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Condensed structure formation in mixed monolayers of anionic surfactants and 2-hydroxyethyl laurate at the air–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects 205 (2002) 249– 260 www.elsevier.com/locate/colsurfa
Condensed structure formation in mixed monolayers of anionic surfactants and 2-hydroxyethyl laurate at the air–water interface Md. Mufazzal Hossain a, Md. Nazrul Islam a, Tomomichi Okano b, Teiji Kato a,* a
Department of Applied Chemistry, Faculty of Engineering, Utsunomiya Uni6ersity, Yoto 7 -1 -2, Utsunomiya 321 -8585, Japan b Better Li6ing Laboratory, Lion Corporation, Tokyo, Japan Received 7 August 2001; accepted 29 January 2002
Abstract A first-order phase transition has been studied by Brewster angle microscopy in mixed monolayers of 2-hydroxyethyl laurate (2-HEL) and two anionic surfactants, sodium 3,6,9,12-tetraoxa-octacosanoate (TOOCNa) and sodium 3,6,9,12-tetraoxa-triacontanoate (TOTCNa) at the air–water interface. Condensed phase domains are observed on the surface of a mixed solution containing 1.0 × 10 − 5 M each of 2-HEL and of TOOCNa or TOTCNa up to 20 °C. This concentration of 2-HEL is much lower than that required for the phase transition in the pure system at the studied temperatures. The phase transition pressures (yt) of all the systems increase with increasing temperature. The yt of 2-HEL-TOOCNa system almost corresponds to that of pure 2-HEL at a definite temperature. In contrast, the yt of 2-HEL-TOTCNa system is lower than the corresponding value of 2-HEL. The shapes and the textures of the domains formed in the mixed system are different to those formed in the pure system. The formed domains in the mixed system have a circular shape with internal segments whereas those of 2-HEL have a fingering pattern with an optical isotropy at the same temperature. A large variety of defects are found in the domains formed in the mixed system. Furthermore, these domains undergo fusion while touching each other. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Brewster angle microscopy; Gibbs monolayer; Mixed monolayer; Phase transition; Line tension
1. Introduction Two-dimensional (2D) phase behavior of amphiphiles in Gibbs monolayers has been of growing interest. The amphiphile molecules undergo * Corresponding author. Tel.: +81-28-689-6170; fax: + 8128-689-6179. E-mail address: [email protected] (T. Kato).
continuous adsorption from an aqueous solution to form Gibbs monolayers that is in equilibrium with the bulk. A change in phase in these monolayers dramatically changes the interfacial properties such as surface tension [1], surface viscosity [2] and foam stability [3]. Thus, a phase transition in these monolayers can distinctly influence the field of application of the surfactants. For instance, the stability of an emulsion against coales-
0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 0 2 4 - 9
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cence and creaming can be enhanced by using a surfactant capable of forming a liquid crystalline phase [4]. Although, a little is known about the phase behavior of water soluble surfactants of medium chain length, the existence of first-order gas-liquid expanded (G-LE) [5– 8], LE-liquid condensed (LE-LC) [9 – 12] and G-LC [13] phase transitions in Gibbs monolayers is now well-established. In most of the literature, a cusp point in the surface pressure-time (y – t) curve followed by a plateau region was considered as an evidence for the first-order phase transition. A second-order phase transition has also been found to occur in Gibbs adsorption layers [13,14]. The morphology of the condensed domains in these monolayers is known to respond to the experimental variables similarly as those reported for Langmuir monolayers [15– 17]. Moreover, Vollhardt et al. [18,19] studied theoretically the adsorption kinetics considering phase transition and determined the 2D crystal structure in Gibbs monolayers by grazing incidence X-ray diffraction (GIXD) measurement. Extensive research has also been performed on mixed surfactant systems, since they show superior performance compared with single surfactant alone. So far the properties of mixed monolayers have been inferred by surface tensiometry [20], neutron reflection [21], light scattering [22] and ESR spectroscopy [23]. Most of the systems studied to date have appeared highly disordered and liquid-like. Shah et al. [24,25] proposed that when two surfactants are mixed with a molar ratio of 1:3 or 3:1, the properties of the surfactants are changed strikingly due to 2D hexagonal packing of molecules. Bain et al. [26– 28] investigated the phase transitions in the mixed monolayers of anionic or cationic surfactants and dodecanol or tetradecane on the solution surfaces by sum-frequency spectroscopy and ellipsometry. Their observation demonstrated that conformationally ordered condensed phase exists in those mixed monolayers. These results lead us to pose the question whether a morphological or a textural change of condensed phase would occur during the phase transition in the mixed systems. In addition, all of the above-cited methods characterize the mixed monolayers indirectly. One of the
goals of this paper is to apply microscopic techniques to achieve direct information about the morphology and the texture of the condensed phases that are formed during the phase transition in the mixed surfactant systems. To date, to the best of our knowledge, no report details morphological features in mixed monolayers. The application of sensitive microscopic techniques, particularly Brewster angle microscopy (BAM) [29,30] has revealed a shape variety or a texture variety of condensed phases in both Langmuir and Gibbs monolayers of pure amphiphiles. Using BAM, Henon et al. [13] carried out experiments on aqueous solutions of sparingly soluble hexadecanoic acid and its mixture with sodium octanoate and observed a first-order G-LC phase transition. They also found approximately the same behavior of the monolayers for both the pure and the mixture of the surfactants and concluded that the monolayer formation in the mixed system is mostly due to the hexadecanoic acid. Vollhardt et al. [31–33] studied the penetration dynamics of protein from aqueous solution into a fluid-like Langmuir monolayer of insoluble amphiphiles and found a first-order phase transition to a condensed phase. These condensed phases have morphological features similar to those of pure insoluble monolayer component indicating that the condensed domains consist of the insoluble amphiphiles. In a recent paper [34] we have showed that two water-soluble surfactants form LC domains in an appropriate mixture. We chose 2-hydroxyethyl laurate (2-HEL) and sodium 3,6,9,12-tetraoxa-octacosanoate (TOOCNa) as amphiphiles. The experiments were carried out mainly by changing the molar ratio of the surfactants at 15 °C. In this article we study the phase behavior of the mixed surfactants in details by changing the temperature. To show the effect of chain length on the phase transition pressures (yt), TOOCNa is replaced by sodium 3,6,9,12-tetraoxa-triacontanoate (TOTCNa) having two more CH2 groups in the alkyl chain. The structures of the amphiphiles used in this study are shown in Fig. 1. We further show that the morphological and the textural features of the domains formed in the mixed systems are different to those of pure 2-HEL.
