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Pressure Effect on the Adsorption of 1,1,2,2-Tetrahydrotridecafluorooctanol at the Hexane/Water Interface Y. Hayami,*,† S. Ono, N. Ikeda,‡ T. Takiue, and M. Aratono Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University 33, Fukuoka 812-8151, Japan Received November 29, 1999. In Final Form: May 30, 2000 The interfacial tension of the hexane solution of 1,1,2,2-tetrahydrotridecafluorooctanol CF3(CF2)5(CH2)2OH (FC8OH) against water was measured as a function of pressure at various concentrations m1 and 298.15 K. There are three series of the break points, two of which result from a phase transition in the adsorbed film at the hexane/water interface and the other one results from a solubility limit of FC8OH in hexane. The curve of the interfacial pressure vs area per adsorbed molecule of FC8OH was drawn and compared with those of the 1,1,2,2-tetrahydroheptadecafluorodecanol CF3(CF2)7(CH2)2OH (FC10OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol CF3(CF2)9(CH2)2OH (FC12OH) systems previously reported. It was concluded that the adsorbed film of FC8OH reveals the first-order phase transitions between a gaseous and an expanded state and that between an expanded and a condensed state. The value of the volume change ∆v associated with the adsorption in the gaseous and expanded state decreases rapidly with an increase in m1, while that in the condensed state decreases very slowly. It was described from the curves of ∆v vs m1 for FC8OH, FC10OH, and FC12OH at 80 MPa that the expanded state region in the interfacial film structure strongly decreases with increasing the chain length and disappears at the FC12OH film and that the ∆v value in the condensed state rapidly decreases with an increase in the chain length. We can explain that the pressure dependence of ∆v in the condensed state is produced by the pressure dependence of the partial molar volume of FC8OH in the hexane solution. We concluded that the deposition is not a pure liquid but FC8OH-rich phase containing small quantity of hexane.
Introduction We have shown that phase transitions take place in many adsorbed films formed at air/water, air/oil, and oil/ water interfaces.1-21 Among such materials, the long-chain * To whom correspondence should be addressed. † Present address: Department of Human Life Science, Chikushi Jogakuen Junior College, Dazaifu, Fukuoka 818-0192, Japan. E-mail address:
[email protected]. ‡ Present address: Department of Environmental Science, Faculty of Human Environmental Science, Fukuoka Women’s University, Fukuoka 813-8529, Japan. (1) Hutchinson, E. J. Colloid Sci. 1948, 3, 219. (2) Hutchinson, E. J. Colloid Sci. 1948, 3, 235. (3) Hutchinson, E.; Randall, D. J. Colloid Sci. 1952, 7, 151. (4) Stauffer, C. E. J. Colloid Interface Sci. 1968, 27, 625. (5) Lutton, E. S.; Stauffer, C. E.; Martin, J. B.; Fehl, A. J. J. Colloid Interface Sci. 1969, 30, 283. (6) Ambwani, D. S.; Jao, R. A.; Fort, T., Jr. J. Colloid Interface Sci. 1973, 42, 8. (7) Lin, M.; Firpo, J. L.; Mansoura, P.; Baret, J. F. J. Chem. Phys. 1979, 71, 2202. (8) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1978, 51, 2800. (9) Matubayasi, N.; Dohzono, M.; Aratono, M.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1979, 52, 1597. (10) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32. (11) Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Colloid Polym. Sci. 1982, 260, 632. (12) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. (13) Matubayasi, N.; Motomura, K. Langmuir 1986, 2, 777. (14) Matubayasi, N.; Matsunaga, R.; Motomura, K. Langmuir 1989, 5, 1048. (15) Matsuguchi, M.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1990, 63, 17. (16) Matubayasi, N.; Azumaya, S.; Kanaya, K.; Motomura, K. Langmuir 1992, 8, 1980. (17) Hayami, Y.; Uemura, A.; Ikeda, N.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1995, 172, 142. (18) Takiue, T.; Yanata, A.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 13743.
