Subphase Conditions Research Articles

PH effect in Langmuir–Blodgett self-assembly of MoS2 and WS2 thin films

We reported Langmuir–Blodgett films of molybdenum disulphide and tungsten disulfide and studied the formation of MoS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Mo}}{\ ext{S}}_{2}$$\\end{document} and WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{WS}}_{2}$$\\end{document} layers. The effect of the subphase pH is investigated. Also, the influence of the subphase conditions on the number of layers was reported. By varying the pH of the buffer solution subphase, the solvent-to-nanoparticle interaction is changed and the compression of the layer during deposition is influenced. The stacking of the MoS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Mo}}{\ ext{S}}_{2}$$\\end{document} and WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{WS}}_{2}$$\\end{document} crystalline structure under the influence of subphase pH and the material concentration were investigated. Langmuir layers assembly is a useful method to manipulate the distribution of WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{WS}}_{2}$$\\end{document} and MoS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Mo}}{\ ext{S}}_{2}$$\\end{document}. Successful deposition of dichalcogenides at the air–water interface on silicon substrate was performed using Balance Langmuir–Blodgett technique. The characterization of WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{WS}}_{2}$$\\end{document} and films MoS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{Mo}}{\ ext{S}}_{2}$$\\end{document} with X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning electron microscopy was carried out.

Effects of ionic strength and ion-specificity on the interface behavior of PDMAEMA-b-PLMA-b-POEGMA triblock terpolymer

Ion-specificity on the stretching degree of diblock copolymers containing poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) at the air/water interface has previously been proved to be inconsistent with the Hofmeister series. In this work, the aggregation behavior of an amphiphilic linear triblock terpolymer poly[2-(dimethylamino)ethyl methacrylate]-block-poly(lauryl methacrylate)-block-poly[oligo(ethylene glycol) methacrylate] (PDMAEMA-b-PLMA-b-POEGMA) on the aqueous subphase with different ionic strengths and ion species (salts) was studied, and the morphologies of its Langmuir−Blodgett (LB) films were explored. At the air/water interface, the terpolymer tends to form surface micelles composed of hydrophobic PLMA cores and mixed hydrophilic coronas of PDMAEMA and POEGMA. Under acidic conditions with low ionic strength, the hydrophilic PDMAEMA blocks of the terpolymer are almost completely electrostatic screened. With the increase of subphase ionic strength, the surface pressure−molecular area isotherms of the terpolymer monolayers at the air/water interface gradually move to large areas due to the increased interfacial stretching degree of POEGMA blocks resulting from the increased salting-out effect and interfacial steric hindrance of PDMAEMA blocks. The hysteresis degrees of the terpolymer monolayers in the presence of salts in the subphase are larger than that in the absence of salt. Under alkaline conditions, the isotherms of the terpolymer monolayers move to large areas with the increase of salt concentration because the introduction of salt makes the hydrophilic blocks more stretched. The experimental results show that the relationship between their interfacial stretching degrees of two kinds of hydrophilic blocks and the types of salt in solution is inconsistent with the Hofmeister series. This may be due to tethering of the corona chains on the hydrophobic core and the configurational constrains (crowding phenomena) induced within the mixed micelle corona. Under different subphase conditions, the LB films of the terpolymer show isolated circular micelles due to the mutual repulsions of the long hydrophilic PDMAEMA and POEGMA blocks.

Purified myelin lipids display a critical mixing point at low surface pressure

Lipids extracted from Purified Myelin Membranes (LPMM) were spread as monomolecular films at the air/aqueous interface. The films were visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (π) and ionic environments. Coexistence of Liquid-Expanded (LE) and cholesterol-enriched (CE) rounded domains persisted up to π ≈ 5 mN/m but the monolayers became homogeneous at higher surface pressures. Before mixing, the domains distorted to non-rounded domains. We experimentally measured the line tension (λ) for the lipid monolayers at the domain borders by a shape relaxation technique using non-homogeneous electric fields. Regardless of the subphase conditions, the obtained line tensions are of the order of pN and tended to decrease as lateral pressure increased toward the mixing point. From the mean square displacement of nested trapped domains, we also calculated the dipole density difference between phases (μ). A non-linear drop was detected in this parameter as the mixing point is approached. Here we quantitively evaluated the π-dependance of both parameters with proper power laws in the vicinity of the critical mixing surface pressure, and the exponents showed to be consistent with a critical phenomenon in the two-dimensional Ising universality class. This idea of bidimensionality was found to be compatible only for simplified lipidic systems, while for whole myelin monolayers, that means including proteins, no critical mixing point was detected.Finally, the line tension values were related with the thickness differences between phases (Δt) near the critical point.

