Effect of metal ions on monolayer collapses

Protein (Lysozyme) Concentration-Dependent Structure, Morphology, and Hysteresis Behavior of a Three-Component (Lysozyme-DMPA-Cholesterol) Protein-Lipid Langmuir Monolayer

Protein (lysozyme)-lipid (DMPA and cholesterol) three-component mixed films (LDC) with varied lysozyme concentration (i.e., LDC_Lx) are investigated at the air-water interface. Elastic modulus-surface pressure (Cs-1-Π) curves derived from Π-A isotherms show that mechanical behavior is strongly dependent on the monolayer composition, and for the same reason, the hysteresis behavior modifies. It is evidenced that the LDC_L0.3 monolayer (lysozyme: 0.3 mg/mL) has significant hysteresis, which is reversible in nature, while the other mixed monolayers do not show such hysteresis behavior. Morphology at the air-water interface via Brewster angle microscopy (BAM) and at the air-solid interface via atomic force microscopy (AFM) shows that the presence of protein in the LDC_Lx monolayer modifies the lateral distribution of molecules, thereby forming a stripe-like pattern at the air-water interface (in optical length scale) with barrier compression or root-like structure on the solid surface at higher Π (in micron length scale), which is not observed in the case of lipid films. Moreover, lysozyme-added LDC_Lx films show an increase in thickness with compression, which is not observed for lipid films, as evidenced from the electron density profiles (EDPs). The morphology modification and thickness variation of LDC_Lx films with compression are most probably due to the reorientation of lysozyme molecules. This structural modification in LDC_Lx films with Π, however, seems to be reversible under expansion, as can be evidenced from the similar in situ morphology observation and similar thickness of the films deposited during both first and second compression. A variation in the strength of interaction forces among film-forming molecules depending on the monolayer composition basically affects the lateral distribution and organizational orientation with surface pressure, thus ultimately influencing macroscopically the monolayer properties such as elastic, hysteresis, morphological, and structural on water and solid surfaces.

Ionic Liquid-Induced Assembly of DNA at Air-Water Interface

DNA nanotechnology is the future of many products in the pharmaceutical and cosmetic industries. Self-assembly of this negatively charged biopolymer at surfaces and interfaces is an essential step to elaborate its field of applications. In this study, the ionic liquid (IL) monolayer-assisted self-assembly of DNA macromolecules at the air-water interface has been closely monitored by employing various quantitative techniques, namely, surface pressure-area (π-A) isotherms, surface potential, interfacial rheology, and X-ray reflectivity (XRR). The π-A isotherms reveal that the IL 1,3-didecyl 3-methyl imidazolium chloride induces DNA self-assembly at the interface, leading to a thick viscoelastic film. The interfacial rheology exhibits a notable rise in the viscoelastic modulus as the surface pressure increases. The values of storage and loss moduli measured as a function of strain frequency suggest a relaxation frequency that depends on the length of the macromolecule. The XRR measurements indicate a considerable increase in DNA layer thickness at the elevated surface pressures depending on the number of base pairs of the DNA. The results are considered in terms of the electrostatic and hydrophobic interactions, allowing a quantitative conclusion about the arrangement of DNA strands underneath the monolayer of the ILs at the air-water interface.

Langmuir monolayer of lysozyme at variable subphase pH conditions: a comprehensive study on structure, morphology and hysteresis behaviour

