Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface

The properties of mixed monolayers composed of the cationic Gemini surfactant ([C(18)H(37)(CH(3))(2)N(+)(CH(2))(3)N(+)(CH(3))(2)C(18)H(37)],2Br(+), abbreviated as 18-3-18,2Br(-1)) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br(-1) and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br(-1) and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a composition X(SA)=0.67 and formed a "cationic-anionic surfactant," i.e., the carboxylate, with 18-3-18,2Br(-1) owing to the electrostatic interaction between the head groups.     

Thermodynamic behavior and relaxation processes of mixed DPPC/cholesterol monolayers at the air/water interface

This study investigated the thermodynamic behavior and relaxation processes of mixed DPPC/cholesterol monolayers at the air/water interface at 37°C. Surface pressure–area isotherms and relaxation curves for the mixed monolayers were obtained by using a computer-controlled film balance. In the thermodynamic analysis of the mixed monolayers, the areas of monolayers exhibited negative deviations from the ideal values at all compositions for lower surface pressures. However, at higher surface pressures, distinctively positive deviations from ideality were observed at lower DPPC contents. Excess free energies of mixing had been calculated and the most stable state of the mixed monolayer with x_DPPC=0.5 or 0.6 was found. Moreover, the relaxation kinetics of the mixed monolayers was investigated by measuring the surface area as a function of time at a constant surface pressure of 40 mN m⁻¹. It was shown that the relaxation processes could be described by the models considering nucleation and growth mechanisms.     

Competition between atmospherically relevant fatty acid monolayers at the air/water interface

Competition and oxidation of fatty acids spread at the air/water interface were investigated using surface-specific, broad-bandwidth, sum frequency generation spectroscopy. At the air/water interface, a monolayer of oleic acid replaced a monolayer of deuterated palmitic acid at equilibrium spreading pressure. Subsequent oxidation of the oleic acid monolayer with ozone resulted in products more water soluble than the palmitic acid; therefore, the palmitic acid monolayer reformed at the surface. Results indicate that the surfactants on the surface of fat-coated tropospheric aerosols will only possess oxidized acyl chains after all less soluble species in the aqueous subphase have been removed through the processes of replacement at the surface and atmospheric oxidation.     

Insoluble monolayers of irisresorcinol at the air/water interface

Monolayer properties of irisresorcinol [5-(cis-10-heptadecenyl) resorcinol] were measured at the air/water interface. TheA-T isobars of the monolayers at 10 and 15 mN/m gave two-dimensional thermal expansivities of 1.4 × 10^–4/K and 1.3 × 10^–4/K at a temperature span from 7–40 C, respectively. The- A isotherms of the material showed only a little dependence on temperature from 5–35 C and onpH except at highpH, where monolayers expanded by ionization of resorcinol headgroups. Some types of saccharose in the subphase exhibited a characteristic interaction with irisresorcinol in monolayers, and there is a possibility that this material will be used for molecular recognition of some saccharoses.     

Nystatin in Langmuir monolayers at the air/water interface

The paper presents a thorough characteristics of Langmuir monolayers formed at the air/water interface by a polyene macrolide antibiotic-nystatin. The investigations are based on the analysis of pi/A isotherms recorded for monolayers formed by this antibiotic at different experimental conditions. A significant part of this work is devoted to the stability and relaxation phenomena. It has been found that nystatin forms at the air/water interface monolayers of the LE state. A plateau region, observed during the course of the isotherm compression, is suggested to be due to the orientational change of nystatin molecules from horizontal to vertical position. Quantitative analysis of the desorption of the monolayer material into bulk water indicates that the solubility of nystatin monolayers increases with surface pressure. At low surface pressures, the desorption of nystatin from a monolayer is controlled both by dissolution and by diffusion. However, at the plateau and in the post-plateau region, the desorption does not achieve a steady state and the monolayer is less stable than in the pre-plateau region. However, the presence of membrane lipids, even at a low mole fraction, considerably increases the stability of nystatin monolayers. This enables the application of the Langmuir monolayer technique to study nystatin in mixture with cellular membrane components, aiming at verifying its mode of action and the mechanism of toxicity.     

