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Thermodynamic Study of Gaseous Adsorbed Films of Sodium Taurocholate at the Air/Water Interface Norihiro Matubayasi,* Makoto Kanzaki, Satoko Sugiyama, and Asako Matuzawa Faculty of Fisheries, Nagasaki University, 1-14 Bunkyoumachi, Nagasaki, 852 Japan Received October 3, 1995X The surface tension of the aqueous solution of sodium taurocholate was measured in the temperature range 15-35 °C at 2.5 °C intervals and concentration range 0-6 mmol kg-1. Sodium taurocholate was strongly adsorbed and formed a saturated adsorbed film at a dilute concentration. It was found that the gaseous/expanded phase transition does not take place in the film, while it does in those of common ionic surfactants. By means of the thermodynamic equations (J. colloid Interface Sci. 1978, 64, 348), thermodynamic quantities of interface formation were obtained and partial molar thermodynamic quantities are evaluated at the infinite dilution. It was seen that partial molar quantity changes associated with adsorption does not depend on the interfacial density of the adsorbed taurocholate ions. The results indicated that the lateral interaction between sodium taurocholate molecules are not strong enough for a phase transition in the adsorbed film to take place.
Introduction The phase behavior of adsorbed films at fluid/fluid interface is a remarkable feature of molecular interaction.1-3 Thermodynamic analyses focused on the phase transition of films, in the past, has demonstrated that the films of typical surfactants with a long hydrocarbon chain generally are accompanied by the gaseous/expanded transition at air/water interface in a very dilute bulk concentration region.3 To date, studies on the gaseous films adsorbed at fluid/fluid interface have been mostly limited to prove the existence of phase transition and gaseous state of the films. Interpretation of the thermodynamic relationships in the gaseous films, based on detailed experimental work, is of interest, as it provides further information on the molecular interaction not only in the adsorbed film but also in the bulk solution. In this study, we choose sodium taurocholate (NaTC), a trihydroxy bile acid, because its aggregation process differs from that of typical surfactants. This salt forms dimers and oligomers in its early aggregation stage; larger aggregation takes place at higher concentration.4,5 This behavior suggests that the attractive molecular interaction of NaTC molecules is weaken than that of typical surfactants though the peculiar behavior of NaTC in aqueous solution is attributed to the steric arrangement in its micells. This paper concerns a thermodynamic analysis of the adsorption of NaTC at the air/water interface by measuring surface tension as a function of temperature and concentration for a better understanding of the gaseous adsorbed film and of the gaseous/expanded phase transition. Materials and Methods Water was twice distilled from an alkaline permanganate solution. NaTC (Sigma) was purified by passing through the * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. Soc., Jpn. 1978, 51, 2800. (2) Matubayasi, N.; Azumaya, S.; Kanaya, K.; Motomura, K. Langmuir 1992, 8, 1980. (3) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. (4) Kratohvil, J. P.; Hsu, W. P.; Aminabhavi, T. M.; Mukunoki, Y. Colloid Polym. Sci. 1983, 261, 781. (5) Funasaki, N.; Ueshiba, R.; Hada, S.; Neya, S. J. Phys. Chem. 1994, 98, 11541.
Figure 1. Surface tension vs temperature relations at the air/ water interface under fixed concentration: (1) m ) 0.00, (2) 0.456, (3) 0.761, (4) 1.247, (5) 1.626, (6) 2.099, (7) 2.737, (8) 3.546, (9) 4.839, (10) 5.963 mmol kg-1. column of LH20 Sephadex Gel recrystalizing from ethanol solution. The surface tension was measured by the drop volume method using a platinum dropping tip, because the use of glass tip results in a somewhat arbitrary value with time. Wetting condition of the tip was, however, important; when the tip is not clean enough, surface tension values vary with the aging of the interface between dropping tip and solution. The measuring cell was immersed in a thermostated bath and kept within 0.01 °C. Calculation was made by using the correction factors of Lando and Oakley.6 The density data7 of pure water was used instead of that of the solution because its concentration is sufficient low. The density of humid air was taken from the literature.8 The experimental error of this method was less than 0.05 mN m-1.
