Langmuir–Blodgett Monolayers of Partially Fluorinated Ionic Liquids

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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Langmuir−Blodgett Monolayers of Partially Fluorinated Ionic Liquids as Two-Dimensional, More Sustainable Functional Materials and Coatings Thorben Sieling, Jens Christoffers, and Izabella Brand* Department of Chemistry, University of Oldenburg, 26111 Oldenburg, Germany

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S Supporting Information *

ABSTRACT: The structure of cations and anions in ionic liquids is precisely selected to tune their properties for applications in catalysis, electrochemistry, or development of sensors. Substitution of a hydrocarbon by a fluorocarbon chain in either the cation or the anion yields fluorinated ionic liquids (FILs). The ionic character combined with hydrophobic and lipophobic properties of fluorocarbon chains ensure extraordinary surface properties of FILs. The use of fluorocarbons with chains longer than six carbon atoms, due to their impact on the environment and bioaccumulative properties, is banned. In this work we demonstrate that more sustainable imidazolium- and triazolium-based FILs containing two short, partially fluorinated chains [(CH2)2(CF2)5CF3] have amphiphilic properties and form stable monolayers at the air−liquid interface which can be transferred onto a gold substrate by the Langmuir−Blodgett (LB) method. X-ray photoelectron and polarization modulation infrared reflection absorption spectroscopies are used to characterize the composition, conformation, and orientation of FILs in LB monolayers on the gold surface. The positively charged heteroaromatic ring is oriented parallel to the gold surface while the fluorocarbon chains are directed toward air. The fluorocarbon chains adopt a helical conformation. In LB monolayers, the tilt of the helix depends on the chemical structure of the cation and the monolayer transfer conditions. Uniform orientation of the amphiphilic cations in the monolayer assembly yields a hydrophobic surface. Two-dimensional LB films of FILs are proposed here as possible sustainable, functional, ultrathin films and coatings. KEYWORDS: Fluorinated ionic liquids, Langmuir−Blodgett monolayer, Two-dimensional films, X-ray photoelectron spectroscopy (XPS), Polarization modulation infrared reflection absorption spectroscopy (PM IRRAS)

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reproduction.1,8 Since the early 2000s, FOCs have attracted scientific and political attention.2 Perfluorocarbons with chains longer than six carbon atoms in one unit will be soon banned. Therefore, science and industry strike with a need to synthesize more sustainable FOCs which contain four to six carbon atoms per unit and in parallel preserve the properties of perfluorinated compounds with longer chains. The use of fluorocarbon−hydrocarbon diblock compounds Y−(CF2)n(CH2)m−X appears as a promising solution for the fabrication of short chain branched perfluoroalkyl polymers3,4,9 and semifluorinated surfactants.9,10 The Y−(CF2)n(CH2)m−X compounds have the following properties:9,11 (i) amphiphilicity which is expressed by a possibility to form a monolayer at the air−water interface; (ii) amphidynamicity which is expressed by a combination of a flexible hydrocarbon with a rigid fluorocarbon chain; and (iii) amphistericity which is

INTRODUCTION A substitution of hydrogen by fluorine atoms leads to dramatic changes of the properties of fluorinated organic compounds (FOCs) compared to their hydrogenated analogues. The C−F bond belongs to the strongest in nature, yielding in resistance of FOCs to hydrolysis, oxidation, and photolysis. FOCs are stable at high temperatures and upon exposure to X-ray radiation.1,2 The chemical and physical stabilities of FOCs contribute to their wide applications as finishing agents for fabrics, components of extinguishing agents, refrigerating liquids, electroplating baths, lubricating oils, superhydrophobic materials, coatings, membranes, detergents, oxygen carriers in blood substitutes, pharmaceuticals, or agrochemicals.1−7 The physical and chemical stability of FOCs makes them resistant to microbial degradation and metabolism by animals and humans.1,2 Perfluorooctanesulfonic acid and perfluorooctanoic acid are the main final products of the environmental degradation of FOCs. They are globally distributed, polluting the environment and accumulating in wildlife and humans. FOCs are classified as carcinogenic, mutagenic, and toxic for © 2019 American Chemical Society

Received: March 15, 2019 Revised: May 3, 2019 Published: May 30, 2019 11593

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

Research Article

ACS Sustainable Chemistry & Engineering caused by differences in the cross-sectional area and chain conformation. The amphistericity of the diblock Y−(CF2)n(CH2)m−X compounds reflects differences in the volume of hydrocarbon and fluorocarbon chains. Fluorocarbon chains are hydrophobic, lipophobic, and fluorophilic while the hydrocarbon chains are hydrophobic, fluorophobic, and lipophilic. The amphiphilicity of diblock Y−(CF2)n(CH2)m−X compounds is expressed by their ability to form either highly ordered aggregates or monolayers at the air−water interface.12−14 The introduction of a terminal anchor group to the hydrocarbon chain (e.g., X = −SH, −SeH, −SiCl3) allows for self-assembly into organized monolayers of the Y−(CF2)n(CH2)m−X compounds on Au, Ag, Cu, or silicon surfaces.15−21 In the past several years, the fluorination effect has been explored to combine the extraordinary properties of fluorocarbon chains with those of ionic liquids (ILs).22−25 Synthesis and macroscopic properties (e.g., viscosity, density, surface energy) of a large number of fluorinated ionic liquids (FILs) containing a perfluorinated chain either in the cation or anion have been described in the literature.23,24,26−29 The amphiphilicity of FILs is reflected by a presence of some order in the bulk of the liquid phase in which the polar ionic moieties are permeated by the hydrophobic chains.30,31 The presence of a fluorocarbon or a hydrocarbon chain either in the cation or in the anion improves the surface activity of FILs. Depending on the length of the fluorocarbon chain, FILs form aggregates in the aqueous phase32 or monolayers at the air−water interface.33 The introduction of a short hydrophobic fluorocarbon chain (C ≤ 6) in FILs gives a unique ability to generate two-dimensional, less toxic, sustainable, hydrophobic surfaces with ionic character.33,34 Recently, Alpers et al.28,29 reported the synthesis of amphiphilic, perfluorinated pyridinium-, imidazolium-, and triazolium-based sustainable and less toxic FILs. The imidazolium and triazolium cations contain two (CH2)2(CF2)5CF3 chains which are substituted to N atoms in the heteroaromatic ring of the cation. In this article we characterize the monolayer properties of imidazolium-, 1,2,3triazolium-, and 1,2,4-triazolium-based FILs. All FILs studied here form stable monolayers at the air−water interface. Langmuir−Blodgett (LB) transfer allows a deposition of ordered, monomolecular films at different substrates. This easy fabrication protocol opens wide application possibilities of two-dimensional FIL-based films. X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) are used to characterize the composition, structure, packing, and orientation of sustainable FILs in LB films. To our knowledge, this is the first study reporting on the structure, packing, and composition of FILs containing partially fluorinated amphiphilic cations in 2-dimensional functional films.

