J. Phys. Chem. B 2005, 109, 16429-16434
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Effect of ω-Hydrogenation on the Adsorption of Fluorononanols at the Hexane/Water Interface: Pressure Effect on the Adsorption of Fluorononanols Takanori Takiue,*,† Daisuke Hirose,† Daiki Murakami,† 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: April 22, 2005; In Final Form: June 27, 2005
The interfacial tension γ of the hexane solution of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analogue, 1H,1H,9H-perfluorononanol (HDFC9OH), against water was measured as a function of pressure and concentration at 298.15 K in order to clarify the effect of ω-dipole on the orientation of fluorononanol molecules from the viewpoint of volume. The adsorbed films of both alcohols exhibit two kinds of phase transitions among three different states: the gaseous, expanded, and condensed states. The partial molar volume changes of adsorption VjH1 - VO1 in the expanded and condensed states were evaluated and compared between the two systems. The VjH1 - VO1 values of both alcohols are negative, and thus the alcohol molecules have smaller volume in the adsorbed film than in the bulk solution. Furthermore, the VjH1 value was obtained through the evaluation of VO1 by the density measurement of the bulk hexane solution. It was found that the VjH1 value of HDFC9OH is smaller than that of FDFC9OH in the condensed state. On the basis of three matters concerning the molecular structure of alcohols, the occupied area at the interface, and the orientation of FDFC9OH in the adsorbed film deduced from the earlier results of X-ray reflectivity measurement, the mean tilt angle of HDFC9OH from the interface normal in the condensed film was estimated to be 15°. The thermodynamic estimation demonstrated here is highly valuable one to provide structure information on an adsorbed film.
Introduction The pressure dependence of interfacial tension provides us fruitful information on the structure and property of and the molecular interaction in the adsorbed films from volumetric viewpoint.1-3 We have clarified the adsorption behavior of various surface-active substances at the hexane/water interface by measuring the interfacial tension under atmospheric to high pressures and evaluating the volume change associated with adsorption from the solution.4-6 In our previous studies on the adsorbed films of 1H,1H,2H,2H-perfluorodecanol (TFC10OH) and its homologous perfluorododecanol (TFC12OH), the effect of the fluorocarbon (FC) chain length on the state and phase transition of the adsorbed film was discussed thoroughly in terms of the partial molar volume change of adsorption.7,8 We found (1) the large pressure dependency of partial molar volume change in the condensed state and (2) the larger partial molar volume change associated with phase transition from the expanded to the condensed state of TFC12OH than that of TFC10OH. These results were explained plausibly from the standpoints such as the difference in the microscopic circumstance between in the condensed film and in the bulk solution, the strengthened dispersion interaction between the FC chains compared to that between hydrocarbon chains, and the more weakened interaction of FC chain with hexane molecules with increasing FC chain length. †
Department of Chemistry. Department of Visual Communication Design. * To whom correspondence should be addressed: Ph +81 92 642 2578, Fax +81 92 642 2607; e-mail
[email protected]. ‡
In the latest paper on the adsorption of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analogue, 1H,1H, 9H-perfluorononanol (HDFC9OH), at the hexane/water interface, we have discussed the effect of ω-dipole of HDFC9OH molecules on the adsorbed film from the viewpoints of entropy and energy obtained through the measurement of temperature dependence of interfacial tension.9 It was suggested that (1) HDFC9OH tends to tilt from 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 and (2) HDFC9OH molecules are energetically unstable in the hexane solution compared to FDFC9OH ones because of the hydrophilic nature of the ω-dipole. In the present study, we aim at confirming the ideas on the orientation of HDFC9OH molecules at the interface claimed in the previous study and also examining the effect of ω-dipole on the orientation of fluorononanols from the viewpoint of volume. Thus, the interfacial tensions of hexane solutions of HDFC9OH and FDFC9OH against water were measured as a function of pressure and concentration at 298.15 K. The partial molar volume change of adsorption is evaluated and compared between the two alcohols. Furthermore, combining the partial molar volume change of adsorption with the partial molar volume of alcohol in the hexane solution obtained from the density measurement, the partial molar volume at the interface can be calculated and then utilized to estimate the tilt angle of HDFC9OH in the condensed film on the basis of the chemical structure of both alcohols, the occupied area in the adsorbed film, and the structure of the adsorbed FDFC9OH film deduced