Results and Discussion Surface tension of the aqueous solution of NaTC was measured in the range 15-35 °C at 2.5 °C intervals and in the range 0-6 mmol kg-1 where aggregation does not occur4 in the aqueous bulk phase. Figures 1 and 2 (6) Lando, J. L.; Oakley, H. T. J. Colloid Interface Sci. 1967, 25, 526. (7) Kell, G. S. J. Chem. Eng. Data 1955, 12, 66. (8) Chemical Society of Japan Kagaku Benran, 3rd ed.; Maruzen: Tokyo, 1984; part 2, pp 2-4.
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Figure 3. Plots of ∆u, ∆f, and T∆s against bulk concentration at 25 °C. Figure 2. Surface tension vs concentration relations under fixed temperature: (1) 15, (2) 25, (3) 35 °C.
illustrate the surface tension (γ) vs temperature (T) relationship at fixed molalities of NaTC solution (m) and the γ vs m relationship at fixed temperatures, respectively. The γ vs T curves are almost linear and their slope decreases gradually with increasing concentration. The γ vs m curves are also almost linear in a dilute concentration region. However their slope decreases gradually with increasing concentration. For typical surface active substances, the gaseous/expanded phase transition of their adsorbed films generally takes place at the air/water interface in the surface tension range of 70 to 65 mN m-1; that is, the γ vs T and γ vs m curves break.3 However, we have observed no break point on both γ vs T and γ vs m curves of the NaTC solution. According to Motomura’s thermodynamic treatment9 the change in surface tension is written as a function of temperature and concentration at constant pressure in the form of
dγ ) -∆sdT - (2RTΓ1H/m) dm
(1)
where ∆s and Γ1H are respectively the entropy change associated with the adsorption and the interfacial density of NaTC. The values of T∆s obtained from a linear γ vs T relation given in Figure 1 are plotted at each concentration in Figure 3 together with the values of the Helmholtz free energy change (∆f) and internal energy change (∆u) associated with the adsorption defined respectively by
∆f ) γ - p∆v
(2)
∆u ) T∆s - p∆v + γ
(3)
and
where p is the pressure and ∆v is the volume change associated with the adsorption. Since the p∆v term is negligibly small1 as compared with the other terms on the right side of eqs 2 and 3, the values of ∆f and ∆u are evaluated by using the values of γ and T∆s. The T∆s value decreases linearly with increasing concentration up to about 1 mmol kg-1 and eventually approaches a constant above 3 mmol kg-1. For ∆u and ∆f, the curves (9) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348.
Figure 4. Dependence of interfacial density, Γ1H, on bulk concentration at 25 °C.
are linear up to the concentration of about 1 mmol kg-1 and then their slopes vary gradually with an increase in m. Applying eq 1 to Figure 2, the Γ1H vs m curves in Figure 4 are obtained. It is seen that NaTC is strongly adsorbed at low bulk concentration in a similar way as typical surfactants such as dodecylammonium chloride10 and sodium dodecyl sulfate.3 The interfacial density, however, immediately reaches a plateau of which the mean area per molecule is 1.3 nm2 at 25 °C. This behavior is in harmony with the previous view that bile salts are not very surface active. It should be emphasized that the increase in concentration brings about the linear variations in ∆u, γ, ∆s, and Γ1H at a dilute concentration region. This linear relation is dependent on temperature; that is, the γ vs m curve departs from the linear relation at the concentration of about 0.5 mmol kg-1 at 15 °C, 0.9 mmol kg-1 at 25 °C, and 1.7 mmol kg-1 at 35 °C. The linear relationship is supposed to suggest that the adsorbed film behaves like an ideal gaseous film in which the interaction between adsorbed ions is insignificant.2 Figure 5 shows the relation between surface pressure (π) and the mean area per adsorbed molecule (A) at 25 °C together with the ideal (10) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32.
