Langmuir Monolayer Properties of Fluorinated Fatty Alcohols and

Chapter 1

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Langmuir Monolayer Properties of Fluorinated Fatty Alcohols and Dipalmitoylphosphatidylcholine (DPPC) Hiromichi Nakahara, Takayoshi Yamada, Chihiro Usui, Shunichi Yokomizo, and Osamu Shibata* Department of Biophysical Chemistry, Graduate School of Pharmaceutical Sciences, Nagasaki International University, 2825-7 Huis Ten Bosch, Sasebo, Nagasaki 859-3298, Japan *E-mail: [email protected]. Phone: +81-956-20-5686. Fax: +81-956-205686. URL: http://www.niu.ac.jp/~pharm1/lab/physchem/indexenglish.html.

The authors have newly synthesized fluorinated amphiphiles with relatively short perfluorocarbon chains to understand their interaction with biomembranes. This chapter describes the monolayer miscibility of perfluorobutylated (F4H11OH) or perfluorohexylated long-chain alcohols (F6H9OH and F6H11OH) with DPPC, which is a major component of native pulmonary surfactants in a mammal. The two-component monolayer has been elucidated from the thermodynamic and morphological aspects. The surface pressure (Π)−molecular area (A) and surface potential (ΔV)−A isotherms for the systems were measured on 0.15 M NaCl at 298.2 K. From the isotherm data, a plot of an excess Gibbs free energy change of mixing versus mole fraction and a two-dimensional phase diagram were constructed to elucidate the miscibility between the two components. The miscibility is also supported by the in situ fluorescence microscopy (FM) and ex situ atomic force microscopy (AFM) after transfer on a mica substrate. Herein, the fluidization of DPPC monolayers containing a small amount of F4H11OH and F6H9OH is induced by increasing surface pressures. On the other hand, the incorporation of F6H11OH undergoes the solidification of DPPC monolayers. The control

© 2015 American Chemical Society Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

of phase states of DPPC monolayers is very important for a pulmonary replacement therapy.


Introduction A molecule substituted with fluorine is endowed with unique properties such as higher hydrophobicity as well as lipophobicity (1), higher gas-dissolving capacity (1, 2), less chemical and biological activities, lower surface tension, and higher fluidity (1, 3). These fascinating properties indicate a potential towards use and application in medicine, disease diagnosis, and therapy (1, 4–6). Indeed, it has been reported that fluorinated materials are potentially applicable as blood substitutes (6–10) and effective additives to lung surfactant preparations (6–14). Despite these favorable properties, there is danger (especially for highly fluorinated compounds) of accumulation in the environment and in the human body (15–18). Among them, the anion type of perfluorooctanoic acid (PFOA) is gaining attention in terms of its sorption by soils and sediments. However, perfluorooctyl bromide (PFOB), which has been developed as the blood substitutes, has been shown to have short organ retention times (1, 2). With the aim of the reduction in the retention time and of the acquirement of the unique properties, the partially fluorinated (or semifluorinated) materials have been investigated using Langmuir monolayers at the air-water interface (19–23). Among the various techniques to investigate the properties of fluorinated amphiphiles, Langmuir monolayers are a considerably simple and optimal model in mimicry of biomembranes. In particular, due to the reduction of one dimension, the monolayer technique shows the advantage of understanding the mutual interaction between two or more components. Furthermore, with the both in situ and ex situ methods such as Brewster angle microscopy (BAM), fluorescence microscopy (FM), atomic force microscopy (AFM), and grazing incidence X-ray diffraction (GIXD), the phase behavior and interaction can be visualized and elucidated in more details. As the simplest partially fluorinated compound in terms of chemical structures, semifluorinated alkanes (CnF2n+1CmH2m+1 or FnHm) have been studied for more than two decades (24). It is a quite surprising property for the alkanes to form Langmuir monolayers on the aqueous subphase because hydrogenated n-alkanes form a droplet instead of spreading as monolayers. Goldmann et al. have subsequently examined the monolayer made of pure FnHm to clarify the FnHm−FnHm and FnHm−substrate interactions (25). Nevertheless, the interactions between fluorocarbon chains and between fluorocarbon and hydrocarbon chains seem to still remain unknown factors. Recently, we have studied the monolayer behavior of partially fluorinated long-chain alcohols (CnF2n+1CmH2m+1OH or FnHmOH) and their interaction with dipalmitoylphosphatidylcholine (DPPC) (20, 26–28). The fluorinated alcohols are less affected by the subphase pH than the compound containing carboxylic or amino groups and show a relatively simple chemical structure among fluorinated amphiphiles. In addition, DPPC is a major component of native pulmonary surfactants (29, 30). The film of pulmonary surfactants at the air-alveolar fluid interface undergoes successive changes in fluidity and rigidity 2 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


