Fatty Acid Monolayers Research Articles

Mechanisms for species-selective oriented crystal growth at organic templates

Langmuir monolayers floating on supersaturated aqueous subphases can act as templates for the growth of oriented inorganic films—a “bioinspired” nucleation process. We have performed in situ grazing incidence x-ray diffraction studies of the selective nucleation of BaClF and BaF2under fatty acid monolayers. The arrangement of the fatty acid headgroups, the monolayer charge, and ion-specific effects all play important roles in selecting the inorganic species. When the monolayer is in a neutral state, both BaClF and BaF2nucleate at the interface and are well aligned, but when the monolayer headgroup is deprotonated, only oriented BaF2grows at the interface. We also observe an enhanced alignment of BaF2crystals during growth from highly supersaturated solutions, presumably due to reorganization of preformed crystals at the organic template. These results show that a delicate interplay between multiple factors governs the oriented growth of inorganic films at organic templates.

Effect of Unsaturation in Fatty Acids on the Main Characteristics of Langmuir Monolayers

The main characteristics of unsaturated acid monolayers deviate considerably from those of the corresponding saturated fatty acids. The monolayers of an unsaturated cis and trans fatty acid with the same hydrocarbon chain length having the double bond at the same position (ω9, Δ13) are compared to obtain detailed information on the effect of the cis and trans double bonds. The studies were performed with erucic acid chosen for the cis form and brassidic acid chosen for the trans form. Some additional studies performed with the homologous cis-nervonic acid (ω9, Δ15) and trans-elaidic acid (ω9, Δ9) allow information about the effect of the hydrocarbon chain length. Phase behavior, domain morphology, and two-dimensional lattice structure of the cis and trans unsaturated fatty acid monolayers are compared, using surface pressure−molecular area (π-A) isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) techniques. The π-A isotherms of the unsaturated fatty acids studied have a fluid/condensed phase coexistence region (plateau) over the measured temperature range. The thermodynamic analysis, performed on the basis of the π-A isotherms, shows that the introduction of unsaturation into the hydrocarbon chain affects the thermodynamic characteristics of the monolayer in a similar way as shortening the alkyl chain length, and it is much larger for the cis isomers rather than for the trans isomers. The morphology of the condense phase domains formed in the fluid/condensed coexistence region reveals considerable differences between the cis and the trans conformations at the same molecular structure of the unsaturated fatty acid. Completely different to the behavior of saturated fatty acid monolayers, for all cis and trans unsaturated fatty acids studied, only a centered rectangular lattice with alkyl chains tilted toward the nearest neighbor (NN) direction is found, and no indication for a phase transition between different condensed phases could be observed independent of different surface pressures and temperatures. The conformity of the cis and trans unsaturated fatty acid monolayers to only one lattice type having a rectangular centered unit cell with alkyl chains tilted in the NN direction correlates obviously with the obstructive effect of the double bond in the alkyl chain on the molecular ordering.

Fatty-Acid Monolayers at the Nematic/Water Interface: Phases and Liquid-Crystal Alignment

The two-dimensional (2D) phases of fatty-acid monolayers (hexadecanoic, octadecanoic, eicosanoic, and docosanoic acids) have been studied at the interface of a nematic liquid crystal (LC) and water. When observed between crossed polarizers, the LC responds to monolayer structure owing to mesoscopic alignment of the LC by the adsorbed molecules. Similar to Langmuir monolayers at the air/water interface, the adsorbed monolayer at the nematic/water interface displays distinct thermodynamic phases. Observed are a 2D gas, isotropic liquid, and two condensed mesophases, each with a characteristic anchoring of the LC zenithal tilt and azimuth. By varying the monolayer temperature and surface concentration we observe reversible first-order phase transitions from vapor to liquid and from liquid to condensed. A temperature-dependent transition between two condensed phases appears to be a reversible swiveling transition in the tilt azimuth of the monolayer. Similar to monolayers at the air/water interface, the temperature of the gas/liquid/condensed triple-point temperature increased by about 10 degrees C for a two methylene group increase in chain length. However, the absolute value of the triple-point temperatures are depressed by about 40 degrees C compared to those of analogous monolayers at the air/water interface. We also observe a direct influence by the LC layer on the mesoscopic and macroscopic structure of the monolayer by analyzing the shapes and internal textures of gas domains in coexistence with a 2D liquid. An effective anisotropic line tension arises from elastic forces owing to deformation of the nematic director across phase boundaries. This results in the deformation of the domain from circular to elongated, with a distinct singularity. The LC elastic energy also gives rise to transition zones displaying mesoscopic realignment of the director tilt or azimuth between adjacent regions with a sudden change in anchoring.

