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J. Phys. Chem. 1996, 100, 10710-10720
2D-3D Transformations of Amphiphilic Monolayers Influenced by Intermolecular Interactions: A Brewster Angle Microscopy Study A. Angelova,*,†,‡ D. Vollhardt,† and R. Ionov§ Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, 12 489 Berlin, Germany, Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. BoncheV Str., Bl. 21, 1113 Sofia, Bulgaria, and Institute of Applied Physics, Technical UniVersity, 1156 Sofia, Bulgaria ReceiVed: February 9, 1996X
The mechanism of the morphological 2D-3D transformations of insoluble supersaturated monolayers of long chain alkanoic acids, and their methyl ester and salt derivatives is studied directly at the air/water interface by means of Brewster angle microscopy. The phase changes occurring upon continuous overcompression of nonionic and charged monolayers are compared. The relationship between the shape of the monolayer isotherms in the region of the attained maximum surface pressure and the morphological features of the 2D-3D transformations is investigated. The effects of the hydrocarbon chain length variation (C14, C18, C22), hydrophilic head groups ionization, strength of interfacial hydrogen bonding, temperature, and compression rate on the morphology of the 3D phase, separated from the overcompressed monolayers, are monitored. It is found that the interaction of the monolayers with the aqueous subphase (through head groups ionization or hydrogen bonding) has the strongest influence on the morphology of the 2D-3D transformations for the shortest chain length (C14) alkanoic acid homologue studied. The obtained results are discussed in relation to the nucleation-growth-collision theory of supersaturated monolayers (Vollhardt, D. AdV. Colloid Interface Sci. 1993, 47, 1.). It can be concluded that the morphological features of the overgrown 3D structures are determined by the balance between the monolayer compression rate and the nucleation and growth rates (sensitive to intermolecular interactions) of the 3D phase.
Introduction Insoluble amphiphilic monolayers at the air/water interface1 have been intensively studied as model membrane systems,2 as well as for the building up of artificial multilayer structures (Langmuir-Blodgett films) with potential applications in various practical fields.3,4 The stability5-7 of the monolayers at high, constant deposition surface pressures is crucial for the preparation of highly ordered layered structures of low concentrations of defects. In this respect, studies of the two-dimensionalthree-dimensional (2D-3D) transformations of insoluble monolayers, compressed at constant surface pressures above the equilibrium surface pressure (ESP),8 have received considerable recent attention both from theoretical8-12 and experimental7,13-17 points of view. It has been indicated that part of the defects18,19 in the deposited Langmuir-Blodgett films is formed already on the liquid surface.5,20 This is due to the fact that the monolayers are in a supersaturated (metastable) state at surface pressures exceeding the ESP and undergo relaxations.13,15,21 Most of the relaxation processes (often referred to as “slow collapse”1,22) lead to nucleation and growth of 3D structures,8 which disturb the homogeneity and integrity of the spread monolayers5 and cause in-plane discontinuities and rearrangement20 of deposited LB multilayers. A subject of the present investigations is the 2D-3D transformations of Langmuir monolayers occurring under their continuous overcompression beyond the limiting densities for close-packed 2D films. These transformations, frequently referred to as “monolayer collapse”,23,24 appear to be phase changes related to the separation of amphiphilic material from * Author for correspondence. † Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. ‡ Bulgarian Academy of Sciences. § Technical University. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00417-0 CCC: $12.00
the 2D monolayers into a 3D phase upon gradual compression. Macroscopically, they are experimentally registered as sharp surface pressure drops (“spikes”24) or plateaus (“walls”24) at the maximum surface pressure, πc, attained during monolayer isotherm (π/A) determination. The magnitude of πc reflects the limit of the monolayer stability with respect to transformations into a 3D phase. It depends on the chemical nature of the amphiphiles, determining the in-plane intermolecular interactions in the monolayers,1 as well as on the interaction of the monolayers with the aqueous subphase25-27 and the dynamics of the compression.28,29 It is well known that the head group polarity and ionization contribute to the enhanced stability of the insoluble monolayers.6-8 From a morphological point of view, the “collapse” process has been considered an irreversible fracture of the monolayers that occurs at the maximum surface pressure, πc. So far, the morphologies of overcompressed monolayers have been visualized mainly after their transfer onto a solid substrate. Scanning and transmission electron microscopies,16,30-32 and recently atomic force microscopy,33-35 have been applied for this purpose. Electron micrographs have registered mostly the final stages36 of the monolayer “breakdown” during the transformations. However, “ordered” multilayer formation37 could also be anticipated during monolayer overcompression. Recently, the “collapse” process of the ionogenic alkanoic acid monolayers has been assigned24 to an “organized” or “irregular” kind, depending on the shape of the π/A isotherms in the region of the maximum surface pressure measured. The “spike” at πc, followed by a “plateau” at lower surface pressure values, has been interpreted as being due to the growth of “organized” trilayers (stabilized by head group hydrogen bonding). The “plateau” at πc has been associated with the irregular fracture of the ionized monolayers. To our knowledge, a direct morphological investigation of the occurrence of such events © 1996 American Chemical Society
