22366
J. Phys. Chem. B 2005, 109, 22366-22370
Effect of ω-Hydrogenation on the Adsorption of Fluorononanols at the Hexane/Water Interface: Miscibility in the Adsorbed Film of Fluorononanols Daiki Murakami,* Youichi Takata, Hiroki Matsubara, Makoto Aratono, and Takanori Takiue Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu UniVersity, Fukuoka 812-8581, Japan ReceiVed: April 13, 2005; In Final Form: August 8, 2005
The interfacial tension 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 the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The existence of ω-dipole in HDFC9OH makes the interfacial density larger in the gaseous and expanded states and smaller in the condensed state compared to FDFC9OH. The phase diagram of adsorption (PDA) was constructed, and the excess Gibbs energy of adsorption (gH,E) was calculated at each state in order to discuss quantitatively the miscibility of FDFC9OH and HDFC9OH in the adsorbed film. We found that the gH,E value is negative in the gaseous state, while it is positive and increases with decreasing interfacial tension in the condensed state. These results are explained mainly by the balance of two effects induced by mixing of two alcohols: (1) Reduction of repulsive interaction between ω-dipoles aligning parallel in the adsorbed film because of the increase in mean distance between HDFC9OH molecules. (2) The loss of effective dispersion interaction between hydrophobic chains due to the fact that the oblique orientation of HDFC9OH molecules at the interface is mixed with the perpendicular one of FDFC9OH. We concluded that the factor (2) is negligible compared to the factor (1) in the gaseous and expanded films and exceeds the factor (1) in the condensed film, in which molecules are closely packed.
Introduction The properties of fluorocarbon (FC) surfactants change drastically with the alternation of its molecular structure, such as chain length, extent of fluorination of hydrophobic chain, and so on. For example, the studies on the surface adsorption and micelle formation of FC surfactants have demonstrated that hydrogenation of the terminal CF3 group of the FC chain to HCF2 increases the critical micelle concentration in aqueous solutions and decreases the saturated surface density.1-5 Penfold et al. have explained this behavior mainly by the hydrophilic nature of ω-dipole caused by substitution of a fluorine atom at the ω-position into a hydrogen atom. However, the effect of ω-dipole on the molecular orientation in the adsorbed film has not yet been clearly shown. In our previous studies of this series, the adsorption of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analogue, 1H,1H,9H-perfluorononanol (HDFC9OH), at the hexane/water interface has been investigated by measuring temperature and pressure dependence of interfacial tension and evaluating the entropy, energy, and volume changes of adsorption.6,7 It was hard to understand the differences in these thermodynamic quantities between FDFC9OH and HDFC9OH systems only by the hydrophilic nature of ω-dipole. Thus, in addition to this hydrophilic nature of ω-dipole, we claimed that HDFC9OH molecules tend to tilt from interface normal to gain effective interaction between ω-dipole and water molecules and between ω-dipole and hydroxyl group of neighboring molecules, and also to reduce the repulsive one between ω-dipoles aligning parallel at the interface. * To whom correspondence should be addressed. Phone: +81 92 642 2578. Fax: +81 92 642 2607. E-mail:
[email protected].
