The cis-bis(decanoate)tin phthalocyanine/DPPC film at the air/water interface

Films made of cis-bis-decanoate-tin(IV) phthalocyanine (PcSn10) and racemic dipalmitoylphosphatidylcholine (DPPC) are studied with compression isotherms and Brewster angle microscopy (BAM) at the air/water interface. Films enriched in PcSn10 present phase separation elliptical-shaped domains. These domains present optical anisotropy and molecular order. They are enriched in PcSn10, and the film outside these domains is enriched in DPPC, as shown in by high-angle annular dark-field transmission electron microscopy on Langmuir-Blodgett (LB) transferred films. Film collapse area and atomic force microscopy images of LB transferred films on mica indicate that the films are actually multilayers. A computational survey was performed to determine how the PcSn10 molecules prefer to self-assemble, in films basically made of PcSn10. The relative energetic stability for several dimeric assemblies was obtained, and a crystal model of the film was developed through packing and repeating the PcSn10 molecules, along the crystallographic directions of the unit cell. Our results contribute to understanding the strong interaction between PcSn10 and DPPC at the air/water interface, where even small quantities of DPPC (~1-2%) can modify the film in an important way.     

Studies of Langmuir-Blodgett films of an ion pair metal complex containing Eu(III)-Ru(II) dual chromophores

A surfactant ion-pair complex, [Ru(bpy)(2)L][Eu(NTA)(4)](2) (in which L = 1-docosyl-2-(2- pyridyl)benzimidazole, bpy = 2,2'-bipyridine, and NTA = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedionato) has been synthesized. The surface pressure-area isotherm measurements show that the complex forms a stable Langmuir film at the air-water interface without adding any electrolytes into the subphase. The monolayers formed at the surface pressures of 5 mN m(-1) and 20 mN m(-1), have been successfully transferred onto glass and quartz substrates with the transfer ratios close to unity. The Langmuir-Blodgett films were studied by UV-visible, infrared, and emission spectroscopies, atomic force microscopy, and cyclic voltammetry. The optical, redox, and morphology properties of the LB films were found to be significantly affected by the target surface pressures used for the film depositions.     

Effects of albumin and erythrocyte membranes on spread monolayers of lung surfactant lipids

Dipalmitoyl phosphatidylcholine (DPPC), one of the main constituents of lung surfactant is mainly responsible for reduction of surface tension to near 0 mN/m during expiration, resisting alveolar collapse. Other unsaturated phospholipids like palmitoyloleoyl phosphatidylglycerol (PG), palmitoyloleoyl phosphatidylcholine (POPC) and neutral lipids help in adsorption of lung surfactant to the air-aqueous interface. Lung surfactant lipids may interact with plasma proteins and hematological agents flooding the alveoli in diseased states. In this study, we evaluated the effects of albumin and erythrocyte membranes on spread films of DPPC alone and mixtures of DPPC with each of PG, POPC, palmitoyloleoyl phosphatidylethanolamine (PE), cholesterol (CHOL) and palmitic acid (PA) in 9:1 molar ratios. Surface tension-area isotherms were recorded using a Langmuir-Blodgett (LB) trough at 37 degrees C with 0.9% saline as the sub-phase. In the presence of erythrocyte membranes, DPPC and DPPC+PA monolayers reached minimum surface tensions of 7.3+/-0.9 and 9.6+/-1.4 mN/m, respectively. Other lipid combinations reached significantly higher minimum surface tensions >18 mN/m in presence of membranes (Newman Keul's test, p<0.05). The relative susceptibility to membrane inhibition was [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)=(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)]. The differential response was more pronounced in case of albumin with DPPC and DPPC+PA monolayers reaching minimum surface tensions less than 2.4 mN/m in presence of albumin, whereas DPPC+PG and DPPC+POPC reached minimum surface tensions of around 20 mN/m in presence of albumin. Descending order of susceptibility of the spread monolayers of lipid mixtures to albumin destabilization was as follows: [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)]>[(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)] The increase in minimum surface tension in presence of albumin and erythrocyte membranes was accompanied by sudden increases in compressibility at surface tensions of 15-30 mN/m. This suggests a monolayer destabilization and could be indicative of phase transitions in the mixed lipid films due to the presence of the hydrophobic constituents of erythrocyte membranes.     

