Spreading Monolayers Research Articles

Reorganization of the outer layer of a model of the plasma membrane induced by a neuroprotective aminosterol

Trodusquemine is an amphipathic aminosterol that has recently shown therapeutic benefit in neurodegenerative diseases altering the binding of misfolded proteins to the cell membrane. To unravel the underlying mechanism, we studied the interactions between Trodusquemine (TRO) and lipid monolayers simulating the outer layer of the plasma membrane. We selected two different compositions of dioleoylphosphatidylcholine (DOPC), sphingomyelin (SM), cholesterol (Chol) and monosialotetrahexosylganglioside (GM1) lipid mixture mimicking either a lipid-raft containing membrane (Ld+So phases) or a single-phase disordered membrane (Ld phase). Surface pressure-area isotherms and surface compressional modulus-area combined with Brewster Angle Microscopy (BAM) provided the thermodynamic and morphological information on the lipid monolayer in the presence of increasing amounts of TRO in the monolayer. Experiments revealed that TRO forms stable spreading monolayers at the buffer-air interface where it undergoes multiple reversible phase transitions to bi- and tri-layers at the interface. When TRO was spread at the interface with the lipid mixtures, we found that it distributes in the lipid monolayer for both the selected lipid compositions, but a maximum TRO uptake in the rafts-containing monolayer was observed for a Lipid/TRO molar ratio equal to 3:2. Statistical analysis of BAM images revealed that TRO induces a decrease in the size of the condensed domains, an increase in their number and in the thickness mismatch between the Ld and So phase. Experiments and MD simulations converge to indicate that TRO adsorbs preferentially at the border of the So domains. Removal of GM1 from the lipid Ld+So mixture resulted in an even greater TRO-mediated reduction of the size of the So domains suggesting that the presence of GM1 hinders the localization of TRO at the So domains boundaries.Taken together these observations suggest that Trodusquemine influences the organization of lipid rafts within the neuronal membrane in a dose-dependent manner whereas it evenly distributes in disordered expanded phases of the membrane model.

Flow-Enhanced Epitaxial Ordering of Brush-Like Macromolecules on Graphite

Long-range orientational order in monolayers of brush-like macromolecules was achieved during spreading of a polymer melt on the surface of highly oriented pyrolytic graphite. The combination of wetting-induced flow and epitaxial adsorption of poly(n-butylacrylate) side chains on graphite led to the formation of large domains of uniaxially oriented rodlike molecules. The domain size varied from ca. 1 to 10 microm which is noticeably larger than the submicrometer-sized mosaic domains typically observed upon adsorption from solution. The increase in the degree of order is attributed to the flow-enhanced diffusion of the macromolecules within spreading monolayers which facilitates the epitaxial alignment of the large macromolecules. The diffusion coefficient was shown to increase linearly with the spreading rate. Even though the ordering occurred during flow, no correlation was observed between the molecular orientation and the flow direction. Thus, the role of the flow was not to induce the molecular orientation but to facilitate the intrinsic ordering process. This finding can inspire and lead to new strategies for constructing large scale ordered structures on surfaces.

水晶結晶微量天秤とＬａｎｇｍｕｉｒ‐Ｂｌｏｄｇｅｔｔ法により評価した油汚れに対する洗浄力（原標題は英語）

The titled assessment was conducted using a quartz crystal microbalance (QCM) and the Langmuir-Blodgett (LB) method. The gold electrodes of QCM were coated with cellulose acetate, nylon 6 or polyethylene film by spin-coating. Spreading monolayers of long-chain fatty acid as oily contaminants on water were transferred onto spin-coated polymer film. LB films of arachidic acid were ultrasonically cleaned in aqueous solution. Detergency was determined based on the frequency change of the QCM due to cleaning. Detergency was greatest for cellulose acetate, followed by nylon 6 and polyethylene. For all films, detergency increased with the addition of ethanol, surfactant and alkali. Detergency is discussed in relation to contact angle of aqueous solution on polymer film. Liquid introduction between LB film and the polymer surface in the zone of contact appeared to be a significant factor in the removal of LB film from the polymer surface.