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tion of 2-HEL than that necessary for the phase transition in pure 2-HEL at the same temperature. The horizontal line in the figure indicates the experimental concentration of 2-HEL used during mixed monolayer formation.
2. Experimental section Fig. 1. Chemical structures of the surfactants used in this study: (a) 2-HEL; (b) TOOCNa; and (c) TOTCNa.
The amphiphile, 2-HEL shows a first-order LELC phase transition up to 25 °C provided that the surface concentrations are sufficiently high [12,17]. Fig. 2 shows the minimum concentration necessary to provide sufficient 2D surface concentration for the phase transition to occur at different temperatures. However, when two surfactants are mixed in appropriate molar ratios, condensed domains are formed by a much lower concentra-
Fig. 2. A plot of minimum concentration (Ct) necessary for the phase transition in pure 2-HEL against temperature. The line was drawn to guide the eye. The data in this figure show a scattering of the order of 90.1× 10 − 5 M at 5 °C. The scattering increases with increasing temperature and shows a value of 9 0.2 ×10 − 5 M at 25 °C. The horizontal line indicates the concentration of 2-HEL used in the mixed monolayer systems.
The amphiphile, 2-HEL was synthesized and purified (]99.5%) following a procedure described elsewhere [35]. Both TOOCNa and TOTCNa were obtained from Lion Corporation, Japan with a purity of above 99% and used without further purification. Unless otherwise stated, the experiments were carried out with an aqueous solution of 2.0×10 − 5 M. To study the phase behavior in the mixed system, a 1:1 mixture of the surfactants was prepared. The solutions of the amphiphiles were prepared separately and then mixed in appropriate volume ratio to attain the 1:1 molar ratio. Ultra-pure water of resistivity 18 MV-cm was used throughout the present study. The instrumental set-up [36] and the experimental procedure [37] were detailed elsewhere and are described here briefly. The surface pressure-time (y– t) curves were measured in a very shallow type Langmuir trough (2 mm depth) with a surface area of 41× 16 cm2. The trough was equipped with a Wilhelmy balance for surface pressure determination and a temperature control system. A small glass plate was used as a Wilhelmy plate. A BAM, mounted on the trough, was used to observe morphology of the monolayers. A beam of p-polarized light from a 20 mW He–Ne laser was directed to the air–water interface at Brewster angle, giving a minimum surface reflectivity. Under such circumstances, coexistence of two phases with different densities was readily observed. Optical anisotropy caused by the differences in molecular orientation in the monolayer is visualized by introducing of an analyzer in the path of the reflected beam. An aqueous solution (either of pure surfactants or their mixtures) was poured into the trough and maintained for 25 min to attain the experimental temperature. The surface of solution was swept
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Fig. 3. y – t adsorption kinetics of 2.0 × 10 − 5 M aqueous solution of 2-HEL (I) and TOOCNa (II) separately and their mixture (III) containing the components at a molar ratio of 1:1 at 15 °C. The arrows indicate the position of the cusp points in the respective curves.
rapidly by the movable teflon barriers and the surface pressure was started acquisition into a computer. The BAM images were recorded with time and stored in a video recorder. Image processing software was used to correct the distortion and to optimize the contrast of the BAM images.
3. Results and discussion
3.1. Inference of phase transition from y– t measurements Condensed domains are not formed in Gibbs adsorption layers even with ] 2.0 × 10 − 5 M solutions of TOOCNa while the critical micelle concentration of this amphiphile is :0.5 ×10 − 5 M at 15 °C. In such a system, an increase in the bulk concentration only results in an increase in the micelle concentration but does not increase the 2D surface concentration. A lower extent of surface concentration causes only a fluid-like phase in Gibbs monolayers. The widely accepted reason behind the absence of densely ordered state in such a monolayer is the hydration, the dipole– dipole repulsion, and the electrostatic repulsion among the long hydrophilic head groups. Fig. 3 shows y –t adsorption kinetics of 2.0× 10 − 5 M
aqueous solution of both 2-HEL and TOOCNa separately and their mixture containing the components at a molar ratio of 1:1 at 15 °C. This concentration is sufficient to form condensed domains when 2-HEL is used alone (shown in Fig. 2). A cusp point followed by a plateau region for the phase transition is indicated in the y– t curve of 2-HEL. This cusp point becomes sharp, if the bulk concentration is high. The rate of adsorption of TOOCNa is faster than that of 2-HEL. The net rate of adsorption for the mixed solution is in between the pure systems. This suggests that both of the surfactants undergo adsorption simultaneously. The y–t curve of the mixture in Fig. 3 shows the existence of a cusp point that is followed by a well-developed plateau region. These are the characteristics of a first-order phase transition in Gibbs monolayers [9–17]. Although the cusp point at which phase transition begins is not well pronounced, however, the simultaneous BAM images indicate clearly the existence of the first-order phase transition. The time necessary for appearing the cusp point in the mixed system is about half of that necessary for 2-HEL. Under these conditions, the concentration of 2-HEL is 1.0× 10 − 5 M which is much lower than that necessary for the formation of condensed domains in pure 2-HEL at 15 °C (: 1.8× 10 − 5 M from Fig. 2). However, the phase transition occurs even on the surface of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 2:3 [34]. In this case, the concentration of 2-HEL is only 0.8× 10 − 5 M. All of these results lead us to conclude that the existence of the phase transition in these systems is due to the interaction of both of the surfactants. The surface pressure of the mixed solution at equilibrium is greater than that of the pure surfactants indicating tight packing of the molecules in the mixed system. Similar results are also observed during equilibrium surface tension measurement of the pure and the mixed systems. The surface tension of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 is less than that of the pure systems. The y–t adsorption profiles of 2.0× 10 − 5 M mixed solutions containing the components at a molar ratio of 1:1 at different temperatures are shown in Fig. 4. All of these curves show the
