Subphase pH Research Articles

Effects of pH and Temperature on the Phase Behavior and Properties of Asphaltene Liquid Films

Crude oil asphaltenes represent a solubility fraction of petroleum that includes the most polar and most interface-active compounds. Asphaltenes have been involved in solid deposition phenomena in oilfield operations and have produced impractical conditions for oil flow in pipelines and reservoir porous rock. In addition, asphaltenes contribute to the formation of both oil-in-water and water-in-oil emulsions. Therefore, the properties of asphaltene interfacial films are crucial to defining the stabilization or breaking of crude oil emulsions. In this work, the results are presented as a contribution to understanding the elasticity and phase behavior of asphaltene liquid films with changes in subphase pH and temperature. n-Pentane-insoluble asphaltenes (C5I) were extracted from a light crude oil and deposited at the air–water interface from a dichloromethane spreading solution to form two-dimensional monolayers. The C5I films were evaluated using a Langmuir trough under compression at a constant rate. Surf...

Effects of subphase pH, temperature and ionic strength on the aggregation behavior of PnBA-b-PAA at the air/water interface

Aggregation behavior of an amphiphilic diblock polyelectrolyte poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at the air/water interface and morphologies of its LB films were characterized by the Langmuir monolayer technique and atomic force microscopy (AFM), respectively. Effects of subphase pH, temperature and ionic strength on the isotherms and hysteresis curves of the PnBA-b-PAA monolayers and the morphologies of its LB films were systematically studied. With the increase of subphase pH, the isotherms shift negatively and the quasi-plateaus disappear under neutral and alkaline conditions. Hysteresis phenomena of the PnBA-b-PAA monolayers on acidic and neutral subphases are quite obvious and similar, while the compression and expansion isotherms under alkaline condition are almost overlapped. The LB films of PnBA-b-PAA transferred from acidic subphase exhibit isolated circular micelles with large size, while those from alkaline subphase exhibit condensed ones with small size. With the rise in subphase temperature, PnBA blocks on the water surface are more likely to aggregate into large cores due to the higher molecular mobility. Furthermore, the totally ringlike nanostructures prepared from alkaline subphase with medium ionic strength are observed for the first time in LB films of block copolymers.

Dynamic Behaviors and Morphology Change of Anionic Phospholipid DPPG Monolayer Caused by Bovine Serum Albumin at Air-Water Interface

The interaction between bovine serum albumin (BSA) and the anionic 1.2-dipalmitoyl-sn-glycero-3-(phospho-rac-(1-glycerol)) (sodium salt) (DPPG) phospholipid at different sub-phase pH values was investigated at air-water interface through surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing surface pressure-mean molecular area (π-A) isotherms, the limiting molecular area in the closed packing state-the concentration of BSA (Alim-[BSA]) curves, the compressibility coefficient-surface pressure (C−1S-π) curves and the difference value of mean molecular area-the concentration of BSA (ΔA-[BSA]) curves, we obtained that the mean molecular area of DPPG monolayer became much larger when the concentration of BSA in the subphase increased at pH=3 and 5. But the isotherms had no significant change at different amount of BSA at pH=10. In addition, the amount of BSA molecules adsorbed onto the lipid monolayer reached a threshold value when [BSA]>5×10−8 mol/L for all pHs. From the surface pressure-time (π-t) data, we obtained that desorption and adsorption processes occurred at pH=3, however, there was only desorption process occurring at pH=5 and 10. These results showed that the interaction mechanism between DPPG and BSA molecules was affected by the pH of subphase. BSA molecules were adsorbed onto the DPPG monolayers mainly through the hydrophobic interaction at pH=3 and 5, and the strength of hydrophobic interaction at pH=3 was stronger than the case of pH=5. At pH=10, a weaker hydrophobic interaction and a stronger electrostatic repulsion existed between DPPG and BSA molecules. AFM images revealed that the pH of subphase and [BSA] could affect the morphology features of the monolayers, which was consistent with these curves. The study provides an important experimental basis and theoretical support to understand the interaction between lipid and BSA at the air-water interface.

