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.     

Langmuir monolayer behavior of an ion pair amphiphile with a double-tailed cationic surfactant

The spread or Langmuir monolayer behavior of an ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), with a double-tailed cationic surfactant, dihexadecyldimethylammonium bromide (DHDAB), at the air/water interface was analyzed with surface pressure-area isotherms, area relaxation curves, and Brewster angle microscope (BAM) images. The surface pressure-area isotherms showed that with increasing the DHDAB molar ratio, X(DHDAB), spread monolayers of HTMA-DS with DHDAB became rigid. In addition, unreasonably small limiting areas per alkyl chain of the molecules in the monolayers were found, especially at X(DHDAB)=0.5, implying the molecular loss from the monolayers at the interface. For spread HTMA-DS/DHDAB monolayers at the interface, a new IPA, DHDA-DS, was proposed to form through the displacement of HTMA(+) from HTMA-DS by DHDA(+), leaving HTMA(+) dissociated. The formation of DHDA-DS and the desorption of dissociated HTMA(+) upon the interface compression were supported by the results obtained from designed monolayer experiments with BAM observations, and were discussed by considering the hydrophilicity, packing efficiency, and headgroup charge characteristic of the species. Moreover, the area relaxation curves of spread HTMA-DS/DHDAB monolayers suggested that the formation of DHDA-DS was strongly related to the improved monolayer stability at the interface, which may have implications for the DHDAB-enhanced physical stability of catanionic vesicles composed of HTMA-DS.     

Edelfosine—A new antineoplastic drug based on a phospholipid-like structure: The Langmuir monolayer study

Edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, Et-18-OCH₃), an anticancer drug based on a phospholipid-like structure, was spread and investigated at the aqueous solution/air interface by means of surface pressure–area (π–A) and electric surface potential–area (ΔV–A) isotherms in addition to Brewster angle microscopy (BAM). The influence of such factors as subphase temperature, ionic strength, speed of compression and number of molecules spread at the surface on the characteristics of the π–A isotherms was studied. Edelfosine was found to form stable Langmuir monolayers which are nearly not influenced by the experimental conditions. The relative reflectivity measurements proved that the thickness of monolayer in the vicinity of collapse is 2.4 nm, which corresponds to length of a vertically oriented molecule. Perpendicular orientation of edelfosine molecules just before the film collapse has been confirmed with the apparent dipole moment value, which attains the maximum value in this region.     

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.     

Statistical thermodynamics approach for estimation of the interaction parameters in the mixed Langmuir films at the water/air interface

The treatment of two-dimensional system of three components of molecules of different molecular sizes, adapted from statistical thermodynamics, is used to derive parameters which could be derive parameters which could be related to the interaction between molecules in the mixed Langmuir films formed at the water/air interface.     

Preparation and characterization of 5,10,15,20‐tetraphenylporphyrin Langmuir films for gas chemsensor applications

In this work is reported the preparation and characterization of 5,10,15,20‐tetraphenylporphyrin (H₂TPP) films at the water‐air interfaces. The surface pressure‐area isotherms (π‐A) and UV‐Vis spectroscopy were used to investigate the effect of the spreading methods and parameters on the porphyrin monolayer formation. Also, Langmuir‐Blodgett (LB) and Langmuir‐Schaefer (LS) films were deposited onto glass substrates in order to study the conformation changes in porphyrin molecular packing. Quartz crystal microbalance (QCM) was utilized as the active solid substrate for the development of the NO₂ gas sensor based on the H₂TPP molecular films. The results of π‐A curves have clearly shown the significant contribution of the preparation methods and processing parameters on the conformation of porphyrin molecular films. The UV‐Vis spectroscopy results using polarized absorption dichroism have indicated different molecular packing for porphyrin films deposited by LB and LS methods, with relative tilted angles of 50° ± 5° and 35° ± 5°, respectively. Moreover, the QCM response has given strong evidence that H₂TPP porphyrin molecular films have performed as NO₂ chemsensor. Copyright © 2011 John Wiley & Sons, Ltd.     

Langmuir monolayer properties of the fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC)

Surface pressure–area (π–A), surface potential–area (ΔV–A), and dipole moment–area (μ–A) isotherms were obtained for the Langmuir monolayer of two fluorinated-hydrogenated hybrid amphiphiles (sodium phenyl 1-[(4-perfluorohexyl)-phenyl]-1-hexylphosphate (F6PH5PPhNa) and (sodium phenyl 1-[(4-perfluorooctyl)-phenyl]-1-hexylphosphate (F8PH5PPhNa)), DPPC and their two-component systems at the air/water interface. Monolayers spread on 0.02 M Tris buffer solution (pH 7.4) with 0.13 M NaCl at 298.2 K were investigated by the Wilhelmy method, ionizing electrode method and fluorescence microscopy. Moreover, the miscibility of two components was examined by plotting the variation of the molecular area and the surface potential as a function of the molar fraction for the fluorinated-hydrogenated hybrid amphiphiles on the basis of the additivity rule. The miscibility of the monlayers was also examined by construction of two-dimensional phase diagrams. Furthermore, assuming the regular surface mixture, the Joos equation for analysis of the collapse pressure of two-component monolayers allowed calculation of the interaction parameter (ξ) and the interaction energy (−Δ) between the fluorinated-hydrogenated hybrid amphiphiles and DPPC. The observations by a fluorescence microscopy also supported our interpretation as for the miscibility in the monolayer state. Comparing the monolayer behavior between the two binary systems, no remarkable difference was found among various aspects. Among the two combinations, the mole fraction dependence in monlayer properties was commonly classified into two ranges: 0 ≤ X ≤ 0.3 and 0.3 < X ≤ 1. Dependence of the chain length of fluorinated part was reflected for the molecular packing and surface potential.     