during compression and expansion. DPPC contributes significantly to the rigidity of pulmonary surfactant films. However, such films can exhibit slow adsorption and poor diffusion at the surfaces. The fluidity of DPPC monolayers is perturbed by the addition of F8HmOH molecules, which depends on their fluorination degree (20, 26–28). Indeed, the F8H5OH incorporation causes a fluidizing effect on DPPC monolayers. On the other hand, the addition of longer F8HmOH (m = 7, 9, and 11) shows an opposite effect of solidification. Especially, in the binary DPPC/F8H7OH system, the two components interact less at low surface pressures whereas a strong interaction occurs at high surface pressures. Thus, fluorinated amphiphiles have potential of controlling the fluidity of DPPC monolayers, however, the interaction between these chains is still documented poorly. The degree of fluorination in partially fluorinated chains may produce an unexpected interaction with lipids. In these days, FnHmOH (n < 8) in the solid state has been reported to show the irregular and interesting behavior for its melting points (19): the melting point of FnHmOH (n < 8) is lower than that of the corresponding hydrogenated alcohols. This unexpected finding is considered to be attributed to the specific properties of fluorocarbons. Furthermore, the study on the monolayer of amphiphiles with fluorocarbon shorter than 8 has been performed rarely (31). Therefore, this article describes recent efforts devoted to understanding the interaction and miscibility of two-component monolayers of long-chain alcohols (F4H11OH, F6H9OH, and F6H11OH) and DPPC. The thermodynamical elucidation of the binary miscibility in the monolayer state was performed by using isotherm data such as surface pressure (π)−molecular area (A) and surface potential (ΔV)−A curves. The phase behavior with respect to monolayer composition and surface pressure was visualized with FM, and AFM.

Experimental Section Materials (Perfluorobutyl)undecanol (F4H11OH), (Perfluorohexyl)nonanol (F6H9OH), and (Perfluorohexyl)undecanol (F6H11OH) were synthesized as reported previously (see Scheme 1) (19). L-α-dipalmitoylphosphatidylcholine (DPPC; purity >99%) was obtained from Avanti Polar Lipids (Alabaster, AL). The fluorescent probe 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4yl)amino]hexanoyl]-sn-glycero-3-phosphocholine (NBD-PC) was from Avanti Polar Lipids. These lipids were used without further purification. Chloroform (99.7%) and n-hexane (>98.5%) were purchased from Kanto Chemical Co., Inc (Tokyo, Japan) and Merck KGaA (Uvasol, Darmstadt, Germany), respectively. Methanol (99.8%) and ethanol (>99.5%) were from nacalai tesque (Kyoto, Japan). These were used as spreading solvents. The chloroform/methanol (2/1, v/v) mixtures for F4H11OH and n-hexane/ethanol (9/1, v/v) for F6H9OH and F6H11OH were used as spreading solvents. Sodium chloride (nacalai tesque) was roasted at 1023 K for 24 h to remove all surface active organic impurities. The substrate solution was prepared using thrice distilled water (surface tension = 72.0 mN m–1 at 298.2 K; electrical resistivity = 18 MΩ cm). 3 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Scheme 1. Chemical Structures of (a) F4H11OH, (b) F6H9OH, and (c) F6H11OH

Methods Surface Pressure–Area Isotherms The surface pressure (π) of monolayers was measured using an automated homemade Wilhelmy balance. The surface pressure balance (Mettler Toledo, AG-245) had a resolution of 0.01 mN m–1. The pressure-measuring system was equipped with filter paper (Whatman 541, periphery = 4 cm). The trough was made from Teflon-coated brass (area = 720 cm2), and Teflon barriers (both hydrophobic and lipophobic) were used in this study. Surface pressure (π)–molecular area (A) isotherms were recorded at 298.2 ± 0.1 K. The spreading solvents were allowed to evaporate for 15 min prior to compression. The monolayer was compressed at a speed of ~0.10 nm2molecule–1min–1. The standard deviations (SD) for molecular surface area and surface pressure were ~0.01 nm2 and ~0.1 mN m–1, respectively (28, 32, 33).

Surface Potential–Area Isotherms The surface potential (ΔV) was recorded simultaneously with surface pressure, when the monolayer was compressed and expanded at the air-water interface. It was monitored with an ionizing 241Am electrode at 1–2 mm above the interface, while a reference electrode was dipped in the subphase. The electrometer (Keithley 614) was used to measure the surface potential. The SD for the surface potential was 5 mV (34, 35).

4 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Fluorescence Microscopy (FM) The film balance system (KSV Minitrough) was mounted onto the stage of an Olympus microscope BX51WI (Tokyo, Japan) equipped with a 100W mercury lamp (USH-1030L), an objective lens (SLMPlan50×, working distance = 15 mm), and a 3CCD camera with a camera control unit (IKTU51CU, Toshiba, Japan). A spreading solution of the co-solubilized samples was prepared, doped with 1 mol% of the fluorescence probe (NBD-PC). Image processing and analysis were carried out using the software, Adobe Photoshop Elements ver. 7.0 (Adobe Systems Incorporated, CA). The total amount of ordered domains (dark contrast regions) was evaluated and expressed as a percentage per frame by dividing the respective frame into dark and bright regions. For the percentage, resolution (or discrimination threshold) was 0.1% and the maximum SD was 8.9%. More details on FM measurements were provided in the previous paper (11, 33).

