J. Phys. Chem. 1996, 100, 13743-13746
13743
Thermodynamic Study on Phase Transition in Adsorbed Film of Fluoroalkanol at the Hexane/Water Interface. 1. Pressure Effect on the Adsorption of 1,1,2,2-Tetrahydroheptadecafluorodecanol Takanori Takiue,* Atsuro Yanata, Norihiro Ikeda,† Kinsi Motomura, and Makoto Aratono Department of Chemistry, Faculty of Science, Kyushu UniVersity 33, Fukuoka 812-81, Japan ReceiVed: February 21, 1996; In Final Form: May 14, 1996X
The interfacial tension γ of hexane solution of 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) against water was measured as a function of pressure p and molality m1 at 298.15 K. The break points were observed on the γ vs p and γ vs m1 curves. The interfacial density ΓH1 increases and the volume change associated with the adsorption ∆ν decreases with increasing m1, and they change discontinuously at concentrations of the break points. By drawing the interfacial pressure π vs area A curve, it is concluded that two types of phase transitions take place from the gaseous to the expanded state and from the expanded to the condensed one in the adsorbed film of FC10OH. By examining our results by the Clapeyron type equations, the order of phase transition was considered first. The partial molar volume change associated with the adsorption of FC10OH was estimated and compared with that of 1-octadecanol (C18OH). It was found that FC10OH molecules were accompanied with the larger decrease in partial molar volume inherent in the interface than C18OH molecules when the phase transition takes place from the expanded to the condensed state in the adsorbed film. This suggests that the microscopic circumstance in the adsorbed film affects appreciably the volume change of adsorption.
Experimental Section
Introduction It is generally known that the interaction between fluorocarbon and hydrocarbon chains is weak and therefore a fluorocarbon surfactant mixes nonideally or often does not mix with a hydrocarbon surfactant in the adsorbed film and micelle.1-6 Thus, the study on the adsorbed films of fluorocarbon compounds at a hydrocarbon oil/water interface is expected to be highly useful for understanding the interaction between fluorocarbon and hydrocarbon chains. In our latest paper,7 we have studied the adsorbed film of 1,1,2,2-tetrahydrohenicosafluorododecanol (FC12OH) at the hexane/water interface from the viewpoints of entropy ∆s and energy ∆u of adsorption and claimed, on the basis of the discontinuous changes of ∆s and ∆u values, that the phase transition takes place at the interface.8-16 Furthermore it was confirmed thermodynamically that the transition is the first order one from the gaseous to the condensed state.7 In order to examine more closely the adsorption of fluoroalkanol from the viewpoint of the volume, this time we employed 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH), because the deposit of FC12OH was observed in the measurement cell at a high-pressure region, and studied the pressure effect on its adsorption at the hexane/water interface. We will report the adsorption behavior of FC12OH in the absence and also in the presence of the deposit in this series.17 The interfacial tension of hexane solution of FC10OH against water was measured as a function of pressure and molality at 298.15 K. The interfacial density and the volume change associated with the adsorption were evaluated by analyzing the experimental results thermodynamically. * To whom correspondence should be addressed. † Present address: Department of Environmental Science, Faculty of Human Environmental Science, Fukuoka Women’s University, Fukuoka 813, Japan. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00525-4 CCC: $12.00
FC10OH purchased from PCR Inc. was recrystallized once from hexane. The purity was estimated to be more than 99.9% by gas-liquid chromatography. Water was distilled three times from dilute alkaline permanganate solution and hexane once in the presence of metallic sodium particles. Their purities were checked by measuring the interfacial tension between them. The interfacial tension was measured by the pendant drop technique.18 The experimental error estimated was within (0.05 mN m-1. The densities of pure hexane and water19,20 were used for the calculation of interfacial tension instead of those of the equilibrium two phases because of their negligibly small mutual solubility and sufficiently low concentration of the hexane solution. Results and Discussion The interfacial tension γ of the hexane solution of FC10OH against water is shown as a function of pressure p at constant molality m1 in Figure 1. It is seen that the γ value increases slightly in a very low concentration and decreases greatly in a high concentration with increasing pressure. At the intermediate concentrations, it was found that the γ vs p curve has one or two break points at which the slope of the curve changes abruptly. Both γ and p values of the break point decrease with increasing concentration, as shown by the two broken lines. The γ value read from Figure 1 at a given p was plotted against m1 in Figure 2; it decreases with increasing m1, and the two break points are clearly recognized (plot b). These results in Figures 1 and 2 suggest the change of states of the adsorbed film. Now we explore the transformation in detail by employing the interfacial pressure π vs the mean area per molecule of FC10OH A curve. For this we evaluate first the interfacial density ΓH1 of FC10OH by applying the equation
ΓH1 ) -(m1/RT)(∂γ/∂m1)T,p © 1996 American Chemical Society
(1)
13744 J. Phys. Chem., Vol. 100, No. 32, 1996
Takiue et al.