双头基两亲分子在气液界面的Langmuir铺展膜结构

用表面压-面积等温线,原子力显微镜（AFM）和X射线衍射（XRD）对两种具有不同取代位置的新型双头基两亲分子（bolaamphiphile）A和B在气液界面形成的Langmuir铺展膜的结构进行了研究,化合物1,20-二十碳二β-萘酯（B）在气液界面形成了拉伸型 Langmuir单分子膜,而化合物1,20-二十碳二α-萘酯（A）在气液界面则形成了具有三层分子厚度的二维结晶膜.     

Incorporation of Molecular Reorientation into Modeling Surface Pressure-Area Isotherms of Langmuir Monolayers

Langmuir monolayers can be assembled from molecules that change from a low-energy orientation occupying a large cross-sectional area to a high-energy orientation of small cross-sectional area as the lateral pressure grows. Examples include cyclosporin A, amphotericin B, nystatin, certain alpha-helical peptides, cholesterol oxydation products, dumbbell-shaped amphiphiles, organic–inorganic nanoparticles and hybrid molecular films. The transition between the two orientations leads to a shoulder in the surface pressure-area isotherm. We propose a theoretical model that describes the shoulder and can be used to extract the energy cost per molecule for the reorientation. Our two-state model is based on a lattice–sublattice approximation that hosts the two orientations and a corresponding free energy expression which we minimize with respect to the orientational distribution. Inter-molecular interactions other than steric repulsion are ignored. We provide an analysis of the model, including an analytic solution for one specific lateral pressure near a point of inflection in the surface pressure-area isotherm, and an approximate solution for the entire range of the lateral pressures. We also use our model to estimate energy costs associated with orientational transitions from previously reported experimental surface pressure-area isotherms.     

Hydrolytic degradation of poly(rac‐lactide) and poly[(rac‐lactide)‐co‐glycolide] at the air–water interface

The understanding of the simultaneous transport and chain‐scission phenomena involved in the hydrolysis of bulk‐degrading polymers requires the experimental separation of chain cleavage and water diffusion. The hydrolytic chain cleavage of poly(rac‐lactide) rac‐(PLA) and poly[(rac‐lactide)‐co‐glycolide] (PLGA) is analysed on the basis of monolayer degradation experiments combined with an improved data reduction procedure. Different, partly contradictory models of the hydrolytic degradation and erosion mechanism of PLA and PLGA, namely random chain scission and chain‐end scission, are discussed in the literature. The instantaneous linear area reduction observed for the polymer Langmuir films indicates a chain‐end scission mechanism. As monolayers of end‐capped and non‐end‐capped polymers degrade with exactly the same rate, the observed differences in the degradation kinetics of bulk samples do clearly result from differences in the water penetration into these polymers. A pronounced ‘auto‐inhibition’ effect is observed for the polymers degraded at initially high pH of the aqueous subphase in the absence of buffers. Copyright © 2007 John Wiley & Sons, Ltd.     

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

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Langmuir单分子膜在海洋飞沫气溶胶表面特性研究中的应用

被表面活性有机物包裹的液相气溶胶,如海洋飞沫气溶胶(SSA),通常具有反胶束的结构,它由有机分子形成的疏水表面和一个水相内核构成.SSA界面有机膜的组分和形态对其物理、化学和光学特性有重要的影响.Langmuir单分子膜是由脂肪酸、脂肪醇和磷脂等具有低挥发性的长链表面活性有机物在空气-水界面上扩散形成的单层分子薄膜.采...     

Mixed‐monolayer of N‐hydroxysuccinimide‐terminated cross‐linker and short alkanethiol to improve the efficiency of biomolecule binding for biosensing

