Langmuir Monolayers Of Stearic Acid Research Articles

Growth of Fe-BTC particles beneath stearic acid Langmuir monolayers and research on its adsorption of methyl orange

Growth of Fe-BTC particles beneath stearic acid Langmuir monolayers and research on its adsorption of methyl orange

Enzyme activity preservation for galactose oxidase immobilized in stearic acid Langmuir-Blodgett films

The adsorption of the enzyme galactose oxidase onto stearic acid Langmuir monolayers was investigated with surface pressure-area isotherms, Brewster Angle Microscopy and polarization-modulated infrared reflection-absorption spectroscopy. The enzyme was incorporated in the monolayer at low surface pressures, expanding the lipid films, and further monolayer compression expelled it from the contact with the air phase and repositioned the enzyme below the lipid polar heads. The enzyme-lipid system was then stabilized by electrostatic interactions, leading the monolayer to condense at higher surface pressures, reducing the number of interfacial aggregates. The lipid-enzyme interactions at the interface made the monolayer less viscoelastic and more disordered as pointed by the reduced compressional modulus and decreased trans/gauche ratio for the lipid acyl chains. Also, the secondary structures of the adsorbed enzyme were kept as demonstrated with infrared spectroscopy. The condensed enzyme-lipid monolayer was transferred to solid supports as Langmuir-Blodgett films and the enzyme activity was detected, being preserved for one month. These results show how the molecular organization provided by the ultrathin films can help maintain the catalytic activity for immobilized enzymes, which is important, general speaking, for nanotechnology and biosensing manufacturing.

Study of aggregation behavior of water insoluble metalloporphyrin (Zn) in LB film

In the present communication, a water insoluble metalloporphyrin, 5,10,15,20-Tetraphenyl-21H,23H-porphine zinc (abbreviated as ZnTPP) has been used to prepare stable Langmuir monolayer at the air–water interface by allowing it to adsorb from the aqueous subphase, into the preformed Langmuir monolayer of stearic acid (SA). Surface pressure versus area per molecule (π-A) isotherm studies gave the thermodynamical behavior of the ZnTPP-SA mixed Langmuir monolayer formed at the air water interface. Access area per molecule (AE) vs molefraction curve indicated the repulsive interaction between the two binary components in the mixed Langmuir monolayer. BAM images gave the visual evidence of the aggregation pattern of the mixed molecules at the air-water interface. UV-vis spectroscopic studies confirmed the formation of prominent J-band in the ultra-thin films.

Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation.

A Langmuir monolayer can be considered as a two-dimensional (2D) sheet at higher surface pressure which structurally deform with mechanical compression depending upon the elastic nature of the monolayer. The deformed structures formed after a certain elastic limit are called collapsed structures. To explore monolayer collapses at lower surface pressure and to see the effect of ions on such monolayer collapses, out-of-plane structures and in-plane morphologies of stearic acid Langmuir monolayers have been studied both at lower (≈6.8) and higher (≈9.5) subphase pH in the presence of Mg^{2+},Ca^{2+},Zn^{2+},Cd^{2+}, and Ba^{2+} ions. At lower subphase pH and in the presence of all cations, the stearic acid monolayer remains as a monolayer before collapse, which generally takes place at higher surface pressure (π_{c}>50mN/m). However, at higher subphase pH, structural changes of stearic acid monolayers occur at relatively lower surface pressure depending upon the specific dissolved ions. Among the same group elements of Mg^{2+},Ca^{2+}, and Ba^{2+}, only for Ba^{2+} ions does monolayer to multilayer transition take place from a much lower surface pressure of the monolayer, remaining, however, as a monolayer for Mg^{2+} and Ca^{2+} ions. For another same group elements of Zn^{2+} and Cd^{2+} ions, a less covered bilayer structure forms on top of the monolayer structure at lower surface pressure, which is evidenced from both x-ray reflectometry and atomic force microscopy. Fourier transform infrared spectroscopy confirms the presence of two coexisting conformations formed by the two different metal-headgroup coordinations and the monolayer to trilayer or multilayer transformation takes place when the coverage ratio of the two molecular conformations changes from the critical value (p_{c}) of ≈0.66. Such ion-specific monolayer collapses are correlated with the 2D lattice percolation model.