2D-3D Transformations of Amphiphilic Monolayers at the air/water interface has not been presented yet. Since the transfer onto a solid substrate may change the structural organization of the overcompressed monolayers, it is desirable for the “collapse” process to be studied directly on the liquid interface. With the initiation of Brewster angle microscopy (BAM)14,38-42 for monolayer characterization at the air/water interface, it has become feasible to visualize details of the inner structure of condensed phase domains, to investigate differences in the morphological structure of condensed monolayer phases, and to follow the morphological peculiarities of the relaxation processes occurring in supersaturated monolayers held at constant surface pressures. The possibility of monitoring the initial stages and the dynamics of the transformation processes in condensed phase monolayers directly at the air/water interface appears to be an essential advantage of BAM studies. This method is applied here for the investigation of the morphological transformation mechanisms of condensed monolayers into a 3D phase upon overcompression beyond the transitions at πc. Knowledge of the sequential stages of the mechanisms of such 2D-3D transformations is essentially necessary when formation of ordered lipid multilayers on liquid surfaces is sought. Despite being very fruitful for the investigation of monolayer phase transitions from the fluid to the condensed state, fluorescence microscopy2,7 still has not allowed for the monitoring of transitions of condensed phase monolayers into a 3D solid phase. In the present study, to elucidate the effects of the chainchain and head group-subphase interactions on the morphological features of the 2D-3D transformation mechanisms, lipid monolayers of simple chemical structures were chosen as examples of nonionic and charged amphiphilic films. The intermolecular interactions in the monolayers were influenced by changing the alkyl chain length of the alkanoic acid derivatives, the ionization state of the carboxylic head groups, and the strength of the interfacial hydrogen bonding. The phase state of the monolayers was affected by temperature variations or interactions with counterions from the aqueous subphase. Special attention here is given to the problems of (i) whether the early stages of the 2D-3D transformations during continuous monolayer compression occur below or exactly at πc and (ii) whether there is a correlation among the ionization (or hydrogen-bonding) state of the monolayers, the shape of the isotherm in the region of the maximum surface pressure πc (“spike” or “plateau”), and the morphology of the separated 3D structures (platelets, granules, terraces, etc.). Experimental Section Myristic acid, stearic acid, arachidic acid, behenic acid, and methyl arachidate were purchased from Merck or Sigma Chemicals, Co. (>99.9% grade). The amphiphilic materials were dissolved in chloroform (Uvasol, for spectroscopy, Merck) to a concentration of 1 × 10-3 M. Deionized ultrapure water, purified by the MilliQ system (Millipore Corp.), was used for the preparation of aqueous subphase solutions. The inorganic salts used, CdCl2 (Fluka), Pb(NO3)2 (Merck), CaCl2 (Fluka), NaSCN (Fluka), were of p.a. grade. The subsolution pH values were adjusted by adding of small volumes of HNO3 (Merck, p.a.) or NaOH (Fluka, p.a.). Perhydrol (30% H2O2, Merck, for analysis ISO, stabilized for higher temperatures) was added to the aqueous subsolutions in some of the monolayer experiments. A fully automated film balance was used for monolayer investigations. It was mounted onto an active, antivibrating X/Y table. Surface pressure was measured by means of the Wilhelmy plate method with a precision of (0.1 mN/m. Before each experiment, the trough was cleaned with chloroform and
J. Phys. Chem., Vol. 100, No. 25, 1996 10711 a hexane/ethanol (1/1, v/v) mixture and rinsed 4-5 times with warm water in order to remove residual organic contaminations from previous monolayer suckings. Spreading solutions were deposited on the clean liquid surface using a Hamilton syringe. Surface pressure/area isotherms of the monolayers were measured at a compression rate of usually 0.8 Å2/(molecule‚min). During the isotherm determinations, the surface pressure increased from zero to a maximum value (defined as πc) and then remained constant or decreased to some intermediate value (for surface densities of the overcompressed layers beyond the limiting density of 2D monolayers). The subphase temperature was varied in the region 8-25 °C by circulating water from a thermostat through the water-flow hood of the Langmuir trough. The principle of operation of BAM has been previously reported.38-40 The design of the BAM1 Plus microscope (Nanofilm Technology, Go¨ttingen, Germany), used in the present study for morphological characterization of insoluble monolayers at the air/water interface, has