10.1021/jp0581375 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/05/2005
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from our previous results of synchrotron X-ray reflection (XR) measurement.10-12 Experimental Section The procedure of purification of HDFC9OH, FDFC9OH, hexane, and water was described in our previous paper.9 The interfacial tensions γ of the hexane solutions of HDFC9OH and FDFC9OH against water were measured as a function of pressure p and molality m1 at 298.15 K by the pendant drop method based on the drop shape analysis described elsewhere.13,14 The densities of pure water and hexane15,16 were used for the calculation of interfacial tension because of sufficiently low concentration of the hexane solution and negligibly small mutual solubility of hexane and water even under high pressure. The error in γ value was estimated within 0.05 mN m-1. The densities F of the hexane solutions of HDFC9OH and FDFC9OH were measured as a function of m1 by using a digital density meter, Anton Paar DMA 60/602, to evaluate the partial molar volume of alcohols in the hexane solution. Results and Discussion The values of interfacial tension γ measured for the HDFC9OH and FDFC9OH systems are plotted against pressure p at 298.15 K in parts a and b of Figure 1, respectively. The γ value increases slightly at low concentrations and decreases steeply at high concentrations with increasing pressure. At intermediate concentrations, the γ vs p curves have one or two break points at which the slope of the curve changes abruptly. The γ values read from Figure 1 at 40 MPa were plotted against molality m1 in Figure 2a. In Figure 2b are magnified the γ vs m1 curves in a low concentration region. It is noted that the extrapolation of the curve from high concentration to zero (broken lines in Figure 2b) does not give the value of the pure hexane/water interfacial tension. Therefore, it was hypothesized that the γ vs p and m1 curves shows two breaks which are due to the phase transitions in the adsorbed film of fluorononanols at the hexane/water interface. This will be examined thermodynamically in the latter part of this paper. The pressure dependence of interfacial tension γeq at the transition points and the corresponding molality meq 1 are shown in Figure 3. The γeq value increases and the meq 1 decreases with increasing pressure. Furthermore, it is noted that the γeq vs p curve of FDFC9OH is shifted to the higher interfacial tension and the corresponding meq 1 vs p to the lower molality compared to those of HDFC9OH. To make clear the states of the adsorbed film, it is useful to draw the interfacial pressure π vs mean area per adsorbed molecule A curves. For doing this, first, the interfacial density ΓH1 is evaluated by applying the equation
ΓH1 ) -(m1/RT)(∂γ/∂m1)T,p
(1)
to the γ vs m1 curves in Figure 2. The results are shown as the ΓH1 vs m1 curve at 40 MPa in Figure 4. It is seen that the ΓH1 value increases with increasing m1 and changes discontinuously at concentrations of the breaks on the γ vs m1 curves. Then the π and A values are calculated by using
π ) γ0 - γ
(2)
A ) 1/NAΓH1
(3)
and
where γ0 is the interfacial tension of the pure hexane/water
Figure 1. (a) Interfacial tension vs pressure curves of HDFC9OH at constant molality: (1) m1 ) 0, (2) 0.101, (3) 0.248, (4) 0.401, (5) 0.601, (6) 0.748, (7) 0.920, (8) 1.057, (9) 1.233, (10) 1.501, (11) 1.748, (12) 1.975, (13) 2.475, (14) 2.991, (15) 3.470, (16) 3.984, (17) 4.478, (18) 4.991, (19) 5.991, (20) 6.968, (21) 8.021, (22) 9.005, (23) 9.950, (24) 11.47, (25) 13.01, and (26) 15.00 mmol kg-1. (b) Interfacial tension vs pressure curves of FDFC9OH at constant molality: (1) m1 ) 0, (2) 0.0998, (3) 0.250, (4) 0.399, (5) 0.498, (6) 0.700, (7) 0.846, (8) 0.996, (9) 1.191, (10) 1.395, (11) 1.738, (12) 1.996, (13) 2.393, (14) 2.984, (15) 4.009, (16) 4.961, (17) 5.981, (18) 7.001, (19) 7.994, (20) 9.810, and (21) 12.01 mmol kg-1.
