1108
Langmuir 1999, 15, 1108-1114
Domain Formation in Gibbs Monolayers at Oil/Water Interfaces Studied by Brewster Angle Microscopy Steffen Uredat and Gerhard H. Findenegg* Iwan-N.-Stranski-Institut fu¨ r Physikalische und Theoretische Chemie, Technische Universita¨ t Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany Received September 16, 1998. In Final Form: November 3, 1998 Brewster angle microscopy (BAM) was applied to a study of temperature-induced phase transitions in Gibbs monolayer films of two long-chain alkanols (octadecanol and 1,1,2,2-tetrahydroperfluorododecanol) at the hexane/water interface. Changes of the interfacial tension along the experimental temperature scans were monitored simultaneously with the BAM micrographs by quasielastic light scattering from the interface. In this way it was possible to locate the phase transition temperature Tt in situ from the observed break in the temperature derivative of the interfacial tension. Domains of the condensed phase are seen below Tt, and the morphological features of these domains show remarkable similarities with those observed in Langmuir films at the free surface of water. The coexistence of the condensed phase with a dilute (gaslike or expanded) phase is not limited to a single temperature (as expected for a first-order phase transition) but extends over a temperture range of ca. 15 K below the transition temperature. This finding is attributed to surface-active impurities which will accumulate in the dilute phase of the interfacial film.
1. Introduction Phase transitions in monolayer films of amphiphilic molecules on a liquid subphase are well-known for more than a century. In recent years sensitive optical techniques, such as fluorescence and Brewster angle microscopy (BAM),1,2 have become powerful tools to investigate the morphology of condensed-phase domains at coexistence with expanded or gaslike states of such films. The typical lateral resolution of these methods is in the order of some micrometers. While fluorescence microscopy demands the addition of a suitable tracer to the system under observation, BAM does not require any artifacts of that kind. Fluorescence microscopy has been used to study films of insoluble amphiphiles (Langmuir films) both at the free surface of water and at hydrocarbon/water interfaces.3 BAM has so far been applied to Langmuir films at the free surface of water (see e.g., ref 4) and to films of strongly adsorbed soluble amphiphiles at the surface of their solution (Gibbs films).5-8 Its application to Langmuir films has a major advantage over fluorescence microscopy, as it proved to be sensitive to molecular tilt orientation within domains of the condensed phase. A recent BAM study of Gibbs films at the water surface revealed the formation of domain structures similar to those observed in Langmuir films during the diffusion-controlled adsorption process of the amphiphile.9 A number of amphiphilic substances exhibit phase transitions in Gibbs films not only at the air/water interface but also at oil/water interfaces.10-12 Consider the situation of a long-chain fatty alcohol such as 1-oc(1) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (2) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (3) Thoma, M.; Mo¨hwald, H. J. Colloid Interface Sci. 1994, 6, 340. (4) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (5) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (6) He´non, S.; Meunier, J. Thin Solid Films 1993, 234, 471. (7) Melzer, V.; Vollhardt, D. Phys. Rev. Lett. 1996, 76, 3770. (8) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (9) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591. (10) Motomura, K.; Matubayasi, N.; Aratono, M.; Matuura, R. J. Colloid Interface Sci. 1978, 64, 357. (11) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1978, 51, 2800.