been described in detail in ref 39. The p-polarized (i.e., in the plane of incidence) light from a He-Ne laser (λ ) 632.8 nm) was incident at the Brewster angle (53.1°) to the air/water interface. Under such conditions of incidence, the reflectivity of the clean water surface was almost zero. The beam, reflected in the presence of an insoluble monolayer, was imaged on a CCD camera and recorded by means of a video recorder. The contrast of the images was provided by gradients of the monolayer density, thickness, and optical dispersion properties.38-40 Continuous variation of the polarization of the reflected light (through rotation of the analyzer in the path of the reflected beam in front of the camera) allowed the optical anisotropy of the monolayers39,40 to be studied. The lateral resolution of BAM was about 4 µm. BAM images were obtained during the continuous, slow monolayer compression simultaneously with the π/A isotherm determinations. The in-plane movement of the Langmuir trough, positioned beneath the microscope onto a X/Y table, allowed for monitoring of the morphological features of the monolayers over large areas of imaging. Results Monolayers of Nonionic Amphiphiles. Methyl arachidate was selected as an example of a nonionic, monolayer-forming non-hydrogen-bonding amphiphile. Its surface pressure/area isotherm (Figure 1a) exhibited a “spike” at the maximum surface pressure πc ) 47 mN/m. The drop in the monolayer stability, at surface densities exceeding the limiting 2D density (18.5 Å2/ molecule), could be associated with a transformation of the ester monolayer into 3D structures (i.e. monolayer “collapse”). However, the BAM image (Figure 1b), corresponding to the steep part of the π/A isotherm (π ) 35 mN/m), showed that the formation of a 3D phase from the condensed, optically homogeneous methyl arachidate monolayer already begins at π < πc. The 3D nuclei, separated from the supersaturated monolayer, were initially of a “point” type (small nuclei looking like dots, not shown). They quickly fuse in a lateral direction upon overcompression, thus forming small 3D “chains” (Figure 1b). The 2D-3D transformation of the monolayer was detected macroscopically as a surface pressure drop at πc when the concentration and size of the 3D nuclei became sufficiently large. The growth of the 3D phase of methyl arachidate ( Figure 1c) occurred predominantly by lateral overlapping of elongated (“chain” type) 3D centers. After the “spike” at πc (Figure 1a), the surface pressure remained nearly constant during the overlapping of the growing 3D islands. Further overcompres-
10712 J. Phys. Chem., Vol. 100, No. 25, 1996
Figure 1. Surface pressure/area isotherm (a) of a methyl arachidate monolayer on pure water subphase (pH 5.6) at 20 °C and BAM images demonstrating (b) nucleation of a 3D phase (bright “dots” and “chains”) in the continuous, supersaturated monolayer at a surface pressure of 35 mN/m from the steep part of the isotherm (i.e., the 2D-3D transformation is observed at surface pressures already below the maximum pressure πc ) 47 mN/m) and (c) growth and overlapping of the 3D nuclei within the “plateau” surface pressure region after the “spike” at πc. The bar represents 200 µm. In this and the following figures, the arrows indicate the surface pressures for which BAM images are shown.
sion of the layers led to the observation of irregular 3D structures. Their overlapping at the maximum surface densities recorded was accompanied by a second increase of the surface pressure (Figure 1a). Monolayers of Ionogenic Amphiphiles. The morphological features of the 2D-3D transformations were compared for alkanoic acid monolayers of different hydrocarbon chain lengths (C14, C18, C22) and different ionization and hydration states of the carboxylic head groups. This allowed for the investigation of the effects of in-plane chain-chain cohesive interactions and the head groups-aqueous subphase interactions on the 2D3D transformation mechanisms. 1. Stearic Acid Monolayers. (a) Unionized Head Groups. Figure 2a presents a typical surface pressure/area isotherm of a stearic acid monolayer spread on an acidified aqueous subphase. The “spike” at πc ≈ 50 mN/m is followed by a sharp surface pressure decrease to constant pressure values within the region of the overcompression (beyond the limiting 2D monolayer density). Analogous to the nonionic methyl arachidate monolayer, the 2D-3D transformation of the uncharged, supersaturated stearic acid monolayer was found to be initiated at surface pressures belonging to the steep part of the π/A isotherm (π < πc). Figure 2b shows the 3D centers of a “point” (granular) type nucleated from the homogeneous, condensed phase monolayer at π above ∼32 mN/m. The “point” centers did not fuse laterally into “chains” of the type shown in the right-hand portion of Figure 1. Further overcompression of the layers led to the growth and overlap of the 3D centers (Figure 2c). During this process, the surface pressure was at a nearly constant value, but the growth of highly ordered multilayers (favored by hydrogen-bondingnetwork formation between the carboxylic head groups24) could not be established by BAM. Upon the maximum piling of the granular 3D structures, the surface pressure increased again (Figure 2a). (b) Effect of Head Groups Ionization. Dissociation of the carboxylic head groups of fatty acid monolayers and their interaction with divalent counterions are used for enhancement
Angelova et al.