Atomic Force Microscopy (AFM) Langmuir-Blodgett (LB) film preparations were carried out with the KSV Minitrough. Freshly cleaved mica (Okenshoji Co., Tokyo, Japan) was used as a supporting solid substrate for film deposition (vertical dipping method). At selected surface pressures, a transfer velocity of 5 mm min–1 was used for singlelayer deposition. The film-forming materials were spread on 0.15 M NaCl at 298.2 K. The transfer occurs so that the hydrophilic part of the monolayer is in contact with mica while the hydrophobic part is exposed to air. LB films with a deposition rate of ~1 were used in the experiments. The AFM experiments were performed in the air at room temperature. The AFM images were obtained using an SPA 400 instrument (Seiko Instruments Co., Chiba, Japan) at room temperature in the tapping mode, which provided both topographical and phase contrast images. Other details about AFM measurements have been mentioned previously (33).

Results and Discussion Thermal Properties Thermal properties of saturated fatty acids or alcohols are expected to depend on their aliphatic chain lengths due to the simplicity of chemical structures (36). For instance, the melting point (Tm) of the acids (or alcohols) increases linearly with respect to the elongation of their chain lengths. Fluorination of the whole hydrogen atoms in a tail group of saturated fatty acids changes or improves their original properties such as melting point, vapor pressure, etc. The Tm values of the perfluorinated fatty acids also increase linearly against hydrophobic chain length although a slope of Tm versus carbon number for the perfluorinated acids is twice smaller than the corresponding hydrogenated acids (37). This fact means that the high fluorination provides original compounds with thermal stability. The variation in melting point allows us easily to estimate the difference in the intermolecular interactions such as dipole-dipole 5 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


interaction and van der Waals interaction between the hydrocarbon−hydrocarbon and fluorocarbon−fluorocarbon chains. In this regard, partially fluorinated amphiphiles are also expected to be endowed with such properties. In fact, the melting point of ω-(perfluorooctyl)alkanol (or F8HmOH) is larger than that of the corresponding hydrogenated alcohol (19); the melting point of perfluorooctylated fatty alcohols increases by ~1 K compared to the corresponding hydrogenated alcohols. However, the melting points of F4H11OH and F6HmOH are lower than those of the corresponding hydrogenated alcohols with the same hydrophobic chain length (19). This may be attributed to dipole-dipole interaction, molecular weight, and restricted motion at the CF2−CH2 linkage, which are open to be disputable. Nevertheless, the irregular phenomenon suggests that the dipole-dipole and van der Waals interactions between the hydrophobic chains of F4H11OH and F6HmOH are different from FnHmOH (n ≥ 8). Langmuir Monolayer of Pure Systems The surface pressure (π)–molecular area (A) and the surface potential (ΔV)–A isotherms of F4H11OH, F6H9OH, F6H11OH, and DPPC monolayers on 0.15 M NaCl at 298.2 K are shown in Fig. 1. The other alcohols with hydrophobic chains shorter than 15 are difficult to form an insoluble (or stable) monolayer at the air-water interface under the same condition due to their high solubility into water (19). At 298.2 K, F4H11OH (curve 1) forms a typical disordered monolayer corresponding to the liquid-expanded (LE) phase of hydrogenated lipid monolayers. A collapse pressure (πc) of F4H11OH monolayers is ~44 mN m−1 at 0.24 nm2. The extrapolated area of highly packed states on the π–A isotherm is ~0.40 nm2, which is based on the cross-sectional area of F-chains (~0.30 nm2) (1). The F4H11OH and F6H9OH (curve 2) monolayers initially have a π value of more than 1 mN m−1 at large molecular areas (A = ~2.0 nm2), where the monolayer is in a disordered phase. However, the isotherm for F6H9OH monolayers has a kink or a transition pressure (πeq) at ~8 mN m−1 (indicated by a dashed arrow), which means a phase transition of the disordered state to an ordered state corresponding to liquid-condensed (LC) phase of the lipid monolayers (19). On further compression, the monolayer collapses at ~47 mN m−1. Considering the same hydrophobic chain length between F4H11OH and F6H9OH, it is found that the replacement of −(CH2)2− by −(CF2)2− improves the rigidity of monolayers. Incidentally, the F4H11OH monolayer comes to indicate the phase transition at the temperature lower than 298.2 K. More detailed elucidation on the phase transition for F4H11OH and F6H9OH monolayers (e.g. entropy and enthalpy changes) has been described in the previous paper (19). Similarly to the thermal behavior of the solid state mentioned above, the slope (∂πeq/∂T) for F4H11OH and F6H7OH monolayers are more sensitive to temperature: the thermal sensitivity of F4H11OH monolayers is twice as large as that of the corresponding fatty alcohol monolayers. On the other hand, F6H11OH including a longer hydrophobic chain has a tendency to form more rigid monolayers. During the compression from ~1 mN m−1 to the collapse pressure of ~52 mN m−1, F6H11OH (curve 3) monolayers keep being in an ordered phase without the transition. The molecular areas at the constant surface pressure (e.g., 15 mN m−1, Fig. 1) indicate that the enhancement 6 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


of fluorination in hydrophobic chains and their elongation produce the condensing effect on monolayers; F4H11OH < F6H9OH < F6H11OH. This result is also supported by that for F8HmOH (m = 5, 7, 9, and 11) monolayers (20, 26–28). That is, it can be said that the F-moiety rather than the hydrocarbon (H) moiety in FnHmOH molecules dominates over the monolayer ordering and orientation at the close-packed state for the monolayers. DPPC monolayers (curve 4) have an LE/LC phase transition at ~11 mN m–1 (dashed arrow) and a collapse pressure at ~55 mN m–1, which has been discussed elsewhere (11, 33, 38).

