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Self-Assembly of Polyoxovanadate-Containing Fluorosurfactants Baofang Zhang, Jie Song, Dong Li, Lang Hu, Craig L Hill, and Tianbo Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02308 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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Self-Assembly of Polyoxovanadate-Containing Fluorosurfactants Baofang Zhang,† Jie Song,‡ Dong Li, † Lang Hu,† Craig L. Hill,‡* and Tianbo Liu†* † Department of Polymer Science, University of Akron, Akron, Ohio, 44325, USA ‡ Department of Chemistry, Emory University, Atlanta, GA, 30322, USA

ABSTRACT: Two novel Polyoxovanadate (POV)-containing fluorosurfactants, each with two hydrophobic fluorinated “tails” and one nanosized, hydrophilic, rigid POV “head group” are synthesized for the first time. They self-assemble into spherical, bilayer vesicles in acetonitrile/water mixed solvents, as evidenced by systemic studies using laser light scattering (LLS) and electron microscopy techniques. The vesicle sizes demonstrate dynamic change over different solvent compositions mainly due to the solvent-swelling of the fluorocarbon chains, although the charge number on the POVs changes over the solvent polarity as well. INTRODUCTION Fluorosurfactants are fluorinated chemical compounds that typically consist of a fluorinated hydrophobic “tail” and a hydrophilic “head group”.1 Fluorine atoms in the structure offer distinct properties from the corresponding hydrogenated counterpart. For instance, fluorosurfactants are not only chemically and thermally more stable, but also more effective at lowering the surface tension of solutions, i.e., they are stronger surfactants than the ones based on hydrocarbon chains.2-4 The special properties of these materials have attracted considerable attention in many

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research fields.5,6 Recently, more sophisticated structures such as nanotubular microstructures7,8 and mixed Langmuir monolayers9 assembled from fluorinated compounds are reported. Their unique structures show great potential in multiple biomedical applications. For example, the fluorinated polymers have been extensively investigated as possible blood substitutes,10 due to their superb oxygen transport property. Polyoxometalates (POMs) are a large group of metal oxide molecular nanoclusters with a broad range of applications in catalysis, medicine, and material science.11-17 These hydrophilic compounds have well-defined size, shape, charge and other structural features and they exhibit the capability of reversible and stepwise multi-electron transfer or storage without significant structural change. Recently these inorganic clusters have been successfully covalently grafted with organic components through various synthetic methods, forming novel amphiphiles of various structures.18-22 For example, inorganic–organic hybrids containing single POM component have been synthesized with one23,24 or two organic tails.25 These hybrids show typical surfactant properties with POMs acting as polar head groups and the hydrocarbon organic portions acting as hydrophobic tails. They form regular23 or reverse vesicles24 in different solvents controlled by the solvophobic interaction. In addition, a series of ‘‘dumb-bell’’ shaped POM–organic–POM hybrids have also been documented recently to display amphiphilic behavior and to form single-layer vesicular structures in solution.26 Herein studies are expanded to POM-containing fluorosurfactants comprising a hydrophilic POM “head group” and covalently linked hydrophobic, fluorinated “tails” in a single complex molecule. The fluorocarbon chains are known to be much more hydrophobic than hydrocarbon chains, and have broad applications from Teflon to absorbing oxygen gas. Hence, the selfassembly behaviors of such novel POM-containing fluorosurfactants in solution may contribute


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to the better understanding of their physicochemical properties and further facilitate the development of applications of such compounds. Here two POM-containing fluorosurfactants involving hexavanadate clusters bearing two fluorocarbon chains of different chain lengths are studied in solution. EXPERIMENTAL SECTION Synthesis

of

POM-containing

fluorosurfactants.

Two

hexavanadate-containing

fluorosurfactants are constructed by incorporating two fluorocarbon tails into one Lindqvist-type polyoxovanadate

[V6O13{(OCH2)3CNH2}]2-,

[(n-

C4H9)4N]2[V6O13{(OCH2)3CNH(COO)CH2C6H4CH2CH2(CF2)5CF3}2]2- ((TBA2·6F-V6) and [(nC4H9)4N]2[V6O13{(OCH2)3CNH(COO)CH2C6H4CH2CH2(CF2)7CF3}2]2-

(TBA2·8F-V6),

respectively, as shown in Scheme 1. The detailed synthesis and characterization are described in supporting information.