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Figure 1. Cation structures of the FILs investigated in this study. The anion is always triflate (trifluoromethanesulfonate, [TfO]−). Residues R = −(CH2)2(CF2)5CF3: (a) [Im]+[TfO]− [1,3-bis(1H,1H,2H,2Hperfluorooctyl)imidazolium triflate]; (b) [1,2,3-Tria]+[TfO]− [1,3bis(1H,1H,2H,2H-perfluorooctyl)-1,2,3-triazolium triflate]; and (c) [1,2,4-Tria]+[TfO]− [1,4-bis(1H,1H,2H,2H-perfluorooctyl)-1,2,4 triazolium triflate]. chloroform/methanol (2:1, v/v). Using a microsyringe (Hamilton, Reno, NV), a few microliters of the FILs solution were placed on the aqueous subphase and left 10 min for the evaporation of the solvent. Deionized water (PureLab Classic, Elga LabWater, Celle, Germany, resistivity of 18.2 MΩ cm) and aqueous 1 M NaCl were used as the subphase. Surface pressure (π) versus area per molecule (A) isotherms were recorded using the KSV LB Mini trough (KSV Ltd., Helsinki, Finland) equipped with two hydrophilic barriers and a Wilhelmy balance with a paper Wilhelmy plate. To protect the experimental setup from dust, it was placed in a laminar flow hood. The accuracy of measurements was ±0.02 nm2 for A and ±0.1 mN m−1 for π. Monolayers of FILs were prepared on a solid substrate surface using Langmuir−Blodgett (LB) transfer. The substrate for LB transfer was a gold film evaporated onto a glass slide. Microscope glass slides (VWR International BVBA, Leuven, Belgium) were cut in pieces of 1.0 × 2.5 cm2 and rinsed with water and 2-propanol. Afterward, the glass slides were dried in a stream of argon. On the cleaned glass surface, 0.7 nm of adhesive Cr and 200 nm Au layers were evaporated using a Tectra MinoCoater instrument (Tectra GmbH, Frankfurt/Main, Germany). Before each LB transfer, the substrates were rinsed with water and ethanol, dried with Ar, and placed in the UV/ozone cleaner (Bioforce Nanoscience Inc., Chicago, IL) for 10 min. FILs monolayers on a pure aqueous subphase were compressed to πt = 15 mN m−1 and transferred by LB vertical withdrawing. The transfer of the FILs monolayer from the 1 M NaClaq subphase was done at πt = 30 mN m−1. The transfer ratio was equal to 1.03 ± 0.05. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectra were measured from the LB monolayers of [Im]+[TfO]− transferred onto the gold surface. The XP spectra were measured using an ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific, East Grinstead, UK). The XPS instrument was operated using a monochromatic Al Kα X-ray radiation source (hν = 1486.6 eV, 288 W). The samples were grounded by connecting carbon tape to the surface and the sample holder. Survey spectra were recorded at a pass energy of 100 eV and a dwell time of td = 50 ms. High-resolution C 1s, F 1s, N 1s, O 1s, and Cl 2p XP spectra were recorded at a pass energy of 30 eV. The S 2p spectra were measured with a pass energy of 100 eV; td was 200 ms for F 1s, 500 ms for S 2p, and 1000 ms for C 1s, N 1s, O 1s, and Cl 2p. It has been shown that FILs on the Au degrade upon irradiation with X-rays.35,36 Thus, the acquisition time was kept rather low to avoid artifacts. The binding energy scale was referenced to the Au 4f7/2 line at 84.0 eV. The spectra were fitted using Avantage v5.982 software (Thermo Fisher Scientific, East Grinstead, UK) by employing Gaussian−Lorentzian functions with a fixed ratio within each spectral region after Shirley-type background subtraction. For the fit of Cl 2p lines, the doublet splitting of 1.60 eV37 was fixed, and the full width at half maxima (fwhm) were constrained to be the same. Polarization Modulation Infrared Reflection Absorption Spectroscopy. PM IRRA spectra were measured with a Vertex 70 spectrometer and an external reflection setup (Bruker, Ettlingen, Germany) containing a photoelastic modulator with the frequency of 50 kHz and a demodulator PMA 50 (Hinds Instruments, Hillsboro, OR). The half-wave retardation was set to 1300 cm−1; the incident angle was 80°. For each sample, 1000 spectra with the resolution of 4 cm−1 were recorded. PM IRRA spectra were processed using OPUS v5.5 software (Bruker, Ettlingen, Germany). Spectra shown in this

EXPERIMENTAL SECTION

Chemicals. High-purity solvents, chloroform, ethanol (SigmaAldrich, Steinheim, Germany), 2-propanol (VWR Chemicals, Fontenay-sous-Bois, France), and methanol (Fisher Scientific U.K. Limited, Loughborough, UK), were used without further purification. NaCl was purchased from Carl Roth GmbH & Co. KG (Karlsruhe, Germany). FILs investigated in this study, [Im]+[TfO]−, [1,2,3Tria]+[TfO]−, and [1,2,4-Tria]+[TfO]−, were synthesized as reported in refs 28 and 29; their constitutions are shown in Figure 1. Langmuir−Blodgett Method. Solutions of FILs were freshly prepared every day. The concentration of an FIL was 1.0 g L−1 in 11594

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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2a). In [Im]+[TfO]− and [1,2,4-Tria]+[TfO]− monolayers, the π increases continuously with a decrease in A, until the monolayer collapse at the limiting area per molecule Alim of 0.55 nm2 and πcoll = 51.9 and 53.7 mN m−1 for [Im]+[TfO]− and [1,2,4-Tria]+[TfO]−, respectively. The monolayer of [1,2,3-Tria]+[TfO]− collapses at Alim = 0.54 nm2 and πcoll = 63.4 mN m−1. In the monolayers of FILs at the 1 M NaClaq−air interface, the Alim is equal to (0.55 ± 0.02) nm2 (Figure 2a). The crosssectional area of the imidazolium moiety is close to 0.50−0.53 nm2.42 It is slightly less than Alim measured here. A fluorocarbon chain occupies the area of 0.26−0.30 nm2.41,43 Therefore, the expected area of an amphiphilic molecule containing two fluorocarbon chains is 0.52−0.60 nm2. The measured Alim indicates that in monolayers the packing of FILs is determined by the arrangement of the fluorocarbon chains of the amphiphilic cations. The isotherm of [1,2,3-Tria]+[TfO]− differs from isotherms of two other FILs (Figure 2a). It shows a phase transition (PT) at APT = 0.69 nm2 and πPT = 21.6 mN m−1 . The compressibility modulus (Ks) is calculated to discuss the physical state of the FILs in monolayer assemblies. Ks values of the IL monolayers are calculated from eq 1.44

paper show absorbance from the organic molecules present on the metal surface, as calculated after the background subtraction and normalization (ΔS) as a function of wavenumbers.38,39 Water Contact Angle. The static water contact angle was measured by the sessile drop method using 9 μL drops of deionized water with the contact angle system OCA 15plus (DataPhysics, Filderstadt, Germany) equipped with a CCD camera and SCA20 software (version 1.0.0). The quoted values are the average of eight measurements on LB monolayers of FILs on a gold surface.

