Characterization of domain growth kinetics in a mixed perfluorocarbon-hydrocarbon Langmuir-Blodgett monolayer

The rate of domain growth in phase-separated, mixed Langmuir-Blodgett (LB) monolayers of arachidic acid, C(19)H(39)COOH (AA) and perfluorotetradecanoic acid, C(13)F(27)COOH (PA) was tracked via atomic force microscope measurements. The growth rate of domains comprised of phase-separated AA was consistent with that predicted by the Lifshitz-Slyozov model for diffusion-limited Ostwald ripening. In addition to Ostwald ripening, some evidence for domain coalescence was also observed when LB films were deposited under conditions of low temperature and short incubation times, though this tendency disappeared at higher deposition temperatures.     

An ensemble and single-molecule fluorescence microscopy investigation of phase-separated monolayer films stained with Nile Red

Phase-separated Langmuir-Blodgett monolayer films prepared from mixtures of arachidic acid (C19H39COOH) and perfluorotetradecanoic acid (C13F27COOH) were stained via spin-casting with the polarity sensitive phenoxazine dye Nile Red, and characterized using a combination of ensemble and single-molecule fluorescence microscopy measurements. Ensemble fluorescence microscopy and spectromicroscopy showed that Nile Red preferentially associated with the hydrogenated domains of the phase-separated films, and was strongly fluorescent in these areas of the film. These measurements, in conjunction with single-molecule fluorescence imaging experiments, also indicated that a small sub-population of dye molecules localizes on the perfluorinated regions of the sample, but that this sub-population is spectroscopically indistinguishable from that associated with the hydrogenated domains. The relative importance of selective dye adsorption and local polarity sensitivity of Nile Red for staining applications in phase-separated LB films as well as in cellular environments is discussed in context of the experimental results.     

Structural and compositional mapping of a phase-separated Langmuir-Blodgett monolayer by atomic force microscopy

The structure and composition of a phase-separated arachidic acid (C19H39COOH) (AA) and perfluorotetradecanoic acid (C13F27COOH) (PA) Langmuir-Blodgett monolayer film was characterized by several different types of atomic force microscopic measurements. At the liquid-air interface, surface pressure-area isotherms show that mixtures of the two acids follow the additivity rule expected from ideal mixtures. Topographic images of the deposited monolayer indicate that the surfactants are oriented normal to the substrate surface, and that the acids undergo phase separation to form a series of discontinuous, hexagonal domains separated by a continuous domain. A combination of lateral force (friction) imaging and adhesion force measurements show that the discontinuous domains are enriched in AA, whereas the surrounding continuous domain is a mixture of both AA and PA. This was further verified by selective, in situ dissolution of AA by n-hexadecane, followed by high-resolution topographical imaging of the discontinuous domains.     

Mechanistic insight into domain formation and growth in a phase-separated Langmuir-Blodgett monolayer

The mechanism of the formation and growth of phase-separated domains in mixed arachidic acid (C19H39COOH) (AA) and perfluorotetradecanoic acid (C13F27COOH) (PA) monolayer films was investigated through a combination of surface pressure-area isotherm measurements and atomic force microscope (AFM) imaging. In the mixed AA-PA monolayer system, distinct discontinuous domains consisting primarily of AA form spontaneously in a surrounding continuous matrix enriched in PA. By varying the monolayer deposition conditions, including temperature, surface pressure, and the mechanical agitation of sample solutions, it was determined that phase-separated nuclei are formed initially in the bulk sample solution and further growth of domains proceeds on the subphase surface via an Ostwald ripening process involving the diffusion of AA from the matrix to the discontinuous domains. In addition, selective dissolution of the arachidic acid followed by in situ AFM imaging has allowed the visualization of the fusion of AA to the phase-separated domains and has highlighted some unusual pattern formation that occurs at low subphase temperatures.     