Molecular dynamic studies on Langmuir monolayers of stearic acid

Compression isotherm for stearic acid was obtained by means of molecular dynamic simulation and compared to experimentally measured values for the Langmuir monolayers. Compared to the previous simulation, the present simulation has provided a method to reproduce the compression of the monolayer. The result is consistent with other experimental results. By analyzing the alkyl tails, the configuration of stearic acid molecules during the compression process was studied and a uniform monolayer was obtained after compression. Stearic acid molecules were observed to form fine organized monolayer from completely random structure. Hexatic order of the arrangement has been identified for the distribution of stearic acid molecules in the monolayer. At the end of the compression, the stearic acid molecules were tightly packed in the gap of two other molecules. At last, the hydrogen bonds in the system were analyzed. The main hydrogen bonds were from stearic acid-water interaction and their intensities constantly decreased with the decreased of surface area per molecule. The weak hydrogen bond interaction between stearic acid molecules may be the reason of easy collapse.

2D “Soap”-Assembly of Nanoparticles via Colloid-Induced Condensation of Mixed Langmuir Monolayers of Fatty Surfactants

We describe a new type of colloidal 2D gels formed in mixed Langmuir monolayers of stearic acid and octadecylamine on a surface of gold hydrosol. The adsorption of gold nanoparticles on the mixed monolayer led to an increase of interactions between oppositely charged surfactants giving a "soap" of mixed fatty salt. The observed effect is equivalent to a virtual "cooling" of floating monolayer, which undergoes rapid condensation on a surface of aqueous colloid. The consequent shrinking and rearrangement of the monolayer resulted in aggregation of nanoparticles into colloidal 2D "soap"-gels, which represented arrested colloidal phases within nonadsorbing organic medium. When sequentially deposited onto solids by Langmuir-Blodgett technique, the 2D "soap"-gels separated into organic and colloidal phases and gave dendrite-like bilateral organic crystallites coated with gold nanoparticles. The reported colloidal "soap"-assembly can offer a new opportunity to design 2D colloidal systems of widely variable chemistry and structures.

Langmuir-Blodgett deposition selects carboxylate headgroup coordination

Infrared reflection-absorption spectroscopy results on stearic acid Langmuir monolayers containing Mn, Co, and Cd ions show that on the water surface, the ions induce unidentate and bidentate (both chelate and bridged) coordination in the carboxylate headgroup with some trace of undissociated acid. Moreover, with Cd and Mn ions in subphase, the preferred coordination is found to be unidentate, whereas for Co, bidentate chelate is most preferred. After transfer onto amorphous substrate, not all coordinations are found to exist in the same ratio for the deposited metal stearate monolayers. More specifically, after transfer, Mn is found to coordinate with the carboxylate group as bidentate chelate, Cd as unidentate and bidentate bridged (with unidentate as the preferred coordination), and Co as preferably bidentate bridged (although all coordinations are present). Results suggest a specific interaction in each case, as the metal-carboxylate pair at the water surface is transferred to the substrate surface during Langmuir-Blodgett deposition.

Solidlike and liquidlike behavior in monolayers and multilayers of metal-bearing amphiphiles

Atomic force microscopy (AFM) of cadmium stearate (CdSt) and cobalt stearate (CoSt) Langmuir-Blodgett films show differences in their in-plane morphologies. CdSt films, with a huge number of in-plane "pinhole" defects, follow self-affine behavior, whereas CoSt films, which are almost void of such in-plane defects, show deviation from self-affinity especially at small length scales, suggesting liquidlike behavior, imparting flexibility to the system, in plane. Phase images of CoSt obtained from tapping mode AFM show gentle undulations or hemispherelike features in contrast to its smooth topography, unlike the CdSt system where both height and phase images show self-affine domains. Near edge x-ray absorption fine structure spectroscopy indicates no preferred in-plane orientation of the head group in CoSt films. The undulating features in CoSt is explained by invoking a radially symmetric orientational distribution in the tilt of adjacent hydrocarbon tails, causing a small in-plane density variation which shows up in the phase image. These orientational disorders in adjacent tails probably allow "filling up" of in-plane defects thereby giving rise to its excellent in-plane coverage and hence a "liquidlike" behavior in CoSt. Brewster angle microscopy shows that parent Langmuir monolayers of stearic acid in the presence of Cd and Co ions in the aqueous subphase behave as two-dimensional "solids" and "liquids," respectively, suggesting the phenomena to be inherent in the amphiphiles and probably independent of their organization as monolayers and multilayers.

