Infrared imaging of a solid phase surfactant monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.     

Permselectivity of Gramicidin A Channels Based on Single‐channel Recordings

The expression mechanism of permselectivity through a gramicidin A (gA) channel between two aqueous phases (W1 and W2) was investigated. When the concentration of CsCl or CsBr in W1 was equivalent to that in W2, the single‐channel current was proportional to the absolute value of the applied membrane potential. Although the single‐channel current linearly increased with increasing electrolyte concentration in W1 and W2 until about 0.1 M (mol dm^?3), it began to level off around 0.1 M, indicating that ion permeation across the channel pore is the rate‐determining step and that the saturation of the transporting ion within the channel pore provokes the leveling off. In the case of asymmetric composition of the electrolyte in W1 and W2, the monovalent cation and the counter anion were transported in the opposite direction through the gA channel pore or the bilayer lipid membrane around the gA channel. Finally, the experimental data was fitted using the Goldman‐Hodgkin‐Katz equation based on the relationship between the membrane potential and the single‐channel current to define the ratio of the diffusion coefficients of Cs⁺, Cl^?, and Br^? as 5.7 : 1.0 : 0.26.     

Bending moduli and spontaneous curvature of the monolayer in a surfactant bilayer

We developed a method to evaluate the mechanical properties of the monolayers in symmetric surfactant bilayers using self-consistent field theory. A specific boundary condition is used to impose the same curvature onto the two opposing monolayers at the surfactant chemical potential equal to that of the corresponding homogeneously curved bilayer. Typically, the spontaneous monolayer curvature not equal 0 and its value depend on the surfactant architecture. This is of importance for the thermodynamics and topology of lamellar surfactant phases. Furthermore, it may be relevant in processes involving biological membranes, for example, the fusion and budding of vesicles and the incorporation of proteins in lipid bilayers.     

Hydrogen bond lifetime dynamics at the interface of a surfactant monolayer

The dynamics of water near the polar headgroups of surfactants in a monolayer adsorbed at the air/water interface is likely to play a decisive role in determining the physical behavior of such organized assemblies. We have carried out an atomistic molecular dynamics (MD) simulation of a monolayer of the anionic surfactant sodium bis(2-ethyl-1-hexyl) sulfosuccinate (aerosol-OT or AOT) adsorbed at the air/water interface. The simulation is performed at room temperature with a surface coverage of that at the critical micelle concentration (78 Angstrom(2)/molecule). Detailed analyses of the lifetime dynamics of surfactant-water (SW) and water-water (WW) hydrogen bonds at the interface have been carried out. The nonexponential hydrogen bond lifetime correlation functions have been analyzed by using the formalism of Luzar and Chandler, which allowed identification of the bound states at the interface and quantification of the dynamic equilibrium between bound and quasi-free water molecules, in terms of time-dependent relaxation rates. It is observed that the water molecules present in the first hydration layer form strong hydrogen bonds with the surfactant headgroups and hence have longer lifetimes. Importantly, it is found that the overall relaxation of the SW hydrogen bonds is faster for those water molecules which form two hydrogen bonds with the surfactant headgroups than those forming one such hydrogen bond. Equally interestingly, it is further noticed that water molecules beyond the first hydration layer form weaker hydrogen bonds than pure bulk water.     

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.     

Hydrocarbon chain conformation in an intercalated surfactant monolayer and bilayer

Cetyl trimethyl ammonium (CTA) ions have been confined within galleries of layered CdPS₃ at two different grafting densities. Low grafting densities are obtained on direct intercalation of CTA ions into CdPS₃ to give Cd_0.93PS₃(CTA)_0.14. Intercalation occurs with a lattice expansion of 4.8 ? with the interlamellar surfactant ion lying flat forming a monolayer. Intercalation at higher grafting densities was effected by a two-step ion-exchange process to give Cd_0.83PS₃(CTA)_0.34, with a lattice expansion of 26.5 ?. At higher grafting densities the interlamellar surfactant ions adopt a tilted bilayer structure.¹³C NMR and orientation-dependent IR vibrational spectroscopy on single crystals have been used to probe the conformation and orientation of the methylene ‘tail’ of the intercalated surfactant in the two phases. In the monolayer phase, the confined methylene chain adopts an essentially all-trans conformation with most of the trans chain aligned parallel to the gallery walls. On lowering the temperature, molecular plane aligns parallel, so that the methylene chain lies flat, rigid and aligned to the confining surface. In the bilayer phase, most bonds in the methylene chain are in trans conformation. It is possible to identify specific conformational sequences containing a gauche bond, in the interior and termini of the intercalated methylene. These high energy conformers disappear on cooling leaving all fifteen methylene units of the intercalated cetyl trimethyl ammonium ion in trans conformational registry at 40 K.     

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.     

