Progress in characterization of Langmuir monolayers by consideration of compressibility

Over decades, information about the rheological properties of the condensed monolayer phases has been obtained by introduction of a two-dimensional compressibility which is defined on the basis of the surface pressure-molecular area (Pi-A) features of the monolayer. Since the last decade, fundamental progress was attained in the experimental determination of the main characteristics of Langmuir monolayers in microscopic and molecular scale. Already smallest changes in the molecular structure of the amphiphile can result in changes in the molecular arrangement in the monolayer and thus, in changes of the main characteristics of the monolayer such as, the surface pressure-area per molecule (Pi-A) isotherms, the shape and texture of the condensed phase domains and the two-dimensional lattice structure. As the classical equations of state allowed only characterisation of the fluid (gaseous, liquid-expanded) state, thermodynamically based equations of state, which consider also the aggregation of the monolayer material to the condensed phase, have been developed. The present review focuses particularly to amphiphilic monolayers, the Pi-A isotherms of which indicate the existence of two condensed phases. For this case, the experimental results of the differences in the structure features and phase properties are discussed. The generalisation of the equation of state for Langmuir monolayers developed for the case that one, two or more phase transitions in the monolayer take place, is in agreement with the experimental results that the two-dimensional compressibility of the condensed phases undergoes a jump at the phase transition, whereas the compressibility is proportional to the surface pressure within one of the condensed phases. An example is presented which explains the procedure of the theoretical analysis of Pi-A isotherms indicating the existence of two condensed phases. An element of the procedure is the application of the general principle that the behaviour of any thermodynamic system is determined by the stability condition. An interesting anisotropy of the compressibility is revealed by GIXD studies of the S-phase of octadecanol monolayers. However, similar studies performed close to the LS-S-phase transition would result in a thermodynamically impossible negative compressibility. Close to this phase transition, the compressibility cannot be determined from the positions of the maxima because the monolayer is in a disordered state attributed to elastic distortions by fluctuations with the structure of the new phase in the surrounding matrix without destroying the quasi-long-range positional order.     

Stabilization of Langmuir monolayer of hydrophobic thiocholesterol molecules

The self-assembled monolayer of the thiocholesterol (TCh) exhibits interesting properties that can be used for various technological applications. TCh is predominantly a hydrophobic molecule, and it does not spread at the air–water interface to form a stable Langmuir monolayer. We have stabilized the TCh molecules in the cholesterol (Ch) monolayer. We find the mixed monolayer to be stable upto 0.75 mole fraction of TCh in Ch. The mixed monolayer shows an initial and a final collapse. On compressing the monolayer beyond the initial collapse, the TCh molecules squeeze out irreversibly from the mixed monolayer phase. The calculation of excess area per molecule for the TCh and Ch mixed monolayer system indicates an attractive interaction between the component molecules. Interestingly, the elasticity of the Ch monolayer reduces to less than half, and the monolayer becomes more fluidic due to the presence of even very minute quantity (5%) of TCh.     

Incorporation conditions guiding the aggregation of a glycosylphosphatidyl inositol (GPI)-anchored protein in Langmuir monolayers

This work investigates the process of incorporation of a glycosylphosphatidyl inositol (GPI)-anchored alkaline phosphatase into Langmuir monolayers of dimyristoyl phosphatidic acid (DMPA). Three different methods of protein incorporation were assayed. When the protein solution was injected below the air–water interface after formation of the lipid monolayer a micro-heterogeneous distribution of alkaline phosphatase throughout the interface was observed. Adsorption kinetics studied by fluorescence microscopy, associated with surface pressure measurements, led to the proposition of a model in which the protein penetration is modulated by the surface packing of the monolayer and intermolecular interactions occurring between the phospholipid and the protein. At initial surface pressures higher than 20 mN m⁻¹, the protein is quickly adsorbed on the interface and the lateral diffusion drives the alkyl chains to turn towards the air phase while the polypeptide moiety faces the aqueous subphase.     