tadecanol, which is soluble in hydrocarbon oils but virtually insoluble in water, at an oil/water interface. In such a three-component, two-phase system the number of thermodynamic degrees of freedom is three, and we adopt temperature T, pressure p, and the concentration of the amphiphile in the oil phase, x, as the independent variables.13 A first-order phase transition in the interface is signified by a break point in a plot of the interfacial tension γ vs one of these independent variables at constant values of the other two variables. In the present work we shall consider temperature scans at constant x and p. For a Gibbs film a discontinuity of the slope dγ/dT at a transition temperature Tt(x,p) can be attributed to a phase transition involving a stepwise change of the adsorbed amount (surface concentration Γ) from low values typical for expanded (or gaslike) films to high values characteristic of condensed monolayer films. For a true three-component system (i.e., perfectly pure amphiphile and solvents), at given x and p, coexistence of the two states of the film is expected only precisely at Tt. In other words, when the temperature is scanned through the transition temperature Tt, two-phase coexistence in the interface will represent a transient phenomenon. In this respect Gibbs films differ from the situation of spread Langmuir films, in which the amphiphilic molecules are pinned to the interface, and thus their surface concentration (or mean area per molecule) represents an additional free variable.14,15 Phase transitions and phase morphologies of adsorbed Gibbs films of slightly water-soluble compounds at the air/water interface have recently been studied by BAM.9 On the other hand, no direct evidence of condensedphase domains in phase transitions of Gibbs films at oil/ water interfaces has been reported so far, although such insights into phase morphologies of adsorbed films at (12) Hayami, Y.; Uemura, A.; Ikeda, N.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1995, 172, 142. (13) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348. (14) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966; Chapter VI. (15) Lutton, E. S.; Stauffer, C. E.; Martin, J. B.; Fehl, A. J. J. Colloid Interface Sci 1969, 30, 283.
10.1021/la981264q CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
Domain Formation in Gibbs Monolayers
Figure 1. Schematic setup of the microscope.
liquid/liquid interfaces are desirable in view of the great relevance of oil/water interfaces in various technological processes. To elucidate morphological details of such systems, we have developed a BAM for investigations of liquid/liquid interfaces. In this instrument the sample is contained in a sealed glass cell, which does not permit simultaneous measurements of the interfacial tension by conventional methods. Instead, the technique of quasielastic light scattering from interfaces (capillary wave spectroscopy16) was used to monitor in situ the temperature dependence of the interfacial tension without pertubating the interface. In this paper we report results for two long-chain 1-alkanols, viz., octadecanol (C18OH) and 1,1,2,2-tetrahydroperfluorododecanol (FC12OH), at the hexane/water interface. For both systems the monolayer phase behavior of the adsorbed Gibbs film has been studied by the Motomura group11,12 by interfacial tension measurements over an extended temperature range. The phase transition in the C18OH system was described as a transition from liquid expanded to a condensed phase,11 and the phase transition in the FC12OH system was described as a transition from gaslike to condensed.12 It was of interest to study the morphological aspects of these transitions by the new technique. 2. Experimental Section Figure 1 shows the setup of the present instrument which combines three different features: (i) Brewster angle microscopy with a lateral resolution of 10 µm; (ii) the measurement of the integral reflectivity of p-polarized light; (iii) a capillary wave spectrometer to monitor changes of the interfacial tension with temperature. The optical setup of the BAM is similar to that developed by He´non and Meunier1 to investigate monolayers at the free surface of water, but it is adapted to the Brewster angle of oil/water systems (θB ) 44.2° for the hexane/water interface at 25 °C). The general setup of the instrument for investigations at ambient temperature is described elsewhere.17 Here we focus on modifications that had to be made for temperature-dependent studies. The two-phase liquid sample is contained in an optical cell (Hellma) with four polished windows, with the interface to (16) Langevin, D. In Light scattering by liquid surfaces and complementary techniques; Langevin, D., Ed.; Marcel Dekker: New York, 1992; Chapter 2. (17) Uredat, S.; Findenegg, G. H. Colloids Surf. A 1998, 142, 323.