Mixed adsorbed film of surface-active substances at the oil/ water interface is of great importance because of its extensive use in industrial and biological fields using emulsion, biomimetic membranes, and so on. We have also discussed the miscibility of molecules in the adsorbed film by constructing a phase diagram of adsorption (PDA) and evaluating the excess Gibbs energy of adsorption.8 In the study on the adsorption of a homologous FC alcohol mixture with different chain length, 1H,1H,2H,2H-perfluorodecanol (TFC10OH)-1H,1H,2H,2H-perfluorododecanol (TFC12OH) at the hexane/water interface, it was found that TFC10OH and TFC12OH mix almost ideally in the gaseous and expanded states and show positive deviation from ideal mixing in the condensed state.9 This demonstrates that the difference in the magnitude of mutual interaction between the same molecules affects appreciably the miscibility in the condensed state in which the molecules are closely packed with each other. Recent application of synchrotron X-ray reflection technique to the adsorbed films formed at oil/water interfaces provides us microscopic information on the electron density, film thickness, interface roughness, and interfacial coverage for the single and/or mixed component systems.10-13 We found that the condensed films of TFC10OH and TFC12OH are regarded to be two-dimensional solid, and both alcohol molecules stand almost upright at the interface. Deducing from this result, it is expected that the FDFC9OH molecules also stand normal to the interface because of a lack of ω-dipole, which is in contrast to the oblique orientation of HDFC9OH. So, the aim of this study is to know how FDFC9OH molecules mix with HDFC9OH, which has almost same molecular structure except for only a hydrogen atom instead of a fluorine atom at ω-position, in the adsorbed film. By performing this, we
10.1021/jp0581227 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2005
Miscibility in the Adsorbed Film of Fluorononanols
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22367
Figure 1. Interfacial tension versus molality curves at constant composition: (1) X2 ) 0 (FDFC9OH), (2) 0.0750, (3) 0.1500, (4) 0.2250, (5) 0.3000, (6) 0.5000, (7) 0.6500, (8) 0.8000, and (9) 1 (HDFC9OH).
understand in more detail the effect of ω-dipole on the miscibility of molecules in the adsorbed film from the viewpoints of molecular packing as well as intermolecular interaction. The interfacial tension of the hexane solution of FDFC9OH and HDFC9OH mixture against water was measured as a function of the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The PDA was constructed, and the excess Gibbs energy of adsorption was evaluated by analyzing the experimental data thermodynamically in order to elucidate the miscibility of solutes in the adsorbed film quantitatively. Experimental Section FDFC9OH and HDFC9OH were purchased from Azmax Co. Ltd. The former was purified by recrystallization three times from hexane solution. In addition, FDFC9OH was mixed with hexane at about 66 °C (slightly higher than the melting point of FDFC9OH) and allowed to stand for a few hours to separate completely into two transparent phases. Then FDFC9OH was recrystallized again from the upper liquid phases because the lower phase was expected to contain more impurities. The latter was purified by recrystallization two times also from its hexane solution. Their purities were checked by observing no time dependence of interfacial tension between the hexane solution of each alcohol and water, and by the 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. The interfacial tension γ was measured as a function of the total molality m and composition of HDFC9OH X2 at 298.15 K under atmospheric pressure by the pendant drop method based on the shape analysis of pendant drops described elsewhere.14,15 Here m and X2 are defined by
m ) m 1 + m2
(1)
X2 ) m2/m
(2)
and
where m1 and m2 are the molalities of FDFC9OH and HDFC9OH in hexane solution, respectively. Experimental error of the γ value was estimated within (0.05 mN m-1. For the calculation of interfacial tension, the densities of pure hexane and water
were used instead of those of hexane solution and water in equilibrium with each other16,17 because the concentration of hexane solution was sufficiently low and mutual solubility between them was negligibly small. Results and Discussion The results of interfacial tension measurement are shown as the γ versus m curves at fixed X2 in Figure 1a. In Figure 1b are magnified the curves at some X2 values at low concentration range. The γ value decreases with increasing m, and every curve shows two break points at which the slope of the curve changes abruptly. The γ versus m curves of the mixture change gradually from the curve of pure FDFC9OH to that of pure HDFC9OH with X2. Furthermore, it is seen that the γ value at given m decreases at low concentrations and increases at high concentrations with increasing X2. This suggests that HDFC9OH is more surface-active in a low concentration range, while less surfaceactive in a high concentration range compared to FDFC9OH. The above view is substantiated by evaluating the total interfacial density, ΓH, defined by the individual interfacial density, ΓiH(i ) 1, 2), as
ΓH ) Γ1H + Γ2H
(3)
The ΓH value was calculated by applying the following equation17
ΓH ) -(m/RT)(∂γ/∂m)T,p,X2
(4)
to the γ versus m curves. The results at some X2 values are plotted against m in Figure 2 in order to see clearly the variation of the ΓH versus m curve with X2. It is found that the curves consist of three regions separated by two discontinuous changes corresponding to the kinks on the interfacial tension curves in Figure 1. Taking account of our previous conclusion that two types of phase transitions from the gaseous to the expanded and from the expanded to the condensed state take place in adsorbed films of pure HDFC9OH and FDFC9OH,6,7 the states of adsorbed films of the mixture are assigned, respectively, to the gaseous, expanded, and condensed states in the order of increasing ΓH. Furthermore, it should be noticed that the ΓH value increases with increasing X2, both in the gaseous and expanded states, which is in contrast to ΓH in the condensed state. This is explained by considering the three factors. (1) First,
22368 J. Phys. Chem. B, Vol. 109, No. 47, 2005
Figure 2. Interfacial density versus molality curves at constant composition: (1) X2 ) 0 (FDFC9OH), (2) 0.2250, (3) 0.5000, (4) 0.8000, and (5) 1 (HDFC9OH).