Modification of hysteresis behaviors of protein monolayer and the corresponding structures with the variation of protein surface charges

Successive compression-decompression cycles of the surface pressure (π) − specific molecular area (A) isotherms of protein (BSA) monolayers show that reversible hysteresis persists if the protein molecules contain effective positive or negative surface charges. However, for neutral condition, i.e., close to the isoelectric point of the protein, irreversibility in the hysteresis behaviour dominates. Out-of-plane structures obtained from the X-ray reflectivity analysis suggest that at lower surface pressure monomolecular layer of BSA is formed on the water surface. With increasing surface pressure, molecules start to lift-up from the water surface in such a way that semi-major axis makes an angle with the water surface. Depending on the surface pressure and surface charge of BSA, monomolecular or bimolecular layer of tilted BSA molecules is formed on the water surface, however, formation of bimolecular layer is observed when the pH is relatively closer to the BSA isoelectric point. After complete decompression, tilted monomolecular or bimolecular structures again transform into monomolecular layer as evidenced from the structural analysis of the films deposited at lower surface pressures in the second compression, however, structural hysteresis varies depending upon the subphase pH or protein surface charge. Structures obtained from the films deposited at first and second compressions at lower pressure implies that although structural dissimilarity is present but structural hysteresis is only present near the isoelectric point of BSA and becomes negligible below and above that pH. Competitive electrostatic and van der Waals interactions are responsible for such hysteresis behaviours and structural modifications.

Interfacial activity of graphene oxide: Anisotropy, loading efficiency and pH-tunability

In this work, we use Langmuir film-balance techniques and high resolution Brewster angle microscopy (BAM) to investigate the surface activity of graphene oxide (GO) at the air-water interface. We focus on GO loading efficiency and packing at the air-water interface. We position two Wilhelmy plate tensiometers in different locations to simultaneously measure anisotropic forces as well as spatial inhomogenities in GO film coverage. We confirm our results with single sheet resolution images of GO at the air-water interface. Evidence is presented to show that GO does not significantly rearrange itself at the air-water interface over several hours. It is shown using a series of successive loading experiments that the GO surface loading efficiency decreases with each successive experiment. We also provide a detailed study of GO surface coverage with pH cycling of the subphase. In addition to subphase pH and GO sheet size we demonstrate the role of subphase pH history in determining GO surface coverage. Finally, we present a hypothesis to complement existing explanations of GO surface activity. This hypothesis explains the role of acidic pH in imparting surface active character to the GO and how GO is driven to the interface. We reconcile previous reports that while there is significant potential in the pH tunability of GO surface activity, there are also limitations pertaining to dynamics and size-selectivity. Our results present a comprehensive and high resolution perspective on the colloidal sheets packing of GO at the air-water interface and examples of how GO films are sensitive to preparation history.

Langmuir monolayers and Langmuir-Blodgett films of pH-sensitive lipid

Colloidal membranous vesicles based on the functionalized liposomes are promising for development of novel efficient systems of controlled and stimuli-triggered drug delivery. In particular, the specific metabolism of cancer cells results in significant acidification in tumors, and thus the corresponding pH value changes can be used as a stimulus for drug release specifically in the tumor areas. pH-sensitive liposomes can be formed via incorporation of special pH-sensitive amphiphilic compounds into the liposomal membrane. Molecules of such compounds undergo a conformational transition caused by certain changes in the pH value of aqueous environment and resulting in corresponding membrane perturbations with rapid leakage of drug content from those liposomes into the areas with corresponding acidity. In the present work the pH-induced changes of Langmuir monolayer compression isotherm and of the structure of corresponding Langmuir-Blodgett films of pH-sensitive synthetic lipid trans-4,5,didodecyloxycarbonyl-trans-2-morpholinocyclohexanol have been observed and studied. The substantial expansion of Langmuir monolayer of that lipid caused by change of the aqueous subphase pH value from 7 to 5 was observed. Corresponding monolayer Langmuir-Blodgett films were deposited on mica substrate surface and studied using AFM technique. The substantial differences in the structure of Langmuir-Blodgett films deposited at pH values 7 and 5 were found. The observed effects directly demonstrate the pH-induced structural transformations in organized condensed planar molecular systems formed by pH-sensitive lipid molecules which underlie the pH-sensitivity of corresponding liposomal bilayer membranes containing such lipids.

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.

Photo-switching of a non-ionic azobenzene amphiphile in Langmuir and Langmuir-Blodgett films.

