Stearic Acid Monolayers Research Articles

Friction of Fatty Acids in Nanometer-Sized Contacts of Different Adhesive Strength

The effects of adhesion, contact area, and pressure on the lubricating properties of self-assembled monolayers on steel have been investigated with friction force microscopy. The adsorbed molecules were fatty acids with varying degrees of unsaturation (0-2 double bonds; stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid), adsorbed from n-hexadecane solution. The friction of these loose-packed monolayers was studied in dry N2 gas and in ethanol. Low adhesion (in ethanol) resulted in a linear increase in friction force at low loads, that is, F = muL, whereas higher adhesion (in N2 gas) gave an apparent area-dependence at low loads of the form F = S(c)A, where S(c) is the critical shear stress. A recent model for the contact mechanics of a compliant elastic film confined between stiffer substrates was applied to the data obtained in dry N2. Using this approach, we obtained interfacial energies of the compliant monolayers in good agreement with van der Waals-Lifshitz theory. With a low monolayer elastic modulus of E'(1)=0.2 GPa, we obtained a slightly higher value of Sc for stearic acid than that established for more close-packed stearic acid monolayers. An increase of mu and S(c) was found with increasing degree of unsaturation of the fatty acid.

Chitosan as a Lipid Binder: A Langmuir Monolayer Study of Chitosan−Lipid Interactions

Owing to its distinct chemico-biological properties, chitosan, a cationic biopolymer, offers a great potential in multifarious bioapplications. One such application is as a dietary antilipidemic supplement to be used to reduce obesity/overweight and to lower cholesterol. The lipid-binding efficiency of chitosan, however, remains debatable. Accordingly, in this study we investigated the interactions of chitosan with selected lipids, cholesterol and fatty acids, the latter including saturated (stearic acid) and unsaturated (oleic, linoleic, alpha-linolenic) acids. The experiments were performed with the Langmuir monolayer technique, in which surface pressure-area isotherms were recorded for the lipid monolayers spread on the acetate buffer pH 4.0 subphase in the absence and presence of chitosan. We found that the presence of chitosan in the subphase strongly influenced the shape and location of the isotherms, proving that there existed attractions between chitosan and lipid molecules. The attractions were revealed by changes of the molecular organization of the monolayers. The common feature of these changes was that all the monolayers studied underwent expansion, in each case reaching saturation with increasing chitosan concentration. In agreement with the lipid molecular structures, the highest expansions were observed for the most unsaturated fatty acids, linoleic and alpha-linolenic, the lowest for stearic acid, with oleic acid and cholesterol being the intermediate cases. By contrast, the main distinguishing feature of these changes was that, although none of the monolayers studied changed its state when completely saturated with chitosan, compared to the parent ones the compactness of the monolayers was modified. The solid monolayers of stearic acid and cholesterol were loosened, whereas those of all the unsaturated acids, liquid in nature, were tightened. On the basis of these results we tentatively propose a mechanism of the chitosan action that includes both electrostatic and hydrophobic lipid-chitosan interactions as well as hydrogen bonding between them.

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.

Incorporation of ovalbumin within cationic octadecylamine monolayer and a comparative study with zwitterionic DPPC and anionic stearic acid monolayer

In this communication we demonstrated the incorporation of water-soluble surface-active protein OVA within an insoluble cationic ODA monolayer and compared with zwitterionic (DPPC) and anionic (SA) monolayer. The incorporation of OVA is found to be more in ODA as compared to that of DPPC and SA. The kinetics of protein adsorption in lipid monolayer gives the idea that unfolding of OVA is less in case of DPPC than SA and ODA. The pi-A isotherm and compressibility study gives the information about the different states of the protein-lipid mixed monolayer. At higher pressure, OVA tend to squeeze out from the lipids monolayer. High-resolution field emission scanning electron microscope (FE-SEM) images confirm this observation. The surface morphology of DPPC-OVA LB film is far better than ODA-OVA and SA-OVA LB film. OVA forms large irregular aggregates on SA and ODA monolayer. Fluorescence study reveals that protein structure is perturbed more in SA and ODA system compared to that of DPPC. The overall results indicate that DPPC monolayer is better to get protein lipid mixed film than SA and ODA monolayer.