Aggregation behavior of the strong amphiphilic cationic diblock polyelectrolytes at the air/water interface

AbstractEffects of subphase temperature and pH on the interfacial behavior of two strong amphiphilic cationic diblock polyelectrolytes, quaternized poly(2‐[dimethylamino]ethyl methacrylate)‐block‐poly(lauryl methacrylate) (qPDMAEMA‐b‐PLMA), were studied by the Langmuir film balance technique for the first time. The morphologies of their Langmuir–Blodgett (LB) films were studied by atomic force microscopy. At the air/water interface, surface micelles with hydrophobic PLMA cores and hydrophilic qPDMAEMA coronas form. With the increase of temperature, the condensed surface pressure‐molecular area isotherms move to large area (right shift). The higher the temperature, the more obvious the isotherm shifts to the right. At low temperature, the isotherms of the two copolymers are almost respectively overlapped under different subphase pH conditions. At high temperature, however, the isotherms of the two copolymers show some subphase pH dependence. All initial LB films obtained at different subphase conditions exhibit isolated circular micelles. Under different subphase pH conditions, the diameters of the micelle cores in the two copolymers' LB films obtained at low temperature are close but larger than those at high temperature, which phenomenon is first reported. This is probably due to the relatively lower mobility of polymer individual chains at low temperature, resulting in their higher local surface concentration and larger aggregation.

Exploring Langmuir-Blodgett technique to investigate effect of various subphase conditions on monolayers formed by amphiphilic block co-polymers tetronic 701 and tetronic 90R4

Tetronics are X-shaped octa block amphiphilic copolymer having multi-stimuli sensitive chemical and physical properties due to the presence of central ethylenediamine, polyethylene oxide (PEO) and polypropylene oxide (PPO) groups. Langmuir monolayers formed at the air-water interface by two tetronics namely tetronic 701 with normal sequence and reverse sequential tetronic 90R4, were studied on a comparative basis. The effects of different subphase conditions on the monolayer at the air-water interface were systematically investigated. Different spreading volumes of tetronics, different pH, temperature and ionic strength of the subphase were the parameters considered in this study. The corresponding Langmuir-Blodgett films were characterized with scanning electron microscopy (SEM). The SEM images of the films at different pH, temperature and salt environment showed a diversity of the copolymer arrangement at the air-water interface which corroborated with the explanation obtained from π-A isotherms analyses. The study revealed that the sequence of the block did not affect the nature of isotherm, instead the ratio of PPO/PEO and molecular weight played significant role in determining the characteristics of the films. This basic interfacial study on tetronics was not only a part of fundamental property-study but would definitely be helpful to design potential application involving tetronics in various fields.

Interfacial properties and aggregates of novel redox-active surfactant to synthesize silver nanoparticles at the air/water interface

An anionic and amphiphilic molecule, AQua (AQ-NH-(CH2)10COOH; where AQ is anthraquinone), which has two stimuli responsive functional-groups, can form self-assembled interfacial aggregates at the air-water interface and, their morphologies can be controlled by changing the subphase conditions such as the pH of the subphase. These interfacial morphologies may be altered from planar structures to wormlike aggregates which is a very rare observation in the literature. Moreover, the unique functional groups of AQua give possibility to reduce metal cations and allow the formation of organic/inorganic nanocomposite structures at the interface. In this context, formation of silver nanoparticles (AgNPs) via AQua-based self-assembled structures at the interface was studied using Langmuir techniques. The kinetics of the reduction process as well as the effect of various parameters were systematically examined. This study demonstrates that interfacial configuration of the AQua molecule at the interface as well as its orientation has a vital role in the morphology of either organic or inorganic self-assemblies formed. The morphology may be adjusted and designed interfacial structures may be obtained. The interfacial properties of AQua, a novel material, may be used in a wide variety of fields such as biotechnology, medicine, energy and pharmaceuticals.