Figure 2. Surface pressure/area isotherm (a) of a stearic acid monolayer on acidified aqueous subphase (pH 3.0) at 20 °C and BAM images of (b) 3D nuclei (bright “dots”) observed at a surface pressure of 40 mN/m within the steep part of the isotherm (i.e., the 3D nucleation occurs at π < πc ) 50 mN/m) and (c) overlap and growth of the 3D nuclei within the “plateau” region of the π/A isotherm (after the pressure “spike” at πc). The bar represents 200 µm.
Figure 3. Surface pressure/area isotherm (a) of a stearic acid monolayer on alkaline aqueous subphase (1 × 10-4 M CaCl2, pH 8.0) at 20 °C and BAM images corresponding to the beginning (b) and the end (c) of the “plateau” surface pressure region at πc ) 63 mN/m, demonstrating nucleation of 3D structures (bright “chains”) and their overlapping into microcrystalline, inhomogeneous textures during continuous overcompression. The bar represents 200 µm.
of the monolayers’ stability. In the investigated case of stearic acid, spread on the alkaline, Ca2+-containing subphase, this was indicated (i) by the higher value of the maximum surface pressure attained during the isotherm determination (Figure 3a) and (ii) by the observation of a “plateau” region at πc, demonstrating the resistance of the monolayer with respect to transformation into a 3D phase. The BAM images, presented in parts b and c of Figure 3, were obtained at the onset and at the end of the “plateau” region of the π/A isotherm, respectively. They demonstrate that the “breakdown” of the charged stearic acid monolayers, at πc, occurs by formation of 3D structures of a “chain” type (Figure 3b). The number of “chains” increased upon continuous overcompression to higher surface densities followed by their overlapping into irregular, microcrystalline 3D structures (Figure 3c). Opposite to the case of the uncharged supersaturated monolayers (Figures 1 and 2), evolution of a 3D phase was not
2D-3D Transformations of Amphiphilic Monolayers
Figure 4. BAM images of condensed phase domains of a stearic acid monolayer on acidified (pH 3.0) aqueous subphase without (a) and with addition of concentrated H2O2 (b). The surface pressure is 8 mN/m, and the subphase temperature is 20 °C. The contrast of the images, adjusted through rotation of the analyzer of BAM, is related to different in-plane azimuthal tilt molecular orientations in the neighboring domains. Sharper interdomain boundaries are observed in the presence of H2O2. The bar represents 200 µm.
detected by means of BAM to occur for surface pressures considerably less than the maximum pressure πc. (c) Effect of Interfacial Hydrogen-Bonding Enhancement or Disturbance. To study the role of the hydrogen bonding in the mechanism of the 2D-3D transformations of unionized fatty acid monolayers, experiments were performed to provide enhancement or disturbance of the interfacial hydrogen-bonding networks formed between the carboxylic head groups and the molecules of the aqueous subphase. It is known43 that some chemical agents, added at high enough concentrations,44,45 may affect the strength of the hydrogen bonds, formed at interfaces or in bulk aqueous solutions, in a “bond-promoting” or “bondbreaking” manner. Perhydrol (H2O2) and the large thiocyanate anions (SCN-) were chosen as hydrogen-bonding promoters or breakers, respectively. They were added to the acidified subphase to a total concentration of 15% H2O2 or 0.1 M NaSCN. Under both kinds of aqueous subphase conditions (H2O2 or NaSCN), the shape of the stearic acid monolayer isotherm did not change (see Figure 2a). At this molecular chain length (C18), the morphology of the 2D-3D transformations also remained uninfluenced by the subphase composition. A rather different result was obtained for the shorter chain homologue (C14), as it will be shown below. The main effect of the addition of perhydrol on the monolayers’ morphology was established for surface pressures below the phase transition kink at π ≈ 25 mN/m. Figure 4 compares the morphologies of the condensed phase stearic acid domains in the absence (Figure 4a) and in the presence of H2O2 (Figure 4b). It is seen that the enhancement of the hydrogen-bonding interaction of the monolayer with the aqueous subphase, induced by the addition of H2O2, results in the observation of much sharper interdomain boundaries (Figure 4b). The different reflectivities of the neighboring domains (seen as areas of different brightness in the BAM images) result from different azimuthal molecular tilt angles42 within the condensed phase domains. While influencing the morphology of the 2D monolayer (Figure 4), the hydrogen-bonding promoting or breaking agents did not influence essentially the 2D-3D transformation of the unionized C18 alkanoic acid monolayer (see, for example, parts b and c of Figure 2).