Transformation of a hydrophilic membrane into semi-super-hydrophobic based on self-assembly of stearic acid monolayer over induced nanostructures on the membrane surface

Both the chemical and structural properties of a surface determine the contact angle. For the formation of super-hydrophobic surfaces, modification of surface chemistry must be always combined with surface roughness enhancement. The used methods to make a super-hydrophobic surface are expensive and need very complicated equipments and cannot be scale up easily. In this study a simple and less expensive method was developed to transform a hydrophilic membrane into a semi-super-hydrophobic. In order to modify the membrane surface geometrically, the required needle-like rugosities were created by boiling the membrane in the water. The chemical modification of the roughened surface was created by the chemical adsorption and controlling the reaction time of stearic acid (STA) on the polymer of the membrane surface. Finally, by controlling the surface roughness, the concentration of the STA solution and duration of reaction time, a semi-super-hydrophobic membrane with the contact angle of 120° was prepared.

Microdomains in mixed monolayers of oleanolic and stearic acids: thermodynamic study and BAM observation at the air–water interface and AFM and FTIR analysis of LB monolayers

Monolayers of oleanolic acid (OLA) mixed with stearic acid (SA) were studied at the air–water interface. The surface pressure–area ( π– A) isotherms, measured over the whole composition range, and BAM observations were used to investigate the phase behaviour and self-organization of these components in a two-dimensional structure. Pure OLA forms a very compressible monolayer, and BAM observation revealed the coexistence of large and irregular solid domains of different thickness dispersed in a gas matrix, compatible with the two most probable orientations of the OLA molecule at the interface. Mixtures of OLA/SA form condensed monolayers from low surface pressures and the thermodynamic analysis indicates that OLA molecules, in the presence of the long-chain SA, orient with the major axis almost perpendicular to the interface. Langmuir–Blodgett (LB) monolayers of pure SA and mixtures were further characterized by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). AFM images of LB mixed monolayers evidenced microphase separation, not observable by BAM. The SA rich domains are 4–6 Å thicker than those rich in OLA. The FTIR spectra of mixed LB films on CaF 2 substrates showed that OLA does not perturb the all- trans conformation of the SA long alkyl chains, up to a mole fraction of 0.4. The carbonyl-stretching band of OLA suggests that the carboxylic groups of neighbour OLA molecules are involved in hydrogen bonds, forming dimers, as in pure solid phase OLA. These interactions seem to prevail over the OLA–water hydrogen bonds.

Interaction and incorporation of ovalbumin with stearic acid monolayer: Langmuir–Blodgett film formation and deposition

In this communication, the surface activity of the ovalbumin (OVA) at the air/water interface was studied to establish the nature of the interaction with the stearic acid (SA) monolayer, based on Langmuir–Blodgett (LB) technique. The interaction was monitored by studying the time ( t) variation of surface pressure ( π) at constant area ( A). The growth of π with time indicates a positive association between the SA and the OVA molecules. The surface compressibility analysis has been performed to specify the phase transition of OVA–SA mixed monolayer. Incorporation/association of OVA within the SA monolayer led to noteworthy changes in surface compressibility and was surface pressure as well as protein concentration dependent. Both the hydrophobic and the Vander wall type interactions are found to be responsible for the association. The quenching of tyrosine band in tryptophan excitation spectrum is observed in steady-state fluorescence spectroscopy. This suggests that the tyrosine is the probable binding site with SA. Due to incorporation of SA, the energy transfer from tyrosine to tryptophan is hindered. At higher pressure, OVA tend to squeeze out from the SA monolayer. The high-resolution field emission scanning electron microscope (FE-SEM) image confirms this observation. Aggregated protein structure observed at high pressure indicates unfolding of protein.