Murakami et al.
Figure 4. Phase diagram of adsorption at constant interfacial tension in condensed state: (s) m versus X2, (- ‚ - ‚ -) m versus X2H curve; (1) γ ) 30, (2) 25, and (3) 20 mN m-1.
should be noted that the m value decreases gradually in the gaseous and expanded states, and on the other hand, it increases in the condensed state with increasing X2. By applying the equation18,19
X2H ) X2 - (X1X2/m)(∂m/∂X2)T,p,γ
(5)
to these curves, we calculated the composition in the adsorbed film X2H defined by
X2H ) Γ2H/ΓH
Figure 3. Phase diagram of adsorption at constant interfacial tension in gaseous and expanded states: (s) m versus X2, (- ‚ - ‚ -) m versus X2H, (- - -) m versus X2eq curve; (1) γ ) 47.5, (2) 44, (3) 42, and (4) 38 mN m-1.
one is the energetically uncomfortable situation of the hydrophilic nature of the ω-dipole of HDFC9OH molecules in the hexane solution. However, the attractive interaction acts advantageously between the ω-dipole of the adsorbed HDFC9OH molecule and water in the interfacial region. Thus, this factor makes the interfacial density of HDFC9OH larger compared to that of FDFC9OH. (2) Second, the repulsive interaction between ω-dipoles arranging parallel in the adsorbed film reduces the interfacial density. (3) Third, the dispersion interaction between the hydrophobic chains is smaller for HDFC9OH than for FDFC9OH because of lower molar mass of HDFC9OH and thus decreases the interfacial density. Judging from the results shown above, it is probable that the factor (1) is dominant in the gaseous and expanded states in which the mean distance between molecules is relatively long, and the factors (2) and (3) mainly determine the property of the condensed film in which the molecules are closely packed. Next, let us consider the miscibility of HDFC9OH and FDFC9OH in each state of the adsorbed film. To do this, first, the m values at given γ read from Figure 1 are plotted against X2 (solid lines) for the gaseous state and the phase transition region in Figure 3 and only for the condensed state in Figure 4. The broken lines represent the total molality at phase transition point meq versus X2 curves on which the m versus X2 curve breaks. It
(6)
and then drew the m versus X2H curves depicted by the dottedchain line in Figures 3 and 4. These figures give the quantitative relationship between the compositions in the bulk and in the adsorbed film at given γ and are called the phase diagram of adsorption (PDA). As expected from the results of interfacial density shown above, the gaseous and expanded films are richer in HDFC9OH, while the condensed state is richer in FDFC9OH, compared to the bulk solution. It is noted that the m versus X2H curve looks convex downward in the gaseous state and upward in the condensed state, respectively. This demonstrates a large difference in the miscibility of HDFC9OH and FDFC9OH in the adsorbed film between different states. The above difference observed is examined in terms of a criterion concerning the ideal mixing of surface-active substances in the adsorbed film developed previously.19,20 For the binary nonionic surfactant system, the ideal mixing in PDA at a given γ is expressed by straight line connecting the molalities of pure components at the same γ as
m ) m10 + (m20 - m10)X2H
(7)
where mi0 is the molality of the pure component i. The deviation from this line means the nonideal mixing of two components in the adsorbed film. It is obvious from PDA that the m versus X2H curve deviates negatively in the gaseous state and positively in the condensed state. In the expanded state, the m versus X2H curve seems to deviate slightly negative. The nonideal mixing found in the present system is examined more quantitatively by calculating the excess Gibbs energy of adsorption gH,E given by18,20
gH,E ) RT(X1H ln f1H + X2H ln f2H)
(8)
Miscibility in the Adsorbed Film of Fluorononanols