Influences of subphase pH and temperature on the interfacial aggregation behavior of poly(lauryl methacrylate)-block-poly(methacrylic acid)

We previously reported that poly(acrylic acid) (PAA) blocks at the air/water interface spontaneously immersed into water at high pH before monolayer compression. To investigate the influence of the hydrophobic methyl groups in poly(methacrylic acid) (PMAA) blocks, in this work, subphase pH and temperature effects on the aggregation behaviors of two pH-responsive amphiphilic diblock copolymers poly(lauryl methacrylate)-block-poly(methacrylic acid) (PLMA-b-PMAA) (PLMA49%-PMAA and PLMA61%-PMAA, composition in weight percent) at the air/water interface were studied. The surface pressure–molecular area isotherms of the two copolymers move to small mean molecular area (mma) as subphase pH increases and a quasi-plateau exists under various subphase conditions, which is quite different from its absence in our previous PAA-containing block copolymer system. The quasi-plateau is attributed to the pancake-to-brush transition as non-ionized PMAA chains submerge into subphase upon monolayer compression. Under acidic condition, PMAA chains of the two copolymers are almost non-ionized, the compression and expansion isotherms show large hysteresis phenomena. Under neutral and alkaline conditions, the isotherms show small hysteresis, which is because more ionized PMAA chains immerse in subphase before compression. The isotherms shift toward small mma with increasing temperature under neutral and alkaline conditions, whereas a slight shift back to large mma is observed under acidic conditions. The plateau surface pressures of the two PLMA-b-PMAA copolymers decrease with increasing temperature under different pH conditions, which is because the micelles move more freely and easier to rearrange at high temperature. The plateau surface pressures of PLMA49%-PMAA are higher than those of PLMA61%-PMAA under the corresponding conditions due to the higher PMAA content in the former. The initial Langmuir–Blodgett (LB) films of the two copolymers transferred under different pH conditions at various temperatures exhibit isolated circular micelles because of the steric hindrance and/or repulsive interactions among the long PMAA chains. Furthermore, temperature has almost no effect on the micelle core diameters of the initial LB films.

Effects of spreading and subphase conditions on the interfacial behavior of an amphiphilic copolymer poly(n-butylacrylate)-b-poly(acrylic acid)

Aggregation behavior of an amphiphilic diblock polyelectrolyte poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at the air/water interface and its Langmuir−Blodgett (LB) films were respectively characterized by the Langmuir film balance technique and atomic force microscopy. Effects of spreading solution concentration and volume, and subphase temperature and ionic strength on the interfacial aggregation behavior of PnBA-b-PAA under alkaline condition were systematically studied. With the increase of spreading concentration, surface pressure−molecular area isotherms shift to small molecular areas at the corresponding spreading volumes. With the rise in subphase temperature, the isotherms shift to small molecular areas due to the decreased pKa value and increased solubility of PAA blocks. With the increase of subphase ionic strength, the isotherms shift to large molecular areas because of the predominant salting out effect. The LB films transferred from alkaline subphase with different ionic strengths are more likely to exhibit isolated circular micelles. With the increase of spreading concentration, diameters of micelle cores first decrease and then increase, a behavior which is attributed to the gradually increased chain entanglements and surface polymer concentration, respectively. For small spreading volume, core diameters in the LB films first decrease and then increase with the rise in subphase temperature. For large spreading volume, however, core diameters increase with temperature due to the higher mobility and weaker entanglements of PAA blocks underwater. With the increase of subphase ionic strength, furthermore, core diameters first increase and then decrease, which is attributed to the predominant electrostatic shielding and salting out effects, respectively.

Continuous Langmuir-Blodgett Deposition and Transfer by Controlled Edge-to-Edge Assembly of Floating 2D Materials.