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RESULTS AND DISCUSSION Langmuir Monolayers of Partially Fluorinated Imidazolium- and Triazolium-Based Ionic Liquids. The amphiphilic cations of FILs [Im]+ [TfO]−, [1,2,3-Tria]+ [TfO]−, and [1,2,4-Tria]+[TfO]− (Figure 1) form monolayers at the liquid−gas interface. On the pure water−air interface, all compounds form unstable, aggregated films (Figure S1). Briefly, the lift-off area (A0, at which the surface pressure starts to increase) is in the range 1.15 < A0 < 0.90 nm2. The collapse of all monolayers is poorly defined and occurs at the π range 35−60 mN m−1. These results indicate that the cations of the FILs are surface active but form poorly defined, aggregated films at the air−water interface. Our results indicate that cations of FILs have some affinity to the aqueous environment. The 1-methyl-3-octadecylimidazolium bis(trifluoromethylsulfonyl) imide IL contains an amphiphilic cation with an 18 C atoms long chain. Compression of the Langmuir isotherm leads to a formation of a monolayer only at low surface pressures. At high surface pressures, the monolayer undergoes a transition to a multilayered film.40 The addition of a salt to the subphase is known to stabilize the Langmuir monolayers of amphiphilic molecules.41 Electrostatic interactions between the anions in the electrolyte solution and the cation of the IL may be responsible for a further stabilization of the monolayer film. Indeed, in the presence of 1 M NaCl in the aqueous subphase, the stability of FIL monolayers significantly increases. Langmuir isotherms of the studied compounds are shown in Figure 2. The lift-off area is close to 1.1 nm2 (Figure

ÄÅ É ÅÅ i 1 yi ∂A yÑÑÑ−1 K s = ÅÅÅ−jjj zzzjjj zzzÑÑÑ ÅÅÇ k A {k ∂π {ÑÑÖ

(1)

Figure 2b shows Ks versus A plots. The Ks of the monolayer of FILs containing [Im]+ and [1,2,4-Tria]+ cations reaches a maximum (150−180 mN m−1) at A proceeding the monolayer collapse, indicating that these monolayers exist in a liquidlike (disordered) state (Figure 2b). In the [1,2,3-Tria]+[TfO]− monolayer Ks versus A plots show two maxima at A1 = 0.74 nm2 (Ks1 = 86 mN m−1) and A2 = 0.59 nm2 (Ks2 = 540 mN m−1), indicating a phase transition in the [1,2,3-Tria]+[TfO]− monolayer. In the [1,2,3-Tria]+ cation, a N atom is present in the position 2 (Figure 1b), whereas in [Im]+ and [1,2,4-Tria]+ cations this position is occupied by a single CH group (Figure 1a,c). The H atom in the CH group is able to interact, via formation of hydrogen bonds, with anions of ILs to form ion pairs.45 In the monolayer, in the plane of the heteroaromatic ring, ion pairs between [Im]+ or [1,2,4-Tria]+ and [Cl]− (or [OH]−) may be formed. It will lead to an increase in the area of the cation−anion pair in the monolayer. Indeed, isotherms show that at the same π the A([1,2,4-Tria]+[TfO]−) is close to A([Im]+[TfO]−) but larger than A([1,2,3-Tria]+[TfO]−). In the [1,2,3-Tria]+[TfO]− monolayer, this kind of interaction is not possible. The counterions have to pack differently to interact with the amphiphilic cation, which may be responsible for the phase transition observed in the [1,2,3-Tria]+[TfO]− monolayer. Two-dimensional LB monolayers of FILs were transferred onto a gold surface. The composition, conformation, and orientation in LB films are discussed below. Composition of Ionic Liquids in Langmuir−Blodgett Monolayers. The survey XP spectrum of FILs transferred from the 1 M NaClaq−air interface reveals the presence of Au, C, N, F, O, and Cl (Figure S2). High-resolution XP spectra of the [Im]+[TfO]− monolayer transferred from the 1 M NaClaq−air interface onto the gold substrate are shown in Figure 3. The [TfO]− anion, originally present in the FIL, contains S, C, O, and F atoms, which should be revealed in the XPS spectra, if [TfO]− anions were present in the LB

Figure 2. (a) Surface pressure and (b) compressibility modulus vs area per molecule plots of [Im]+[TfO]− (black), [1,2,3-Tria]+[TfO]− (red), and [1,2,4-Tria]+[TfO]− (blue) on the 1 M NaCl aqueous subphase. 11595

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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Figure 3. High-resolution XP spectra of LB monolayer of [Im]+-based FIL transferred from 0.1 M NaClaq−air interface at πt = 30 mN m−1 onto the gold substrate: (a) S 2p, (b) O 1s, (c) Cl 2p, (d) N 1s, (e) C 1s, and (f) F 1s. Residuals (R) are shown below spectra.

of amphiphilic cations (anions) of ionic liquids offers a new experimental approach for the exchange of water-soluble counterions. In consequence, the composition of FILs in the LB monolayers changes. Due to this fact, FILs in LB monolayers are named as follows: [Im]+ [X]−, [1,2,3Tria]+[X]−, and [1,2,4-Tria]+[X]−, where [X]− corresponds to [Cl]−, [OH]−, and/or [TfO]− anions. C, F, and N atoms are present in the amphiphilic cations of the studied FILs and give photoelectron lines in the XP spectrum (Figure 3d−f). In the [Im]+[X]− monolayer, the N 1s photoemission line appears at EB = 400.4 eV (Figure 3d, Table 1). The fwhm is equal to 2.5 eV. In the [Im]+[X]− monolayer transferred from the aqueous subphase the EB of the N 1s line is equal to 400.1 eV (fwhm = 3.3 eV). The two N atoms in imidazolium-based ILs adsorbed on metallic surfaces

monolayer. The high-resolution spectra show zero intensity in the binding energy region between 168 and 171 eV where photoemission lines of S 2p electrons of the sulfonyl group are expected (Figure 3a).46 The high-resolution O 1s XP spectrum shows a broad (fwhm = 3.4 eV) photoemission line at EB = 530.8 eV (Figure 3b). In [TfO]−-based ILs, the O 1s photoemission line appears at the EB = 532.0 eV.46 In our spectrum, the O 1s peak is downshifted by 1.4 eV. This line is assigned to hydroxide ions which accumulate in the monolayer and on the gold surface during the LB transfer. The absence of S 2p and O 1s lines characteristic for [TfO]− indicates that these anions are not present in the LB monolayer transferred from 1 M NaCl solution. The XPS spectra reveal the presence of Cl atoms in the LB monolayers of FILs transferred onto the gold substrate (Figure 3c). The high-resolution Cl 2p XP spectrum shows a well-resolved doublet at EB = 197.2 eV (Cl 2p3/2) and EB = 198.8 eV (Cl 2p1/2). These two Cl 2p lines are ascribed to [Cl]− adsorbed directly on the gold surface.47,48 The second, weak doublet is shifted to higher binding energy: EB = 199.7 eV (Cl 2p3/2) and EB = 201.2 eV (Cl 2p1/2). It is assigned to [Cl]− present in the adlayer on the gold surface,47,48 providing a source of counterions to the imidazolium-based amphiphilic cations. XPS results indicate clearly that the [TfO]− anion, originally present in the FIL, is absent in the LB monolayer. Instead [Cl]− and probably [OH]− are found in the LB monolayer transferred onto the gold surface. On the aqueous subphase, prior to the LB transfer, exchange of anions takes place. In the LB monolayers of FILs transferred from the pure aqueous subphase, the XP spectra show that, next to Au, C, F, N, and O, traces of S are present (Figure S3). This indicates that in this case small amounts of [TfO]− anions are transferred onto the Au surface. In conclusion, the LB transfer

Table 1. Binding Energy (EB) and Full Width at Half Maximum (fwhm) of the O 1s, Cl 2p, N 1s, C 1s, and F 1s Photoemission Lines in the LB Monolayer of [Im]+[X]− Transferred from the 0.1 M NaClaq Subphase XPS line