The concept of programmable and reconfigurable soft matter has emerged in science in the last few decades and can be realized by photoisomerization of azobenzene derivatives. This possibility results in great application potential of these compounds in optical storage devices, molecular junctions of electronic devices, command layers of liquid crystal displays or holographic gratings. In this paper, we present the results of a study on the organization and isomerization of the non-ionic and amphiphilic methyl 4-[(E)-2-[4-(nonyloxy)phenyl]diazen-1-yl]benzoate (LCA) in a 2D layer architecture of Langmuir and Langmuir-Blodgett (LB) films supported by spectroscopic studies on LCA chloroform solutions. Our investigation has shown a significantly different molecular organization of LCA depending on the ratio of trans and cis isomers in the monolayers. Taking advantage of a relatively low packing density and aggregation strength in the cis-LCA monolayer, we demonstrated the reversible isomerization in the LB film initially formed of LCA molecules in the cis form, while in the trans-LCA monolayer this effect was not observed. Our approach allows the formation of a switchable monolayer made of the amphiphilic LCA showing liquid crystalline properties without introducing an ionic group into the molecule structure, mixing with another compound or changing the subphase pH to provide free space for the molecules' isomerization.

Tuning the Structure and Rheology of Polystyrene Particles at the Air-Water Interface by Varying the pH.

We form films of carboxylated polystyrene particles (C-PS) at the air-water interface and investigate the effect of subphase pH on their structure and rheology by using a suite of complementary experimental techniques. Our results suggest that electrostatic interactions drive the stability and the structural order of the films. In particular, we show that by increasing the pH of the subphase from 9 up to 13, the films exhibit a gradual transition from solid to liquidlike, which is accompanied by a loss of the long-range order (that characterizes them at lower values of pH). Direct optical visualization of the layers, scanning electron microscopy, and surface pressure isotherms indicate that the particles deposited at the interface form three-dimensional structures involving clusters, with the latter being suppressed and a quasi-2D particle configuration eventually reached at the highest pH values. Evidently, the properties of colloidal films can be tailored significantly by altering the pH of the subphase.

A ‘modified’ Langmuir–Blodgett technique for transfer of graphene oxide monolayer sheets on solid substrates

A ‘modified’ Langmuir–Blodgett (MLB) technique has been developed as a simple, low cost, energy efficient and scalable alternative to conventional Langmuir–Blodgett (LB) technique for the transfer of graphene oxide (GO) monolayer sheets from air–water interface on to solid substrates. This technique uses a simple apparatus consisting of a glass or teflon reservoir and a vertically held stationary substrate. The transfer of GO sheets from the air–water interface onto hydrophilic substrates takes place by a downward movement of the meniscus, through controlled draining of subphase. The MLB technique facilitates the transfer of well defined, uniformly distributed, morphologically flat and adherent GO sheets under a wide range of conditions, namely subphase pH in the range, 3.5–6.5, target pressure in the range, 0.5–8 mN m−1 and meniscus speed in the range, 0.5–3 mm min−1 on RCA-1 treated Si and SiO2/Si. Compared to LB technique, it offers a substantially larger tolerance for subphase and transfer conditions, even on ‘weakly’ hydrophilic (RCA-1treated) Si substrates, as neither crumpling of GO sheets nor transfer of GO agglomerates and multilayers are observed in a wide window of operation. Subsequent chemical reduction of the MLB transferred GO sheets with hydrazine vapor followed by thermal treatment at 400 °C results in reduced graphene oxide (RGO) sheets, which contain ∼70% graphitic carbon, with O/C ratio ∼0.16 and display Raman G-mode ∼1590 cm−1, together with I(D)/I(G) ratio ∼1. The electrical conductivity and field effect mobility values of RGO sheets were measured in back gated field effect geometry and are comparable to those for RGO sheets obtained by LB as well as other approaches, followed by chemical/thermal reduction. Thus the newly developed MLB technique opens a simple, versatile and scalable route to obtain device quality GO and RGO sheets on solid substrates.

Collapsed States of Langmuir Monolayers.

Langmuir monolayers of amphiphilic molecules at an air-water interface can be compressed laterally to achieve high surface density. However, compression beyond a certain threshold causes the monolayer to become unstable, which may lead to the formation of collapsed states with topographical differences that are associated with the structures and mechanical properties of the constituent molecules of the monolayer. The mechanisms and collapsed structures can differ owing to differences in experimental conditions, i.e., temperature, ion-content, the pH of subphase, or compression rate; in addition, the type of constituent molecules, i.e., biological lipids or chemical surfactants, has an effect. In this review, we compare studies concerning several aspects of collapse, from basic concepts and theoretical mechanisms to experimental visualization of the monolayer topography. In addition, techniques often employed to study this subject are discussed in this review.