The Langmuir-Blodgett technique is one of the most controlled methods to deposit monomolecular layers of floating or surface active materials but has lacked the ability to coat truly large-area substrates. In this work, by manipulating single-layer dispersions of graphene oxide (GO) and thermally exfoliated GO into water-immiscible spreading solvents, unlike traditional Langmuir-Blodgett deposition which requires densification achieved by compressing barriers, we demonstrate the ability to control the 2D aggregation and densification behavior of these floating materials using a barrier-free method. This is done by controlling the edge-to-edge interactions through modified subphase conditions and by utilizing the distance-dependent spreading pressure of the deposition solvent. These phenomena allow substrates to be coated by continuous deposition and substrate withdrawal-enabling roll-to-roll deposition and patterning of large-area substrates such as flexible polyethylene terephthalate. The aggregation and solvent-driven densification phenomena are examined by in situ Brewster angle video microscopy and by measuring the local spreading pressure induced by the spreading solvent acting on the floating materials using a Langmuir-Adam balance. As an example, the performance of films deposited in this way is assessed as passivation layers for Ag nanowire-based transparent conductors.

Effect of Subphase Conditions on the Formation of Graphene Langmuir layers

This paper presents a study of the formation of graphene layers by Langmuir-Blodgett (LB) technique. The effect of subphase pH and temperature on the graphene Langmuir isotherms are investigated. Also, the influence of subphase conditions on the surface potential of a monolayer was reported. A variation in the pH of the water subphase can lead to change in the electrostatic interactions that influence the compression of the layers. The mechanical characteristics of graphene layers variation under the influence of the subphase pH and temperature was investigated. Langmuir layers assembly is a useful method to manipulate the distribution of graphene layers at the air-water interface. Successful deposition of graphene layers at the air-water interface on a silicon substrate was performed by using Langmui-Schaefer (LS) technique.

Transparent and Hydrophobic “Reduced Graphene Oxide–Titanium Dioxide” Nanocomposites for Nonwetting Device Applications

Graphene oxide–titanium dioxide (GO–TiO2) nanocomposite sheets were transferred onto Si and quartz substrates by Langmuir–Blodgett (LB) technique at different subphase TiO2 concentrations and pH values. The effects of subphase and heat treatment conditions on the composition, surface morphology, microstructure, and hydrophobic performance of the composite sheets were investigated by XPS, Raman, AFM, HR-TEM, and contact-angle measurements. The wetting behavior of composite sheets before and after vacuum heat treatment was analyzed in conjunction with the extent of TiO2 uptake, distribution of nanoparticles over GO sheets, and overall surface roughness. The degree of subphase ionization significantly affects the uptake and aggregation behavior of TiO2, which tends to be dominated by the interaction of TiO2 with carboxylic acid functional groups at the edges of the GO sheets. Wettability studies revealed improvement in the hydrophobicity of composite sheets compared to the GO sheets, which is attributed prim...

Влияние состава субфазы и внешних условий на формирование и свойства ленгмюровского монослоя

Влияние состава субфазы и внешних условий на формирование и свойства ленгмюровского монослоя

A ‘modified’ Langmuir–Blodgett technique for transfer of graphene oxide monolayer sheets on solid substrates

A ‘modified’ Langmuir–Blodgett (MLB) technique has been developed as a simple, low cost, energy efficient and scalable alternative to conventional Langmuir–Blodgett (LB) technique for the transfer of graphene oxide (GO) monolayer sheets from air–water interface on to solid substrates. This technique uses a simple apparatus consisting of a glass or teflon reservoir and a vertically held stationary substrate. The transfer of GO sheets from the air–water interface onto hydrophilic substrates takes place by a downward movement of the meniscus, through controlled draining of subphase. The MLB technique facilitates the transfer of well defined, uniformly distributed, morphologically flat and adherent GO sheets under a wide range of conditions, namely subphase pH in the range, 3.5–6.5, target pressure in the range, 0.5–8 mN m−1 and meniscus speed in the range, 0.5–3 mm min−1 on RCA-1 treated Si and SiO2/Si. Compared to LB technique, it offers a substantially larger tolerance for subphase and transfer conditions, even on ‘weakly’ hydrophilic (RCA-1treated) Si substrates, as neither crumpling of GO sheets nor transfer of GO agglomerates and multilayers are observed in a wide window of operation. Subsequent chemical reduction of the MLB transferred GO sheets with hydrazine vapor followed by thermal treatment at 400 °C results in reduced graphene oxide (RGO) sheets, which contain ∼70% graphitic carbon, with O/C ratio ∼0.16 and display Raman G-mode ∼1590 cm−1, together with I(D)/I(G) ratio ∼1. The electrical conductivity and field effect mobility values of RGO sheets were measured in back gated field effect geometry and are comparable to those for RGO sheets obtained by LB as well as other approaches, followed by chemical/thermal reduction. Thus the newly developed MLB technique opens a simple, versatile and scalable route to obtain device quality GO and RGO sheets on solid substrates.

