Stearic Acid Monolayers Research Articles

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.

Decorating a Liquid Interface Promotes Splashing

We report on the role of a surfactant monolayer in the impact of small spheres on an air-water interface. We observe that an air cavity (splashing) is induced above a threshold impact velocity. We explore the dependence of this threshold velocity on the bath surface tension and on the bath viscosity using water-ethanol and water-glycerol mixtures, respectively. Interestingly, the threshold velocity for air entrainment is reduced by the presence of a stearic acid monolayer because the hydrophobic tails favor the forced entry of air during the sphere's impact. More generally, we show that this threshold velocity is determined by the wettability of the sphere by the bath, which can equivalently be controlled by tuning the bath properties (presence of a monolayer) or the sphere surface properties (via a surface treatment). These results provide insight into recently developed vesicle production techniques based on the impact of drops or jets on a lipid layer.

An Alkaline Hydrothermal Method to Synthesize Super-Hydrophobic Wood

A layer of sword-like super-hydrophobic film was fabricated on wood surface through an alkaline hydrothermal method. The super-hydrophobic property of the wood surface was measured by contact angle measurements (CA). The microstructure and chemical composition of the super-hydrophobic film was analyzed by scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR). Analytical characterization revealed that the micro-scale roughness sword-like particles uniformly erect on the wood surface and create the rough surface. Also, a monolayer of stearic acid was adsorbed on the rough surface which contributes to reduce the surface energy. With the combination of roughness and low surface energy of wood surface, the wood has turned its wetting property from hydrophilic into super-hydrophobic with a water contact angle 151o, and the sliding angle is about 5o.

Headgroup Effects of Template Monolayers on the Adsorption Behavior and Conformation of Glucose Oxidase Adsorbed at Air/Liquid Interfaces

Stearic acid (SA) and octadecylamine (ODA) monolayers at the air/liquid interface were used as template layers to adsorb glucose oxidase (GOx) from aqueous solution. The effect of the template monolayers on the adsorption behavior of GOx was studied in terms of the variation of surface pressure, the evolution of surface morphology observed by BAM and AFM, and the conformation of adsorbed GOx. The results show that the presence of a template monolayer can enhance the adsorption rate of GOx; furthermore, ODA has a higher ability, compared to SA, to adsorb GOx, which is attributed to the electrostatic attractive interaction between ODA and GOx. For adsorption performed on a bare surface or on an SA monolayer, the surface pressure approaches an equilibrium value (ca. 8 mN/m) after 2 to 3 h of adsorption and remains nearly constant in the following adsorption process. For the adsorption on an ODA monolayer, the surface pressure will increase further 1 to 2 h after approaching the first equilibrium pressure, which is termed the second adsorption stage. The measurement of circular dichroism (CD) spectroscopy indicates that the Langmuir-Blodgett films of adsorbed GOx transferred at the first equilibrium state (π = 8 mN/m) have mainly a β-sheet conformation, which is independent of the type of template monolayers. However, the ODA/GOx LB film transferred at the second adsorption stage has mainly an α-helix conformation. It is concluded that the specific interaction between ODA and GOx not only leads to a higher adsorption rate and adsorbed amount of GOx but also induces a conformation change in adsorbed GOx from β-sheet to α-helix. The present results indicate that is possible to control the conformation of adsorbed protein by selecting the appropriate template monolayer.

Phase diagram of mixed monolayers of stearic acid and dimyristoylphosphatidylcholine. Effect of the acid ionization

The aim of this work is to study the phase diagram of mixed monolayers composed of dimyristoylphosphatidylcholine (DMPC) and stearic acid (SA) at different ionic strength and bulk pH of the aqueous subphase. In this way, the effect of ionization of SA on the interaction and thus on phase separation with the DMPC matrix can be analyzed. To this purpose, we first determined the ionization state of pure SA monolayers as a function of the bulk subphase pH. The SA monolayers are nearly fully ionized at pH 10 and essentially neutral at pH 4 and the mixture of DMPC and SA was studied at those two pHs. We found that the DMPC-enriched phase admits more SA if the SA monolayer is in a liquid-expanded state, which is highly related to the acid ionization state, and thus to the bulk pH and ionic strength. At pH 4 the molecules hardly mix while at pH 10 the mixed monolayer with DMPC can admit between 30 and 100% of SA (depending on the lateral pressure) before phase separation is established. The addition of calcium ions to the subphase has a condensing effect on SA monolayers at all pHs and the solubility of SA in the DMPC matrix does not depend on the bulk pH in these conditions. The observed phase diagrams are independent on the manner in which the state of the mixed film is reached and may thus be considered states of apparent equilibrium.