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typical nature of the existence of the first-order phase transition in Gibbs monolayers. In the case of the y –t curve at 20 °C, the concentration of 2-HEL is about one-third of the minimum concentration necessary for the phase transition in pure 2-HEL (:2.8 × 10 − 5 M from Fig. 2). Again, a clear effect of the TOOCNa molecules on the existence of the phase transition in the mixed monolayers can be concluded. It is very likely that the incorporated 2-HEL molecules shield the repulsive interactions among the long hydrophilic head groups of TOOCNa molecules and thereby allow the existence of high density phase in the mixed monolayer systems. The existence of the phase transition in the mixed monolayers becomes further evident when TOOCNa is replaced by TOTCNa, a surfactant having two more CH2 groups in the hydrophobic tail. Fig. 5 shows the y– t adsorption kinetics at different temperatures. Although, a phase transition was found to exist on the surface of 2.0× 10 − 5 M mixed solution, we carried out the experiment using a mixed solution of 2.5× 10 − 5 M containing the components at a ratio of 1:1. In the former case, lower rate of adsorption causes an uncertainty in the position of the cusp points in the y– t curves. Increased bulk concentration (2.5 × 10 − 5 M) provides a relatively greater rate of adsorption and indicates the cusp points
Fig. 4. y – t adsorption kinetics of 2.0 ×10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at different temperatures. The arrows indicate the position of the cusp points in the respective curves.
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Fig. 5. y – t adsorption kinetics of 2.5 × 10 − 5 M mixed solution containing 2-HEL and TOTCNa at a molar ratio of 1:1 at different temperatures. The arrows indicate the position of the cusp points in the respective curves.
clearly. All of these curves show the existence of the phase transition in 2-HEL-TOTCNa systems at different temperatures.
3.2. Effect of temperature and chain length of amphiphiles on the phase transition pressure Fig. 6 shows plots of phase transition pressure (yt) against temperature for pure 2-HEL and the mixed systems of 2-HEL-TOOCNa and of 2HEL-TOTCNa. In all of the cases, the yt increases linearly with increasing temperature. The values of yt of 2-HEL-TOOCNa systems at different temperatures almost correspond to those of
Fig. 6. Plots of phase transition pressures of pure 2-HEL and its mixture with TOOCNa or TOTCNa against temperature. The lines were drawn by the linear regression.
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2-HEL, whereas those for 2-HEL-TOTCNa systems are always smaller than the corresponding values of 2-HEL. With increase in the size of the hydrophilic head groups, yt increases whereas it decreases when the hydrophobic chain length is increased. TOOCNa molecules have a longer chain length and a larger head group compared with those of 2-HEL. Thus, two opposing effects balance each other and the resultant yt remains unaffected. The decrease in the yt for 2-HELTOTCNa systems can be attributed to an increase in the chain length of TOTCNa by two CH2 groups than that of the TOOCNa molecules.
3.3. Growth of condensed phase Fig. 7 shows the growth of the condensed phase domains in a Gibbs monolayer of 2.2×10 − 5 M aqueous solution of pure 2-HEL at 15 °C. The domains at this temperature have a fingering pattern with optical isotropy all over the domains (image A). With time the size of the domains increases (image B). The domains preserve their fingering pattern before the complete monolayer formation (image C). The domains do not fuse at the beginning stage of phase transition, but show a uniform layer of condensed phase at the equilibrium stage by a constrained pressure exerted by the adsorbed molecules. The in situ BAM observation of 2.0× 10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C is shown in Fig. 8(b). The times at which images were taken are indicated on the corresponding y – t curve (Fig. 8(a)). At the beginning stage where the surface pressure increases, only a dark phase (image A) is observed. However, just after the cusp point, formation of circular condensed domains surrounded by a dark phase is found (image B). With time the size of the domains increases (images C, D) and at the latter stage where the surface pressure increases further, the surface is fully covered by the bright monolayer phase. The coexistence of two surface phases at the plateau region of the y– t curve clearly support the existence of the first-order phase transition in the mixed monolayers.
Fig. 7. Growth of domains in a Gibbs monolayer of 2.2 × 10 − 5 M aqueous solution of pure 2-HEL at 15 °C with time: (A) 1610 s; (B) 2125 s; and (C) 3420 s. Image size: 400 ×300 mm2.
3.4. Shapes and textures of the domains Fig. 9 presents the BAM images of pure 2-HEL and of mixed surfactants containing 2-HEL and TOOCNa at different temperatures. For pure 2-
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HEL, the domains have a circular shape with stripe texture at lower temperatures (image A) and a fingering pattern with uniform brightness all over the domain at 15 °C (image B). A further increase in the temperature causes more branching in the domain shape (image C). The optical isotropy of the domains at higher temperature indicates that the molecules in the domains have the same tilt orientation. A detailed investigation of the texture of the domains of 2-HEL at differ-
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ent temperatures has been carried out in a previous paper [17]. The formation of isotropic domains in Gibbs monolayers of 2-HEL at ] 15 °C was explained by considering the fact that the molecules are oriented almost normal to the surface at higher temperatures. The branching in the domain of 2-HEL at higher temperatures was reported to be caused by a decrease in the line tension with increasing temperature [12,17] Although the fingering pattern in pure 2-HEL
Fig. 8. (a) y – t adsorption kinetics of 2.0 ×10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ration of 1:1 at 15 °C. The points A, B, and so forth in the figure indicate the position of BAM observation shown in part b. (b) BAM images with time showing a first-order phase transition. Image size: 400 ×300 mm2.