Figure 1. The π–A and ΔV–A isotherms of F4H11OH (curve 1), F6H9OH (curve 2), F6H11OH (curve 3), and DPPC (curve 4) monolayers on 0.15 M NaCl at 298.2 K.

The surface potential (ΔV) of monolayers can be considered as a combination of dipole moments of molecules in the subphase (layer 1), polar head group (layer 2), and terminal group of hydrophobic chain (layer 3). Independent dipole moments and effective local dielectric constants are attributed to each of the three layers. Thus, the ΔV–A isotherms of the monolayers indicate changes in molecular orientation upon lateral compression. It is widely accepted that the ΔV–A isotherm of monolayers consisting mainly of H-chains or F-chains exhibits positive or negative variation against lateral compression, respectively (28, 39, 40). The negative ΔV value of F-chains is attributed to the electronegativity of a fluorine, which has been discussed thoroughly (21, 22, 39–42). The ΔV value decreases with decreasing molecular areas and finally approaches a minimum ΔV value (ΔVmin) at the close-packed state, where monolayers begin to collapse. 7 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

The ΔVmin value for F4H11OH monolayers is around −650 mV. On the other hand, both the values for F6H9OH and F6H11OH monolayers are concentrated to nearly −700 mV. In this connection, F8HmOH (m = 5, 7, 9, and 11) monolayers indicate almost ΔVmin = −750 mV independent of total chain lengths (20, 26–28). Considering few contributuns of dipole moments (layers 1 and 2) to the ΔV value, these result becomes important evidence that F-moiety (accurately, terminal CF3− group) in FnHmOH molecules is exposed to the air similarly to typical lipid monolayers and that the monolayer ordering and orientation depend on the F-moiety rather than the H-moiety at the close-packed state for the monolayers.


Two-Component Monolayers with DPPC The π–A and ΔV–A Isotherms The two-component interaction and miscibility between DPPC and the fluorinated fatty alcohols in the monolayer state can be elucidated from the π–A and ΔV–A isotherms measured by varying the monolayer composition. The isotherms of binary monolayers often lie in compositional order between the isotherms of each pure monolayer due to the extensive variable of molecular area (or surface concentration). The π–A and ΔV–A isotherms for the binary DPPC/F4H11OH, DPPC/F6H9OH, and DPPC/F6H11OH monolayers are shown in Fig. 2. Herein, the binary monolayers including both H-chains and F-chains indicate the more distinct and larger variation in ΔV–A isotherms due to their opposite ΔV signs (20, 26–28). The criteria for assessing the interaction of binary monolayers are based on a variation in πeq as well as πc with regard to composition (43). In particular, the ΔV–A isotherm is useful in determination of the pressures (πeq and πc). The disordered/ordered phase transition on the π–A and ΔV–A isotherms is indicated by dashed arrows (19, 44). In Fig. 2A, the π–A isotherms shift to smaller areas with increasing the mole fraction of F4H11OH (XF4H11OH) and accordingly the πeq value increases. The increment in πeq means that there is a fluidizing effect of F4H11OH on DPPC monolayers (28). In addition, the πc value apparently changes as a function of XF4H11OH. These variation in πeq and πc suggests a miscibility between DPPC and F4H11OH. The ΔV–A isotherms shift more considerably from positive to negative values (~600 to −600 mV for ΔVmin) with regard to XF4H11OH. Shown in Fig. 2B are the isotherms for DPPC/F6H9OH monolayers. Both the pressures (πeq and πc) vary with the mole fraction of F6H9OH (XF6H9OH). However, the mode of πeq variation is somewhat complicated differently to the DPPC/F4H11OH system. This is attributed to the fact that both pure components in the DPPC/F6H9OH system exhibit the phase transition. Nevertheless, in the small XF6H9OH region, it is found that the addition of F6H9OH to DPPC monolayers induces the fluidizing effect. Whereas, the π–A isotherms for the DPPC/F6H11OH system in the larger XF6H11OH region indicate the reduced πeq value as the mole fraction of F6H11OH (XF6H11OH) increases (Fig. 2C). This means a solidifying effect of F6H11OH on DPPC monolayers. In the case of the binary DPPC/F8HmOH monolayers (20, 26–28), the incorporation of F8HmOH also generates fluidizing or solidifying effects depending on m: fluidization for m = 5 and solidification for m = 7, 9, 11. The boundary factor to 8 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


induce the fluidizing or solidifying effects must be related deeply to a mismatch between the hydrophobic chain lengths of DPPC and the fluorinated alcohols. It is noticed that the additional effect of the alcohols on DPPC monolayers changes in direct opposition only by two methylene groups. Consequently, the miscibility for the three systems here is suggested by the fact of the variation in surface pressures (πeq and πc) against mole fraction.