Kinetic analysis of the ozone processing of an unsaturated organic monolayer as a model of an aerosol surface

It has been suggested that an organic aerosol containing unsaturated organic compounds at the surface would likely be processed by atmospheric ozone. The ozonolysis of oleic acid gives rise to the formation of products having shorter chain lengths than the starting molecule, and are consequently more water soluble than oleic acid. Hence, the exposure of a monolayer of oleic acid to ozone should lead to a decrease in surface activity at the air-water interface. A model system is used for real-time measurements of surface tension changes due to ozone exposure of a pendant drop that is coated by a fatty acid monolayer. The surface tension is measured based on an analysis of the shape profile of acquired images of the drop. A study of the kinetics of the gas-surface reaction is presented. Assuming that the uptake of ozone is dominated by the reaction at the surface, the measured reactive uptake coefficient of ozone gamma(meas) is estimated to be (2.6 +/- 0.1) x 10(-6).

Aggregation-governed oriented growth of inorganic crystals at an organic template

X-ray studies performed during the growth of CdCO(3) and MnCO(3) crystals from supersaturated aqueous solutions, at fatty acid monolayer templates, reveal that the nucleates are nearly three-dimensional powders below a threshold supersaturation. However, at higher supersaturations, the crystals are preferentially oriented with the {0 1 2} direction vertical. Scanning electron microscope images of samples transferred to substrates show discrete crystals at low concentrations, while at higher concentrations the crystals self-aggregate to form linear chains and sheets. The authors speculate that preferential alignment at the organic-inorganic interface is enhanced as a consequence of oriented aggregation of crystals. The role of monolayer-ion interactions in governing the morphologies and the resulting orientation of the inorganic nucleate is discussed.

Microscale and nanoscale hierarchical structured mesh films with superhydrophobic andsuperoleophilic properties induced by long-chain fatty acids

Inspired by the lotus effect, we fabricate new microscale and nanoscale hierarchicalstructured copper mesh films by a simple electrochemical deposition. After modification ofthe long-chain fatty acid monolayer, these films show superhydrophobic and superoleophilicproperties, which could be used for the effective separation of oil and water. The length ofthe fatty acid chain strongly influences the surface wettability of as-prepared films. It isconfirmed that the cooperative effect of the hierarchical structure of the copperfilm and the nature of the long-chain fatty acid contribute to this unique surfacewettability.

Synthesis and characterization of partially fluorinated stearolic acid analogs: Effect of their fluorine content on the monolayer at the air–water interface

Novel stearolic acid analogs (i.e., 9-octadecynoic acid analogs: 1a– d) containing the shorter perfluoroalkyl groups, CF 3, C 2F 5, n-C 3F 7 or n-C 4F 9 group were synthesized. Equilibrium spreading pressures ( π es) of their monolayers at the air–water interface were measured in order to demonstrate how the fluorine content has an effect on the stability of the fatty acid monolayers. As the fluorine content in stearolic acid molecule increased, its melting points was lowered indicating the solid bulk phase of stearolic acid became thermally unstable, while its monolayer stability evaluated by π e at 25 °C, dramatically increased and subsequently leveled off above a certain fluorine content. Under this condition, the replacement of at least five hydrogen atoms at the terminal hydrophobic segment in stearolic acid molecule by fluorine atoms (CF 3CF 2 group) was required to alter the bulk property of stearolic acid and exhibit the stabilization of monolayers, whereas further fluorination of stearolic acid had a minor effect on the monolayer stability. This behavior suggests the terminal fluorinated hydrophobic segment exclusively controls the interfacial stability of fatty acid monolayers.

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.

Complementary Control by Additives of the Kinetics of AmorphousCaCO3Mineralization at an Organic Interface:In-SituSynchrotron X-Ray Observations

The kinetics of biomimetic mineralization at a fatty acid monolayer interface have been measured in situ by synchrotron x-ray reflectivity. The formation of biologically relevant amorphous calcium carbonate films is affected by soluble macromolecules, supersaturation rate of change, and Mg cations. We find that these solution conditions influence mineral film formation in a complementary fashion. Poly(sodium acrylate) extends the lifetime of metastable amorphous calcium carbonate, solution saturation controls the mineral film growth rate, and Mg cations create a longer induction time. This is the first quantification of potentially competitive biomineralization mechanisms that addresses nucleation and growth of the amorphous mineral phases, which are important in biomineralization.