Behavior of maltose-neopentyl glycol-3 (MNG-3) at the air/aqueous interface

Integral membrane proteins (IMPs) performing a variety of cell functions comprise a significant fraction of the proteins encoded in an organism’s genome. Maltose-neopentyl glycol-3 (MNG-3) was invented as a novel detergent to extract IMPs from the cell membranes; MNG-3 has an inherent flexibility originating from its central carbon and its arms comprising alkyl chains and maltose groups. Herein, we demonstrate the reverse-scissoring behavior of MNG-3 through the use of different subphase conditions at the air/water interface. The origin of its high performance was investigated by means of Langmuir techniques, X-ray reflectivity (XRR) analysis of its Langmuir–Schaefer (LS) film, Brewster angle microscope (BAM) and atomic force microscope (AFM) and computational simulation.

Effect of substrate and subphase conditions on the surface morphology of graphene oxide sheets prepared by Langmuir–Blodgett technique

Graphene oxide (GO) sheets were transferred by Langmuir–Blodgett technique on Si and SiO2/Si substrates. The surface pressure – area isotherms of ‘amphiphilic’ GO sheets at the air–water interface are found to be highly sensitive to subphase pH, which significantly affects their hydrophilicity and the interaction between the sheets. The surface morphology of GO sheets transferred under different subphase and transfer conditions has been studied. RCA-1 (NH4OH:H2O2:H2O – 1:1:2) treatment of Si is found to be necessary for the transfer of GO sheets. The deposition on RCA-1 treated Si, is also highly sensitive to the compressibility of the Langmuir monolayer. Thus, transfer of well defined, uniformly distributed and flat GO sheets on Si is possible only in a narrow window of process parameters, namely, subphase pH, target pressure and lifting speed. In contrast, the transfer behavior and morphology of GO sheets on RCA-1 treated SiO2/Si is practically insensitive to changes in subphase pH, target pressure and lifting speed. This behavior is attributed to the uniformly strong hydrophilic character of SiO2/Si surface, compared to that of Si. The different behavior of Si surface, even after RCA-1 treatment, is attributed to its microscopic chemical and topographical inhomogeneities.

Effect of cyclodextrins on the behaviour of insulin at the air–water interface: A Langmuir monolayer study

Herein we report the effect of β-cyclodextrins on the behaviour of bovine insulin Langmuir monolayers spread at the air–water interface. The insulin–β-cyclodextrin interaction was studied using two different approaches: first, the macrocycles were dissolved in the aqueous subphase, and second, they were included in the insulin spreading solutions. The experiments were performed under various subphase conditions of pH and salt content. The behaviour of the insulin monolayers in the presence of β-cyclodextrins was investigated by measuring the isotherms of the surface pressure (π) as a function of the area per molecule and the surface compressional modulus–π curves. Brewster angle microscopy was applied to confirm the formation of insulin aggregates at the air–water interface. Our results indicate that the characteristics and behaviour of the insulin monolayers are influenced by β-cyclodextrins and their effect on the monolayers depends on the location of the macrocycles.