Reduction of oxygen and hydrogen peroxide on electrodes with adsorbed monolayer of aliphatic compounds

Cathodic reduction of oxygen and hydrogen peroxide on amalgamated platinum electrodes, which are coated with monolayers of long-chain aliphatic compounds cetyl alcohol (CA) and stearic acid (SA), is retarded as compared with the same reactions on clean mercury (or amalgam) surface. The oxygen reduction kinetics differ from that on mercury. The difference is explained by that oxygen diffuses into the monolayer and is reduced in it at a certain distance from the metal surface and only at the limiting current the reaction is forced onto the monolayer surface. In contrast to the oxygen reduction, the hydrogen peroxide reduction kinetics on electrodes with SA and CA monolayers is much closer to that on mercury, but with some quantitative distinctions. All results favor the H2O2 reduction at the monolayer/solution interface. The difference in the behavior of O2 and H2O2 is explained by different polarity of these molecules: it is significantly more difficult to penetrate the hydrocarbon monolayer for polar H2O2 molecule than for nonpolar O2 molecule.

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.

Infrared Imaging of a Solid Phase Surfactant Monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.

Hydrogen peroxide reduction on amalgamated platinum electrodes covered with a monolayer of stearic acid

The reduction of hydrogen peroxide and, for comparison, oxygen on an amalgamated platinum electrode covered with a monolayer of stearic acid is studied by methods of polarization curves and impedance spectroscopy. In contrast with the oxygen reduction, the reduction of dissolved hydrogen peroxide occurs predominantly on the monolayer surface, rather than inside it. This is explained by the difficulty of penetration of the polar molecule of hydrogen peroxide into hydrocarbon environment.

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.

Brewster Angle Microscope – An Excellent Tool for Nanoscience Researchers

An indigenous development of Brewster Angle Microscope (BAM) is completed at National Chemical Laboratory (NCL), India that is real time imaging instrument capable of imaging the monolayer while compressing the barrier trough simultaneously. The instrument will be useful for the basic research in the area of nanoscience. BAM has proven to be an excellent tool for the characterization of the morphology of insoluble monolayers of the surfactant molecules at air‐water interface, which are known as Langmuir films. BAM technique that was developed more than a decade ago globally, is well‐known optical method for visualizing extremely thin films. BAM can reveal inhomoginities, particularly phase domains in ultra thin layer, without the use of special probes. The reflected light is the basis of the Brewster Angle Microscopy of the ultra‐thin films. Imaging of these monolayers yield to many morphological/phase transition based study of these monolayers. The development set up consist of p‐polarized light, designed optics, frame grabber card, CCD camera as detector and teflon trough with barrier mechanism. Movement of a teflon barrier is controlled with stepper motor. An intelligent module based on single chip microcontroller 89C51 is developed for controlled barrier movement. With BAM, the real time image of a monolayer can be viewed on PC monitor. Stored images can be analyzed further through software package developed under VC++. In house software development has been completed at NCL, for instrument control, image acquisition, and image analysis. Standard APIs available with frame grabber cards can not be made applicable for this real time application. A special algorithm has been developed for finding out the area of the interested domain marked by the user. Other analysis software function includes file management, magnification of the image, contrast and brightness control, etc. A standard monolayer of Stearic acid is used for testing purpose. The instrumentation performance has been tested with an amount of 50 microliter of concentration 1 mg/ml Stearic acid spread on the subphase and found satisfactory. Sub phase used was high purity water (Millipore Milli‐ Q) contained in Teflon trough. The monolayer is compressed using teflon barrier. Simultaneously real time imaging was done to record the first order phase behavior of the monolayer. The instrument testing was done with the help of Nanoscience group at NCL. Recently instrument automation has been updated using LabVIEW platform. With LabVIEW addition, simultaneous viewing of compressed monolayer image and barrier movement control is possible beside other functions.