11596

EB/eV

fwhm/eV

O 1s Cl 2p(3/2,1/2) Cl 2p(3/2,1/2) N 1s C 1s C 1s

530.8 197.2, 198.8 199.7, 201.2 400.5 284.1 285.7

3.4 1.2, 1.2 1.2, 1.2 2.5 1.3 1.7

C 1s C 1s C 1s C 1s F 1s

287.7 290.1 291.0 293.2 688.0

1.9 0.9 1.1 1.0 1.9

assignment −

[OH] [Cl]− on Au [Cl]− in adlayer Naromatic‑ring Cadventitious CNaromatic ring, CH2N, CH2CF2 Cadventitious CF2CH2 CF2 CF3 CF2 and CF3

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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Figure 4. PM IRRA spectra in the 1700−900 cm−1 spectral region of (a) randomly distributed [Im]+[X]− in a monolayer thick film (1.6 nm) at the surface coverage marked in the figure; and in LB monolayers on the gold surface of (b) [Im]+[X]− (c) [1,2,4-Tria]+[X]−, and (d) [1,2,3Tria]+[X]−. Monolayer transfer conditions are given in the figure. Numbering of modes in parts c and d is omitted for clarity.

three C 1s lines which, in accordance with the literature,16,18,20 are ascribed to CF2 adjacent to the methylene groups (EB = 290.0 eV), CF2 of the fluorinated chain (EB = 291 eV), and CF3 (EB = 293.3 eV) groups (Table 1). In XP spectra, C 1s photoemission lines at EB = 284.1 eV and EB = 287.7 eV are present (Figure 3e). They are ascribed to adventitious carbon in the monolayer.18 XPS results indicate clearly that the amphiphilic cations are transferred onto the gold surface. They form monolayers in which the cations are directed toward the metal surface. Conformation and Orientation of Partially Fluorinated Cations in LB Monolayers. Figure 4 shows PM IRRA spectra of [Im]+[X]−, [1,2,4-Tria]+[X]−, and [1,2,3-Tria]+[X]− FILs in LB monolayers on the gold surface and spectra of a randomly distributed [Im]+ cation in monolayer thick films of variable surface coverage (Θ). The latter were calculated from isotropic optical constants of [Im]+[I]− (Figure S4) at Θ corresponding to the LB transfer conditions (Table S1). [I]− ions do not absorb the IR light; thus, all IR absorption modes originate exclusively from the cation. Several overlapped IR absorption modes contribute to the PM IRRA spectrum of the LB monolayers of FILs (Figure 4). The assignment of individual IR absorption modes in the PM IRRA spectra shown in Figure 4 is based on the quantum chemical calculations (Figure S5 and Table S3) and literature data.16,19,53−55 It is summarized in Table 2. In the 1700−900 cm−1 spectral region, the aromatic ring, methylene groups, and fluorocarbon chains absorb the IR light. The IR absorption mode at 1575 cm−1 (mode 1 in Figure 4) is assigned to the in-plane stretching mode of the imidazolium ring.56 In the spectrum of randomly distributed [Im]+ cations, the δ(CH2) mode has two components at 1457 and 1433 cm−1

give the N 1s photoemission line at 400 > EB > 402 eV.35,46,49,50 In macroscopic films of imidazolium bis(trifluoromethylsulfonyl)imide, the N 1s photoemission line of the imidazolium cation appears at EB = 402.0 eV.35 At submonolayer coverages of the gold surface by an imidazolium-based IL, the EB of the N 1s line decreases. A 1.0−1.4 eV downshift of the EB of the N 1s line is ascribed to a direct interaction of the N atoms in the imidazolium ring with the metal surface.35 The EB of the N 1s line indicates that in LB monolayers the imidazolium rings of the amphiphilic partially fluorinated cations make a direct contact with the gold surface. Large values of fwhm of the N 1s lines may reflect differences in the chemical environment of the cation. The C 1s and F 1s photoemission lines originate from fluorinated chains of the amphiphilic cation. In the LB monolayer, the EB of the F 1s is equal to 688.5 eV (Table 1). This value is in excellent agreement with the EB of the F 1s photoemission line of partially fluorinated alkanethiols in monolayers on gold and silver surfaces.16,18,51,52 No individual components originating from CF3 and CF2 moieties can be distinguished (Figure 3f). The high-resolution C 1s XP spectrum shows five components (Figure 3e), which are assigned to differently substituted carbon atoms in the amphiphilic cation (Table 1). The imidazolium cations give the C 1s line at EB = 285.7 eV (Figure 3e, Table 1).35 The ethylene (−CH2CH2−) fragment in the [Im]+ cation contains two methylene groups. One methylene group is adjacent to the fluorocarbon chain while the second methylene group is connected to the N atom in the imidazolium ring. In the XP spectrum shown in Figure 3e, the C atoms of methylene groups are overlapped with the C 1s line of the imidazolium ring. Fluorinated hydrocarbon chains give 11597

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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Table 2. Wavenumber, Assignment, and Direction of the Transition Dipole Moment Vector (μ) of the IR Absorption Modes of the Amphiphilic, Partially Fluorinated [Im]+ Cation in the 1700−900 cm−1 Spectral Regiona wavenumber of the IR absorption mode/cm−1 no.

random

1

1576 1560 1457 1431 1366 1352 1322 1302 1266

LB monolayer πt = 30 mN m−1 (1 M NaClaq)

LB monolayer πt = 15 mN m−1 (H2O)

mode assignment

direction of μ (based on calculation results)

n.o.

n.o.

i.p. stretching mode of the imidazolium ring

plane of the ring

1444

1444

δ(CH2)

bisector of the methylene group

1365 1355 sh. 1322 1302 sh. 1268 sh.

6

1247 1233 sh.

1249 1230 sh.

1249 1230 sh.

7

1216

1213

1213

νs(CF2)progression ν(CC)helix νs(CF2)progression ν(CC)helix twist (CH2) νas(CF3) def. (CF2) ν(CC) twist (CH2) νas(CF3) νas(CF2) def. (CF2) ν(CC) twist (CH2) νas(CF3)

parallel to the helix axis

sh. sh.

1365 1355 sh. 1322 1302 sh. 1269 sh.

2 3 4 5

sh.

sh.