Rippled domain formation in phase-separated mixed Langmuir-Blodgett films

The morphology and composition of phase-separated Langmuir and Langmuir-Blodgett films of stearic acid (C17H35COOH) (SA) mixed with perfluorotetradecanoic acid (C13F27COOH) (PA) have been investigated using a combination of atomic force microscopy (AFM) measurements and surface pressure-area isotherms. At elevated surface pressures, the mixed film phase-separated to form a distinct series of lines (ripples), as opposed to the hexagons that have previously been observed with mixed films with longer alkyl chain fatty acids. At low surface pressures, phase separation is still observed, though a range of different domain structures was formed. The chemical composition of the phase-separated domains has been investigated by AFM-based compositional mapping, which has allowed unambiguous identification of the chemical composition of the domains. A simple mechanistic model describing how domain formation takes place in this system is presented.     

Mixed films of poly(n-hexyl isocyanate) and poly(vinyl acetate) at the air-water interface

We study interfacial properties of rigid-rod-like poly(n-hexyl isocyanate) (PHIC), flexible poly(vinyl acetate) (PVAc), and mixed films of PHIC and PVAc spread at the air-water interface as a function of the molar fraction of PHIC by surface pressure measurements and fluorescence microscopy. From the plots of the experimental mean area of the mixed polymer films at a constant surface pressure as a function of the molar fraction of PHIC in the mixed films, the binary mixtures of PHIC/PVAc were concluded to be compatible at the air-water interface. This means that the hydrophobic hexyl group of PHIC takes a horizontal orientation to the air-water interface rather than a perpendicular one, leading to PHIC and PVAc having the same interfacial orientation. Compatibility of the binary mixtures of PHIC/PVAc at the air-water interface is also confirmed by their fluorescence microscopic images, since PHIC proves to be inhomogeneous and PVAc is homogeneous with the aid of a fluorescence probe, respectively.     

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.     

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.     

Molecular organization in two-dimensional films of liquid II. Langmuir and Langmuir-Blodgett films of a perylene-like dye mixed with liquid crystals having a terminal cyano group crystalline mixtures

Langmuir and Langmuir-Blodgett (LB) films of a perylene-like compound and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT) have been studied. On the basis of the surface pressure-area isotherms, the molecular organization on the air-water interface has been estimated. Information about the miscibility or the phase separation of components in the binary mixtures has been obtained. The spectroscopic study of the LB films has allowed conclusions to be drawn about the arrangement of the molecules on the quartz slides. The fluorescence spectra of the LB films of the perylene-like compound have revealed the formation of self-aggregates.     

Properties of mixed monomolecular films of poly(benzyl-methacrylate) and arachidic acid

Binary mixed monomolecular films of poly(benzyl-methacrylate) and arachidic acid at the water/air interface have been studied with respect to compatibility and stability. The surface pressure — area isotherms indicate compatibility of the two components. However, the miscible state is unstable at high surface pressures. This is demonstrated by the constant pressure relaxation of the mixtures. There is a mechanism of separation and nucleation of the arachidic acid from the film for mixtures with high polymer content at a surface pressure of 30 mN/m. For lower concentrations of polymer in the mixtures and at a surface pressure of 20 mN/m the fatty acid is stabilized by the polymer.     

Molecular organization in two-dimensional films of liquid II. Langmuir and Langmuir-Blodgett films of a perylene-like dye mixed with liquid crystals having a terminal cyano group crystalline mixtures

Langmuir and Langmuir-Blodgett (LB) films of a perylene-like compound and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT) have been studied. On the basis of the surface pressure-area isotherms, the molecular organization on the air-water interface has been estimated. Information about the miscibility or the phase separation of components in the binary mixtures has been obtained. The spectroscopic study of the LB films has allowed conclusions to be drawn about the arrangement of the molecules on the quartz slides. The fluorescence spectra of the LB films of the perylene-like compound have revealed the formation of self-aggregates.     

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.     

