Brewster Angle Microscopy Observation Research Articles

Interactions of l-arginine with Langmuir monolayers of common phospholipids at the air-water interface

The interactions of l-arginine (l-arg) with Langmuir monolayers of three most common phospholipids, which are sodium salt of dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE), have been investigated at the air-water interface. The surface pressure-area (π-A) isotherms of these monolayers have been measured with a film balance and monolayer morphology has been observed by a Brewster angle microscopy (BAM). The DPPG monolayers on pure water do not show any phase transition but show irregular shaped condensed phases formed just after evaporation of the solvent at 20 °C. However, this monolayer on l-arg solution subphase indicates a first-order phase transition from liquid expanded to liquid condensed (LE-LC) phases and forms LC domains at the same temperature. With an increase in the l-arg concentration in the subphase up to 5.0 × 10−4 M, the π-A shows an overall increasingly greater expansion in the molecular area. All of the π-A isotherms recorded on ≥5.0 × 10−4 M l-arg solution subphases almost coincide with each other. These changes in the phase behavior have been explained by the fact that l-arg having guanidinium cationic group undergoes strong hydrogen bonding interaction with the anionic phosphatidylglycerol (PG-) head group. The bonding between two molecules is further strengthened by electrostatic attraction between cationic l-arg and anionic PG- ions. The BAM observation of the monolayer morphology supports this explanation. On the other hand, a very negligible interaction has been observed between l-arg and DPPC or DPPE monolayers. The π-A isotherms in the presence of l-arg for both the amphiphiles show a very little expansion only in the LE phase region, but coincide in the so called solid phase region. The monolayer morphology of both the monolayers also supports these results. This little effect of expansion in the LE region may be explained by the ion-pair formation between cationic l-arg and anionic head groups in the monolayers at lower pressures. However, due to compression at high pressure, the l-arg molecules are squeezed out from the amphiphile head groups.

Morphology Observation of Novel 3, 4-Bis-Alkyloxycarbonyl-Hexanedioic Acid Monolayer on Water Surface by Dropping Method

We have investigated the morphology of novel 3, 4-bis-alkyloxycarbonyl-hexanedioic acid monolayer (alkyl chain number n = 12, 14, and 16) formed on the water surface by dropping method using Surface Tension measurement (STm) and Brewster Angle Microscopy (BAM). Surfactant of 3, 4-bis-alkyloxycarbonyl-hexanedioic acid has a gemini type structure. π-A isotherm curves (STm) of two gemini surfactants of n = 12 and 14 showed gradual increase in surface pressure, while that of n = 16 showed gradual increase, a stagnant range and finally steep increase. BAM observation showed each image corresponding to the result of STm measurement. From STm and BAM results of surfactants, it is found that surfactant of n = 12 and 14 form Liquid-Expand (LE) type monolayer, while that of n = 16 formed Liquid-Condensed (LC) type monolayer including LE-LC phase transition.

Study of the collapse mechanism of selected fluorinated surfactants

Two semifluorinated alkanes (SFAs), differing in their fluorinated segment lengths (F6H18 and F10H19), were compared with typical surfactants (F8H10OH and C18OH) as regards their collapse mechanism. It has been found that analysis of the π – A isotherms under different experimental conditions is insufficient for a reliable description of the collapse phenomenon, and the importance of the relaxation experiments complemented with Brewster angle microscopy observation has been emphasized in this respect. The applied nucleation–growth–collision model points out differences between typical (alcohols) and nontypical (SFAs) surfactants as regards the collapse mechanism. Our results indicate that for the collapse process the presence of the polar head group in the molecule is more important than the structure of the hydrophobic chain.