Figure 2. The π–A isotherms of the two-component DPPC/F4H11OH (A), DPPC/F6H9OH (B), and DPPC/F6H11OH (C) monolayers on 0.15 M NaCl at 298.2 K. Adapted with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society.

Excess Gibbs Free Energy Change The mutual interactions between DPPC and the fluorinated alcohols in the monolayer state can be analyzed thermodynamically with the excess Gibbs free ), which is calculated with the following equation energy change of mixing ( (Eq. (1)) (43):

where Ai and Xi are the molecular area and mole fraction of component i, respectively, and A12 is the mean molecular area of the binary monolayer. For identical interactions between the two components, is zero. In this state, they are either ideally mixed in the monolayer or are not mixed completely, resulting in patch-like packing state (45, 46). A negative value indicates that an attractive interaction exists between the two components. The − XF4H11OH plot for DPPC/F4H11OH at representative surface pressures is shown in Fig. 3A. The values in 0 < XF4H11OH ≤ 0.7 are almost negative at all surface pressures. In addition, they decrease with an increase in surface pressure. The −XF4H11OH profile reaches −850 J mol−1 at XF4H11OH = 0.3 as a minimum at 10 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


45 mN m−1. This indicates that the attractive force between hydrophobic chains of DPPC and F4H11OH is enhanced upon compression and their affinity at high surface pressures becomes the largest value at XF4H11OH = 0.3. At XF4H11OH = 0.9, values exhibit positive values, which means existence of a repulsive the force or a steric hindrance between the two components. However, the values are relatively small in terms of the interaction between the two components. Thus, the monolayer at XF4H11OH = 0.9 is considered to take a looser packing. As for the DPPC/F6H9OH system (Fig. 3B), the values are negative at all surface pressures over the whole XF6H9OH and reach nearly −1600 J mol−1 at XF6H9OH = 0.6 as the surface pressure increases to 45 mN m−1. On the other hand, the DPPC/F6H11OH system indicates the minimum value of around −600 J mol−1 at XF6H11OH = 0.3 (Fig. 3C). It is noticed that the value for the DPPC/F6H9OH system is more than twice as small as that for DPPC/F6H11OH system, which indicates that DPPC molecules prefer F6H9OH in miscibility to F6H11OH. This is considered to be due to the similarity in monolayer phase states between DPPC (LE/LC) and analysis is based on the additivity rule F6H9OH (disordered/ordered). The so that it does not provide direct relationships with interaction modes such as fluidization and solidification.

Figure 3. Excess Gibbs free energy changes of mixing ( ) of the binary DPPC/F4H11OH (A), DPPC/F6H9OH (B), and DPPC/F6H11OH (C) monolayers as a function of XFnHmOH at typical surface pressures on 0.15 M NaCl at 298.2 K. Adapted with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society.

Two-Dimensional Phase Diagram Two-dimensional phase diagrams, which illustrate variations between the phases of monolayers with respect to surface pressure and composition under the thermodynamically equiliburium state, are constructed by plotting the πeq and πc values for the binary monolayers against composition at 298.2 K. In the 11 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


DPPC/F4H11OH system (Fig. 4A), the πeq values are kept almost constant below XF4H11OH = 0.3. However, they change positively as XF4H11OH increases from 0.3 to 0.7. As shown in Fig. 4B, the πeq value in the DPPC/F6H9OH system increases with increasing XF6H9OH smaller than 0.6. Then, it decreases to the πeq value of pure F6H9OH (~8 mN m−1). On the other hand, for the DPPC/F6H11OH system (Fig. 4C), the reduction in πeq is caused with an increase in XF6H11OH. These variations in πeq against monolayer composition are a confirmation of miscibility between the two compositions and suggest the interaction modes of fluidization (DPPC/F4H11OH and DPPC/F6H9OH) and solidification (DPPC/F6H11OH). In the high surface pressure region, the experimental πc values also vary against mole fraction. If the surface pressure remains constant with regard to mole fraction, two components can be said to be immiscible with each other. Accordingly, the two components are found to be miscible in the monolayer state.

Figure 4. Two-dimensional phase diagrams based on the variation of the transition pressure (πeq: open circle) and collapse pressure (πc: solid circle) on 0.15 M NaCl at 298.2 K as a function of XFnHmOH. The dashed lines were calculated according to Eq. (2) for ξ = 0. The solid line at high surface pressures was obtained by curve fitting of experimental collapse pressures to Eq. (2). “M” indicates a mixed monolayer formed by DPPC and FnHmOH species, whereas “Bulk” denotes a solid phase of DPPC and FnHmOH (“bulk phase” may be called “solid phase” not monolayer state). Adapted with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society.

