Molecular Orientation and Multilayer Formation in the Adsorbed Film

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5078

J. Phys. Chem. C 2008, 112, 5078-5084

Molecular Orientation and Multilayer Formation in the Adsorbed Film of 1H,1H,10H,10H-Perfluorodecane-1,10-diol at the Hexane/Water Interface; Temperature Effect on the Adsorption of Fluoroalkane-diol Takanori Takiue,*,† Tsubasa Fukuda,† Daiki Murakami,† Hideaki Inomata,† Hiroyasu Sakamoto,‡ Hiroki Matsubara,† and Makoto Aratono† Department of Chemistry, Faculty of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan, and Department of Visual Communication Design, Faculty of Design, Kyushu UniVersity, Fukuoka 815-8540, Japan ReceiVed: September 17, 2007; In Final Form: January 5, 2008

The adsorption of 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol) at the hexane/water interface was investigated by the measurement of temperature dependence of interfacial tension and the thermodynamic data analysis in order to know the effect of two hydroxyl groups at both ends of the hydrophobic chain and the rigidity of the hydrophobic chain on the adsorption of fluorocarbon alcohol at the interface. The curves of interfacial tension versus temperature and concentration show break points corresponding to the phase transitions in the adsorbed FC10diol film. The interfacial pressure versus mean area per adsorbed molecule curve shows three kinds of states connected by two discontinuous changes. The area value after the first phase transition is very close to the calculated cross-sectional area of the FC10diol molecule along its major axis, and thus the FC10diol molecules form a condensed monolayer with molecular orientation parallel to the interface. Another noticeable point is that the value after the second phase transition point decreases furthermore to 0.12 nm2, which is much smaller than the cross-sectional area of the fluorocarbon chain, 0.28 nm2, with increasing interfacial pressure. This suggests that FC10diol molecules pile spontaneously and successively form a multilayer above the second phase transition. Furthermore, the partial molar entropy and energy change of adsorption in the expanded and condensed states were evaluated and compared to those of 1H,1H,2H,2H-perfluorodecanol (TFC10OH), which orients almost perpendicular to the interface. In addition to the contact of two hydroxyl groups with hexane in the bulk solution, the results are explained by the dependence of partial molar entropy and energy at the interface on the following factors resulting from the parallel orientation of FC10diol at the interface; (a) hydrogen bonding of two hydroxyl groups with water molecules, (b) hydrogen bonding between two hydroxyl groups facing each other, and (c) the fluorocarbon chain-water contact. The adsorbed FC10diol film is stabilized by factors a and b, which overwhelm the energetic disadvantage caused by factor c. Furthermore, the entropy change of adsorption ∆s in the multilayer is compared to the ∆scal calculated on the assumption that the condensed monolayer piles to form the multilayer. It was suggested that FC10diol molecules are not so densely packed in the multilayer compared to the first condensed monolayer and therefore the multilayer is not simply formed by the piling of condensed monolayers.

Introduction The adsorption of long-chain hydrocarbon (HC) alcohols at oil/water interfaces has been studied by many researchers from the macroscopic viewpoint through the interfacial tension measurement.1-8 Recently, the microscopic structure of the alcohol monolayer has become apparent by using optical and spectroscopic techniques such as Brewstar angle microscopy (BAM)9 and synchrotron X-ray reflection.10,11 Among others, in the study on the adsorption of fluorocarbon (FC) alcohols at the hexane/water interface,12-14 it was found that the adsorbed film of FC alcohol shows different types of phase transitions among gaseous, expanded, and condensed states. Especially, FC alcohols with carbon numbers larger than 8 form condensed film (two-dimensional solid) under atmospheric pressure in which alcohol molecules are closely packed with each other. * To whom correspondence should be addressed. Address: Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan. Phone: +81 92 642 2578. Fax: +81 92 642 2607. E-mail: [email protected]. † Department of Chemistry, Faculty of Sciences, Kyushu University. ‡ Department of Visual Communication Design, Faculty of Design, Kyushu University.