fluoroalkanols indicate the strong condensing effect on the adsorbed film behavior at the hexane/water interface.17-20 In general, the behavior of an adsorbed film at the oil/water interface is characterized by the interactions among constituent molecules. The strong condensing effect of the long-chain fluoroalkanol is supposed to result mainly from the strong cohesive force between the fluorocarbon chains.22 Probably, the interaction between the hydroxyl group of fluoroalkanol and water molecule takes the role to arrange the molecules in the adsorbed film to effectively act such cohesive force at the hexane/water interface. Furthermore, the weaker interaction between fluorocarbon chains and hexane molecules, compared to the interaction between fluorocarbon chains and that between hexane molecules, leads to enhance the adsorption from the hexane solution. Thus, it is very interesting to investigate the adsorption behavior of fluoroalkanol at a hydrocarbon oil/water interface. Recently, Schlossman et al. have investigated the structure of 1,1,2,2-tetrahydrohenicosafluorododecanol CF3(CF2)9(CH2)2OH (FC12OH) monolayer at the hexane/water interface by using synchrotron X-ray reflectivity and concluded the interfacial density change at the transition temperature.23 In our previous papers, we have studied the adsorption behavior of FC12OH and 1,1,2,2-tetrahydroheptadecafluorodecanol CF3(CF2)7(CH2)2OH (FC10OH) at the hexane/ water interface by measuring the temperature and (19) Takiue, T.; Yanata, A.; Ikeda, N.; Hayami, Y.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 20122. (20) Takiue, T.; Uemura, A.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. 1998, 102, 3724. (21) Hayami, Y.; Findenegg, G. H. Langmuir 1997, 13, 4865. (22) Rowlinson, J. S.; Swinton, F. L. Liquids and Liquid Mixtures, 3rd ed.; Butterworth: London, 1982; Chapter 5. (23) Zhang, Z.; Mitrinovic, D. M.; Williams, S. M.; Huang, Z.; Schlossman, M. L. J. Chem Phys. 1999, 110, 7421.
10.1021/la9915466 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000
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pressure dependence of the interfacial tension and evaluating the thermodynamic quantities associated with the adsorption.17-20 We have shown that the pressure dependence of interfacial tension gives quite useful information about the structure of the interface and the molecular interaction in the adsorbed film from the viewpoint of the volume. In this study, we employed 1,1,2,2-tetrahydrotridecafluorooctanol CF3(CF2)5(CH2)2OH (FC8OH) to elucidate the influence of fluorocarbon chain length on the adsorption behavior at the hexane/water interface. The interfacial tension of the hexane solution of FC8OH against water was measured as a function of pressure p at various concentrations and 298.15 K. The thermodynamic quantities are evaluated to clarify the pressure effect on the adsorption behavior of FC8OH at the hexane/water interface. Experimental Section Materials. The impurity of FC8OH (>97%) purchased from PCR Inc. was extracted by hexane. After four times extraction, the purity was more than 99.9% by GLC. Hexane and water were purified as described previously18 and saturated with each other before the measurements. The solubility change of hexane in water in this pressure and temperature range can be assumed to be negligibly small.24 The purity of hexane and water was checked by the value of the interfacial tension between them. Method. The equilibrium interfacial tension was measured by the pendant drop method25 within an experimental error of 0.05 mN m-1 up to a concentration of about 80 mmol kg-1. The pressure was exerted on the two-phase system with an oil/water interface, which is in a quartz glass cell equipped with a movable plunger and separated from the surrounding fluids. The densities of pure hexane and water26,27 were used for the calculation of the interfacial tension.
