Effect of Na+ and Ca2+ Ions on a Lipid Langmuir Monolayer: An Atomistic Description by Molecular Dynamics Simulations

Studying the effect of alkali and alkaline‐earth metal cations on Langmuir monolayers is relevant from biophysical and nanotechnological points of view. In this work, the effect of Na⁺ and Ca²⁺ on a model of an anionic Langmuir lipid monolayer of dimyristoylphosphatidate (DMPA^?) is studied by molecular dynamics simulations. The influence of the type of cation on lipid structure, lipid–lipid interactions, and lipid ordering is analyzed in terms of electrostatic interactions. It is found that for a lipid monolayer in its solid phase, the effect of the cations on the properties of the lipid monolayer can be neglected. The influence of the cations is enhanced for the lipid monolayer in its gas phase, where sodium ions show a high degree of dehydration compared with calcium ions. This loss of hydration shell is partly compensated by the formation of lipid–ion–lipid bridges. This difference is ascribed to the higher charge‐to‐radius ratio q/r for Ca²⁺, which makes ion dehydration less favorable compared to Na⁺. Owing to the different dehydration behavior of sodium and calcium ions, diminished lipid–lipid coordination, lipid–ion coordination, and lipid ordering are observed for Ca²⁺ compared to Na⁺. Furthermore, for both gas and solid phases of the lipid Langmuir monolayers, lipid conformation and ion dehydration across the lipid/water interface are studied.     

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.     

Thin films of discotic liquid crystals and their applications

ABSTRACT

Discotic liquid crystals (DLCs) are considered as fascinating systems due to their unique property of self-assembly to yield different columnar structures. DLCs are organic semiconductors and create pathways for the development of numerous optical and electrical devices. The thin films of DLCs can be considered as low dimensional system which can exhibit remarkable optical and physical properties. In this article, we present a review on ultrathin films of some interesting DLC molecules at air–water and air–solid interfaces. The Langmuir monolayer and Langmuir–Blodgett films of DLC molecules are extensively studied. The ultrathin films of DLC molecules can yield highly anisotropic layer wherein the molecular orientation and aggregation can have large impact on the physicochemical properties of the film. Different surface phases with different molecular orientations as function of surface density and temperature can be obtained by forming the Langmuir monolayer of the DLC molecules at the air–water interface. The Langmuir monolayer in a particular phase can be deposited onto the active area of a device layer-by-layer by employing a highly controlled Langmuir–Blodgett technique. Here, we report some interesting results related on molecular orientation of the DLC molecules at different interfaces. Such aggregation of DLC molecules in ultrathin films may find applications in thin film-based electro-optical devices.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

Orientation of the Azobenzene Spacer of Carboxylic Methyl Ester Gemini Surfactants in Langmuir Monolayer

The Langmuir monolayer of carboxylic methyl ester Gemini surfactants with the azobenzene spacer, referred to as MC_m(azo)MC_m, was prepared and the π‐A isotherms were measured. The result revealed an orientational picture of the azobenzene spacer at the air/water interface. Before irradiation, the planar trans‐azobenzene group adopted an approximate configuration parallel to the interface and lay on the air/water interface. After UV‐light irradiation, the spacer became the twisted cis‐one. Because of the location of the polar headgroups at the air/water interface, the two phenyl rings were also forced to take a near‐parallel orientation with the interface to reduce the free energy. As a result, the spacer thin‐layer was "thickened", which caused an elevation in collapse pressure.     

Free Energy of Bare and Capped Gold Nanoparticles Permeating through a Lipid Bilayer

Herein, we study the permeation free energy of bare and octane‐thiol‐capped gold nanoparticles (AuNPs) translocating through a lipid membrane. To investigate this, we have pulled the bare and capped AuNPs from bulk water to the membrane interior and estimated the free energy cost. The adsorption of the bare AuNP on the bilayer surface is energetically favorable but further loading inside it requires energy. However, the estimated free‐energy barrier for loading the capped AuNP into the lipid membrane is much higher compared to bare AuNP. We also demonstrate the details of the permeation process of bare and capped AuNPs. Bare AuNP induces the curvature in the lipid membrane whereas capped AuNP creates an opening in the interacting monolayer and get inserted into the membrane. The insertion of capped AuNP induces a partial unzipping of the lipid bilayer, which results in the ordering of the local lipids interacting with the nanoparticle. However, bare AuNP disrupts the lipid membrane by pushing the lipid molecules inside the membrane. We also analyze pore formation due to the insertion of capped AuNP into the membrane, which results in water molecules penetrating the hydrophobic region.     

