H17COOH and 3,5,5-trimethylhexanoic acid were dripped to NaOH solutions under stirring, respectively. The solutions were adjusted to pH = 5, and then ZnCl2 solutions were dripped. The precipitates were filtered and washed by water and ethanol successively until no Cl− ions were detected. The precipitates were freeze-dried at about −50 °C. For the products obtained, Na+ was detected within 0.3 wt % to ∼1 wt % of the total metal ions on a WP1 atomic emission spectrometer. Phase Behavior. Phase behavior was studied through visual inspection with the help of crossed polarizers. Samples were obtained by dissolving various amounts of anionic surfactants into C14DMAO micellar aqueous solutions under ultrasonication; these samples were then allowed to be equilibrated

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RESULTS AND DISCUSSION Detection of Hydrophobic Interaction. For the detection of the hydrophobic interaction, we mixed three Zn2+ surfactants with different hydrophobic chains, zinc perfluoropelargonate [(C8F17COO)2Zn], zinc pelargonate [(C8H17COO)2Zn], and zinc 3,5,5-trimethylcaproate [(bC8H17COO)2Zn], with 100 mmol L−1 C14DMAO. The 100 mmol L−1 C14DMAO was chosen for two reasons: (i) the coordinated interaction is inconspicuous at low C14DMAO concentrations and (ii) when the concentration is above 100 mmol L−1, based on the phase behavior reported by Hoffmann et al., the C14DMAO solution begins an overlap range, in which the interaction between the micelles is repulsive in nature, exhibiting a complicated state.31 Due to the presence of divalent counterions, all of the Zn2+ surfactants used have high Krafft points (above 50 °C) and show poor solubility in water at room temperature. When these surfactants were added to the C14DMAO micellar solutions,31 the combination of the coordination between Zn2+ and the N → O group of 3539

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phase separation occurred with a thick turbid phase appearing at the bottom of the two-phase (marked as L1/Lα′) solution. Figure S2a of the Supporting Information shows the fragments of irregularly shaped short bilayers in the bottom phase, which stack to separate from the transparent L1 phase, indicating a transition from micelles to bilayers. A similar bilayer structure was reported by Miguel et al. in catanionic vesicles that were charged by opposite polyelectrolytes.33 After the two-phase region, a three phase (marked as L1/Lα′/Lα) occurred, with a new slightly turbid phase in the middle. For the birefringent middle phase, the longer curly membranes were observed from the cryo-TEM image (Figure S2b of the Supporting Information); this can be ascribed to the growth of the stacked short unshaped bilayers. In the transparent upper phase, the solubilization of Sudan II (not shown) demonstrated the existence of the micelles. After the three-phase, the upper L1 phase disappeared and the middle phase extended to form another two-phase solution (marked as Lα/Lα′), indicating the achievement of the transition from micelles to bilayers. With the addition of more (C8F17COO)2Zn, a slightly turbid Lα phase with obvious birefringence appeared (Figure 3a). Finally, (C8F17COO)2Zn reached excess and precipitated at the bottom of the test tube. For the three studied systems, the phase transition process was similar, exhibiting a phase sequence of L1 phase, L1/Lα′ two phase, L1/Lα/Lα′ three phase, Lα/Lα′ two phase, Lα phase, and precipitates. However, there were also differences between the three systems. The regions of the different phases formed by (C8F17COO)2Zn, (C8H17COO)2Zn, and (b-C8H17COO)2Zn in 100 mmol L−1 C14DMAO solution are shown in Figure 2.

C14DMAO and the hydrophobic interaction resulted in rich aggregation behavior. In a discussion of the aggregation behavior, the self-assembly of amphiphilic molecules in aqueous solutions is usually described using the packing parameter, p, which is defined by the equation32 p = v /la0 = a tail /a0

(3)

where v and l are the volume and length of a hydrophobic chain, respectively, a0 is the area of the hydrophilic group of an amphiphilic molecule, and atail is the cross-sectional area of the hydrophobic tail. When p increases, spherical micelles (p ≤ 1/ 3), rodlike or wormlike micelles (1/3 < p ≤ 1/2), bilayers (1/2 < p ≤ 1), and reverse structures (p > 1) can be obtained. The surface tension results (Figure S1 of the Supporting Information) were used to calculate a0 from eqs 1 and 2. The results, shown in Table 1, demonstrated that the addition of Table 1. Surface Properties of Zn2+ Surfactants/C14DMAO Systems mixtures of different molar ratios (r)