Immbolization of uricase enzyme in Langmuir and Langmuir-Blodgett films of fatty acids: Possible use as a uric acid sensor

Preserving the enzyme structure in solid films is key for producing various bioelectronic devices, including biosensors, which has normally been performed with nanostructured films that allow for control of molecular architectures. In this paper, we investigate the adsorption of uricase onto Langmuir monolayers of stearic acid (SA), and their transfer to solid supports as Langmuir–Blodgett (LB) films. Structuring of the enzyme in β-sheets was preserved in the form of 1-layer LB film, which was corroborated with a higher catalytic activity than for other uricase-containing LB film architectures where the β-sheets structuring was not preserved. The optimized architecture was also used to detect uric acid within a range covering typical concentrations in the human blood. The approach presented here not only allows for an optimized catalytic activity toward uric acid but also permits one to explain why some film architectures exhibit a superior performance.

On the formation of calcium carbonate thin films under Langmuir monolayers of stearic acid

In this publication, we describe the growth of thin films of calcium carbonate beneath Langmuir monolayers of stearic acid. The size and shape of the crystalline structures were systematically studied by means of different microscopic techniques including Brewster angle microscopy, atomic force microscopy and scanning electron microscopy. In a series of experiments, we explored the calcium carbonate crystallization process for different lipid monolayers and subphases. The observed phenomena support a crystallization process which is induced by a thin, film-like structure of a precursor phase. The basic processes of crystal and aggregate formation can be represented by a simple model which is based on electrostatic interactions between the surfactant film and the inorganic calcium carbonate structures.

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.

Growth of a collapsing Langmuir monolayer

Langmuir monolayers of stearic acid on Co ions in the aqueous subphase have been deposited at different stages of constant pressure collapse, on hydrophilic Si(001) using a modified version of the inverse Langmuir-Schaefer method of horizontal deposition. The electron density profiles (EDPs) along the depth of the deposited films, extracted from the x-ray reflectivity data, show that a monolayer to bi-molecular layer transformation takes place after collapse. The molecules in the lower monolayer have asymmetric configurations with head groups touching water and tails in air, whereas molecules in the upper layer are in symmetric configurations with tails on both sides of the heads. Atomic force microscopy images of the deposited films after collapse, however, show nearly circular islands of height more than that of the bimolecular layer observed in the EDP. As pressure increases, ridges are seen to coexist with these islands. Although the coverage of such islands and ridges is low, they play an important role in determining the growth mode. The growth of the wetting and island layers, taken together, has a striking similarity with the Stranski-Krastanow mode, observed usually for heteroepitaxial growth.

Chain orientation and headgroup structure in Langmuir monolayers of stearic acid and metal stearate (Ag, Co, Zn, and Pb) studied by infrared reflection-absorption spectroscopy

The monolayers of stearic acid at the air-water interface on pure water and ion-containing subphases have been studied using infrared reflection-absorption spectroscopy. In the presence of Co(2+), Zn(2+), and Pb(2+), ordered hydrocarbon chains and hexagonal subcell structure remain almost unchanged in comparison with those for the monolayer on pure water at the surface pressure of 20 m/Nm. In the cases of Co(2+) and Zn(2+), the H-bonded monodentate and unsymmetric bidentate chelating structure within the headgroups were formed, and in the case of Pb(2+), three types of structures, bidentate chelating, unsymmetric chelating, and bidentate bridging coordinations, were formed. The hydrocarbon chains in the monolayers are uniaxially oriented at a tilt angle of approximately 0 degrees with respect to the surface normal in contrast to a tilt angle of approximately 20 degrees on pure water surface at 20 m/Nm by the computer simulation of theoretical calculation to experimental data. In the presence of Ag(+), multilayers were developed with the highly ordered hydrocarbon chains in a triclinic subcell structure and a bidentate bridging structure within the headgroups. The multilayers were composed of three monolayers and the hydrocarbon chains in each monolayer were oriented at an angle of approximately 30 degrees away from the surface normal with their C-C-C planes almost perpendicular to the water surface.

