Two-dimensional facets in Langmuir monolayers of 1-O-hexadecyl-rac-glycerol at the air-water interface

We study the surface phase behavior in Langmuir monolayers of 1-O-hexadecyl-rac-glycerol (C16G) by film balance and Brewster angle microscopy over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressure-area (pi-A) isotherm indicates a first-order phase transition between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the air-water interface. A wide variety of condensed domains are found to form just after the appearance of the cusp point. The observed surface morphology was compared with that of ethylene glycol mono-n-hexadecyl ether (C16E1) that bears an ethylene oxide (EO) unit in the head-group. As usually observed, the domains of C16E1 are found to be circular at lower temperatures and fractal at higher temperatures. Contrary to this usual behavior, the domains of C16G are found to be strip-like structures at lower temperatures, which attain increasingly compact shape as the temperature increases and finally attain faceted structures at > or = 25 degrees C. It is concluded that a higher degree of dehydration around the head-group region of C16G appreciably reduces the hydration-induced repulsive interactions between the head-groups and imparts to the molecules an increase in hydrophobicity, thereby a closer molecular packing. As a result, the molecules form increasingly compact domains as the temperature increases. Since the head-group of C16E1 is much smaller than that of C16G, dehydration effect cannot appreciably increase its hydrophobic character. Rather, increases in subphase temperature result in a decrease in the line tension of the interface giving fractal structures at higher temperatures. In addition, the changes in enthalpy (deltaH) and entropy (deltaS) values were also calculated to understand the thermodynamic nature of condensation of the molecules in the LE-LC transition region.     

Effect of temperature on the surface phase behavior and micelle formation of a mixed system of nonionic/anionic surfactants

The adsorption and micellar behavior of diethylene glycol mono-n-tetradecyl ether (C14E2), sodium 3,6,9,12-tetraoxaoctacosanoate (TOOCNa), and their mixture at a 1:1 molar ratio have been studied by film balance, Brewster angle microscopy (BAM), and surface tensiometry at different temperatures. The monolayers of pure C14E2 and its mixture with TOOCNa show a first-order phase transition with a conspicuous cusp point in their respective adsorption isotherms. This is further confirmed by the observation of bright two-dimensional condensed phase domains visualized by BAM just after the appearance of the phase transition. It is interesting to note here that for C14E2, condensed domains are observed up to 19 degrees C, while in the mixed system, they are observed up to 22 degrees C. To understand why in the mixed system the domains are observed at higher temperatures than for pure C14E2, we have measured the temperature dependency of the equilibrium surface tension at > or = cmc (gammacmc) values of both the pure and the mixed systems. The gammacmc values of pure C14E2 remain almost constant, while those of pure TOOCNa and its mixture with C14E2 decrease appreciably with increasing temperature. It is concluded that higher degree of dehydration of the ethylene oxide (EO) chain reduces the head-group size of TOOCNa, which outweighs the combined effect of the repulsive interactions between the head-groups and the thermal motion of the adsorbed molecules. Furthermore, C14E2 being inserted into the TOOCNa monolayer reduces the electrostatic repulsions between the charged heads, and consequently, the adsorbed monolayers attain closer molecular packing. As a result, the gammacmc values of both pure TOOCNa and its mixture with C14E2 decrease with increasing temperature. This facilitates the formation of condensed domains in the mixed system at higher temperatures, whereas none of the individual members can show any indicative feature of phase transition under the same experimental conditions.     

气/液界面Langmuir单分子膜的原位拉曼光谱

提出了一种原位测量气/液界面Langmuir单分子膜拉曼光谱的新方法, 即利用SERS技术, 通过降低亚相的方法来获得气/液界面Langmuir单分子膜的原位拉曼光谱. 利用这种方法, 用原位拉曼光谱测量系统得到了信噪比较好的十八胺及二棕榈酰磷脂酰胆碱单分子膜的拉曼光谱, 在分子水平上获取了单分子膜中的结构信息.     

