Langmuir 2000, 16, 3345-3348
3345
Higher-Order Structure Formation in Adsorbed Monolayers at Aqueous Solution Surfaces Studied by Brewster Angle Microscopy Md. Mufazzal Hossain, Masaaki Yoshida, and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received April 27, 1999. In Final Form: December 9, 1999 The first-order phase transitions in adsorbed monolayers of 2-hydroxyethyl laurate have been studied by film balance and Brewster angle microscopy (BAM). A cusp point in the surface pressure-time (π-t) curves indicates the commencemnet of phase transition. The critical surface pressure necessary for the phase transition increases with increasing temperature. Large size circular domains having characterishtic internal stripes are observed up to 10 °C. The width of the stripes increases with increasing termperature. The separate stripes in a domain undergo continuous narrowing into a core defect which lies on the domain peripheral line at 2 °C. The core defect seems to be also present at higher temperatures but remains far out of the doman boundary at 10 °C. Two surface phases observed after the cusp points by BAM are pseudo equilibrium state and the global equilibrium is attained with the formation of one condensed surface phase after a long time.
Introduction Langmuir monolayers of insoluble amphiphiles have been investigated for a long time by a wider class of experiments. The measurement of π-A isotherms suggested early the existence of a plateau region on the isotherms, which is due to the first-order phase transitions in the spread monolayers. The introduction of highly sensitive Brewster angle microscopy (BAM) has made it possible to observe in situ the monolayer morphologies at the air-water interface.1,2 It is also effective to detect the internal anisotropy of condensed domains, caused by the tilt azimuthal angle of the molecules, which appears because of the difference of the surface reflectivity of the p-polarized light.3,4 Although kinetics of the formation of a uniform Gibbs adsorption layer5 on the aqueous solution surfaces has been studied for a long time, condensed domain formation in this monolayer has been observed recently. He´non et al.6,7 first reported the formation of condensed domains during adsorption from aqueous solutions at the air-water interface by BAM. However, they also stated that the presence of trace impurities of less solubility are responsible for the formation of such domains.8 Quite recently, Vollhardt et al.9-12 studied phase transitions and subse* To whom correspondence should be addressed. Phone: +8128-689-6170. Fax: +81-28-689-6179. E-mail:
[email protected]. (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) Weidemann, G.; Vollhardt, D. Langmuir 1996, 12, 5114 (4) Tsao, M.-W.; Fischer, T. M.; Knobler, C. M. Langmuir 1995, 11, 3184. (5) Duchin, S. S.; Kretzchmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Mobius, D., Miller, R., Eds.; Elsevier: Amsterdam, 1995. (6) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (7) Rivie`re, S.; He´non, S.; Meunier, J. J Phys. Rev. E 1994, 49, 1375. (8) He´non S.; Meunier, J. Thin Solid Films 1992, 210/211, 121. (9) Melzer, V.; Vollhardt, D. Phys. Rev. Lett. 1996, 76, 3770. (10) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (11) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591. (12) Melzer, V.; Weidemann, G.; Wagner, R.; Vollhardt, D.; DeWolf, C.; Brezesinski, G.; Mo¨hwald, H. Chem. Eng. Technol. 1998, 21, 44.