Effect of Metal Ions on Monolayer Collapses

A Langmuir monolayer of stearic acid on pure water and in the presence of certain divalent metal ions such as Cd and Pb at pH approximately 6.5 of the subphase water collapses at constant area, while for other divalent ions such as Mg, Co, Zn, and Mn at the same subphase pH the monolayer collapses nearly at constant pressure. Films of stearic acid with Cd, Pb, Mn, and Co in the subphase (at pH approximately 6.5) have been transferred onto hydrophilic Si(001) using a horizontal deposition technique, just after and long after collapse. Electron density profiles obtained from X-ray reflectivity analysis show that a three-molecular-layer structure starts to form just after constant area collapse, where in the lowest molecular layer, in contact with the substrate, molecules are in asymmetric configuration, i.e., both hydrocarbon tails are on the same side of the metal-bearing headgroup that touches the substrate, while the molecules above the first layer are in symmetric conformation of the tails with respect to the headgroups. Further along collapse, when the surface pressure starts to rise again with a decrease in area, more layers with molecules in the symmetric configuration are added, but the coverage is poor. On the other hand, only bimolecular layers form after constant pressure collapse, with the lower and upper layers having molecules in asymmetric and symmetric configurations, respectively, and the upper molecular layer density increases with compression of the monolayer after collapse. A "Ries mechanism" for constant area collapse and a "folding and sliding mechanism" for constant pressure collapse have been proposed.

Crystallization of calcium oxalate beneath Langmuir monolayers in the presence of urinary inhibitors

The crystallization of calcium oxalate (CaOxa) beneath Langmuir monolayers of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC) was investigated by environmental scanning electron microscopy (ESEM) and X-ray diffraction (XRD) analysis. The headgroups of the monolayer materials, the surface pressure of the monolayers and the biomolecules in subphases affected the formation of CaOxa crystals. More calcium oxalate monohydrate (COM) crystals were observed beneath SA monolayers than that beneath DPPC monolayers. The crystals grown at SA monolayers exhibited unusual morphology, while beneath DPPC monolayers the COM crystals were oriented with the elongated hexagonal face parallel to the plane of the monolayers. More crystals were observed at lower surface pressure of DPPC monolayers. In the presence of l-glutamic acid ( l-glu), irregular polyhedron crystals and three-dimensional hexagonal prismatic crystals were obtained. While urinary macromolecule chondroitin sulfate A (C 4S) made the elongated ( 1 ¯ 0 1 ) face of COM much longer and the crystals became thinner. C 4S can also delay the two-dimensional (2D) growth of the ( 1 ¯ 0 1 ) face of COM crystals.

Measurement of the Adsorption of Drug Ions at Model Membranes by Scanning Electrochemical Microscopy

We present quantitative measurements of the adsorption of drug ions at Langmuir monolayers, which are considered as a model for biological membranes. The technique we have employed is based on scanning electrochemical microscopy using a capillary probe filled with organic electrolyte solution and operated in an inverted submarine arrangement in a Langmuir trough. In this experimental arrangement, we have made potential step chronoamperometric measurements, with the probe adjacent to the Langmuir monolayer deposited at the air/water interface, to extract information on the adsorption process. We have measured the adsorption of a cationic β-blocker drug, propranolol, at a Langmuir monolayer of stearic acid. By developing a theoretical simulation that assumes that the equilibrium at the interface is controlled by the competition between the acid/base equilibrium of the stearic acid molecules at the surface and the electrostatically induced adsorption of the positively charged propranolol ion, we have been able to extract from our data a drug adsorption constant of (5 ± 1) × 10-5 mol dm-3 at the monolayer surface pressure of 10 mN m-1.