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Fig. 9. Typical shape and texture of the domains of pure 2-HEL (images A – C) and of the mixed monolayers of 2-HEL and TOOCNa (image D) at different temperatures: (A) 10 °C; (B, D) 15 °C; and (C) 20 °C. The shape and texture of the domains in the mixed systems are almost similar at different temperatures. Image size: 400 ×300 mm2.
monolayer at 15 °C is a growth shape, it could not relax to a circular shape within 1 h. In contrast, the domains of mixed monolayers are circular with inner segments (image D) at 15 °C. The change in the shape and the texture of the domains in the mixed monolayers clearly suggests that the domains contain both of the surfactants. A wide variety of defects are observed in the domains of the mixed systems. Fig. 10 represents a few examples of texture of the domains in these systems. The domains of the mixed monolayers at different temperatures are almost similar in shape as well as in texture as shown in Fig. 10. In the mixed systems, the condensed phase might be dominated by the 2-HEL, but a considerable number of TOOCNa molecules are incorporated into the domains. Since, the latter
amphiphile contains a 16-carbon chain; a higher value of line tension is possible when it is introduced into the domains. One of the reason for appearing circular domains in mixed systems is this high line tension [38–40]. Another reason is the higher rate of relaxation of the domains in the mixed monolayers, which will be discussed in the next section. At ]15 °C, the 2-HEL molecules remain almost perpendicular to the surface, which causes uniform brightness in pure 2-HEL monolayers (Fig. 7). On the other hand, the TOOCNa molecules containing a longer carbon chain and a larger head group should be tilted. The tilted phase domains having star defects were observed even in hexadecanoic acid [13]. Thus overall interactions among these molecules cause texture in the domains. We are aware of the fact that a large
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difference in the tilt angle between the two types of mixing molecules would result in complete incompatibility in the condensed phase. Thus, we expect a relatively little tilt angle of the TOOCNa molecules in the condensed phase. The weak contrast among the segments in a domain also indi-
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cates that the molecules are slightly tilted. Furthermore, the domains of the mixed systems show a variety of patterns. This also occurs in a condensed phase containing slightly tilted alkyl chains. One of the interpretations for the appearance of a variety of textures is the fusion of the
Fig. 10. Few examples of a variety of textures of the domains in the mixed monolayers containing 2-HEL and TOOCNa at 15 °C. Image size: 400 ×300 mm2.
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Fig. 11. A typical fusion mechanism of the domains observed on the surface of a 2.0 × 10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C (images A –C). The time interval between images A and C is about 2 min. Image D shows a fusion between two domains by the regions of different tilt azimuthal orientation. Image size: 400 ×300 mm2.
domains. In Fig. 11, a textural change in the domains is shown by fusion.
3.5. Fusion of the domains One of the interesting characteristics of the domains of the mixed system is their fusion when they are in close contact. This happens not only at 15 °C, but also at 10 °C or at higher temperatures. Fig. 11 shows two examples of fusion of the domains on the surface of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C. This type of fusion is rather a common phenomenon during the course of the domain formation in these monolayers. Images A to C present the same focus region of BAM and illustrate the fusion mechanism of the domains.
Clearly, two domains touch each other and form a dumbbell shaped structure (image A). The dumbbell shaped domain undergoes a quick relaxation to form a circular domain within 2 min. In contrast, the relaxation of the domains in pure 2-HEL takes more than 1 h (Fig. 7). These results lead us to conclude that the incorporated TOOCNa molecules cause the observed quick relaxation in the mixed system. In any event, fusion produces a new stripe inside the newly formed relaxed domain and thereby changes the internal texture of the domains. Moreover, the two fused domains do not have similar tilt oriented molecules at the regions of their contact. This conclusion is corroborated by image D where it is shown that two domains fuse even by the regions of different brightness.
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The relaxation of the dumbbell shaped domains formed by fusion can be explained by an increase in the line tension by the incorporation of TOOCNa molecules. The line tension in the mixed system is such that only a circular domain is stable and, therefore, a distorted shaped domain undergoes a quick relaxation. In addition, incorporated TOOCNa molecules help to cause a rapid relaxation of the domains. Mo¨ hwald et al. [41] found that a favorable relative orientation of the colliding domains is necessary for fusion to occur. He´ non et al. [13] described a number of fusion processes in a monolayer of the mixture of sodium octanoate and hexadecanoic acid. They even found that fusion could occur between domains having different tilt azimuthal directions. Our observation of fusion of domains in the mixed systems is, thus, consistent with the report of Henon et al.
4. Conclusions A first-order phase transition from a fluid-like phase to a condensed phase is observed on the surface of a mixed solution containing 2-HEL and TOOCNa or TOTCNa at a molar ratio of 1:1 at different temperatures. This conclusion is drawn from y– t measurement, as well as from BAM observation. The phase transition pressures, yt of all three systems increase linearly with increasing temperature. The yt of 2-HEL-TOOCNa systems almost corresponds to that of pure 2-HEL at the same temperature. In contrast, the yt of 2-HELTOTCNa systems is less than the corresponding value of 2-HEL. The domains of the mixed systems are circular with a variety of internal textures at the studied temperatures. In contrast, the domains of pure 2-HEL are circular with stripe texture at 5 10 °C but are of fingering pattern with optical isotropy at ]15 °C. The change in shape as well as in texture of the domains in the mixed systems suggests that these domains contain both of the components. However, the exact composition of the mixed condensed domains is still unknown. Both increased line tension and increased rate of relaxation allow the molecules to form circular domains in the mixed monolayers.
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The incorporation of TOOCNa molecules into the domains increases the mobility of the molecules. This helps to cause a rapid relaxation of the dumbbell shaped domains formed by fusion. The fusion of the domains is responsible to show irregularly textured domain by forming new segments.