Characteristics of surface-active substances with the FC chain are (i) a large polarization effect induced by the strong electronegative nature of the fluorine atom, (ii) very weak mutual interaction with the HC chain, and (iii) the rigidity of the hydrophobic chain due to steric hindrance between fluorine atoms at n and n + 2 positions on a carbon chain.15 Therefore, the study on the adsorption of FC alcohol at the HC oil/water interface is of great interest from the viewpoint of basic science.16 In our previous studies on the adsorbed film of 1H,1H,2H,2H-perfluorodecanol (TFC10OH), which has one hydroxyl group at the end of the hydrophobic chain, at the hexane/ water interface, we found that TFC10OH molecules form gaseous, expanded, and condensed films and exhibit phase transitions among three states on the basis of the interfacial tension measurement and its thermodynamic data analysis.13,17 Taking account of the fact that the condensed film appears only for the adsorbed film of HC alcohols with carbon numbers larger than 18, the enhanced interaction between FC chains by effect ii makes the condensed film formation easier for TFC10OH compared to HC alcohol with the same chain length at the hexane/water interface.

10.1021/jp077456o CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008

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Figure 1. Interfacial tension vs time plots at given concentration and temperature: (a) m1 ) 0.115 mmol kg-1 and T ) 298.15 K, (b) m1 ) 0.080 mmol kg-1 and T ) 288.15 K, (c) m1 ) 0.205 mmol kg-1 and T ) 308.15 K, (d) m1 ) 0.205 mmol kg-1 and T ) 298.15 K.

The structure of the adsorbed TFC10OH film was examined further by means of X-ray reflectivity measurement.18-20 The phase transition between the expanded and condensed states was confirmed by a drastic change in the interfacial coverage defined by the fraction of interface occupied by condensed phase domain. It was claimed that TFC10OH molecules are densely packed with their molecular orientation almost perpendicular to the interface, judging from the electron density and film thickness values. Furthermore, we have studied the adsorption behavior of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analog 1H,1H,9H-perfluorononanol (HDFC9OH) in order to clarify the effect of the ω dipole generated by effect i on the state of the adsorbed film from the viewpoints of entropy, energy, and volume change of adsorption.21,22 It was concluded that FDFC9OH molecules orient almost normal to the interface, while HDFC9OH tend to tilt from the interface normal for ω dipoles to interact effectively with water molecules in the interfacial region and to reduce the repulsive interaction between neighbors arranging parallel in the adsorbed film. The tilt angle was estimated to be around 15° by evaluating the mean partial molar volume of alcohols at the interface. Bolaform surfactants have hydrophilic head groups at both ends of the hydrophobic chain. The adsorption of such surfactants at the air/water interface has been investigated by interfacial tension measurement.23-25 It was suggested that dodecane-1,12-bis(trimethylammonium bromide) shows “wicket-like” conformation in which the hydrophobic chain forms a loop and two head groups are anchored at the interface under saturated adsorption because of the flexible nature of the HC chain. In this study, we aim at elucidating the effect of the rigidity of the hydrophobic chain (factor iii) in addition to that of two terminal head groups on the adsorption behavior of the fluorinated compound at the hexane/water interface from the

viewpoint of the temperature dependence of interfacial tension. Therefore, we employed 1H,1H,10H,10H-perfluorodecane-1,10-diol (FC10diol), which has two hydroxyl groups at both ends of the hydrophobic chain because the FC chain is more stiff than HC chain and thus the molecular conformation in the adsorbed film is expected to be considerably different not only from those of FC alcohols such as TFC10OH but from those of bolaform surfactants. The interfacial tension between the hexane solution of FC10diol and water was measured as a function of temperature and concentration under atmospheric pressure. The partial molar entropy and energy of adsorption was evaluated and compared to those of TFC10OH. Experimental Section 1. Materials. 1H,1H,10H,10H-Perfluorodecane-1,10-diol (FC10diol) was purchased from Azmax Co., Ltd. and purified by recrystallization once from chloroform solution. Its purity was checked by observing no time dependence of equilibrium interfacial tension between the hexane solution of FC10diol and water and by liquid-gas chromatography. Hexane (Aldrich Chemical Co. Inc.) was distilled once and water three times; the second and third stages were done from alkaline permanganate solution. 2. Interfacial Tension Measurement. The equilibrium interfacial tension, γ, of the hexane solution of FC10diol against water was measured as a function of temperature T and the molality of alcohol m1 under atmospheric pressure by the pendant drop method based on the shape analysis of pendant drops described elsewhere.26 The γ values were monitored with time at given T and m1 until γ reaches a constant value, which is regarded as an equilibrium value. The time necessary for equilibrium was 30-90 min. For the calculation of interfacial tension, the densities of pure hexane and water27,28 were used instead of the hexane solution and water in equilibrium because

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Figure 3. Interfacial tension vs molality curves at constant temperature: T ) (1) 288.15, (2) 293.15, (3) 298.15, (4) 303.15, (5) 308.15 K.

