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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Aggregation of Perfluoroalkyl Groups Can Win the Hydrogen Bonding between Amides Takafumi Shimoaka, Hironori Ukai, Kana Kurishima, Koutaro Takei, Norihiro Yamada, and Takeshi Hasegawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07435 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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The Journal of Physical Chemistry

Molecular Aggregation of Perfluoroalkyl Groups Can Win the Hydrogen Bonding between Amides Takafumi Shimoaka1, Hironori Ukai1, Kana Kurishima2, Koutaro Takei2, Norihiro Yamada2 and Takeshi Hasegawa1*

1

Laboratory of Chemistry for Functionalized Surfaces, Division of Environmental Chemistry,

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan 2

Laboratory of Chemistry, Faculty of Education, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba

263-8522, Japan

*Corresponding Author E-mail: [email protected] (T.H.). Phone: +81 774 38 3070

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract The hydrogen bonding is, in general, recognized to have a much stronger molecular interactive force than the dipole-dipole interaction that is one of the van der Waals forces. The molecular interaction between perfluoroalkyl (Rf) chains is driven by a two-dimensional dipole-dipole interaction network because of a large dipole moment along the C–F bond and a helical conformation about the Rf chain axis, which generates the Rf-specific tight and closed molecular packing. The polarization of a molecular aggregate on a macroscopic scale comprehensively explains the Rf compound-specific properties represented by the high melting point. This cooperative interaction in the two-dimensional network gives us an impression that the dipole-dipole interaction can win the H-bonding in a molecular aggregate. In the present study, amphiphilic compounds having an Rf group and an amide group are prepared, and the molecular aggregation factor is investigated by means of surface chemistry and vibrational spectroscopic techniques. In fact, we show that the dipole-dipole interaction becomes the dominant factor of the molecular aggregation of the amide-containing compound.

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Introduction

The intermolecular interaction between perfluoroalkyl (Rf) chains is dominated by the dipole-dipole interaction generated by the large dipole moment along each C−F bond1-3 and the Rf-specific helical conformation about the chain axis3-8 in a cooperative manner.

These

two

characters

on

interaction

and conformation

induce

a

two-dimensional tight molecular packing of Rf chains, in which dipole-arrays with different directions are generated.2,3 On a macroscopic scale, these dipole arrays having different directions (vectors) are summed up (i.e., a collection of dipole moments) to yield a small polarization,1 which results in a small surface energy determining the material property of an Rf compound. In this manner, the bulk property of Rf compounds is totally different from a single-molecular character,2, 9, 10 which is known as the stratified-dipole arrays (SDA) theory.2,3 The SDA theory comprehensively explains the Rf compounds-specific ‘bulk’ properties represented by the water- and oil-repellency. A high melting point (m.p.) is one of the representative properties of Rf compounds. For example, the m.p. of polytetrafluoroethylene (PTFE) is 327°C.2,3 Molecular interaction in a non-ionic organic compound is dominated by

3



hydrogen(H)-bonding and the van der Waals (vdW) interaction.11 The dipole-dipole interaction is one of the vdW forces, and the interaction energy is roughly 10 times weaker than that of H-bonding.11,12 It is of interest, however, to see a representative H-bonding dominating polymer, polyvinyl alcohol, which exhibits a much lower melting point (ca. 200°C) than that of PTFE.13 This suggests that the two-dimensional dipole-dipole interaction network of Rf chains can win the H-bonding. In the present study, some compounds consisting of an Rf chain and a H-bonding part of an amide group with a different length-ratio are prepared, and the molecular interactions of both interacting parts are competed by using a surface chemical technique and infrared (IR) spectroscopy.