Formation of a pure Langmuir monolayer of lysozyme at the air-water interface and its investigation by means of a surface pressure (π)-mean molecular area (A) isotherm has been accomplished under different subphase pH conditions. A normalized area-time curve confirms the stable nature of the lysozyme monolayer whose compressibility variation with an increased surface pressure at specific subphase pH has also been studied from π-A isotherms. The monolayers exhibit irreversible hysteresis behaviour irrespective of subphase pH conditions, as evidenced from successive compression-expansion π-A isotherm cycles. Comparison of surface thermodynamics under hysteresis with subphase pH variation confirms that the monolayer at subphase pH ≈ 4.0 involves a greater amount of energy to attain and retain the ordered and compact monolayer than the other two pH conditions (pH ≈ 7.0 and 9.5). In situ visualization of lysozyme monolayers by Brewster angle microscopy suggests the homogeneous and stripe-like pattern formation at lower and higher surface pressure respectively. Further investigations of lysozyme films at solid surfaces have been carried out with atomic force microscopy and X-ray reflectivity (XRR) analysis. Structural reversibility of lysozyme molecules under compression-expansion-compression of the monolayer is revealed from the comparison of height profiles of AFM images and electron density profiles as extracted from XRR analysis of the films deposited during both first and second compressions of the monolayer. The mechanism of the structural rearrangement of lysozyme molecules with surface pressure variation at different subphase pH is explored, correlating macroscopic and microscopic information.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

Spectroscopic Analysis of Cu(II)-Complexed Thin Films to Characterize Molecular-Level Interactions and Film Behavior

We report on the structure and dynamics of a Cu²⁺-complexed arachidic acid (AA) monolayer formed by Langmuir-Blodgett (LB) deposition. Infrared reflection-absorption spectroscopy (IRRAS) was used to characterize aliphatic chain -CH₂ symmetric and asymmetric stretching modes and determine the chain tilt angle and order as a function of subphase pH. Monolayer structure is controlled by metal ion-amphiphile interactions. At low subphase pH (<5), film buckling at high surface pressure is observed, while for high subphase pH (≥5), monolayer buckling is not observed. This finding is correlated to monolayer structural mediation by metal ion-amphiphile interactions. Dynamics and mobility of a fluorophore incorporated into the monolayer were also affected by Cu²⁺-AA interactions, determined by fluorescence recovery after photobleaching (FRAP) measurements. These data are consistent with the formation of a rigid film due to Cu²⁺ coordination to AA headgroups, with the extent of headgroup protonation being determined by the pH of the subphase during monolayer deposition.

Structure, morphology and reversible hysteresis nature of human serum albumin (HSA) monolayer on water surface

Compression-decompression surface pressure (π)-specific molecular area (A) isotherm cycle of human serum albumin (HSA) monolayer is performed on water surface at four different subphase pH conditions, i.e., below and above the isoelectric point (pI ≈ 4.7) of HSA molecule. For all pH conditions, the decompression curve nearly follows the compression curve, however, at pH ≈ 5.0, hysteresis is observed at higher surface pressure. Out-of-plane structures and in-plane morphologies obtained from the X-ray reflectivity and AFM studies show that only the film thickness variation takes place with the change in surface pressure, which is also evidenced from the BAM images. With increase in surface pressure, the oblate-shaped HSA molecules start tilting making an angle with the water surface and as the monolayer is decompressed the molecules regain their initial untilted monomolecular configuration. Depending upon the subphase pH and local surface charge of the specific protein molecule, electrostatic repulsive interaction dominates over the van der Waals attraction and as a result decompression curve follows the compression curve as the molecules repel each other, however, closer to the isoelectric point as strength of the interactions reverses, a hysteresis is obtained at higher surface pressure and accordingly monolayer behaviour modifies on the water surface.

Electrochemical Quantification of Corrosion Mitigation on Iron Surfaces with Gallium(III) and Zinc(II) Metallosurfactants

We have recently described a new potential use for Langmuir-Blodgett films of surfactants containing redox-inert metal ions in the inhibition of corrosion and have shown good qualitative results for both iron and aluminum surfaces. In this study we proceed to quantify electrochemically the viability of gallium(III)- and zinc(II)-containing metallosurfactants [Ga^III(L^N2O3)] (1) and [Zn^II(L^N2O2)H₂O] (2) as mitigators for iron corrosion in saline and acidic media. We evaluate their charge transfer suppression and then focus on potentiodynamic polarization and impedance spectroscopy studies, including detailed SEM data to interrogate their metal dissolution/oxygen reduction rate mitigation abilities. Both complexes show some degree of mitigation, with a more pronounced activity in saline than in acidic medium.