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Figure 5. Surface pressure vs area isotherms at 25 °C. The thin line is calculated by πA ) 2RT.
isotherm given by πA ) 2RT. An equation of state of the ionic charged film is reviewed by Adamson;12 a linear relation between γ and m is experimental evidence for the ideal adsorbed film.2 With a decrease in A, the film is found to be much expanded compared with the ideal film and have the limiting area of 1.3 nm2, that is, the area saturated by NaTC molecules. According to the quasithermodynamic treatment of interface, the entropy change associated with the adsorption (∆s) has been shown as
∆s ) Γ1H(s1H - s1w) + ΓwI (swI - swW) + ΓaI (saI - saA) (4) where ΓwI and ΓaI are the numbers of moles of water and air inherent in the interface per unit area.9 (s1H - s1W), (swI - swW), and (saI - saA) are the partial molar entropy changes of adsorption of NaTC, water, and air, respectively. In order to examine numerically what part of ∆s should be attributed to the solute, we define the apparent molar entropy change (∆sΦ) as
∆s ) Γ1H∆sΦ + ∆s°
(5)
where the superscript ° represents that Γ1H is zero. At infinite dilution, ∆sΦ should approach the partial molar entropy change of adsorption, that is, s1H,o - s1W. In Figure 6a ∆sΦ is given against Γ1H at 25 °C. It must be noted that the ∆sΦ value is negative and remains almost constant. This indicates that decrease in ∆s shown in Figure 3 is attributed to the negative change of partial molar entropy accompanied by the adsorption of NaTC molecules which does not depend on the interfacial density and bulk concentration. The apparent molar Helmholtz free energy change (∆fΦ) and energy change (∆uΦ) accompanied by the adsorption are similarly evaluated and plotted against Γ1H in Figure 6b, where the T∆sΦ vs m curve is also drawn. Both ∆uΦ and ∆fΦ values are also found to be negative (11) Roda, A.; Hofmann, A. F.; Mysels, K. J. J. Biol. Chem. 1983, 258, 6362. (12) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons, Inc.: New York, 1990; p 169. (13) Motomura, K.; Matubayasi, N.; Aratono, M.; Matuura, R. J. Colloid Interface Sci. 1978, 64, 356.
Figure 6. (a) Plots of ∆sΦ against interfacial density at 25 °C. (b) Plots of ∆uΦ, ∆fΦ, and T∆sΦ against interfacial density at 25 °C.
and remain almost constant. This finding indicates that the partial molar energy and partial molar Helmholtz free energy changes are not affected by increasing interfacial density or bulk concentration. These results suggest that molecular interaction between NaTC molecules in the gaseous film is insignificant, although the π vs A isotherm deviates from that of the ideal one. The phase transition data can be used to discuss qualitatively the strength of molecular interaction in the adsorbed film. For almost all ionic surfactants which have a long hydrocarbon chain as hydrophobic group, a gaseous/ expanded phase transition is observed to take place at the air/water interface. It has been shown that the attractive force between hydrophobic groups plays an important role in determining the π and m values at which two phases coexist, although the repulsive force between ionic groups is also important.3 On comparison of the limiting area with the cross section of NaTC molecule, the adsorbed film is presumed to be in an expanded state at a high pressure. However, as seen from Figures 1 and 2, the γ vs T and γ vs m curves do not possess break points. Therefore we can say that the adsorbed film of NaTC changes its state from a gaseous to an expanded state without passing through the critical point. Since the occurrence of the phase transition is evidence for the strong lateral interaction in the film, we are forced to conclude that the three hydroxy substituents of NaTC weaken the strong attractive force arising from the rigid structure of the steroid nucleus. This conclusion explains the intriguing aggregation process of NaTC in its bulk aqueous solution. LA950832O
