Range Of Surface Pressures Research Articles

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air–water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression–expansion cycle of the monolayer is not reversible and the second π– A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.

Dynamic viscoelastic properties of spread monostearin monolayer in the presence of glycine

The monostearin monolayer at the air-aqueous interface is more expanded in presence of glycine and at higher temperature from both the surface pressure—area per molecule ( π– A) isotherms and static elasticity—surface pressure ( E s – π) curves. The dilational viscoelastic properties of monostearin monolayer spread on the subphase of glycine solution have been determined by the dynamic oscillation method and discussed as a function of surface pressure, temperature, and frequency. At the frequency of 50 mHz, the monostearin monolayer on pure water shows negative dilational viscosity and is viscoelastic at some surface pressures, while the monostearin monolayer in the presence of glycine is nearly elastic over a wide range of surface pressure, especially at 25 °C. Both positive and negative loss angle tangent can be deduced as a function of surface pressure. The negative dilational viscosity can be attributed to the phase transitions induced by the propagation of the surface waves during the dynamic oscillation. It can be convinced that the interactions between monostearin and glycine play an important role in the formation and rheological behavior of the monolayer. On the other hand, temperature has effect on the dilational elasticity and the dilational viscosity of the monostearin monolayer in different extents. Furthermore, at the surface pressure of 20 mN/m, the monostearin monolayer on the glycine solution at 18 °C is essentially elastic at lower frequency (<100 MHz) and shows viscoelastic behavior at higher frequency. These phenomena should be associated with the complicated monolayer structure and structural reorganization due to the interactions between monostearin and glycine in presence of glycine.

Surface Dilatational Behavior of Pulmonary Surfactant Components Spread on the Surface of a Captive Bubble. 3. Dipalmitoyl Phosphatidylcholine, Surfactant Protein C, and Surfactant Protein B

Spread monolayers containing dipalmitoyl phosphatidylcholine (DPPC), surfactant protein C (SP-C), and surfactant protein B (SP-B) were characterized by dynamic and equilibrium surface pressure/area isotherms, surface dilatational rheological measurements, and transient stress relaxation. The maximum surface pressure which can be realized by monolayer compression depends significantly on the compression rate. The surface rheological behavior is strongly influenced by the presence of SP-B, which reduces the characteristic time of stress relaxation. Therefore the surface pressure of a layer containing SP-B (DPPC + SP-B or DPPC + SP-B + SP-C) changes within some seconds after a transient surface pressure jump to a constant value which corresponds to an equilibrium surface pressure plateau at about 51 mN/m, whereas DPPC + SP-C relaxes within tens of seconds to a steady-state value of surface pressure. In the frequency range of human breathing, the surface dilatational viscosity is weak and nearly independent of surface pressure and frequency for all systems investigated. The surface rheological behavior of the phospholipid/surfactant layers in this range is mainly elastic, with maximum elasticity at a surface pressure slightly below the equilibrium surface pressure plateau, whereas the compressibility of the pulmonary film in the surface pressure range of breathing crosses a stationary compressibility maximum. This study suggests that the presence of SP-B is essential for expiration and the presence of both proteins, SP-B and SP-C, is essential for the inspiration process of the breathing cycle.

Miscibility behavior of dipalmitoylphosphatidylcholine with a single-chain partially fluorinated amphiphile in Langmuir monolayers

Surface pressure–area, surface potential–area, and dipole moment–area isotherms were obtained for monolayers made from a partially fluorinated surfactant, (perfluorooctyl)undecyldimorpholinophosphate (F8H11DMP), dipalmitoylphosphatidylcholine (DPPC), and their combinations. Monolayers, spread on a 0.15 M NaCl subphase, were investigated at the air/water interface by the Wilhelmy method, ionizing electrode method, and fluorescence microscopy. Surface potentials were analyzed using the three-layer model proposed by Demchak and Fort. The contribution of the dimorpholinophosphate polar head group of F8H11DMP to the vertical component of the dipole moment was estimated to be 4.99 D. The linear variation of the phase transition pressure as a function of F8H11DMP molar fraction ( X F8H11DMP) demonstrated that DPPC and F8H11DMP are miscible in the monolayer. This result was confirmed by deviations from the additivity rule observed when plotting the molecular areas and the surface potentials as a function of X F8H11DMP over the whole range of surface pressures investigated. Assuming a regular surface mixture, the Joos equation, which was used for the analysis of the collapse pressure of mixed monolayers, allowed calculation of the interaction parameter ( ξ=−1.3) and the energy of interaction (Δ ε=537 J mol −1) between DPPC and F8H11DMP. The miscibility of DPPC and F8H11DMP within the monolayer was also supported by fluorescence microscopy. Examination of the observed flower-like patterns showed that F8H11DMP favors dissolution of the ordered LC phase domains of DPPC, a feature that may be key to the use of phospholipid preparations as lung surfactants.