Antibiotic interaction with phospholipid monolayers

We studied the interactions of tetracycline (TC) antibiotic molecules with phospholipid monolayers with the two-fold aim of elucidating the mechanism of action and providing a first step for the realization of bio-mimetic sensors for such drugs by means of the Langmuir–Blodgett technique. We examined spreading monolayers of three phospholipids in the presence of tetracycline in the subphase by means of surface pressure–area and surface potential–area isotherms as a function of bulk pH. We selected phospholipids with hydrophobic chains of the same length but polar head groups differing either in dimensions and protonation equilibria, i.e. dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidic acid (DPPA). The interaction of tetracycline with the three phospholipids was found to be highly dependent on the electric charge of the antibiotic and on the ionization state of the lipid. Significant interactions are established between the negatively charged form of dipalmitoylphosphatidic acid and the zwitterionic form of tetracycline. The drug was found to migrate at the interface where it is adsorbed underneath or/and among the head groups, depending on the surface pressure of the film, whereas penetration through the hydrophobic layer was excluded for all the three phospholipids.

Drugs solubilization in ascorbyl–decanoate micellar solutions

Alkanoyl-6- O -ascorbic acid esters are obtained upon esterification of ascorbic acid's primary hydroxyl groups with fatty acids. Being more hydrophobic than vitamin C, they dissolve in lipophilic media, and behave as surfactants in water, where they produce micellar aggregates or spreading monolayers, depending on the side chain length. These amphiphiles keep the same antioxidant activity of vitamin C, and can be used for solubilization and protection of hydrophobic species from oxidative degradation. In this paper we report a study on the micellar aggregates formed by decanoyl-6- O -ascorbic acid in water, through surface tension, conductivity, and solubility experiments, and on its coagels produced at low temperatures, by means of differential scanning calorimetry and scanning electron microscopy. Solubilization of some hydrophobic molecules (phenacetin, danthron, and griseofulvin) in ascorbyl–decanoate (ASC10) micelles confirms that the supramolecular assemblies significantly enhance the solubility of these drugs in the liquid phase.

Spreading monolayers and Langmuir–Blodgett films of a novel type of optically active macrocyclic poly(ether sulfone)s containing 1,1′-bi-2-naphthol moiety

Aimed at investigating the monolayers and Langmuir–Blodgett (LB) films of compounds without any substituted alkyl chains and constructing chiral ultrathin films, a new group of optically active macrocyclic poly(ether sulfone)s containing a binaphthyl group, which have no substituted alkyl chains, were synthesized. The spreading behavior and the ultrathin film fabrication of these compounds was studied. Stable monolayers with collapse surface pressures up to 40 mN m −1 were formed at the air/water interface and uniform LB films were fabricated by using the horizontal lifting method. The optical activities of the compounds were maintained in the LB films. Transferred LB films were characterized by UV-vis, fluorescence and circular dichroism spectroscopy. It was found that the 1B b, 1L a and 1L b bands of the compounds showed a red shift compared with the corresponding solutions. While no split of the Cotton effect was observed for the dimeric compound, strong exciton couplings that could be ascribed to the interactions between the naphthyl groups were observed for the trimeric macrocyclic compounds.

Applying the Langmuir-Blodgett Technique to a Well Defined Model Detergent System

The Langmuir-Blodgett technique is used to obtain a well defined model detergent system. Spreading monolayers of arachidic acid as a model organic contaminant are transferred onto glass plates untreated and treated with two silane coupling agents, γ-aminopropyltriethoxysilane (APTES) and methyltriethoxysilane (MTES). Brewster angle microscopic images show that the LB films of arachidic acid are uniformly deposited on the substrate. Glass plates with the LB films are ultrasonically washed in various aqueous solutions, and the detergency is evaluated from the change in FTIR spectra of the substrate due to washing. The efficiency of detergency decreases as a result of the silanization of glass, especially for the APTES treated glass. The wettability and the surface charge of the substrate play important roles in the detergency, suggesting that the LB films of arachidic acid are removed directly from the substrate. Removal of LB films from the APTES-treated glass increases with increasing surfactant or ethanol concentration, and the removal rate strongly depends on temperature. As a removal technique without washing liquids, exposure of the substrates with LB films to 172 nm ultraviolet light is also attempted.