多肽Langmuir膜控制下方解石晶体的生长

本文研究了聚β-苯甲基天门冬氨酸(PBA)Langmuir膜对碳酸钙成核和生长的诱导控制作用。在PBA Langmuir单分子膜作用下,我们得到形状规则、边缘清晰的花瓣状碳酸钙晶体。XRD分析表明所得碳酸钙晶体为沿(104)和(208)晶面取向生长的方解石晶型;AFM、TEM和SEM观察显示了方解石形貌的变化过程。结果表明多肽PBA Langmuir单分子膜对碳酸钙的结晶、生长取向和形貌具有良好的诱导和调控作用。     

Subphase pH effect on the limiting molecular area of amphiphilic β-diketones in Langmuir monolayers

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Theoretical modeling of photo-induced wave propagation in liquid-crystalline Langmuir monolayers

A phenomenological model of wave propagation in photo-excited liquid-crystalline Langmuir monolayers is constructed. The spontaneous splay deformation of the liquid-crystalline order and the anisotropy of photo-excitation of molecules are taken into account in this model. Numerical simulations of the model well reproduce qualitative features of the wave propagation phenomenon observed in recent experiments. A linear stability analysis of the model equations reveals that an interplay between the spontaneous splay deformation and the anisotropy of the photo-excitation can lead to the wave propagation.     

Langmuir monolayers from perfluorobutyl-n-eicosane

Perfluorobutyl-n-eicosane (abbreviated as F4H20) was spread at the air/water as Langmuir monolayers and studied under different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (π-A isotherms) have been complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers’ structure and estimation of the film thickness at different stages of compression. It has been found that the molecules are oriented almost vertically (with respect to the interface) in the vicinity of film collapse. The negative sign of the measured surface potential, ΔV, is evidence for the orientation of F4H20 molecules with their perfluorinated parts exposed towards the air. In the case of F4H20 a limited fluorination relative to perfluoroeicosane also results in monolayer formation, in contrast to eicosane itself, which forms lenses.     

Interaction of curcumin with lipid monolayers and liposomal bilayers

Curcumin shows huge potential as an anticancer and anti-inflammatory agent. However, to achieve a satisfactory bioavailability and stability of this compound, its liposomal form is preferable. Our detailed studies on the curcumin interaction with lipid membranes are aimed to obtain better understanding of the mechanism and eventually to improve the efficiency of curcumin delivery to cells. Egg yolk phosphatidylcholine (EYPC) one-component monolayers and bilayers, as well as mixed systems containing additionally dihexadecyl phosphate (DHP) and cholesterol, were studied. Curcumin binding constant to EYPC liposomes was determined based on two different methods: UV/Vis absorption and fluorescence measurements to be 4.26 × 10⁴ M⁻¹ and 3.79 × 10⁴ M⁻¹, respectively. The fluorescence quenching experiment revealed that curcumin locates in the hydrophobic region of EYPC liposomal bilayer. It was shown that curcumin impacts the size and stability of the liposomal carriers significantly. Loaded into the EYPC/DPH/cholesterol liposomal bilayer curcumin stabilizes the system proportionally to its content, while the EYPC/DPH system is destabilized upon drug loading. The three-component lipid composition of the liposome seems to be the most promising system for curcumin delivery. An interaction of free and liposomal curcumin with EYPC and mixed monolayers was also studied using Langmuir balance measurements. Monolayer systems were treated as a simple model of cell membrane. Condensing effect of curcumin on EYPC and EYPC/DHP monolayers and loosening influence on EYPC/DHP/chol ones were observed. It was also demonstrated that curcumin-loaded EYPC liposomes are more stable upon interaction with the model lipid membrane than the unloaded ones.     

亚相pH值对磷脂单层下甘氨酸结晶过程的影响

亚相pH值对磷脂单层下甘氨酸结晶过程的影响;Langmuir单分子层; 结晶; 取向; 形貌     

Phenomenological surface characterization of cationic-lipid monolayers in the presence of oppositely charged polyions

The thermodynamic properties of monolayers of double chain cationic lipids DOTAP at the air–water interface have been investigated by means of surface pressure and surface potential measurements. We studied the interfacial properties of the film in the liquid-expanded regime during the isothermal compression in the presence of oppositely charged linear polyions (poly(acrylate)sodium salt, [NaPA]) of different molecular weights. The influence of the ionic character of the aqueous subphase on the polyion adsorption has been studied in different environmental conditions, considering different subphase compositions, ranging from a polyion solution at different concentrations to a salty polyion solution, containing different amount of simple added salt [NaCl]. The data are compared to the ones when only NaCl salt is present in the subphase. The results have been analyzed according to an osmotic-type equation of state and the characteristic parameter associated with the water activity has been evaluated as a function the different molecular weight polyion content. The influence of the simple salt in the adsorption process has been discussed in the light of current scaling theories of polyelectrolyte solutions and the critical salt concentrations inducing a polyion desorption in the different experimental conditions investigated have been estimated.     