Structural control of Langmuir–Blodgett films containing metal cations by ligands exchange

The structure of multilayer gadolinium stearate Langmuir–Blodgett (LB) films has been studied in dependence on the gadolinium salt nature and aqueous subphase composition used for the LB film preparation. Stearic acid Langmuir monolayers were formed on the surface of aqueous gadolinium acetate or gadolinium chloride solutions (10 −4 M) with different monovalent electrolyte concentrations at the same pH value (5.65) at which all monolayer stearic acid molecules were ionized and bound with Gd 3+ cations. The structure of the deposited films was characterised by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). It was found that Gd stearate LB films formed with the use of gadolinium acetate solution as a subphase had quasi-crystalline domain morphology with ordered layered structure, in contrast to the rather disordered structure of LB films formed using GdCl 3 solution as a subphase. The structure characteristic for the Gd stearate LB film formed with the use of Gd acetate solution as a subphase can be obtained using GdCl 3 solution with an excess of acetic acid (9×10 −4 M). The substantially different morphology of Gd stearate LB film characterized by the perfect layered and planar terrace structure was obtained in the presence of NaCl (10 −2 M) in Gd acetate or Gd chloride solutions used as subphases. The data obtained show that the formation of polyvalent metal cation complexes with Langmuir monolayer and bulk phase ligands along with ligands exchange and substitution interactions are substantial factors determining structural organization of the corresponding deposited metal-containing multilayer LB films.

Elasticity of Langmuir monolayer detected by dynamic oscillation method

A dynamic oscillation method has been used to measure the dynamic elasticity as a function of mean molecular area for Langmuir monolayers of stearic acid on the subphases of pure water, CdCl2 and TbCl3 solution, respectively. The elasticity-area curves nicely revealed the phase transition(s) taking place during the compression. A local sudden fall in elasticity was found for stearic acid monolayer in liquid state on water as well as on CdCl2 subphase. By contrast, no such discontinuities could be observed in the case of TbCl3 subphase. It is convinced that the nature of subphase plays an important role in the formation of the monolayer. However, from the suggested molecular structure model, it can be concluded that the elasticity of monolayer is mainly dominated by the interaction between long hydrocarbon chains of stearic acid molecules, but not the static electric interaction between the positive metal ions.

Dependence of the Amplification of Giant Raman Scattering and Fluorescence on the Distance between an Adsorbed Molecule and a Metal Surface

We have studied the effect of the long-range amplification of secondary radiation of the molecules of oxazine 1 adsorbed on the surface of thin silver film covered by 1–7 Langmuir monolayers of stearic acid. We show that the intensity of the giant Raman scattering (GRS) changes proportionally to r−1 with removal of a molecule from the surface of a metal, which is in good agreement with the theory. The distances over which the strength of the electric field enhanced by the surface makes a substantial contribution to the processes mentioned are of the order of 10 nm.

Two-Dimensional Mixtures of Stearic Acid and Partially Fluorinated Amphiphilic Molecule: A Grazing Incidence X-ray Diffraction Study

We report a grazing incidence X-ray diffraction (GIXD) study of mixed Langmuir monolayers of stearic acid (ST) and a fluorinated amphiphilic molecule (FEP). The mixing behavior of the two components was investigated at two different surface pressures, 5 and 30 mN/m. The ST molecules are found to be in the L2 mesophase at 5 mN/m and in the LS mesophase at 30 mN/m, whereas the FEP molecules appear disordered. In the mixture, the GIXD measurements, associated with previous thermodynamical and optical microscopy results, show that in the whole range of molar ratio, the two molecules are completely phase-separated. Also, we observed at 5 mN/m, relaxation in the pure ST monolayer and in the mixtures rich in ST; the tilt angle of the molecules and the rectangular distortion of the 2D-lattice decrease. By comparing mixed films and pure ST films, this relaxation could be correlated to the size of the domains.