Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.     

Occurrence, shape, and dimensions of large surface hemimicelles made of semifluorinated alkanes. Elongated versus circular Hemimicelles. Pit- and tip-centered hemimicelles

The formation of large (approximately 20-35 nm) surface hemimicelles in monolayers of semifluorinated alkanes, C(n)F(2)(n)(+1)C(m)H(2)(m)(+1) (FnHm), observed after transfer onto silicon wafers, is a general phenomenon. F6H16 and F8H14 exclusively form highly monodisperse circular hemimicelles, organized in a hexagonal array. The other FnHm investigated form both circular and elongated hemimicelles. The longer FnHm is, the larger the area fraction of elongated micelles; both the hydrocarbon block (H-block) and the fluorocarbon block (F-block) affect this area fraction. The length of the elongated micelles increases with the total length of the diblocks. The diameter of the circular micelles increases with the length of the H-block but, unexpectedly, not with that of the F-block. Model calculations account for these observations. Close examination of the circular micelles showed that they generally present a pit or a tip at their center. The width of the elongated micelles is comparable to the radius of the circular micelles, suggesting that the latter arise from a partition of elongated micelles, followed by coalescence of the edges of the resulting fragments. The elongated micelles become shorter and fewer when surface pressure increases, further suggesting a conversion of elongated into circular micelles. This conversion is reversible. The surface pressure-molecular area isotherms do not present any feature that forebears the existence of hemimicelles. The obtaining of stable surface patterns from simple, "nonpolar" molecular fluorocarbon/hydrocarbon diblocks opens a new approach for producing featured nanostructures from organic templates.     

Gramicidin A channel in a matrix from a semifluorinated surfactant monolayer

Gramicidin A, a polypeptide antibiotic forming transmembrane ion channels, has been incorporated into a Langmuir monolayer formed by a semifluorinated alkane (SFA). In this work, partially fluorinated tetracosane, perfluorohexyloctadecane (F6H18), has been applied, aiming at finding a suitable matrix for gramicidin A to be transferred onto solid support for a biosensor design. For this purpose, the physiological conditions were of special interest (mixed monolayers containing low gramicidin proportion and the surface pressure of 30 mN/m). Mixed monolayers of gramicidin and SFA were found to be miscible within the whole range of mole fractions. A very significant increase of the stability of SFA monolayer has been found in the presence of gramicidin, even at such a low proportion as X(gramicidin) = 0.1, which is reflected in a 3.5-fold increase of the collapse pressure value of mixed monolayer as compared to the film from pure SFA. This interesting phenomenon has been interpreted as being due to the existence of a strong dipole-dipole interaction between both film-forming molecules. Opposite sign of the measured electric surface potential for gramicidin and SFA, resulting from different directions of the dipole moment vectors in both film molecules, implies that the ordered, antiparallel orientation of the dipole moments in the mixed gramicidin/SFA system can be responsible for its extremely high stability.     

Self-assembly of asymmetric fan-shaped dendrimers with different generation numbers at the air-water interface

Monolayer formation of two dendrimers containing a hydrophilic core group (COOH) and hydrophobic peripheral groups (anthracene and aryl ether tail groups), 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid (G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid (G2), were studied. To understand the mechanism of the self-assembly of these molecules, we measured the surface pressure-surface area (Pi- A) isotherm and investigated the surface texture of Langmuir-Blodgett monolayers transferred onto hydrophilic silicon wafers. Both dendrimers form circular domains at the onset point of surface pressure as a result of the difference in hydrophobicity between the core group and the peripheral end group. The core group has a functional group at the end of dendrimer and can be anchored on the water surface. Upon further compression, monolayer of G1 shows a domain of molecules whereas a monolayer of G2 is aligned in the direction of compression at 10 mN/m. At higher surface pressure (20 mN/m), G1 molecules have several aggregates of domains, but G2 molecules maintain their ordering. These results were confirmed by the electron density profile of G1 and G2 monolayers transferred to silicon substrates, as measured by X-ray reflectivity.     

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.     