quent domain formation in the adsorbed monolayers of a series of acid amide compounds. They were mainly concerned with the phase transitions and subsequent domain formation in adsorbed monolayers. We have also reported on the existence of a phase transition and highly ordered domain formation in the adsorbed monolayers of 2-hydroxyethyl laurate (2-HEL).13 In this paper we discuss the detailed formation process of the monolayers during adsorption of the amphiphile from the bulk of aqueous solutions up to nearly equilibrium. Highly ordered structure formation in condensed domains is also studied extensively at different temperatures. Experimental Section The amphiphile 2-HEL was synthesized and purified in hexane in our laboratory.14 Purity of the sample was checked by 400MHz 1H NMR (Varian Unity INOVA) and was much higher than 99.5% because there was no small impurity peak detectable in comparison with the satellite peaks of 13C. The experimental setup consisted of a computer-controlled Langmuir trough which was very shallow (2-mm depth), above which a Brewster angle microsope (BAM) was mounted. Surface pressure was measured by the Wilhelmy method using a small glass plate. The BAM is composed of a 25-mW semiconductor laser, a Glan-Thompson polarizer, an analyzer, a zooming microscope with a CCD camera of higher sensitivity, and a video recording system. The BAM images were distorted because of the oblique glancing of the microscope (Brewster angle ) 53.1°); that is, the circular domains apparently looked elliptical. This distortion was not corrected in our BAM system. Details of instrumentation were published elsewhere.15 The experiments were performed by pouring a definite amount of aqueous solution into the Langmuir trough for 25 min to attain equilibrium at the temperature of the system. The molecules of 2-HEL already adsorbed were completely removed by sweeping off the surface rapidly with movable PTFE barriers. Under such circumstances the surface concentration can be considered to be approximately zero and the corresponding surface pressure also (13) Hossain, M. M.; Yoshida, M.; Iimura, K.; Suzuki, N.; Kato, T. Colloids Surf. A, in press. (14) Bevan, T. H.; Malkin, T.; Smith, D. B. J. Chem. Soc. 1955, (pt I), 1043. (15) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34 (part.2, No. 7B), L911.
10.1021/la990519c CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000
3346
Langmuir, Vol. 16, No. 7, 2000
Figure 1. π-t adsorption profiles of 1.4 × 10-5 M aqueous solution of 2-HEL at different temperatures: (I) 2 °C, (II) 5 °C, (III) 10 °C, (IV) 15 °C, and (V) 20 °C. The arrows indicate the cusp points for the phase transition. to be zero. The increase in surface pressure was then monitored with time and the surface of the aqueous solution was observed by BAM simultaneously. Ultrapure water of resistivity 18 MΩ cm (Elgastat, UHQ-PS) was used throughout the present study.
Results and Discussion Figure 1 shows the adsorption kinetics of 1.4 × 10-5 M aqueous solution of 2-HEL at different temperatures. A cusp point followed by a pronounced plateau region was observed in each of the π-t curves up to 10 °C. π-A isotherms of some other insoluble amphiphiles at the airwater interface also have similar types of cusp points followed by plateau regions which are widely accepted for first-order phase transition. Thus, the cusp points in the π-t curves are due to the first-order phase transitions in the adsorbed monolayers.9-13 The initial rapid rise in the surface pressure corresponds to the adsorption of the amphiphile in a fluid-like phase. The discontinuity in the π-t curves indicates that a condensed phase is formed, which is in coexistence with the fluid-like phase. Above 10 °C, each of the π-t curves shows a continuous rise, leading to its final equilibrium value after a long time. With increasing temperature the time necessary to reach equilibrium decreases. This kind of continuous curve is typical for uniformly distributed adsorption of amphiphiles.5 It is also clear from the figure that the critical surface pressure, πc, necessary for the phase transition increases with increasing temperature. At 15 °C and above, the monolayers could not show the cusp points in the π-t curves under experimental conditions, although increased concentrations of 2-HEL showed the cusp points for the phase transitions (data are not shown). This fact reveals that the melting temperature of the condensed phase is above 15 °C and the absence of a cusp point in the π-t curves at higher temperatures is due to low twodimensional surface concentrations of the molecules. The results suggest that if the two-dimensional surface concentration reaches a critical value on the way to the global equilibrium, some condensed domains form in the adsorbed monolayers. Thus, the two-dimensional critical surface concentration of the amphiphilic molecules under certain conditions is an essential criterion for the phase transition to form condensed domains. Figure 2a presents the π-t curve of 1.4 × 10-5 M aqueous solution of 2-HEL at 2 °C and Figure 2b shows the BAM images taken at the times marked by arrows on the π-t curve. The images of the 2-HEL monolayer clearly illustrate the coexistence of the fluid-like phase and the condensed phase during adsorption from the bulk. Similar
Hossain et al.