Experimental Amphiphilic compounds (N+C10-Azo-Gly-OC2Rfn: denoted as NAGFn) shown in Figure 1a were prepared, which have a quaternary ammonium cation acting as the hydrophilic part (–N+(CH3)3), and the hydrophobic part (–(CH2)10–) as well as two molecular interacting parts, i.e., the amide group (Gly) and an Rf chain (CF3–(CF2)n–). By introducing the azobenzene (Azo) group having a rigid skeleton, the molecule keeps a linear shape,14-16 which plays an important role to generate a spread monolayer with a

4


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high molecular density on water (Langmuir (L) film). For the L film formation, the hydrophilic head group works as the vertical-position adjusting unit at the air/water interface. If the molecules are packed with a perpendicular stance as shown in Figure 1b, each interacting part is positioned at the same height from the water surface, which helps the molecules interacted accurately between neighboring molecules. In this situation, the amide group plays the best performance by forming the cooperative H-bonding by a linear alignment of the amide groups (•••H–N–C=O•••H–N– C=O•••).17,18 In a similar manner, the Rf chains also form two dimensional dipole-dipole interaction network as schematically shown by the red circle in Figure 1b. As a result, by generating an aggregate as shown in Figure 1b, the H-bonding and the dipole-dipole interaction in NAGFn L films can be competed.

Figure 1

(a) Chemical structure of NAGFn. (b) Schematic image of a molecular aggregation of NAGFn

molecules on water.

5



For a comprehensive understanding, two samples having an Rf chain with a different length of n = 7 and 9 were prepared, and the molecular aggregation was analyzed at 15 and 25°C considering the Rf-specific phase transition at 19°C.6 By controlling the two parameters of the chain length and the temperature, aggregation strength of Rf chains can be controlled as predicted by the SDA theory, which should influence the H-bonding formation. The two parameters directly influence the twisting angle of the Rf chain (θt) summarized in the 1st column in Table 1,2,3,19 which determines the aggregation property.2,3 Table 1 Limiting molecular area (LMA) of myristic acid having an Rf group (MA-Rf) and NAGF LB films

θt / °

Limiting molecular area / nm2 molecule-1 MA-Rf

NAGF

120 (n = 9, 15°C)

0.330

0.406

103 (n = 9, 25°C)

0.336

0.379

90 (n = 7, 15°C)

0.401

0.421

77 (n = 7, 25°C)

0.421

0.388

In the present study, molecular aggregation properties in an L film are discussed by measuring surface pressure (π)-surface area (A) isotherms. The L film was transferred onto a solid substrate by the Langmuir-Blodgett (LB) technique to make a

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single monolayer (1ML), and IR spectroscopic analysis of the four LB films was carried out, which focuses on the dipole-dipole interaction of Rf groups and the H-bonding of the amide group individually. Through the IR spectroscopic analysis of the LB films dependent on θt, a competition of dipole-dipole interaction and H-bonding in NAGFn aggregate is discussed, which readily shows a fact that the molecular aggregation of the Rf groups can be stronger than the H-bonding.

Sample preparation: One of the authors (N. Y.) has already reported the synthetic method of the ammonium amphiphiles that contain an Azo group, an alaninate residue and a hydrocarbon chain with a chain length of 6~13 carbons.14 The same synthetic approach,

using

glycine

and

perfluoro-alcohols

instead

of

L-alanine

and

hydrocarbon-alcohols, was implemented to the syntheses of NAGF7 and 9. The procedure in detail is described in Supporting Information.

Chemicals: Chloroform (ACS Spectra Grade, ≥99.8%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Methanol (G.R., ≥99.8%) was purchased from Nacalai tesque (Kyoto, Japan). They were used as is without further purification. NAGFns were dissolved in a mixed solvent of chloroform and methanol in volume ratio of 10:1 with a concentration of 0.38 mM for preparation of L films. Pure water for 7



subphase was obtained by a Millipore (Molsheim, France) Elix UV-3 pure-water generator and a Yamato (Tokyo, Japan) Autopure WT100U water purifier (a compatible model with Milli-Q). The water showed an electric resistivity of 18.2 MΩ cm or higher. The surface tension of the water exhibited 72.5 mN m-1 at 25 °C, which was measured by using a Kyowa Interface Science Co., Ltd. (Saitama, Japan) DropMaster, DM-501Hy, contact angle meter.

π-A isotherms measurements: Surface pressure-surface area (π–A) isotherms were measured by using a USI system (Fukuoka, Japan) FSD–220 Langmuir-Blodgett system. The surface pressure was measured by the Wilhelmy method using a paper plate (1 × 1 cm2). The temperature of the subphase water and ambient air were both fixed at 15 °C or 25 °C to keep the phase II (19 °C) of the Rf parts, respectively. The Langmuir monolayer was prepared by spreading a solution of NAGFn on water. Each isotherm was measured with a compression rate of 0.312 nm2 molecule-1 min-1. All the measurements were repeated three times to confirm the quantitative reproducibility.