Figure 2. Interfacial tension vs temperature curves at constant molality: m1 ) (1) 0, (2) 0.002, (3) 0.004, (4) 0.005, (5) 0.010, (6) 0.020, (7) 0.035, (8) 0.0499, (9) 0.0599, (10) 0.0685, (11) 0.078, (12) 0.0845, (13) 0.090, (14) 0.0924, (15) 0.097, (16) 0.111, (17) 0.125, (18) 0.135, (19) 0.140, (20) 0.145, (21) 0.155, (22) 0.168, (23) 0.176, (24) 0.182, (25) 0.190, (26) 0.195, (27) 0.200, (28) 0.205 mmol kg-1.

the concentration was sufficiently low and the mutual solubility between two phases was negligibly small. The experimental error of the γ value was estimated to be within (0.05mN m-1. Results and Discussion Figure 1a shows the time, t, dependence of interfacial tension γ measured at m1 ) 0.115 mmol kg-1 and 298.15 K. The γ value decreases with time, and a plateau region appears on the γ versus t plots at around ∼35.9 mN m-1. Vollhardt et al. have studied the adsorption of 1-dodecanol at the air/aqueous solution interface at a given temperature and concentration by measuring the time dependence of surface pressure.29 The surface pressure versus time curves have a plateau region that follows a conspicuous break point before adsorption equilibrium. According to the phase rule for the system with a planar interface,30 the degree of freedom is two when two kinds of interfacial phases coexist at the interface and hence the γ value is only a function of temperature T and pressure p. Therefore, the plateau on the γ versus t curve is caused by the coexistence of two interfacial phases during phase transition in the adsorbed film. Similar plots were obtained at other temperatures; the γ values in the plateau regions are ∼36.0 mN m-1 at m1 ) 0.08 mmol kg-1 and 288.15 K (Figure 1b) and ∼35.3 mN m-1 at m1 ) 0.205 mmol kg-1 and 308.15 K (Figure 1c). In Figure 1d are shown γ versus t plots observed at m1 ) 0.205 mmol kg-1. The γ value decreases with time, and an oscillation region appears at around ∼32.9 mN m-1, which is due to a type of phase transition different from that observed in Figure 1a. Furthermore, another plateau was observed in the course of adsorption equilibrium. The equilibrium interfacial tensions γ are plotted against temperature T under atmospheric pressure in Figure 2. The γ value at the pure hexane/water interface decreases slightly, and that at the solution/water interface increases with increasing

temperature. Taking note of the fact that plateau regions appear on the γ versus t plots, it is hypothesized that the γ versus T curves in a high concentration region have one or two break points because of phase transitions in the adsorbed FC10diol film. This will be examined thermodynamically in the latter part of this paper. The first phase transition is shown by a short dashed line and the second transition by a long dashed line. The γ values of the first and second phase transitions correspond, respectively, to the plateau and oscillation region shown in Figure 1a and d. Furthermore, the slope of the curve below the second phase transition at high m1 tends to become steeper with decreasing temperature. In Figure 3 are shown the γ versus m1 curves at given temperatures. The γ value decreases gradually with increasing m1. Because the extrapolation of the curves between two broken lines to zero concentration does not give the value of the pure hexane/water interfacial tension, one or two break points are expected to exist on the γ versus m1 curves depending on temperature. The γ values at the break points decrease and the corresponding m1 values increase with increasing temperature as shown by the dashed lines. Here let us mention briefly thermodynamic equations for analyzing experimental results. By introducing two dividing planes, which make the interfacial excess numbers of moles of hexane and water zero concurrently, the total differential of interfacial tension γ for the present system is expressed as31

dγ ) -∆s dT + ∆V dp - Γ1H(RT/m1)dm1

(1)

when the hexane solution is assumed to be ideally dilute. Here ∆s and ∆V are, respectively, the entropy and volume changes associated with the adsorption of alcohol from the hexane solution defined by