Surface phase behavior in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface

The surface phase behavior of condensed-phase domains formed during a first-order phase transition in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface has been investigated by Brewster angle microscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). A variety of two-dimensional (2D) structures are observed just after the appearance of the phase transition at different temperatures. At 10 and 15 degrees C, the domains are found to be small nuclei of irregular structures. Spiral structures are observed at 20 and 22 degrees C, while striplike structures at 24 degrees C. The spiral domains attain increasingly compact shape with increasing temperature, and finally become circular at >or=26 degrees C. Increases in temperature result in dehydration in the ethylene oxide chain, which increases the hydrophobicity, and impart to the molecules a longer-chain-like character. As a result line tension increases with increasing temperature, which probably outweighs the dipole-dipole repulsions showing circular domains at higher temperatures. The PM-IRRAS measurement reveals that the nu(as)(CH(2)) mode moves to lower wave numbers indicating that the LE-LC (liquid expanded-liquid condensed) phase transition during the compression of the monolayer involves changes in the conformational order of the molecules with a preferential increase in the planner trans zigzag conformation of the hydrocarbon chains. The nu(as)(CH(2)) mode in the LC region of the isotherm shows a constant value around 2917.8 cm(-1) indicating a stable state of the monolayer with an almost all-trans conformation of the hydrocarbon chains. The downward band at 1124 cm(-1) assigned to the nu(as)(C-O-C) mode indicates that the corresponding transition dipole moment is oriented perpendicular to the water surface.     

Comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, at the air-water interface

A comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, C16TAB, at the air-water interface is made using the specular reflection of neutrons. The phenyl ethanol is more surface active than the benzyl alcohol, and competes more effectively with the C16TAB for the interface. The structure of the C16TAB component in the mixed monolayer is compared with the structure of the pure C16TAB monolayer at an equivalent area per molecule. The addition of the aromatic alcohol subtly alters the conformation of the C16TAB and draws it closer to the aqueous subphase. The center of the alcohol distribution is located in the interface adjacent to the C6 group of the C16TAB alkyl chain closest to the headgroup. Compared to the benzyl alcohol, the more hydrophobic phenyl ethanol is slightly farther away from the headgroup, and has a greater impact on the conformation of the alkyl chain of the C16TAB.     

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.     

Effect of cholesterol and phospholipid on the behavior of dialkyl polyoxyethylene ether surfactant (2C18E12) monolayers and bilayers

Surface pressure-area isotherm, neutron specular reflection, and small-angle neutron scattering studies have been carried out to determine the effects of added cholesterol and distearoylphosphatidylcholine (DSPC), on the molecular structures of monolayers and vesicles containing the dialkyl polyoxyethylene ether surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(alpha-dodecaethylene glycol) (2C18E12). Previous neutron reflectivity studies on 2C18E12 monolayers at the air/water interface have shown them to possess a thickness of approximately 24 angstoms and highly disordered structure with significant intermixing of the polymer headgroups and alkyl chains. SANS studies of 2C18E12 vesicles gave a bilayer thickness of approximately 51 angstroms. Addition of cholesterol to 2C18E12 monolayers (1:1 molar ratio), produced a marked condensing effect coupled with an increased the layer thickness of approximately 7 angstroms, and in vesicles, increased bilayer thickness by approximately 16 angstroms. Monolayers consisting of 2C18E12:DSPC:cholesterol (1:1:2 molar ratio), showed a layer thickness of approximately 31 angstroms, whereas in vesicles, three-component bilayer was found to be only approximately 9 angstroms thicker than those possessed by vesicles composed solely of 2C18E12. Mixing between the molecules in three-component monolayers was shown to be ideal through analysis of the neutron reflectivity data. These findings are discussed in relation to increased ordering and decreased headgroup/hydrophobe intermixing within both monolayers and vesicle bilayers containing 2C18E12. The inferred increase in molecular order within vesicles composed of 2C18E12 with additional cholesterol and phospholipid is used as a model for explaining theoretical differences in bilayer permeability.     