Langmuir-Blodgett (LB) film: LB films (1ML) were prepared by transferring an L film onto a CaF2 substrate by using the LB (vertical dipping) method at a surface 8


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pressure of 45 mN m−1. The CaF2 substrate was purchased from Pier Optics (Gunma, Japan) and cleaned in a conventional manner: successive sonication in pure water, ethanol, acetone, and dichloroethane for about 1 min each. All the solvents were guaranteed reagents. The LB films were prepared by using a Biolin Scientific (Espoo, Finland) KSV-NIMA Minitrough Langmuir-Blodgett system. The transfer ratios of the samples were in the range of 0.94~1.23, which is acceptable for the transfer at a high surface pressure.

IR spectrum measurements: Infrared transmission measurements were performed on a Thermo Fischer Scientific (Madison, WI) Nicolet iS50 FT-IR spectrometer. The modulation frequency of the IR ray was 60 kHz, and the transmitted light was detected by a liquid-N2-cooled MCT (Hg-Cd-Te) detector. The wavenumber resolution was 4 cm−1. The number of accumulations of the interferogram was 1000.

Results and Discussion

π-A isotherm measurements. Figure 2a presents four π-A isotherms of monolayers of the two compounds (NAGF7 and 9) measured at 15 and 25°C. For making comprehensive discussion of the isotherms of the structurally complicated compounds, much simpler compounds of the myristic acid (MA) skeleton having the common Rf 9



groups (n = 7, 9) without the amide and azobenzene moieties were prepared (CF3(CF2)n(CH2)m–COOH; MA-Rfn, where n + m = 12).2,3 The π-A isotherms of the simple compounds at the same temperatures are presented in Figure 2b. On the monolayer compression, all the curves of MA-Rfn keep the surface pressure of nil down to the surface area of A ≈ 0.4~0.5 nm2 molecule-1, followed by a rapid increase of the surface pressure, which indicates that a stiff domains are formed at an early stage of the compression, and the domains are collected in the small surface-area region. The stiff domain at a large surface area is attributed to the spontaneous molecular aggregation as expected by the SDA theory. 2,3

10


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70

(a) NAGFn

60 50 45 mN m–1

40

Surface Pressure / mN m-1

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


(i)

(ii)

30 20 10

(iii)

0

(b) MyAcidn MAｰRfn

50 40 30

n = 9, 15ºC n = 9, 25ºC n = 7, 15ºC n = 7, 25ºC

20 10 0 0

1

2 2

Surface Area / nm molecule Figure 2

3 -1

π-A isotherms of a Langmuir monolayer of (a) NAGFn and (b) MA-Rfn dependent on the Rf chain

length (n), and temperature.

On the other hand, NAGFn yields largely different results from MA-Rfn: the surface pressure lifts off at a much larger surface area of about 2.0 nm2 molecule-1, and the pressure gradually increases when the surface area is decreased down to A ≈ 1.2 nm2 molecule-1 (marked as (i); liquid expansion (LE) phase in Figure 2a). In the LE region, the molecules are not aggregated with each other, but they still lie on the water surface.20-22 This lying orientation is totally different from the perpendicular stance of MA-Rfn, which is understandable as follows. (1) The spacer azobenzene and amide 11



moieties should disturb the spontaneous molecular aggregation on water because of the steric effect, which results in the un-aggregated molecules on water. (2) An un-aggregated single Rf chain is exposed to the air, which should potentially interact with the water surface via dipole-dipole interaction as experimentally proved in a previous paper.2,10 In other words, the Rf group in NAGFn also works as another hydrophilic group as well as the terminal end (–N+(CH3)3). As a result, the molecules take the lying stance on water in the LE phase. On a further monolayer compression after the LE phase, an apparent plateau appears for all the curves down to A ≈ 0.5 nm2 molecule-1 (phase (ii)). In this irreversible compression process, the molecules are gradually rising up by the compression. Note that the small but apparent difference between n = 7 and 9 is recognized in this range: the plateau of n = 9 (at both 15 and 25°C) stays at a lower surface pressure than those of n = 7. This difference indicates that n = 7 needs a higher compression in phase (ii) to attain phase (iii) than n = 9. In other words, NAGF9 is easier to be aggregated with an orientation change on the compression. Since the direct molecular contacts of the Rf chains are not allowed because of the steric effect of the spacer group, this difference implies that the Rf chains with n = 9 has advantages for