Variations of surfactant monolayer surface density in induced steady wave regime

Fluctuations of an insoluble surfactant concentration along the free liquid surface induced by steady surface waves are considered theoretically. The energy of a waved surface is assumed to consist of surface tension, curvature, and van der Waals energy components. Dependencies of the surface tension and the bending stiffness versus the surfactant concentration are assumed to be linear relative to some reference level. The van der Waals energy is taken in the form of interaction term for a thin film. Minimization of the total energy allows the expression for the deviations of concentration to be obtained. The distribution of a surfactant concentration relative to some reference level has been found to be periodic, with a period that is half of the wave period, and the amplitude of oscillations is a function of a wave number that is very similar to the Landau expansion of the free-energy near the critical point in phase transitions.     

Gels from a semifluorinated n-alkane in fluorinated solvents as a probe for intermolecular interactions

Diblock semifluorinated n-alkanes can form aggregates and gels in fluorinated solvents. We have investigated the thermal behavior of binary mixtures comprising F(CF₂)₈(CH₂)₁₆H and fluorinated solvents. The solvents were perfluorohexane, perfluoroheptane, perfluorooctane, perfluorooctyl bromide, perfluorodecalin, and perfluorotributylamine. The phase diagrams were used to calculate the activity coefficients of the two components and the main excess thermodynamic functions. The solubility and self-assembly behavior of F₈H₁₆ in the fluorinated solvents are related to the different solute–solvent dispersion interactions that depend on the polarizabilities and ionization potentials of the interacting species, and on the structural properties of the solvent.     

Nucleation and growth in the collapsed langmuir monolayers from semifluorinated alkanes

The 3D phase formation was monitored in relaxation experiments of the collapsed Langmuir monolayers of selected partially fluorinated tetracosanes, that is, F6H18, F8H16, and F10H14. To carry out these experiments, the classical method of surface manometry, such as pi-A isotherms registration and the molecular area-time dependencies, under quasi-static monitoring conditions were applied. The evolution of 3D structures at the water/air interface was observed with Brewster angle microscopy (BAM). The obtained data were interpreted according to the nucleation-growth-collision theory model. It occurred that, even though the investigated chemicals are not classical surfactants and do not possess any polar headgroup, their evolution from a 2D monolayer to 3D structures can be successfully modeled with the above-mentioned theory. The influence of the subphase temperature on the nucleation process is also discussed.     

Langmuir monolayer characteristics of a perfluoroaryl surfactant: 10-Perfluorobenzyl-decan-1-ol (PBD)

A surfactant containing the perfluorobenzyl moiety attached at the 10 position of decanol (10-perfluorobenzyldecan-1-ol, PBD) was synthesized and employed for Langmuir monolayer characterization, using surface pressure (π) and electric surface potential (ΔV)-mean molecular area (A) isotherms. Both static and dynamic stability experiments demonstrate that the monolayers formed by PBD are of satisfactory stability. Characteristic dendritic structures were observed with Brewster Angle Microscope (BAM) upon the whole compression up to the vicinity of collapse, where 3D crystallites appeared. The monolayer thickness is found to be comparable to the PBD molecule length. It occurs that ΔV has negative value even at large molecular areas, and decreases gradually along the compression, reaching a minimum of ca. −500 mV. Using the Helmholtz model, the apparent dipole moment was calculated upon film compression. The measurements were complemented with semi-empirical calculations involving molecular geometry optimization and dipole moment estimation.     

Theoretical conformational analysis of the Gramicidin a transmembrane channel. I. Helix sense and energetics of head-to-head dimerization

Conformational energy calculations are presented for the head-to-head dimerized β helices for Gramicidin A transmembrane channel structures. The calculations take into account both left- and right-handed β helices, and various side-chain conformations. The energetics of the dimerization is studied by considering various docking geometries. It is concluded from these vacuum-energy calculations that the lowest energy conformation for the channel dimer is that comprised of left-handed β helices.     

A novel viscoelastic system from a cationic surfactant and a hydrophobic counterion

The phase behavior of 2-hydroxy-1-naphthoic acid (2,1-HNC) mixed with cetyltrimethylammonium hydroxide (CTAOH) is reported. This novel system is compared with the published one of 3-hydroxy-2-naphthoic acid (3,2-HNC) mixed with CTAOH. We investigated the phase behavior and properties of the phases in aqueous solutions of 100 mM CTAOH with 2,1-HNC. In both systems a multilamellar vesicle phase is formed when the naphthoate/surfactant ratio (r) reaches unity. When an increasing amount of 2,1-HNC is mixed with a micellar solution of 100 mM CTAOH, an isotropic low-viscous micellar solution, a viscoelastic gel (consisting of rodlike micelles), a turbid region (two-phase region), and a viscoelastic liquid crystalline gel (consisting of multilamellar vesicles, MLV) were formed. The vesicular phase is highly viscoelastic and has a yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/2,1-HNC over a two-phase region, where micelles and vesicles coexist. Also it was noticed that 2,1-HNC is dissolved in 100 mM CTAOH until the naphthoate/surfactant ratio reaches approximately 1.5, and the liquid crystalline phases were found to change their color systematically when they were viewed between two crossed polarizers. The vesicles have been characterized by differential interference contrast microscopy, freeze-fracture electron microscopy, and cryo-electron microscopy (cryo-TEM). The vesicles were polydisperse and their diameter ranged from 100 to 1000 nm. The interlamellar spacing between the bilayers was determined with small angle neutron scattering and agrees with the results from different microscopical methods. The complex viscosity rises by six orders of magnitude when rodlike micelles are formed. The complex viscosity decreases again in the turbid region, and then rises approximately six orders of magnitude above the water viscosity. This second rising is due to the formation of the liquid crystalline MLV phase.     