The coexistence phase boundary of the monolayer/bulk states of the molecules spread on a surface can be theoretically simulated using the Joos equation (47, 48) under the assumption of a regular surface mixture:



where and denote the respective molar fractions of components 1 and 2 in the two-component monolayers; and are the respective collapse pressures of components 1 and 2; is the collapse pressure of the two-component monolayer at a given composition of (or ); ω1 and ω2 are the corresponding molecular areas at the collapse; ξ is the interaction parameter. As seen in Fig. 4, the solid curve at higher surface pressures was drawn by adjusting ξ in Eq. (2) to obtain the best fit for the experimental values of collapse pressures. All the systems here are of the positive azeotropic type, which indicates the stronger interaction between heterogeneous monolayers. The interaction energy can be calculated as the following equation,

where z is the number of nearest neighbors, equal to 6, in a close-packed monolayer, and the interaction energy is Δε = ε12 – (ε11 + ε22)/2 (48); εij denotes the potential energy of interaction between components i and j. In the small mole fraction region, the ξ value for the DPPC/F4H11OH system is largest among the three systems. This is found to depend on the fluidity of monolayers for pure components: F4H11OH > F6H9OH > F6H11OH. In the large mole fraction region, the two components nearly cause the ideal interaction or very weak interaction due to the small ξ value (near zero). On the other hand, the ξ value for the DPPC/F8HmOH systems except for m = 5 is quite larger in magnitude in the small XF8HmOH region. That is, the values for the F8HmOH systems are almost 2−3 times larger in magnitude than those for the F4H11OH and F6HmOH systems. It is suggested that the interaction mode of alcohols with F-chains shorter than 8 is quite different from that of F8HmOH. Considering that the ξ value is estimated at the close-packed state of monolayers, it is found that the F-chains are not restrained strongly by the hydrophobic chains of DPPC. That is, it can be said that the F4- and F6-moiety in the alcohols possess the degree of freedom for the molecular motion at the linkage between H- and F-chains compared to a F8HmOH molecule (44).

Fluorescence Microscopy (FM) FM observations at the air-water interface can provide morphological information on the phase behavior of monolayers in relatively high resolution and magnification. FM measurements require a fluorescent probe, which is incorporated into the monolayer. The bright and dark contrasts are respectively assigned to LE (or disordered) and LC (or ordered) phases of lipid monolayers (49), which is based on the fact that the probe generally forms disordered monolayers at the interface due to its spatially bulky moiety (or fluorescent part). However, there is a worry of the possibility that the probe (or dye) affects the original phase behavior of monolayers. Thus, the resulting FM image should be checked in validity that the incorporation of FM probes has no influence on the original isotherm and the original domain shape in the image captured with Brewster angle microscopy (BAM). The BAM image is observed in situ at the 13 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


air-water interface and the capture requires no exogenous materials. The BAM observation is simply based on the difference in refractive index of monolayer phases caused by a change in the molecular density or packing at the surface (50, 51). It has been reported that the addition of less than 1 mol% FM probes produces few effects on the monolayer behavior (38, 52). Thus, the FM images presented here have been checked by being compared to the corresponding BAM images to have no influence of the probe on the original phase behavior (data not shown).

Figure 5. Fluorescent micrographs of the binary DPPC/F4H11OH monolayers on 0.15 M NaCl (298.2 K) at 12, 15, and 25 mN m−1. The monolayers contain 1 mol% of fluorescent probe (NBD-PC). The scale bar in the lower right represents 100 μm. Reproduced with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society.

The FM micrographs of the binary DPPC/F4H11OH monolayers are shown in Fig. 5. As for the DPPC monolayer, the images exhibit LC domains coexistent with LE regions in a clear and sharp contrast. The LC domain with counterclockwise arms typically at 15 mN m−1 is characterized at DPPC monolayers (20, 28, 53–55). In common, a domain formation is controlled by balance of a line tension at the boundary between disordered and ordered domains and a long-range dipole-dipole interaction between ordered domains (49, 56–61). When a small amount of F4H11OH is added into DPPC monolayers (XF4H11OH = 0.1), the ordered domain become larger in size and the domain shape changes to a nearly circular or bean-like form. This means that the F4H11OH addition 14 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


enhances contribution of the line tension at the phase boundary. However, the further addition induces the size reduction of ordered domains and modification of the shape as shown at XF4H11OH = 0.3 and 0.4. At 25 mN m−1 (for XF4H11OH = 0.3) and 15 mN m−1 (for XF4H11OH = 0.4), the specific modification of ordered domain shape is observed: the edge of the ordered domains changes into disordered phase. These phenomena correspond to the fluidizing effect on DPPC monolayers (28). Moreover, it indicates that a fluidizing effect of F4H11OH on DPPC monolayers is induced by surface pressures. This effect is considered to be characterized at the incorporation of fluorinated amphiphiles with F-chains shorter than 8 because there are many papers on the fluidizing effect induced by surface compositions in the binary systems containing the fluorinated amphiphiles with longer F-chains (23, 28, 31, 41). Apparently, the present system exerts not only the surface pressure-induced effect but also the surface composition-induced effect, which is shown in Fig. 4. At XF4H11OH > 0.4, the FM image remains homogeneously bright regardless of surface pressure (data not shown).