TBA2·6F-V6

TBA2·8F-V6

Scheme 1. Molecular structures of the POV-containing fluorosurfactant with hexavanadate as the large polar head and two fluorinated tails as nonpolar termini, [(n(TBA2·6F-V6) and [(nC4H9)4N]2[V6O13{(OCH2)3CNH(COO)CH2C6H4CH2CH2(CF2)5CF3}2] C4H9)4N]2[V6O13{(OCH2)3CNH(COO)CH2C6H4CH2CH2(CF2)7CF3}2] (TBA2·8F-V6).


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Preparation of surfactant solutions. The powder of each surfactant was first dissolved in acetonitrile (MeCN, a good solvent for both surfactants) at room temperature, and then deionized water was added dropwise with gentle stirring until the desired solvent composition was achieved. The solutions were stored at room temperature for use. RESULTS AND DISCUSSION Static light scattering (SLS) measurements showed a very low scattered intensity at ∼70 kcps for 0.1 mg/mL of TBA2·6F-V6 in MeCN (the scattered intensity of pure solvent is ∼65 kcps), indicating that the hybrid surfactant molecules remain as individual molecules in pure MeCN solution due to their good solubility in this solvent. However, when water was introduced to the MeCN solutions with the amount of 65 – 95 vol% of total mixed solvent, the scattered intensity started to increase significantly over time (Figure 1A) and large assemblies were observed (Figures 1B and C) as revealed by dynamic light scattering (DLS) measurements. For TBA2·6FV6 in MeCN/water mixed solvent containing 95 vol% of water, a very narrow peak in the CONTIN analysis appears with an average hydrodynamic radius (Rh) of 43 ± 2 nm. Furthermore, the Rh value is not angular dependent, implying that the assemblies are likely spherical (Figure 1B). From SLS measurements, the average radius of gyration (Rg) of the assemblies measured at different angles is ca. 42 ± 2 nm. The relationship of Rg = Rh for spherical structures indicates that the assemblies are probably hollow spherical vesicles. The TEM image (Figure 2A) confirms the vesicular structures by clearly showing a contrast between their walls and centers. The SEM image (Figure 2B) shows some defects on the vesicular surfaces, reflecting their hollow nature; these defects could be generated when the samples were placed under vacuum.


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The formation of vesicular structures was further supported through AFM measurements when vesicle solutions were coated and dried on a flat surface (see Figures 2C, D, E, and F). The average diameter of vesicles measured by the AFM was ca. 80 nm, comparable to the DLS results (86 ± 4 nm), while the smaller height of ~24 nm from the AFM measurement suggesting the partial collapse of vesicle membranes onto solid surface. Considering that two layers of vesicle membranes are included in this value, each membrane’s thickness is up to 12 nm, which is close to the sum of the sizes of two POMs (0.7 nm each) and the zig-zag length of the fluorocarbon tail (8.5nm). All the above experimental results and analysis confirm that the assemblies formed by the hybrid surfactants in MeCN/water mixed solvents are bilayer vesicle structures (see the model in Scheme 2).

(B)

(A) 6000

30 degree 45 degree 60 degree 75 degree 90 degree

0.8

5500

0.6

ΓG(Γ)

scattered intensity (kcps)

6500

5000

4500

0.4

0.2

4000 0.0

3500

1

3000 0

5

10

15

20

10

R h (nm )

100

1000

25

time(days)

(C)

0 .6

2 days 4 days 9 days 21 days 23 days

0 .4

ΓG(Γ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 .2

0 .0 1

10

10 0

1000

R h (n m )

Figure 1. (A) The change of the scattered intensity over time in water/MeCN mixed solvent with 5 vol% MeCN for the hybrid surfactant TBA2·6F-V6; (B) the Rh distribution of the vesicles of TBA2·6F-V6 in water/MeCN mixed solvent containing 5 vol% MeCN at different angles, obtained from the CONTIN analysis; (C) the Rh distribution of the vesicles of TBA2·6F-V6 in water/MeCN mixed solvent containing 5 vol% MeCN at different time, obtained from the CONTIN analysis.