Results The interfacial tension γ was measured as a function of pressure p at constant molality m1 of FC8OH at 298.15 K. The γ value is plotted against pressure in Figure 1: it increases linearly in a low concentration region and decreases in a high concentration region with increasing p. The curves in a high concentration and high-pressure region are concave upward. Clearly, most of the curves are seen to have a break point. There are three series of the break points. Two of them indicate a phase transition in the adsorbed film and are drawn by the broken line in the higher and lower interfacial tension region. The third type of the break points appears in the lowest interfacial tension region and the γ values at the points are practically invariant. We conclude that this type of the break point is caused by the solubility limit of FC8OH in hexane because we observed many small deposits, separated from the hexane solution, on both the surface of pendant drop and of the measurement cell window at pressures above the break points. The γ value at a given pressure was read from Figure 1 and plotted against m1 in Figure 2, where Figure 2b is the enlargement of the low concentration region. We see the first and second type of the break points indicated by the arrows in Figure 2b and in Figure 2a, respectively. (24) Since the molar volume of hexane v(0) is about 130 mL/mol and then (v(0) - v(p)) ) 0.13 mL/mol when (v(0) - v(p))/v(0) ) 1 × 10-3, the approximated equation c(p)/c(0) ) exp[p(v(0) - v(p))/RT] yields the ratio of the solubility c(p)/c(0) ) 1.005, where c(p) and c(0) mean the solubility at the pressure (p) and the atmospheric pressure (0). (25) Matubayasi, N.; Motomura, K.; Kaneshina, S.; Nakamura, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1977, 50, 523. (26) Eduljee, H. E.; Newitt, D. M.; Weale, K. E. J. Chem. Soc. 1951, 3086. (27) Fine, R. A.; Millero, F. J. J. Phys. Chem. 1973, 59, 5529.
Figure 1. Interfacial tension vs pressure curves at 298.15 K and constant molality for m1: (1) 0 mmol kg-1, (2) 0.250, (3) 0.350, (4) 0.500, (5) 0.750, (6) 1.000, (7) 1.500, (8) 2.000, (9) 2.500, (10) 3.001, (11) 4.001, (12) 4.999, (13) 5.997, (14) 6.970, (15) 8.001, (16) 9.998, (17) 12.50, (18) 15.00, (19) 17.49, (20) 19.97, (21) 22.47, (22) 24.99, (23) 29.96, (24) 34.99, (25) 39.98, (26) 45.00, (27) 49.96, (28) 59.99, and (29) 80.01.
The slope of the curve is found to change at the phase transition point. It is noted in Figure 2a that the γ value is practically independent of m1 at concentrations above the third break point. The molality meq 1 at the first and second break points and mL1 at the third break point are plotted against pressure p in Figure 3: they decrease with increasing p. Discussion Let us now evaluate the thermodynamic quantity changes associated with the adsorption of FC8OH. Adopting the convention employing the two dividing planes,28,29 we obtain the fundamental equation describing the interfacial tension as
dγ ) -sH dT + vHdp - ΓH 1 dµ1
(1)
where sH, vH, and ΓH 1 are the interfacial excess entropy, volume, and number of moles of solute per unit area defined with respect to the two dividing planes making the excess number of moles of water and hexane zero, respectively. Choosing temperature T, p, and the concentration m1 as the experimental variables and assuming an ideal behavior of the FC8OH solution in this experimental range, we have the equation relating the interfacial (28) Motomura, K.; Aratono, M. Langmuir 1987, 3, 304. (29) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348.
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Figure 2. (a) Interfacial tension vs molality curves at constant pressure: (1) 0.1 MPa, (2) 20 MPa, (3) 40 MPa, (4) 60 MPa, (5) 80 MPa, (6) 100 MPa, and (7) 120 MPa. The arrows indicate the break points. (b) Interfacial tension vs molality curves at constant pressure: (1) 0.1 MPa, (2) 20 MPa, (3) 40 MPa, (4) 60 MPa, (5) 80 MPa, (6) 100 MPa, and (7) 120 MPa. The arrows indicate the break points.