Figure 6. Fluorescent micrographs of the binary DPPC/F6H9OH monolayers on 0.15 M NaCl (298.2 K) at 12, 15, and 25 mN m−1. The monolayers contain 1 mol% of fluorescent probe (NBD-PC). The scale bar in the lower right represents 100 μm. 15 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


In the binary DPPC/F6H9OH system at XF6H9OH ≤ 0.1 (Fig. 6), the phase behavior is almost the same as the DPPC/F4H11OH system in terms of the domain shape. With the further addition of F6H9OH (XF6H9OH = 0.2), the surface pressureinduced fluidization, which corresponds to unclearness of the edge of ordered domains, is observed at 25 mN m−1. The fluidizing effect is observed in smalleramount addition compared to the F4H11OH system, which implies that the F6 moiety tends more to disturb the hydrophobic chain in DPPC at middle surface pressures. On the other hand, the DPPC/F6H11OH system does not exert the surface pressure-induced fluidization as seen in Fig. 7. The solidifying effect is, rather, observed with increasing XF6H11OH to 0.2. Interestingly, the ordered domain fuses with each other at XF6H11OH = 0.2. The domain fusion indicates attenuation of repulsive force between the domains, which is based on the long-range dipoledipole interaction. This is considered to result from the reduced polarization or dispersed dipole moment inside each ordered domain. Thus, the domain fusion and growth in size strongly support the mutual miscibility between DPPC and F6H11OH in the monolayer state.

Figure 7. Fluorescent micrographs of the binary DPPC/F6H11OH monolayers on 0.15 M NaCl (298.2 K) at 12, 15, and 25 mN m−1. The monolayers contain 1 mol% of fluorescent probe (NBD-PC). The scale bar in the lower right represents 100 μm. 16 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.




Figure 8. Surface-pressure dependence of the ratio of ordered domain area (dark contrast) in FM images of the binary DPPC/F4H11OH (A), DPPC/F6H9OH (B), and DPPC/F6H11OH (C) monolayers. Adapted with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society.

Figure 9. Typical AFM topographic images of the binary DPPC/F4H11OH monolayers for XF4H11OH = 0.5 at 20 and 35 mN m−1. The scale bar in the lower right represents 500 nm. The cross-sectional profiles along the scanning line (white line) are given just below the respective AFM images. The height difference between the arrows is indicated in the cross-sectional profile. Reproduced with permission from reference (44). Copyright 2015 Japan Oil Chemists’ Society. 18 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 10. Typical AFM topographic images of the binary DPPC/F6H9OH monolayers for XF6H9OH = 0.3 at 20 and 35 mN m−1. The scale bar in the lower right represents 500 nm. The cross-sectional profiles along the scanning line (white line) are given just below the respective AFM images. The height difference between the arrows is indicated in the cross-sectional profile. A percentage of the ordered domain per frame of FM images is plotted as a function of surface pressure in Fig. 8. As for the DPPC/F4H11OH system (Fig. 8A), the ordered domain ratio in the range of 0 ≤ XF4H11OH ≤ 0.2 increases monotonously and finally approaches ~95% upon compression, which suggests that the small-amount incorporation of F4H11OH does not induce the fluidizing 19 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


effect. However, the percentage at XF4H11OH = 0.3 and 0.4 does not reach the value near ~95% due to the fluidizing effect on the ordered domains. In the case of the DPPC/F6H9OH system (Fig. 8B), the percentage at XF6H9OH = 0, 0.05, and 0.1 reaches nearly 100% with an increase in surface pressure. However, at XF6H9OH = 0.2, it is found that the ordered domain transfers rapidly into disordered phases within 5 mN m−1 beyond π = 20 mN m−1. The monotonous increase in the percentage occurs in the binary DPPC/F6H11OH system from XF6H11OH = 0 to 0.2 (Fig. 8C). The profile shows no reduction with respect to surface pressure in percentage as opposed to the DPPC/F4H11OH and DPPC/F6H9OH systems. Considering the fact that the fluidizing effect induced by surface pressures is not observed for the binary DPPC/F8HmOH system (20, 26–28), it is suggested that the compact corn-swing motion of F4- and F6-moieties in the alcohols as the fulcrum in the CH2−CF2 linkage disturbs or disperses the ordered domain at the phase boundary to transfer it to the disordered state at middle surface pressures.