12


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spontaneous aggregation over the steric hindrance, which reaches in a relatively long distance. After passing through the plateau region, the surface pressure rapidly increases in phase (iii) as also found in the curves of MA-Rfn, i.e., the solid phase. To discuss the molecular packing in the solid phase quantitatively, the limiting molecular area (LMA), that is the cross-sectional area of a molecule, is measured. The LMAs of MA-Rfn and NAGFn are presented in the 2nd and 3rd columns of Table 1, respectively, against the corresponding twisting angle, θt. MA-Rfn at θt = 120° (n = 9, 15°C) exhibits the smallest LMA of 0.330 nm2 molecule-1, which corresponds to the closed hexagonal packing,2,3 and LMA becomes larger with decreasing θt as expected by the SDA theory. NAGFn exhibits a larger LMA (0.379-0.421 nm2 molecule-1) than those of MA-Rfn, which straightforwardly implies that the steric spacer group is the major disturbing factor for the close packing even in the solid state. It is worth noting that, however, the collapse pressure attains more than 60 mN m-1 that is larger than those of MA-Rfn (40~50 mN m-1), which indicates that NAGFn forms a highly stable molecular aggregate in spite of the steric disturbance. The molecular aggregation in a NAGFn monolayer is, in fact, not simply driven by the interaction between the Rf part only as shown by the unsystematic change on θt (Table 1). 13



On closer inspection of NAGFn films, however, the following two tendencies are recognized in the table: (1) NAGF9 exhibits a closer packing than NAGF7 at each temperature, (2) the L films at 15°C are in a looser packing than those at 25°C for both n = 7 and 9. Judging from the SDA theory, therefore, the molecular aggregation of NAGFn cannot be attributed to the spontaneous aggregation of the Rf chains alone, but another molecular interaction, i.e., H-bonding at the amide group, should also be taken into account. To reveal the molecular interaction in detail, the L film was transferred onto a solid substrate to make 1ML (not multilayer) with the LB technique for IR spectroscopic analysis. The LB transfer is carried out at π ≈ 45 mN m-1, which is the low-end of the linear part of all the isotherms (a magnified figure is not shown).

Molecular interaction analysis in LB films using IR spectroscopy. Four LB films as a function of the Rf length (n = 7 and 9) and the temperature (15 and 25°C) were prepared. Figure 3 presents normal-incidence IR transmission (Tr) spectra of the four LB films. This region involves key bands for discussing both H-bonding and the dipole-dipole interactions represented by the amide I (ca. 1640 cm-1) and the CF2 symmetric vibration (νs(CF2); ca. 1150 cm-1) bands, respectively.

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νs(CF2) 1151.6

0.001

1151.5

1647.4

amide I

n = 9, 15ºC

n = 9, 25ºC

1149.1

1643.2

1644.0

Absorbance

1148.9

n = 7, 15ºC 1639.9

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


n = 7, 25ºC

1800

1600

1400

1200

Wavenumber / cm Figure 3

1000 -1

IR transmission (Tr) spectra of four LB monolayers of NAGFn deposited on a CaF2 substrate at the

common surface pressure of 45 mN m-1.