The goal of this study was to use a novel surface chemistry for modifying gold surfaces to decrease the steric hindrance, minimize the nonspecific bindings while providing directed immobilization of proteins for advancing the transducer property and to provide a biosensing platform for surface plasmon resonance (SPR) applications. Mixed self‐assembled monolayers (mSAMs) were prepared using 3,3′‐Dithiodipropionic acid di (N‐hydroxysuccinimide ester) (DSP) and 6‐mercapto‐1‐hexanol (MCH) and the selected model proteins bovine serum albumin (BSA) and lysozyme were tested for binding efficiency. First, binding of these two proteins at constant concentration to different DSP:MCH mSAMs were compared to deduce the best molar ratio for forming mSAM using a continuous flow system coupled to SPR. Coincidently the maximum protein binding DSP:MCH mSAM were the same for both proteins. The change in Response Unit (∆RU) signal due to protein binding between DSP SAM and maximum protein binding DSP:MCH mSAM for lysozyme binding was more in comparison to BSA binding. Second, the effect of BSA and lysozyme concentration on binding efficiency to maximum protein binding DSP:MCH mSAM were compared and discussed. Lysozyme and BSA were shown to reach saturations on the same monolayer at concentrations of 5.7x10⁻⁵ and 8.96x10⁻⁶ [M] respectively, hence the molar ratio for limit concentrations is 6:1. The DSP SAM, MCH SAM, and DSP:MCH mSAMs where maximum and minimum protein binding occurs were also characterized with XPS and Attenuated total reflectance‐Fourier transform infrared (ATR‐FTIR) spectroscopy. Blank gold surface, maximum protein binding DSP:MCH mSAM and BSA immobilized DSP:MCH mSAM on gold surface were also investigated utilizing tapping mode AFM.     

Langmuir Analysis of Nanoparticle Polyvalency in DNA‐Mediated Adsorption

Many nanoparticle adsorption processes are dictated by the collective interactions of surface‐bound ligands. These adsorption processes define how nanoparticles interact with biological systems and enable the assembly of nanoparticle‐based materials and devices. Herein, we present an approach for quantifying nanoparticle adsorption thermodynamics in a manner that satisfies the assumptions of the Langmuir model. Using this approach, we study the DNA‐mediated adsorption of polyvalent anisotropic nanoparticles on surfaces and explore how deviations from model assumptions influence adsorption thermodynamics. Importantly, when combined with a solution‐based van’t Hoff analysis, we find that polyvalency plays a more important role as the individual interactions become weaker. Furthermore, we find that the free energy of anisotropic nanoparticle adsorption is consistent across multiple shapes and sizes of nanoparticles based on the surface area of the interacting facet.     

Surface Chemistry Studies of Quantum Dots (QDs) Modified with Surfactants

Since luminescent CdSe quantum dots (QDs) have shown great potential in biological labeling, the surface chemistry behavior of QDs at interfaces is of great research interest. In the present study, CdSe QDs with green luminescence were modified with hydrophobic chains of varying lengths [from C6 to C18]. These modified QDs can be utilized to form stable monolayers at the air/water interface. Surface pressure-area isotherms of modified QDs have been measured and limiting molecular areas have also been extrapolated in order to analyze the size of the QDs. UV absorption spectra of modified QDs at various surface pressures were also determined. Surface chemistry, as well as the topographic properties, of modified QDs in Langmuir and L-B films was discussed.     

Molecular Arrangement of a Bolaamphiphilic Anthrancene Derivative in Langmuir-Blodgett Film

Langmuir monolayer at the air/water interface is the basis to understand the molecular arrangement and to fabricate the organized molecular films1. Bolaamphiphiles describes the molecules in which two head functional groups are linked by one or two hydrophobic chains2,3. In comparison with the one-headed amphiphile, abundant configurations of Langmuir monolayer are expected in bolaamphiphiles. Generally, three kinds of configurations of the Langmuir monolayers of bolaamphiphile at the air/w…     

Ionic liquid self‐assembled monolayer and ion exchange for surface force control

Adhesion and lateral force were controlled by the formation of a self‐assembled monolayer combined with the surface ion exchange of ionic liquids on a silicon surface. In this study, the functionalized imidazolium ionic liquids were designed and synthesized with the aim of controlling surface force. N‐[3‐(Trimethoxylsilyl)propyl] ethylenediamine molecules were first self‐assembled onto a surface as an anchor layer and then 1‐propionic acid‐3‐methylimidazolium chloride were successfully grafted onto the amino‐modified surface. The surface force was changed by surface ion exchange in various anionic solutions. The self‐assembly and ion exchange processes were detected by means of attenuated total reflectance‐Fourier transform infrared spectrometry and further confirmed by X‐ray photoelectron spectra. Adhesion and friction behaviors were systematically investigated by atomic force/friction force microscope. The results indicated that anions played a great role in determining surface properties. Furthermore, surface adhesion and friction can be possibly quantitatively determined by the counter‐anions on the surface. Copyright © 2011 John Wiley & Sons, Ltd.     

Self‐assembled Azobenzenethiol Monolayer on Electrode Surfaces: Effect of Photo‐switching on the Surface and Electrical Property

Azobenzenethiol molecules carrying different para‐substituents were used to form mixed monolayers with n‐alkanethiol molecules on Au and Ag surfaces. UV‐ and visible light irradiation of the surfaces resulted in reversible alternation of contact angle and characteristic infrared absorption peak intensities, as well as the work function of the metal surfaces. The alternations can be correlated with the cis‐trans isomerization of the azobenzene moieties at the surface. Electron transport from the metal electrode to a redox center in a contacting solution was measured and analyzed based on the change in the work function of the electrode as well as the monolayer film structure upon isomerization.