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(A) (B)

(C)

(D)

E

nm

nm

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µm

F

µm

Figure 2. (A) TEM image of vesicular structures of surfactant TBA2·6F-V6 in water/MeCN mixed solvent containing 5vol% MeCN; (B) SEM image of vesicular structures of TBA2·6F-V6 in water/MeCN mixed solvent containing 5 vol% MeCN; (C) and (D) AFM images of vesicular structures of TBA2·6F-V6 in water/MeCN mixed solvent containing 5 vol% MeCN; (E) and (F) Section analysis corresponding to particles in (C) and (D), respectively.


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MeCN/Water 2-

2POM-containing fluorosurfactant

Scheme 2.

+

Counterion TBA

Cartoon of hollow and bilayer vesicle formation occurring in the presence of POM-containing

fluorosurfactant and the TBA countercation in MeCN/water mixture.

Time-resolved scattered intensity of the hybrid surfactants in solution was recorded by SLS to determine the mechanism of the self-assembly process. For vesicles, the scattered intensity I is determined by two factors, the vesicle concentration (C) and vesicle radius (R) with I ∝ CR2. Interestingly, the current system took three weeks to reach equilibrium (Figure 1A), but the vesicle size determined by the DLS did not change during the whole process (Figure 1C). This observation implies that the scattered intensity increment should be attributed to the increase of vesicle number instead of vesicle size. We speculate that the driving force for the vesicle formation of POV-containing fluorosurfactants in MeCN/water solutions is the hydrophobic interaction, similar to the self-assembly of the Mn-anderson-C6 surfactants in MeCN/water.23 Such surfactants have a stiff cluster structure, along with one tail sticking out from each side of the cluster. In order to form vesicular structures, the two fluorocarbon chains have to bend into the solvent phobic layer (Scheme 2). Therefore, a high bending energy is needed. Here, we deduce that the mechanism of the vesicle formation was through a closed association mechanism


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and did not experience any intermediate state (Scheme 2), and needed high activation energy caused by the high bending energy. The last point had been proved in our early work23-24. More interestingly, the vesicle size can be tuned accurately by adjusting the solvent polarity. Solutions of 0.1 mg/mL TBA2·6F-V6 in MeCN/water mixed solvents containing 95, 90, 80, 75 and 70 vol% of water were prepared, respectively. Larger vesicles measured by the DLS are observed from solutions containing less water content (i.e., less polar). The vesicle sizes from these solutions show a linear relationship with the inverse of the solvent dielectric constant. This is a typical feature for charge-regulated self-assembly processes.27 TBA2·8F-V6 shows similar assembly size dependence on the solvent polarity which implies both hybrids follow similar selfassembly mechanisms (Figure 3A). There are several interesting observations from Figure 3A. First, in the most polar solvent (containing 95 vol% water), the two hybrid surfactants show almost identical assembly sizes with TBA2·8F-V6 being slightly larger. This is reasonable because for both surfactants, the fluorocarbon chains have to stay away from the very hydrophilic environment by packing very tightly to each other in the bilayer. Secondly, the average vesicle size increases with decreasing solvent polarity, as a typical charge-regulated assembly process. Zeta potential analysis confirms that the Zeta potential values of the surfactant assemblies become less negative (i.e., carry less effective charges) in less polar solvents (Figure 3B). The weaker electrostatic repulsion between adjacent POV clusters on vesicle surface helps to decrease the curvature of the vesicles, i.e., leading to larger vesicle sizes. On the other hand, in a less polar solvent, the hydrophobic vesicle bilayers will be more solvated, which makes the packing of hydrophobic fluorocarbon chains in the hydrophobic domain less compact. This will favor smaller vesicle sizes. Overall, the trend in Figure 3 confirms that the first effect, i.e., charge-regulated effect, is more dominant. Finally, the


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vesicle size of TBA2·8F-V6 increases faster than that of TBA2·6F-V6 vesicles with decreasing solvent polarity, i.e., larger slope for the linear regression in Figure 3A. The only difference between these two surfactants is the length of the fluorocarbon chains. It is more difficult to solvate longer fluorocarbon chains for a given water/MeCN composition, therefore the vesicles of TBA2·8F-V6 tend to becomes larger.