Fatty-acid monolayers and metal ions: First step towards a new look

We have studied the surface pressure versus specific molecular area ( π– A) isotherms of heneicosanoic acid Langmuir monolayers, varying the pH and concentration of Cd-ions in the aqueous subphase. We have measured the discontinuities in A (Δ A) at the L 2 - L ′ 2 phase transition and in κ, the isothermal compressibility, i.e., Δ κ at the L ′ 2 - S phase transition, and have shown that these have distinct zeroes at specific values of pH, which depend on the Cd-ion concentration. From these we have constructed the pCd–pH phase diagram, where pCd = −log 10[Cd], [Cd] being the molar concentration of Cd-ions. This phase diagram clearly shows that this complex two-dimensional system has three distinct behavioral regions.

Characterization of Anomalous Flow and Phase Behavior in a Langmuir Monolayer of 2-Hydroxy-tetracosanoic Acid

Langmuir monolayers of long-chain fatty acids display a wide range of interesting flow and phase behaviors, but the correlation between these behaviors varies. In this paper, we report two interesting behaviors in a Langmuir monolayer of 2-hydroxy-tetracosanoic acid: phase coexistence between the L(2h) and L(2)' phases over a large pressure range and a peak in the viscosity and elastic modulus of the material as a function of pressure. The two behaviors are confirmed over a range of temperatures for which the location of the undistorted lattice appears to vary relative to the range of coexistence for the two phases. Evidence for a correlation between the phase behavior and the peak in the mechanical properties is presented.

A brief review of the relationships between monolayer viscosity, phase behavior, surface pressure, and temperature using a simple monolayer viscometer.

The two-dimensional surface shear viscosity, eta, of fatty acid monolayers of different chain lengths, measured using a simple magnetic needle viscometer, strongly correlates with the molecular organization in condensed phases and the absolute temperature. eta can increase by orders of magnitude at phase boundaries associated with tilted to untilted molecular order, providing the underlying order is semicrystalline. Hence, untilted, long-range ordered CS phases are the most viscous films. However, despite being untilted, the LS rotator phase is less viscous than certain laterally ordered tilted phases, suggesting a decrease of the van der Waals interactions due to molecular rotation. In certain regions of the L2 phase, eta reaches a maximum before the L2-LS transition, an anomalous behavior correlated with the change in the lattice symmetry of the headgroup. Surface shear viscosity, even when measured with a macroscopic probe, is particularly sensitive to the microscopic organization of monolayers.

Evidence of surface reconstruction during ‘bioinspired’ inorganic nucleation at an organic template

X-ray diffraction studies performed during the nucleation of hydrocerussite (PbCO 3 · Pb(OH) 2) from a supersaturated aqueous solution, at a floating fatty acid monolayer template, reveal (√7 × √7)- R19.1° reconstruction at the (0 0 1) surface of hydrocerussite. Reordering of surface atoms into epitaxial superstructures is thought to require extremely clean surfaces (cleaned electrochemically or under UHV). At this organic–inorganic interface, the organic monolayer relaxes to form an epitaxial match with the reconstructed lattice just below it. We speculate that this lattice match, along with the ion-specific interactions of the monolayer, provide the clean nucleation environment.

In Situ Determination of Orientational Distributions in Langmuir Monolayers by Total Internal Reflection Fluorescence

A new application of the polarized total internal reflection fluorescence (PTIRF) technique to study the orientation distribution of a fluorophore within a Langmuir monolayer in situ on an aqueous subphase is described. The technique utilizes the measurement of polarized fluorescence, excited by the evanescent field appearing upon total internal reflection. The excitation by the evanescent field is achieved by launching the beam into a prism that is brought into contact with the monolayer from above. We also show here that a combination of PTIRF of monolayers on water and those freshly deposited onto the prism by horizontal lift in the same experiment provide enough data to determine the dielectric constant of the actual local environment of the fluorophore in the monolayer to eliminate the ambiguity of the orientation determination, arising from uncertainty in the normal component of excitation field. The new technique was applied to several model systems: fatty acid monolayers containing amphiphilic dyes DiI or BODIPY and also a monolayer of a synthetic amphiphilic porphyrin-binding peptide AP0. This technique is more accurate than polarized epifluorescence (PEF) in determining the fluorophore orientation distribution due to the much higher normal component of the excitation, achievable in the evanescent field, and to the lack of surface vibrations caused by capillary waves. Comparison of the new PTIRF approach with PEF shows that the monolayer structure is not disturbed by weak van der Waals attachment to the hydrophobic substrate.