types of domains are also observed at different temperatures up to 10 °C. The condensed domains are subdivided by parallel stripes, showing a very characteristic surface pattern. There are two different types of domains, bright and dark as a whole (image C). The bright domain can be changed to a dark one by rotating the analyzer. Because the domains look dark or bright on the whole, it is expected that the tilt azimuthal directions of the molecules are almost the same in a particular domain. The domains having tilt azimuth of the molecules in the p-plane look bright whereas those containing molecules with tilt azimuth orthogonal to the p-plane look dark when the analyzer is in the p-plane. Initially, two or three stripes are observed and then the number increases as the domain becomes larger. The width of the stripes in a domain is almost the same except for the width of the outermost growing stripes. However, several stripes near one edge of a domain disappear by a continuous narrowing into a core defect like a vertex of boojum. At 2 °C, this core defect seems to be present on the boundary of the domains. With increasing temperature, however, it seems that the vertex shifts to out of the boundary as shown in the next figure. The size of domains and the number of stripes in a domain can vary from one domain to another. At a certain temperature, the number of stripes depends on the size of the domains. A maximum of eight or nine stripes are observed at 2 °C in some experiments (image D). The stripe structure in a domain is due to the sudden jump of the tilt azimuthal direction of the molecules across the defect line, separating stripes from their neighbors.7,13 Since all the stripes in a domain in our case appear with almost identical brightness, the degree of jump of the molecular tilt azimuthal angle is estimated to be not so large. Across the stripes, the tilt azimuth of the molecules varies continuously to the extent that the contrast between the two ends of the stripe is weakly detectable by BAM. A similar orientation of the molecules repeats in the neighboring stripes. With time the domains become gradually larger by the sacrifice of the fluid-like phase (Figure 2b). The domains start to touch each other and deformation of the domains begins. The surface pressure in the π-t curve starts to rise again at this stage after a prolonged plateau region. The domains are easily deformed toward the accessible fluid-like phase region and deformation can take place by a number of different ways. Images E and F show two different types of such deformations. In the former one, part of the domain tends to extend toward available space by adding a new narrow stripe to the outermost matured stripe, and in the latter one, the whole domain tends to be longer as the fluid-like phase is available along these directions. The focused domain in the images D and E represents the same one and illustrates that the deformation increases the number of stripes in the domains (from 8 to 9 stripes). Although there were some other domains around the accessible fluid-like phase region in images E and F, the focused ones are in a favored position to be deformed. These results demonstrate that growth of the domain preferentially takes place along the orthogonal direction of the stripes by forming a new stripe at the outermost side of the domain. As the adsorption continues, the surface of the solution is covered with the deformed condensed-phase domains (image G). Even then, fusion of the domains does not occur and we can see staggered stripe structures. Surface pressure in the π-t curve approaches some global equilibrium value asymptotically after a long time. The fluidlike phase has completely disappeared and the surface is
Adsorbed Monolayers at Aqueous Solution Surfaces
Langmuir, Vol. 16, No. 7, 2000 3347
a
b
Figure 2. (a) π-t adsorption profile of a 1.4 × 10-5 M aqueous solution of 2-HEL at 2 °C. (b) BAM images of the monolayer observed in situ during adsorption from aqueous solution. The arrows on the π-t curve indicate the time at which images were taken whereas those on the images E and F indicate the directions of the domain deformation.
3348
Langmuir, Vol. 16, No. 7, 2000
Hossain et al.