Anomalous Damping of the Capillary Waves at the Air−Water Interface of a Soluble Triblock Copolymer

The dilational viscoelasticity of Gibbs and Langmuir monolayers of pluronic F-68 (PEO76−PPO29−PEO76) has been studied by capillary wave techniques over a broad frequency range. The results show that the behaviors of Gibbs and Langmuir monolayers are equivalent in the low surface pressure range. For concentrated monolayers, the dynamic elasticity ε shows a noticeable frequency dependence, and the concentration dependence of ε (at constant frequency) is not compatible with the classical Lucassen−Van den Tempel model. Moreover, in this concentration range, the dilational viscosity takes apparent negative values. The spectra of light scattered by thermal capillary waves have been analyzed in terms of two recently proposed dispersion equations. The equation of Buzza et al. (J. Chem. Phys. 1998, 109, 5008) is able to fit the experimental spectra only if one assumes that the thickness of the monolayer takes unrealistic values of the order of 0.1 μm. The equation of Hennenberg et al. (J. Colloid Interface Sci. 2000, 230, 216) is able to fit the experimental spectra, although the physical meaning of its new coupling parameter remains obscure.

Mixed monolayers made from dipalmitoyl phosphatidylcholine and a fluorinated amphiphile

We have recently reported that water-in-fluorocarbon (FC) reverse emulsions and microemulsions could be used as delivery systems of bioactive materials to the lung, in particular for local administration using pressurized metered-dose inhalers. The surfactant involved in these reverse emulsions is a perfluoroalkylated amphiphile derived from the dimorpholinophosphate polar head group C8F17(CH2)11OP(O)[N(CH2CH2)2O]2 (F8H11DMP). The interaction of F8H11DMP with the lung surfactant is unknown, as well as that of reverse FC microemulsions. In this study, based on film balance measurements and fluorescence microscopy, we report on the mixed monolayer behavior of F8H11DMP and dipalmitoylphosphatidylcholine (DPPC) used as a lung surfactant model at the air/water interface. F8H11DMP/DPPC mixtures formed miscible, liquid expanded (LE) mixed monolayers in the whole range of F8H11DMP molar fractions investigated (XF8H11DMP≥0.33). Repulsive interactions were observed for high surface pressures. Miscible monolayers in LE state were also formed when DPPC/F8H11DMP mixtures were put in contact with perfluorooctyl bromide (PFOB). In this case, however, attractive interactions were observed (for XF8H11DMP=0.33 and 0.5) in the all range of surface pressures investigated. In addition, it was found that spreading a water-in-PFOB microemulsion formulated with F8H11DMP on a DPPC monolayer led to the adsorption of F8H11DMP at the interface and formation of a mixed DPPC/F8H11DMP monolayer.

Langmuir–Blodgett films of polyethylene

The possibility to obtain surface layers on water and prepare solid multilayer Langmuir–Blodgett films of medium-density polyethylene is shown. The polymer film on water is stable, demonstrates a reversible surface pressure-area isotherm up to 15 mN/m, and can be deposited onto a substrate using the Langmuir–Blodgett technique in a wide range of surface pressures. The thickness of a single deposited layer is 5.1 nm on average. The dielectric and optical constants of multilayer films are near their bulk values. The films exhibit high dielectric strength of at least 200 MV/m.

Scanning electrochemical microscopy (SECM) feedback approach for measuring lateral proton diffusion in langmuir monolayers: theory and application

An SECM method for measuring lateral proton diffusion at Langmuir monolayers deposited at a water/air (W/A) interface is described. The approach utilises the electrogeneration of a base by the reduction of weak acid (producing hydrogen) at an inverted ‘submarine’ ultramicroelectrode (UME) placed close to the monolayer in the aqueous subphase of a Langmuir trough. The electrogenerated base diffuses to the monolayer and titrates away protons, resulting in conversion of the base back to its initial form. The acidic monolayer is thus depronated locally which creates a concentration gradient for lateral proton diffusion. A numerical model is developed for the SECM mass transport problem and the effect of lateral proton diffusion on the UME current response is established. The method is demonstrated experimentally with measurements of lateral proton diffusion along a stearic acid monolayer at a wide range of surface pressures. The lateral proton diffusion coefficient is found to be fairly uniform (∼8 × 10−6 cm2 s−1) when the surface pressure of the monolayer (π) is in the range of 1–20 mN m−1, but decreases to immeasurably low values when π is greater than 25 mN m−1.