Influence of molecular environment on the analysis of phospholipids by time-of-flight secondary ion mass spectrometry

Understanding the influence of molecular environment on phospholipids is important in time-of-flight secondary ion mass spectrometry (TOF-SIMS) studies of complex systems such as cellular membranes. Varying the molecular environment of model membrane Langmuir-Blodgett (LB) films is shown to affect the TOF-SIMS signal of the phospholipids in the films. The molecular environment of a LB film of dipalmitoylphosphatidylcholine (DPPC) is changed by varying the film density, varying the sample substrate, and the addition of cholesterol. An increase in film density results in a decrease in the headgroup fragment ion signal at a mass-to-charge ratio of 184 (phosphocholine). Varying the sample substrate increases the secondary ion yield of phosphocholine as does the addition of proton-donating molecules such as cholesterol to the DPPC LB film. Switching from a model system of DPPC and cholesterol to one of dipalmitoylphosphatidylethanolamine (DPPE) and cholesterol demonstrates the ability of cholesterol to also mask the phospholipid headgroup ion signal. TOF-SIMS studies of simplistic phospholipid LB model membrane systems demonstrate the potential use of these systems in TOF-SIMS analysis of cells.     

Interfacial organizations of gel phospholipid and cholesterol in bovine lung surfactant films

Pulmonary surfactants stabilize the lung by way of reducing surface tension at the air-lung interface of the alveolus. 31P NMR, thin-layer chromatography, and electrospray ionization mass spectroscopy of bovine lipid extract surfactant (BLES) confirmed dipalmitoylphosphatidylcholine (DPPC) to be the major phospholipid species, with significant amounts of palmitoyl-oleoylphosphatidylcholine, palmitoyl-myristoylphosphatidylcholine, and palmitoyl-oleoylphosphatidylglycerol. BLES and DPPC spread at the air-water interface were studied through surface pressure area, fluorescence, and Brewster angle microscopy measurements. Langmuir-Blodgett films of monomolecular films, deposited on mica, were characterized by atomic force microscopy. BLES films displayed shape, size, and vertical height profiles distinct from those of DPPC alone. Calcium ions in the subphase altered BLES film domain structure. The addition of cholesterol (4 mol %) resulted in the destabilization of compressed BLES films at higher surface pressures (>40 mN m-1) and the formation of multilayered structures, apparently consisting of stacked monolayers. The studies suggested potential roles for individual surfactant lipid components in supramolecular arrangements, which could be the contributing factors in pulmonary surfactant to attain low surface tension at the air-water interface.     

Surface chemistry studies of (CdSe)ZnS quantum dots at the air-water interface

Trioctylphosphine oxide- (TOPO-) capped (CdSe)ZnS quantum dots (QDs) were prepared through a stepwise synthesis. The surface chemistry behavior of the QDs at the air-water interface was carefully examined by various physical measurements. The surface pressure-area isotherm of the Langmuir film of the QDs gave an average diameter of 4.4 nm, which matched very well with the value determined by transmission electron microscopy (TEM) measurements if the thickness of the TOPO cap was counted. The stability of the Langmuir film of the QDs was tested by two different methods, compression/decompression cycling and kinetic measurements, both of which indicated that TOPO-capped (CdSe)ZnS QDs can form stable Langmuir films at the air-water interface. Epifluorescence microscopy revealed the two-dimensional aggregation of the QDs in Langmuir films during the early stage of the compression process. However, at high surface pressures, the Langmuir film of QDs was more homogeneous and was capable of being deposited on a hydrophobic quartz slide by the Langmuir-Blodgett (LB) film technique. Photoluminescence (PL) spectroscopy was utilized to characterize the LB films. The PL intensity of the LB film of QDs at the first emission maximum was found to increase linearly with increasing number of layers deposited onto the hydrophobic quartz slide, which implied a homogeneous deposition of the Langmuir film of QDs at surface pressures greater than 20 mN.m(-1).     

Effect of lysozyme adsorption on the interfacial rheology of DPPC and cholesteryl myristate films

A model tear film lipid layer composed of a binary mixture of cholesteryl myristate (CM) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was characterized using surface tension measurements, Brewster angle microscopy (BAM) and interfacial stress rheology (ISR). Isotherms showed that films containing >or=90 mol % CM have a 17-fold greater % area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Lipid films with or=50 mol % CM became elastic at higher surface pressures. Increasing CM content reduced the surface pressure at which the mixed film became elastic. Lysozyme adsorption into a CM film increased the compressibility and resulted in a more expanded film. Lysozyme increased the ductility of the CM/DPPC films with no film breakdown occurring up to the highest pressure measured (40 mN/m). In summary, CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption increased the ductility and decreased the isotherm hysteresis for CM/DPPC films.     