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Condensed structure formation in mixed monolayers of anionic surfactants and 2-hydroxyethyl laurate at the air–water interface Md. Mufazzal Hossain a, Md. Nazrul Islam a, Tomomichi Okano b, Teiji Kato a,* a
Department of Applied Chemistry, Faculty of Engineering, Utsunomiya Uni6ersity, Yoto 7 -1 -2, Utsunomiya 321 -8585, Japan b Better Li6ing Laboratory, Lion Corporation, Tokyo, Japan Received 7 August 2001; accepted 29 January 2002
Abstract A first-order phase transition has been studied by Brewster angle microscopy in mixed monolayers of 2-hydroxyethyl laurate (2-HEL) and two anionic surfactants, sodium 3,6,9,12-tetraoxa-octacosanoate (TOOCNa) and sodium 3,6,9,12-tetraoxa-triacontanoate (TOTCNa) at the air–water interface. Condensed phase domains are observed on the surface of a mixed solution containing 1.0 × 10 − 5 M each of 2-HEL and of TOOCNa or TOTCNa up to 20 °C. This concentration of 2-HEL is much lower than that required for the phase transition in the pure system at the studied temperatures. The phase transition pressures (yt) of all the systems increase with increasing temperature. The yt of 2-HEL-TOOCNa system almost corresponds to that of pure 2-HEL at a definite temperature. In contrast, the yt of 2-HEL-TOTCNa system is lower than the corresponding value of 2-HEL. The shapes and the textures of the domains formed in the mixed system are different to those formed in the pure system. The formed domains in the mixed system have a circular shape with internal segments whereas those of 2-HEL have a fingering pattern with an optical isotropy at the same temperature. A large variety of defects are found in the domains formed in the mixed system. Furthermore, these domains undergo fusion while touching each other. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Brewster angle microscopy; Gibbs monolayer; Mixed monolayer; Phase transition; Line tension
1. Introduction Two-dimensional (2D) phase behavior of amphiphiles in Gibbs monolayers has been of growing interest. The amphiphile molecules undergo * Corresponding author. Tel.: +81-28-689-6170; fax: + 8128-689-6179. E-mail address: [email protected] (T. Kato).
continuous adsorption from an aqueous solution to form Gibbs monolayers that is in equilibrium with the bulk. A change in phase in these monolayers dramatically changes the interfacial properties such as surface tension [1], surface viscosity [2] and foam stability [3]. Thus, a phase transition in these monolayers can distinctly influence the field of application of the surfactants. For instance, the stability of an emulsion against coales-
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cence and creaming can be enhanced by using a surfactant capable of forming a liquid crystalline phase [4]. Although, a little is known about the phase behavior of water soluble surfactants of medium chain length, the existence of first-order gas-liquid expanded (G-LE) [5– 8], LE-liquid condensed (LE-LC) [9 – 12] and G-LC [13] phase transitions in Gibbs monolayers is now well-established. In most of the literature, a cusp point in the surface pressure-time (y – t) curve followed by a plateau region was considered as an evidence for the first-order phase transition. A second-order phase transition has also been found to occur in Gibbs adsorption layers [13,14]. The morphology of the condensed domains in these monolayers is known to respond to the experimental variables similarly as those reported for Langmuir monolayers [15– 17]. Moreover, Vollhardt et al. [18,19] studied theoretically the adsorption kinetics considering phase transition and determined the 2D crystal structure in Gibbs monolayers by grazing incidence X-ray diffraction (GIXD) measurement. Extensive research has also been performed on mixed surfactant systems, since they show superior performance compared with single surfactant alone. So far the properties of mixed monolayers have been inferred by surface tensiometry [20], neutron reflection [21], light scattering [22] and ESR spectroscopy [23]. Most of the systems studied to date have appeared highly disordered and liquid-like. Shah et al. [24,25] proposed that when two surfactants are mixed with a molar ratio of 1:3 or 3:1, the properties of the surfactants are changed strikingly due to 2D hexagonal packing of molecules. Bain et al. [26– 28] investigated the phase transitions in the mixed monolayers of anionic or cationic surfactants and dodecanol or tetradecane on the solution surfaces by sum-frequency spectroscopy and ellipsometry. Their observation demonstrated that conformationally ordered condensed phase exists in those mixed monolayers. These results lead us to pose the question whether a morphological or a textural change of condensed phase would occur during the phase transition in the mixed systems. In addition, all of the above-cited methods characterize the mixed monolayers indirectly. One of the
goals of this paper is to apply microscopic techniques to achieve direct information about the morphology and the texture of the condensed phases that are formed during the phase transition in the mixed surfactant systems. To date, to the best of our knowledge, no report details morphological features in mixed monolayers. The application of sensitive microscopic techniques, particularly Brewster angle microscopy (BAM) [29,30] has revealed a shape variety or a texture variety of condensed phases in both Langmuir and Gibbs monolayers of pure amphiphiles. Using BAM, Henon et al. [13] carried out experiments on aqueous solutions of sparingly soluble hexadecanoic acid and its mixture with sodium octanoate and observed a first-order G-LC phase transition. They also found approximately the same behavior of the monolayers for both the pure and the mixture of the surfactants and concluded that the monolayer formation in the mixed system is mostly due to the hexadecanoic acid. Vollhardt et al. [31–33] studied the penetration dynamics of protein from aqueous solution into a fluid-like Langmuir monolayer of insoluble amphiphiles and found a first-order phase transition to a condensed phase. These condensed phases have morphological features similar to those of pure insoluble monolayer component indicating that the condensed domains consist of the insoluble amphiphiles. In a recent paper [34] we have showed that two water-soluble surfactants form LC domains in an appropriate mixture. We chose 2-hydroxyethyl laurate (2-HEL) and sodium 3,6,9,12-tetraoxa-octacosanoate (TOOCNa) as amphiphiles. The experiments were carried out mainly by changing the molar ratio of the surfactants at 15 °C. In this article we study the phase behavior of the mixed surfactants in details by changing the temperature. To show the effect of chain length on the phase transition pressures (yt), TOOCNa is replaced by sodium 3,6,9,12-tetraoxa-triacontanoate (TOTCNa) having two more CH2 groups in the alkyl chain. The structures of the amphiphiles used in this study are shown in Fig. 1. We further show that the morphological and the textural features of the domains formed in the mixed systems are different to those of pure 2-HEL.