Langmuir, Vol. 15, No. 4, 1999 1109 be studied placed exactly in the center plane of the sample cell. The cell has a quadratic inner cross-section of 2 cm × 2 cm and a length of 5 cm. It is sealed by a stainless steel plate, using an oil-resistant rubber seal. The cell is in thermal contact with a copper block, which is thermostated by a water bath, allowing a control of the sample temperature in a range from 5 to 45 °C. A Pt100 sensor, immersed in the aqueous phase, allows temperature measurements inside the cell. The expanded beam of a 50 mW continuous wave Nd:YAG laser (λ ) 532 nm) is directed to the interface by means of a mirror, passing the cell windows almost at right angles. A GlanThompson prism is used as a polarizer to ensure that the incident beam is p-polarized. The reflected beam is imaged to a CCD camera by a microscope lens. A rotatable dichroic sheet polarizer is placed as an analyzer between the lens and camera. In addition to the images the mean intensity of the reflected light (Ir) was measured by averaging the intensity over the entire sensing area of the camera. The thermostat and the camera are connected to a computer, allowing automatic temperature variation and storage of the microcraphs to a hard-disk. The capillary wave spectrometer uses the incident laser beam of the BAM as the light source butsunlike the conventional surface light scattering geometrysthe light scattered by the interface is detected in transmission geometry, i.e., around the transmitted beam (see Figure 1). Due to the temperature dependence of the refractive indices of the coexistent liquid phases, the vertical position of the spots of the refracted and scattered light in the detection plane is varying with temperature in accordance with Snell’s law. To account for this effect, the light-sensitive detector is attached to a position-sensitive photoresistor and mounted on a motor-driven positioning table. The position-sensitive photoresistor is arranged in such a way that it meets the transmitted beam in the detection plane. An electronic controller adjusts the position of the device in such a way that its resistance remains constant. In this way it is ensured that the light-sensitive detector remains always in the same position relative to the transmitted beam, and thus the scattering vector q of the scattered light remains constant when the sample temperature is changed. The capillary wave spectra were fitted by a single Lorentzian function. For samples exhibiting high interfacial tension and low viscosities of the two coexisting phases, as in the present situation, and neglecting instrumental broadening effects, the center frequency of the Lorentzian function, νmax, is related to the interfacial tension by16
νmax )
1 2π
x
γq3 F1 + F2
(1)
where F1 and F2 denote the mass densities of the two bulk phases. Hence by monitoring νmax one has a direct measure of the interfacial tension γ in an experiment.
3. Materials A sample of 1,1,2,2-tetrahydroperfluorododecanol (FC12OH) was supplied by D. Prescher, University of Potsdam/Germany. The octadecanol (C18OH) was purchased from Fluka (>99% purity) and used without further purification. Hexane was purchased from Merck (p.a., >99% purity) and further purified by passing through a chromatographic column filled with basic alumina. Water was taken from a Milli-Q 50 water supply system (Millipore). 4. Results As mentioned in the Introduction, accurate measurements of the interfacial tension of hexane solutions of FC12OH and C18OH against water have been reported by the Motomura group.11,12 For both alcohols the graphs of γ vs T for given alcohol concentrations in the hexane phase (mole fractions x) exhibit a pronounced break point which is a signature of the interface phase transition, and the transition temperature Tt was found to increase with
1110 Langmuir, Vol. 15, No. 4, 1999
Figure 2. Interfacial tension (measured by the pendant drop technique) vs temperature: 0, FC12OH (x ) 7.3 × 10-5); b, C18OH (x ) 1.21 × 10-3); ×, pure hexane/water interface.
Figure 3. Reflected intensity Ir (9, 0) and peak frequency νmax of capillary wave spectra (b, O) of a Gibbs film of FC12OH (x ) 7.3 × 10-5) at the hexane/water interface vs temperature T: full symbols, stepwise decrease of T; open symbols, stepwise increase of T.