Figure 1. Interfacial tension vs pressure curves at constant molality (m1, mmol kg-1) as follows: (1) 0, (2) 0.190, (3) 0.343, (4) 0.496, (5) 0.739, (6)1.001, (7) 1.272, (8) 1.497, (9) 1.755, (10) 2.090, (11) 2.521, (12) 2.990, (13) 3.527, (14) 4.000, (15) 4.523, (16) 5.034, (17) 6.023, (18) 7.040, (19) 9.040, (20) 9.979, (21) 12.017, (22) 15.030.
to the γ vs m1 curve in Figure 2 and then calculate the π and A values by
π ) γ0 - γ
(2)
A ) 1/NAΓH1
(3)
where γ0 is the interfacial tension between pure hexane and water and NA the Avogadro number. The results are shown as ΓH1 vs m1 and π vs A curves at several pressures in Figures 3 and 4, respectively. It is seen that the ΓH1 value increases with increasing m1 and changes discontinuously at a concentration of the break point on the γ vs m1 curve. From Figure 4 one realizes that all curves exhibit the two discontinuous changes among the three different states and, therefore, that the two types of phase transitions take place in the adsorbed film. The three different states are identified by comparing the π vs A curve of FC10OH with that of C18OH;12 the example at 80 MPa is shown in Figure 5. Taking account of our previous conclusion that the phase transiton of C18OH takes place from the expanded to the condensed state, it is concluded that the almost vertical region corresponds to the condensed state, the intermediate area to the expanded, and the large to the gaseous. It is important to note that the limiting area of FC10OH found to be 0.3 nm2 is very close to the cross-sectional area of the fluorocarbon chain. Next we discuss the adsorption behavior of FC10OH from the viewpoint of the volume. The volume change associated with the adsorption ∆ν is evaluated by applying the equation
∆ν ) (∂γ/∂p)T,m1
(4)
to the γ vs p curves shown in Figure 1. The results are shown as the ∆ν vs m1 curves at various pressures in Figure 6. It is seen that there are two discontinuous changes on ∆ν vs m1 curves at concentrations corresponding to the phase transition points. In the gaseous state, the ∆ν value does not change appreciably from that of pure hexane/water interface. In the expanded state, it decreases gradually from the positive to the
Figure 2. Interfacial tension vs molality curves at constant pressure (p, MPa) as follows: (Plot a, top) (1) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, (6) 100; (Plot b, bottom) (1) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, (6) 100.
Figure 3. Interfacial density vs molality curves at constant pressure (p, MPa) as follows: (1) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, (6) 100.
negative value with increasing m1. We note that the condensed state is characterized by the large negative ∆ν value and its strong dependence on pressure. The decrease in the ∆ν value with increasing molality indicates that the adsorption of FC10OH makes a negative contribution to ∆ν. This negative contribution of the adsorption of a solute to the ∆ν value has been successfully examined in terms of the partial molar volume change associated with the adsorption νjH1 - νO1 .21 Its value is estimated by using the ΓH1 and ∆ν values and the following equation:21,22
νjH1 - νO1 ) ∆ν/ΓH1 - (1 - ΓH1 a1)(∂γ/∂p)T,ΓH1
(5)
Phase Transition in Adsorbed Fluoroalkanol Film
J. Phys. Chem., Vol. 100, No. 32, 1996 13745
Figure 4. Interfacial pressure vs mean area per molecule curves at constant pressure (p, MPa) as follows: (1) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, (6) 100.