8

1187

1188

1188

9

1147

1147

10

1124

1124

1148 1138 sh. 1124

11

1079

1072

1072

12

1039

1046

1030

νas(CF2) ν(CC) twist (CH2) νas(CF2) wagg. (CH2) νs(CF2) δ(CF2) νs(CF2) wagg. (CH2) νs(CF2) νs(CF3) wagg. (CH2) νs(CF2) νs(CF3) wagg. (CH2)

a small tilt from the helix axis almost perpendicular to the helix axis

almost perpendicular to the helix axis

not well-defined, in the direction of the helix axis

perpendicular to the helix axis perpendicular to the helix axis almost perpendicular to the helix axis in the direction of the helix axis

in the direction of the helix axis

a

n.o., not observed; sh., shoulder; i.p., in-plane.

depend on the fluorocarbon chain length and its conformational order. In amorphous films the number of these modes increases, and their intensities decrease. Figure 4 shows that the absorption modes at 1147 cm−1 (mode 9) and 1080 cm−1 (mode 11), characteristic for helical chains with uniform conformational order, have weak shoulders (modes 10, 11). The symmetric CF2 stretching modes involving CC stretching modes of the helix appear in the 1400−1300 cm−1 spectral region.54 Two strong modes around 1380−1360 cm−1 and 1330−1320 cm−1 are expected in fluorocarbon chains having a high degree of conformational order. An increase in the chain disorder gives a large number of weak IR absorption modes.54 In PM IRRA spectra shown in Figure 4 (modes 3, 4), two modes centered at 1366 and 1322 cm−1 are clearly seen. Both modes have weak shoulders at the low-wavenumber side of the spectrum. The analysis described above indicates that six carbon atoms long fluorocarbon chains of FILs in LB monolayers adopt a helical conformation. Indeed, the helical conformation of fluorocarbons is stable, even in chains whose length is shorter than a full turn of the helix (13−15 carbon atoms).9,19,54,58

(mode 2, Table 2), which are ascribed to the bending modes of the methylene groups attached to the imidazolium ring and fluorocarbon chains, respectively. In LB monolayers, the δ(CH2) mode has an enhanced intensity compared to the calculated spectrum (Figure 4). However, it is broad and poorly defined. Stretching and deformation modes involving fluorocarbon chains contribute to IR absorption modes in the 1380−900 cm−1 spectral region (Figure 4, Table 1). Fluorocarbon chains give the strongest IR absorption modes in the 1280−1200 cm−1 spectral region. These modes are complex in nature and involve CF3 and CF2 asymmetric stretching, CC stretching, and CF2 deformation modes of fluorocarbon chains having a helical conformation.16,19 The repulsive F−F interaction and large (compared to the H atom) van der Waals diameter of the F atom (0.294 nm) are responsible for a twist of a C−C−C−C− chain sequence by ca. 12°, resulting in the formation of helices (15/7 as well as 13/ 6) by fluorocarbon chains.9,57,58 In the 1150−800 cm−1 spectral region, progression modes involving asymmetric CF2 stretching modes of fluorocarbon chains appear in the IR spectrum.54 The number and frequency of these modes 11598

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Table 3. Calculated θ Angles between the Transition Dipole Vector of the Analyzed IR Absorption Modes and the Surface Normal in the LB Monolayer of FILs on the Gold Surfacea FIL +

−

[Im] [X] [1,2,4-Tria]+[X]− [1,2,3-Tria]+[X]− [Im]+[X]− [1,2,4-Tria]+[X]− [1,2,3-Tria]+[X]− [1,2,3-Tria]+[X]−

πt/mN m−1

subphase

θ3/° mode 3 ∥

θ4/° mode 4 ∥

θ9/° mode 9 ⊥

15 15 15 30 30 15 30

H2O H2O H2O 1 M NaClaq 1 M NaClaq 1 M NaClaq 1 M NaClaq

45 57 58 48 42 41 42

57 51 63 29 42 23 27

57 54 56 45 55 62 60

a

The orientation of the transition dipole vector with respect to the helix axis is marked.

modes of fluorocarbon chains (1270−1220 cm−1) are very difficult for the quantitative analysis. The transition dipole vector of these modes has a preferential, but not exactly perpendicular to the helix axis orientation (results of the quantum chemical calculations). In addition, in LB monolayers transferred from the aqueous subphase, small amounts of [TfO]− are present (Figure S3). The νas(SO3) mode of the sulfonyl groups in the [TfO]− anion is located in the 1280− 1250 cm−1 spectral region,60 overlapping with the νas(CF2) and νas(CF3) modes. For the orientation analysis of the aromatic ring and fluorocarbon helices in LB monolayers of FILs, the IR absorption modes marked in Figure 4 with 1, 3, 4, and 9 are selected. The in-plane ring stretching mode (1575 cm−1) (mode 1, Figure 4) is well-defined in the spectrum of randomly distributed molecules while in the LB films it cannot be distinguished from the background. The transition dipole vector of this mode is located in the plane of the heteroaromatic ring. The decrease in the intensity of this mode to zero indicates that in the LB monolayers the aromatic rings of the cations orient themselves parallel to the gold surface. The analysis of the helix orientation is more complex. The transition dipole vectors of the IR absorption modes centered at 1366 and 1322 cm−1 (modes 3, 4 in Figure 4) are preferentially oriented parallel to the helix axis (Figure S6).19,54 Thus, the calculated θ angle gives directly the tilt angle of the helix with respect to the surface normal. The transition dipole vector of the IR absorption mode centered at 1147 cm−1 (mode 9) is oriented perpendicular to the helix axis (Figure S6).19,54 Deconvolution of the PM IRRA spectra (Figure S7) provides integral intensities of these IR absorption modes, which, after substitution in eq 3, give the θ values (Table 3). Results summarized in Table 3 show that in the LB monolayers of FILs transferred from the pure aqueous subphase all θ angles (independent of the orientation of the transition dipole vector versus helix axis) give an average value of (55 ± 4)°. This value is very close to 54.7° (magic angle) corresponding to the random distribution of the fluorocarbon chains in the studied film. This result indicates that the fluorocarbon helices of FILs in LB monolayers do not display any long-range orientation order with respect to the surface normal. However, in LB monolayers of FILs transferred from the 1 M NaCl subphase, a long-range order in packing of the amphiphilic cations is observed (Table 3). The packing of the FILs in LB monolayers depends on the structure of the cation. The transition dipole vector of the mode 3 is located along the helix axis (Figure S6); thus, the θ3 values calculated for this mode provide the tilt of the fluorocarbon chain. The transition dipole vector of the mode 4 has a small tilt from the helix axis (Figure S6). Table 3 shows that the calculated θ3 values are ca.

Figure 4 shows that the intensities of the IR absorption modes of FILs in monolayers depend on the structure of the amphiphilic cation and LB transfer conditions. The PM IRRA spectra of the monolayers transferred from the aqueous subphase are characterized by higher intensities of IR absorption modes compared to the monolayers transferred from the 1 M NaCl subphase. According to the surface selection rule of IRRAS,59 in an anisotropic film, the integrated intensity of an IR absorption mode ΔS dṽ depends on the surface concentration of molecules in this film (Γ) and on the orientation of the transition dipole vector of a given absorption mode (μ⃗) versus electric field vector (E⃗ ) (always perpendicular to the surface),

∫ ΔS dv ̃ = Γ|μ⃗ ·E⃗|2 = Γ |μ⃗ |2 |E⃗|2 cos2 θ

(2)

where θ is the angle between μ⃗ and E⃗ vectors. In the monolayer at the NaClaq−air interface at the πt = 30 mN m−1, the Γ([Im]+[X]−) = Γ([1,2,4-Tria]+[X]−) and is equal to 2.4 × 10−10 mol cm−2 while the Γ([1,2,3-Tria]+[X]−) is equal to 2.7 × 10−10 mol cm−2. In the monolayers of FILs transferred at πt = 15 mN m−1, the Γ is close to 2.1 × 10−10 mol cm−2 (Table S1). Obviously, the Γ is lower in monolayers transferred from the air−water interface (πt = 15 mN m−1) than from the air−1 M NaClaq interface (πt = 30 mN m−1). According to eq 2, the integrated intensity of an IR absorption mode is directly proportional to Γ. Thus, in LB monolayers transferred from the air−water interface (at lower Γ) an attenuation of the intensity of the IR absorption modes compared to the monolayers transferred from the 1 M NaClaq interface (at higher Γ) is expected. The PM IRRA spectra in Figure 4 show an increase in the intensity of a large number of IR absorption modes of the LB monolayers transferred at lower surface pressure (lower values of Γ). It indicates that the average orientation (expressed by θ) of the amphiphilic cations in LB monolayers transferred from both subphases is different. The θ, describing the direction of a transition dipole moment vector of a given IR absorption mode versus surface normal, is calculated from eq 3, (LB) 1 ∫ ΔSexp dv ̃ cos θ = (iso) 3 ∫ ΔScal dv ̃ 2