Proton diffusion at phospholipid assemblies.

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME. Local deprotonation of the monolayer creates a concentration gradient for lateral proton diffusion. A numerical model has been developed, taking into account the potential-dependent association/dissociation constant of the interfacial acid groups. A comparison is made of monolayers comprising either acidic DL-alpha-phosphatidyl-L-serine, dipalmitoyl (DPPS) or zwitterionic L-alpha-phosphatidylcholine, dipalmitoyl (DPPC) monolayers at a range of surface pressures. It is demonstrated that lateral proton fluxes at DPPS are significant, but the lateral proton diffusion coefficient is lower than in bulk solution. In contrast, lateral proton diffusion cannot be detected at DPPC, suggesting that the acid/base character of the phospholipid is important in determining the magnitude of interfacial proton fluxes.

Surface Shear Rheology of a Polymerizable Lipopolymer Monolayer

We have used an interfacial stress rheometer to study the changes in the mechanical shear properties of a Langmuir monolayer of a linear polymer with reactive side chains. The polymer investigated had a linear hydrophilic backbone consisting of poly(ethylene imine-co-ethyl oxazoline) and contained 20% hydrophobic sorbyl side chains, making it amphiphilic and capable of forming stable monolayers at the air−water interface. The sorbyl moiety in the side chain was photopolymerizable at 254 nm. Upon exposure of the film to UV light, the surface dynamic shear modulus, Gs‘, which is a measure of the elasticity, increased 1000-fold to ∼2 mN/m. This large increase in elasticity arises from the formation of a network by reaction of side chains on different polymer backbones. Rubber-like properties were deduced from the frequency dependence of the moduli and from creep experiments, where elastic recoil could be observed upon removing the applied stress. Such a network could be formed over a wide range of surface pressures ranging from 5 to 25 mN/m. We investigated the cross-linking kinetics upon UV exposure as a function of temperature, initial surface pressure, and light intensity. We have developed a lumped kinetic model that agrees with the transient behavior of the storage modulus, our measure of the density of cross-link points. The reaction was independent of temperature (over 10−30 °C) and exhibited a first-order dependence on light intensity. Since the reactive side groups must be in close enough proximity for the interpolymer side chain reaction to occur, a threshold side chain density is required to form junction points. After extensive UV exposure, the elasticity of the network degraded, and the rate of degradation was faster for films polymerized at lower initial surface pressures.

In situ measurement of the displacement of protein films from the air/water interface by surfactant.

The displacement of spread protein films from the air/water interface by surfactant was followed using Brewster angle microscopy (BAM) and interfacial rheology. The displacement of beta-lactoglobulin and beta-casein by a nonionic surfactant was monitored as a function of both surface pressure and time. In both cases, protein displacement occurred over the same surface pressure range that had been observed previously by atomic force microscopy (AFM). In the case of the beta-lactoglobulin, surfactant domains grew large enough in the protein film to be visible in the BAM images. The shapes of the domains were very similar to those seen previously by AFM in the late stages of displacement. The results from both proteins confirm the results published previously while highlighting some implications for the application of the "orogenic" model of displacement for large protein films. The surface rheological data showed that the beta-lactoglobulin/surfactant mixed film retained much of its elasticity until the latter stages of displacement. This indicates that at least in the early stages of displacement, the mixed film was dominated by the behavior of the protein in the film.

Distribution of Ganglioside GM1 in l- α-Dipalmitoylphosphatidylcholine/Cholesterol Monolayers: A Model for Lipid Rafts

The distribution of low concentrations of ganglioside GM1 in l- α-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol monolayers supported on mica has been studied using atomic force microscopy (AFM). The monolayers studied correspond to a pure gel phase and a mixture of liquid-expanded (LE) and liquid-condensed (LC) phases for DPPC and to a single homogeneous liquid-ordered phase for 2:1 DPPC/cholesterol. The addition of 2.5–5% GM1 to phase-separated DPPC monolayers resulted in small round ganglioside-rich microdomains in the center and at the edges of the LC domains. Higher amounts of GM1 (10%) give numerous filaments in the center of the LC domains and larger patches at the edges. A gel phase DPPC monolayer containing GM1 showed large domains containing a network of GM1-rich filaments. The addition of GM1 to a liquid-ordered 2:1 DPPC/cholesterol monolayer gives small, round domains that vary in size from 50 to 150 nm for a range of surface pressures. Larger amounts of GM1 lead to coalescence of the small, round domains to give longer filaments that cover 30–40% of the monolayer surface for 10 mol % GM1. The results indicate that biologically relevant GM1 concentrations lead to submicron-sized domains in a cholesterol-rich liquid-ordered phase that is analogous to that found in detergent-insoluble membrane fractions, and are thought to be important in membrane microdomains or rafts. This demonstrates that AFM studies of model monolayers and bilayers provide a powerful method for the direct detection of microdomains that are too small for study with most other techniques.