Figure 7. Partial molar volume change of adsorption vs molality curves at 0.1 and 80 MPa: (1) FC10OH and (2) C18OH at 0.1 MPa; (3) FC10OH and (4) C18OH at 80 MPa.
Figure 5. Interfacial pressure vs mean area per molecule curves at 80 MPa: (1) FC10OH, (2) C18OH.
Figure 8. Values of coefficient of dp of eqs 6 and 9 vs pressure curves: (s) eq 6 (O) eq 9; (1) gaseous T expanded and (2) expanded T condensed.
Figure 6. Volume change associated with the adsorption vs molality curves at constant pressure (p, MPa) as follows: (1) 0.1, (2) 20, (3) 40, (4) 60, (5) 80, (6) 100.
where νO1 represents the partial molar volume of the alcohol in the hexane phase, νjH1 the mean partial molar volume inherent in the interface, and a1 the partial molar area of the alcohol, respectively. In eq 5 the second term on the right hand side originates from the contribution of water and hexane molecules to ∆ν, which becomes negligibly small in the condensed state, and the a1 value is assumed to be 0.3 nm2 for the FC10OH molecule and 0.2 nm2 for the C18OH molecule. The results obtained for the expanded and condensed states are shown as νjH1 - νO1 vs m1 curves in Figure 7. It should be noted that the νjH1 - νO1 value is negative, except the positive value of FC10-
Figure 9. Values of coefficient of dp of eqs 10 and 11 vs pressure curves: (s) eq 10 (O) eq 11; (1) gaseous T expanded and (2) expanded T condensed.
OH at very low concentration and 80 MPa, and changes discontinuously at the phase transition point. Therefore it is said that FC10OH molecule at the interface has smaller volume than in the bulk solution. We also note that the νjH1 - νO1 value at 0.1 MPa is more negative than that at 80 MPa. This explains why the ∆ν value at a lower pressure is more negative than that at a higher pressure, although the interfacial density at the lower pressure is smaller than that at the high pressure. In Figure 7 is clearly shown the fluorination effect of the alkyl chain of the alcohol molecule on the partial molar volume change of adsorption. At the transition point the magnitude of
13746 J. Phys. Chem., Vol. 100, No. 32, 1996
Takiue et al.
the discontinuous change in νjH1 - νO1 value is the difference in νjH1 values between the expanded and the condensed states, - νjH,e νjH,c 1 1 . It is noted that the values and their pressure dependence of FC10OH are larger than those of C18OH. This result is understood by considering as follows; there exists a number of hexane molecules in the adsorbed film of the expanded state, while only alcohol molecules are virtually present in the adsorbed film of the condensed state. Taking into account that the mixing of hydrocarbon and fluorocarbon compounds is generally accompanied by a large increase in volume,23 therefore it is probable that the transformation from the condensed to the expanded state of FC10OH is attended by a larger increase in νjH1 than that of C18OH. In this respect, we may say that a microscopic circumstance in the adsorbed film is substantially reflected in the volumetric behavior found in this study. The interfacial tension at the break point γeq on the γ vs m1 curves was found to be determined solely by pressure at a given temperature and then written by
dγeq ) (∂γeq/∂T)p dT + (∂γeq/∂p)T dp
(6)
This means that the degree of freedom is diminished from 3 to 2 and suggests that, judging from the phase rule for the systems with interfaces,24 the two kinds of phases coexist in the interface at the point. Now we suppose that the two states, say R and β, coexist and write the equations eq eq dγeq ) -∆sR dT + ∆νR dp - ΓH,R 1 (RT/m1 ) dm1
(7)
eq eq dγeq ) -∆sβ dT + ∆νβ dp - ΓH,β 1 (RT/m1 ) dm1
(8)
where ∆s is the entropy change associated with the adsorption. By solving eqs 7 and 8, we have H,β H,R R H,R dγeq ) - {(∆sβ/ΓH,β 1 - ∆s /Γ1 )/(1/Γ1 - 1/Γ1 )} dT + H,β H,R R H,R {(∆νβ/ΓH,β 1 - ∆ν /Γ1 )/(1/Γ1 - 1/Γ1 )} dp (9)