Interfacial interaction between dextran sulfate and lipid monolayers: an electrochemical study

The interaction between dextran sulfate (DS) with zwitterionic dipalmitoylphosphatidylcholine (DPPC) and negatively charged dipalmitoylphosphatidic acid monolayers at different surface pressures at air-liquid and liquid-liquid interfaces was studied using Langmuir-Blodgett (LB) and electrochemical techniques. The negatively charged DS can bind to phospholipids via calcium ions. To investigate the mechanism of the adsorption of DS on lipid monolayers, compression isotherms (pi-A) and capacitance-potential curves were measured, and a theoretical model was developed to interpret the capacitance data. The compression of lipid monolayers in the presence of DS led to a more condensed hybrid layer, removing the LE-LC phase transition of DPPC. Lower surface pressures improved the binding of DS on the lipid monolayers via calcium bridges due to the electrostatic attraction. Alternating current voltammetry and cyclic voltammetry were used to monitor the transfer of a cationic beta-blocker (metoprolol) across lipid monolayers in the absence and presence of the polyelectrolyte and to compare with the transfer of the standard probe, tetraethylammonium cation. Results showed a strong dependence on (i) the surface pressure, (ii) the applied potential, and, (iii) in the case of the hybrid layer, the charge of the phospholipid headgroup. Finally, results were also confirmed by attenuated total reflection Fourier transform infrared spectroscopy, performed after transferring lipid multilayers onto a solid substrate by the LB method.     

Deposition and photopolymerization of phase-separated perfluorotetradecanoic acid-10,12-pentacosadiynoic acid Langmuir-Blodgett monolayer films

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.     

On the low surface tension of lung surfactant

Natural lung surfactant contains less than 40% disaturated phospholipids, mainly dipalmitoylphosphatidylcholine (DPPC). The mechanism by which lung surfactant achieves very low near-zero surface tensions, well below its equilibrium value, is not fully understood. To date, the low surface tension of lung surfactant is usually explained by a squeeze-out model which predicts that upon film compression non-DPPC components are gradually excluded from the air-water interface into a surface-associated surfactant reservoir. However, detailed experimental evidence of the squeeze-out within the physiologically relevant high surface pressure range is still lacking. In the present work, we studied four animal-derived clinical surfactant preparations, including Survanta, Curosurf, Infasurf, and BLES. By comparing compression isotherms and lateral structures of these surfactant films obtained by atomic force microscopy within the physiologically relevant high surface pressure range, we have derived an updated squeeze-out model. Our model suggests that the squeeze-out originates from fluid phases of a phase-separated monolayer. The squeeze-out process follows a nucleation-growth model and only occurs within a narrow surface pressure range around the equilibrium spreading pressure of lung surfactant. After the squeeze-out, three-dimensional nuclei stop growing, thereby resulting in a DPPC-enriched interfacial monolayer to reduce the air-water surface tension to very low values.     

Langmuir-Blodgett films and chiroptical switch of an azobenzene-containing dendron regulated by the in situ host-guest reaction at the air/water interface

An amphiphilic dendron containing an azobenzene ring at the focal point and the l-glutamate peripheral groups was designed. Its monolayer formation and host-guest reaction with cyclodextrins at the air/water interface and the properties of the transferred Langmuir-Blodgett (LB) films were investigated. The individual dendron, although without any long alkyl chains, could still form a stable monolayer at the air/water interface because of the good balance between hydrophilic and hydrophobic parts within the molecule. When cyclodextrin (CyD) was added to the subphase, a host-guest reaction occurred in situ at the air/water interface. The inclusion of the focal azobenzene moiety into the cavity of cyclodextrin decreased the packing of the aromatic ring and also led to the diminishment of the molecular area. Both the films formed at the surface of pure water and aqueous cyclodextrins were transferred onto solid substrates. Nanofiber structures were obtained for the film from the water surface as a result of the packing of the azobenzene groups, and circular domains were obtained for the film transferred from the aqueous CyD phases. The film transferred from the water surface showed an exciton couplet in the absorption band of azobenzene, whereas a negative Cotton effect was obtained for the film from CyD subphases. It was found that the supramolecular chirality in the LB film transferred from water was due to the transfer of the molecular chirality to the assemblies whereas that from the CyD subphase was due to the inclusion of azobenzene into the chiral cavity. Interestingly, the film from the water surface was photoinactive, whereas a reversible optical and chiroptical switch could be obtained for the film from the α-CyD subphase. The work provided a way to regulate the assembly and functions of organized molecular films by taking advantage of the interfacial host-guest reaction.     