Structure and phase behavior of archaeal lipid monolayers

We report X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) measurements of archaeal bipolar tetraether lipid monolayers at the air-water interface. Specifically, Langmuir films made of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at three different temperatures, i.e., 68, 76, and 81 °C, were examined. The dependence of the structure and packing properties of PLFE monolayers on surface pressure were analyzed in a temperature range between 10 and 50 °C at different pH values. Additionally, the interaction of PLFE monolayers (using lipids derived from cells grown at 76 °C) with the ion channel peptide gramicidin was investigated as a function of surface pressure. A total monolayer thickness of approximately 30 ? was found for all monolayers, hinting at a U-shaped conformation of the molecules with both head groups in contact with the interface. The monolayer thickness increased with rising film pressure and decreased with increasing temperature. At 10 and 20 °C, large, highly crystalline domains were observed by GIXD, whereas at higher temperatures no distinct crystallinity could be observed. For lipids derived from cells grown at higher temperatures, a slightly more rigid structure in the lipid dibiphytanyl chains was observed. A change in the pH of the subphase had an influence only on the structure of the lipid head groups. The addition of gramicidin to an PLFE monolayer led to a more disordered state as observed by XRR. In GIXD measurements, no major changes in lateral organization could be observed, except for a decrease of the size of crystalline domains, indicating that gramicidin resides mainly in the disordered areas of the monolayer and causes local membrane perturbation, only.     

Temperature dependence of poloxamer insertion into and squeeze-out from lipid monolayers

F68, a triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), is found to effectively seal damaged cell membranes. To better understand the molecular interaction between F68 and cells, we have modeled the outer leaflet of a cell membrane with a dipalmitoylphosphatidylcholine (DPPC) monolayer spread at the air-water interface and introduced poloxamer into the subphase. Subsequent interactions of the polymer with the monolayer either upon expansion or compression were monitored using concurrent Langmuir isotherm and fluorescence microscopy measurements. To alter the activity of the poloxamer, a range of subphase temperatures from 5 to 37 degrees C was used. Lower temperatures increase the solubility of the poloxamer in the subphase and therefore lessen the amount of material at the interface, resulting in a lower equilibrium spreading pressure. Additionally, changes in temperature affect the phase behavior of DPPC. Below the triple point, the monolayer is condensed at pertinent polymer insertion pressures; for temperatures immediately above the triple point, the monolayer is a heterogeneous mix of liquid expanded and condensed phase; for the highest temperature measured, the DPPC monolayer remains completely fluid. At all temperatures, F68 inserts into DPPC monolayers at its equilibrium spreading pressure. Upon compression of the monolayer, polymers are squeezed-out at surface pressures notably higher than those for insertion, with higher temperatures leading to a higher squeeze-out pressure. An increase in temperature decreases the solvent quality of water for the poloxamer, lowering solubility of the polymer in the subphase and thus increasing its propensity to be maintained within the monolayer to higher pressures.     

Studies of monolayer/substrate adhesion as function of the monolayer headgroup charge: DMPE and DMPA

The variation of the work of adhesion between lipid monolayers and a plane silicon oxide surface in a typical LB-configuration is measured as function of the subphase pH. The adhesion energy is deduced via fluorescence microscopy from the equilibrium meniscus height. With increasing pH the negative headgroup charge of both, dimyristoylphosphatidylethanolamine (DMPE) and dimyristoylphosphatidic acid (DMPA) monolayers increases. The increasing charge of DMPE is reflected in a measured decrease of the work of adhesion at higher pH. The DMPA/SiO₂ interaction is not affected by increasing headgroup charges. These results are qualitatively understood in terms of an electrostatic double layer interaction between charged surfaces. It predicts decreasing adhesion for increasing, but low surface charge densities (DMPE). whereas the adhesion is constant for high surface charge densities (DMPA).     