CMC (mmol L−1)

γcmc (mN m−1)

Γ (μmol m−2)

a0 (Å2)

C14DMAO (C8F17COO)2Zn/ C14DMAO (r = 1:10) (C8F17COO)2Zn/ C14DMAO (r = 3:10) (b-C8H17COO)2Zn/ C14DMAO (r = 1:10) (b-C8H17COO)2Zn/ C14DMAO (r = 3:10)

0.14 0.0078

31.77 22.41

2.806 7.471

59.18 22.23

0.0061

21.52

8.226

20.19

0.036

25.64

4.226

39.29

0.020

24.14

5.478

30.31

Zn2+ surfactants to the C14DMAO solutions caused a reduction in a0, due to the formation of coordination between Zn2+ and C14DMAO. Moreover, the combination of Zn2+ surfactants and C14DMAO induced an increase in the cross-sectional area of the hydrophobic tail, atail. The increasing atail and the decreasing a0 led to an increase in the p value, and correspondingly, the growth of aggregates; this in turn induced a series of phase transitions. In Figure S1 of the Supporting Information and Table 1, the surface property of the (C8H17COO)2Zn/ C14DMAO system is not shown, as the aggregation only appears at a rather high temperature, above 55 °C. The aggregation behavior of the (C 8 F 17 COO) 2 Zn/ C14DMAO system was selected to be studied in detail. As shown in Figure 1, at low (C8F17COO)2Zn concentrations, the (C8F17COO)2Zn molecules inserted into the C14DMAO micelles and formed mixed micelles due to the coordination between Zn2+ and C14DMAO, which induced an increasing p value and the growth of aggregates. With the continuously increasing concentration of (C8F17COO)2Zn, a macroscopic

Figure 2. Phase regions of different Zn2+ surfactants dissolved in 100 mmol L−1 C14DMAO. The different phases are marked in the following manner: L1 phase (empty rectangle), L1/Lα′ two-phase (rectangle with forward-slanted stripes), L1/Lα/Lα′ three-phase (horizontal-striped rectangle), Lα/Lα′ two-phase (rectangle with backward-slanted stripes), Lα phase (background-filled rectangle with forward-slanted stripes), and precipitates (pattern-filled rectangle).

For the (C8H17COO)2Zn/C14DMAO mixtures, only the L1 phase and precipitates appeared at 25 °C. When heated to above 55 °C, the phase behavior was similar to that of (C8F17COO)2Zn/100 mmol L−1 C14DMAO but with a narrower Lα phase region. When the linear hydrophobic chain was replaced with a branched chain, in the (bC8H17COO)2Zn/100 mmol L−1 C14DMAO mixtures, the birefringent Lα phase formed at room temperature. The phase behavior demonstrates that when coordinated with C14DMAO, due to the differences in the structure of the hydrophobic chains, the Zn2+ surfactants of fluorocarbon and branched hydrocarbon chains are more favored to form an Lα phase than those of linear hydrocarbon chains. The result should be ascribed to the different steric effects and

Figure 1. Phase behavior of the (C8F17COO)2Zn/100 mmol L−1 C14DMAO/H2O system at 25 °C. The photos are without (up) and with (below) crossed polarizers. 3540

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had a spherical shape and were hardly deformed. For the (C8H17COO)2Zn in 100 mmol L−1 C14DMAO solutions at 60 °C, the diameter of the polydisperse vesicles ranged from about 50 nm to several hundred nanometers (Figure 3b). Moreover, many bilayer membranes were strongly elongated and deformed, revealing a flexible property. For the (bC8H17COO)2Zn/100 mmol L−1 C14DMAO system, a few elliptical and elongated vesicles with diameters from several tens of nanometers to about or even more than 1 μm were found coexisting with unordered bilayers (Figure 3c). The bilayers joined with each other to form closed districts with irregular shapes and borders with both positive and negative curvatures, indicating a random undulation that should be induced by the high flexibility of the bilayers. In accordance with the curvature free-energy equation,35−37