Interactions of a fluoroaryl surfactant with hydrogenated, partially fluorinated, and perfluorinated surfactants at the air/water interface

A novel surfactant containing pentafluorophenyl moiety attached at the terminal position of undecanol (11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, abbr. PBD) was synthesized and employed for the Langmuir monolayer characterization and miscibility studies with a semifluorinated alkane (perfluorodecyleicosane, abbr. F10H20) and four alcohols differing in the degree of fluorination in their hydrophobic chains: octadecanol (C18OH), perfluorooctyldecanol (F8H10OH), perfluoroisononyldecanol (iF9H10OH) and 1H,1H-perfluorooctadecanol (F18OH). Pure monolayers of all of the investigated surfactants as well as their mixtures were investigated with surface pressure-area isotherms complemented by Brewster angle microscopy (BAM) images. PBD was found to form stable Langmuir monolayers of liquid-expanded character. Characteristic dendritic structures were formed at the very early stage of compression and remained up to the vicinity of collapse, where 3D crystallites appeared. 2D miscibility studies revealed that PBD forms mixed monolayers with the investigated semifluorinated alkane (F10H20) as well as with perfluorinated alcohol (F18OH) within the whole composition range, do not mix with octadecanol to the fully hydrogenated alcohol, whereas it is partially miscible (up to a certain surface pressure value) with the studied semifluorinated alcohols. The analysis of the miscibility derived from the surface pressure-area isotherms (collapse pressure vs composition dependencies) agrees well with BAM images. Molecular interactions in the investigated systems have been quantified with interaction parameter, alpha.     

Structural studies of the monolayers and bilayers formed by a novel cholesterol-phospholipid chimera

Langmuir isotherm, neutron reflectivity, and small angle neutron scattering studies have been conducted to characterize the monolayers and vesicular bilayers formed by a novel chimeric phospholipid, ChemPPC, that incorporates a cholesteryl moeity and a C-16 aliphatic chain, each covalently linked via a glycerol backbone to phosphatidylcholine. The structures of the ChemPPC monolayers and bilayers are compared against those formed from pure dipalmitoylphoshatidylcholine (DPPC) and those formed from a 60:40 mol % mixture of DPPC and cholesterol. In accord with previous findings showing that very similar macroscopic properties were exhibited by ChemPPC and 60:40 mol % DPPC/cholesterol vesicles, it is found here that the chimeric lipid and lipid/sterol mixture have very similar monolayer structures (each having a monolayer thickness of ～26 ?), and they also form vesicles with similar lamellar structure, each having a bilayer thickness of ～50 ? and exhibiting a repeat spacing of ～65 ?. The interfacial area of ChemPPC, however, is around 10 ?(2) greater than that of the combined DPPC/cholesterol unit in the mixed lipid monolayer (viz., 57 ± 1 vs 46 ± 1 ?(2), at 35 mN·m(-1)), and this difference in area is attributed to the succinyl linkage which joins the ChemPPC steroid and glyceryl moieties. The larger area of the ChemPPC is reflected in a slightly thicker monolayer solvent distribution width (9.5 vs 9 ? for the DPPC/cholesterol system) and by a marginal increase in the level of lipid headgroup hydration (16 vs 13 H(2)O per lipid, at 35 mN·m(-1)).     

Cationic Gemini surfactant at the air/water interface

The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C(18)H(37)(CH(3))(2)N(+)CH(2)C(6)H(4)CH(2)N(+)(CH(3))(2)C(18)H(37)],2Br(-), abbreviated as 18-Ar-18,2Br(-1)), at the air/water interface were investigated. It is found that the surface pressure-molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual "kink" before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir-Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV-vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV-vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a pi-pi aromatic stacking. This pi-pi interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.     

Langmuir-Blodgett-Kuhn and self-assembled films of asymmetrically substituted poly(paraphenylene)

Asymmetrically substituted poly(paraphenylene) (PhPPP) with hydrophilic and hydrophobic side chains was investigated. The polymer behavior at the air-water interface was studied on the basis of surface pressure-area (pi-A) isotherms and compression/expansion hysteresis measurements. PhPPP can form stable monolayers with an area per repeat unit of A=0.20+/-0.02 nm2 and a collapse pressure in the range of pi=25 mN/m. Then, Langmuir-Blodgett-Kuhn (LBK) films of PhPPP were prepared by horizontally and vertically transferring the Langmuir monolayers onto hydrophilic solid substrates at pi=12 mN/m. Cross-section analysis of the AFM tapping-mode topography images of a single transferred monolayer reveals a thickness of d0=0.9+/-0.1 nm. Taking into account the obtained monolayer thickness, curve-fitting calculations of angular scan data of LB monolayers measured using surface plasmon resonance (SPR) spectroscopy lead to a value for the refractive index of n=1.78+/-0.02 at lambda=632.8 nm. Next, the spontaneous formation of a PhPPP monolayer by adsorption from solution was studied ex situ by atomic force microscopy and UV-vis spectroscopy and in situ by using SPR spectroscopy. Stable self-assembled monolayers of PhPPP can be formed on hydrophilic surfaces with a thickness similar to that of the monolayer obtained using the LB method. The characterization results confirmed the amphiphilic character and the self-assembly properties of PhPPP, as well as the possibility of preparing homogeneous monolayer and multilayer films.     