Phase behaviour of binary mixtures involving tristearin, stearyl stearate and stearic acid: thermodynamic study and BAM observation at the air–water interface and AFM analysis of LB films

The binary mixtures involving tristearin (TS), stearyl stearate (SS) and stearic acid (SA) were studied by surface pressure–area ( π– A) measurements and by Brewster angle microscopy (BAM), at the air–water interface, and the Langmuir–Blodgett (LB) monolayers, transferred onto mica substrates, were analysed by AFM. The thermodynamic analysis indicated miscibility in the whole composition range for the system SA/TS, and partial miscibility for systems SA/SS and TS/SS. This behaviour was further confirmed by BAM observation and AFM analysis of LB films. The AFM imaging of collapsed monolayers revealed domains with a multilayered structure varying with system and composition. The layers thickness determined by cross section analysis are consistent with estimated molecular lengths and conformations proposed for the molecules, assuming nearly perpendicular or tilted orientations of the hydrocarbon chains to the interface.

Instability of Adsorbed Films of 1-Dodecanol and is Mixture with Sodium Dodecyl Sulfate at the Air/Water Interface

Properties and structures of adsorbed films on mixed aqueous solutions of sodium dodecyl sulfate (SDS) and a trace amount of 1-dodecanol (DD) were investigated by film balance experiments, a Brewster angle microscopy (BAM), and a quick X-ray reflectometry (q-XR) at 20 C. Pouring the aqueous solutions of the surfactants into a trough, the surface pressure first increased up to certain maximum values due to adsorption of the surfactants at the air/solution interface, but after a while it started to decrease probably due to evaporation of DD. BAM observation showed formation of holes of fluid phase in homogeneous condensed-phase monolayers in the course of surface pressure decrease. The q-XR technique was applied to the adsorbed films at the beginning stage of destabilization in order to characterize the film structures at the molecular level.

Monolayer behavior of a pyridyl head-group-containing amphiphile and its miscibility with poly( d, l-lactide-co-glycolide) on different pH subphase

In this paper, we investigated the Langmuir film and Langmuir–Blodgett (LB) monolayer film of a nonionic amphiphilic molecule, 4-(6- p-pyridyloxyl)hexyloxyl-4′-dodecyloxylazobenzene (C 12AzoC 6Py) and its mixture with poly( d, l-lactide-co-glycolide) (PLG) at different subphase pH values (2.0, 2.6, 3.3, 4.4, and 6.5, respectively) by surface pressure–area ( π – A ) isotherms, in situ interface Brewster angle microscopy (BAM), and ex situ atomic force microscopy (AFM). For pure C 12AzoC 6Py, its π – A isotherms display a plateau when the subphase pH value is lower than 3.0. The pressure of the plateau increases with the decrease of pH until 2.0. Over the plateau, the π – A isotherms become almost identical to the one under neutral conditions. The appearance of such a plateau can be explained as the coexistence of protonation and unprotonation of pyridyl head groups of the employed amphiphile. In contrast to the homogeneous surface morphology of pure C 12AzoC 6Py near the plateau by BAM observation, the surface in the case of its mixing with PLG exhibits a dendritic crystalline state under low surface pressure at subphase pH lower than 3.0. The crystalline state becomes soft and gradually melts into homogeneous aggregates with surface pressure increasing to a higher value than that of the plateau. Meanwhile, the hydrolysis of PLG in the mixture system at the interface has been affirmed to be restrained to a very large extent. And the PLG was believed to be compelled to the up layer of the LB film due to the phase separation, which is examined by AFM. Based on the experimental results, the corresponding discussion was also performed.

Kinetic Appearance of First-Order Gas−Liquid Expanded and Liquid Expanded−Liquid Condensed Phase Transitions below the Triple Point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (pi-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 degrees C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the pi-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air–water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 °C, is constructed by measuring the surface pressure (π)–time (t) adsorption isotherms. Below 25.8 °C, each of the π–t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the π–t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G–LE (liquid expanded) phase transition, which is followed by another first-order LE–I phase transition. The second-order phase transition is an I–LC phase transition. Contrary to these results, at 36 °C both the π–t measurements and the BAM observation present only two first-order phase transitions, which are G–LE at zero surface pressure and LE–LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 °C to an elongated at 24 °C and finally to a circular shape at 36 °C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.