hydrophobicity of the hydrophobic chains. The surfactant parameter can be calculated for a fluorocarbon surfactant using the formula, atail/a0 = 31.3 Å2/a0, and the surfactant parameter for the hydrocarbon analogue can be calculated by 21.3 Å2/a0.34 This indicates that to have the same packing parameter, hydrocarbon surfactants need a smaller a 0 than the fluorocarbon analogues; similarly, branched hydrocarbon surfactants have larger atail values than linear ones. Thus, the fluorocarbon and branched hydrophobic surfactants prefer structures with little curvature (e.g., vesicles). It should be pointed that due to the combination of the two surfactant hydrophobic chains with different proportions, we could not obtain an accurate v or atail for the mixtures of Zn2+ surfactants and C14DMAO and can only deduce the aggregation behavior from the variation tendency of atail/a0. For each system, a sample in the middle region of the Lα phase was selected to be characterized using the cryo-TEM. The images are shown in Figure 3. For the Lα phase formed in

2 ⎛1 1 ⎞ f = 2K ⎜ − ⎟ R0 ⎠ ⎝R

(4)

where f is the curvature energy per unit area of vesicle bilayer, K is an effective bending constant, R is the vesicle radius, and R0 is the radius of the vesicle with minimum energy, the microstructures of the vesicles are mainly determined by (i) the spontaneous curvature (1/R0) of the surfactant bilayers, which chooses the optimum radius (R0) of vesicles and then decides the vesicle sizes and (ii) the effective bending constant (K), which represents the curvature elasticity or rigidity of the vesicle bilayers and governs the polydispersity of the vesicles.38 The microstructures can be reflected in different macroscopic properties (e.g., the rheological property). For vesicle solutions with high-volume fractions, the total energy of the system depends on the interaction between vesicles and vesicle deformability, which influences the rheological behavior and the elastic modulus, G′.39 When K is low and on the same order of kBT (kB is the Boltzmann’s constant), the vesicle interaction is dominated and is mainly induced by the repulsion of thermal undulation. However, for a high K, G′ is the major result of the energy stored in the deformation of vesicle bilayers, which was

Figure 3. Cryo-TEM images of the Lα phase formed by (a) 30 mmol L−1 (C8F17COO)2Zn, (b) 18 mmol L−1 (C8H17COO)2Zn, and (c) 30 mmol L−1 (b-C8H17COO)2Zn in 100 mmol L−1 C14DMAO at 25 °C, 60 °C, and 25 °C, respectively.

the (C8F17COO)2Zn/100 mmol L−1 C14DMAO system, the cryo-TEM image (Figure 3a) was densely polydisperse uni- and multilamellar vesicles with diameters from several 10 nm to near 1 μm. Most of the vesicles were unilamellar, and a few small unilamellar vesicles were also found to be enclosed in big vesicles. In general, the vesicle bilayers in the cryo-TEM image

Figure 4. Oscillatory rheograms of the Lα phase formed by (a) 30 mmol L−1 (C8F17COO)2Zn, (b) 18 mmol L−1 (C8H17COO)2Zn, and (c) 30 mmol L−1 (b-C8H17COO)2Zn in 100 mmol L−1 C14DMAO at 25.0, 60.0, and 25.0 °C, respectively. 3541