Relaxation behaviors of monolayers of octadecylamine and stearic acid at the air/water interface

This study investigated the relaxation behaviors of octadecylamine (ODA), stearic acid (SA), and SA/ODA mixed monolayers at the air/water interface. Area relaxations of monolayers at constant surface pressure were studied by a nucleation and growth mechanism and by direct observation using a Brewster angle microscope (BAM). The results showed that ODA and SA monolayers exhibit different characteristics in the area loss and in the BAM morphology. In the initial relaxation stage, SA monolayer illustrates a more stable characteristic than ODA. But at the later stage, the area loss of SA monolayer increases more quickly than that for ODA due to significant nucleation and growth of 3D aggregates. The BAM results demonstrated that 3D aggregates of large scale domains are likely to form on a SA monolayer even when the area loss is insignificant. On the contrary, only dotlike aggregates of low density were found on the ODA monolayer when relaxation is carried out at higher surface pressure. The relaxation behavior of SA monolayer can be described by the Vollhardt model. However, the relaxation of ODA monolayer does not follow the nucleation model described by Vollhard but can reasonably be attributed to the effect of dissolution. For the SA/ODA mixed monolayers, the relaxation behaviors in the initial and final stages follow different mechanisms, which is attributed to the formation of distinct phases as observed from the BAM. This result also implied that SA and ODA are not completely miscible to be a homogeneous phase. Phases of various compositions were formed in the mixed monolayers, and thus, the relaxation mechanism was shifted during the relaxation process as dominated by different relaxation behaviors of various phases.     

The equilibria of phosphatidylcholine-fatty acid and phosphatidylcholine-amine in monolayers at the air/water interface

Monolayers of phosphatidylcholine, fatty acid and amine and binary mixtures phosphatidylcholine-fatty acid or phosphatidylcholine-amine were investigated at the air/water interface. Phosphatidylcholine (lecithin, PC), stearic acid (SA), palmitic acid (PA), decanoic acid (DA) and decylamine (DE) were used to the experiment. The surface tension values of pure and mixed monolayers were used to calculate π-A isotherms. The surface tension measurements were carried out at 22°C using an improved Teflon trough and a Nima 9000 tensiometer. The Teflon trough was filled with a subphase of triple-distilled water. Known amounts of lipid dissolved in 1-chloropropane were placed at the surface using a syringe. The interactions between lecithin and fatty acid as well as phosphatidylcholine and amine result in significant deviations from the additivity rule. An equilibrium theory to describe the behaviour of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC-SA, PC-PA, PC-DA and PC-DE complexes. We considered the equilibrium between the individual components and the complex and established that lecithin and fatty acid as well as phosphatidylcholine and amine formed highly stable 1:1 complexes.     

Complete immiscibility in binary cholesteryl-n-carboxylic acid ester/stearic acid monolayers at the water/air interface

Applying different surface and spreading techniques to form binary monolayers in a different mixing state, the mixing behavior of the three binary systems cholesteryl formiate/stearic acid, cholesteryl acetate/stearic acid, and cholesteryl-n-propionate/stearic acid were investigated and compared.Analyzing the force ()/area (a) isotherms and the equilibrium spreading pressures (^e of the binary monolayers, it can be concluded that the components of the three binary systems do not mix within the whole concentration range. The lipids in the binary monolayers are completely immiscible.     

Surface viscoelasticity of phospholipid monolayers at the air/water interface

The surface viscoelastic properties of monolayers of two phospholipids DPPC (L--dipalmitoylphosphatidylcholine) and DMPE (L--dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity _s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.Results indicate a strong dependence of _s andE upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states.     