increasing alcohol concentration in the bulk phase. These authors11,12 have presented a detailed thermodynamic analysis of their data, including the adsorption isotherms Γ(x) and π-A isotherms. We have repeated the interfacial tension measurements γ(T) for selected concentrations x, using the pendant drop technique. Typical temperature scans γ(T) for fixed mole fractions of FC12OH and C18OH are shown in Figure 2. For C18OH these results agree within experimental accuracy with those given in the literature.11 For FC12OH, however, the interfacial tension γ(T) for given sample concentrations are significantly lower (by ca. 6 mN/m) and the break points are shifted downward by ca. 4 K relative to the results reported by Hayami.12 These shifts indicate that the sample of FC12OH used in our study contains surface active impurities. For an ideal first-order phase transition from a dilute (gaslike or liquid expanded) film to a condensed monolayer, one expects a stepwise increase of the reflectivity Ir of p-polarized light at the transition temperature Tt, from the value for the dilute film to that of the uniform condensed layer. However, such a behavior was not found for the present systems. Figure 3 shows results for Ir and νmax (a measure of the interfacial tension, see eq 1) as a function of temperature for a sample of FC12OH (x ) 7.3 × 10-5), measured along a stepwise temperature ramp in a temperature range above and below the expected transition temperature Tt. Temperature steps of 1 K where
Uredat and Findenegg
chosen, and the system was allowed to equilibrate for 30 min after each step. For this sample νmax does not vary significantly in the temperature range above 24 °C. Below this temperature further cooling causes a decrease, while heating envokes an increase of νmax. These changes in νmax vs T resemble the dependence of γ vs T observed in the pendant drop experiment (Figure 2). Incidentally, it was generally observed that νmax exhibits a more regular temperature dependence along the positive temperature ramp as compared with the negative ramp (stepwise decrease of T). Interfacial tensions calculated from the capillary wave spectra by a more sophisticated analysis17 than that indicated by eq 1 gave values which agree with those obtained by the pendant drop measurements within an experimental error of 5%. No attempt was made to improve the accuracy of the in situ measurement of the interfacial tension since all adjustments of the cuvette where made in order to get good BAM images from the interface, which implies some variance of q for different samples. Therefore we present the results obtained by capillary wave spectroscopy only in the form of νmax(T). For the sample of Figure 2 the transition temperature Tt is located at 24 °C. Above this temperature the interface exhibits a weak reflectivity, as to be expected for a gaslike film of low (excess) optical density. As the temperature is lowered below Tt the reflected intensity Ir increases roughly proportional to the temperature increment Tt T. This increase of Ir is attributed to the formation of the condensed monolayer phase as proposed by Hayami et al.12 As will be shown below, the condensed phase does not form as a uniform film at Tt, but the area covered by the condensed phase increases gradually as temperature is decreased. These gradual changes of Ir below Tt occur in an analogous way during the positive temperature ramp (see Figure 3). The temperature-dependent behavior of νmax and Ir shown in Figure 3 was observed for all samples of FC12OH and C18OH that should undergo a first-order phase transition according to the results of the Motomura group. The morphological features of the adsorbed films of FC12OH and C18OH at the hexane/water interface as revealed by BAM observations will now be presented. 4.1. FC12OH. Figure 4 presents a sequence of BAM micrographs showing different states of the Gibbs film of FC12OH during the temperature scan documented in Figure 3. The letters a-f refer to the respective letters in Figure 3. Parts a-d of Figure 4 represent domain patterns along the descending scan, all of which correspond to temperatures below the phase transition (Tt ) 24 ( 1 °C). One notes that at 21 °C (4 K below Tt) small domains of condensed film exist at the interface, apparently in coexistence with a gaslike state of the monolayer film (Figure 4a). These domains tend to form closer packed ordered arrays as temperature is decreased (Figure 4b). Further decrease of temperature causes a gradual increase of the size of individual domains and an increasing portion of the interface covered by the condensed phase, but only at the lowest experimental temperature (13 K below Tt) the interface appears to be covered completely by a uniform film of the condensed phase (Figure 4d). Parts e and f of Figure 4 show states of the Gibbs film along the ascending temperature scan (see Figure 3): Figure 4e reveals the formation of “holes” of gaslike film within the condensed phase still well below the phase transition (Tt - T ≈ 10 K). Finally, at the nominal phase transition temperature, small domains of the condensed phase (diameter ca. 10 µm) coexist with the gaslike phase (Figure 4f) and these domains disappear as the temperature is increased further. All these domain patterns are stable at a given
Domain Formation in Gibbs Monolayers
Langmuir, Vol. 15, No. 4, 1999 1111
Figure 4. BAM images of an adsorbed film of FC12OH at different temperatures: (a) 21 °C, (b) 20 °C, (c) 15 °C, (d) 12 °C, (e) 15 °C, (f) 23 °C; (a-d) stepwise decrease of T; (e, f) stepwise increase of T.