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described by Milner and Safran.40 In the systems examined in this study, the fluorocarbon chain, C8F17COO−, was much stiffer than the hydrocarbon chains, C8H17COO− and bC 8 H 1 7 CO O − . Therefo re, vesicles formed in the (C8F17COO)nM/C14DMAO system had a smaller R0 and a larger K and were therefore more likely to be smaller sized unilamellar vesicles. The oscillatory rheological measurements show that both the elastic modulus (G′) and viscous modulus (G″) were independent of oscillatory frequency, and G′ was about one order higher than G″, exhibiting the elastic dominance (Figure 4a). For the Lα phase formed by (C8H17COO)2Zn and (b-C8H17COO)2Zn in 100 mmol L−1 C14DMAO, the flexible hydrocarbon chains induced a larger R0 and a smaller K, leading to the polydisperse vesicles with irregular shapes. The Lα phase exhibited lower viscoelasticity than in the (C8F17COO)nM/C14DMAO system (Figure 4, panels b and c). In the (b-C8H17COO)2Zn/C14DMAO system in particular, both G′ and G″ were very low and strongly dependent on the oscillatory frequency. Moreover, G″ was higher than G′ before they intersected at a certain frequency value; the relaxation time (τR), obtained from their intersection, demonstrated the faster recovery process. In general, the different compositions and structures of the hydrophobic chains induced variation in the rigidity and microstructure of the bilayers (i.e., shape and size distribution). The elasticity of the Lα phase in the (C8F17COO)nM/C14DMAO system should be mainly ascribed to the deformability of the bilayers. In the (C 8 H 17 COO) 2 Zn/C 14 DMAO and (b-C 8 H 17 COO) 2 Zn/ C14DMAO systems, due to the decrease of bilayer rigidity, the contribution of the undulation repulsion increased in the elastic energy. Detection of Metal Ions. The mixtures of (C8F17COO)nM (Mn+ = Ca2+, Ni2+, Al3+, Fe3+, La3+, Eu3+, and Tb3+) and 100 mmol L−1 C14DMAO were studied to detect the effect of metal ions. For anionic surfactants with monovalent metal ions as counterions, the metal ions move freely in the solutions and the Lα phase cannot form due to the absence of the metalcoordinated interaction.6 For this phase of the study, several divalent (Zn2+, Ca2+, and Ni2+) and trivalent (Al3+, Fe3+, La3+, Eu3+, and Tb3+) metal ions were selected. The obviously different phase behavior (Figure 5) revealed the importance of metal-binding in these systems. For the (C8F17COO)2M/C14DMAO systems (Mn+ = Zn2+, Ca2+, and Ni2+), shown in Figure 5a, the Lα phase appeared at the lowest (C8F17COO)2Zn concentration and had the largest range in the (C8F17COO)2Zn/C14DMAO system. In the (C8F17COO)2Ca/C14DMAO mixtures, the Lα phase occurred within the smallest region of (C8F17COO)2Ca concentration. Figure 6 shows the microstructures of the Lα phase formed by (C8F17COO)2Ca and (C8F17COO)2Ni in 100 mmol L−1 C14DMAO, respectively. Uni- and multilamellar vesicles were found with diameters from about 100 nm to more than 500 nm. In the (C8F17COO)2Ni/C14DMAO system, the vesicles were perfectly spherical. However, in the (C8F 17COO)2Ca/ C14DMAO system, there were slightly deformed elliptical multilamellar vesicles. The oscillatory results (Figure 6c) showed that the Lα phase of the (C8F17COO)2Ca/C14DMAO system demonstrated the lowest elasticity of the three systems. The differences in aggregation behavior and rheological properties should correlate with the different binding potentials of the metal ions to the ligands. Combined with the phase behavior study, the results suggest that compared with Zn2+ and Ni2+, Ca2+ has a weaker binding to C14DMAO.

Figure 5. Phase regions of (a) M2+ surfactants and (b) M3+ surfactants in 100 mmol L−1 C14DMAO aqueous solution at 25 °C. The different phases are marked in the following manner: L1 phase (empty rectangle), L1/Lα′ two-phase (rectangle with forward-slanted stripes), L1/Lα/Lα′ three phase (horizontal-striped rectangle), Lα/Lα′ twophase (rectangle with backward-slanted stripes), Lα phase (background-filled rectangle with forward-slanted stripes), and precipitates (patteren-filled rectangle).

Figure 6. Cryo-TEM images of the Lα phase formed by (a) 17 mmol L−1 (C8F17COO)2Ca and (b) 17 mmol L−1 (C8F17COO)2Ni in 100 mmol L−1 C14DMAO and the oscillatory rheograms of the Lα phase formed by 17 mmol L−1 (C8F17COO)2M (M = Zn2+, Ca2+, and Ni2+) in 100 mmol L−1 C14DMAO.