density ΓH 1 of FC8OH to the experimental results:
ΓH 1 ) -(m1/RT)(∂γ/∂m1)T,p
(2)
The values of ΓH 1 , obtained by applying eq 2 to Figure 2, are plotted in the form of ΓH 1 vs m1 in Figure 4. It shows value increases with increasing m1 and that the ΓH 1 changes discontinuously at the break point concentration shown in Figure 2. To view such a behavior more clearly, the interfacial pressure π is plotted against the area per adsorbed molecule A of FC8OH at several pressures in Figure 5. The values of π and A are defined, respectively, by the equations 0
π)γ -γ
(3)
A ) 1/NAΓH 1
(4)
and
of Figure 5, the π-A isobar of FC8OH is compared with the corresponding curves of FC10OH and FC12OH at the hexane/water interface at 80 MPa. In the studies of these systems,17-20 it has been established that the phase transitions of FC10OH are the one from a gaseous to an expanded and the one from an expanded to a condensed state, while the phase transition of FC12OH is the one from a gaseous to a condensed state. Clear similarity between the behavior of FC8OH and FC10OH supports our classification that the adsorbed monolayer of FC8OH reveals a gaseous, an expanded, and a condensed state. We next consider the volume change associated with the adsorption ∆v defined by Ο ∆v ) vH - ΓH 1 v1
and calculated by the use of
∆v ) (∂γ/∂p)T,m1
where γ0 is the interfacial tension of the pure hexane/ water interface and NA is Avogadro’s constant. The π-A curve shows that the gaseous-expanded phase transition occurs at low pressure (80 MPa). Although the type of the phase transition varies with the pressure, the locus of π-A value is hardly dependent on the pressure within the experimental error. In the inset
(5)
(6)
where vΟ 1 is the partial molar volume of FC8OH in the hexane solution.29 The ∆v vs m1 curve at constant pressure is shown in Figure 6. It is seen that the ∆v values in the gaseous state are positive while those in the expanded and condensed states are negative and that they change discontinuously at the phase transition points represented by the broken lines. A large negative ∆v in the condensed film is attributable to that the adsorption of FC8OH at the hexane/water interface from the hexane solution ac-
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OH molecules in the gaseous and expanded states are surrounded by hexane molecules as in the bulk phase, then the pressure dependence of the volume in the gaseous and expanded states is probably very similar to that in the bulk phase. As a result the ∆v at the gaseous and expanded states is almost independent of pressure. On the other hand the molecular atmosphere of FC8OH in the condensed state is very different from that in the bulk phase, namely the FC8OH molecules in the condensed state are separated from the hexane molecules except the uppermost perfluoromethyl group, and the volume in the condensed state is less likely to change than that in the bulk phase. Thus, the ∆v at the condensed state is dependent on pressure. Consider now the adsorption at a concentration above mL1 . In Figures 1 and 2, it is shown that the γ value decreases very slightly with increasing p and hardly depends on m1. Assuming that the deposit is pure liquid FC8OH and substituting its chemical potential
dµ1 ) -sL1 dT + vL1 dp
(7)
into eq 1, we have the total differential of γ L eq vs p curves at 298.15 K: (1) Figure 3. meq 1 , m1 , and γ gaseous-expanded transition, (2) expanded-condensed transition, and (3) saturated concentration.
dγ ) -∆s(L)dT + ∆v(L)dp
Here the thermodynamic quantity change associated with the adsorption from the pure liquid state ∆y(L) defined by L ∆y(L) ) yH - ΓH 1 y1 y ) s, v
Figure 4. Interfacial density vs molality curves at constant pressure: (1) 0.1 MPa, (2) 20 MPa, (3) 40 MPa, (4) 60 MPa, (5) 80 MPa, (6) 100 MPa, and (7) 120 MPa.