The equilibrium shapes of the domains have been reported to be governed in part by a competition between the line tension and dipole-dipole repulsion between the molecules inside the domains.16,17 Higher line tension compared to repulsive interaction favors circular domains.18,19 Although line tension decreases with increasing temperature as well as surface pressure, it is sufficient to cause circular domains up to 10 °C but reduces to give a fingering pattern at and above 15 °C in the case of 2-HEL. An anisotropic contribution to the line tension is essential to stabilize a defect point inside or at the boundary of the domains,6 which is in fact observed during the experiment (Figure 2b, image C). This anisotropy of line tension prevents the domains from being fused at a lower temperature, even at higher surface pressure (Figure 2b, images G and H). Experimental results show that only one surface phase can exist in adsorbed monolayers of 2-HEL at the final equilibrium state below or above the critical temperature after a long time. This means that the two surface phase coexistent state observed after the cusp points in the adsorbed monolayers is not the truly equilibrated one but is a metastable or transient state that appeared on the way to the global equilibrium by the kinetic reason of when the temperature is lower than a certain critical value. Conclusions
Figure 3. BAM images of adsorbed monolayers of 2-HEL at different temperatures: A, 2 °C; B, 5 °C; C, 10 °C.
covered with one surface phase at this final stage. In this region, we can observe some bright nuclei along the contact line of the domains (image H). Figure 3 shows BAM images of a 1.4 × 10-5 M aqueous solution of 2-HEL at different temperatures. In almost equal size domains, seven, six, and three stripes are shown in the figure at 2, 5, and 10 °C, respectively. The width of the stripes is almost the same for those in the domains at the same temperature but increases with increasing temperature. The BAM images reveal that the widths of the matured stripes are of the order of 50, 70, and 100 µm at 2, 5, and 10 °C, respectively. However, a core defect as observed at 2 °C seems to be present also at 5 °C, but that for 10 °C is either absent or present far out of the domains. At 15 °C and above, we could not observe the condensed domains under the experimental conditions. The result is also compatible with the corresponding π-t data as described above. However, condensed domains are observed if the experiment is carried out with higher concentrations of 2-HEL at 15 °C and above. The domains grow in a shape of fingering patterns without any inner structure at this temperature. The results with higher concentrations of the aqueous solutions of 2-HEL at higher temperatures will be presented elsewhere.
The first-order phase transition in adsorbed monolayers indicated by the cusp point in the π-t curves is further confirmed by simultaneous BAM observations of the condensed domain formation during adsorption on the aqueous solution surfaces. With a temperature increase, the critical surface pressures necessary for the phase transition increase. Highly ordered circular domain formation is always observed up to 10 °C under the experimental conditions. We discuss the formation of stripe textured domains in adsorbed monolayers of a highly purified amphiphile in detail. The main criterion for the existence of condensed domains in adsorbed monolayers is the critical twodimensional surface concentration and acquiring of this concentration largely depends on the bulk concentration at a certain temperature. Presently, we can only assume that the texture of the domains are similar with that reported by Rivie`re et al.7 but are unable to give the order parameters such as tilt angle, tilt azimuthal angle of the molecules in the domains. However, research with grazing incidence X-ray diffraction (GIXD) using synchrotron orbital radiation is in progress to focus on the matter, cooperative with investigators of the Max-Planck Institute in Potsdam. Finally, we wish to emphasize here that the condensed domain formation in the adsorbed monolayers is not a truly equilibrated phenomenon, rather a transient one on the way to the global equilibrium of the system. Acknowledgment. Part of this work was supported financially by the Venture Business Laboratory of Utsunomiya University. This was much appreciated. LA990519C (16) Keller, D. J.; McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1986, 90, 2311. (17) Andelman, D.; Brochard, F.; Joanny, J. F. J. Chem. Phys. 1987, 86, 3673. (18) Suresh, K. A.; Nittmann, J.; Rondelez, F. Europhys. Lett. 1988, 6, 437. (19) Siegel, S.; Vollhardt, D. Thin Solid Films 1996, 284/285, 424.