For the (C8F17COO)3M/C14DMAO systems, the Lα phase could not be obtained in the (C8F17COO)3Al/C14DMAO 3542

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complexes of different geometry, usually octahedral (Ni2+, Co2+, etc.) or tetrahedral (Zn2+, etc.).42 However, we should note that in addition to the ion binding, the metal−ligand complexes are also affected by the hydrophobic interaction that induces the distortion of the coordination geometry when the hydrophobic chains arrange to form bilayers. Thus, it is very difficult to describe accurately how the binding strength, the hydrophobic interaction, and the coordinated geometry affect the aggregation behavior and rheological properties. All of these factors are worth further research in the surfactant sciences. Here, a rough speculation about the mechanism of the different metal ions is offered. The mixtures of (C8F17COO)2M (M = Zn2+, Ca2+, and Ni2+) and C14DMAO, Ni2+ have 3d8 electronic configurations in the outmost shell and these tend to form strong complexes, inducing tight binding to the ligands. In the case of Zn2+, although the 3d orbital is fully filled (3d10), the stable Zn2+-coordinated complexes can also be formed by accepting electrons in its higher unoccupied orbitals.43 The sblock ion, Ca2+, has a weaker binding to the ligand than Ni2+ and Zn2+, due to its weak-coordinated potential. For Al3+ and Fe3+, the unfilled d orbital (3d5) of Fe3+ encourages the more tight binding of Fe3+ with the N → O group than Al3+ due to the empty 3s and 3p orbitals of Al3+. For the lanthanide ions, La3+, Eu3+, and Tb3+, our finding that the likelihood of vesicle bilayers formation increases with the atomic number reveals an enhancement of the metal ions binding to the ligands, probably correlated with the lanthanide contraction. Due to an incomplete shielding of the nuclear charge by the 4f electrons of lanthanide ions,44,45 the filling of the 4f shell across the lanthanide series is accompanied by a considerable decrease in the ionic radius, a phenomenon known as the lanthanide contraction. The decrease in ionic radius strengthens the interaction between the lanthanide ions and ligands, as the atomic number increases. It should be noted that we only provide a general idea of the metal-coordinated interaction. The real coordinated states are probably much more complicated. In aqueous solutions, the metal ions are hydrated with various numbers of H2O molecules. When coordinated with C14DMAO, some of the H2O molecules should be replaced by the N → O groups of C14DMAO molecules. The formation of the bilayers should be affected by the competition of the H2O and C14DMAO molecules, as they combine with metal ions. Thus, the coordination numbers of Mn+ and the geometry of the coordination complexes are unlikely to be accurately confirmed by experimental methods. For deeper understanding, the theoretical calculations should be helpful. However, the complicated electronic configuration of metal ions and the

mixtures at the studied condition (Figure 5b), but could form easily in the (C8F17COO)3Fe/C14DMAO system; this demonstrated the feeble binding between Al3+ and C14DMAO. The Lα phase of the (C8F17COO)3Fe/C14DMAO system, consisting of regularly spherical vesicles (Figure S3a of the Supporting Information), exhibited a rather high viscoelasticity (Figure S3b of the Supporting Information). For the three selected lanthanide ions, La3+, Eu3+, and Tb3+, no Lα phase was obtained in the (C8F17COO)3La/C14DMAO mixtures. The Lα phase was enlarged in range with the increase in atomic number in the (C8F17COO)3Eu/C14DMAO and (C8F17COO)3Tb/C14DMAO systems. The Lα phase of the (C8F17COO)3Eu/C14DMAO system consisted of deformed multilamellar vesicles (Figure 7a) and exhibited very weak

Figure 7. Cryo-TEM images of the Lα phase formed by (a) 10 mmol L−1 (C8F17COO)3Eu and (b) 10 mmol L−1 (C8F17COO)3Tb in 100 mmol L−1 C14DMAO.

viscoelasticity (Figure 8a), implying a strong fluid property of the vesicle bilayers. In the (C8F17COO)3Tb/C14DMAO system, the Lα phase was composed of spherical vesicles without obvious deformations (Figure 7b). Compared with the (C8F17COO)3Eu/C14DMAO system, this Lα phase represented a higher viscoelasticity with an elastic dominant property (Figure 8b). The above results demonstrate that metal ions play a significant role in the studied process due to the different binding degrees of the metal ions with the N → O group of C14DMAO. The binding between the metal ions and the C14DMAO decreases the a0, leading to an increase in p, which is favorable to the formation of bilayers. Thus, different metal ions will induce significant differences in aggregation behavior, due to the different-coordinated degree. Two crucial factors should be considered in this process. First, the trend toward coordinated strength increases with the charges of the Mn+ ions and decreases with ionic radius, which correlates with the replacement of the water molecules that are bonded to the metal ions by ligands.41 Second, the participation of metal ions in the coordination bonding can enhance the tendency to form