The strength of the H-bonding between the amide groups can be discussed by using the band position of the amide I band.23-26 For the H-bonding free amide group, the amide I band appears at ca. 1680 cm-1; whereas a H-bonded one appears at a lower wavenumber position, and the shift has a correlation with the H-bonding strength. Thus far, a variety of amphiphilic molecules having 1~4 amino acid residuals (mono- ~ tetra-peptide) have already been studied for discussing the role of the amide group on 15



the molecular aggregation by using IR spectroscopy.26,27 When no Rf chain is introduced, the amide group forms a cooperative H-bonding (•••H–N–C=O•••H–N– C=O•••), which gives the amide I band at a fairly low position.26 A mono-peptide compound in the solid state, for example, gives the amide I band at 1640 cm-1, which is a typically low position specific to the cooperative H-bonding. In the spectrum of the NAGF7 LB film at 25°C (yellow line in Figure 3), the amide I band appears at 1639.9 cm-1, which is almost the same position as the typical cooperative H-bonding for the mono-peptide. This strongly implies that the cooperative H-bonding dominates the molecular aggregation in NAGF7 at 25°C. The position of the νs(CF2) band at 1148.9 cm-1 (NAGF7, 25°C) has information about the strength of the dipole-dipole interaction of the Rf chains.2,3 Before the discussion, we have to pay attention that the νs(CF2) band position also depends on the Rf chain length.2,19,28 A solid compound having the Rf length of n = 7 is known to give the νs(CF2) band at 1146.7 cm-1, and the band shifts to a higher position for an weaker interaction. Thus, the shift of the νs(CF2) band position from that of the reference position of the close packing ( ∆ Dipole-Dipole = 1146.7 −ν s ( CF2 ) ) is useful for evaluating the strength of the dipole-dipole interaction. If this index is negative, the Rf packing is looser than the close packing. Since the νs(CF2) band position of the NAGF7 16


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film at 25°C (1148.9 cm-1) is found as ∆ Dipole-Dipole = −2.2 , the molecular packing is thus found to be looser than the close packing. For n = 9, ∆ Dipole-Dipole is defined by using a reference position of 1149.5 cm-1 as follows: ∆ Dipole-Dipole = 1149.5 −ν s ( CF2 ) . As a result, the NAGF9 film at 25°C yields

∆ Dipole-Dipole = −2.0 , which is smaller than NAGF7. Although the change is minute, the band position can be discussed in the accuracy of 0.1 cm-1 under the present condition of IR measurements, and the νs(CF2) band position exhibits a quite high reproducibility.29 In this manner, the νs(CF2) band shift ( ∆ Dipole-Dipole = −2.0 ) implies a slight improvement of the dipole-dipole interaction as expected on the SDA theory. To discuss the H-bonding competing with the dipole-dipole interaction in the NAGFn aggregate, a similar index of the H-bonding is defined as the band shift from that of the cooperative H-bonding of the mono-peptide (1640.0 cm-1) as follows:

∆ H-bonding = 1640.0 −ν ( amide I ) . In the same manner as ∆ Dipole-Dipole , a negative value of ∆ H-bonding indicates a weak H-bonding. By using these two indices ( ∆ Dipole-Dipole and ∆ H-bonding ), all the spectra in Figure 3 can comprehensively be discussed in a competing manner of the two interactions. The indices of all the samples are plotted in Figure 4. Since the upper direction of the ordinate axis corresponds to a stronger H-bonding, NAGF7 at 25°C is found to

17



have the strongest H-bonding. On the other hand, the right direction of the abscissa axis corresponds to the stronger dipole-dipole interaction, and therefore the NAGF9 at 25°C has the strongest dipole-dipole interaction. It is worth noting that even the strongest one is at ∆ Dipole-Dipole

−2 cm-1, which means that the Rf groups in the NAGF samples do

not attain the closest SDA packing ( ∆ Dipole-Dipole = 0 ) because of the steric disturbance by the bulky spacer group, as reflected by the LMA in Table 1. In spite of the steric disturbance, an apparent correlation between ∆ Dipole-Dipole and θt is found at a common temperature as marked by the pink and blue circles, which confirms that the SDA interaction works on the Rf group in NAGF. Next, a competition of the two interactions dependent on the Rf chain length is discussed, and the temperature (phase difference) dependence is also discussed.

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n=7

77°

0

∆ H-bond / cm-1

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


25°C

-2 -4

103°

90°

15°C

-6

n=9 -8

120°

-2.4

-2.2

∆ Dipole-Dipole / cm Figure 4

Plot of the index of the H-bonding,

∆ H-bonding ,

-2 -1

against that of the dipole-dipole interaction,

∆ Dipole-Dipole . The corresponding θt of the Rf chain are added.

Rf chain length dependence.