气/液界面Langmuir单分子膜的原位拉曼光谱

提出了一种原位测量气/液界面Langmuir单分子膜拉曼光谱的新方法, 即利用SERS技术, 通过降低亚相的方法来获得气/液界面Langmuir单分子膜的原位拉曼光谱. 利用这种方法, 用原位拉曼光谱测量系统得到了信噪比较好的十八胺及二棕榈酰磷脂酰胆碱单分子膜的拉曼光谱, 在分子水平上获取了单分子膜中的结构信息.     

山嵛酸Langmuir膜结合氨气扩散控制生长碱式硝酸锌的分形结构

以山嵛酸(BA) Langmuir膜为仿生矿化模板, 结合氨气动态扩散控制方法, 在气-液界面上制备了取向单一的Zn5(OH)8(NO3)2•2H2O分形结构材料. 利用扫描电子显微镜(SEM)和X射线衍射(XRD)分别对样品形貌及其晶型进行了表征. 结合π-A曲线研究了分形结构生长的驱动力, 并详细分析了分形生长的过程和机理. 认为Langmuir膜的存在导致了材料单一的晶面取向, 而氨气在气-液界面的动态扩散提供了分形结构形成的驱动力. 这有可能为控制合成一系列具有特殊结构的无机晶体材料提供一种可行的方法.     

Surface pressure‐area curves compression isotherm of sericin Langmuir monolayer

The Langmuir monolayer of sericin protein was studied by means of surface pressure (π)—molecular area (A) isotherms at different pH subphase. The monolayer of sericin exhibits typical phase transition phenomena at pH 2, pH 4.8, pH 7 and pH 11, respectively, including from gas state to gas‐liquid state and finally to condensed solid state. However, the monolayer of sericin on pH 11 subphase appears to be solid state. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of molecular structure in Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate by IR external reflection spectroscopy

 In situ polarized Fourier transform IR external reflection spectra of Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate on a water surface were recorded for various surface areas, and their molecular structures were estimated. In the zinc stearate monolayer, the wavenumbers and the absorbances of the antisymmetric and symmetric methylene stretching bands did not change during monolayer compression, which means that orientational and conformational changes of the hydrocarbon chain did not occur. However, wavenumber changes of the antisymmetric and symmetric carboxylate stretching bands were observed during surface compression. The change in the binding nature of the zinc cation to the carboxylate group was speculated. Moreover, it was elucidated that the structure of the hydrocarbon chain in the zinc 12-hydroxystearate monolayer was different from that in the zinc stearate monolayer. Received: 21 March 2001 Accepted: 6 July 2001     

Langmuir films of mixtures of C70 and arachidic acid

Surface pressure-area isotherms of Langmuir films of C₇₀/arachidic acid mixed films were measured at the water (pH=2)-air interface, at several temperatures and concentrations. Our results for the case of 25 °C and 1:1 mixtures, differ from those presented by Jehoulet, et al. (1992), and are closer to the results for C₆₀/arachidic acid mixed films presented by those authors.     

Properties of monolayers of crown ethers bearing an azo and azoxy group in the macrocycle

Two methods are described for preparing monomolecular layers of crown ethers with an azo or azoxy group in the macrocycle. When the molecules used to build the monolayer are soluble in aqueous solutions, adsorptive preconcentration on mercury electrodes was used to prepare the monolayer coating. The monolayer was electroactive due to the presence of the azo or azoxy unit in the molecules. Monolayers of crown ethers bearing an azo group in the macrocycle were shown to recognize alkali metal cations present in the solution. Changes of the parameters of the voltammetric reduction peaks - peak potential and peak width, served as an indication of specific interactions of the monolayer of 13-membered and 16-membered azocrown ethers with Na⁺ and K⁺ cations, respectively.The monolayers capable of recognizing cations have also been prepared on the aqueous solution-air interface, using the Langmuir technique. In this approach, amphiphilic derivatives of the azocrowns were synthesized and the monolayer has been assembled on the subphase containing metal cations. Binding of the cation by the macrocycle has a stabilizing effect on the monolayer and higher collapse pressures are achieved than on the pure water subphase. The monolayer was transferred from the air-water interface on the solid substrate using the Langmuir-Blodgett technique. Thin mercury film electrodes on the Ag substrate, or An films evaporated on glass slides were employed as the electrode substrates. The former gave monolayer modified electrodes of higher stability.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.     