Helical Superstructures from a Poly(γ-benzyl-l-glutamate)−Poly(l-glutamic acid) Amphiphilic Diblock Copolymer: Monolayer Formation on Water and Its Specific Binding of Amino Acids

This study examines the monolayer properties of a novel amphiphilic diblock polymer of poly(γ-benzyl-l-glutamate) (PBLG)-block-poly(l-glutamic acid) (PLGA) (PBLG-b-PLGA) at the air−water interface and the binding of α-amino acids such as tryptophan, tyrosine, and phenylalanine to the monolayer surfaces. The block polymer, PBLG-b-PLGA, was synthesized via two steps: at first the PLGA block was prepared by polymerization of γ-benzyl-l-glutamate-N-carboxylic anhydride (BLG-NCA) with propylamine and then by removal of benzyl group, and subsequently the polymerization of BLG-NCA initiated with the amino group at the chain end of the PLGA was performed and gave a final product. PBLG-b-PLGA formed stable monolayers, in which the PLGA segment took an α-helix structure with above 95% helicity over the wide range of surface pressures and pHs. The helix orientaion was in almost perpendicular to the surface, being variable slightly by changing the surface pressure. These helix assemblies were found to have the abili...

Reflection–absorption FT–IR spectroscopy of vinyl octadecanoate at the air/water interface

Reflection–absorption Fourier transform infrared (RA FT–IR) spectra of spread monolayers of vinyl octadecanoate have been obtained for a range of surface pressures at 292 K. Unpolarised and s- and p-polarised radiation at 30° angle of incidence have been used. Results in the CH stretching region show that even at very low surface pressures the monolayer appears to consist of condensed domains where the acyl chains are highly ordered and densely packed. At higher surface pressures the average packing density is higher, but otherwise the spectra are unaltered, showing that the acyl chains remain highly ordered with few gauche bonds throughout the accessible range of surface pressure (0.1 to 18 mN m −1). There was no appreciable change in the spectrum when the monolayer passed through the LC to S phase transition.

Disorder in Langmuir Monolayers: 2. Relation between Disordered Alkyl Chain Packing and the Loss of Long-Range Tilt Orientational Order

A loss of long-range tilt orientational order in 1,2-hexadecandiol monolayers is observed by Brewster angle microscopy. It coincides with a coexistence of two different lattices in the monolayer over a large surface pressure range detected by grazing incidence X-ray diffraction. In one lattice the alkyl chains are tilted toward nearest neighbors and in the other toward next-nearest neighbors. In monolayers of 2-hexadecanol a condensed phase with short-range tilt orientational order is formed. The diffraction patterns show a superposition of the peaks from lattices which differ with respect to the tilt azimuth of the alkyl chains. The behavior of 1,2-hexadecandiol monolayers is related to that of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol. In monolayers of observed 2-hydroxypalmitic acid/palmitic acid mixtures a crossover from one behavior to the other is observed. The loss of order in Langmuir monolayers is initiated by a misfit of alkyl chains and headgroups. Chains can to headgroups which are of their packing. If the size of the headgroup perpendicular to the tilt direction exceeds a particular value, a disordered packing results. If substances with smaller headgroups are added to such monolayers, initially the spacing to the chain tilt direction is optimized then second the polar tilt angle is reduced.

Reflection–absorption FT-IR spectroscopy of pentadecanoic acid at the air/water interface

Reflection–absorption Fourier transform infrared (RA FT-IR) spectra of spread monolayers of pentadecanoic acid have been obtained for a range of surface pressures at 292 and 297 K. Unpolarised and s- and p-polarised radiation at 30° angle of incidence have been used. Results in the C–H stretching region reveal disorder in the arrangement of the acyl chains with numerous gauche bonds when the monolayer is in the liquid expanded (LE) phase and highly ordered chains with mostly trans bonds in the condensed phases (LC and S). The transition from one form to the other occurs at nearly constant surface pressure in the LE↔LC coexistence region.