The production and verification of pristine semi-fluorinated thiol monolayers on gold

In this work the interaction between human serum albumin (HSA) and a monofluorinated phospholipid, 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine] (F-DPPC), was studied by using Langmuir monolayer and Brewster angle microscopy (BAM) techniques. Different amounts of F-DPPC were spread on a previously formed HSA monolayer located at the air/water interface at 25 °C and the mixed monolayers thus obtained showed the existence of a liquid expanded-liquid condensed (LE-LC) phase transition (at 14 mN/m), attributed to the pure F-DPPC monolayer, coexisting with a second transition (at 22-24 mN/m) corresponding to the protein conformational change from an unfolded state to another in “loops” configuration. Relative thickness measurements recorded during the compression of the mixed monolayers showed the existence of an “exclusion” surface pressure (π_exc), above which the protein is squeezed out the interface, but not totally. BAM images reveal that some protein molecules in a packed “loops” configuration remain at the interface at surface pressures higher than the “exclusion” surface pressure. The application of the Defay-Crisp phase rule to the phase diagram of the F-DPPC/HSA system can explain the existence of certain regions of surface pressure in which the mixed monolayer components are miscible, as well as those others that they are immiscible.     

Swelling and deswelling of adsorbed microgel monolayers triggered by changes in temperature, pH, and electrolyte concentration

The formation and characterization of close-packed monolayers of negative, poly(N-isopropylacrylamide)-based microgel particles onto positively charged silicon wafers is described. The silicon wafers were rendered positive by first oxidizing their surface to silica and then adsorbing a layer of poly(ethyleneimine). The thickness of the deposited microgel monolayers (under aqueous conditions) has been determined by spectroscopic ellipsometry as a function of temperature (20-60 degrees C), pH (3-8), and added NaCl concentration (0-1 M). No actual desorption of the microgel particles was evident on changing the conditions, but a swelling/deswelling transition was observed around 32 degrees C. The thickness of the monolayer has been compared with the hydrodynamic diameter of the free microgel particles, dispersed in aqueous solution. For the poly(N-isopropylacrylamide) microgel particles, without any bulk ionisable comonomer groups present, the temperature dependence of the ellipsometric thickness of the monolayer mirrors closely that of the hydrodynamic diameter of the free particles. When ionizable (-COOH) groups are introduced into the microgel particles, however, this correspondence is largely lost because the microgel particles forming the deposited monolayer now contract strongly onto the oppositely charged substrate surface.     

Formation of 2D spherulites in Langmuir films of amphiphilic T-shaped liquid crystals

Langmuir films of facial T-shaped amphiphilic liquid crystals were studied at the air-water interface. The liquid crystals were composed of three incompatible segments: a central rigid rodlike p-terphenyl (TP) group, two flexible hydrophobic n-alkyl terminal chains of identical length linked through ether bonds, and one hydrophilic lateral chain of three ethylene oxide units with a carboxyl end group. In order to determine the influence of the alkyl chain length on the characteristics of condensed films three TPs having n-alkyl chains with eight (TP8/3), ten (TP10/3) or 16 (TP16/3) carbon atoms were investigated. Surface pressure - mean molecular area isotherms revealed clear differences. TP8/3 and TP10/3 exhibit an extended plateau region where a phase transition from monolayer to multilayer takes place. On the other hand, the TP16/3 isotherm showed a distinct maximum ('spike') corresponding to a surprising surface crystallization process which is reported for the first time for a Langmuir film of a liquid crystal. Brewster angle microscopy clearly confirmed these differences: TP8/3 and TP10/3 formed circular domains with liquid crystalline order, while TP16/3 formed well-defined two-dimensional polycrystalline spherulites which are fractured after further compression. The film thickness determined by X-ray reflectivity measurements correlated with a multilayer formation for TP10/3. The morphology of Langmuir-Blodgett (LB) films transferred onto silicon wafers and studied by atomic force microscopy also confirmed the striking different behavior (multilayer formation vs. 2D crystallization) of the TPs under investigation.     