J. Phys. Chem., Vol. 100, No. 25, 1996 10713
Figure 5. Surface pressure/area isotherms of arachidic acid monolayers on acidified aqueous subphase (pH 3.0), demonstrating the change of the shape of the isotherm in the region of πc upon the increase of the compression velocity: (1) 0.8 Å2/(molecule‚min); (2) 4 Å2/(molecule‚ min); (3) 12 Å2/(molecule‚min).
2. Arachidic and Behenic Acid Monolayers. (a) Effect of the Compression Rate. The shape of the π/A isotherms in the high surface pressure region can be influenced by variations of the monolayer compression rate.29,32,46,47 Such an example is demonstrated here for the case of unionized arachidic acid monolayers spread on acidified aqueous subphases. Figure 5 shows three surface pressure/area isotherms recorded at increasing compression velocities. A transition from a surface pressure “spike”, at πc, to a “plateau” was induced by faster monolayer compression. A similar effect was observed upon the ionization of the monolayer head groups. (b) Effect of Monolayer Interaction with the Aqueous Subphase. Surface pressure/area isotherms of behenic acid monolayers on acidified and alkaline aqueous subphases are compared in Figure 6. The shape of the isotherms in the high surface pressure region (“plateau”) and the maximum πc value of 64 mN/m are nearly identical for both the uncharged and the ionized monolayers. This shows that the stability of the unionized behenic acid monolayers is higher compared to that of the shorter-chain homologues. The main effect of the head group ionization and interaction with divalent (Ca2+) counterions on the π/A isotherms was expressed in a depression of the phase transition kink from about 30 mN/m (Figure 6, curve 1) to a nearly zero surface pressure (Figure 6, curve 2). At this molecular chain length (C22), essential effects of the head group ionization on the morphological features of the 2D3D transformations were not established. For both the acidified and the alkaline subphases, the transformations occurred within the “plateau” region of the isotherms upon overcompression at πc. They began with the formation of “chain” type structures (Figure 6a), which organized into networks upon further compression (Figure 6b), corresponding to the middle of the “plateau” region. The highest degree of overcompression resulted in the observation of irregular, microcrystalline textures similar to that shown in Figure 3c. The addition of H2O2 or NaSCN to the aqueous subphase for enhancement or disturbance of the interfacial hydrogenbonding networks did not show an essential effect on the mechanism of the 2D-3D transformation of the uncharged behenic acid monolayers. The obtained results indicated that the head group ionization state and the interaction with the subphase play a minor role in the determination of the shape of the π/A isotherms in the region of πc and the mechanism of the 2D-3D transformations when the stability of the alkanoic acid monolayers is dominated by cohesive interactions of the sufficiently long (C22) hydrocarbon chains.
10714 J. Phys. Chem., Vol. 100, No. 25, 1996
Angelova et al.
Figure 6. Surface pressure/area isotherms of behenic acid monolayers on acidified (pH 3.0) aqueous subphase (1) and alkaline 1 × 10-4 M CaCl2 (pH 8.0) subsolution (2) at 20 °C. The presented BAM images are typical for the initial (a) and the intermediate stage (b) of the 2D3D transformation occurring at πc ) 64 mN/m within the “plateau” region of the isotherms. The morphological features of the transformations are independent of the ionization state of the monolayer head groups. The final stage of the overcompression is similar to that shown in Figure 3c. The bar represents 200 µm.
Figure 7. Surface pressure/area isotherms of myristic acid monolayers on acidified aqueous subphase (pH 3.0) at 24 °C (1), 20.5 °C (2), and 10 °C (3). The kinks represent the L1/L2 (curve 1), L1/L2 and L2/LS (curve 2), and L2/LS (curve 3) phase transitions49,50 of the monolayers.