Hysteresis behavior of amphiphilic model peptide in lung lipid monolayers at the air-water interface by an IRRAS measurement

Pulmonary functions such as rapid adsorption, respreading, and hysteresis behavior of pulmonary surfactants are very important for respiratory movement. The interfacial behavior of pulmonary preparations containing an amphiphilic peptide (Hel 13-5) has recently investigated. An orientation of hydrophobic chains in a dipalmitoylphosphatidylcholine (DPPC) with or without palmitic acid (PA) is associated with a collapse of alveoli during respiration process. Therefore, the present study focused on the acyl chain orientation in model pulmonary surfactants (DPPC/Hel 13-5 and DPPC/PA/Hel 13-5). A successive change in the orientation during cyclic compression and expansion of films at the air-water interface can be probed directly by an infrared reflection-absorption spectrometry (IRRAS) technique. The hysteresis behavior, one of very important pulmonary functions, was previously observed in surface pressure (pi)-molecular area (A) isotherms for the both model pulmonary surfactant systems (Langmuir 22(2006)1182-1192 and Langmuir 22(2006)5792-5803). In addition, it was reported that Hel 13-5 was squeezed-out of the surface on compression like native pulmonary surfactant proteins. The data obtained for the binary and ternary systems were compared with those of the equivalent pure DPPC and DPPC/PA mixtures, respectively. For an asymmetric methylene stretching vibration (nu(a)-CH(2)) RA intensity, the absolute RA values increased with shifting to small surface area, monotonously. For the corresponding wavenumber, on the other hand, the values gradually decreased into approximately 2920cm(-1). However, they were kept constant in the squeeze-out region in spite of a further decrease of surface area. These results suggested that the orientation of hydrophobic chains in DPPC and DPPC/PA mixtures became in the most packed state soon after emergence of the squeeze-out process of Hel 13-5 and then the packed orientation was retained up to the collapse state. This indicated that the squeezed-out Hel 13-5 stabilized monolayers left at the interface. For the DPPC/PA/Hel 13-5 system, in particular, dissociated PA molecules were excluded together with Hel 13-5 and the surface monolayers were refined to DPPC and undissociated PA components during the compression process. And the similar behavior in the second and third cycles supported the good respreading ability of the monolayers containing Hel 13-5.     

Langmuir monolayer and Langmuir–Blodgett films of amphiphilic triblock copolymers with water-soluble middle block

Langmuir monolayers and Langmuir–Blodgett (LB) film morphology of amphiphilic triblock copolymers are studied using surface pressure-area measurements and atomic force microscopy (AFM), respectively. The triblock copolymers are composed of long water-soluble poly(ethylene oxide) (PEO) chains as middle block with very short poly(perfluorohexylethyl methacrylate) (PFMA) end blocks. The surface pressure-area isotherms show phase transitions in the brush regime. This phase transition is due to a rearrangement of PFMA block at the air–water interface. It becomes more significant with increasing PFMA content in the copolymer. LB films transferred at low surface pressures from the air–water interface to hydrophilic silicon substrates show surface micelles in the size range of 50–100 nm. A typical crystalline morphology of the corresponding PEO homopolymer is observed in LB films of copolymers with very short PFMA blocks, transferred in the brush region at high surface pressure. This crystallization is hindered with increasing PFMA content in the copolymer.     

Micellization of PEO/PS block copolymers at the air/water interface: a simple model for predicting the size and aggregation number of circular surface micelles

Isotherms of monolayers of poly(ethylene oxide) (PEO) and polystyrene (PS) triblock copolymers spread at the air/water interface were obtained by film balance technique. In a low concentration regime, the PEO segments surrounding the PS cores behave the same way as in monolayers of PEO homopolymers. Langmuir-Blodgett (LB) films prepared by transferring the monolayers onto mica at various surface pressures were analyzed by atomic force microscopy (AFM). The results reveal that these block copolymers form micelles at the air/water interface. Within the micelles, the PS blocks act as anchoring structures at the interface. In several cases, aggregation patterns were modified by the dewetting processes that occur in Langmuir-Blodgett films transferred to solid substrates. High transfer surface pressures and metastable states favored these changes in morphology. A flowerlike surface micelle model is proposed to explain the organization of the surface circular micelles. The model can be generalized and applied to diblock copolymers as well. The model permits prediction of the aggregation number and the size of circular surface micelles formed by PEO/PS block copolymers at the air/water interface.     