Soybean Oleosomes Behavior at the Air–Water Interface

Soy milk is a highly stable emulsion, the stability being mainly due to the presence of oleosomes or oil bodies, spherical structures filled with triacylglycerides (TAGs) and surrounded by a monolayer of phospholipids and proteins called oleosins. For oleosomes purified from raw soymilk, surface pressure investigations and Brewster angle microscopy have been performed to unveil their adsorption, rupture and structural changes over time at different subphase conditions (pH, ionic strength). Such investigations are important for (industrial) food applications of oleosomes, but are also useful for the understanding of the general behavior of proteins and phospholipids at interfaces. In addition a better comprehension of the highly stable oleosomes can lead to advancements in liposome manufacturing, e.g., for storage and transport applications. Although oleosomes have their origin in food systems, their unique stability and physical behavior show transferable characteristics which lead to a much better understanding of the description of any kind of emulsion. This study is one of the first steps toward the comparison of natural emulsification concepts based on different physical structures: e.g., the animals' low density lipoproteins, where apolipoproteins with phospholipids are located only at the interface and plant oleosomes with its oleosins, which are embedded in a phospholipid monolayer and reach deep inside the oil phase.

Nanopatterned Cadmium Selenide Langmuir–Blodgett Platform for Leukemia Detection

We present results of the studies relating to preparation of Langmuir-Blodgett (LB) monolayers of tri-n-octylphosphine oxide-capped cadmium selenide quantum dots (QCdSe) onto indium-tin oxide (ITO) coated glass substrate. The monolayer behavior has been studied at the air-water interface under various subphase conditions. This nanopatterned platform has been explored to fabricate an electrochemical DNA biosensor for detection of chronic myelogenous leukemia (CML) by covalently immobilizing the thiol-terminated oligonucleotide probe sequence via a displacement reaction. The results of electrochemical response studies reveal that this biosensor can detect target DNA in the range of 10(-6) to 10(-14) M within 120 s, has a shelf life of 2 months, and can be used about 8 times. Further, this nucleic acid sensor has been found to distinguish the CML-positive and the control negative clinical patient samples.

Surface Phase Behavior and Domain Topography of Ascorbyl Palmitate Monolayers

Ascorbyl palmitate (ASC(16)) is a molecule of potential pharmacological interest due to its antioxidant properties and amphiphilic nature. The surface behavior of ASC(16) was studied using Langmuir monolayers and Brewster angle microscopy. This molecule formed stable monolayers at room temperature that showed phase transition from a liquid-expanded to liquid-condensed or crystalline phase, depending on the subphase conditions. Using a theoretical approach, we were able to explain the behavior of the ASC(16) film at different bulk pH values and salt conditions based on the surface pH and the dissociation fraction of the film. Both condensed phases corresponded to highly packed conditions with the crystalline phase occurring at a low charge density, showing molecular tilting and preferential growth at characteristic angles, while the liquid-condensed phase formed in highly charged surfaces revealed small flowerlike domains probably as a consequence of internal dipole repulsion. A smaller perpendicular dipole moment was observed for the crystalline than the liquid-condensed phase which may explain the domain features. In conclusion, ASC(16) showed a complex surface behavior that was highly sensitive to subphase conditions.

Bovine Insulin–Phosphatidylcholine Mixed Langmuir Monolayers: Behavior at the Air–Water Interface

The behavior of the binary mixed Langmuir monolayers of bovine insulin (INS) and phosphatidylcholine (PC) spread at the air-water interface was investigated under various subphase conditions. Pure and mixed monolayers were spread on water, on NaOH and phosphate-buffered solutions of pH 7.4, and on Zn(2+)-containing solutions. Miscibility and interactions between the components were studied on the basis of the analysis of the surface pressure (π)-mean molecular area (A) isotherms, surface compression modulus (C(s)(-1))-π curves, and plots of A versus mole fraction of INS (X(INS)). Our results indicate that intermolecular interactions between INS and PC depend on both the monolayer state and the structural characteristics of INS at the interface, which are strongly influenced by the subphase pH and salt content. Brewster angle microscopy (BAM) was applied to investigate the peptide aggregation pattern at the air-water interface in the presence of the studied lipid under any experimental condition investigated. The influence of the lipid on the INS behavior at the interface strongly depends on the subphase conditions.