Spontaneous collapse of palmitic acid films on an alkaline buffer containing calcium ions

Understanding the interaction of ions with fatty acids is important to identify their roles in various bioprocesses and to build novel biomimetic systems. In this study, the molecular organization of palmitic acid (PA) films on alkaline buffer solutions (pH 7.4) with and without divalent Ca²⁺ was measured at a constant surface area using Langmuir troughs coupled with microscopy and X-ray interfacial techniques. Without Ca²⁺, PA molecules remained a monolayer organization; however, with Ca²⁺, formation of the inverted bilayers of PA-Ca²⁺ superstructures caused a spontaneous 2D to 3D transformation under no compression due to the strong interaction between PA and the divalent cation. Self-assembly of this highly-organized inverted bilayer superstructure involved a two-step process of nucleation and nuclei growth. During nucleation, densely packed PA and Ca²⁺ monolayer firstly corrugated and some of PA and Ca²⁺ molecules ejected out from the monolayer; the ejected molecules then reorganized and formed the inverted bilayer nuclei. Nucleation was followed by nuclei growth, during which PA and Ca²⁺ in the monolayer kept integrating into the inverted bilayer structure through molecule migration and PA rotation around Ca²⁺.

3D texturing of the air-water interface by biomimetic self-assembly

A simple, insoluble monolayer of fatty acid is shown to induce 3D nanotexturing of the air-water interface. This advance has been achieved through the study of monolayers of a methyl-branched long chain fatty acid, analogous to those found on the surface of hair and wool, directly at the air-water interface. Specular neutron reflectometry combined with AFM probing of deposited monolayers shows pronounced 3D surface domains, which are absent for unbranched analogues and are attributed to hydrocarbon packing constraints. The resulting surface topographies of the water far exceed the height perturbation that can be explained by the presence of capillary waves of a free liquid surface. These have hitherto been considered the only source of perturbation of the flatness of a planar water interface under gravity in the absence of topographical features from the presence of extended, globular or particulate matter. This amounts to a paradigm shift in the study of interfacial films and opens the possibility of 3D texturing of the air-water interface.

Surface Activities of a Lipid Analogue Room-Temperature Ionic Liquid and Its Effects on Phospholipid Membrane

There are great efforts of synthesizing imidazolium-based ionic liquids (ILs) for developing new antibiotics as these molecules have shown strong antibacterial activities. Compared to a single-hydrocarbon-chained IL, the lipid analogues (LAs) with two chains are more effective. In the present study, the LA molecule ^MeIm(COOH)^Me(Oleylamine)Iodide has been synthesized and its surface activities along with the effectiveness in restructuring of a model cellular membrane have been quantified. The molecule is found to be highly surface active as estimated from the area-pressure isotherm of a monolayer of the molecules formed at the air-water interface. The X-ray reflectivity (XRR) studies of a monolayer dip-coated on a hydrophilic substrate have shown the structural properties of the layer which resembles to those of unsaturated phospholipids. The LA molecules are observed to fluidize a phospholipid bilayer formed by the saturated lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). At a lower surface pressure, the lipid monolayer of DPPC has exhibited a thickening effect at a low concentration of added LA and a thinning effect at higher concentration. However, at a high surface pressure of the monolayer, the thickness is found to decrease monotonically. The in-plane pressure-dependent interaction of LA molecules with model cellular membrane and the corresponding perturbation in the structure and physical properties of the membrane may be linked to the strong lysing effect of these types of molecules.