J. Phys. Chem. B, Vol. 109, No. 47, 2005 22369 mainly by the latter interaction. In the mixed system, mean distance between adsorbed HDFC9OH molecules and, therefore, the distance between ω-dipoles is expected to be lengthened by mixing of HDFC9OH and FDFC9OH molecules in the adsorbed film, and as a result, the repulsive interaction become weaker compared to that in the pure HDFC9OH system. In the condensed film, on the other hand, positive value of gH,E reflects weaker interaction between different species. In addition to this, the gH,E value tends to increase with decreasing γ, which gives positive excess area per adsorbed molecule, AH,E, calculated by18,20
AH,E ) - (1/NA)(∂gH,E/∂γ)T,p,X2H
Figure 5. Excess Gibbs energy versus composition curves at constant interfacial tension: (1) γ ) 47.5, (2) 30, (3) 25, and (4) 20 mN m-1.
Here, fiH is the activity coefficient defined symmetrically as fiH f 1 when XiH f 1, and is calculated by
fiH ) mXi/mi0XiH
(9)
When fiH ) 1, eq 9 yields the ideal mixing line of eq 7. Making use of these equations and PDA, we obtained the gH,E value at given γ in the gaseous and condensed states. The results are shown as the gH,E versus X2H curves in Figure 5. In the gaseous state, it is realized that the gH,E value is negative (curve 1 in Figure 5). Although the gH,E value is not able to be calculated in the expanded state, the fiH value is expected to be less than unity from the shape of PDA. These results indicate the stronger mutual interaction between the different species than between the same ones, that is, the stabilization of the mixed adsorbed film compared to the pure film. As mentioned above, it is likely that the miscibility of HDFC9OH and FDFC9OH in the adsorbed film is determined by the balance between the attractive dispersion interaction of neighboring molecules and the repulsive interaction between ω-dipoles aligning parallel in the adsorbed film. Since the dispersion interaction is short-range and the dipole-dipole interaction acts even at long distance between the molecules,21 the miscibility in the gaseous and expanded states is governed
(10)
This result shows that the mixing of HDFC9OH and FDFC9OH in the condensed film causes an increase in mean distance between the molecules. Since the mixing of both alcohols reduces the repulsive interaction between ω-dipoles at the interface, other factor(s) may cause the positive gH,E in the condensed state. Remembering our previous conclusion that the HDFC9OH molecules tend to tilt from interface normal compared to the FDFC9OH, the mixing state of the oblique and perpendicular orientation of molecules lowers the stability of the condensed film and eventually causes a loss of effective dispersion interaction between molecules. This effect is superior to the stabilization effect by the reduction of repulsive force due to the mixing of HDFC9OH and FDFC9OH in the condensed film. The direct structural evidence of molecular packing may be obtained by spectroscopic measurements, such as synchrotron X-ray reflection (XR) and grazing incidence X-ray diffraction (GIXD). Comparison between the film thickness obtained by XR and calculated molecular length would help us to evaluate the tilt angle of molecules in the adsorbed film. Furthermore, GIXD would provide us the information concerning molecular arrangement as well as tilt angle more directly. The study on film structure by means of these techniques will be our future works. Finally, we verify the existence of phase transition in the adsorbed film thermodynamically. For two states, R and β, coexisting in equilibrium in the adsorbed film at constant temperature and pressure, the total differential of interfacial tension, dγeq, is written as
Figure 6. Value of both sides of eq 13: (s) lhs, (O) rhs; (a) gaseous-expanded transition, (b) expanded-condensed transition.