companies a decrease in the volume. It is noted that the ∆v value of the gaseous and expanded state are almost independent of pressure within the experimental error, while that in the condensed state is strongly dependent on it. The contrast between a variety of ∆v values in the condensed state and almost constant ∆v values in the gaseous and expanded states accompanied by pressure change in Figure 6 is caused by the difference in the molecular states of FC8OH monolayer. Namely, the FC8-
(8)
(9)
was introduced, where y1L is the molar thermodynamic quantity of pure liquid FC8OH. The ∆v(L) value was calculated from the slope of the γ vs p curves at a concentration above mL1 and shown by the dotted line in Figure 6. In addition, we reported previously a pure solidphase deposition of FC12OH system.19 It is noteworthy that the ∆v(L) value for FC8OH is slightly negative and independent of pressure and concentration, while the ∆v(S) for FC12OH is slightly positive (about 0.03 mm3 m-2) as previously reported.19 To clarify the influence of fluorocarbon chain length on the volumetric behavior of fluoroalkanol at the hexane/ water interface, we compared the ∆v vs m1 curve for FC8OH with those for FC10OH and FC12OH at 80 MPa in Figure 7.18,19 The curve 3 in the gaseous state on the inset of Figure 7 is overlapped on the curve 2 in the gaseous state. A glance at Figure 7 shows the role of the fluorocarbon chain on the interfacial behavior of fluoroalkanol: the ∆v value in the condensed state becomes more negative with an increase in the chain length. In addition, the region of the expanded state decreases with increasing the chain length and disappears in the FC12OH film. Namely the multipolar intermolecular force and dispersion forces of the fluorocarbon chain produces a very strong interaction between fluoroalkanol molecules and thus the intermediate interaction region of the expanded state cannot exist any more in the FC12OH film at that temperature and pressure.22 We next examine whether the phase transitions are first-order. Denoting the two phases existing in equilibrium by the superscripts R and β, we have the relations:
dγ ) -∆sR dT + ∆vR dp - ΓH,R 1 (RT/m1) dm1 (10) and
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Figure 5. Interfacial pressure vs area per molecule curves at constant pressure: (1) 0.1 MPa, (2) 20 MPa, (3) 40 MPa, (4) 60 MPa, (5) 80 MPa, (6) 100 MPa, (7) 120 MPa, (8) FC8OH at 80 MPa, (9) FC10OH at 80 MPa, and (10) FC12OH at 80 MPa.
Figure 6. ∆v and ∆v(L) vs molality curves at constant pressure: (s) ∆v; (‚‚‚) ∆v(L); (1) 0.1 MPa, (2) 20 MPa, (3) 40 MPa, (4) 60 MPa, (5) 80 MPa, (6) 100 MPa, and (7) 120 MPa.
dγ ) -∆sβ dT + ∆vβ dp - ΓH,β 1 (RT/m1) dm1
(11)
where ∆s is the entropy change associated with the adsorption. By using the relations γR ) γβ ) γeq at the equilibrium and at the condition of constant temperature, we can derive the following equation from eqs 10 and 11: eq H,β β R - ΓH,R (∂meq 1 /∂p)T ) [(m1 /RT)(∆v - ∆v )]/(Γ1 1 ) (12)
where the superscript eq means the equilibrium value at the phase transition points. The value of the left-hand side of eq 12 is given by the slope of the meq 1 vs p curve obtained from Figure 3. The value of the right-hand side is evaluated from Figures 4 and 6. In a similar manner, we obtain the pressure dependence of the equilibrium interfacial tension γeq as follows:
Figure 7. Volume change vs molality curves at 80 MPa: (1) FC8OH, (2) FC10OH, and (3) FC12OH. R H,R H,β (∂γeq/∂p)T ) (∆vβ/ΓH,β - 1/ΓH,R 1 - ∆v /Γ1 )/(1/Γ1 1 ) (13)
The value of the left-hand side of eq 13 is given by the slope of the γeq vs p curve shown in Figure 3. The value of the right-hand side is obtained from Figures 4 and 6. A comparison of both sides of eqs 12 and 13 is made in Figure 8. The criterion whether the transition is the firstorder is as follows: First the phase transition is assumed to be the first-order one. Then we can calculate the thermodynamic quantities inherent in the interfaces such as the interfacial density of surfactant, entropy change associated with adsorption, volume change associated with adsorption, and so on, for the respective two interfacial phases. Therefore, the right-hand sides of eqs 12 and 13 are estimated numerically by using thermodynamics. On the other hand, the left-hand sides of eqs 12 and 13 are evaluated separately and directly by using the experimental results without using thermodynamics. Then the coincidence of the numerical values between the left and
Adsorption of 1,1,2,2-Tetrahydrotridecafluorooctanol
Langmuir, Vol. 16, No. 17, 2000 7011
Figure 8. Values of the left-hand side and the right-hand side for eqs 9 and 10: (s) value of the left-hand side; (O) value of the right-hand side; (1) gaseous-expanded transition for eq 9, (2) expanded-condensed transition for eq 9, (3) gaseousexpanded transition for eq 10, and (4) expanded-condensed transition for eq 10.