Similarly the molality at the break point meq 1 is written by eq eq dmeq 1 ) (∂m1 /∂T)p dT + (∂m1 /∂p)T dp
(10)
and eqs 7 and 8 give eq H,β H,R β R dmeq 1 ) - {[(m1 /RT)(∆s - ∆s )]/(Γ1 - Γ1 )} dT + H,β H,R β R {[(meq 1 /RT)(∆ν - ∆ν )]/(Γ1 - Γ1 )} dp (11)
Then, if the coefficients of dp of eq 6 and that of eq 9 are evaluated separately and proved to have equal values, our working hypothesis that the values ∆ν and ΓH1 change discontinuously and therefore the first order phase transition takes place at the break point is substantiated. The parallel discussion is
also applied to eqs 10 and 11. Here we note that our consideration relies on the analogue of the Clapeyron equation of first order phase transition and not on that of the KeesomEhrenfest equation of higher order phase transition.25 The (∂γeq/∂p)T value was obtained from the slope of the γeq vs p curve given in Figure 1 and the coefficient of dp of eq 9 from the ∆ν and ΓH1 values in Figures 3 and 6; they are plotted against pressure in Figure 8. It is said that the agreement is satisfactory for the two types of phase transitions. Furthermore, the coefficients of dp of eqs 10 and 11 were evaluated and shown in Figure 9. The agreement of them is also fairly good for the two types of phase transitions. Therefore, it is probable that the first order phase transition takes place in the adsorbed film of FC10OH at the hexane/water interface. Acknowledgment. The present paper was supported by Grant-in-Aid for Scientific Research (B) No. 06453057 from the Ministry of Education, Science, and Culture. References and Notes (1) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (2) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (3) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (4) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K. J. Colloid Interface Sci. 1992, 150, 331. (5) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K. J. Colloid Interface Sci. 1992, 154, 454. (6) Aratono, M.; Ikeguchi, M.; Takiue, T.; Ikeda, N.; Motomura, K. J. Colloid Interface Sci. 1995, 174, 156. (7) Hayami, Y.; Uemura, A.; Ikeda, N.; Aratono, M.; Motomuta, K. J. Colloid Interface Sci. 1995, 172, 142. (8) Hutchinson, E. J. Colloid Sci. 1948, 3, 219. (9) Stauffer, C. E. J. Colloid Interface Sci. 1968, 27, 625. (10) Lutton, E. S.; Stauffer, C. E.; Martin, J. B.; Fehl, A. J. J. Colloid Interface Sci. 1969, 30, 283. (11) Lin, M.; Firpo, J. L.; Mansoura, P.; Baret J. F. J. Chem. Phys. 1979, 30, 2202. (12) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1978, 51, 2800. (13) Matubayasi, N.; Dohzono, M.; Aratono, M.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1979, 52, 1597. (14) Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Colloid Polym. Sci. 1982, 260, 632. (15) Matubayasi, N.; Motomura, K. Langmuir 1986, 2, 777. (16) Matsuguchi, M.; Aratono, M.; Motomura, K. Bull Chem. Soc. Jpn. 1990, 63, 17. (17) Takiue, T.; Yanata, A.; Ikeda, N.; Hayami, Y.; Motomura, K.; Aratono, M. To be submitted for publication. (18) Matubayasi, N.; Motomura, K.; Kaneshina, S.; Nakamura, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1977, 50, 523. (19) Eduljee, H. E.; Newitt, D. M.; Weale, K. E. J. Chem. Soc. 1951, 3086. (20) Fine, R. A.; Millero, F. J. J. Phys. Chem. 1973, 59, 5529. (21) Motomura, K.; Matubayasi, N.; Aratono, M.; Matuura, R. J. Colloid Interface Sci. 1978, 64, 356. (22) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348. (23) Rowlinson, J. S.; Swinton, F. L. Liquids and Liquid Mixtures, 3rd ed.; Butterworth: London, 1982; Chapter 5. (24) Defay, R.; Prigogine, I. Surface Tension and Adsorption; Everett, D. H., Translator; Longmans: London, 1966; Chapter 6. (25) Prigogine, I.; Defay, R. Chemical Thermodynamics; Everett, D. H., Translator; Longmans: London, 1954; Chapter 19.
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