3. Myristic Acid Monolayers. The phase state and the pattern formation during phase transitions of myristic acid monolayers are very sensitive to the subphase temperature variations48 and the monolayer interaction with species from the aqueous subphase. For this reason, experiments were performed to elucidate the effects of these factors on the mechanism of the 2D-3D transformations of the myristic acid monolayers. (a) Unionized Head Groups. Effect of Subphase Temperature on the Condensed Phase Domains and 3D Crystallites Morphologies. Surface pressure/area isotherms of myristic acid monolayers recorded at three different temperatures of the aqueous subphase are presented in Figure 7. It is seen that moderate temperature variations may influence essentially the main phase transition pressure and the stability of the mono-
Figure 8. BAM images demonstrating the growth and fusion of condensed phase domains in myristic acid monolayers, preceding the 3D nucleation on acidified (pH 3.0) aqueous subphase. The bar represents 200 µm. Rotation of the analyzer was applied to study the anisotropic internal structure of the condensed phase domains. (a) Shown are circular, condensed phase domains formed at π ≈ 17 mN/m within the first-order phase coexistence region (Figure 7, curve 2). The subphase temperature is 20.5 °C. At small domain radii (∼80 µm), some of the domains are subdivided into two or three plane segments of uniform surface reflectivity. These segments are separated by straight line boundaries passing usually through the center of the domains. (b) Shown are condensed phase domains of large radii (∼200 µm), growing at π ≈ 19 mN/m (Figure 7, curve 2) and developing an unstable pattern of their internal structure at 20.5 °C. The subdomains boundaries adopt curved shapes and do not pass through the domain center. The reflectivity within each segment is not homogeneously distributed. At very large radii, the outer shape of the domains begins to deviate from a circular one. (c) Shown are condensed phase domains growing at π ≈ 10 mN/m at a subphase temperature of 10 °C (Figure 7, curve 3) and exhibiting an internal structure of a spiral type. (d) Shown is the 3D nucleation upon fusion of condensed phase domains in the high surface pressure regions of the isotherms (Figure 7). The LS phase of the monolayers appears as a homogeneously reflecting surface. (The superimposed interference pattern results from the optical system of the microscope used.) The nuclei of the myristic acid 3D phase, seen as small bright granules at the places of the interdomain fusion, are formed at surface pressures below πc.
layers (represented by the maximum surface pressure values, πc, measured under the same compression rates). In all cases, the π/A isotherms of the uncharged myristic acid monolayers were terminated by surface pressure “spikes” at πc (Figure 7). The morphological features of the 2D-3D transformations of myristic acid monolayers on acidified aqueous subphases are presented in Figures 8 and 9. Both the domain pattern and the internal structure of the condensed phase domains (see, for
2D-3D Transformations of Amphiphilic Monolayers
J. Phys. Chem., Vol. 100, No. 25, 1996 10715
Figure 10. (a) Surface pressure/area isotherm of a myristic acid monolayer on 1 × 10-4 M CdCl2 (pH 5.6) aqueous subphase at 22 °C; (b) BAM images of the condensed phase domains growing at π ≈ 5 mN/m within the first-order phase coexistence region; (c) 3D crystalline structures without compact shape, growing at overcompression after the pressure “spike” at πc of 40 mN/m. The 3D nucleation is observed at a surface pressure of about 30 mN/m. The bar represents 200 µm.
Figure 9. BAM images demonstrating the growth of compact 3D structures of myristic acid during the 2D-3D transformations of the monolayers on an acidified aqueous subphase: (a) plane 3D crystallites separated from the monolayers at a subphase temperature of 10 °C; (b) areas of different contrast on the surface of the crystallites formed at 22 °C and related to orientational or thickness gradients; (c) growth of very large, compact 3D crystallites of irregular surface structure upon the addition of 0.1 M NaSCN to the aqueous subphase (22 °C). The bar represents 200 µm.
example, the bright structures in Figure 8) were strongly dependent on the subphase temperature. The outer shape of the domains changed from branched, finger-like (at temperatures above 24 °C) to compact, rounded one (at temperature below ∼22-23 °C). The inner structure of the circular, condensed phase domains was characterized by straight or curved (sharp at 19 °C or smeared at 20-21 °C) boundaries between the segments of different azimuthal tilt angle (top two parts of Figure 8), or it was of a spiral type (at