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.

XPS analysis of down stream plasma treated wool: Influence of the nature of the gas on the surface modification of wool

A microwave plasma treatment in a down stream configuration was used to modify the natural hydrophobocity of untreated wool fibers. This property is a consequence of the presence of a Fatty acid monolayer (F-layer) on the outermost part of the fiber surface. The wool fibers treated with plasma were analyzed by means of X-ray photoelectron spectroscopy (XPS) without previous exposure to the air. Experiments have been carried out with air, water vapor, oxygen and nitrogen as plasma gas. The “in situ” analysis of the treated samples has permitted to differentiate between the plasma effects and those other linked to the exposure of the fibers to the air after their treatment. The results have evidenced the effects induced by the different active species generated by plasma from the different components of the air. In general, the intensity of C C peaks decreases and that of the C O, C O and O C O increases when using a gas containing oxygen species. Simultaneously, the intensity of the S S groups decreases and that of the sulphonate (SO 3 −) increases. Other changes are also detected in the intensity of the N 1s level. The extent and characteristics of the oxidation and functionalisation of the hydrocarbon chains of the F-layer depend on the nature of gas. Thus, whereas treatments with plasmas of air and water vapor strongly affect the hydrocarbon chains of the F-layer, oxygen is less effective in the oxidation process. It has been also noted that the active species formed in the nitrogen plasma do not induce any significant change in the surface composition of the wool fibers.

Small-polaron hopping via defect centres: anomalous temperature and voltage dependenceof current through fatty-acid monolayers

We study electron hopping in thin metal–insulator–metal structures which involves two defectcentres with a strong electron–phonon coupling. We calculate the dependences of the current,J, on voltage,V, andtemperature, T, and show that they are consistent with those observed in molecular monolayers of fattyacids. We analyse in detail an unusual, near-exponential temperature dependence of thecurrent: observed at K in eicosanoic acid (C20) organic monolayers sandwiched between Pt electrodes, where the parameterT0 increaseswith the bias voltage. We show that at relatively high voltages the two-defect small-polaron hopping resultsin N-shape current–voltage characteristics which were observed in some organic molecularmonolayers.

Oxygen Permeability of Fully Condensed Lipid Monolayers

The oxygen permeation resistance of highly condensed monolayers composed of a homologous series of saturated diacyl phosphatidylcholine lipids was measured using a novel technique that combined micromanipulation and electrochemical techniques. The use of lipid monolayer-coated air microbubbles allowed measurement of the oxygen permeation resistance of fully condensed lipid monolayers that were previously unavailable using classical film balance techniques. Fully condensed lipid monolayers were found to significantly impede oxygen transport from the gas core, an effect that increased with lipid acyl chain length. The magnitudes of the measured oxygen permeation resistances, 10 2 to 10 3 s/cm, agree with literature values for various gases permeating through highly condensed fatty acid and alcohol monolayers. Such high resistances can account for the previously observed hindered dissolution of lipid-stabilized microbubbles in degassed media. Additionally, the ability to probe a lipid monolayer in its fully condensed state leads to new physical insights into monolayer permeation.

Interaction of Fatty Acid Monolayers with Cobalt Nanoparticles

Stable colloidal suspension of magnetic nanoparticles is challenging owing to both van der Waals forces and magnetic dipolar interactions. Thus, it is essential to coat magnetic nanoparticles with ...

Superlattice Formation in Fatty Acid Monolayers on a Divalent Ion Subphase: Role of Chain Length, Temperature, and Subphase Concentration

We report the study of behenic acid Langmuir monolayers spread over chloride salts solutions of cadmium, lead, magnesium, or manganese. These monolayers were investigated by means of surface pressure−area isotherms and grazing incidence X-ray diffraction (GIXD). The effect of the concentration of cations in the subphase on the structure of the monolayers was probed at room temperature and for three different subphase pHs (5.5, 7.5, 10.5). A threshold in subphase concentration is detected for the formation of a superlattice structure corresponding to an inorganic organized layer in addition to the ordered behenic acid monolayer. This threshold is shown to strongly depend on the cation and the subphase pH. Above the threshold, the superstructure is independent of both the cation concentration and the pH. Below the threshold, the ions are disordered but induce a condensing effect on the fatty acid molecules, which is more or less pronounced depending on the ions. Moreover, the combination of isotherms and GI...