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tion of 2-HEL than that necessary for the phase transition in pure 2-HEL at the same temperature. The horizontal line in the figure indicates the experimental concentration of 2-HEL used during mixed monolayer formation.
2. Experimental section Fig. 1. Chemical structures of the surfactants used in this study: (a) 2-HEL; (b) TOOCNa; and (c) TOTCNa.
The amphiphile, 2-HEL shows a first-order LELC phase transition up to 25 °C provided that the surface concentrations are sufficiently high [12,17]. Fig. 2 shows the minimum concentration necessary to provide sufficient 2D surface concentration for the phase transition to occur at different temperatures. However, when two surfactants are mixed in appropriate molar ratios, condensed domains are formed by a much lower concentra-
Fig. 2. A plot of minimum concentration (Ct) necessary for the phase transition in pure 2-HEL against temperature. The line was drawn to guide the eye. The data in this figure show a scattering of the order of 90.1× 10 − 5 M at 5 °C. The scattering increases with increasing temperature and shows a value of 9 0.2 ×10 − 5 M at 25 °C. The horizontal line indicates the concentration of 2-HEL used in the mixed monolayer systems.
The amphiphile, 2-HEL was synthesized and purified (]99.5%) following a procedure described elsewhere [35]. Both TOOCNa and TOTCNa were obtained from Lion Corporation, Japan with a purity of above 99% and used without further purification. Unless otherwise stated, the experiments were carried out with an aqueous solution of 2.0×10 − 5 M. To study the phase behavior in the mixed system, a 1:1 mixture of the surfactants was prepared. The solutions of the amphiphiles were prepared separately and then mixed in appropriate volume ratio to attain the 1:1 molar ratio. Ultra-pure water of resistivity 18 MV-cm was used throughout the present study. The instrumental set-up [36] and the experimental procedure [37] were detailed elsewhere and are described here briefly. The surface pressure-time (y– t) curves were measured in a very shallow type Langmuir trough (2 mm depth) with a surface area of 41× 16 cm2. The trough was equipped with a Wilhelmy balance for surface pressure determination and a temperature control system. A small glass plate was used as a Wilhelmy plate. A BAM, mounted on the trough, was used to observe morphology of the monolayers. A beam of p-polarized light from a 20 mW He–Ne laser was directed to the air–water interface at Brewster angle, giving a minimum surface reflectivity. Under such circumstances, coexistence of two phases with different densities was readily observed. Optical anisotropy caused by the differences in molecular orientation in the monolayer is visualized by introducing of an analyzer in the path of the reflected beam. An aqueous solution (either of pure surfactants or their mixtures) was poured into the trough and maintained for 25 min to attain the experimental temperature. The surface of solution was swept
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Fig. 3. y – t adsorption kinetics of 2.0 × 10 − 5 M aqueous solution of 2-HEL (I) and TOOCNa (II) separately and their mixture (III) containing the components at a molar ratio of 1:1 at 15 °C. The arrows indicate the position of the cusp points in the respective curves.
rapidly by the movable teflon barriers and the surface pressure was started acquisition into a computer. The BAM images were recorded with time and stored in a video recorder. Image processing software was used to correct the distortion and to optimize the contrast of the BAM images.
3. Results and discussion
3.1. Inference of phase transition from y– t measurements Condensed domains are not formed in Gibbs adsorption layers even with ] 2.0 × 10 − 5 M solutions of TOOCNa while the critical micelle concentration of this amphiphile is :0.5 ×10 − 5 M at 15 °C. In such a system, an increase in the bulk concentration only results in an increase in the micelle concentration but does not increase the 2D surface concentration. A lower extent of surface concentration causes only a fluid-like phase in Gibbs monolayers. The widely accepted reason behind the absence of densely ordered state in such a monolayer is the hydration, the dipole– dipole repulsion, and the electrostatic repulsion among the long hydrophilic head groups. Fig. 3 shows y –t adsorption kinetics of 2.0× 10 − 5 M
aqueous solution of both 2-HEL and TOOCNa separately and their mixture containing the components at a molar ratio of 1:1 at 15 °C. This concentration is sufficient to form condensed domains when 2-HEL is used alone (shown in Fig. 2). A cusp point followed by a plateau region for the phase transition is indicated in the y– t curve of 2-HEL. This cusp point becomes sharp, if the bulk concentration is high. The rate of adsorption of TOOCNa is faster than that of 2-HEL. The net rate of adsorption for the mixed solution is in between the pure systems. This suggests that both of the surfactants undergo adsorption simultaneously. The y–t curve of the mixture in Fig. 3 shows the existence of a cusp point that is followed by a well-developed plateau region. These are the characteristics of a first-order phase transition in Gibbs monolayers [9–17]. Although the cusp point at which phase transition begins is not well pronounced, however, the simultaneous BAM images indicate clearly the existence of the first-order phase transition. The time necessary for appearing the cusp point in the mixed system is about half of that necessary for 2-HEL. Under these conditions, the concentration of 2-HEL is 1.0× 10 − 5 M which is much lower than that necessary for the formation of condensed domains in pure 2-HEL at 15 °C (: 1.8× 10 − 5 M from Fig. 2). However, the phase transition occurs even on the surface of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 2:3 [34]. In this case, the concentration of 2-HEL is only 0.8× 10 − 5 M. All of these results lead us to conclude that the existence of the phase transition in these systems is due to the interaction of both of the surfactants. The surface pressure of the mixed solution at equilibrium is greater than that of the pure surfactants indicating tight packing of the molecules in the mixed system. Similar results are also observed during equilibrium surface tension measurement of the pure and the mixed systems. The surface tension of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 is less than that of the pure systems. The y–t adsorption profiles of 2.0× 10 − 5 M mixed solutions containing the components at a molar ratio of 1:1 at different temperatures are shown in Fig. 4. All of these curves show the