right-hand sides within the experimental error justifies the assumption that the phase transition is the first-order one. The agreement between the values is good enough to conclude that the phase transitions are first-order. Taking account of the high interfacial density of FC8OH in the condensed film, we have the approximation at the concentration above mL1 : I Ι I ∆v(L)/ΓH j o - vΟ j Ιw - vW 1 ) [Γo(v o ) + Γw(v w) + L H ΓH jH 1 (v 1 - v1 )]/Γ1 L ≈ vj H 1 - v1
(14)
where ΓIj is the number of moles per unit area inherent in the interface, vj Ιj and vj H 1 are the mean partial molar volume of the solvent component j and FC8OH inherent in the interface, and vj Jj is the partial molar volume of the component j in its bulk phase J, respectively. Here the L values of ΓH 1 above m1 were assumed to be equal to the L ones at m1 (solid circles in Figure 4). Judging from the fact that the values of A of FC8OH and FC12OH at mL1 are almost same, the condensed state of both adsorbed films are very similar and resemble the solid state.19 Since the value of ∆v(L)/ΓH 1 for FC8OH is small compared with the molar volume of FC8OH (ca. 217 cm3 mol-1) as is shown in Figure 9, the molar volume in the liquid state in this experimental condition is very close to that in the condensed state at the interface. This is also confirmed by the finding that the pressure dependence of ∆v(L)/ΓH 1 is negligibly small,19,30 i.e., the compressibility of the liquid state is small and very similar to that of the condensed film at the interface. Furthermore, we evaluated the partial molar volume change accompanied by the transfer of FC8OH from the (30) Prigogine, I.; Defay, R. Chemical Thermodynamics; Everett, D. H., Translator; Longmans: London, 1954; Chapter 12.
O L Figure 9. ∆v(L)/ΓH 1 and v1 - v1 vs pressure curves at 298.15 H O L L K: (s) ∆v(L)/Γ1 ; (‚‚‚) v1 - v1 calculated by eq 15; (O) vO 1 - v1 calculated by eq 16.
L pure liquid to its hexane solution, vO 1 - v1 , by
L L L vO 1 - v1 ) -(RT/m1 )(∂m1 /∂p)T
(15)
where an ideal dilute solution at mL1 was assumed. The L values of vO 1 - v1 , obtained by applying eq 15 to Figure 3, L are plotted in the form of vO 1 - v1 vs p by the dotted line O L in Figure 9. The v1 - v1 value decreases with increasing p. As we mentioned above the pressure dependence of vL1 L is negligibly small, hence the decrease in vO 1 - v1 with O increasing pressure is caused by the decrease in v1 . In the same way we can explain that the pressure dependence of ∆v in the condensed state shown in Figure 6 results from the pressure dependence of vO 1 in the hexane solution. L This quantity vO 1 - v1 is also evaluated by using eqs 5 and 9 as L H vO 1 - v1 ) -[∆v - ∆v(L)]/Γ1
(16)
L O L where ΓH 1 and ∆v are the values at m1 . The v1 - v1 values calculated by eqs 15 and 16 are plotted by the dots and the open circles in Figure 9, respectively: the positive value is in accord with the general finding that the mixing of hydrocarbon and fluorocarbon accompanies an increase L in volume and therefore vO 1 is larger than v1 . The disagreement between the dots and open circles suggests that the deposit is not a pure liquid but FC8OH-rich phase containing small quantities of hexane. However, the qualitative description of volumetric behavior mentioned above is not greatly affected.
Acknowledgment. This work was supported in part by the Grant-in-Aid for Encouragement of Young Scientists of The Ministry of Education, Science, Sports and Culture(No.10740326) and in part by the Kurata foundation. LA9915466