A Molecular Approach for Mitigation of Aluminum Pitting based on Films of Zinc(II) and Gallium(III) Metallosurfactants

The use of metallosurfactants to prevent pitting corrosion of aluminum surfaces is discussed based on the behavior of the metallosurfactants [Zn^II (L^N2O2 )H₂ O] (1) and [Ga^III (L^N2O3 )] (2). These species were deposited as multilayer Langmuir-Blodgett films and characterized by IR reflection absorption spectroscopy and UV/Vis spectroscopy. Scanning electron microscopy images, potentiodynamic polarization experiments, and electrochemical impedance spectroscopy were used to assess corrosion mitigation. Both metallosurfactants demonstrate superior anticorrosion activity due to the presence of redox-inactive 3d¹⁰ metal ions that enhance the structural resistance of the ordered molecular films and limit chloride mobility and electron transfer.

Monolayer formed by l-Asp-based gemini surfactants self-assembled in 1D nanostructures

Herein, studies on the surface activities of newly synthesized l-Asp-based gemini surfactants, both nonionic and anionic, are presented. Conductometry, tensiometry, and the Langmuir-Blodgett (LB) film technique were applied for this purpose. π-A isotherms were obtained with a Langmuir trough and Wilhelmy balance. The structures of the monolayers assembled at the air/water interface and those deposited as LB films were studied via Brewster angle microscopy (BAM) and atomic force microscopy (AFM). The 2D films formed by the anion-active compounds show a well-known pattern of a monolayer film, whereas the nonionogenic amphiphiles have been found to be 1D structures with nano-widths and micro-lengths that align with each other during the process of compression; this is the first study where the organization of 1D fibrils in 2D films during compression is reported. The scanning electron microscopy (SEM) study reveals that 1D nanostructure formation is an intrinsic tendency of these molecules as not only nonionogenic surfactants, but also the anion active representatives have been constructed in the solid state by fibrillary structures.

Behaviour of protein (BSA)-lipid (DMPA) mixed monolayer on the spreading order of the individual component

Surface pressure (π) - mean molecular area (A) isotherms of protein (BSA) - lipid (DMPA) mixed films are examined by varying their ratio and altering the spreading order of BSA and DMPA on the water surface to study the protein-lipid interactions and the corresponding structures and patterns at different interfacial conditions. π-A isotherms and compression-decompression isotherm cycles of protein-lipid mixed monolayers below and above of the isoelectric point of BSA (pI ≈ 4.8) are also examined. Below the isoelectric point of BSA (pH ≈ 4.0), i.e., when BSA is weakly hydrophobic and has net positive charge shows low hysteresis irrespective of the spreading order of the molecules. However, at pH ≈ 7.0, i.e., when the overall charge of BSA is negative and is strongly hydrophobic the protein-lipid mixed films display higher hysteresis value. Besides the properties of the isotherms, the surface morphology and secondary conformations of protein inside the mixed films are obtained from X-ray reflectivity, atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy respectively after depositing the mixed films on solid substrates. Nearly similar information is obtained after altering the spreading order of BSA and DMPA, which indicates that the spreading of molecules on the water surface is one of the better ways of forming the lipid-protein mixed film at the air-water interface.

Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films

Sea spray aerosol (SSA) is highly enriched in marine-derived organic compounds during seasons of high biological productivity, and saturated fatty acids comprise one of the most abundant classes of molecules. Fatty acids and other organic compounds form a film on SSA surfaces, and SSA particle surface-area-to-volume ratios are altered during aging in the marine boundary layer (MBL). To understand SSA surface organization and its role during dynamic atmospheric conditions, an SSA proxy fatty acid film and its individual components stearic acid (SA), palmitic acid (PA), and myristic acid (MA) are studied separately using surface pressure–area ( Π − A ) isotherms and Brewster angle microscopy (BAM). The films were spread on an aqueous NaCl subphase at pH 8.2, 5.6, and 2.0 to mimic nascent to aged SSA aqueous core composition in the MBL, respectively. We show that the individual fatty acid behavior differs from that of the SSA proxy film, and at nascent SSA pH the mixture yields a monolayer with intermediate rigidity that folds upon film compression to the collapse state. Acidification causes the SSA proxy film to become more rigid and form 3D nuclei. Our results reveal film morphology alterations, which are related to SSA reflectivity, throughout various stages of SSA aging and provide a better understanding of SSA impacts on climate.