Monolayer Studies on an Acetic Acid Substituted Porphyrin and Its Complexes with Aliphatic Amines or Quaternary Ammonium Salts at the Air−Liquid Interface

Monolayers of 5,10,15,20-tetra-4-oxy(acetic acid)phenylporphyrin (TAPP) and its complexes formed with n-octadecylamine (ODA) in molar ratios of 1:1, 1:2, 1:3, and 1:4, n-hexadecylamine (HDA) (1:4), dimethyldihexadecylammonium bromide (DMDAB) (1:4), and tetrahexadecylammonium bromide (THDAB) (1:4) at the air−Cd2+ solution interface have been studied. The formation of the complexes was confirmed by UV−vis spectra of spreading solutions and surface pressure−area (π−A) isotherms of monolayers. π−A isotherms of spreading monolayers and UV−vis spectra of transferred monolayers show that the molecular areas and the maxima of the Soret bands of these porphyrin complexes changed significantly and regularly with the mixed molar ratios and the different mixed species. However, the porphyrin ring orientation angles that were measured by linear dichronic measurements are similar for TAPP and the complexes in different monolayers. These results indicate that the number and the length of the attached alkyl chains on the porphyrin periphery have no influence on the orientation of the rings, but have great influences on the organization of the rings. Brewster angle microscopy (BAM) observations show different monolayer behaviors for these systems, indicating that the introduction of aliphatic amines or quaternary ammonium salts greatly affects the intermolecular interactions.

Comparative investigation of Langmuir–Blodgett films and Self-Assembled Monolayers on metal surfaces

We studied a single chain thiol and a double chain dithiocarbammate with alkyl chains of comparable length and sulphur containing head-groups. We preliminary characterised the spreading monolayers of both compounds at the water–air interface. The formation of self-assembled films on metal substrates as a function of lipid concentration and immersion time was investigated by contact angle, ellipsometric and FT-IR measurements. The same techniques were used to characterise Langmuir–Blodgett (LB) films of the same substances. The results showed that the behaviour at water–air interface strongly conditioned the properties of the resulting LB films, whereas the structural features of Self-Assembled Monolayers (SAM) samples depended mainly on the adsorption sites distribution of the metal surface.

Reactions in two-dimensional state

An enzymatic hydrolysis reaction of a diglyceride, able to give spreading monolayers, was followed by surface pressure measurements as a function of the time. The present study reports the characterisation of 1,2-dicaprin (DCP) monolayers, as substrates of the reaction, by means of spreading isotherms at four temperatures, surface potential and ellipsometric/spectroscopic measurements. The results show that: (a) the hydrolysis reaction can take place only for definite values of surface pressure, corresponding to the condensed phase of the lipid film; (b) the reaction is of zero-order and it has an activation energy of about 1400 cal/moles; (c) the study of the inhibition of this reaction, by means of β-lactoglobulin (β-LG), shows that the inhibition does not derive from a direct interaction protein-enzyme, but the protein modifies the lipid substrate on which the enzyme acts.

Langmuir Monolayer Formation of Zinc Complex from 8-Hydroxyquinoline Amphiphilic Ligand

The behavior of 2-octadecylcarbamoyl-8-hydroxyquinoline(HL) spreading monolayers was investigated as a function of the metal ion concentration and counterions of the subphase. Stable complex monolayers were formed when the subphase contained more than 0.1 mM Zn(II) ion. LB films transferred from subphase containing ZnCl2 were characterized by XPS, UV-vis spectra and low-angle X-ray diffraction.