Investigation in monolayers of double-chain fluorocarbon amphiphiles Effect of headgroups on molecular packing and interaction between hydrocarbon and fluorocarbon monolayers

The surface pressure-area diagrams of double-chain fluorocarbon amphiphiles with different headgroup compositions show that the amphiphiles arrange almost perpendicularly to the water subphase and the structure of headgroups exerts significant influence on the amphiphile packing. Strong hydrogen bonding and weak electrostatic interaction favor the formation of stable monolayers. Perfluorooctanoic acid (FOA) cannot form monolayer at water/air interface and can only form liquid monolayer in subphase of calcium nitrate solution. Complete phase separation of palmitic acid and a fluorocarbon amphiphile with shorter hydrocarbon spacer group, 1, could be demonstrated in monolayers by using the phase rule of Crisp. The creation of phase-separated monolayers is possible when the monolayer is composed of a mixture of palmitic acid and a fluorocarbon amphiphile with longer hydrocarbon spacer group, 2. It can be suggested that the miscibility of hydrocarbon amphiphiles with fluorocarbon amphiphiles is determined by the hydrocarbon fraction of fluorocarbon amphiphiles.     

Temperature dependent dendritic domain shapes in Langmuir monolayers of tetradecanoyl N-ethanolamide at the air-water interface

The effect of temperature on the surface phase behavior of tetradecanoyl N-ethanolamide (NHEA-14) in Langmuir monolayers at the air-water interface has been investigated by film balance and Brewster angle microscopy (BAM). It has been observed that dendritic domains are formed in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) phases at different temperatures. At 10 and 15°C, the domains are four-armed dendrites having wide arms which have a tendency to be fractal while growing in size. At 20°C, five-armed dendritic domains are formed. At a temperature higher than 20°C, the domains are mainly six-armed dendrites having very narrow and sharp arms. The formation of dendritic domains should be due to the presence of interfacial hydrogen bonding among the head groups of the amphiphile. Increased dehydration of the head groups with an increase in the temperature should be responsible for the temperature dependency of the dendritic domain shapes in the monolayers of NHEA-14.     

Compression induced helical nanotubes in a spreading film of a bolaamphiphile at the air/water interface

Inspired by the elegant helical structures endowed by mother nature, we designed an L-glutamic acid terminated bolaamphiphile and obtained helical nanotubes through the manipulation on the two-dimensional Langmuir films at the air/water interface. It has been found that on the subphase with a pH value lower than 3, stable monolayers with plateau regions were obtained for the bolaamphiphile. Although a flat and uniform morphology was observed for the film deposited at a surface pressure below the plateau region, helical nanotube structures were obtained when the film was compressed over the plateau region. It was suggested that the compression of the monolayer at the air/water interface caused the one end of the bolaamphiphile to leave from the water surface and form an intermediate monolayer in which one end group attached on the water surface and the other extruded in the air. Such an intermediate monolayer subsequently rolled into a helical structure due to the chiral nature of the L-glutamic acid headgroup.     

Design and interfacial assembly of a new series of gemini amphiphiles with hydrophilic poly(ethyleneamine) spacers

A new series of gemini amphiphiles containing two Schiff base moieties linked by the poly(ethyleneamine) with different lengths were designed, and their interfacial assemblies were investigated. Condensed monolayers were obtained on nearly neutral subphase where the hydrophilic spacers were found to immerse into the subphase. On strong alkaline and acidic subphase, the headgroup and the spacer of the gemini amphiphiles underwent dissociation and protonation, respectively, resulting in the enlargement of the molecular areas. Flat and uniform domains were obtained for the monolayers from nearly neutral subphase; flower-like or dendritic domains were observed for the films transferred from strong acidic subphase. On the other hand, when an anionic tetrakis(4-sulfonatonphenyl)porphine (TPPS) was added into an acidic subphase, an in situ complex formation between the gemini amphiphiles and TPPS occurred. The complex monolayers were transferred onto solid substrate and TPPS existed predominantly as J-aggregate in the complex films. Due to the multisited positive charges in the spacer on acidic subphase, the complex films of gemini amphiphiles with TPPS appeared as short fiber or nanorod structures and formed two-dimensional (2D) conglomerate chiral domains.     