Observation of current rectification by the new bimetallic iron(iii) hydrophobe [Fe(LN4O6)] on Au|LB-molecule|Au devices

Targeting the development of stimulus-responsive molecular materials with electronic functionality, we have synthesized and studied the redox and electronic properties of a new bimetallic iron hydrophobe [FeIII2(LN4O6)] (1). The new H6LN4O6 ligand displays bicompartmental topology capable of accomodating two five-coordinate HSFeIII ions bridged by tetraaminobenzene at a close distance of ca. 8 Å. We show that the metal-based reduction processes in (1) proceed sequentially, as observed for electronically coupled metal centers. This species forms a well-defined Pockels-Langmuir film at the air-water interface, with collapse pressure of 32 mN m-1. Langmuir-Blodgett monolayers were deposited on gold substrates and used to investigate current-voltage (I-V) measurements. This unprecedented bimetallic hydrophobe [FeIII2(LN4O6)] (1) shows unquestionable molecular rectification and displays a rectification ratio RR between 2 and 15.

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.

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

Electromodified Langmuir–Blodgett films of a cobalt-based metallosurfactant support water oxidation at turnover numbers of up to 54 000 h⁻¹.

Langmuir–Blodgett films of salophen-based metallosurfactants as surface pretreatment coatings for corrosion mitigation

Langmuir–Blodgett films of metallosurfactants prevent up to 30% of acidic and saline corrosion, showing potential as surface pretreatment coatings.

Dynamics driven by lipophilic force in Langmuir monolayers: In-plane and out-of-plane growth

While monolayer area fraction versus time (A(n)-t) curves obtained from surface pressure-area (π-A) isotherms for desorption-dominated (DD) processes in Langmuir monolayers of fatty acids represent continuous loss, those from Brewster angle microscopy (BAM) also show a two-dimensional (2D) coalescence. For nucleation-dominated (ND) processes both techniques suggest competing processes, with BAM showing 2D coalescence alongside multilayer formation. π enhances both DD and ND processes with a lower cutoff for ND processes, while temperature has a lower cutoff for DD but negligible effect on ND processes. Hydrocarbon chain length has the strongest effect, causing a crossover from DD to ND dynamics. Imaging ellipsometry of horizontally transferred films onto Si(100) shows Stranski-Krastanov-like growth for ND process in an arachidic acid monolayer resulting in successive stages of monolayer, trilayer, and multilayer islands, ridges from lateral island coalescence, and shallow wavelike structures from ridge coalescence on the film surface. These studies show that lipophilic attraction between hydrocarbon chains is the driving force at all stages of long-term monolayer dynamics.

Self-assembly of long chain fatty acids: effect of a methyl branch

The morphology and molecular conformation of monolayers of straight chain and methyl-branched fatty acids have been investigated by VSFS and AFM, revealing domains in the latter case, due to inverse micellar packing constraints.

Understanding the collapse mechanism in Langmuir monolayers through polarization modulation-infrared reflection absorption spectroscopy

The collapse of films at the air-water interface is related to a type of 2D-to-3D transition that occurs when a Langmuir monolayer is compressed beyond its stability limit. Studies on this issue are extremely important because defects in ultrathin solid films can be better understood if the molecular mechanisms related to collapse processes are elucidated. This paper explores how the changes of vibration of specific groups of lipid molecules, as revealed by polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS), are affected by the monolayer collapse. Different mechanisms of collapse were studied, for those lipids that undergo constant-area collapse (such as stearic acid) and for those that undergo constant-pressure collapse (such as DPPC, DPPG, and DODAB). Lipid charges also affect the mechanism of collapse, as demonstrated for two oppositely charged lipids.

Surface pressure and elasticity of hydrophobin HFBII layers on the air-water interface: rheology versus structure detected by AFM imaging

Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the air-water interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layer's tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.