Figure 8. Oscillatory rheograms of the Lα phase formed by (a) 10 mmol L−1 (C8F17COO)3Eu and (b) 10 mmol L−1 (C8F17COO)3Tb in 100 mmol L−1 C14DMAO solutions. 3543

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(6) Song, A.; Jia, X.; Teng, M.; Hao, J. Ca2+− and Ba2+−Ligand Coordinated Unilamellar, Multilamellar, and Oligovesicular Vesicles. Chem.Eur. J. 2007, 13, 496−501. (7) Tian, H.; Ding, Q.; Zhang, J.; Song, A.; Hao, J. Effect of Hydrophilic Groups of Ca Surfactants and Hydrophobic Chains of CnDMAO on Coordinated Vesicle Formation. Langmuir 2010, 26 (24), 18652−18658. (8) Song, A.; Hao, J. Self-Assembly of Metal-Ligand Coordinated Charged Vesicles. Curr. Opin. Colloid Interface Sci. 2009, 14, 94−102. (9) Yan, Y.; Huang, J. Hierarchical Assemblies of Coordination Supramolecules. Coord. Chem. Rev. 2010, 254, 1072−1080. (10) Hao, J.; Zemb, Th. Self-Assembled Structures and Chemical Reactions in Room-Temperature Ionic Liquids. Curr. Opin. Colloid Interface Sci. 2007, 12, 129−137. (11) Versluis, F.; Tomatsu, I.; Kehr, S.; Fregonese, C.; Tepper, A. W. J. W.; Stuart, M. C. A.; Ravoo, B. J.; Koning, R. I.; Kros, A. Shape and Release Control of a Peptide Decorated Vesicle through pH Sensitive Orthogonal Supramolecular Interactions. J. Am. Chem. Soc. 2009, 131, 13186−13187. (12) Wang, Y.; Ma, N.; Wang, Z.; Zhang, X. Photocontrolled Reversible Supramolecular Assemblies of an Azobenzene-Containing Surfactant with α-Cyclodextrin. Angew. Chem., Int. Ed. 2007, 46, 2823−2826. (13) Gazzaz, H. A.; Robinson, B. H. Kinetics Involving Divalent Metal Ions and Ligands in Surfactant Self-Assembly Systems: Applications to Metal-Ion Extraction. Langmuir 2000, 16, 8685−8691. (14) Owen, T.; Pynn, R.; Hammouda, B.; Butler, A. MetalDependent Self-Assembly of a Microbial Surfactant. Langmuir 2007, 23, 9393−9400. (15) Friese, V. A.; Kurth, D. G. Soluble Dynamic Coordination Polymers as a Paradigm for Materials Science. Coord. Chem. Rev. 2008, 252, 199−211. (16) Kurth, D. G. Metallo-Supramolecular Modules as a Paradigm for Materials Science. Sci. Technol. Adv. Mater. 2008, 9, 014103. (17) Rowan, S. J.; Beck, J. B. Metal-Ligand Induced Supramolecular Polymerization: A Route to Responsive Materials. Faraday Discuss. 2005, 128, 43−53. (18) Dobrawa, R.; Wurthner, F. Metallo Supramolecular Approach Toward Functional Coordination Polymers. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4981−4995. (19) Bergman, S. D.; Wudl, F. Mendable Polymers. J. Mater. Chem. 2008, 18, 41−62. (20) Beck, J. B.; Ineman, J. M.; Rowan, S. J. Metal/Ligand-Induced Formation of Metallo-Supramolecular Polymers. Macromolecules 2005, 38, 5060−5068. (21) Chiper, M.; Hoogenboom, R.; Schubert, U. S. Toward Main Chain Metallo-Terpyridyl Supramolecular Polymers: “The Metal Does the Trick”. Macromol. Rapid Commun. 2009, 20, 565−578. (22) Fox, J. D.; Rowan, S. J. Supramolecular Polymerizations and Main-Chain Supramolecular Polymers. Macromolecules 2009, 42, 6823−6835. (23) Przybyla, D. E.; Chmielewski, J. Metal-Triggered Collagen Peptide Disk Formation. J. Am. Chem. Soc. 2010, 132, 7866−7867. (24) Micklitsch, C. M.; Knerr, P. J.; Branco, M. C.; Nagarkar, R.; Pochan, D. J.; Schneider, J. P. Zinc-Triggered Hydrogelation of a SelfAssembling b-Hairpin Peptide. Angew. Chem., Int. Ed. 2011, 50, 1577− 1579. (25) Yang, H.; Pritzker, M.; Fung, S. Y.; Sheng, Y.; Wang, W.; Chen, P. Anion Effect on the Nanostructure of a Metal Ion Binding SelfAssembling Peptide. Langmuir 2006, 22, 8553−8562. (26) Zapf, A.; Beck, R.; Platz, G.; Hoffmann, H. Calcium Surfactants: A Review. Adv. Colloid Interface Sci. 2003, 100 −102, 349−380. (27) Alargova, R.; Petkov, J.; Petsev, D.; Ivanov, I. B.; Broze, G.; Mehreteab, A. Light Scattering Study of Sodium Dodecyl Polyoxyethylene-2-sulfonate Micelles in the Presence of Multivalent Counterions. Langmuir 1995, 11, 1530−1536. (28) Berton, M.; Mancin, F.; Stocchero, G.; Tecilla, P.; Tonellato, U. Self-Assembling in Surfactant Aggregates: An Alternative Way to the