ΔS(LB) exp

(3)

ΔS(iso) cal

where change dṽ and dṽ are the integral intensities of the given IR absorption mode in the LB film and in the monolayer of randomly distributed molecules, respectively. To find the average tilt of the fluorocarbon helix axis with respect to the surface normal, the direction of the transition dipole vector of a selected mode within the cation has to be known (Table 2). The strongest IR absorption 11599

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

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ACS Sustainable Chemistry & Engineering 10−15° higher than the θ4 values, showing the difference in the orientation of both transition dipole vectors. In the monolayers of FILs, the average tilt of the helix axis is equal to (43 ± 3)°. Molecular Scale Picture of FILs in LB Monolayers. Results of the XPS and PM IRRAS studies provide detailed information on the composition, conformation, and orientation of FILs in LB monolayers on the gold surface as schematically shown in Figure 5.

LB monolayer. It is worth emphasizing that recent studies show that six carbon atom long partially fluorinated compounds accumulate to a significantly less extent in the environment, wildlife, and humans than their C8 analogues.61,62 These compounds appear to be noncarcinogenic. Summarizing, LB transfer of monolayers of FILs onto solid supports opens a new, simple approach for the fabrication of sustainable ultrathin materials and coatings composed of short chain partially fluorinated compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01496. Langmuir isotherms, XP spectra, isotropic optical constants, details and results of the quantum chemical computation, deconvolution of the PM IRRA spectra, and contact angle measurement data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49-441798-3973. Fax: +49-441-798-3979.

Figure 5. Schematic representation of the packing and orientation of [Im]+[X]− in the LB monolayer transferred onto the gold surface from (a) water and (b) the 1 M NaClaq−air interface.

ORCID

Izabella Brand: 0000-0002-7710-0021 Notes

The authors declare no competing financial interest.

The LB monolayers transferred from the aqueous subphase have a complex composition. At the aqueous subphase, the ion exchange is partial. The [OH]− and trace amounts of [TfO]− are found as the counterions of the amphiphilic cations. The aromatic ring of the amphiphilic cation is oriented parallel to the gold surface. The fluorocarbon chains exist in a helical conformation; however, their order with respect to the surface normal is poorly defined (Figure 5a). In LB monolayers transferred form 1 M NaClaq solution [Cl]− and [OH]− are present. The analysis of the XP spectra indicates an excess of [Cl]− ions, compared to the amount of the amphiphilic cations of the FILs in the LB monolayer on the gold surface. It is due to a specific adsorption of [Cl]− ions on the gold surface. In the LB monolayer the anions make a contact to the gold surface and to positively charged amphiphilic cations. The aromatic rings adopt a parallel to the gold surface orientation (Figure 5b). The fluorocarbon chains exist in a helical conformation and are oriented toward air. Macroscopic Properties of LB Monolayers. Our results indicate that LB transfer conditions have a large impact on the packing and orientation of the amphiphilic cations of FILs in monolayers on the gold surface. Measurement of the water contact angle (Table S4 and Figure S8) allows the analysis of the relationship between the molecular scale structure and macroscopic properties of the two-dimensional films. Water contact angles of the FILs in LB monolayers on the gold surface are equal to (94 ± 2)° and (109 ± 2)° for films transferred from aqueous and 1 M NaCl subphases, respectively. In a thick film of [Im]+[TfO]− deposited on the glass surface, the water contact angle is equal to (70 ± 1)°.28 The water contact angle of a single monolayer is ca. 40° higher than the water contact angle of a thick film of randomly deposited FILs. This result indicates that the conformational order and packing order of the amphiphilic cations in the monolayer influence the macroscopic surface properties of the

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ACKNOWLEDGMENTS I.B. acknowledges the financial support from the DFG project BR-3961-5. I.B. and T.S. thank Prof. V. Zamlynny (Acadia University, Canada) for valuable discussions and for the possibility to use the Fresnel program. Calculations have been performed on the HPC Cluster CARL, located at the University of Oldenburg.

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REFERENCES

(1) Lewandowski, G.; Meissner, E.; Milchert, E. Special applications of fluorinated organic compounds. J. Hazard. Mater. 2006, A136 (3), 385−391. (2) Lindstrom, A. B.; Strynar, M. J.; Libelo, E. L. Polyfluorinated compounds: Past, present, and future. Environ. Sci. Technol. 2011, 45 (19), 7954−7961. (3) Dichiarante, V.; Martinez Espinoza, M. I.; Gazzera, L.; Vuckovac, M.; Latikka, M.; Cavallo, G.; Raffaini, G.; Oropesa-Nuñez, R.; Canale, C.; Dante, S.; Marras, S.; Carzino, R.; Prato, M.; Ras, R. H. A.; Metrangolo, P. A short-chain multibranched perfluoroalkyl thiol for more sustainable hydrophobic coatings. ACS Sustainable Chem. Eng. 2018, 6 (8), 9734−9743. (4) Sundaram, H. S.; Cho, Y.; Dimitriou, M. D.; Finlay, J. A.; Cone, G.; Williams, S.; Handlin, D.; Gatto, J.; Callow, M. E.; Callow, J. A.; Kramer, E. J.; Ober, C. K. Fluorinated amphiphilic polymers and their blends for fouling-release applications: The benefits of a triblock copolymer surface. ACS Appl. Mater. Interfaces 2011, 3 (9), 3366− 3374. (5) Lebaupain, F.; Salvay, A. G.; Olivier, B.; Durand, G.; Fabiano, A. S.; Michel, N.; Popot, J. L.; Ebel, C.; Breyton, C.; Pucci, B. Lactobionamide surfactants with hydrogenated, perfluorinated or semifluorinated tails: Physical-chemical and biochemical characterization. Langmuir 2006, 22 (21), 8881−8890. (6) Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly, D. J.; Meanwell, N. A. Applications of Fluorine in Medicinal Chemistry. J. Med. Chem. 2015, 58 (21), 8315−8359. 11600