Nucleation of bulk phases in the HCl/H2O system

We report experimental results on the low-temperature uptake of HCl on H(2)O ice (ice). HCl was deposited on the surface at greater than monolayer amounts at 85 K, and the ice substrate was heated. The temperature dependence of the HCl vapor pressure from this phase was measured from 110 to 150 K, with the nucleation of a bulk hydrate phase observed at 150 K. Measurements were conducted in a closed system by simultaneous application of gas phase mass spectrometry and surface spectroscopy to characterize vapor/solid equilibrium and the nucleation of bulk hydrate phases. Combining the nucleation data reported here with data we reported previously (180 to 200 K) and data from two other laboratories (165 and 170 K), the thermodynamic boundaries for the nucleation of both the metastable bulk solution and bulk hydrate phases subsequent to monolayer adsorption of HCl have been determined. The nucleation of the metastable bulk solution phase occurs promptly at monolayer coverage at the ice/liquid coexistence boundary on the binary bulk phase diagram. The nucleation of the bulk hexahydrate occurs from this metastable solution along a locus of points defining a state of constant solution free energy. This measured free energy is -51.2 +/- 0.9 kJ/mol. Finally, the temperature dependence of the HCl vapor pressure from the low-temperature phase is reported here for the first time and is consistent with that of the metastable solution predicted by this thermodynamic model of uptake, extending the range of validity of this model of adsorption followed by bulk solution and hydrate nucleation to a lower bound in temperature of 110 K.     

Study on the Film Properties and Recognition for Metal Ions of Calix[4]arenes Derivative by Langmuir Monolayer

Calixarene molecules are very powerful ligand for ions and small molecules, and have been studied with several techniques as models for host‐guest systems. In this approach, the formation of Langmuir monolayer properties of three kinds calix[4]arene derivative were characterized and one of them, p‐tert‐butylthiacalix[4]arene (TCA), was chosen as object to study its Langmuir monolayer affected by different subphase conditions. The purpose of this study is to investigate the molecular recognition ability of TCA for metal ions at the water‐air interface. Changing the composition of aqueous subphase (containing various metal ion solutions respectively) produced strong variations on the monolayer parameters, indicating a different selectivity of the TCA ligand for the different metal cations. In particular, high selectivity for transition metal ions was found. Limiting area values are discussed in relation to the orientation of the cone‐shaped molecules at the water‐air interface.     

Rod–disc oligomeric liquid crystal based on 4-cyanobiphenyl and truxene core

A facile synthesis of a novel covalently linked disc–rod mesogen is reported consisting of a truxene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers. The compound formed a stable Langmuir monolayer at the air–water interface. The atomic force microscope study on the Langmuir–Blodgett film of the molecule reveals a tilted orientation at air–solid interfaces.     

Mechanochemical Tuning of the Binaphthyl Conformation at the Air–Water Interface

Gradual and reversible tuning of the torsion angle of an amphiphilic chiral binaphthyl, from ?90° to ?80°, was achieved by application of a mechanical force to its molecular monolayer at the air–water interface. This 2D interface was an ideal location for mechanochemistry for molecular tuning and its experimental and theoretical analysis, since this lowered dimension enables high orientation of molecules and large variation in the area. A small mechanical energy (<1 kcal mol^?1) was applied to the monolayer, causing a large variation (>50 %) in the area of the monolayer and modification of binaphthyl conformation. Single‐molecule simulations revealed that mechanical energy was converted proportionally to torsional energy. Molecular dynamics simulations of the monolayer indicated that the global average torsion angle of a monolayer was gradually shifted.     

Substrate‐induced formation of a recognition structure in a four‐polypeptide–lipid monolayer system

Poly(γ‐methyl L ‐glutamate)s with Ser, His, Asp, and Glu residues at the amino terminal as the serine protease catalytic site were prepared. The number‐average degree of polymerization of the polypeptides was 51. A dipalmitoylphosphatidylcholine monolayer containing the polypeptides was formed at the air–water interface and was transferred onto gold‐deposited glass plates. The binding of N‐acetyltyrosine ethyl ester, a typical substrate of the serine protease, to the monolayer was characterized by surface plasmon resonance measurements. The four‐polypeptide–lipid monolayer system conditioned on an aqueous solution containing the substrate N‐acetyltyrosine ethyl ester exhibited Langmuir‐type binding of the substrate. Its binding constant of 6.1 × 10⁴ M⁻¹ was about 20 times larger than that observed for a monolayer prepared on pure water. The behavior may have arisen from a substrate‐induced rearrangement of the four kinds of polypeptides in the monolayer, forming a substrate‐binding structure similar to that found in serine protease. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2186–2191, 2000     

Interaction of DMPC liposomes with a DMPA monolayer: related study of Langmuir-Blodgett films