Protection of iron corrosion by stearic acid and stearic imidazoline self-assembled monolayers

A type of stearic imidazoline (IM) inhibitor was prepared using stearic acid (SA) and diethylenetriamine (DETA) as raw materials. The monolayers of IM and SA were assembled on the iron surface. The electrochemical characterization of stearic acid (SA) and stearic imidazoline (IM) on an oxide free iron surface had been studied. The monolayers of IM inhibitor were characterized by electrochemical impedance spectroscopy (EIS), electrochemical polarization curves, double layer capacitance, X-ray photoelectron spectroscopy (XPS) and molecular simulation. The results of electrochemical studies had illustrated that the inhibition efficiency of IM was higher than SA. XPS showed that the IM molecules adsorbed on the iron surface. The molecular simulation calculations showed that the IM molecules were tilted at an angle on the iron surface.

Deposition of densely packed gold film on an alkane derivative monolayer modified graphite surface

The growth of thin metal films is an important step in the fabrication of electronic and magnetic devices. In this work, an atomically flat graphite surface was used as a model system to understand the details of gold film growth mechanisms and kinetics. Ordered assembling monolayers of 1-octadecanethiol and stearic acid are used to modify the surface and uniform, densely packed ultrathin gold film with the thickness less than 5 nm are formed on these monolayer-modified graphite surfaces in a large area. The amount of gold needed to be deposited in order to form a continuous gold film is significantly reduced as compared to that needed on a bare graphite surface. Copyright © 2006 John Wiley & Sons, Ltd.

Biomimetic synthesis of carbonated hydroxyapatite thin films

Free-standing continuous carbonated hydroxyapatite (HA) films were synthesized at room temperature through a biomimetic pathway. A stearic acid monolayer was utilized as a template for inducing deposition from supersaturated calcium phosphate solutions. Crystalline films could be collected at the water–air interface after just 8 h at room temperature when the Ca/P molar ratio of the supersaturated solution is 1:1, whereas the period of time necessary to obtain a crystalline film lengthens to 12 days when the Ca/P of the supersaturated solution is 9:1. The films obtained from this last solution display smoother surfaces, contain a greater amount of carbonate substituted to phosphate ions, and exhibit a greater Ca/P molar ratio, which slightly increases with time. Heat treatment up to 700 °C allows to obtain continuous films of HA crystals preferentially oriented almost orthogonal to the film surface. On the other hand, heat treatment of the films deposited from solution with a Ca/P ratio of 1:1 allows to get bi-phasic films constituted of smooth HA/β-tricalcium phosphate aggregates.

Interparticle and particle–matrix interactions in polyethylene reinforcement and viscoelasticity

Composites of surface treated and untreated non-colloidal CaCO 3 particles and high-density polyethylene (HDPE) with different filler loading (0–30 vol%) were prepared. Their viscoelastic properties were studied by dynamic strain sweep and small amplitude oscillatory shear and correlated to the particle–particle and particle–matrix interactions. The results gave insight into the mechanism of polymer reinforcement by solid inclusions and the factors that lead to the often observed solid-like response in the terminal zone. With increasing filler volume fraction, the particles tend to agglomerate and build clusters (local structures) that can be disintegrated by shearing. Up to 30 vol% no evidence for a space-filling particle network could be found. The presence of clusters increases the viscosity, the moduli and the viscoelastic non-linearity of the composites. Coating the filler surface by a stearic acid monolayer reduces its tendency to agglomerate as well as the adhesion between the particles and the polymer, leading to lower viscosity and interfacial slippage with increasing strain amplitude. Solid inclusions increase the storage modulus more than the loss modulus, hence decrease the material damping. The hydrodynamic reinforcement is frequency independent and dominates at high frequency. Polymer adsorption on the particles surface results in a transient filler–polymer network, which together with the topological restraints exerted by the inclusions on the polymer chain reptation leads to slow relaxation. These slow relaxation processes are sensitive to the oscillation frequency and strongly contribute to the polymer reinforcement at low frequencies. Agglomerates differ in shape and packing from the nearly spherical primary particles, and exert strong restraints on the polymer chain relaxation, hence offer an additional contribution to the composite's moduli. The sum of these effects results in higher moduli and a shift of the crossover (liquid-like to solid-like) frequency to lower values with increasing filler volume fraction. They also lead to the often-observed tendency towards a solid-like response in the terminal zone before a space-filling filler network is formed. Hydrodynamic and micromechanical models can only predict the hydrodynamic reinforcement, provided that the polymer strongly adheres to the inclusions.