Study of adsorption and penetration of E2(279-298) peptide into Langmuir phospholipid monolayers

The peptide corresponding to the sequence (279-298) of the Hepatitis G virus (HGV/GBV-C) E2 protein was synthesized, and surface activity measurements, pi-A compression isotherms, and penetration of E2(279-298) into phospholipid monolayers spread at the air-water interface were carried out on water and phosphate buffer subphases. The results obtained indicated that the pure E2(279-298) Langmuir monolayer exhibited a looser packing on saline-buffered than on pure water subphase and suggest that the increase in subphase ionic strength stabilizes the peptide monolayer. To better understand the topography of the monolayer, Brewster angle microscopy (BAM) images of pure peptide monolayers were obtained. Penetration of the peptide into the pure lipid monolayers of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) and into mixtures of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) at various initial surface pressures was investigated to determine the ability of these lipid monolayers to host the peptide. The higher penetration of peptide into phospholipids is attained when the monolayers are in the liquid expanded state, and the greater interaction is observed with DMPC. Furthermore, the penetration of the peptide dissolved in the subphase into these various lipid monolayers was investigated to understand the interactions between the peptide and the lipid at the air-water interface. The results obtained showed that the lipid acyl chain length is an important parameter to be taken into consideration in the study of peptide-lipid interactions.     

Flexibility of the triblock copolymers modulating their penetration and expulsion mechanism in Langmuir monolayers of dihexadecyl phosphoric acid

The surface activity of the poly–[block (ethylene oxide)]–poly [block (propylene oxide)]–poly [block (ethylene oxide)] copolymers (EO)_x–(PO)_y–(EO)_x adsorbed together with dihexadecyl phosphoric acid (DHP), a synthetic phospholipid, is analyzed from their surface pressure and surface potential isotherms. The block copolymers of (EO)_x–(PO)_y–(EO)_x with variable molecular weight (1100–14 000) were dissolved in the subphase for DHP monolayers. The concentration of the copolymers within the aqueous subphase were selected to render an initial surface tension of 60 mN/m. The simultaneous adsorption of the copolymer and DHP is attested by the observation of a liquid expanded state at large areas, absent for pure DHP monolayers. Above some critical surface pressure all copolymers cited above are expelled from the interface. The surface potential isotherms, which give information on the component of the molecular dipole moment normal to the plane of the monolayer, are interpreted in terms of changes in the copolymer conformation as well as in terms of the copolymer desorption from the air–liquid interface. For an equal hydrophobic/hydrophilic ratio, the size of the chains or molecular weight is decisive in the mechanism of the copolymer expulsion from the air–liquid interface.     

Sum-frequency spectroscopy analysis of two-component langmuir monolayers and the associated interfacial water structure

Sum-frequency spectroscopy (SFS) in the CH and OH stretching regions was employed to obtain structural information about Langmuir monolayers on the H(2)O subphase of the model lipid dioctadecyldimethylammonium bromide (DOMA) and of the neutral surfactant methyl stearate (SME) and their mixtures and about the interfacial water structure underneath the films. These results were compared with the sum-frequency spectra of the interface between Langmuir monolayers of stearic acid and stearic acid-DOMA monolayers and water to prove that the uncompensated headgroup charge of DOMA at the interface is the reason for structuring of interfacial water close to the studied monomolecular films. Sum-frequency spectra on D(2)O subphase were also studied to account for the interference between the CH and OH spectral signatures because of the coherent nature of the SFS signals. Interfacial water structure proved to be a determining factor in the behavior of the mixed lipid monolayers. A mixing induced amplification in the surface potential DeltaV observed in our previous work was explained with total increase of the dipole moment for the mixed films, bigger than the arithmetic average for DOMA and SME monolayers alone. The increase is due to the better packing of the molecules in the mixed films and to the decrease in the interfacial water dipole moment arising from a more disordered water structure underneath the mixed monolayers.     