Phase and miscibility behavior of binary lipid systems

Differential scanning calorimetry was applied to study the phase diagrams of the following binary lipid systems: myristic acid (C₁₃COOH) / pentadecanoic acid (C₁₄COOH); palmitic acid methyl ester (C₁₅COOMe) / heptadecanoic acid methyl ester (C₁₆COOMe); palmitic acid methyl ester (C₁₅COOMe) / stearic acid methyl ester (C₁₇COOMe); palmitic acid methyl ester (C₁₅COOMe) / arachidic acid methyl ester (C₁₉COOMe). A distinct succession in the phase diagram types and phase regions was observed, according to the chemical structure of the mixing components.In the systems C₁₃COOH/C₁₄COOH; C₁₅COOMe/C₁₆COOMe and C₁₆COOMe/ C₁₇COOMe, both components are completely miscible in the low- and high-temperature phase. Contrasting with these three binary lipid systems, the system C₁₅COOMe/ C₁₄COOMe shows complete miscibility only in the high-temperature phase, but almost complete demixing in the solid state. In the phase diagram an incongruent melting 11 complex is built up. This complex forms an eutectic mixture with the phase of C₁₅COOMe and a peritectic system with C₁₉COOMe.     

Incorporation and exclusion of long chain alkyl halides in fatty acid monolayers at the air-water interface

Mixed monolayers of deuterated palmitic acid C(15)D(31)COOH (dPA) and deuterated stearic acid C(17)D(35)COOH (dSA) with 1-bromoalkanes of different alkyl chain length (C(4) to C(16)) at the air-water interface were investigated. Alkanes and 1-chlorohexadecane ClC(16)H(33) (ClHex) were also studied to compare the effects of the halogen on the mixed monolayers. Surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to obtain the organization and phase behavior, providing a macroscopic view of the mixed monolayers. A molecular-level understanding of the interfacial molecular organization and intermolecular interactions was obtained by using vibrational sum frequency generation (SFG) spectroscopy and infrared reflection-absorption spectroscopy (IRRAS). It was found that from the alkyl halide molecules investigated 1-bromopentadecane, BrC(15)H(31) (BrPent), 1-bromohexadecane, BrC(16)H(33) (BrHex), and ClHex incorporate into the fatty acid monolayers. Alkanes of 15- and 16-carbon chain length do not incorporate into the fatty acid monolayer, which suggests that the halogen is needed for incorporation. Isotherms and spectra suggest that BrHex molecules are squeezed out, or excluded, from the fatty acid monolayer as the surface pressure is increased, while BAM images confirm this. Additionally, SFG spectra reveal that the alkyl chains of both fatty acids (dPA and dSA) retain an all-trans conformation after the incorporation of alkyl halide molecules. BAM images show that at low surface pressures BrHex does not affect the two-dimensional morphology of the dPA and dSA domains and that BrHex is miscible with dPA and dSA. We also present for the first time BAM images of BrHex deposited on a water surface, which reveal the formation of aggregates while the surface pressure remains unchanged from that of neat water.     

Polymer-surfactant layered heterostructures by electropolymerization of phenosafranine in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of the water-soluble dye phenosafranine (PS) have been prepared by its adsorption from aqueous dye solution to an arachidic acid (AA) monolayer at the air-water interface. Atomic force microscopy (AFM) images of the LB films revealed the effect of change in pH of deposition on the degree of complexation of AA with the PS dye. Well-defined circular islands and holes were observed which disappeared with the increase in pH. Polarized absorption studies indicated that the dye molecules are oriented uniaxially with their long axis titled at a constant angle to the surface normal of the LB film. Within the restricted geometry of the LB film, the PS dye was electropolymerized to form a two-dimensional film of poly(phenosafranine) sandwiched between arachidic acid layers. The film was characterized by IR spectroscopy, cyclic voltammetry, and AFM. X-ray diffraction studies reveal the presence of a layer structure in the AA-PS LB film before and after polymerization. The polymer film showed highly anisotropic electrical conductivity of ca. 10 orders of magnitude. This indicates the formation of two-dimensional polyPS layers between arachidic acid layers resulting in a layered heterostructure film having alternate conducting and insulating regions. Also, the conductivity of the polyPS prepared from LB film was found to be approximately 2.5 times higher than the conductivity of polyPS prepared by solution polymerization method.     