Mixed monolayers involving DPPC, DODAB and oleic acid and their interaction with nicotinic acid at the air–water interface

The behaviour of binary mixtures involving dipalmitoylphosphatidylcholine (DPPC), dioctadecyldimethylammonium bromide (DODAB) and oleic acid (OA) was investigated at the air–water interface by surface pressure–area ( π– A) measurements and by Brewster angle microscopy (BAM). Thermodynamic analysis indicates for the system DPPC/DODAB miscibility with strong negative deviations from the ideal behaviour, from low to high surface pressures over all the composition range. For systems DODAB/OA and DPPC/OA, thermodynamic analysis and BAM observation indicate miscibility from low to intermediate surface pressures, and phase separation in a limited range of composition at high surface pressures. The interaction of nicotinic acid (NA) with pure lipids and with selected compositions of mixed systems was investigated. Significant positive deviations of π– A isotherms in the presence of NA indicate attractive interactions between NA and the polar groups of DPPC and DODAB. NA easily penetrates in expanded regimes while it tends to be segregated from condensed regimes in mixed monolayers.

Phase behaviour and morphology of binary mixtures of DPPC with stearonitrile, stearic acid, and octadecanol at the air −water interface

The behaviour of dipalmitoylphosphatidylcholine (DPPC), mixed with stearonitrile (SN), was investigated at the air–water interface by surface pressure–area ( π– A) measurements and by direct visualisation of monolayers by Brewster angle microscopy (BAM). The π– A– X diagram of system DPPC/SN was compared with the corresponding diagrams of systems DPPC/stearic acid (SA) and DPPC/octadecanol (OD) at 20 °C. Monolayers of the three systems reach the closest packing of alkyl chains in the 0.4−0.6 range of X DPPC. Thermodynamic analysis indicates miscibility in the three binary systems with negative deviations from the ideal behaviour. Morphological features of system DPPC/SN change significantly with X DPPC and temperature in the range 10–30 °C. At 10 and 20 °C mixed monolayers form condensed states from low π all over the composition range. At 30 °C, the liquid-expanded (LE)−liquid-condensed (LC) phase transition occurs at increasing π with X DPPC. The shape and size of condensed domains change with X DPPC and π. Contrarily to the behaviour of pure components, mixed monolayers of DPPC/SN exhibit orientational order in the 0.2−0.6 mol fraction range of DPPC. BAM observation confirmed the partial miscibility indicated by G E data in a limited range of compositions at 30 °C.

Spontaneous two-dimensional crystallization and photochemical reaction of a novel bolaamphiphilic naphthol ester in the spreading Langmuir film at the air/water interface and transferred Langmuir–Blodgett film

A bolaamphiphilic compound containing photoactive naphthyl group, 1,20-bi(α-naphthol)octadecane-dicarboxylate (C20BNp) was newly synthesized. The spreading property of the compound at the air/water interface was investigated by the surface pressure (π)–area ( A) isotherm and in situ Brewster angle microscopy (BAM). It has been found that C20BNp could self-aggregate into a two-dimensional (2D) crystalline film on water surface. Upon irradiation (365 nm) in N 2 atmosphere, photochemical reaction in the Langmuir film occurred as detected from the surface pressure–time measurement and in situ BAM observation. The 2D crystal film on water surface could be deposited onto solid substrates by vertical dipping to form a well-organized Langmuir–Blodgett (LB) film, as verified from the X-ray diffraction measurement. Upon irradiation, photochemical reaction could also occur in the LB film as verified from the ultraviolet and fluorescence measurement. While the atomic force microscopy image of the LB film appeared as a flat rectangle crystal, many holes have been observed after photoirradiation.

Photoinduced Dimerization of ap-Phenylenediacrylic Acid Derivative in a Langmuir Monolayer Mixed with Stearic-d35Acid on a Water Surface

Photoinduced cycloaddition of a p-phenylenediacrylic acid derivative, 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid, was conducted successfully in a mixed Langmuir monolayer with stearic-d35 acid on a water surface at room temperature. The reaction process was investigated by UV−visible and infrared spectroscopy, gel permeation chromatography (GPC), and Brewster-angle microscopy (BAM). The product was identified as the cyclobutane dimer. The formation of the stable monolayer and the dimerization reaction on the water surface were evaluated at the molecular level by observation of the monolayer's behavior (surface pressure, monolayer area, and change of surface morphology). The BAM observation and GPC analysis of the photoproducts obtained under different states of the monolayer reveal that the molecular arrangement with higher mobility gives a much higher reaction speed.