Properties of Monolayers of Long-Chained Aliphatic Compounds Adsorbed at an Amalgamated Platinum Electrode

Electrochemical impedance spectra are obtained in a wide potential range for electrodes covered by monolayers of cetyl alcohol and stearic acid. The conditions for the determination of true values of the monolayer’s capacitance and ohmic resistance are found. It is shown that the restructuring of the stearic acid monolayer in alkaline solutions slightly increases its true resistance and consequently does not involve an increase in its dielectric constant. This validates the assumption on the thinning of the monolayer, which follows from the increase of its capacitance. The short-term adsorption of the above aliphatic compounds results in the formation of a monolayer whose chains are orientated parallel to the electrode surface while during the long-term adsorption, the chains are normal to the surface.

Negative Dipole Potentials of Uncharged Langmuir Monolayers Due to Fluorination of the Hydrophilic Heads

The dipole potential, affecting the structure, functions, and interactions of biomembranes, lipid bilayers, and Langmuir monolayers, is positive toward the hydrocarbon moieties. We show that uncharged Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) exhibit large negative dipole potentials, while the nonfluorinated docosyl ethyl ether (DEE) forms films with positive dipole potentials. Comparison of the Delta V values for these ethers with those of the previously studied(37-39) monolayers of trifluoroethyl ester (TFEB) and ethyl ester of behenic acid (EB) shows that the reversal of the sign of Delta V causes the same change Delta(Delta V) = -706 +/- 16 mV due to fluorination of heads. The Delta V values of both TFEB and EB films differ by -122 +/- 16 mV from those of DFEE and DEE monolayers, respectively, with the same density. Such quantitative coincidence points to a common mechanism of reversal of the sign of the dipole potential for the ether and ester films despite the different structure of their heads. The mechanical properties and phase behaviors of these monolayers show that both fluorinated heads are less hydrated, suggesting that the change of the sign of Delta V could, at least partially, be related to different hydration water structure. The same negative contribution of the carbonyl bond in both TFEB and EB films contrasts with the generally accepted positive contribution of the C(delta+)=O(delta-) bond in condensed Langmuir monolayers of fatty acids, their alcohol esters, glycerides, and phospholipids but concurs with the theoretical analysis of Delta V of stearic acid monolayers. Both results question the literature values of the molecular dipole moments of these substances calculated via summation of bonds and atomic group contributions. Mixed monolayers of DFEE and DEE show smooth monotonic variation of Delta V from +450 to -235 mV, indicating a way for adjustment of the sign and magnitude of the dipole potential at the membrane-water boundary and regulation of such membrane behaviors as binding and translocation rate of hydrophobic ions and ion-carriers, adsorption and penetration of amphiphilic peptides, polarization of hydration water, and short-range repulsion. The interaction of the hydrophobic ions tetraphenylboron TPhB- and tetraphenylphosphonium TPhP+ with DFEE and DEE monolayers qualitatively follows the theory of binding of such ions to lipid bilayers, but the shifts Delta(Delta V) from the values obtained on water are much smaller than those for DPPC monolayers. This difference seems to be due to the solid (polycrystalline) character of the DFEE and DEE films that hampers the penetration of TPhB- and TPhP+ in the monolayers and reduces the attractive interaction with the hydrophobic moiety. This conclusion orients the future synthesis of amphiphiles with fluorinated heads to those which could form liquid-expanded Langmuir monolayers.