Molecular control of pentacene/ZnO photoinduced charge transfer

Photoinduced charge transfer modifies the device properties of illuminated pentacene field effect transistors (FETs) incorporating ZnO quantum dots at the gate insulator/pentacene interface. The transferred charge is trapped on electronic states associated with the ZnO quantum dots, with a steady state population approximately proportional to the rate of organic-inorganic charge transfer. Trapped charge shifts the threshold voltage of the FETs, providing the means to evaluate the rate of organic/inorganic charge transfer and the effects of interface modification. Monolayers of the wide-gap alkane stearic acid and the conjugated oligomer terthiophene attached to the ZnO suppress or permit charge transfer, respectively.

A Facile Method To Prepare Superhydrophobic Coatings by Calcium Carbonate

In this work, we have developed a facile and inexpensive method to fabricate a superhydrophobic surface with hierarchical nano- and microstructures. The superhydrophobic surface was prepared through spin coating of a mixture of nano- and microsized calcium carbonate (CaCO3) suspensions on a substrate, followed by modification of a low-surface-energy monolayer of stearic acid. The key parameters of the fabrication of suitable surface morphologies, including the content of CaCO3 powder and the percentage of nano- and microsized CaCO3 powder, were also investigated. Results showed that the optimum CaCO3 content was 40 wt % and the self-cleaning property of the film was achieved only by the addition of 2 wt % microsized CaCO3 powder and 38 wt % nanosized CaCO3 powder. After modification with a self-assembled monolayer of stearic acid, the as-prepared coating showed self-cleaning properties with a water contact angle as high as 152.8° and a sliding angle of 7.8°.

Reversible switching of nanostructured cobalt hydroxide films from superhydrophobic to superhydrophilic state

Superhydrophobic cobalt hydroxide films with flower-like micro-nano structure prepared by chemical-bath deposition undergo a change in wettability from superhydrophobic to superhydrophilic state either by immersion in ethyl acetate or by heat treatment at 200°C and above. The superhydrophobicity of the film can be regained by immersion in an ethanolic solution of stearic acid. The superhydrophobicity is attributed to the combined effect of micro-nano binary surface morphology and the low surface energy of the self-assembled monolayer of stearic acid. It is proposed that switching from superhydrophobicity to superhydrophilicity is caused by removal of the adsorbed layer of stearic acid by solvent or heat treatment.

Incorporation of pepsin within zwitterionic, anionic, and cationic lipid monolayers: A comparative study

The incorporation of water-soluble surface-active enzyme pepsin (PEP) within insoluble zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and anionic stearic acid (SA) monolayer is studied. Furthermore, the results are compared with cationic octadecylamine (ODA). Adsorption of PEP is found to be higher in ODA as compared to that of DPPC and SA. PEP adsorption kinetics in lipids monolayer follows two-step process: diffusion and unfolding. The unfolding of PEP is lower in the case of DPPC than in SA and ODA. π-A isotherm together with high-resolution field emission scanning electron microscope (FE-SEM) images indicate that at higher pressure, PEP molecules tend to squeeze out from the lipids monolayer. PEP forms larger irregular intermolecular aggregates by increment of β-component on SA and ODA monolayer. However, in zwitterionic (DPPC) matrix the α-helix increases with smaller intermolecular aggregates. The overall results specify that zwitterionic (DPPC) monolayer is better choice to obtain protein lipid mixed film than anionic (SA) and cationic (ODA) monolayer.

Surface plasmon resonance instrument as a refractometer for liquids and ultrathin films