Organogels from a vitamin C-based surfactant

A new double chained surfactant, 2-octyl-dodecanoyl-6-O-ascorbic acid (8ASC10), with a L-ascorbic acid unit as the polar headgroup was synthesized for the first time. The behavior of the compound in the dry solid state has been characterized through DSC, XRD, and SAXS measurements. The surfactant forms stable viscous organogels in the presence of suitable organic solvents and also water-induced organogels upon addition of water to the organogel. These mixtures show shear-thinning properties and are birefringent. The behavior and properties of the organogels have been studied through rheology, DSC, and SAXS experiments. The organogels possess the same antioxidant properties of the original L-ascorbic acid ring and can be used to solubilize and protect valuable organic molecules.     

Switchable dielectric permittivity with temperature and Dc-bias in a semifluorinated azobenzene derivative

The thermodynamic, optical, structural, and dynamic properties of the semifluorinated (E)-1-(4-octylphenyl)-2-(4-(perfluorooctyl)phenyl)diazene (4) and the corresponding (E)-1,2-bis(4-octylphenyl)diazene (5) have been studied with differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dielectric spectroscopy. 4 combines the azobenzene properties with the fluorophobic effect and gives rise to a responsive material with a temperature and dc-bias-driven switchable dielectric permittivity within the narrower nematic phase. This is caused by the nematic potential that inevitably brings some fluorocarbon chains in proximity to the hydrocarbon chains from adjacent molecules. Frustration is alleviated by reducing the nematic-to-isotropic transition temperature and by increasing the crystalline-to-nematic transition temperature, thus limiting the stability of the nematic phase. Unlike the normal isotropic phase of compound 5, the isotropic phase of compound 4 contains dipoles with short-range orientation correlations. Optimizing the type of interactions may result in materials with applications as molecular switches and electrooptic devices.     

In situ assembly, regeneration and plasmonic immunosensing of a Au nanorod monolayer in a closed-surface flow channel

Herein, a simple and effective approach is reported for the in situ generation and regeneration of a Au nanorod (AuNR) monolayer inside a glass/silica-based, closed-surface flow channel. The density of the AuNR monolayer in the flow channel can be easily modified by varying the concentration of the AuNR and the cetyltrimethylammonium bromide as well as the incubation time. The fabricated AuNR monolayer in the flow channels was stable under harsh conditions, such as in extreme pH, organic solvents and at a fast flow rate. In addition, the flow channel could be reused by removing the immobilized AuNRs via the injection of diluted aqua regia or potassium iodide; the AuNR monolayer can subsequently be regenerated. The AuNRs in the closed flow channel were further exploited as a label-free detection method for a clinical biomarker, neutrophil gelatinase-associated lipocalin (NGAL), based on single-nanoparticle plasmonic assay. The corresponding limit of detection for NGAL was measured to be 8.5 ng mL(-1) (~340 pM) based on a signal-to-noise ratio of 3. The estimated recovery of NGAL in human serum and urine was higher than 80%, which indicates that this technique could potentially be used for the diagnosis of acute kidney injury.     

Spontaneous non-linear surface tension oscillations in the presence of a spread surfactant monolayer at the air/water interface

The phenomena accompanying the dissolution of a surfactant droplet under the water/air interface covered by a spread monolayer are studied experimentally and theoretically. It is shown that the variation of the initial surface coverage changes the way of the system evolution. With respect to the character of changes of the interfacial tension with time one can distinguish between three different regimes which replace each other by increase of the initial surface coverage: (i) single oscillation followed by a long period of the monotonous decrease of the surface tension after which repeated non-linear oscillations develop spontaneously; (ii) repeated non-linear oscillations of the surface tension (without period of the monotonous decrease); (iii) monotonous decrease of the surface tension without any oscillation. The hydrodynamics of the observed regimes are discussed.     

Photochemical reactions of surfactant molecules in condensed monolayer assemblies--environmental control and modification of reactivity.

An investigation of the effects of a controlled microenvironment on reactivity has been carried out by the synthesis and study in organized monolayers of surfactant molecules containing photochemically reactive chromophores. Reactions where substantial environmental effects have been observed include the cis-trans isomerization of olefins, ketone photoeliminations, ligand substitution processes, excimer and photodimer formation, and photoredox reactions. Both reactions occurring within the organized assemblies as well as interfacial phenomena have been investigated. Results obtained in these studies suggest possible extensions and applications for the development of new catalyst systems, biological models and controlled syntheses.