Subphase pH induced monolayer to multilayer collapse of fatty acid Salt Langmuir monolayer at lower surface pressure

Abstract Subphase pH induced monolayer to multilayer collapse at very low surface pressure of barium stearate Langmuir monolayer has been confirmed from the surface pressure versus specific molecular area ( π – A ) isotherms, X-ray reflectivity (XRR) and atomic force microscopy (AFM) studies. From the π – A isotherms it is clear that stearic acid molecules form a monolayer on the water surface in the presence of Ba 2+ ions at pH ≈ 5.5 (low) and ≈6.8 (moderate) and remain as a monolayer before collapse pressure ( π c = 55 mN/m). However, at high subphase pH (≈9.5), 2D–3D structural transition starts to occur from the very low surface pressure as indicated by the π – A isotherm and is verified by both XRR and AFM as the barium stearate multilayer structure deposits in the single up stroke of hydrophilic Si substrate at π = 25 mN/m. Subphase pH induced monolayer to multilayer formation at such lower surface pressure is unusual. Fourier transform infrared spectroscopy results confirm that the formation of bidentate chelate coordination in the metal containing headgroup is the reason for such monolayer to multilayer transition. Relatively longer chain fatty acid molecules like arachidic and behenic acids do not show such type of monolayer collapse.

PH dependence of daunorubicin interactions with model DMPC:Cholesterol membranes

Mixed monolayers composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC):Cholesterol 7:3 prepared by the Langmuir-Blodgett method were used as model membranes to investigate the influence of the anticancer drug daunorubicin (DNR) on the properties of the lipid membrane. The Langmuir monolayer experiments revealed that drug – membrane interactions are pH-dependent. The changes in monolayer organization at subphases of pH 7.4 containing daunorubicin visualized by Brewster Angle Microscopy showed that in the presence of the drug the typical morphology observed for phospholipid layers containing cholesterol was no longer seen. It supports the explanation of the mechanism of the drug incorporation into the layers in terms of the competition between DNR molecules and cholesterol in the layer. Increasing surface pressure with time and increasing value of limiting surface pressure with increasing drug concentration in the subphase confirmed incorporation of the drug into the membranes. The interactions between the lipid monolayer and the drug at pH 5.4 were of electrostatic nature between the negative part of the DMPC molecule and positively charged drug, while at pH 7.4 contribution of interactions of daunorubicin with cholesterol was observed. Large differences of the surface–pressure vs. time plots were observed at both pH values when the DMPC:Cholesterol monolayer was not well organized yet. The voltammograms recorded for DMPC:Cholesterol monolayers transferred from the air-water interface onto gold electrode confirmed the presence of the drug in the lipid layer. Based on the charge of the oxidation-reduction peaks corresponding to the redox processes of quinone-hydroquinone group in daunorubicin, the initial surface concentration of the drug in the membrane and the drug release profile to the solution were evaluated.

The effect of perfluorooctadecanoic acid on a model phosphatidylcholine–peptide pulmonary lung surfactant mixture

Fluorinated surface active molecules have demonstrated potential for applications in pulmonary surfactant replacement therapy. To characterize molecular-level interactions between constituents in model replacement formulations, the mixing thermodynamics, surfactant recovery and morphology of a mixed surfactant monolayer containing a phospholipid, a perfluorinated fatty acid and a peptide-based mimetic of a pulmonary surfactant protein has been investigated at the air–water interface. The synthetic peptide (amino acid sequence KLLKLLLKLCLKLLKLLL; KLL) was added to mixed films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F) on a simplified lung fluid mimic subphase (pH 7.4, 150 mM NaCl), and compression isotherms of the films were used to extract excess Gibbs free energies of mixing, along with hysteresis (surfactant recovery) response data. It was observed that, at low peptide content, KLL mixed non-ideally with the other film components, exhibiting negative deviations from ideality. Surfactant recovery upon repeated compression-expansion cycles for ternary films that contained DPPC, C18F and KLL were comparable with mixed DPPC–C18F films, indicating that this performance parameter is only minimally improved by the addition of the peptide. Film morphologies were significantly different for the peptide-containing mixed monolayers in comparison with control samples containing DPPC alone or DPPC–C18F, with films consisting of discrete DPPC-rich domains that tended to fuse as a function of increased film compression. Mixing thermodynamics, film structures and behavior upon compression are discussed in the context of affiliated systems described previously in the literature.