Mixed alkaline phosphatase/sphingomyelin monolayer at the air-buffer interface: phase behavior and morphology

The glycosylphosphatidyl inositol(GPI)-anchored proteins are localized on the outer of the plasma membrane and clustered in membrane microdomain known as lipid rafts. Among them, mammalian alkaline phosphatase(AP) is an enzyme widely distributed. So, it has important biological significance to study the combination of AP with lipid monolayer. In our work, the interaction between AP and sphingomyelin has been studied at the air-buffer interface as a biomimetic membrane system by the Langmuir film technique and atomic force microscopy. The surface pressure-area isotherm for the mixed alkaline phosphatase/sphingomyelin monolayer shown the presence of a transition from a liquid-expanded phase to the liquid-expanded/liquidcondensed coexist phase. And the surface compressional modulus suggested the mixed alkaline phosphatase/sphingomyelin monolayer has larger compressibility compared with the pure sphingomyelin monolayer. Besides, according to the micrographs, we inferred when combined with lipid monolayer at the air-buffer interface, the AP molecules formed polymer not multilayer or micelle. And, according to the limiting molecules area of AP, we inferred that 12 AP molecules formed a hexagon polymer unit.     

X-ray investigation of monolayers formed at the soft air/water interface

Gibbs or Langmuir monolayers formed at the soft air/liquid interface are easy to handle and versatile model systems for material and life sciences. The phase state of the monolayers can be modified by lateral compression of the film while the layer structural changes are monitored by highly sensitive surface characterization techniques. The use of high brilliant synchrotron light sources for X-ray experiments is essential for the monolayer research. The present review highlights the recent achievements recorded in the monolayer field with a special emphasis on different synchrotron based X-ray characterizing methods as: grazing incidence X-ray diffraction, X-ray reflectivity and total reflection X-ray fluorescence. Some examples of single-chain surfactants, special sugar lipids, and semifluorinated compounds are given. Additionally, thin layers formed by peptides, polymers or nanoparticles are highlighted.     

A new liquid surface neutron reflectometer and its application to the study of DPPC in a monolayer at the air/water interface

A constant wavelength neutron reflectometer is described. Using this reflectometer, the neutron reflectivities from phosphatidylcholine monolayers in the highly condensed LC phase on ultra pure H₂O and D₂O have been measured on a Wilhelmy film balance. The neutron reflectivities have been carefully compared with those obtained by the X-ray method applied to the same monolayer under similar conditions. A new approach to analyzing a combined set of data composed of X-ray and neutron reflectivities has been used. From the analysis it is concluded that despite their limited q_z range neutron reflectivities are as essential as X-ray reflectivities for the unique determination of the monolayer structure.     

Anomalous spreading behaviour of polyethyleneglycoldistearate monolayers at air/water interface

Spreading behaviour of the dimeric surfactant polyethylene-glycoldistearate (PEGDS) monolayer at air/water interface has been studied using surface pressure-area (π-A) isotherms as a function of temperature. The isotherms show a plateau suggesting a transition between a liquid expanded (LE) and a condensed state. The condensed state possibly arises due to nucleation and growth of multilayers from the monolayer. Isobaric measurements of bothA-T and π-T at constant area show transitions atT = 295 K. These plots suggest a melting followed by formation of condensed microcrystallites. Structure optimization carried out using various angles of orientation of the alkyl tails with respect to the backbone in PEGDS reveals tilt transitions of the tails in different states which can be related to the packing behaviour seen in the isotherms. Optical microscopy has been used to confirm the structures in these states.     

The influence of pH on phosphatidylcholine monolayer at the air/aqueous solution interface

The measurements of the interfacial tension at the air/aqueous subphase interface as the function of pH were performed. The interfacial tension of the air–aqueous subphase interface was divided into contributions of individuals. A simple model of the influence of pH on the phosphatidylcholine monolayer at the air/hydrophobic chains of phosphatidylcholine is presented. The contributions of additive phosphatidylcholine forms (both interfacial tension values and molecular area values) depend on pH. The interfacial tension values and the molecular areas values for LH⁺, LOH⁻ forms of phosphatidylcholine were calculated. The assumed model was verified experimentally.     