∆y ) yH - Γ1Hy1O,

y ) s,V

(2)

where yH, Γ1H, and y1O are the interfacial excess thermodynamic quantity, the interfacial density of FC10diol per unit area, and the partial molar thermodynamic quantity of alcohol in the bulk solution, respectively. To characterize the state of the adsorbed film, first, the interfacial density of alcohol, Γ1H, was calculated by applying the equation

Γ1H ) -(m1/RT)(∂γ/∂m1)T,p

(3)

to the γ versus m1 curves in Figure 3. The results are shown as Γ1H versus m1 curves in Figure 4 at constant temperature. It is seen that the Γ1H value increases with increasing m1 and

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Figure 4. Interfacial density vs molality curves at constant temperature: T ) (1) 288.15, (2) 293.15, (3) 298.15, (4) 303.15, (5) 308.15 K.

Figure 6. Entropy change associated with adsorption vs molality curves at constant temperature: T ) (1) 288.15, (2) 293.15, (3) 298.15, (4) 303.15, (5) 308.15 K; (---) ∆scal.

Figure 5. Interfacial pressure vs mean area per adsorbed molecule curves at constant temperature: T ) (1) 288.15, (2) 298.15, (3) 308.15 K; (4) the curve of TFC10OH at 298.15 K.

In the FC10diol system, it is noted that the π versus A curve just after the first phase transition is almost vertical as seen in the condensed state of the TFC10OH system; the compressibility of the film is extremely low just like the two-dimensional solid state. However, the A value of FC10diol is around 0.82 nm2, which is very much larger than 0.3 nm2 of the condensed TFC10OH film and is very close to the calculated cross-sectional area of the FC10diol molecule along its major axis. Thus, FC10diol molecules are expected to form a condensed monolayer in which molecules are fully extended and closely packed with their molecular orientation parallel to the interface. This conformation is different from the “wicket-like” one of the bolaform surfactant with the HC chain, primarily because of the two terminal hydroxyl groups and the rigidity of the FC chain. Another noticeable point is that the A value after the second phase transition point decreases furthermore to 0.12 nm2, which is much smaller than the cross-sectional area of the fluorocarbon chain, around 0.28 nm2, with increasing π. This further decrease in A verifies a type of condensed layer different from that of FC alcohols. Because the contact of hydroxyl groups with hexane molecules is energetically unfavorable when FC10diol molecules stand upright in the adsorbed film, it is likely that FC10diol molecules spontaneously form a multilayer in which molecules pile with their orientations’ parallel to the interface. An X-ray reflectivity measurement is undertaken to characterize the structure of the multilayer as well as the condensed monolayer in terms of interfacial thickness and coverage of them. Next, let us consider the adsorption behavior of FC10diol from the viewpoint of entropy. The entropy change associated with adsorption ∆s is evaluated by applying the equation

becomes almost constant around 2 µmol m-2 above the first phase transition point. Furthermore, the Γ1H value changes discontinuously at the second transition and takes around 6 µmol m-2 there. The sudden increase in Γ1H value after the second phase transition is an unusual observation for a condensed monolayer. The state of and molecular orientation in the adsorbed film are inferred by drawing the interfacial pressure, π, versus the mean area per adsorbed molecule, A, curve. The π and A values are calculated, respectively, by

π ) γ0 - γ

(4)

A ) 1/NAΓ1H

(5)

and

where γ0 is the interfacial tension at the pure hexane/water interface and NA is Avogadro’s number. The π versus A curves at three temperatures are shown in Figure 5 together with the curve of TFC10OH at 298.15 K. These curves consist of three parts connected by two discontinuous changes. In the case of the TFC10OH system, it was concluded that the adsorbed film exhibits two kinds of phase transitions among the gaseous, expanded, and condensed states.13,17 Because the A value in the condensed state is very close to the cross sectional area of fluorocarbon chain (A ≈ 0.28 nm2), TFC10OH molecules are assumed to orient almost normal to the interface. This was confirmed by the fact that the film thickness obtained by the X-ray reflectivity measurement coincided with the calculated fluorocarbon chain length with all-trans conformation.18-20

∆s ) -(∂γ/∂T)p,m1

(6)

to γ versus T curves in Figure 2. The ∆s values obtained are plotted against m1 in Figure 6. The ∆s value decreases from positive to negative with increasing adsorption and changes discontinuously at the phase transition points. Particularly, ∆s in the multilayer state is largely negative and decreases steeply with increasing m1 in a narrow concentration range. According to our thermodynamics of interfaces,31 the ∆s is expressed in the form

∆s ) ΓoI(sjoI - soO) + ΓwI(sjwI - swW) + Γ1H(sj1H - s1O) (7) where ΓiI (i ) o, w) is the number of moles of solvent components inherent in the interface, jsiI (i ) o, w) and js1H are, respectively, the mean partial molar entropy of solvents and that of alcohol inherent in the interface, and sjR (R ) O, W; j

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Figure 7. Partial molar entropy change of adsorption vs interfacial density curves at 298.15 K: (1) FC10diol; (2) TFC10OH.