髓鞘碱性蛋白对细胞膜脂质分子DPPC、DPPE单层膜影响机理研究

本文通过Langmuir单层膜的表面压力-平均分子面积(π-A)曲线的测定与分析,分别对髓鞘碱性蛋白(MBP)与细胞膜中不同头部基团脂质分子二棕榈酰基磷脂胆碱(DPPC)和二棕榈酰基磷脂酰乙醇胺(DPPE)在空气/液体界面上的相互作用过程进行了系统研究.实验结果表明:(1)当界面上脂质含量一定时,亚相中随着MBP浓度的增大,DPPC、DPPE单层膜的等温线向平均分子面积较大的方向移动;(2)在单层膜表面压力为10 mN/m时,一个MBP分子分别结合140±3个DPPC分子和100±3个DPPE分子,随着表面压力增大,当MBP分子分别与两种磷脂分子相互作用时,MBP插入到磷脂单层界面的个数逐渐减少;(3)随着蛋白质浓度的增加,脂分子形成的单层膜变得较为疏松,且MBP分子易于插入到分子头部较小的DPPE单层膜中;(4)蛋白质的存在使DPPC单层膜的表面压力逐渐减小,且蛋白质浓度越大表面压力降低越多,DPPC被MBP带入到亚相中越多;(5)对于DPPE单层膜,蛋白质通过与DPPE相互作用插入到界面膜中,引起表面压力增大,且蛋白质浓度越高,压力变化量越大.     

Langmuir-Blodgett films of cellulose nanocrystals: preparation and characterization

The goal of this work is the preparation of monolayers of cellulose I nanocrystals providing flat crystalline cellulose surfaces. Suspensions of cellulose nanocrystals were prepared by hydrolyzing ramie and tunicin fibers with sulfuric acid. Due to surface grafted sulfate groups, the negatively charged, rod-like cellulose nanocrystals were found to form stable layers at the air-water interface in the presence of a cationic amphiphilic molecule such as dioctadecyldimethylammonium (DODA) used in this work. These layers were formed at different cellulose-DODA weight ratios, compressed and analyzed by tensiometry, ellipsometry and Brewster angle microscopy. At low cellulose concentrations the layers are discontinuous, becoming dense and homogeneous upon reaching a critical weight ratio, which depends on the aspect ratio of the cellulose nanocrystals. After transfer onto silicon wafers, the surface composition and morphology as well as the thickness of the films were examined by X-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. The results indicate that they are monolayer films, well structured, relatively smooth and pure. These films offer a crystalline and easily reproducible model cellulose surface.     

Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

Two-dimensional miscibility studies of alamethicin and selected film-forming molecules

Alamethicin (ALM), a 20-amino acid antibiotic peptide (peptaibol) from fungal sources, was mixed in Langmuir monolayers with six different surfactants: semifluorinated (F6H18, F10H19, F8H10OH, F6H10SH) and hydrogenated (C18SH and DODAC), aimed at finding appropriate molecules for ALM incorporation for nanodevice construction. Alamethicin-containing mixed monolayers were investigated by means of surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM). Our results show that only semifluorinated alkanes can serve as an appropriate material since they form miscible and homogeneous monolayers with ALM within the whole concentration range. All the remaining surfactants, possessing polar groups, were found to demix with ALM. This effect was explained as being due to the existence of strong polar interactions between vertically oriented surfactant molecules, which tend to separate from horizontally oriented alpha-helices of the peptide. On the contrary, semifluorinated alkanes, lacking any polar group in their structure and bearing a large dipole moment, interact with ALM, also possessing a huge cumulative dipole moment. These dipole-dipole interactions between ALM and SFAs are more attractive than those between SFA molecules in their pure monolayers, causing the large ALM molecule, situated parallel to the interface, to be surrounded by SFA molecules in perpendicular orientation, leading to the formation of a highly organized binary mixed monolayer. BAM images of the ALM monolayer indicate that this peptide collapses with the nucleation and growth mechanism, like the majority of surfactants, which contradicts the model of ALM collapse by desorption, previously published in the literature.     

Preparation of microscopic and planar oil-water interfaces that are decorated with prescribed densities of insoluble amphiphiles