Fabrication and characterization of charge-transfer complex films by a bias-voltage LB technique

We have investigated the effect of bias voltage applied to an electrically conductive substrate on the formation of adsorption Langmuir–Blodgett (LB) films. In fabricating the adsorption LB films, we used tetracyanoquinodimethane (TCNQ) with a long alkyl chain as a molecule for spreading monolayers on the subphase and tetramethyl- p-phenylenediamine (TMPD) as a material for adsorbing from the subphase to the monolayers. It is well known that TCNQ and TMPD are acceptor-like and donor-like molecules, respectively. In the visible absorption spectra and Fourier transform infrared reflection–absorption spectra of the adsorption LB films fabricated under several bias conditions, the peaks assigned to TCNQ show a clear dependence on the bias voltage. The electrical conductivity corresponding to the quantity of charge-transfer complex was also obtained. These results demonstrate that the bias-voltage LB technique enables us to control the formation of the charge-transfer complex.

Thermodynamic and spectroscopic properties of mixtures of β-lactoglobulin and dioleylphosphatidylcholine

The behavior of the single spreading monolayers of β-lactoglobulin (β-LG) and dioleylphosphatidylcholine (DOPC), as well as their mixtures, has been studied on subphases containing Na+ or Ca++ ions. The results show an influence of temperature and subphase on the studied systems. The behavior of the areas as a function of the weight fraction of the two components shows significative and prevalently positive deviations from the additivity and their bidimensional miscibility. The variation of ΔGex, ΔHex and ΔSex calculated for the DOPC-β-LG mixture having maximum deviation on two different supports allows to deduce that the interactions are prevalently repulsive.

Structural characterization of Langmuir-Blodgett films of nickel complexes with amphiphilic compounds

Spreading monolayers and Langmuir-Blodgett (LB) multilayers are appropriate supramolecular structures for molecular recognition. Specifically oriented ligands in the head group of the amphiphiles in the monolayer can selectively interact with the ions to be detected in solution. We investigated spreading monolayers of dioctadecyldithiocarbamate (DODTC) and nonadecylpiridyne (NDP), on subphases containing transition metal ions, by means of π- and Δ V-area isotherms. UV-Vis spectroscopy absorption and electron spectroscopy for chemical analysis measurements performed on LB films of DODTC and NDP transferred from subphases containing nickel ions indicated that the Ni ions combine with the monolayer to form stable adducts. The structure of the resulting LB films were further characterized using X-ray diffraction and atomic force microscopy. Both experiments indicated that in the presence of the transition metal ion there is a drastic variation in the structural organization of the monolayers and in the morphology of the LB films.

Investigation of 2-(Docosylamino)-5-nitropyridine Monolayers and Langmuir−Blodgett Films

The behavior of 2-(docosylamino)-5-nitropyridine (DCANP) spreading monolayers was investigated as a function of temperature, pH, and subphase composition. The interactions of DCANP monolayers with copper ions dissolved into the subphase at different concentrations were also studied. The investigations were carried out by measuring surface pressure and surface potential-area isotherms. Langmuir-Blodgett (LB) films transferred from pure water and copper aqueous subphases were also studied by Fourier transform infrared (FTIR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, X-ray photoelectron spectroscopy (XPS), ellipsometry, and refractive index measurements. The interactions between copper ions and DCANP molecules were investigated by means of various techniques in the bulk phase, in monolayers, and in LB films. Clear evidence ofinteractions between DCANP and copper ions was provided only from EPR spectroscopy in the LB films, because of the very small molar ratio of bound/free DCANP in the LB films. The refractive index of Cu-doped LB films was shown to be increased by the presence of metal ions.