) o, w, 1) is the partial molar entropy of component j in the bulk phases. Thus, the partial molar entropy change of alcohol associated with the adsorption js1H - s1O provides further insight into the adsorption behavior of FC10diol at the hexane/water interface. The js1H - s1O value is estimated by the following equation31

js1H - s1O ) [∆s + (1 - Γ1Ha1)(∂γ/∂T)p,Γ1H]/Γ1H

(8)

where a1 is the partial molar area of the alcohol molecule. The second term in the square bracket represents the contribution of solvents to ∆s.30 Here it should be noted that there are some possibilities of a1 value because a1 cannot be determined experimentally. One of the possibilities is a1 ) 0.82 nm2, which is the cross-sectional area of the FC10diol molecule along its major axis, and another is the value obtained by using a1 ) 1/Γ1H. Because the expanded state is regarded as a mixture of FC10diol and solvent molecules, we used 0.82 nm2 as the a1 value, since employing a1 ) 1/Γ1H means that the contribution of water and hexane molecules to the js1H - s1O value is not taken into account. Concerning the condensed monolayer, we employed a1 ) 1/Γ1H because of negligible contribution of solvents to ∆s. In Figure 7 are compared js1H - s1O versus Γ1H curves in the expanded and condensed states of FC10diol with the corresponding curves of TFC10OH at 298.15 K. The js1H - s1O value is negative and changes discontinuously at the phase transition points. The negative js1H - s1O value means smaller entropy in the adsorbed film than in the bulk solution because of the restricted orientation of molecules at the interface. It is noted that the value of FC10diol in the expanded film is smaller than that of TFC10OH, whereas those in the condensed state are comparable with each other. This could be explained by considering the effect of molecular structure and orientation on entropy in the adsorbed film and bulk solution. Because the FC10diol molecule has two hydroxyl groups at terminal positions of the hydrophobic chain, the entropy of mixing of FC10diol with hexane is larger than that of TFC10OH with hexane in the bulk solution. Therefore, s1O is assumed to be larger for FC10diol than for TFC10OH. In the adsorbed film, taking account of the parallel orientation of FC10diol molecules, the js1H value is dependent mainly on (a) hydrogen bonding between two hydroxyl groups and water molecules, (b) hydrogen bonding between hydroxyl groups facing each other, and (c) fluorocarbon chain-water contact in the condensed monolayer of flat layering of FC10diol as described above. Because hydrogen bonding restricts an orientation of FC10diol molecules in the adsorbed film, factors a and b reduce the js1H value. Factor c is expected