interface and NA Avogadro’s number. Figure 5 shows the π vs A curves at 40 MPa of both alcohols together with the curve obtained for the 1H,1H,2H,2H-perfluorodecanol (TFC10OH) system.7 The π vs A curves consist of three parts connected by two discontinuous changes, which is due to the two types of phase transitions in the adsorbed film. The A value in almost vertical region of the curve is about 0.3 nm2. Since this value is very close to the cross-sectional area of fluorocarbon chain, it is concluded that this region corresponds to the condensed state. By comparing the π vs A curves of fluorononanols to that of TFC10OH which shows two types of phase transitions from the gaseous to the expanded and from the expanded to the condensed state, the regions with intermediate and large A values are assigned to the expanded and gaseous films, respectively. Furthermore, it is noted that the interfacial pressure at the
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Figure 2. Interfacial tension vs molality curves at 40 MPa: (1) HDFC9OH, (2) FDFC9OH.
transition point πeq is larger and the corresponding area Aeq smaller for HDFC9OH than for the FDFC9OH system. This difference relies on two factors induced by the substitution of a fluorine atom at the ω-position of FDFC9OH into a hydrogen atom; the repulsive interaction between parallel oriented ω-dipoles in the adsorbed HDFC9OH film and the weaker dispersion interaction between HDFC9OH molecules compared to that between FDFC9OH ones because of lower molecular weight of HDFC9OH than FDFC9OH. Next, let us consider the adsorption behavior of both alcohols from a viewpoint of volume. The volume change associated with adsorption ∆V was evaluated by applying the equation
∆V ) (∂γ/∂p)T,m1
(4)
to the γ vs p curves in Figure 1. The results are shown as the ∆V vs m1 curves in Figure 6. In both systems, the ∆V value decreases with increasing adsorption in the gaseous and expanded states and changes discontinuously at the phase transition points. It should be noted that the ∆V values in the condensed state are largely negative and strongly depend on pressure. A similar feature has been observed in the condensed film of other fluoroalkanols such as TFC10OH at the hexane/ water interface.7 The negative contribution of adsorption of fluorononanols to the ∆V value is examined in terms of the partial molar volume change of adsorption VjH1 - VO1 . This value is estimated by using the following equation17
VjH1 - VO1 ) [∆V - (1 - ΓH1 a1)(∂γ/∂p)T,ΓH1 ]/ΓH1
Figure 3. (a) Equilibrium interfacial tension vs pressure curves: (1) gaseous-expanded transition for HDFC9OH, (2) gaseous-expanded transition for FDFC9OH, (3) expanded-condensed transition for HDFC9OH, (4) expanded-condensed transition for FDFC9OH. (b) Equilibrium molality vs pressure curves: (1) gaseous-expanded transition for HDFC9OH, (2) gaseous-expanded transition for FDFC9OH, (3) expanded-condensed transition for HDFC9OH, (4) expandedcondensed transition for FDFC9OH.
(5)
Figure 4. Interfacial density vs molality curves at 40 MPa: (1) HDFC9OH, (2) FDFC9OH.
where VjH1 is the mean partial molar volume of alcohol inherent in the interface, VO1 the partial molar volume in the hexane solution, and a1 the partial molar area of fluoroalkanol, respectively. The a1 value is assumed to be 0.3 nm2 for the present systems.7,8 The results evaluated at 0.1 and 40 MPa for the expanded and condensed states are shown as the VjH1 - VO1 vs ΓH1 curves in
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Figure 5. Interfacial pressure vs mean area per adsorbed molecule curves at 40 MPa: (1) HDFC9OH, (2) FDFC9OH, (3) TFC10OH.
Figure 7. Partial molar volume change of adsorption vs interfacial density curves: (a) HDFC9OH, (b) FDFC9OH; (1) p ) 0.1 and (2) 40 MPa.
TABLE 1: Partial Molar Volume of Alcohols in the Condensed State
HDFC9OH FDFC9OH
Figure 6. Volume change associated with adsorption vs molality curves at constant pressure: (a) HDFC9OH, (b) FDFC9OH; (1) p ) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, and (6) 100 MPa.