participation of the H2O molecules as ligands, especially for the lanthanide ions, should be a significant challenge for both molecular dynamics simulation and the ab initio method.

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CONCLUSIONS Both hydrophobic chains and metal ions play important roles in the self-assembly behavior of metal-coordinated surfactant systems. The aggregation behavior of metal−ligand coordination systems is strongly affected by the comprehensive equilibrium between metal coordination and hydrophobic interaction. Several properties of metal−ligand complexes formed by Mn+ surfactants and C14DMAO can be identified. (i) Bilayers are more likely to form in the presence of fluorocarbon chains than in the presence of corresponding hydrocarbon ones, and the Lα phase formed by fluorocarbon chains has a higher viscoelasticity than that formed by hydrocarbon chains. (ii) Branched chains are more favorable for the formation of bilayers than linear chains of the same component. The bilayers formed by branched chains are more flexible than those formed by linear chains, and the Lα phase exhibits very weak viscoelasticity. (iii) The Lα phase forms more easily in the presence of metal ions with higher-coordinated capability, and the viscoelasticity of the Lα phase is affected by the coordinated capability. On the basis of these results, we can construct an Lα phase with anticipative properties by choosing surfactants with appropriate structures.

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ASSOCIATED CONTENT

S Supporting Information *

Surface tension versus the total surfactant concentration of Zn2+ surfactants/C14DMAO systems and cryo-TEM 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]. Tel: +86-531-88563532. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (Grant 21173132), the NSF of Shandong Province (Grant ZR2010BM015), and by the Independent Innovation Foundation of Shandong University (IIFSDU, Grant 2012TS001).

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REFERENCES

(1) Nguyen, S. T.; Gin, D. L.; Hupp, J. T.; Zhang, X. Supramolecular Chemistry: Functional Structures on the Mesoscale. Proc. Natl. Acad. Sci. U.S.A 2001, 98, 11849−11850. (2) Shimizu, T.; Masuda, M.; Minamikawa, H. Supramolecular Nanotube Architectures Based on Amphiphilic Molecules. Chem. Rev. 2005, 105, 1401−1443. (3) Nitschke, J. R.; Schultz, D.; Bernardinelli, G.; Gerard, D. Selection Rules for Helicate Ligand Component Self-Assembly: Steric, pH, Charge, and Solvent Effects. J. Am. Chem. Soc. 2004, 126, 16538− 16543. (4) Giordani, C.; Wakai, C.; Okamura, E.; Matubayasi, N.; Nakahara, M. Dynamic and 2D NMR Studies on Hydrogen-Bonding Aggregates of Cholesterol in Low-Polarity Organic Solvents. J. Phys. Chem. B 2006, 110, 15205−15211. (5) Wang, J.; Song, A.; Jia, X.; Hao, J.; Liu, W.; Hoffmann, H. Two Routes to Vesicle Formation: Metal-Ligand Complexation and Ionic Interactions. J. Phys. Chem. B 2005, 109, 11126−11134. 3544