to raise partial molar entropy change js1H - s1O because of the positive entropy value of ∆s of interface formation between fluorocarbon and water.32 In the expanded state, in addition to the interaction of two hydroxyl groups with water molecules, hydrogen bonding is expected to occur between hydroxyl groups facing each other. Thus, the js1H value is smaller for FC10diol than for TFC10OH even under the factor (c); smaller js1H - s1O value of FC10diol is attributable to smaller js1H and larger s1O compared to those of TFC10OH. The discontinuous change in js1H - s1O associated with the phase transition from the expanded to the condensed state gives a partial molar entropy change between expanded and condensed states, js1H,c - js1H,e. Because the value is quite small for the FC10diol system compared to the TFC10OH system, the js1H value in the condensed FC10diol film is similar to that in the expanded film. This may come from the situations that the discontinuous change in Γ1H is comparatively small and that the fluorocarbon-water contact still remains in the condensed monolayer. In contrast to this, we remember that TFC10OH molecules in the condensed state orient almost normal to the hexane/water interface with hexagonal array; one fluorocarbon chain is surrounded by another six fluorocarbons. This leads to a large reduction of js1H accompanied by expanded-condensed phase transition. Although s1O is larger for FC10diol than for TFC10OH, the reduction of js1H accompanied by phase transition is greater for TFC10OH than for FC10diol, and eventually the js1H - s1O value of the FC10diol is comparable to that of TFC10OH. Here, let us briefly comment that the increase in js1H - s1O of TFC10OH is not due to an increase in js1H but an increase in s1O. In the case of the TFC10OH system, the increase in s1O by the entropy of mixing is estimated to be R ln(m1II/m1I) ) 0.011 kJ K-1 mol-1 from the phase transition point (m1I ) 4 mmol kg-1) to the saturated adsorption (m1II ) 15 mmol kg-1). This is almost comparable to the increase in js1H - s1O. Alternatively, in the case of the FC10diol system, the increase in s1O is 0.001 kJ K-1 mol-1 at most, and thus a decrease in js1H is dominant to a decrease in js1H - s1O. In the the case of the multilayer state, because FC10diol molecules may not be necessarily densely packed in the multilayer as in the condensed monolayer, the contribution of solvent molecules to ∆s should be estimated in order to evaluate the js1H - s1O value. Although a1 value should be less than 0.82 nm2 for a multilayer, a1 ) 1/Γ1H cannot be employed. So, at this moment, it is hard to evaluate the js1H - s1O value correctly because of the lack of an appropriate value of a1 in the multilayer state. Instead of this, in Figure 6, the ∆s value in the multilayer is compared to the calculated ∆scal value on the basis of the assumption that condensed monolayer piles and forms a multilayer by

∆scal ) Γ1H(sj1H - s1O)first

(9)

where the (sj1H - s1O)first(≈ -0.151 kJ K-1 mol-1) value of the condensed monolayer is used for the calculation. The ∆s value is smaller just after the phase transition but becomes larger than ∆scal at higher concentrations. This means that the entropy in the multilayer is different from that in the condensed monolayer. The notable difference between the first and the upper layers is that the fluorocarbon chain is in contact with water molecules in the former, while the latter is surrounded by the fluorocarbon chain or hexane molecules. Thus, (sj1H - s1O)first > (sj1H s1O)upper is expected if other situations are similar to each other. This is consistent with ∆scal > ∆s at the phase transition point.

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Figure 8. Energy change associated with adsorption vs molality curves at constant temperature: T ) (1) 288.15, (2) 293.15, (3) 298.15, (4) 303.15, (5) 308.15 K.

Figure 9. Partial molar energy change of adsorption vs interfacial density curves at 298.15 K: (1) FC10diol; (2) TFC10OH.

The inverse ∆scal < ∆s at higher concentrations may suggest that FC10diol molecules are not so densely packed in the multilayer compared to the first condensed monolayer and therefore the multilayer is not formed simply by the piling of condensed monolayers. The appearance of plateau region after the condensed monolayer-multilayer transition on the γ versus t plot in Figure 1d may attribute to a change in molecular arrangement in multilayer state. Next, let us discuss the adsorption behavior of FC10diol from the viewpoint of energy. The energy change associated with adsorption ∆u is evaluated by

∆ u ) γ + T∆ s - p∆V ≈ γ + T∆ s

(10) (11)

The result is demonstrated as ∆u versus m1 curves in Figure 8. Here, the p∆V term in eq 10 is approximated to be negligible compared to the other two terms under atmospheric pressure. The ∆u value decreases from positive to negative with increasing concentration and changes discontinuously at phase transition points. In case of interface-parallel orientation of FC10diol, the adsorption replaces the water-HC (hexane) contact by the water-FC contact. Taking account of that, the interfacial tensions at alkane/water and perfluoroalkane/water interfaces are, respectively, around 50 and 55 mN m-1 (mJ m-2), the energy loss of 5 mJ m-2 by the replacement is relatively small compared to the energy gain (-30 to -45 mJ m-2) by a decrease in ∆s due to hydrogen bonding between the hydroxyl group and water and between hydroxyl groups facing each other. Furthermore, the partial molar energy change of adsorption, uj1H - u1O, was calculated by using

uj1H - u1O ) γa1 + T(sj1H - s1O) - p(Vj1H - V1O) (12) ≈ γa1 + T(sj1H - s1O)

(13)