Thermodynamic and structural characterization of a mixed perfluorocarbon-phospholipid ternary monolayer surfactant system

Pulmonary lung surfactant is a mixture of surfactants that reduces surface tension during respiration. Perfluorinated surfactants have potential applications for artificial lung surfactant formulations, but the interactions that exist between these compounds and phospholipids in surfactant monolayer mixtures are poorly understood. We report here, for the first time, a detailed thermodynamic and structural characterization of a minimal pulmonary lung surfactant model system that is based on a ternary phospholipid-perfluorocarbon mixture. Langmuir and Langmuir-Blodgett monolayers of binary and ternary mixtures of the surfactants 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) and perfluorooctadecanoic acid (C18F) have been studied in terms of miscibility, elasticity and film structure. The extent of surfactant miscibility and elasticity has been evaluated via Gibbs excess free energies of mixing and isothermal compressibilities. Film structure has been studied by a combination of atomic force microscopy and fluorescence microscopy. Combined thermodynamic and microscopy data indicate that the ternary monolayer films were fully miscible, with the mixed films being more stable than their pure individual components alone, and that film compressibility is minimally improved by the addition of perfluorocarbons to the phospholipids. The importance of these results is discussed in context of these mixtures' potential applications in pulmonary lung surfactant formulations.     

A monolayer study on interactions of docetaxel with model lipid membranes

Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.     

X-ray and neutron reflectometry investigation of Langmuir-Blodgett films of cellulose ethers

Langmuir-Blodgett films of a series of cellulose ethers are investigated by X-ray and neutron reflectometry. Two types of samples are considered: simple alkyl ethers of cellulose and derivatives obtained by the alkylation of (2-hydroxypropyl)cellulose (HPC). All of the cellulose ethers form stable monolayers at the air-water interface. Significant differences are, however, found in the surface pressure-area compression isotherms. Ethers prepared from HPC typically exhibit larger limiting molecular areas and higher surface pressures than the corresponding simple cellulose ethers. The ease of monolayer transfer to hydrophobic silicon substrates differs greatly from one type of molecule to another. Successful transfer conditions are found only for ethers that form stable monolayers at sufficiently high surface pressures. Surprisingly, deuterated HPC ethers, prepared for neutron reflectivity measurements, exhibit monolayer properties significantly different from those of their hydrogenated analogues. Although essentially identical limiting molecular areas are found, the surface pressure corresponding to a characteristic plateau transition in the compression isotherm is found to decrease by about 8-10 mN m(-1) upon side chain deuteration. X-ray reflectivity results show a linear increase of film thickness with the number of deposited layers, indicating a regular and reproducible transfer. Observed average layer spacings are, however, significantly smaller than the calculated length of fully extended side chains. Neutron reflectivity curves recorded for composite LB films composed of both deuterated and hydrogenated polymers exhibit regular Keissig fringes, but no Bragg peak. This result indicates that these LB films do not possess an internal periodic structure and the initial layer-by-layer organization is lost by large interlayer diffusion.     

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.     

Structures of palmitoyl-L and DL-lysine monolayers at the air-water interface--polarization modulation infrared reflection absorption spectroscopic study

Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) was applied to measure the IR spectra of palmitoyl-DL-lysine (L-PL) and palmitoyl-DL-lysine (DL-PL) at the air-water interface. The spectra in the amide I and II regions were simulated by using the extinction coefficients of the amide I and II bands of L-PL and DL-PL determined by the analyses of the IR external reflection spectra of the Langmuir-Blodget (LB) films prepared on a Ge plate (Yasukawa et al. J. Mol. Struct. 2005, 735-736, 53), indicating the angle between the plane of the secondary amide group (the amide plane) and the surface normal in the L-PL monolayer to be about 20 degrees and the angle in the DL-PL monolayer to be about 37 degrees. Comparison of the tilt angles with the corresponding angles in the LB films (about 20 degrees for the LB film of L-PL; about 49 degrees for the LB film of DL-PL) indicated that, upon being transferred to the solid substrate from the air-water interface, the L-PL monolayer keeps the orientation of the amide plane virtually unchanged, while the DL-PL monolayer changes the orientation appreciably to a horizontal direction. The orientation change of the amide plane was interpreted as due to the accommodation of irregularly oriented palmitoyl groups into the LB films of DL-PL on the solid substrate.