temperature. Specifically, the regular pattern of small domains shown in Figure 4b was observed over a period of several days. Domain patterns such as those exhibited in Figure 4 where found in the whole range of sample concentrations (mole fractions of FC12OH in the hexane phase from 5.5 × 10-5 to 1.1 × 10-4). As the phase transition temperature Tt increases with the concentration of the alcohol, the evolution of domain patterns is also shifted to higher temperatures with increasing sample concentration, indicating that the size and shape of domain structures depend predominantly on the increment Tt - T. 4.2. C18OH. As for the FC12OH system, the temperature derivative of the interfacial tension of hexane solutions of C18OH against water exhibits a pronounced discontinuity at the respective phase transition temperature Tt(x). However, unlike the former system, the derivative (∂γ/∂T) is still positive at temperatures above Tt (see Figure 2). This has been taken as an indication that in this system the phase transition occurs from the condensed phase (at T < Tt) not to a gaslike but a liquid expanded phase (at T > Tt).11 It is also noticed that C18OH has a significantly higher solubility than FC12OH in the hexane phase. We have studied several samples of this system with C18OH mole fractions in hexane from 1.2 × 10-3 to 1.9 × 10-3, in a temperature range down to the respective solubility limit Ts(x), i.e., the temperature at which solid alcohol appears in the hexane phase. Generally, the domains of condensed phase of C18OH at the hexane/water interface are considerably larger than those for FC12OH, typically of the order of 30 µm, and thus a greater variety of morphological features could be resolved with the present BAM instrument. Some of these features are presented in Figures 5-7. All micrographs shown in these figures result from a sample of mole fraction
x ) 1.21 × 10-3, for which Tt is 24 ( 0.5 °C (see Figure 2) and Ts ≈ 10 °C. Figure 5 shows the evolution of domain patterns of the Gibbs film of C18OH at the hexane/water interface along a descending temperature scan. As in the case of FC12OH, distinct domains of condensed phase coexist with regions of the dilute (liquid expanded) phase down to temperatures well below Tt. Domains of approximately circular shape are dominating at temperatures down to Tt - T ≈ 12 K (Figure 5a), but continuous regions of condensed phase coexist with these circular domains at lower temperatures (Figure 5b). It is of interest that these two types of domains coexist over periods of several days, although the shape of the continuous regions is strongly influenced by convective flow within the interface, revealing a fluidlike behavior of these regions of condensed phase. As an example, Figure 5c shows bandlike shapes of continuous structures and highly deformed separate domains oriented along the direction of flow within the interface (from bottom to top in Figure 5c). Figure 5 again shows that the fraction of interfacial area covered by the condensed phase increases as temperature is decreased. Down to Tt - T ≈ 12 K the mean coverage was essentially time-independent, but as temperature was further decreased, the entire interfacial area was gradually covered with the condensed phase. Parts c-f of Figure 5 show this time evolution of the interface after lowering the temperature from 12 to 10 °C. The micrographs reveal that the individual domains tend to assume polyhedral shape as the distance between neighboring domains decreases. After approximately 2 h almost the entire interface is covered by a film of condensed phase although the domain boundaries are still clearly visible. Finally, a fast formation of three-dimensional crystals commences: Within a few seconds the whole image area
1112 Langmuir, Vol. 15, No. 4, 1999
Uredat and Findenegg
Figure 5. C18OH at the hexane/water interface, domain structures along a descending scan below the phase transition temperature (Tt ) 24 °C): (a) 15 °C; (b) 12 °C; (c-f) 10 °C, illustrating the evolution of domain patterns as a function of time after the temperature step from 12 to 10 °C for (c) 45 min, (d) 100 min, (e) 108 min, and (f) 111 min. The white bar represents a length of 100 µm.