Langmuir monolayers (monolayers of insoluble molecules formed at the surface of water), and associated Langmuir-Blodgett/Schaefer monolayers prepared by transfer of Langmuir films to the surfaces of solids, are widely used in studies aimed at understanding the physicochemical properties of biological and synthetic molecules at interfaces. In this article, we report a general and facile procedure that permits transfer of Langmuir monolayers from the surface of water onto microscopic and planar interfaces between oil and aqueous phases. In these experiments, a metallic grid supported on a hydrophobic solid is used to form oil films with thicknesses of 20 mum and interfacial areas of 280 mum x 280 mum. Passage of the supported oil films through a Langmuir monolayer is shown to lead to quantitative transfer of insoluble amphiphiles onto the oil-water interfaces. The amphiphile-decorated oil-water interfaces hosted within the metallic grids (i) are approximately planar, (ii) are sufficiently robust mechanically so as to permit further characterization of the interfaces outside of the Langmuir trough, (iii) can be prepared with prescribed and well-defined densities of amphiphiles, and (iv) require only approximately 200 nL of oil to prepare. The utility of this method is illustrated for the case of the liquid crystalline oil 4-pentyl-4'-cyanobiphenyl (5CB). Transfer of monolayers of either dilauroyl- or dipalmitoylphosphatidylcholine (DLPC and DPPC, respectively) to the nematic 5CB-aqueous interface is demonstrated by epifluorescence imaging of fluorescently labeled lipid and polarized light imaging of the orientational order within the thin film of nematic 5CB. Interfaces prepared in this manner are used to reveal key differences between the density-dependent phase properties of DLPC and DPPC monolayers formed at air-water as compared to that of nematic 5CB-aqueous interfaces. The methodology described in this article should be broadly useful in advancing studies of the interfacial behavior of synthetic and biological molecules at liquid-liquid interfaces.     

Effect of the conditions of transfer on the structure and optical properties of Langmuir graphene oxide films during deposition on a substrate

The effect the solvent and transfer pressure of graphene oxide (SLGO) Langmuir–Blodgett films on the physicochemical properties of monolayers, and on their structural and optical properties, is studied. Examination of the physicochemical properties of SLGO monolayers on subphase surfaces that are formed from SLGO dispersions in different organic solvents reveals that monolayer behavior is virtually independent of the solvent. Electron microscope and optical studies show that the monolayers formed from SLGO dispersions in DMF and acetone have the highest transfer coefficients. It is concluded that the structural heterogeneity of the surfaces of graphene oxide films results from simultaneous effect of electrostatic interactions between graphene oxide particles and Van der Waals interactions with the solvation shell of the particles. Studies focusing on the effect the pressure of transferring a graphene oxide monolayer onto the surface of a solid substrate has on structural features of LB films show that films produced at low surface pressures have more homogeneous structures.     

Study of Langmuir and Langmuir-Blodgett films of odorant-binding protein/amphiphile for odorant biosensors

To make ultrathin films for the fabrication of artificial olfactory systems, odorant biosensors, we have investigated mixed Langmuir and Langmuir-Blodgett films of odorant-binding protein/amphiphile. Under optimized experimental conditions (phosphate buffer solution, pH 7.5, OBP-1F concentration of 4 mg L(-1), target pressure 35 mN m(-1)), the mixed monolayer at the air/water interface is very stable and has been efficiently transferred onto gold supports, which were previously functionalized by self-assembled monolayers (SAMs) with 1-octadecanethiol (ODT). Atomic force microscopy and electrochemical impedance spectroscopy were used to characterize mixed Langmuir-Blodgett (LB) films before and after contact with a specific odorant molecule, isoamyl acetate. AFM phase images show a higher contrast after contact with the odorant molecule due to the new structure of the OBP-1F/ODA LB film. Non-Faradaic electrochemical spectroscopy (EIS) is used to quantify the effect of the odorant based on the electrical properties of the OBP-1F/ODA LB film, as its resistance strongly decreases from 1.18 MOmega (before contact) to 25 kOmega (after contact).     

Molecular interactions of cord factor with dipalmitoylphosphatidylcholine monolayers: implications for lung surfactant dysfunction in pulmonary tuberculosis

Molecular interactions between mycobacterial cell wall lipid, cord factor (CF) and the abundant surfactant lipid, dipalmitoylphosphatidylcholine (DPPC) were investigated using Langmuir monolayers at physiological temperatures (37 degrees C). Surface topography of the films was visualized by atomic force microscopy (AFM). Thermodynamic behavior of the mixed monolayers was evaluated by investigating the molecular area excess, excess Gibbs free energy of mixing and maximum compressibility modulus (SCM(max)). Cord factor formed immiscible and thermodynamically unstable monolayers with DPPC. Monolayer presence of cord factor altered the physical state of DPPC monolayers from liquid condensed to liquid expanded with the lowering of SCM(max) from 160 to 40 mN/m, respectively. AFM imaging exhibited smooth homogenous surface topography of DPPC films which in the presence of cord factor was markedly altered with the appearance of aggregates and increased surface roughness. The results highlight the capacity of cord factor to disturb DPPC monolayer organization and structure. Interfacial presence of cord factor results in DPPC monolayer fluidization. Lung surfactant function is attributed to its ability to form well packed low compressibility films. Such molecular interactions suggest a dysfunction of lung surfactant in pulmonary tuberculosis due to surfactant monolayer fluidization.