Surface chemistry and spectroscopy of the β-galactosidase Langmuir monolayer

The changes of interfacial properties of β-galactosidase introduced into different pH environments are investigated through surface chemistry and in situ spectroscopy. Conditions for an optimal Langmuir monolayer formation were firstly obtained by varying the subphase salt concentration and the surface-pressure area isotherm was used to extrapolate the limiting molecular area of the enzyme monolayer to be around 42,000Å2molecule−1. Surface pressure stability measurements held at 20mN/m for 90min along with compression–decompression cycles revealed no aggregate formation at the air–water interface. Consistent with the data obtained from the isotherm, in situ UV–Vis and fluorescence spectroscopy shows a steep rise in absorbance and photoluminescence intensity correlating to with a switch from a liquid-expanded to a liquid-condensed phase. A decrease in subphase pH increased the electrostatic repulsion as the enzyme was protonated, leading to an expanded monolayer. Infrared absorption–reflection spectroscopy demonstrates that the enzyme adopts mainly β-sheet conformation at the air–water interface before and during the compression.

Effect of ionization on the behavior of n-eicosanephosphonic acid monolayers at the air/water interface. Experimental determinations and molecular dynamics simulations.

Monolayers of n-eicosanephosphonic acid, EPA, were studied using a Langmuir balance and a Brewster angle microscope at different subphase pH values to change the charge of the polar headgroups (Zav) from 0 to -2. Molecular dynamics simulations (MDS) results for |Zav| = 0, 1, and 2 were compared with the experimental ones. EPA monolayers behave as mixtures of mutually miscible species (C20H41-PO3H2, C20H41-PO3H(-), and C20H41-PO3(2-), depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |Zav|, while go from strongly interconnected by phosphonic-phosphonic hydrogen bonds (|Zav| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Zav| < 1.6) to an entirely ionic monolayer (|Zav| ≈ 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Zav| ≈ 1 fingering domains were identified. When Z ≈ 2, the headgroups are more disordered and distanced, not only in the xy plane but also in the z direction, forming a rough layer and responding to compression with a large plateau in the isotherm. The monolayers collapse behavior is consistent with the structures and domains founds in the different ionization states and their consequent in-plane rigidity: there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases. The film area in the close-packed state increases relatively slow when the polar headgroups are able to form hydrogen bonds but increases to near twice that this value when |Zav| ≈ 2. Other nanoscopic properties of monolayers were also determined by MDS. The computational results confirm the experimental findings and offer a nanoscopic perspective on the structure and interactions in the phosphonate monolayers.

Fabrication of ultrathin films containing the metal organic framework Fe-MIL-88B-NH 2 by the Langmuir–Blodgett technique

Abstract In this work, the fabrication of ultrathin films containing the metal organic framework (MOF) Fe-MIL-88B-NH 2 by the Langmuir–Blodgett (LB) technique has been explored. MOF crystals of two different sizes (1.5 ± 0.3 and 0.07 ± 0.01 μm) have been synthesized and assembled at the air–liquid interface by the LB method. The effect of the subphase pH and particle size on the film formation process has been studied. Moreover, for the first time, mixed MOF + polymer (the commercial soluble polyimide Matrimid ® ) LB films containing different MOF loadings have been fabricated. These experiments show that it is possible to obtain ultrathin MOF + polymer films with a controlled MOF density. Furthermore, MOF particles are homogeneously distributed in the polymer matrix, even with very large amounts of MOF (up to 95 wt%). LB films have been incorporated into materials of different nature, including glass and mica substrates and also polymeric membranes based on polysulfone Udel ® and PIM-1 (polymer of intrinsic microporosity), and the modification of water contact angle after LB film deposition has been analyzed.