Figure 6. C18OH at the hexane/water interface: Evolution of domain structures after taking the sample of Figure 5 from a temperature of 10 °C (cf. Figure 5f) to 14 °C (a) after 25 min and (b) after 120 min.
becomes shining bright, indicating that the entire interface is being covered by a 3D crystalline layer. The in situ observation of a crystallization front advancing within the plane of the interface proves that the crystalline layer is formed directly at the interface and is not caused by precipitation from the bulk phase. Figure 5f shows a snapshot of the advancing crystallization front. Domain patterns occurring after an increase of the temperature from 10 to 14 °C (Tt - T ≈ 10 K) are shown in Figure 6. After the disappearance of the threedimensional crystals (cf. Figure 5f), the morphological features of the Gibbs film can again be seen. As in the descending temperature scan, coexistence of individual domains and continuous regions of condensed phase with regions of liquid-expanded phase are observed. In particular, holes of the expanded phase within the condensed matrix can be seen next to circular domains of condensed phase within the expanded phase.
BAM studies of Gibbs films5 and Langmuir films4,18 at the air/water interface have proved that BAM is sensitive to the orientational order of molecules within tilted phases. The image of the interface exhibits maximum brightness for regions in which the tilt direction is parallel to the analyzer. In our work a rotation of the analyzer did never reveal such morphological features within the domains of condensed phase. Accordingly, our results give no hints at regions of uniform molecular tilt within the domains. However, the contrast between regions of condensed and expanded phase can be inverted by a rotation of the analyzer, as shown in Figure 7. In all cases a rotation of the analyzer revealed only two distinct states within the interfacial film. It is unlikely that these two states correspond to different tilt orientations, as this would imply that only two tilt orientations exist. Instead, the (18) Weidemann, G.; Vollhardt, D. Langmuir 1996, 12, 5114.
Domain Formation in Gibbs Monolayers
Langmuir, Vol. 15, No. 4, 1999 1113
Figure 7. C18OH at the hexane/water interface: Contrast inversion resulting from a rotation of the analyzer.
two states are believed to represent the condensed phase and the dilute (expanded or gaslike) phase, respectively. We can distinguish between these two states using the criterion that the area covered by the condensed phase will increase with decreasing temperature. Accordingly, in Figures 4-6 and 7a the brighter regions represent the condensed phase. This assignment is consistent with the observation that condensed phase regions were always brighter than the dilute phase regions when the analyzer was positioned parallel to the plane of incidence. We attribute the observed contrast inversion caused by a rotation of the analyzer to an incomplete p-polarization in the incident light (presumably due to birefringence of the cell windows and the effect of adsorption of the alcohol at the hydrophilic glass wall), which causes the BAM instrument to perform like an imaging zero-ellipsometer. This contrast inversion strongly supports our conjecture that the observed domain patterns result from structures having a thickness of molecular length; otherwise, if the layer thickness of the interfacial structures would vary beyond this scale, no contrast inversion would occur because the layer thickness and the angle of incidence would vary over rough or curved structures. 5. Discussion The domain patterns observed in the present Gibbs films at the hexane/water interface resemble in several respects those of Langmuir films at the air/water interface. This similarity is brought out by a comparison of the present results with compression-decompression cycles of Langmuir films of tetradecane at the water surface.4 In those experiments a compression of an expanded film leads to a uniform condensed film via a two-phase coexistence region which exhibits domain patterns of the condensed phase. On decompression of the film the formation of holes of expanded phase within domains of condensed phase was observed. This similarity of the observed domain patterns suggests a close relationship between Langmuir films and the Gibbs films presented in this work. Apparently a decrease of temperature in the present systems is equivalent to a compression of a Langmuir film and vice versa. However, as explained in the Introduction, such a behavior is not expected for a first-order surface phase transition of a Gibbs film of pure amphiphiles, for which two-phase coexistence is expected to occur only at a single temperature Tt(x,p). Contrary to this expectation the BAM micrographs show the coexistence of condensed-phase domains with either a gaslike or an expanded monolayer phase over an extended temperature range below the