It is of another interest to correlate ∆ Dipole-Dipole with

∆ H-bonding in the same plot. The indices of NAGF7 at 25°C is plotted at ∆ Dipole-Dipole = −2.2 and ∆ H-bonding = +0.1 . By referring to the rule of Figure 4 as stated above, the positive ∆ H-bonding index and the relatively smaller ∆ Dipole-Dipole index apparently indicate that the cooperative H-bonding dominates the molecular interaction in the NAGF7 LB film; whereas the contribution of the dipole-dipole interaction is minor. This significance of the H-bonding over the dipole-dipole interaction is quite 19



understandable, since the dipole-dipole interaction as a van der Waals interaction is mostly much weaker than the H-bonding. Nevertheless, on the other hand, the NAGF9 at 25°C has an apparently larger

∆ Dipole-Dipole indicating a stronger dipole-dipole interaction than NAGF7. At the same time, the H-bonding is remarkably weakened. This means that the robust H-bonding is second to the van der Waals interaction; otherwise the strength of H-bonding should stay unmoved. The reason of this unusual force balance is explained as follows. Rf groups are characterized by the spontaneous aggregation due the dipole-dipole interaction, which develops two-dimensionally.2,3 The two-dimensional interactive network is fairly strong, which can win a linear (one-dimensional) cooperative H-bonding in total. A similar discussion is also possible at 15°C highlighted by light blue in Figure 4. When Table 1 is referred to discuss Figure 4, a good correlation between LMA and the indices are found: a larger dipole-dipole interaction of NAGF9 makes LMA smaller than that of NAGF7 at both 15 and 25°C. This implies that the two-dimensional dipole-dipole interaction should be more favorable for a closer packing than the H-bonding.

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Influence of Phase Change. On the SDA theory, the Rf chains at 15°C (phase II) should have a stronger aggregation than 25°C (phase IV) for both n = 7 and 9, which should make ∆ Dipole-Dipole larger. Figure 4 indicates, however, an opposite result that the dipole-dipole interaction in the film at 15°C becomes weaker than that at 25°C for both

n = 7 and 9. The hint for this matter is found in another fact that H-bonding also becomes weaker on cooling. To explain the unexpectedly weak interaction in the NAGFn aggregate at 15°C, a ‘molecular mobility’ dependent on temperature should be taken into account. As mentioned in the section of the π-A measurements, the NAGFn molecules largely change the orientation on the monolayer compression. The amide groups in NAGFn aggregates, however, do not necessarily align linearly to form the cooperative H-bonding. For making the linear alignment, the molecules on water require some molecular mobility to change the orientation of the amide group. A lower temperature is unfavorable for molecular rearrangement to attain the linear alignment because of a lower molecular mobility on water. The poor molecular mobility on water should also influence the two-dimensional dipole-dipole interactive network formation. In fact, these weaker interactions are consistent with the larger LMA at 15°C exhibiting a looser packing. 21



SUMMARY

To investigate H-bonding competing with the Rf compounds-specific dipole-dipole interaction network, amphiphilic compounds having the amide group and an Rf chain are used, and the contribution of the two interactions on the molecular aggregation dependent on the twisting angle, θt, are discussed by π-A isotherms measurements of L films and IR spectroscopic analyses of the LB films. The LMA derived from molecular packing exhibits poor correlation with θt. This indicates that the aggregation cannot solely be attributed to dipole-dipole interaction of the Rf chains, but it also depends on H-bonding. IR spectroscopic analysis of the LB film readily explained the unusual LMA considering the two interactions, and the two-dimensionally interactive dipole network of Rf chains is found to be able to win the cooperative H-bonding.

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Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx/xxx.

Detailed procedure for the synthesis of NAGF7 and 9

ACKNOWLEDGMENTS

This work was financially supported by a Grant-in-Aid for Scientific Research (A) (No. 15H02185 (TH)) and Grant-in-Aid for Young Scientists (B) (No. 17K14502 (TS)) from the Japan Society for the Promotion of Science (JSPS).

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References

1.

Hasegawa, T. Understanding of the Intrinsic Difference between Normal- and Perfluoro-Alkyl

Compounds

toward

Total

Understanding

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Molecular Aggregation of Perfluoroalkyl Groups ... - ACS Publications

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