Inherent and Cooperative Photomechanical Motions in Monolayers of an Azobenzene Containing Polymer at the Air−Water Interface

Photoinduced expansion and contraction response in monolayers consisting of a poly(vinyl alcohol) derivative having an azobenzene (Az) side chain at the air−water interface was investigated in detail by surface potential measurements, Brewster angle microscopy (BAM), UV−visible absorption spectroscopy, and the macroscopic area estimations with a Langmuir film balance. Surface potential measurements confirmed the model of photostimulated motions previously supposed (Seki et al., Bull. Chem. Soc. Jpn. 1996, 69, 2375). The BAM observation combined with a movable minitrough allowed precise evaluations of the mechanical response of an isolated domain monolayer at zero pressure, which can be regarded as providing the inherent attributes. In comparison with the corresponding UV−visible spectroscopic data, the expansion process on UV light (365 nm) illumination was found to show a nonlinear response with photoreaction, which is characterized by the existence of an induction period. Initiation of the film expansio...

Monolayer Behavior of Poly(amic acid) Alkylamine Salts Containing the Dimethylsiloxane Structure and Their Langmuir−Blodgett Films

The behavior of monolayers of poly(amic acid) alkylamine salts containing different dimethylsiloxane chain lengths at the air−water interface was investigated. The limiting areas estimated from surface pressure−area isotherms were much larger than those calculated from both the alkyl chain number per repeating unit and the cross-sectional area of the alkyl chain. The direct observation of monolayers was carried out by Brewster angle microscopy (BAM). The BAM observation was affected by the length of the dimethylsiloxane chain in the poly(amic acid)s. The layered structure of the LB films of poly(amic acid) alkylamine salts with multichains was investigated by X-ray diffraction, Fourier transform infrared−reflection absorption spectroscopy, and X-ray photoelectron spectroscopy. Consequently, it was found that the LB films consist of bilayer (Y-type) structures in which the alkyl chains are appreciably tilted from the normal direction and that the long dimethylsiloxane chain is concentrated in the outer surface of the LB films.

Host−Guest Monolayers of Metal Complexes of Octaalkoxyphthalocyanines with Long-Chain Normal Alkanes on a Water Surface

Mixed monolayers consisting of octakis(octadecyloxy)phthalocyanine metal complexes (C18PcM; M = Mg, Co, Cu, Zn) and long-chain alkanes on a water surface were investigated by measurements of π−A isotherms and visible absorption spectra and Brewster angle microscopy observation. C18PcCu and octadecane (C18) in a 1:7 molar ratio gave a uniform monolayer, where phthalocyanine rings float in parallel with a water surface, and one C18PcCu molecule incorporates seven C18 molecules to form a molecular complex. It was found that the formation of two-dimensional molecular complexes consisting of C18PcCu and a long-chain alkane shows critical temperature dependence, reflecting the melting point of guest hydrocarbons.

Visualization of Photoresponsive Monolayers Comprised of Photochromic Polymers.

Photomechanical response of the polymer monolayers having an azobenzene (Az) side chain was successfully visualized by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). In situ BAM observation that covers the sub-millimeter scale range revealed clear photoinduced morphological and rheological changes. By following the photoinduced morphological changes of an isolated single domain, a couple of hitherto unknown properties have been unveiled, for example, non-linearity involved in the film expansion process, intrinsic magnitude of film expansion at zero pressure, dependence on light intensity and so forth. By AFM that observes the ranges of micrometer to nanometer levels, it was found that even on a mica surface, the Az containing monolayer shows a reversible photoinduced film deformation under highly humid conditions. These visualization techniques provided a great deal of complementary unique information which covers the scale ranges between the macroscopic Langmuir trough experiments and scale spectroscopic observations at a molecular level.