transition temperature Tt as determined from the break in the temperature derivative of the interfacial tension. As these domain structures persist over long periods of time at constant temperature and occur in similar form during descending and ascending temperature scans, they are believed to represent true thermodynamic equilibrium states of the film. A likely explanation for this apparent violation of the phase rule is that the alcohols contain surface-active impurities which must be regarded as additional components and thus increase the thermodynamic variance of the system.14,15 Assume that the alcohol (A) contains a single surface active impurity (B) which is insoluble in the condensed surface phase of A, while A and B form an ideal mixture in the dilute (gaslike or expanded) surface phase. In this case the mole fraction of the impurity in the dilute surface phase (xBs) is given approximately by
-ln(1 - xBs) ≈ xBs )
∆H (T ° - T) RTt°T t
(2)
where ∆H is the molar enthalpy of the surface phase transition of pure component A and Tt° is the transition temperature in the absence of the impurity. According to this simple relation the mole fraction of the impurity in the dilute phase must increase as T is decreased. Now, when temperature is lowered both components will be adsorbed from the subphase, but while the majority component (A) can be accommodated in the condensed phase, the impurity (B) remains in the dilute phase. Accordingly, as the temperature is lowered the fraction of surface covered by the condensed phase will increase while in the remaining part of the surface the mole fraction of the impurity will increase in accordance with eq 2. Above it has been assumed that the impurity comes with the alcohol, but amphiphilic impurities contained in the oil or water would cause exactly the same effects. In fact, oil/ water interfaces are even more sensitive to such impurities in the majority components in view of the small amounts of alcohol in the present systems. Whereas eq 2 applies to a simple first-order phase transition in the interfacial film, one may take into account explicitly that the condensed phase is a structured phase in itself and thus the chemical potential will depend on the size of the domains. Such an ansatz has been used recently by Fainerman, Vollhardt, and Melzer19 to explain the observed gradual increase of the film pressure in the (19) Fainerman, V. B.; Vollhardt, D.; Melzer, V. J. Phys. Chem. 1996, 100, 15478.
1114 Langmuir, Vol. 15, No. 4, 1999
two-phase region of the Π-A isotherms of Langmuir films. However, the present results are not sufficiently elaborate to justify a quantitative analysis of the two-phase coexistence region. 6. Conclusions The present work has shown that the BAM technique can be adapted to study temperature-induced phase transitions of Gibbs monolayer films at oil/water interfaces. Changes of the interfacial tension along the experimental temperature scans can be monitored simultaneously with the BAM micrographs by the technique of quasielastic light scattering from the interface (capillary wave spectroscopy). In this way it was possible to locate the phase-transition temperature Tt of the Gibbs film in situ for any given sample concentration from the observed break in the temperature derivative of the interfacial tension. For the two long-chain alcohols studied in this work, FC12OH and C18OH, it was verified that the appearance of domains of condensed phase coincides with the phase transition temperature as determined from the break point of the interfacial tension in previous studies.11,12 The morphological features observed in the two-phase region of the Gibbs films below Tt are similar in many respects to those observed in Langmuir films of insoluble lipids during compression/expansion cycles at the free water surface. Specifically, we observe circular domains and continuous regions of the condensed phase, as well as holes of the dilute phase within the condensed phase. The shape of small domains and larger patches of condensed phase
Uredat and Findenegg
is rather flexible and is affected significantly by convective flows within the interface. On the other hand, the present work gives no hints of the formation of subdomains of uniform molecular tilt within the condensed phase (as have been observed, for example, in circular domains at the free water surface). Further work is needed to clarify if such subdomains do indeed not exist at oil/water interfaces. A significant finding of the present work is that the two surface phases can coexist over a rather wide temperature range below Tt. From the observed mean intensity of reflected light it is seen that the fraction of the interface covered by the condensed phase increases in an approximately linear way with the temperature increment Tt - T. Such a behavior is not expected for a true firstorder phase transition of the Gibbs film. It is attributed to surface-active impurities which accumulate in the dilute surface phase of the interfacial film and can be rationalized in a qualitative manner in terms of a surface eutectic behavior of the alcohol and the impurity. It is anticipated that the coexistence of patches of condensed phase and dilute phase below the nominal phase transition temperature is a characteristic property of real (nonideal) interfaces which invariably contain surface-active impurities. This conclusion may be of importance for the understanding of the mass transport across interfaces in such systems. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (Grant No. Fi 235/11-3). LA981264Q