Effect of the graft chain length and density on the morphology of radiation-modified polysilane monolayers at the air/water interface

The variation in the morphology of monolayers at the air/water interface is investigated for two kinds of radiation-modified polysilanes with different structures: poly(diethyl fumarate)-grafted poly(methyl-n-propylsilane) (PMPrS-g-PDEF) and maleic anhydride-grafted PMPrS (PMPrS-g-MAH). PMPrS-g-PDEF has long but sparsely-attached PDEF graft chains, while PMPrS-g-MAH has short but densely-attached MAH graft units. Surface pressure-area measurements indicate that PMPrS-g-PDEF monolayers extensively spread at the air/water interface though PMPrS homopolymer hardly spreads. AFM observation reveals that PMPrS-g-PDEF monolayers have an inhomogeneous structure containing string-like microstructures. This result suggests that PMPrS main chains are detached from the water surface to aggregate together and only PDEF chains spread over the water surface. In contrast, PMPrS-g-MAH forms uniform monolayers with a smooth surface. PMPrS main chains of PMPrS-g-MAH are anchored to the water surface by densely grafted MAH units. It is also demonstrated that only the PMPrS-g-MAH monolayers are successfully deposited layer-by-layer on a solid substrate by the Y-type deposition.     

Catalase monolayers at the air/water interface I. Reversibilities and irreversibilities

/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pH-value of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules.A model is proposed to explain the nature of the critical/A-value occurring in the/A-isotherms: on compression beyond _c, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure is determined, not only by the surface itself, but also by this subsurface layer.     

Interactions between phosphatidylcholines and cholesterol in monolayers at the air/water interface

Mixtures of cholesterol and synthetic phospholipids, differing in saturation of phosphatidylcholine (PC) acyl chains, such as distearoyl phosphatidylcholine (DSPC), stearoyl-oleoyl phosphatidylcholine (SOPC) and dioleoyl phosphatidylcholine (DOPC) have been studied as floating Langmuir monolayers at the air/water interface. In order to examine the influence of a polar group, distearoyl phosphatidylethanolamine (DSPE) was chosen. The films were spread at room temperature on aqueous subphases and characterized by the surface pressure-area (pi-A) isotherms and compression modulus (C(s)(-1)) values. The interactions were examined by analyzing the mean molecular areas and quantified by the excess free energy of mixing values. The obtained results indicate that the affinity of cholesterol to saturated/unsaturated phosphatidylcholines does not differ significantly, and revealed strong influence of the kind of a polar group on the cholesterol-phospholipid interactions. On the other hand, the apolar group structure was found to modify the stoichiometry of sterol-PC complexes.     

Gelation of octadecyltrimethoxysilane at the air/water interface: effect of perfluorotetradecanoic acid and divalent cadmium ions

The effect of an acidic surfactant, perfluorotetradecanoic acid (PFTDA), on the gelation of octadecyltrimethoxysilane (ODTMS) monolayers at the air/water interface was investigated using a Langmuir film balance, attenuated total reflection (ATR) infrared spectroscopy, and atomic force microscopy (AFM). The gelation of ODTMS was greatly accelerated by adding only 2 mol % PFTDA into the monolayer; in this case, the hydrolysis of ODTMS was completed within a few minutes, whereas otherwise it took 18 h. ATR-IR spectroscopy clearly demonstrated that the gelation of a 49:1 ODTMS/PFTDA monolayer proceeded on a pure water subphase mainly via hydrolysis followed by condensation. In the presence of PFTDA, the local acidity at specific sites is presumed to be very high, catalyzing the hydrolysis of ODTMS. The extremely fast rate is attributed not only to the lateral diffusion of PFTDA in the mixed monolayer but also to proton propagation where the proton(s) involved in the initial hydrolysis are transposed rapidly to the nearby methoxy groups for subsequent hydrolysis. The catalytic activity of PFTDA can be neutralized, however, simply by the addition of multivalent metallic ions such as Cd2+ to form saltlike complexes with PFTDA; the rate of 2-D sol-gel processes can thus be easily regulated by a minute amount of PFTDA and/or Cd2+ added into the reaction system.