The formation of a phospholipidic layer was achieved in two steps: (1) a dimyristoyl-l-α-phosphatidic acid (DMPA) Langmuir monolayer was formed by spreading a chloroform/methanol DMPA solution onto an aqueous subphase; after a 10 min period, the monolayer was compressed at 5mNm⁻¹; and (2) keeping the area of the DMPA monolayer constant, a dimyristoyl-l-α-phosphatidylcholine (DMPC) liposomal suspension was added. The progressive incorporation of DMPC molecules into the DMPA monolayer was studied by monitoring the variation of surface pressure with time at constant film area. Three parameters involved in the formation of the interfacial layer DMPA/liposomal DMPC (DMPA/ lip-DMPC) were studied: liposome addition, aqueous subphase composition and initial surface pressure of the DMPA monolayer. The transfer of this mixed layer was controlled through a traceable fluorescent probe incorporated in the liposomes. The thickness and homogeneity of the Langmuir-Blodgett films thus obtained were assessed through Fourier transform infra-red spectroscopy and Nomarski microscopy, respectively. This study shows that the DMPA/lip-DMPC monolayer could be transferred without dragging of aggregates or mesophases.     

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.     

An ab initio study of water clusters in gas phase and bulk aqueous media: (H2O)n,n=1–12

Geometries of several clusters of water molecules including single minimum energy structures of n‐mers (n=1–5), several hexamers and two structures of each of heptamer to decamer derived from hexamer cage and hexamer prism were optimized. One structural form of each of 11‐mer and 12‐mer were also studied. The geometry optimization calculations were performed at the RHF/6‐311G* level for all the cases and at the MP2/6‐311++G** level for some selected cases. The optimized cluster geometries were used to calculate total energies of the clusters in gas phase employing the B3LYP density functional method and the 6‐311G* basis set. Frequency analysis was carried out in all the cases to ensure that the optimized geometries corresponded to total energy minima. Zero‐point and thermal free energy corrections were applied for comparison of energies of certain hexamers. The optimized cluster geometries were used to solvate the clusters in bulk water using the polarized continuum model (PCM) of the self‐consistent reaction field (SCRF) theory, the 6‐311G* basis set, and the B3LYP density functional method. For the cases for which MP2/6‐311++G** geometry optimization was performed, solvation calculations in water were also carried out using the B3LYP density functional method, the 6‐311++G** basis set, and the PCM model of SCRF theory, besides the corresponding gas‐phase calculations. It is found that the cage form of water hexamer cluster is most stable in gas phase among the different hexamers, which is in agreement with the earlier theoretical and experimental results. Further, use of a newly defined relative population index (RPI) in terms of successive total energy differences per water molecule for different cluster sizes suggests that stabilities of trimers, hexamers, and nonamers in gas phase and those of hexamers and nonamers in bulk water would be favored while those of pentamer and decamer in both the phases would be relatively disfavored. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 90–104, 2001     

Interfacial Water Structure in Langmuir Monolayer and Gibbs Layer Probed by Sum Frequency Generation Vibrational Spectroscopy

Langmuir monolayer and Gibbs layer exhibit surface‐active properties and it can be used as simple model systems to investigate the physicochemical properties of biological membranes. In this report, we presented the OH stretching vibration of H₂O in the 4′′‐n‐pentyl‐4‐cyano‐p‐terphenyl (5CT), nonadecanenitrile (C₁₈CN) Langmuir monolayer and compared them with CH₃CN Gibbs layer at the air/water interface with polarization SFG‐VS. This study demonstrated that the hydrogen bond network is different in the Langmuir monolayer of 5CT, C₁₈CN from CH₃CN Gibbs layer at the air/water interface which showed two different water structures on the different surface layer. The results provided a deeper insight into understanding the hydrogen bond on the interfaces.     

Multi‐responses optimization of simultaneous adsorption of methylene blue and malachite green dyes in binary aqueous system onto Ni:FeO(OH)‐NWs‐AC using experimental design: derivative spectrophotometry method

In this research, response surface methodology (RSM) approach using Central Composite Design (CCD) coupled by derivative spectrophotometry method was applied to develop mathematical model and optimize process parameters for simultaneous adsorption of methylene blue (MB) and malachite green (MG) from aqueous solution using Ni:FeO(OH) ‐ NWs‐AC. The optimal conditions to adsorption of MB and MG in binary mixture solution from aqueous solution were found at pH 8.0, MB concentration 20 mg L^‐1, MG concentration 20 mg L^‐1, adsorbent dosage 0.033 g and contact time 40 min. At these conditions, high adsorption efficiency (99.39% and 100.0% for MB and MG, respectively) was achieved. Among experimental equilibrium, Langmuir isotherm model fitted well with maximum monolayer adsorption capacity of 28.6 and 29.8 mg g^‐1 for MB and MG, respectively. The adsorption kinetic data followed pseudo second‐order kinetics for MB and MG dyes.     

Orientational changes in dipalmitoyl phosphatidyl glycerol Langmuir monolayers

Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure-area (pi-A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 A as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.     

Molecular organization of a water-insoluble iridium(III) complex in mixed monolayers

In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.