Mesogenic metal alkylthioporphyrins for the formation of Langmuir-Blodgett films

The spreading monolayers at the air-water surface of the mesogenic, discotic copper(II) derivatives of 2,3,7,8,12,13,17,18-octakis(alkylthio)porphyrin (CuTAP) have been characterized with the aim of finding the best experimental conditions for fabricating Langmuir-Blodgett (LB) films with columnar aggregates having significant mono- and/or bidimensional properties. The spreading isotherms of pure metal compounds were interpreted in terms of inhomogeneous, rigid surface films, not suitable for LB film deposition. A better transfer was reached with mixtures ( e.g. 1 20 molar ratio) of Cu-TAP with the film forming stearic acid, thus obtaining stable and reproducible LB films with more than 300 layers. The film maintained a disordered state of columnar aggregates as proved by electron spin resonance and UV-visible spectroscopy.

Monolayer and multilayer films of differently substituted metal tetraazaannulene complexes

The properties of monolayers and multilayers of nickel(II) compounds with the dinaphtho derivative of the macrocycle ligand dinaphtho{ b,i}5,7,12,14-tetramethyltetraaza[14]annulene with one and two hexadecanoyl chains on the water surface and onto a solid support have been studied from a thermodynamic and spectroscopic point of view and the results were compared with those from the dibenzo analogues. The molecular plane showed a tilted edge-on orientation at the air-water interface. The spreading monolayers were stable with time and their stability was almost independent of the compound used. The presence of hysteresis in the compression-expansion-compression cycles indicated an incomplete reversibility. Films prepared with the monohexadecanoyl macrocycle metal derivatives were regular and homogeneous whereas the occurrence of two alkyl chains on the ring resulted in a monolayer too viscous to be transferred. The effect of the film iodination was investigated through UV-visible and electron spin resonance spectroscopies.

Potassium ion selective membranes built by the Langmuir—Blodgett technique

Spreading monolayers of the ionophore valinomycin in mixtures with methyl stearate with different molar ratios were studied at the air/water interface in the 15–30 C temperature range. The bidimensional miscibility of the two components was deduced by recording spreading isotherms, which allowed also the evaluation of the thermodynamic parameters Δ G mix, Δ H mix and TΔ S mix. Languir—Blodgett (LB) films of mixtures with various molar ratios were built by transferring spreading monolayers from the air/water interface on to hydrophobic and hydrophilic substrates, and their electrical resistance was determined using KCl and NaCl solutions in the 10 −3-5·10 −1 M concentration range. It was verified that valinomycin-containing mixtures showed K − ion selectivity in LB films, at an opportune molar ratio between the two components.

A study of the bidimensional miscibility of ceramide and dioleoylphosphatidylcholine

Some parameters related to the bidimensional interactions between ceramide (CER) and diolcoylphosphatidylcholine (DOPC) molecules, alone and in mixtures of different weight fractions, were deduced from the study of spreading monolayers at the air-water interface. Spreading isotherms were recorded using both 0.1 M NaCl (pH 5.6) and a phosphate buffer solution (pH 7) as subphases in the 15–30°C temperature range. The bidimensional phases of the two components were obtained and their bidimensional miscibility was deduced as a consequence of positive deviations of the surface areas from the additivity relationship and by applying the phase rule at the collapse pressure. Excess surface Gibbs energies, enthalpies and entropies of mixing were also computed. The results obtained showed that the two components were miscible under the experimental conditions at all temperatures investigated, using both the Na +-ion-containing subphase and the buffered one.

Generation of the displacement current by the transformation of J-aggregates in spreading monolayers of squarylium dye

Maxwell displacement currents generated from monolayers of squarylium dye with propyl groups (SQ) and from mixed monolayers with arachidic acid were investigated during the course of monolayer compression in connection with the formation of J-aggregates in the monolayers. Abrupt changes in the generation of displacement current were observed for monolayers of SQ due to the transformation between two types of J-aggregates with different absorption spectra. In contrast, for mixed monolayers with arachidic acid which show no transition of J-aggregates, abrupt changes in the displacement current were not observed. It was concluded that displacement current measurement is effective in the detection of the transformation of the molecular arrangement in aggregates.