The uj1H - u1O versus Γ1H curve of FC10diol is shown together with the curve of TFC10OH at 298.15 K in Figure 9. The negative value of uj1H - u1O substantiates that energetic stabilization by hydrogen bonding exceeds the disadvantage caused by the contact of FC chain with water in the adsorbed film. The difference in uj1H - u1O value in the expanded state is not so large compared to that of js1H - s1O shown in Figure 7. In eq 13, the second term represents a contribution arising from the entropy change of adsorption. The values are -42.6 for FC10diol and -31.6 kJ mol-1 for TFC10OH at Γ1H ) 1.7 µmol m-2

Figure 10. Values of coefficient of dT of eqs (O) 14 and (-) 15 vs temperature curves: (1) expanded-condensed transition; (2) condensedmultilayer transition.

and 298.15 K. Alternatively, the values of the first term are 17.1 for FC10diol and 8.3 kJ mol-1 for TFC10OH. Because the a1 is larger for FC10diol than for TFC10OH, it is suggested that the energy increase due to the contact of the FC chain with water at the interface dissipates a decrease in uj1H by hydrogen bonding formation in the adsorbed film. In the condensed state, the uj1H - u1O value is larger for the FC10diol system than for the TFC10OH system. This results from a larger decrease in uj1H of TFC10OH than FC10diol by enhanced dispersion interaction accompanied by a large increase in Γ1H due to phase transition from the expanded to the condensed state. It is noteworthy that the ∆u value is largely negative in the multilayer state compared to that of the condensed TFC10OH film (ca. -210 mJ m-2). The gap of ∆u values between these two alcohols corresponds to be around 20 kJ mol-1 and is mainly due to hydrogen-bond formation between hydroxyl groups facing each other. Finally, we examine the expanded-condensed phase transition thermodynamically. When two kinds of states R and β coexist at the interface, the equilibrium interfacial tension γ eq is given by

dγ eq ) -[(∆ sR/Γ1H,R - ∆ sβ/Γ1H,β)/(1/Γ1H,R - 1/Γ1H,β)]dT + [(∆VR/Γ1H,R - ∆Vβ/Γ1H,β)/(1/Γ1H,R - 1/Γ1H,β)]dp (14) Because γ eq is a function of temperature and pressure, we have

dγ eq ) (∂γ eq/∂T)p dT + (∂γ eq/∂p)T dp

(15)

5084 J. Phys. Chem. C, Vol. 112, No. 13, 2008 The coefficients of dT in eqs 14 and 15 are evaluated separately by using the Γ1H and ∆s values at the phase transition point shown in Figures 4 and 6 and by the slope of the γ eq versus T curve (short dashed line) in Figure 2, respectively. The results are shown in Figure 10. It is realized that because of the agreement of both coefficients for both phase transitions two kinds of phase transitions take place in the adsorbed FC10diol film at least from the viewpoint of thermodynamic quantities. Conclusions The adsorption behavior of 1H,1H,10H,10H-perfluorodecane1,10-diol (FC10diol) at the hexane/water interface was investigated. The curves of interfacial tension versus temperature and concentration showed break points corresponding to the phase transitions in the adsorbed FC10diol film. The interfacial pressure versus mean area per molecule curve showed three kinds of states connected by two discontinuous changes. The area just before the first phase transition suggested that the molecules are fully extended and closely packed with molecular orientation parallel to the interface in the condensed monolayer. This conformation is due to two terminal hydroxyl groups and the rigidity of FC chain and different from the “wicket-like” conformation of the bolaform surfactant with the HC chain. A sudden rise in interfacial density to the extremely larger values after the second-phase transition point suggested that the FC10diol molecules spontaneously pile and form a multilayer at the interface. The adsorbed film of FC10diol is stabilized by hydrogen bonding between the hydroxyl group and water and between hydroxyl groups facing each other, which overcome energetic disadvantage caused by the contact of the FC chain with the water at the interface. Furthermore, the discrepancy between the entropy of adsorption ∆s in the multilayer and the calculated one ∆scal, based on the assumption that condensed monolayers pile to form a multilayer, suggested that FC10diol molecules are not so densely packed in the multilayer compared to the first condensed monolayer. Acknowledgment. This work was supported in part by the Grant-in Aid for Scientific Research (C) of Japan Society for the Promotion of Science (No. 19550021) and The Mitsubishi Foundation. References and Notes (1) Hutchinson, E. J. Colloid Sci. 1948, 3, 219. (2) Jasper, J. J.; Houseman, B. L. J. Phys. Chem. 1965, 69, 310.

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