22370 J. Phys. Chem. B, Vol. 109, No. 47, 2005
dγeq ) - (ΓH,RRT/meq)dmeq - (ΓH,RRT/X1X2)(X2H,R - X2)dX2 (11) and
dγeq ) - (ΓH,βRT/meq)dmeq - (ΓH,βRT/X1X2)(X2H,β - X2)dX2 (12) respectively. By eliminating dγeq from eqs 11 and 12, we obtain the following equation
(∂meq/∂X2)T,p ) - (meq/X1X2)[X2 - (ΓH,RX2H,R - ΓH,βX2H,β)/(ΓH,R - ΓH,β)] (13) The left-hand-side (lhs) of this equation can be evaluated from the slope of the meq versus X2 curve in Figure 3. The righthand-side (rhs) can be calculated by using the ΓH and X2H values at the transition points obtained from Figures 2 and 3. Their values are plotted against X2 in Figure 6 for comparison. The agreement of both sides is good enough within the experimental error, and thus we concluded that the phase transitions take place in the adsorbed film of FDFC9OH and HDFC9OH mixture 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).
Murakami et al. References and Notes (1) Downer, A.; Eastoe, J.; Pitt, A. R.; Penfold, J.; Heenan, R. K. Colloids Surf. A 1999, 156, 33. (2) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591. (3) Eastoe, J.; Paul, A.; Rankin, A.; Wat, R.; Penfold, J.; Webster, J. R. P. Langmuir 2001, 17, 7873. (4) Ravey, J. C.; Stebe, M. J. Colloids Surf., A 1994, 84, 11. (5) Matsukubo, T. unpublished data. (6) Takiue, T.; Murakami, D.; Tamura, T.; Matsubara, H.; Aratono, M. J. Phys. Chem. B 2005, 109, 14154. (7) Takiue, T.; Hirose, D.; Murakami, D.; Matsubara, H.; Aratono, M. J. Phys. Chem. B 2005, 109, 16429. (8) Aratono, M.; Takiue, T. In Mixed Surfactant Systems; Abe, M., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 2005; p 1. (9) Takiue, T.; Fukuta, T.; Matsubara, H.; Ikeda, N.; Aratono, M. J. Phys. Chem. B 2001, 105, 789. (10) Zhongjian, Z.; Mitrinovic, D. M.; Williams, S. M.; Huang, Z.; Schlossman, M. L. J. Chem. Phys. 1999, 110, 7421. (11) Mitrinovic, D. M.; Zhang, Z.; Williams, S. M.; Huang, Z.; Schlossman, M. L. J. Phys. Chem. B 1999, 103, 1780. (12) Tikhonov, A. M.; Li, M.; Schlossman, M. L. J. Phys. Chem. B 2001, 105, 8066. (13) Pingali, S. V.; Takiue, T.; Luo, G.; Tikhonov, A. M.; Ikeda, N.; Aratono, M.; Schlossman, M. L. J. Phys. Chem. B 2005, 109, 1210. (14) Sakamoto, H.; Murao, A.; Hayami, Y. J. ITE Jpn. 2002, 56, 1643. (15) Murakami, R.; Sakamoto, H.; Hayami, Y.; Matsubara, H.; Takiue, T.; Aratono, M. J. Colloid Interface Sci. Submitted for publication. (16) Kell, G. S.; Whally, E. Philos. Trans. R. Soc. London, Ser. A 1965, 258, 565. (17) Orwoll, R. A.; Flory, P. J. J. Am. Chem. Soc. 1967, 89, 6814. (18) Aratono, M.; Ohta, A.; Minamizawa, H.; Ikeda, N.; Iyota, H.; Takiue, T. J. Colloid Interface Sci. 1999, 217, 128. (19) Todoroki, N.; Tanaka, F.; Ikeda, N.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1993, 66, 351. (20) Aratono, M.; Villeneuve, M.; Takiue, T.; Ikeda, N.; Iyota, H. J. Colloid Interface Sci. 1998, 200, 161. (21) Israelachivili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 4.