Synergism in mixed monolayers of cationic and anionic surfactants: A thermodynamic analysis of miscibility at the air–water interface

The miscibility of cationic and anionic surfactants when spread at the air–water interface was investigated at 298 K. At this temperature, the cationic dioctadecyldimethylammonium bromide (DODAB) forms an expanded monolayer, while the anionic sodium hexadecylsulfate (SHS) and sodium dodecylsulfate (SDS) are water-soluble and do not remain as monolayers when compressed at the air–water interface. When SHS (or SDS) is added to DODAB up to equal molar ratio, the mixture remains in the monolayer. As the anionic mole fraction, x 2, increases, the π– A isotherms of the mixed monolayers shift to lower areas and become more condensed. Very steep isotherms form at x 2=0.5 with high collapse surface pressures, around 60 mN m −1. For x 2>0.5, the excess of the anionic component dissolves in the water subphase, while the equimolar mixed monolayer (complex 1:1 or salt C +A −) remains at the interface. In the range 0.1< x 2⩽0.4 two collapses are visible in the π– A isotherms: the first plateau appears at approximately 47 mN m −1, and the final collapse around 60 mN m −1. The plateau region was ascribed to a phase separation process into a DODAB-rich phase, which is squeezed out of the monolayer, and a phase rich in the equimolar complex, which collapses at higher surface pressures. A thermodynamic analysis, performed in terms of molecular areas and compressibilities as a function of x 2, showed evidence of an increase in the condensing effect until x 2=0.5. The miscibility of DODAB with the complex 1:1 was investigated in the range of surface pressures below the first collapse. Negative excess molecular areas and negative Gibbs energies of mixing indicate miscibility with strong excess interactions for both systems. For the system HTAC–SHS (HTAC is hexadecyltrimethylammonium chloride), where both components are water-soluble, only the equimolar composition is stable at the air–water interface; for non-equimolar concentrations, the component in excess dissolves into the subphase.

Equilibrium Size Distributions of Circular Domains in Amphiphilic Monolayers

An expression is derived for the free energy of a surfactant monolayer at the interface between two immiscible bulk phases for a range of surface pressures in which two surface phases coexist. The condensed phase is assumed to be present in dispersed form, viz., as circular domains embedded in the less dense phase. The domains are kept in fixed positions as a result of electrostatic trapping. A thermodynamic analysis is presented on the basis of the free energy expression for the case of a single amphiphilic component using a mass-action equilibrium condition, which yields the equilibrium distribution of domain sizes. The distribution function is shown to be the result of a competition between a collective entropy effect (which favors dispersion), line tension, electrostatic edge effects, and interdomain repulsion, while a possible contribution due to bending elasticity of the phase boundary is taken into account. A numerical example illustrates that for practical systems the distributions are fairly narrow and that both the domain size and the width of the distribution increases with the fractional surface coverage of the condensed phase, but in such a way that the degree of polydispersity decreases.

Maxwell displacement current due to phase transitions in liquid crystals on a water surface

The phase transitions of Langmuir films of the liquid crystals (LCs) 4′-n-pentyl-4-cyano-biphenyl (5CB), 4′-n-octyl-4-cyano-biphenyl (7CB), 4′-n-octyl-4-cyano-biphcnyl (8CB) and 4′-n-decyi-4-cyano-biphenyl (10CB), have been investigated by a Maxwell displacement current (MDC) measuring technique, A phase transition of LC molecules from the planar surface alignment phase to the polar one was clearly detected in the range of low surface pressures during monolayer compression. A phase transition between a stable monolayer phase and a stable three-layer phase of the liquid crystal film was detected during further monolayer compression.

Monolayers of 14-H-perfluorotetradecane-1-carbonic acid

Monolayers of 14-H-perfluorotetradecane-1-carbonic acid (PFTDA) at the air-water interface with different metal cations (K +, Cd +2, Cu 2+, A1 3+, Fe 3+) dissolved in a water subphase were studied. The dependence of the surface pressure P( A) and the jump of the surface potential ΔV on the area per molecule A in the monolayer were studied. Some compression isotherms of the PFTDA monolayers within a wide surface pressure range obey the equation of state P(A−A 0) = kT n , where A o is the effective area of interaction of the molecules in the monolayer, n = 1 − 4 and it depends on the interaction of the polar groups with one another or with the cations of the subphase. On the whole, this is the evidence of a weak cohesive interaction between the hydrophobic radicals in the monolayer.