Figure 7. The VjH1 - VO1 values except for those at high pressures of the FDFC9OH system are negative, and therefore the alcohol molecule has a smaller volume in the adsorbed film than in the hexane solution. It is noted that the VjH1 - VO1 value in the condensed state is larger at 40 MPa than at 0.1 MPa. Taking into account that the condensed film is regarded as a two-dimensional solid which consists of only fluoroalkanol and the hexane solution is a mixture of fluoroalkanol and hexane, the above result is due to the larger decrease in VO1 with rising pressure compared to the corresponding decreases in VjH1 . This would explain why the ∆V value is less negative at high pressure than at low pressure. Furthermore, it is realized that the
m1/ mmol kg-1
ΓH1 / µmol m-2
VO1 / cm3 mol-1
VjH1 / cm3 mol-1
5.60 2.80
5.30 5.30
264 277
230 240
discontinuous change in the VjH1 - VO1 value at the phase transition points, which gives the partial molar volume change accompanied by phase transition VjH,c jH,e 1 - V 1 is also negative. This confirms that the expanded state is a kind of twodimensional mixture of fluorononanol and hexane having a larger partial molar volume, while the condensed state is a twodimensional solid of fluorononanol with a relatively smaller one. Now, let us try to bring out more definitely the effect of ω-dipole on the adsorbed film of fluorononanols from the partial molar volume at the interface VjH1 . For doing this, first, the densities of the hexane solutions of both alcohols were measured as a function of m1 at 298.15 K under atmospheric pressure in order to evaluate the partial molar volume in the hexane solution VO1 . Then the VjH1 values are calculated by subtracting VO1 from VjH1 - VO1 . The results obtained at ΓH1 ) 5.30 µmol m-2 are listed in Table 1. The VjH1 value of HDFC9OH is smaller than that of FDFC9OH in the condensed state. Taking account of the idea claimed from the energetic viewpoint that HDFC9OH tends to tilt from interface normal for the ω-dipoles to interact effectively with water molecules in the interfacial region and to reduce the repulsive interaction between neighbors,18 the difference in VjH1 values between the HDFC9OH and FDFC9OH systems seems to be due to the difference in molecular
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orientation such as a tilt angle. Therefore, we will try to estimate roughly the tilt angle of fluorononanol molecules in the condensed film. For this calculation, we rely on the following three plausible matters about the structure of the condensed film and the molecular structure of HDFC9OH and FDFC9OH. The first expectation is related to the orientation of FDFC9OH molecules in the condensed film. In the recent study on the structure of adsorbed film of TFC10OH at the hexane/water interface by means of synchrotron X-ray reflection (XR),10-12 we obtained the structural parameter such as layer thickness, electron density, interfacial coverage, and so on, by fitting the reflectivity plots in terms of a layer model. The normalized electron density obtained is about 1.85, which is almost equal to the value of solid fluoroalkane, and the layer thickness 1 ( 0.1 nm is also very close to the calculated fluorocarbon chain length. These results suggest that TFC10OH molecules in the condensed film stand nearly perpendicular to the interface. Deducing from this, the FDFC9OH molecule, which is also a ω-fluorinated alcohol like the TFC10OH molecule, is expected to stand upright at the interface. The second is concerned with the occupied area in the condensed film. Judging from the π vs A curves in Figure 5, the A value in the condensed state is almost equal for both alcohols, and thus the occupied area of HDFC9OH AH is regarded to be equal to that of FDFC9OH, AF; AH ) AF ) A. Third, we assume that the molecular length of HDFC9OH LF is same as that of FDFC9OH LH; LH ) LF ) L because the only difference in the chemical structure between them is one atom at the ω-position, and thus the difference between C-H and C-F bond is about 0.03 nm,19 which is less than 3% of LF and LH. Now, we can roughly estimate the tilt angle of HDFC9OH molecule in the condensed film. The partial molar volume in the condensed film VjH1 is given approximately by the following equations
interfacial tension for the two states in equilibrium is given by1,7
VjH1 (FDFC9OH) ≈ NAAFL ) NAAL
eq eq dγeq ) -∆sR dT + ∆VR dp - ΓH,R 1 (RT/m1 )dm1
(8)
eq eq dγeq ) -∆sβ dT + ∆Vβ dp - ΓH,β 1 (RT/m1 )dm1
(9)
Figure 8. Values of coefficient of dp of eqs 10 and 11 vs pressure curves: (O) eq 10; (-) eq 11; (a) HDFC9OH, (b) FDFC9OH; (1) gaseous-expanded transition; (2) expanded-condensed transition.