The forces and physical properties of polymer particulate monolayers at air/aqueous interfaces

Polystyrene particles with poly[2-(dimethylamino)ethylmethacrylate] or poly[2-(diethylamino) ethylmethacrylate] hairs (PDMA-PS or PDEA-PS) formed monolayers at the air/water interface. We measured the forces between the particle monolayers and a positively charged hydrophilic particle attached to an atomic force microscope cantilever (probe particle) in the water subphase. Force–distance curves were recorded as a function of the surface pressure at different pH. In this way, we determined the effect of the particle charge, hydrophobicity and packing density on the forces. PDMA-PS and PDEA-PS particles strongly adhered to the water surface and formed stable monolayers at pH 5.8 and pH 9.0. However, the particles did not attach to the water surface strongly at pH 2.5 and therefore only formed incomplete monolayers. This result was explained by the fact that the particles were highly positively charged and hydrophilic at pH 2.5, but became less charged and therefore more hydrophobic as the subphase pH was increased to pH 5.8 and pH 9.0. The effective stiffness of the air/water interface that was determined from the force measurements decreased with a surface pressure increase (particle packing density increase) for pH 5.8 and pH 9.0. A higher particle density gave a lower stiffness, due to the lower surface tension. The adhesion between the probe and the monolayer and the effective stiffness of the monolayer increased when the subphase pH was increased from 2.5 to pH 5.8 or 9.0. This increase was explained by the increased ability of the particles to adsorb to the air/aqueous interface, which resulted from the lower charge and therefore higher hydrophobicity of the particles. Increasing the charge density of the monolayer of the partially hydrophobic particles by decreasing the subphase pH from 9.0 to 5.8 tended to increase the effective stiffness in the wrinkling phase. This result was explained by the increased electrostatic repulsion between the particulate monolayer and the probe, which caused the probe to contact the monolayer less and therefore to physically perturb the monolayer less. Increasing the hydrophobicity of the particles increased the stiffness of the monolayer, due to the stronger inter-particle hydrophobic attractions.

Dependence of interfacial film organization on lipid molecular structure.

Combination of surface analytical techniques was employed to investigate the interfacial behavior of the two designed lipids-N-stearoylglycine (1) and its bulky neutral headgroup-containing derivative N-stearoylvaline ethyl ester (2)-at the air-solution interface and as transferred layers on different substrates. Formation of monolayers at the air-water interface was monitored on pure water and on aqueous solutions of different pH. Crystallization effects were visualized at pure water by recording the hystereses in the Langmuir-Blodgett (LB) isotherms and by transferring the layers onto mica, gold (111), and ITO (indium-tin oxide on glass) electrodes. Subphase pH affects the morphology and patch formation in monolayers of 1, as evidenced by BAM measurements. At pH 8.2, formation of well-ordered crystallites is observed, which upon compression elongate according to predominantly 1-D growth mechanism to form a dense layer of crystallites. This effect is not observed in monolayers of 2, whose headgroup is not protonated. The orientation of layers of 1 transferred to the solid supports is also pH dependent, and their stability can be related to formation of a hydrogen-bonded networks. AFM images of 1 exhibited platelets of multilayer phase. The IR spectra of the ITO substrates covered by 1 indicated formation of hydrogen bonds between the amide groups. The nature of the adsorption layer and its organization as a function of potential were studied in-depth by EC STM using Au(111) as the substrate. A model showing the arrangement of hydrogen bonds between adsorbed molecules is presented and related to the observed organization of the layer.

Miscibility of Perylene Derivatives with Fatty Acid in Two-Dimensional Binary Mixture

Two component mixtures of a uorescent perylene dye (derivative of 3,4,9,10-perylenetetracarboxylic acid) with arachidic acid in the whole range of mole fraction have been investigated in form of molecular monolayers at the air water interface at di erent pH of the water subphase. Miscibility of the components in the Langmuir monolayers have been examined for pH 3 and pH 13 of the subphase. Using the Langmuir Blodgett technique the monolayers were transferred onto freshly cleaved mica sheets and morphology of the lms was analyzed using atomic force microscopy working in tapping mode. Investigation of the components miscibility in monolayers, determined by mean molecular area and surface pressure analysis reveals di erent behavior of the mixtures depending on pH of the subphase. Atomic force microscopy morphology images of the Langmuir Blodgett lms strongly support conclusion about the components mixing and coverage of the mica surface. It has been found that onto acidic subphases components are only miscible in vestigial form, whereas on alkaline the miscibility is strongly a ected by the dye concentration.