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typical nature of the existence of the first-order phase transition in Gibbs monolayers. In the case of the y –t curve at 20 °C, the concentration of 2-HEL is about one-third of the minimum concentration necessary for the phase transition in pure 2-HEL (:2.8 × 10 − 5 M from Fig. 2). Again, a clear effect of the TOOCNa molecules on the existence of the phase transition in the mixed monolayers can be concluded. It is very likely that the incorporated 2-HEL molecules shield the repulsive interactions among the long hydrophilic head groups of TOOCNa molecules and thereby allow the existence of high density phase in the mixed monolayer systems. The existence of the phase transition in the mixed monolayers becomes further evident when TOOCNa is replaced by TOTCNa, a surfactant having two more CH2 groups in the hydrophobic tail. Fig. 5 shows the y– t adsorption kinetics at different temperatures. Although, a phase transition was found to exist on the surface of 2.0× 10 − 5 M mixed solution, we carried out the experiment using a mixed solution of 2.5× 10 − 5 M containing the components at a ratio of 1:1. In the former case, lower rate of adsorption causes an uncertainty in the position of the cusp points in the y– t curves. Increased bulk concentration (2.5 × 10 − 5 M) provides a relatively greater rate of adsorption and indicates the cusp points
Fig. 4. y – t adsorption kinetics of 2.0 ×10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at different temperatures. The arrows indicate the position of the cusp points in the respective curves.
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Fig. 5. y – t adsorption kinetics of 2.5 × 10 − 5 M mixed solution containing 2-HEL and TOTCNa at a molar ratio of 1:1 at different temperatures. The arrows indicate the position of the cusp points in the respective curves.
clearly. All of these curves show the existence of the phase transition in 2-HEL-TOTCNa systems at different temperatures.
3.2. Effect of temperature and chain length of amphiphiles on the phase transition pressure Fig. 6 shows plots of phase transition pressure (yt) against temperature for pure 2-HEL and the mixed systems of 2-HEL-TOOCNa and of 2HEL-TOTCNa. In all of the cases, the yt increases linearly with increasing temperature. The values of yt of 2-HEL-TOOCNa systems at different temperatures almost correspond to those of
Fig. 6. Plots of phase transition pressures of pure 2-HEL and its mixture with TOOCNa or TOTCNa against temperature. The lines were drawn by the linear regression.
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2-HEL, whereas those for 2-HEL-TOTCNa systems are always smaller than the corresponding values of 2-HEL. With increase in the size of the hydrophilic head groups, yt increases whereas it decreases when the hydrophobic chain length is increased. TOOCNa molecules have a longer chain length and a larger head group compared with those of 2-HEL. Thus, two opposing effects balance each other and the resultant yt remains unaffected. The decrease in the yt for 2-HELTOTCNa systems can be attributed to an increase in the chain length of TOTCNa by two CH2 groups than that of the TOOCNa molecules.
3.3. Growth of condensed phase Fig. 7 shows the growth of the condensed phase domains in a Gibbs monolayer of 2.2×10 − 5 M aqueous solution of pure 2-HEL at 15 °C. The domains at this temperature have a fingering pattern with optical isotropy all over the domains (image A). With time the size of the domains increases (image B). The domains preserve their fingering pattern before the complete monolayer formation (image C). The domains do not fuse at the beginning stage of phase transition, but show a uniform layer of condensed phase at the equilibrium stage by a constrained pressure exerted by the adsorbed molecules. The in situ BAM observation of 2.0× 10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C is shown in Fig. 8(b). The times at which images were taken are indicated on the corresponding y – t curve (Fig. 8(a)). At the beginning stage where the surface pressure increases, only a dark phase (image A) is observed. However, just after the cusp point, formation of circular condensed domains surrounded by a dark phase is found (image B). With time the size of the domains increases (images C, D) and at the latter stage where the surface pressure increases further, the surface is fully covered by the bright monolayer phase. The coexistence of two surface phases at the plateau region of the y– t curve clearly support the existence of the first-order phase transition in the mixed monolayers.
Fig. 7. Growth of domains in a Gibbs monolayer of 2.2 × 10 − 5 M aqueous solution of pure 2-HEL at 15 °C with time: (A) 1610 s; (B) 2125 s; and (C) 3420 s. Image size: 400 ×300 mm2.
3.4. Shapes and textures of the domains Fig. 9 presents the BAM images of pure 2-HEL and of mixed surfactants containing 2-HEL and TOOCNa at different temperatures. For pure 2-
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HEL, the domains have a circular shape with stripe texture at lower temperatures (image A) and a fingering pattern with uniform brightness all over the domain at 15 °C (image B). A further increase in the temperature causes more branching in the domain shape (image C). The optical isotropy of the domains at higher temperature indicates that the molecules in the domains have the same tilt orientation. A detailed investigation of the texture of the domains of 2-HEL at differ-
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ent temperatures has been carried out in a previous paper [17]. The formation of isotropic domains in Gibbs monolayers of 2-HEL at ] 15 °C was explained by considering the fact that the molecules are oriented almost normal to the surface at higher temperatures. The branching in the domain of 2-HEL at higher temperatures was reported to be caused by a decrease in the line tension with increasing temperature [12,17] Although the fingering pattern in pure 2-HEL
Fig. 8. (a) y – t adsorption kinetics of 2.0 ×10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ration of 1:1 at 15 °C. The points A, B, and so forth in the figure indicate the position of BAM observation shown in part b. (b) BAM images with time showing a first-order phase transition. Image size: 400 ×300 mm2.