(6) and
and
VjH1 (HDFC9OH) ≈ NAAHL′ ) NAAL cos θ
(7)
where NA is Avogadro’s number, θ the tilt angle from the interface normal, L the molecular length, and L′ the projection of L on interface normal direction. By using eqs 6 and 7, and the VjH1 values listed in Table 1, the mean tilt angle of HDFC9OH molecules at the interface is estimated to be around 15°. Although the grazing incidence X-ray diffraction (GIXD) will be, in principle, the most suitable spectroscopic measurement to evaluate the tilt angle of molecules of HDFC9OH, its application to the present system is very difficult at the moment because the absorption of X-ray by the hexane phase is too much to get enough scattering intensity. Thus, the thermodynamic estimation demonstrated here is highly valuable one to provide structure information on an adsorbed film. The difference in the tilt of the fluorononanol molecules in the adsorbed film may affect the miscibility of HDFC9OH and FDFC9OH in the adsorbed film. This point will be investigated in detail by constructing phase diagram of adsorption and evaluating the excess Gibbs energy of adsorption in the other paper of this series.20 Finally, let us examine the phase transition thermodynamically. Assuming that the two states R and β coexist in the interface at the phase transition point, the total differential of
respectively. By eliminating dmeq 1 from eqs 8 and 9, we obtain H,β H,R R H,R dγeq ) -[(∆sβ/ΓH,β 1 - ∆s /Γ1 )/(1/Γ1 - 1/Γ1 )] dT + R H,R H,β H,R [(∆Vβ/ΓH,β 1 - ∆V /Γ1 )/(1/Γ1 - 1/Γ1 )] dp (10)
The γeq is also written as a function of pressure and temperature by
dγeq ) (∂γeq/∂T)p dT + (∂γeq/∂p)T dp
(11)
The coefficient of dp of eq 10 is calculated by using the ∆V and ΓH1 values at the phase transition points, and that of eq 11 is obtained from the slope of the γeq vs p curve in Figure 3. Their values are plotted against pressure in Figure 8. The agreement of both values is good enough to claim that two types of phase transitions occur in the adsorbed fluorononanol films at the hexane/water interface. Acknowledgment. This work was supported in part by the Grant-in Aid for Scientific Research (C) and (B) of Japan Society for the Promotion of Science (Nos. 16550017 and 16350075).
16434 J. Phys. Chem. B, Vol. 109, No. 34, 2005 References and Notes (1) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1978, 51, 2800. (2) Motomura, K.; Iyota, H.; Aratono, M.; Yamanaka, M.; Matuura, R. J. Colloid Interface Sci. 1983, 93, 264. (3) Lin, M.; Firpo, J. L.; Mansoura, P.; Baret, J. F. J. Chem. Phys. 1979, 30, 2202. (4) Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Colloid Polym. Sci. 1982, 260, 632. (5) Aratono, M.; Yamanaka, M.; Matubayasi, N.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1980, 74, 489. (6) Iyota, H.; Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1983, 65, 2402. (7) Takiue, T.; Yanata, A.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 13743. (8) Takiue, T.; Yanata, A.; Ikeda, N.; Hayami, Y.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 20122. (9) Takiue, T.; Murakami, D.; Tamura, T.; Matsubara, H.; Aratono, M. J. Phys. Chem. B, in press. (10) Zhang, Z.; Mitrinovic, D. M.; Williams, S. M.; Huang, Z.; Schlossman, M. L. J. Chem. Phys. 1999, 110, 7421.
Takiue et al. (11) Tikhonov, A. M.; Li, M.; Schlossman, M. L. J. Phys. Chem. B 2001, 105, 8065. (12) Pingali, S. V.; Takiue, T.; Luo, G.; Tikhonov, A. M.; Ikeda, N.; Aratono, M.; Schlossman, M. L. J. Phys. Chem. B 2005, 109, 1210. (13) Sakamoto, H.; Murao, A.; Hayami, Y. J. Inst. Image Inf. TeleVision Eng. 2002, 56, 1643. (14) Murakami, R.; Sakamoto, H.; Hayami, Y.; Matsubara, H.; Takiue, T.; Aratono, M. J. Colloid Interface Sci., submitted for publication. (15) Eduljee, H. E.; Newitt, D. M.; Weale, K. E. J. Chem. Soc. 1951, 3086. (16) Fine, R. A.; Millero, F. J. J. Phys. Chem. 1973, 59, 5529. (17) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348. (18) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 4. (19) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 7. (20) Murakami, D.; Takata, Y.; Matsubara, H.; Aratono, M.; Takiue, T. J. Phys. Chem. B, submitted for publication.