Isotherms of phospholipid monolayers measured by a pendant drop technique

The Axisymmetric Drop Shape Analysis (ADSA) has been used to study the surface pressure/area isotherms of insoluble surfactant monolayers. The continuous measurement of surface tension as a function of surface area by increasing and decreasing the drop volume allows to investigate the phase transitions in monolayers. The isotherms of two phospholipids, dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl ethanolamine (DMPE), show good agreement with those measured by using a conventional Langmuir-Blodgett film balance, except in the coexistence region. The observed disagreements are discussed in terms of differences in compression rate, curvature of the surface and effect of impurities. Evidence of possible geometric effects on monolayer domain formation and growth is given on the basis of BAM images.Due to the small total surface area, the ADSA technique provides advantages as regards homogeneity of temperature, surface pressure, surface concentration and the symmetry of area changes.     

Langmuir isotherm analysis of novel branched per-fluorinated surfactants and their interactions with single stranded DNA

The efficient synthesis and surface properties of new fluorinated gemini surfactants are described. The aim of this study was to investigate the relationships between the molecular structure and the Langmuir layer properties of these fluorinated gemini lipids. The electrostatic ssDNA binding interactions of amino groups included on the spacer were also investigated. The synthesis corresponds to the substitution of vinyl fluorine atom of fluoro-unsaturated esters by a diethylene-oxide diamine via a Michael addition followed by a fluoride elimination reaction. For the spread layers, the measurements of surface pressure versus molecular area were performed with or without ssDNA in the subphase. The monolayers characteristics depend on the hydrophobic chain length, the polar-head, the pH of the subphase and the flexibility of the spacer. The introduction of ssDNA in the subphase seems to show a low interaction between the surfactants and the oligonucleotide.     

下缘芳香取代结构对杯[4]芳烃Langmuir膜的影响

The Langmuir monolayer properties of lower rim aromatically substituted calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis（2-naphth-1＇-ylacetylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BNAEC）, 5,11,17,23-tetra-tert- butyl-25,27-bis（2-benzoylamino ethoxy）-26,28-dihydroxylcalix[4]arene （BBAEC） and 5,11,17,23-tetra-tert-butyl- 25,27-bis（2-cinnamoylaminoethoxy）-26,28-dihydroxylcalix[4]arene （BCAEC）, have been studied. Film balance measurements and Brewster angle microscopy （BAM） observation demonstrate that all the compounds can form Langmuir monolayers with different molecular limiting areas. BNAEC or BBAEC monolayer is able to form condensed domains during compression, while BCAEC monolayer can never form condensed domain. BNAEC monolayer is more readily to form condensed domain than BBAEC monolayer. Moreover, BNAEC monolayer can form the total condensed phase during compression even when T=28℃, while BBAEC monolayer can not when T 〉 10 ℃. The results imply that different lower rim aromatic substitutions affect essentially the intermolecular interaction and molecular packing in the monolayer at air/water interface.     

Compressibility study of quaternary phospholipid blend monolayers

The mechanical properties of liposome membranes are strongly dependent on type and ratio of lipid compounds, which can have important role in drug targeting and release processes when liposome is used as drug carrier. In this work we have used Brewster's angle microscopy to monitor the lateral compression process of lipid monolayers containing as helper lipids either distearoyl phosphatidylethanolamine (DSPE) or dioleoyl phophatidylethanolamine (DOPE) molecules on the Langmuir trough. The compressibility coefficient was determined for lipid blend monolayers containing the helper lipids above, cholesterol, distearoyl phosphatidylcholine (DSPC) and pegylated-DSPE at room temperature. Two variables, the cholesterol fraction and the ratio ρ between the helper lipid (either DSPE or DOPE) and the reference lipid DSPC, were studied by multivariate analysis to evaluate their impact on the compressibility coefficient of the monolayers. The cholesterol level was found to be the most significant variable for DSPE blends while the ratio ρ was the most significant one for DOPE blend monolayers. It was also found that these two variables can exhibit positive interaction and the same compressibility value can be obtained with different blend compositions.