Atomic Force Microscopy (AFM) The in situ microscopic observation at the air-water interface has limitations of resolution and magnification to catch the clear two contrasts at least on the image. As mentioned above, the phase morphology of monolayers at high mole fractions of the alcohols could not be visualized with BAM and FM in the present study. Therefore, the phase behavior and distribution have been observed at the nano-meter scale with AFM, which is herein the ex situ microscopy employing the Langmuir-Blodgett (LB) technique. However, the LB technique includes a possibility of changes of the native monolayer structures at the interface by the electric charge between the samples and substrates, and by the physical factors during the deposition procedure. AFM images may therefore not provide completely correct information on the phase behavior at the air-water interface but nevertheless allow us to understand phase morphologies that can’t be caught with in situ microscopic techniques (BAM and FM). The representative AFM images at XF4H11OH = 0.5 in the DPPC/F4H11OH system are shown in Fig. 9. There are many stripes (bright contrast) composed mainly of DPPC (44). The height difference between the stripe and the surrounding network (dark contrast) composed mainly of F4H11OH is ~0.5 nm irrespective of an increase in surface pressure from 20 to 35 mN m−1. These stripes are quite small in size compared to the ordered domains shown in Fig. 5. This is attributed to the transformation of ordered domains into disordered domains, which is induced by F4H11OH. Shown in Fig. 10 are the AFM micrographs at XF6H9OH = 0.3 in the DPPC/F6H9OH system. The image at 20 mN m−1 indicates the coexistence state of bright and dark domains. The bright domain is made mainly of DPPC because of the molecular length and the fact that the domain reduces in size and its occupied ratio in a frame also decreases at XF6H9OH = 0.5 (data not shown). Moreover, similarly to the DPPC/F4H11OH system, the height difference of ~0.5 nm remains regardless of surface pressure. However, the bright stripe is fused upon further compression to 35 mN m−1. The fusion of the stripes do not support the surface pressure-induced fluidization at the nano-meter scale. This may be because that F6H9OH dissolves 20 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


into the DPPC-rich domain by the surface pressure increase on the basis of the binary miscibility. This dissolution is also supported by the fact of reduction in occupied area of the domain with dark contrast with increasing surface pressures (Fig. 10). The AFM topographic images for the DPPC/F6H11OH monolayers shown in Fig. 11 are apparently different from the former two systems. As seen in the images at 10 mN m−1, the dark region increases in occupied area with increasing XF6H11OH. Therefore, it is found that the bright contrast is expressed by the DPPCrich domains. At XF6H11OH = 0.3, the DPPC-rich domain disturbs the formation of the network of F6H11OH at 10 mN m−1. The network is more difficult to be formed by the rise in surface pressure. Conparison to the AFM images in Fig. 10 allows us easily to understand the solidifying effect of F6H11OH on DPPC monolayers (XF6H11OH = 0.3). When F6H11OH is added further (XF6H11OH = 0.5 and 0.7), the F6H11OH network comes to be formed to disperse the DPPC-rich domain finely. Nevertheless, the ratio of bright domains per frame is kept high irrespective of the fine dispersion of DPPC-rich domains. The similar solidification have been reported for the systems containing partially fluorinated amphiphiles (20, 26).

Figure 11. Typical AFM topographic images of the binary DPPC/F6H11OH monolayers for XF6H11OH = 0.3, 0.5, and 0.7 at 10 and 35 mN m−1. The scale bar in the lower right represents 500 nm. The cross-sectional profiles along the scanning line (white line) are given just below the respective AFM images. The height difference between the arrows is indicated in the cross-sectional profile. 21 Wang and Leblanc; Recent Progress in Colloid and Surface Chemistry with Biological Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Conclusion The lateral interaction between DPPC and partially fluorinated alcohols having perfluorobutyl or perfluorohexyl moieties has been elucidated employing the monolayer technique at the air-water interface. The two-component DPPC/F4H11OH, DPPC/F6H9OH, and DPPC/F6H11OH systems are found thermodynamically and morphologically to be miscible in the monolayer state. The mode of interaction for all the systems presented here is very similar in interaction parameter and energy, which are estimated from the isotherm at high surface pressures. However, the miscibility behavior at low and middle surface pressures is significantly different among the three systems. The DPPC/F4H11OH and DPPC/F6H9OH systems indicate a fluidizing effect on DPPC monolayers. In particular, at the specific compositions, the fluidization is induced by the increment in surface pressure. On the other hand, a solidifying effect on DPPC monolayers occures in the DPPC/F6H11OH system. Considering the difference in total carbon number between F6H9OH and F6H11OH, it is found that only the two methylene groups are the key to the two opposite effects. Generally, the amphiphiles with the Fn moiety (n ≤ 8) are accumulated less in the human body and the environment compared to perfluorinated and highly fluorinated amphiphiles. Therefore, perfluorobutylated or perfluorohexylated compounds are hoped for a potential use and application in medical and pharmaceutical fields as well as in industrial and materials science. With the aim of application particular in artificial pulmonary surfactant preparations, it is quite important to control the softness and hardness of pulmonary surfactant monolayers, especially DPPC. Herein it is demonstrated that the rigidity of DPPC monolayers is possible to be adjusted by the addition of the partially fluorinated amphiphiles. Moreover, the rigidity can be controlled by composition of monolayers as well as lateral pressure. These findings will necessitate a further understanding of interaction of fluorinated compounds with biomembranes and their constituents.

Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research 26350534 from the Japan Society for the Promotion of Science (JSPS).

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