A surface plasmon resonance (SPR) setup in Kretschmann configuration is being utilized as a refractometer for both liquids as well as ultrathin films. The SPR signal detection technology used is based on a goniometer approach providing a wide angular scan range which facilitates highly accurate liquid and gas phase measurements. Attention was paid to improve sample handling and preparation. In order to avoid cross-contamination between measurements an easily removable and exchangeable molded PDMS flow cell was used during the measurements. By careful choice of components for liquid handling the dead volume of the system could be reduced down to some microliters. The angular change and thus the refractive index for sucrose, ethylene glycol (EG) and ethanol solutions with different concentrations, the thickness and refractive index of deposited Langmuir-Blodgett (LB) films, and the interaction kinetics between a biotin containing self-assembled monolayer (SAM) and streptavidin were determined. The measured refractive indices of sucrose, EG and ethanol solutions corresponded well with literature values. LB films were characterized by measuring the complete SPR curve in an angular scan range from 40 to 78°. A two-color SPR approach combined with two-media measurements was successfully employed for simultaneous and unambiguous determination of both refractive index and thickness of stearic acid monolayers. The thickness obtained for the stearic acid monolayer was 2.66 nm, and the refractive indices at 635 and 670 nm were 1.5800 and 1.4138, respectively. The developed sensor-plate holder enabled functionalization of the SPR gold chip outside the instrument, therefore making the sample handling more flexible. The affinity constant obtained for the streptavidin–biotin interaction was 1.01 × 10 −8 M. The total angle SPR method used in this study clearly shows its potential to be used as a refractometer for both liquids and ultrathin films, as well as for traditional liquid phase biomolecular kinetic studies.

Investigations of RhB Langmuir monolayer by Fluorescence Imaging Microscopy

This communication reports the surface pressure vs area per molecule isotherm and Fluorescence Imaging Microscopic studies of the formation of domain structure in the mixed Langmuir monolayer of RhB and Stearic acid (SA) at the air-water interface. Strong repulsive interaction between the unlike components leads to the phase separation and formation of microcrystalline domains at the air-water interface of the Langmuir monolayer. These domains can be directly visualized using fluorescence imaging microscope.

Dipolar electrostatic energy effect on relaxation process of monolayers at air-water interface: Analysis of thermodynamics and kinetics

In order to understand the effect of electrostatic energy on phase transition from monolayer to multilayer, isobaric relaxation process of Langmuir monolayers composed of stearic acid or ferroelectric polyvinylidene fluoride and trifluoroethylene copolymer with various vinylidene fluoride (VDF) ratios is investigated in terms of thermodynamic and kinetic analysis. A monotonous decreasing tendency of material loss with respect to temperature is observed for stearic acid monolayer, which is due to thermal activation effect on phase transition from monolayer to multilayer. In contrast, for the ferroelectric monolayer it presents a nonmonotonous behavior of losing materials with a peak position near the Curie temperature, which is not only owing to thermal activation but also dipole moment change. This observation is confirmed for the copolymer monolayers with other VDF content ratios. Amazingly, for the ferroelectric monolayers a good correspondence is found for critical temperatures evaluated from several independent methods including the analysis on slow collapse. This finding again tells the fact that the relaxation process, namely phase transition from monolayer to multilayer, is greatly influenced by dipolar electrostatic energy. Moreover, the study of time dependent relaxation process reveals a diffusionlike behavior of multilayer structure formation, which cannot be interpreted by classical models. Hence a new model based on diffusion-driven material transfer is proposed and diffusivity of the copolymer molecules is estimated with a value of 0.4x10(-5) cm(2)/s. As a whole, this research reflects the importance of dipolar electrostatic energy for phase transition of monolayers at air-water interface.

The influence of stearic acid coating on the properties of magnesium hydroxide, hydromagnesite, and hydrotalcite powders

Hydrated filler-type flame retardants were coated with approximately a monolayer of stearic acid using a solvent technique. Compared to the uncoated powders, the BET surface area was lower, the powder packing density was improved, and the thickening effect on white oil was significantly reduced. The latter two observations are rationalized in terms of a reduction in the attractive interactions between the powder particles. The viscosity of white oil slurries containing 25 wt% solids showed shear-thinning non-Newtonian behavior. The coated powders showed significantly lower viscosities at low shear rates although the difference diminished at high shear rates. The lower viscosities shown by the coated powders indicate that the surface modification facilitated the break-up of agglomerates and the dispersion of individual particles in the fluid.

Observation of an Amorphous Calcium Carbonate Precursor on a Stearic Acid Monolayer Formed during the Biomimetic Mineralization of CaCO3

The Langmuir monolayer is widely regarded as a model for investigating biomineralization on biological membranes, but so far the crystallization process has been elusive. In this work, we study the crystallization process of CaCO(3) on a stearic acid monolayer at the air-water interface by in situ Brewster angle microscopy and ex situ electron microscopy. It is shown that the final CaCO(3) crystals are transformed from a particle precursor rather than directly from the solvated ions under the compressed stearic acid monolayer. SEM and TEM images reveal that the precursors are uniform spherical particles of amorphous calcium carbonate (ACC) with sizes smaller than 100 nm. The amorphous particles are produced in an early stage of mineralization and can be stabilized for at least 0.5 h. Subsequently, the ACC particles tend to aggregate with increasing population and are eventually transformed into the crystalline calcite phase with the elapse of time. This work is the first to clarify the crystallization process of calcium carbonate under the monolayer at the air-water interface by a direct observation of the ACC and therefore establish the roles of stearic acid in the process.