Thermodynamic and spectroscopic studies on the nickel arachidate-RNA polymerase Langmuir-Blodgett monolayer

The Langmuir-Blodgett (LB) monolayers offer a unique system to study molecular interaction at the air-water interface with reduced dimensionality. In order to develop this further to follow macromolecular interactions at equilibrium, we first characterized the Ni (II)-arachidate (NiA) monolayer at varying conditions. Subsequently, the interaction between NiA and histidine-tagged RNA polymerase (HisRNAP) were also studied. LB films of arachidic acid-NiA and NiA-RNAP with different mole fractions were fabricated systematically. Surface pressure versus area per molecule (P-A) isotherms were registered, and the excess Gibbs energy of mixing was calculated. The LB films were then deposited on solid supports for Fourier transform infrared (FTIR) spectroscopic measurements. The FTIR spectra revealed the change in the amount of incorporated Ni (II) ions into the arachidic acid monolayer with the change in pH and the increasing mole fraction of RNAP in the NiA monolayer with its increasing concentration in the subphase. The system developed here seems to be robust and can be utilized to follow macromolecular interactions.     

Spectroscopic studies of Langmuir-Blodgett films of 3,4,9,10-tetra(heptyloxycarbonyl)perylene and its mixtures with a liquid crystal

Langmuir (L) and Langmuir-Blodgett (LB) films of 3,4,9,10-tetra(heptyloxycarbonyl)perylene and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4'-cyano-p-terphenyl (5CT) have been studied. On the basis of the surface pressure-mean molecular area isotherms of L films, the alignment of the molecules on the air-water interface has been estimated. The L films were transferred onto quartz plates at surface pressures below the collapse point. The absorption and fluorescence spectra of LB films, obtained using unpolarized and linearly polarized light, were recorded. The results obtained have led to conclusions on the arrangement of the dye and liquid crystal molecules on the air-solid substrate interface. The fluorescence spectra revealed the formation of excited dimers in LB films.     

N,N,N＇,N＇-四苄基取代方块菁染料LB膜研究

方块菁染料在有机光导材料^[1,2]、有机太阳能存储^[2,3]、光记录^[4、有机光盘中红外吸收器^[4]以及光纤识别功能薄膜等领域中有着广泛应用前景.     

Effect of subphase temperature on the phase-separated structures of mixed Langmuir and Langmuir-Blodgett films of fatty acids and hybrid carboxylic acids

We examined the phase-separated structures of the mixed Langmuir-Blodgett (LB) films of C(k)H(2k+1)COOH (HkA: k=17, 21) and C(m)F(2m+1)C(10)H(20)COOH (FmH10A: m=6, 8) fabricated isothermally or after isobaric thermal treatments. Under isothermal fabrication conditions, disks and wire-type domains formed in the H17A/F6H10A LB films at high and low fabrication temperatures, respectively, because the line tension and dipole-dipole interaction were comparable with each other. The thermodynamically stable phases of H21A/F6H10A LB films at high and low fabrication temperatures were disks and polygonal domains, respectively. Isobaric thermal treatments of the Langmuir films affected the domain size and not the domain shape when the transfer temperature was the same. Isobaric thermal treatments were effective in controlling the domain size. The thermodynamically stable phases of both H17A/F8H10A LB films and H21A/F8H10A LB films were nanowires in the range of the fabrication temperatures studied. Isobaric thermal treatments of the Langmuir films did not affect the domain shape significantly and affected the domain size in both the LB films studied. The change in the value m of FmH10A was more effective in controlling the phase-separated structures of the mixed LB films than the value of k of HkA.     

Molecular organization in two-dimensional films of liquid crystalline mixtures III. Langmuir films of binary mixtures of liquid crystal materials with terminal CN o r NCS group

Langmuir films of binary mixtures of the following liquid crystal materials: 4-octyl-4'-cyano'biphenyl (8CB) or 4-pentyl-4"-cyano-p-terphenyl (5CT) with 4-(trans-4'-octylcyclohexyl)isothiocyanatobenzene (8CHBT), trans-4-octyl(4'-cyanophenyl)cyclohexane (8PCH) or 4-octyl4'-isothiocyanatobiphenyl (8BT) were investigated. Surface pressure-mean molecular area isotherms were recorded at various mixture compositions. It was found that only liquid crystal materials for which the molecules have a terminal -CN group are able to form a stable monolayer at the air-water interface. Moreover, information about the miscibility or the phase separation of the two components in the mixures was obtained by using the excess area criterion and surface phase rules.