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Fig. 9. Typical shape and texture of the domains of pure 2-HEL (images A – C) and of the mixed monolayers of 2-HEL and TOOCNa (image D) at different temperatures: (A) 10 °C; (B, D) 15 °C; and (C) 20 °C. The shape and texture of the domains in the mixed systems are almost similar at different temperatures. Image size: 400 ×300 mm2.
monolayer at 15 °C is a growth shape, it could not relax to a circular shape within 1 h. In contrast, the domains of mixed monolayers are circular with inner segments (image D) at 15 °C. The change in the shape and the texture of the domains in the mixed monolayers clearly suggests that the domains contain both of the surfactants. A wide variety of defects are observed in the domains of the mixed systems. Fig. 10 represents a few examples of texture of the domains in these systems. The domains of the mixed monolayers at different temperatures are almost similar in shape as well as in texture as shown in Fig. 10. In the mixed systems, the condensed phase might be dominated by the 2-HEL, but a considerable number of TOOCNa molecules are incorporated into the domains. Since, the latter
amphiphile contains a 16-carbon chain; a higher value of line tension is possible when it is introduced into the domains. One of the reason for appearing circular domains in mixed systems is this high line tension [38–40]. Another reason is the higher rate of relaxation of the domains in the mixed monolayers, which will be discussed in the next section. At ]15 °C, the 2-HEL molecules remain almost perpendicular to the surface, which causes uniform brightness in pure 2-HEL monolayers (Fig. 7). On the other hand, the TOOCNa molecules containing a longer carbon chain and a larger head group should be tilted. The tilted phase domains having star defects were observed even in hexadecanoic acid [13]. Thus overall interactions among these molecules cause texture in the domains. We are aware of the fact that a large
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difference in the tilt angle between the two types of mixing molecules would result in complete incompatibility in the condensed phase. Thus, we expect a relatively little tilt angle of the TOOCNa molecules in the condensed phase. The weak contrast among the segments in a domain also indi-
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cates that the molecules are slightly tilted. Furthermore, the domains of the mixed systems show a variety of patterns. This also occurs in a condensed phase containing slightly tilted alkyl chains. One of the interpretations for the appearance of a variety of textures is the fusion of the
Fig. 10. Few examples of a variety of textures of the domains in the mixed monolayers containing 2-HEL and TOOCNa at 15 °C. Image size: 400 ×300 mm2.
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Fig. 11. A typical fusion mechanism of the domains observed on the surface of a 2.0 × 10 − 5 M mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C (images A –C). The time interval between images A and C is about 2 min. Image D shows a fusion between two domains by the regions of different tilt azimuthal orientation. Image size: 400 ×300 mm2.
domains. In Fig. 11, a textural change in the domains is shown by fusion.
3.5. Fusion of the domains One of the interesting characteristics of the domains of the mixed system is their fusion when they are in close contact. This happens not only at 15 °C, but also at 10 °C or at higher temperatures. Fig. 11 shows two examples of fusion of the domains on the surface of a mixed solution containing 2-HEL and TOOCNa at a molar ratio of 1:1 at 15 °C. This type of fusion is rather a common phenomenon during the course of the domain formation in these monolayers. Images A to C present the same focus region of BAM and illustrate the fusion mechanism of the domains.
Clearly, two domains touch each other and form a dumbbell shaped structure (image A). The dumbbell shaped domain undergoes a quick relaxation to form a circular domain within 2 min. In contrast, the relaxation of the domains in pure 2-HEL takes more than 1 h (Fig. 7). These results lead us to conclude that the incorporated TOOCNa molecules cause the observed quick relaxation in the mixed system. In any event, fusion produces a new stripe inside the newly formed relaxed domain and thereby changes the internal texture of the domains. Moreover, the two fused domains do not have similar tilt oriented molecules at the regions of their contact. This conclusion is corroborated by image D where it is shown that two domains fuse even by the regions of different brightness.
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The relaxation of the dumbbell shaped domains formed by fusion can be explained by an increase in the line tension by the incorporation of TOOCNa molecules. The line tension in the mixed system is such that only a circular domain is stable and, therefore, a distorted shaped domain undergoes a quick relaxation. In addition, incorporated TOOCNa molecules help to cause a rapid relaxation of the domains. Mo¨ hwald et al. [41] found that a favorable relative orientation of the colliding domains is necessary for fusion to occur. He´ non et al. [13] described a number of fusion processes in a monolayer of the mixture of sodium octanoate and hexadecanoic acid. They even found that fusion could occur between domains having different tilt azimuthal directions. Our observation of fusion of domains in the mixed systems is, thus, consistent with the report of Henon et al.
4. Conclusions A first-order phase transition from a fluid-like phase to a condensed phase is observed on the surface of a mixed solution containing 2-HEL and TOOCNa or TOTCNa at a molar ratio of 1:1 at different temperatures. This conclusion is drawn from y– t measurement, as well as from BAM observation. The phase transition pressures, yt of all three systems increase linearly with increasing temperature. The yt of 2-HEL-TOOCNa systems almost corresponds to that of pure 2-HEL at the same temperature. In contrast, the yt of 2-HELTOTCNa systems is less than the corresponding value of 2-HEL. The domains of the mixed systems are circular with a variety of internal textures at the studied temperatures. In contrast, the domains of pure 2-HEL are circular with stripe texture at 5 10 °C but are of fingering pattern with optical isotropy at ]15 °C. The change in shape as well as in texture of the domains in the mixed systems suggests that these domains contain both of the components. However, the exact composition of the mixed condensed domains is still unknown. Both increased line tension and increased rate of relaxation allow the molecules to form circular domains in the mixed monolayers.
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The incorporation of TOOCNa molecules into the domains increases the mobility of the molecules. This helps to cause a rapid relaxation of the dumbbell shaped domains formed by fusion. The fusion of the domains is responsible to show irregularly textured domain by forming new segments.
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