Interfacial rheology of stable and weakly aggregated two-dimensional suspensions

The interfacial rheological properties of stable and weakly aggregated two-dimensional suspensions are studied experimentally using a magnetic rod interfacial rheometer. Particle monolayers with well controlled structures were prepared. Charged polystyrene particles create two-dimensional colloidal crystals at the water-decane interface over a wide range of concentrations. Under similar conditions a predominantly liquid structure is obtained at the water-air interface for the same particles. The addition of appropriate combinations of the anionic surfactant sodiumdodecylsulfate (SDS) and sodium chloride (NaCl) to the aqueous subphase leads to a destabilization of these monolayers with the formation of fractal aggregates at low concentrations and a heterogeneous gel forming as the surface coverage is increased. After the structures have been built up a reproducible structure can be obtained, of which the interfacial rheological properties can be investigated using a magnetic rod stress rheometer. In all cases, numerical calculations were used to assess the importance of instrumental artifacts and the effect of the coupling between surface and subphase flows. The rheology of aggregated suspensions was compared to the reference case of a colloidal crystal. The two-dimensional aggregated suspensions display rheological features which are similar to their three-dimensional counterparts. These include an elastic response with small linearity limits, a power law dependence on surface coverage and a dependence on the strength of attraction. The results shed some light on the possible role of interfacial rheology on the stability of particle laden high interface systems. Additionally, the 2D suspensions could present fundamental insights in the rheological properties of dense colloidal suspensions.     

Adsorption of nonionic surfactant mixtures at the hydrophilic solid-solution interface

The adsorption of the mixed nonionic surfactants, monododecyl triethylene glycol (C2EO3) and monododecyl octaethylene glycol (C12EO8), at the hydrophilic silica-solution interface has been studied by specular neutron reflectivity. The adsorption at the solid-solution interface is compared with that previously measured at the air-solution interface. The marked differences that are observed are explained in terms of the different packing constraints or preferred curvature arising from the disparity in the respective headgroup dimensions.     

Calorimetric study on the temperature dependence of the formation of mixed ionic/nonionic micelles

The differential excess enthalpy of mixed micelle formation was measured at different temperatures by mixing nonionic hexa(ethylene glycol) mono n-dodecyl ether with anionic sodium dodecyl sulfate or cationic dodecylpyridinium chloride. The experimental data were obtained calorimetrically by titrating a concentrated surfactant solution into a micellar solution of nonionic surfactant. The composition and the size of the mixed nonionic/ionic micelles at different surfactant concentrations were also determined. Pronounced differences in both composition and excess enthalpy were found between the anionic and the cationic mixed system. For both systems, the excess enthalpies become more exothermic with increasing temperature, but for the anionic mixed system an additional exothermic contribution was found which was much less temperature dependent. Temperature dependence of the excess enthalpy was attributed to the effect of the ionic headgroup on the hydration of the ethylene oxide (EO) groups in the mixed corona. Ionic headgroups located in the ethylene oxide layer cause the dehydration of the EO chains resulting in an additional hydrophobic contribution to the enthalpy of mixing. A high affinity of sodium dodecyl sulfate for nonionic micelles and an extra exothermic and less temperature dependent contribution to the excess enthalpy found for the SDS-C(12)E(6) system might be attributed to specific interactions (hydrogen bonds) between the sulfate headgroup and the partly dehydrated EO chain.