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

Research Article

ACS Sustainable Chemistry & Engineering

Fluorinated ionic liquids: Properties and applications. ACS Sustainable Chem. Eng. 2013, 1 (4), 427−439. (25) Linder, T.; Sundermeyer, J. Three novel anions based on pentafluorophenyl amide combined with two new synthetic strategies for the synthesis of highly lipophilic ionic liquids. Chem. Commun. 2009, No. 20, 2914−2916. (26) Ganapatibhotla, L. V. N. R.; Wu, L.; Zheng, J.; Jia, X.; Roy, D.; McLaughlin, J. B.; Krishman, S. Ionic liquids with fluorinated blockoligomer tails: Influence of self-assembly on transport properties. J. Mater. Chem. 2011, 21 (48), 19275−19285. (27) de Almeida, R. F. M.; Loura, L. M. S.; Fedorov, A.; Prieto, M. Sphingomyelin/phosphatidylcholine/cholesterol phase diagram: boundaries and composition of lipid rafts. Biophys. J. 2003, 85 (4), 2406−2416. (28) Alpers, T.; Schmidtmann, M.; Muesmann, T. W. T.; Temme, O.; Christoffers, J. Perfluorinated pyridinium and imidazolium ionic liquids. Eur. J. Org. Chem. 2017, 2017 (29), 4283−4290. (29) Alpers, T.; Muesmann, T. W. T.; Temme, O.; Christoffers, J. Perfluorinated 1,2,3- and 1,2,4-triazolium ionic liquids. Eur. J. Org. Chem. 2018, 2018 (31), 4331−4337. (30) Konieczny, J. K.; Szefczyk, B. Structure of alkylimidazoliumbased ionic liquids at the interface vacuum and water - A molecular dynamics study. J. Phys. Chem. B 2015, 119 (9), 3795−3807. (31) Wang, Y.; Voth, G. A. Unique spatial heterogeniety in ionic liquids. J. Am. Chem. Soc. 2005, 127 (35), 12192−12193. (32) Luis, A.; Shimizu, K.; Araujo, J. M. M.; Carvalho, P. J.; LopesDa-Silva, J. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Coutinho, J. A. P.; Freire, M. G.; Pereiro, A. B. Influence of Nanosegregation on the Surface Tension of Fluorinated Ionic Liquids. Langmuir 2016, 32 (24), 6130−6139. (33) Shimizu, K.; Canongia Lopes, J. N.; Goncalves da Silva, A. M. P. S. Ionic liquid films at the air-water interface: Langmuir isotherms of tetra-alkylphosphonium-based ionic liquids. Langmuir 2015, 31 (30), 8371−8378. (34) Tindale, J. J.; Mouland, K. L.; Ragogna, P. J. Thiol appended, fluorinated phosphonium ionic liquids as covalent superhydrophobic coatings. J. Mol. Liq. 2010, 152 (1), 14−18. (35) Cremer, T.; Stark, M.; Deyko, A.; Steinrück, H. P.; Maier, F. Liquid/solid interface of ultrathin ionic liquid films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(111). Langmuir 2011, 27 (7), 3662−3671. (36) Cremer, T.; Wibmer, L.; Caderon, S. K.; Deyko, A.; Maier, F.; Steinrück, H. P. Interfaces of ionic liquids and transition metal surfacesadsorption, growth, and thermal reactions of ultrathin [C1C1Im][Tf2N] films on metallic and oxidised Ni(111) surfaces. Phys. Chem. Chem. Phys. 2012, 14 (15), 5153−5163. (37) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray photoelectron spectroscopy, 1st ed.; Perkin-ElmerCorporation: Minessota, 1992. (38) Zamlynny, V.; Lipkowski, J. Quantitative SNIFTIRS and PM IRRAS of organic molecules at electrode surfaces. In Adv. Electrochem. Sci. Eng.; Alkire, R. C., Kolb, D. M., Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 9, pp 315−376. (39) Zamlynny, V.; Zawisza, I.; Lipkowski, J. PM FTIRRAS Studies of Potential-Controlled Transformations of a Monolayer and a Bilayer of 4-Pentadecylpyridine a Model Surfactant, Adsorbed on a Au(111) Electrode Surface. Langmuir 2003, 19 (1), 132−145. (40) Filipe, E. J. M.; Morgado, P.; Teixeira, M.; Shimizu, K.; Bonatout, N.; Goldmann, M.; Canongia Lopes, J. N. Crystalline-like structures and multilayering in Langmuir films of ionic liquids at the air-water interface. Chem. Commun. 2016, 52 (32), 5585−5588. (41) Rogalska, E.; Bilewicz, R.; Brigaud, T.; El Moujahid, C.; Foulard, G.; Portella, C.; Stebe, M. J. Formation and properties of Langmuir and Gibbs monolayers: a comparative study using hydrogenated and partially fluorinated amphiphilic derivatives of mannitol. Chem. Phys. Lipids 2000, 105 (1), 71−91. (42) Cordes, D. B.; Smiglak, M.; Hines, C. C.; Bridges, N. J.; Dilip, M.; Srinivasan, G.; Mertlen, A.; Rogers, R. D. Ionic liquid-based routs

(7) Okazoe, T. Overview on the history or organofluorine chemistry from the viewpoint of material industry. Proc. Jpn. Acad., Ser. B 2009, 85 (8), 276−289. (8) Lim, X. The fluorine detectives: Researchers are battling to identify and assess a worrying class of persistent chemicals. In Nature Magazine; Scientific American: London, U.K., 2019; Vol. 566 (1), pp 26−29. (9) Krafft, M. P.; Riess, J. G. Chemistry, physical chemistry, and uses of molecular fluorocarbon-hydrocarbon diblocks, triblocks, and related compounds unique “apolar” components for self-assembled colloid and interface engineering. Chem. Rev. 2009, 109 (5), 1714− 1792. (10) Park, D.; Weinman, C. J.; Finlay, J. A.; Fletcher, B. R.; Paik, M. Y.; Sundaram, H. S.; Dimitriou, M. D.; Sohn, K. E.; Callow, M. E.; Callow, J. A.; Handlin, D. L.; Willis, C. L.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Amphiphilic surface active triblock copolymers with mixed hydrophobic and hydrophilic side chains for tuned marine fouling-release properties. Langmuir 2010, 26 (12), 9772−9781. (11) Bernardini, C.; Stoyanov, S. D.; Arnaudov, L. N.; Stuart, M. A. C. Colloids in Flatland: a perspective on 2D phase-separated systems, characterisation methods, and lineactant design. Chem. Soc. Rev. 2013, 42 (5), 2100−2129. (12) Fontaine, P.; Goldmann, M.; Muller, P.; Faure, M. C.; Konovalov, O. V.; Krafft, M. P. Direct evidence for highly organized networks of circular surface micelles of surfactants at the air-water interface. J. Am. Chem. Soc. 2005, 127 (2), 512−513. (13) Gaines, G. L. J. Surface activity of semifluorinated alkanes: F(CF2)m(CH2)nH. Langmuir 1991, 7 (12), 3054−3056. (14) Eastoe, J.; Paul, A.; Rankin, A.; Wat, R.; Penfold, J.; Webster, J. R. P. Fluorinated nonionic surfactants bearing either CF3- or H-CF2terminal groups: Adsorption at the surface of aqueous solutions. Langmuir 2001, 17 (25), 7873−7878. (15) Tsao, M. W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Studies of molecular orientation and order in self-assembled semifluorinated n-alkanethiols: Single and dual component mixtures. Langmuir 1997, 13 (16), 4317−4322. (16) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M.; Tamada, K.; Colorado, R. J.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Structure of self-assembled monolayers of semifluorinated alkanethiols on gold and silver substrates. Isr. J. Chem. 2000, 40 (2), 81−97. (17) Klein, R. J.; Fischer, D. A.; Lenhart, J. L. Thermal and mechanical aging of self-assembled monolayers as studied by near edge X-ray absorption fine structure. Langmuir 2011, 27 (20), 12423−12433. (18) Shaporenko, A.; Cyganik, P.; Buck, M.; Ulman, A.; Zharnikov, M. Self-assembled monolayers of semifluorinated alkaneselenolates on noble metal substrates. Langmuir 2005, 21 (18), 8204−8213. (19) Lu, H.; Zeysing, D.; Kind, M.; Terfort, A.; Zharnikov, M. Structure of self-assembled monolayers of partially fluorinated alkanethiols with a fluorocarbon part of variable length on gold substrate. J. Phys. Chem. C 2013, 117 (37), 18967−18979. (20) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R. J.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Molecular packing of semifluorinated alkanethiol self-assembled monolayers on gold: Influence of alkyl spacer length. Langmuir 2001, 17 (6), 1913−1921. (21) Meiners, F.; Ross, J. H.; Brand, I.; Buling, A.; Neumann, M.; Köster, P. J.; Christoffers, J.; Wittstock, G. Modification of silicon oxide surfaces by monolayers of an oligoethylene glycol-terminated perfluoroalkyl silane. Colloids Surf., A 2014, 449 (1), 31−41. (22) Bastos, J. C.; Carvalho, S. F.; Welton, T.; Canongia Lopes, J. N.; Rebelo, J. P. N.; Shimizu, K.; Araujo, J. M. M.; Pereiro, A. B. Design of task-specific fluorinated ionic liquids: nanosegregation versus hydrogen-bonding ability in aqueous solutions. Chem. Commun. 2018, 54 (29), 3524−3527. (23) Xue, H.; Shreeve, J. M. Ionic liquids with fluorine-containing cations. Eur. J. Inorg. Chem. 2005, 2005 (13), 2573−2580. (24) Pereiro, A. B.; Araújo, J. M. M.; Martinho, S.; Alves, F.; Nunes, S.; Matias, A.; Duarte, C. M. M.; Rebelo, L. P. N.; Marrucho, I. M. 11601

DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602

Research Article

ACS Sustainable Chemistry & Engineering to conversion or reuse of recycled ammonium prechlorate. Chem. Eur. J. 2009, 15 (48), 134441−13448. (43) Blanco, E.; Pineiro, A.; Miller, R.; Ruso, J. M.; Prieto, G.; Sarmiento, F. Langmuir monolayers of a hydrogenated/fluorinated cataionic surfactant: From the macroscopic to the nanoscopic size scale. Langmuir 2009, 25 (14), 8075−8082. (44) Smaby, J. M.; Momsen, M.; Kulkarni, V. S.; Brown, R. E. Cholesterol - induced interfacial area condensations of galactosylceramides and sphingomyelins with identical acyl chains. Biochemistry 1996, 35 (19), 5696−5704. (45) Yokozeki, A.; Kasprzak, D. J.; Shuflett, M. B. Thermal effect on C-H stretching vibrations of the imidazolium ring in ionic liquids. Phys. Chem. Chem. Phys. 2007, 9 (36), 5018−5026. (46) Villar-Garcia, I. J.; Smith, E. F.; Taylor, A. W.; Qiu, F.; Lovelock, K. R. J.; Jones, R. G.; Licence, P. Charging of ionic liquid surfaces under X-ray irradiation: the measurement of absolute binding energies by XPS. Phys. Chem. Chem. Phys. 2011, 13 (7), 2797−2808. (47) Gao, W.; Baker, T. A.; Zhou, L.; Pinnaduwage, D. S.; Kaxiras, E.; Friend, C. M. Chlorine adsorption on Au(111): Chlorine overlayer or surface chloride? J. Am. Chem. Soc. 2008, 130 (11), 3560−3565. (48) Kastanas, G. N.; Koel, B. E. Interaction of Cl2 with Au(111) surface in the temperature range of 120 to 1000 K. Appl. Surf. Sci. 1993, 64 (3), 235−249. (49) Buchner, F.; Forster-Tonigold, K.; Bozorgchenani, M.; Gross, A.; Behm, R. J. Interaction of a self-assembled ionic liquid layer with graphite(0001): A combined experimental and theoretical study. J. Phys. Chem. Lett. 2016, 7 (2), 226−233. (50) Beattie, D. A.; Harmer-Bassell, S. L.; Ho, T. T. M.; Krasowska, M.; Ralston, J.; Sellapperumage, P. M. F.; Wasik, P. Spectroscopic study of ionic liquid adsorption from solution onto gold. Phys. Chem. Chem. Phys. 2015, 17 (6), 4199−4209. (51) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. The effect of sulfur-metal bonding on the structure of self-assembled monolayers. Phys. Chem. Chem. Phys. 2000, 2 (15), 3359−3362. (52) Ford, K.; Battersby, B. J.; Wood, B. J.; Gentle, I. R. The production and verification of pristine semi-fluorinated thiol monolayers on gold. J. Colloid Interface Sci. 2012, 370 (1), 162−169. (53) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R. J.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Microstructure, wettability, and thermal stability of semifluorinated self-assembled monolayers (SAMs) on gold. J. Phys. Chem. B 2000, 104 (31), 7417−7423. (54) Hsu, S. L.; Reynolds, N.; Bohan, S. P.; Strauss, H. L.; Snyder, R. G. Structure, crystallization, and infrared spectra of amorphous perfluoro-n-alkane films prepared by vapor condensation. Macromolecules 1990, 23 (21), 4565−4575. (55) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F. Structural investigation of molecular organization in self-assembled monolayers of a semifluorinated amidethiol. Langmuir 1994, 10 (12), 4610−4617. (56) Heimer, N. E.; Del Sesto, R. E.; Meng, Z.; Wilkes, J. S.; Carper, W. R. Vibrational spectra of imidazolium tetrafluoroborate ionic liquids. J. Mol. Liq. 2006, 124 (1−3), 84−95. (57) Alves, C. A.; Porter, M. D. Atomic force microscopic characterization of a fluorinated alkanethiolate monolayer at gold and correlations to electrochemical and infrared reflection spectroscopic structural descriptions. Langmuir 1993, 9 (12), 3507−3512. (58) Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang, J.; Xiang, M.; Char, K.; Ober, C. K.; Bubeck, R. A.; Fischer, D. A.; Graupe, M.; Colorado, R. J.; Shmakova, O. E.; Lee, T. R. Molecular orientation of single and two-armed monodendron semifluorinated chains on “soft” and “hard” surfaces using NEXAFS. Macromolecules 2000, 33 (16), 6068−6077. (59) Moskovits, M. Surface selection rules. J. Chem. Phys. 1982, 77 (9), 4408−4416. (60) Bauer, T.; Mehl, S.; Brummel, O.; Pohako-Esko, K.; Wasserscheid, P.; Libuda, J. Ligand effects at ionic liquid-modified

interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111). J. Phys. Chem. C 2016, 120 (8), 4453−4464. (61) Anderson, J. K.; Luz, A. L.; Goodrum, P.; Durda, J. Perfluorohexanoic acid toxicity, part II: Application of human health toxicity value for risk characterization. Regul. Toxicol. Pharmacol. 2019, 103, 10−20. (62) Luz, A. L.; Anderson, J. K.; Goodrum, P.; Durda, J. Perfluorohexanoic acid toxicity, part I: Development of a chronic human health toxicity value for use in risk assessment. Regul. Toxicol. Pharmacol. 2019, 103, 41−55.

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DOI: 10.1021/acssuschemeng.9b01496 ACS Sustainable Chem. Eng. 2019, 7, 11593−11602