(A) 240

(B)

-36

TBA.6F-V6

200

-38

TBA.8F-V6 -40

160

ζ(mv)

Rh(nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120

-42

-44

80

-46

40 0.0130

0.0135

0.0140

0.0145

0.0130

1/ε

0.0135

0.0140

1/ε

Figure 3. (A) Plot of the average vesicle radius (in Rh) versus the inversed dielectric constant (1/ε) of the solvent for TBA2·6F-V6 and TBA2·8F-V6 in water/MeCN mixed solvent; (B) Zeta potential values (mv) versus the inversed dielectric constant (1/ε) of the solvent for TBA2·6F-V6 in water/MeCN mixed solvents.

CONSLUSIONS Two unique polyoxovanadate-containing fluorosurfactants with hydrophobic, fluorinated “tails” and large, hydrophilic POM “head groups” in one hybrid molecule were synthesized for the first


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time. The self-assembly of such novel surfactants into spherical, bilayer and hollow vesicles in water/MeCN mixed solvents is investigated by using multiple characterization methods. The vesicle formation is a charge-regulated process. The vesicle size is controlled by a collective effects of the electrostatic repulsion among the charged polar head groups and the hydrophobic interaction among the fluorocarbon tails. The length of the fluorocarbon tails on the hybrid surfactants has a major effect on the vesicle sizes. The fundamental understanding of the selfassembly of such novel inorganic POM-fluorosurfactants in solution would facilitate the development of new catalytic materials, especially given the capability of trapping oxygen molecules in the fluorinated polymer chains. Particularly, considering the potential applications of the polyoxovanadate-containing fluorosurfactants in the catalysis of water splitting reactions, their solubility in water could be improved by increasing the charge of the polar head groups.

ASSOCIATED CONTENT Supporting Information. Further experimental details, 1H, 13C, 19F and 51V NMR spectra, FTIR, UV-VIS absorption, preparation of TEM, SEM and AFM samples, and DLS, SLS. The supporting information is available free of charge on the ACS Publications website at XXX. AUTHOR INFORMATION Corresponding Author †* E-mail: [email protected]. ‡* E-mail: [email protected]. ACKNOWLEDGMENT


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TL acknowledges support from the National Science Foundation (CHE1305756 and CHE1607138) and The University of Akron. CLH wishes to thank the Army Research Office grant number W911NF-12-1-0136 for support of this research.

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(26) Misdrahi, M. F.; Wang, M.; Pradeep, C. P.; Li, F. Y.; Lydon, C. ; Xu, L.; Cronin L.; Liu, T. Amphiphilic Properties of Dumb-bell-shaped Inorganic-Organic-Inorganic Molecular Hybrid Materials in Solution and at Interface, Langmuir 2011, 27, 91939202. (27) Verhoeff, A. A.; Kistler, M. L.; Bhatt, A.; Pigga, J.; Groenewold, J.; Klokkenburg, M.; Veen, S.; Roy, S.; Liu, T.; Kegel, W. K. Charge Regulation as a Stabilization Mechanism for Shell-like Structures, Phys. Rev. Lett. 2007, 99, 066104.

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Two polyoxometalate-containing fluorosurfactants with hydrophobic fluorinated “tails” and large hydrophilic POM “polar head groups” are found to form spherical, bilayer vesicles in water/MeCN mixed solvents. The vesicular size increases with increasing MeCN content. The length of the very hydrophobic fluorocarbon chains is also critical in determining the vesicle formation and vesicular size.


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