dx.doi.org/10.1021/la4003669 | Langmuir 2013, 29, 3538−3545

Langmuir

Article

Realization of Fluorescence Chemosensors for Cu(II) Ions. Langmuir 2001, 17, 7521−7528. (29) Coulombeau, H.; Testard, F.; Zemb, T.; Larpent, C. Effect of Recognized and Unrecognized Salt on the Self-Assembly of New Thermosensitive Metal-Chelating Surfactants. Langmuir 2004, 20, 4840−4850. (30) Alargova, R.; Danov, K. D.; Kralchevsky, P. A.; Broze, G.; Mehreteab, A. Growth of Giant Rodlike Micelles of Ionic Surfactant in the Presence of Al3+ Counterions. Langmuir 1998, 14, 4036−4049. (31) Hoffmann, H.; Oetter, G.; Schwandner, B. The Aggregation Behaviour of Tetradecyldimethylaminoxide. Prog. Colloid Polym. Sci. 1987, 73, 95−106. (32) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of SelfAssembly of Hydrocarbon Amphipiles into Micelles and Bilayers. J. Chem. Soc. Faraday Trans. 2 1976, 72, 1525−1568. (33) Antunes, F. E.; Marques, E. F.; Gomes, R.; Thuresson, K.; Lindman, B.; Miguel, M. G. Network Formation of Catanionic Vesicles and Oppositely Charged Polyelectrolytes. Effect of Polymer Charge Density and Hydrophobic Modification. Langmuir 2004, 20, 4647−4656. (34) Wang, G. K.; Karlsson, G.; Almgren, M. Aggregation Behavior of Cationic Fluorosurfactants in Water and Salt Solutions. A Cryo-TEM Survey. J. Phys. Chem. B 1999, 103, 9237−9246. (35) Safran, S. A.; Pincus, P.; Andelman, D. Theory of Spontaneous Vesicle Formation in Surfactant Mixtures. Science 1990, 248, 354−356. (36) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. The Origins of Stability of Spontaneous Vesicles. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353−1362. (37) Helfrich, W. Elastic Properties of Lipid Bilayers: Theory and Possible Experiments. Z. Naturforsch. 1973, 28, 693−703. (38) van Zanten, R.; Zasadzinski, J. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 261−268. (39) Haas, K. H.; Blom, C.; van den Ende, D.; Duits, M. H.; Haveman, B.; Mellema, J. Rheological Behavior of a Dispersion of Small Lipid Bilayer Vesicles. Langmuir 1997, 13, 6658−6668. (40) Milner, S. T.; Safran, S. A. Dynamical Fluctuations of Droplet Microemulsions and Vesicles. Phys. Rev. A: At., Mol., Opt. Phys. 1987, 36, 4371−4379. (41) Stendahl, J. C; Rao, M. S.; Guler, M. O.; Stupp, S. I. Intermolecular Forces in the Self-Assembly of Peptide Amphiphile Nanofibers. Adv. Funct. Mater. 2006, 16, 499−508. (42) Rulõsek, L.; Vondrasek, J. Coordination Geometries of Selected Transition Metal Ions (Co2+, Ni2+,Cu2+, Zn2+, Cd2+, and Hg2+) in Metalloproteins. J. Inorg. Biochem. 1998, 71, 115−127. (43) Sato, J.; Kobayashi, H.; Ikarashi, K.; Saito, N.; Nishiyama, H.; Inoue, Y. Photocatalytic Activity for Water Decomposition of RuO2Dispersed Zn2GeO4 with d10 Configuration. J. Phys. Chem. B 2004, 108, 4369−4375. (44) Bagus, P. S.; Lee, Y. S.; Pitzer, K. S. Effects of Relativity and of the Lanthanide Contraction on the Atoms from Hafnium to Bismuth. Chem. Phys. Lett. 1975, 33, 408−411. (45) Küchle, W.; Dolg, M.; Stoll, H. Ab Initio Study of the Lanthanide and Actinide Contraction. J. Phys. Chem. A 1997, 101, 7128−7133.

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