In Situ Observation of γ-Fe2O3Nanoparticle Adsorption under Different Monolayers at the Air/Water Interface

We studied the adsorption of gamma-Fe 2O 3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of gamma-Fe 2O 3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed zeta potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (pi/ A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.

Structural Features of Mixed Monolayers of Oleanolic Acid and Stearic Acid

The features of oleanolic acid at interfaces and its effect on another component of the epicuticular wax are studied using a binary monolayer system consisting of oleanolic acid (OLA) and stearic acid (SA). A combination of lateral pressure studies, grazing incidence X-ray diffraction (GIXD), and specular X-ray reflectivity (SR) measurements, is proven to be optimal in this respect. In the case of the OLA monolayer, the LS phase exists in the whole accessible pressure region, whereas the SA monolayer develops the phase sequence from the nearest neighbor tilted L2 phase to the next nearest neighbor tilted OV phase and, finally, to the hexagonal LS phase with nontilted molecules with increasing lateral pressure. The cross-section area of 44.1 A2·molecule−1 obtained for the pure OLA monolayer from the position of the diffraction peak agrees well with the circular cross-section of the OLA molecular data. The thicknesses of the OLA monolayer determined by GIXD and XR coincide perfectly. The structure data obta...

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

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

Poly (3-dodecylthiophene) Langmuir–Blodgett films: Preparation and characterization

Poly(3-dodecylthiophene) (P3DDT)/stearic acid (SA) monolayer and multilayers have been successfully deposited onto indium tin oxide (ITO) substrates through Langmuir–Blodgett (LB) technique. Surface pressure–mean molecular area ( π– A) isotherms, X-ray diffraction (XRD) pattern, absorption spectrum, fluorescence spectrum, atomic force microscope (AFM), and photo-assistant conducting AFM (PC-AFM) have been used for characterization. The AFM topographic images of P3DDT/SA LB film showed high coverage morphology with some aggregated islands dispersed uniformly in the film when deposited at surface pressure of 40 mN/m. Compared with the absorption and fluorescence spectra of P3DDT in toluene, the spectra of P3DDT/SA LB films showed an obvious red-shift phenomena, which were due to the formation of P3DDT aggregates in LB films. It was confirmed by XRD measurement that P3DDT/SA LB films formed an ordered structure with an average thickness of 2.9 nm. The results from PC-AFM measurements showed that the P3DDT/SA LB films had much higher conductivity under UV light irradiation, which indicated that P3DDT molecules had a good photocurrent response in such ordered structure.

Fatty acid monolayer dissociation and collapse: Effect of pH and cations

Surface pressure versus specific molecular area ( π– A) isotherms of stearic acid monolayers have been studied by varying the pH of the aqueous subphase containing monovalent (Na) or different divalent (Mg, Ca, Ba, Zn and Cd) cations. The lowering of the tilted to untilted phase transition pressure ( π t) with increasing pH of the subphase is correlated with an increase of the headgroup dissociation of the acid monolayer. On pure water, the surface pressure drops after the collapse point ( π c) to a lower pressure value ( π l). The pressure-gap Δ π = π c − π l changes with increasing dissociation, depending upon the particular cation. After a certain pH, no pressure gap is observed and π remains constant or increases slowly after π c. This transition from constant area collapse to constant pressure collapse of the monolayer is observed for all monovalent and divalent cations near total monolayer dissociation.

Structure and dynamics of water at liquid–vapour interfaces covered by surfactant monolayers of neutral stearic acid and charged stearate ions

The behaviour of water molecules at liquid–vapour interfaces with a surfactant monolayer of either stearic acid molecules or anionic stearate ions is investigated by means of molecular dynamics simulations. The density and dipolar orientational profiles and also the dynamics of translational and rotational motion of interfacial water molecules are calculated in the present work and the results are compared with the bulk liquid water and also of liquid–vapour interface of surfactant-free water. The present simulation results are also compared with available experimental results of similar interfacial systems with a monolayer of either neutral or ionic surfactants.