From two-dimensional to three-dimensional at the air/water interface: the self-aggregation of the acridine dye in mixed monolayers

The formation of well-defined supramolecular structures on the nanoscopic scale is a fundamental step in nanotechnology. The fine control of the layer-by-layer growth of the supramolecular assemblies at interfaces is most desirable. The collapse of a mixed monolayer composed of two surfactants in an equimolar ratio (the organic dye N-10-dodecyl acridine (DAO) and stearic acid (SA)) is analyzed herein. The collapse process of the DAO/SA mixed monolayer has been monitored using surface pressure-molecular area (π-A) and surface potential isotherms, UV-visible reflection spectroscopy, polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), Brewster angle microscopy (BAM), and synchrotron-based in situ X-ray reflectivity (XRR) measurements. The collapse of the DAO/SA mixed monolayer leads to an ordered trilayer. The growth of anisotropic 2D domains of micrometric size is observed during the formation of the trilayer, related to the ordering of the acridine polar headgroups. The trilayer is organized with the first and third monolayers displaying the polar headgroups pointing to the aqueous subphase, whereas the intermediate layer displays the polar headgroups pointing to the air. The trilayer is stabilized by the strong self-aggregation acridine dye group of the DAO molecule. The controlled transition from a monolayer to a trilayer described herein is proposed as a model for further interfacial supramolecular structures of tunable thickness comprising organic dyes.

Study of relaxation process of dipalmitoyl phosphatidylcholine monolayers at air-water interface: effect of electrostatic energy

The instability of organic monolayer composed of polar molecules at the air-water interface has been a spotlight in interface science for many decades. However, the effect of electrostatic energy contribution to the free energy in the system is still not understood. Herein, we investigate the mechanical and electrical properties by studying the isobaric relaxation process of a dipalmitoyl phosphatidylcholine monolayer on water subphase with various concentrations of divalent ions to reveal the effect of electrostatic energy on thermodynamics and kinetics of the collapse mechanism. Our results demonstrate that electrical energy among the dipolar molecules plays an important role in the stability of monolayer and enhances the formation of micelles into subphase under high pressure. In addition, to confirm the electrostatic energy contribution, the well-known thermal effect on the stability of the film is compared. Hence, the general description of the monolayer free energy with contribution of electrostatic energy is suggested to describe the phase transition.

Influence of cationic composition and pH on the formation of metal stearates at oil-water interfaces

We study the formation of layers of metal stearates at the interface between a decane solution of stearic acid and aqueous salt solutions of variable composition and pH by monitoring the evolution of their mechanical, optical, and chemical properties as a function of time after formation of the interface. For values of the pH below the pK(a) of stearic acid hardly any interfacial activity is observed. For pH > pK(a), stearic acid deprotonates at the interface and forms metal stearates, eventually leading to the formation of macroscopic solid layers. Dynamic interfacial tension measurements reveal that the process takes place in several stages, which we attribute to the successive formation of dilute and dense monolayers followed by three-dimensional growth. In the presence of divalent ions, the solid layers display a significant increase in the dilatational storage modulus. Experiments performed with an aqueous phase containing multiple cation species (artificial seawater) give rise to particularly pronounced growth of solid layers, which preferentially incorporate Ca(2+) as revealed by X-ray photoelectron and infrared spectroscopy. Our results highlight in particular the importance of the complex synergistic effects of simultaneously present monovalent and divalent cation species on the interfacial adsorption.

Crossover from layering to island formation in Langmuir-Blodgett growth: role of long-range intermolecular forces

Combined studies by atomic force microscopy, x-ray reflectivity, and Fourier transform infrared spectroscopy on transition-metal stearate (M-St, M = Mn, Co, Zn, and Cd) Langmuir-Blodgett films clearly indicate association of bidentate coordination of the metal-carboxylate head group to layer-by-layer growth as observed in MnSt and CoSt and partially in ZnSt. Crossover to islandlike growth, as observed in CdSt and ZnSt, is associated with the presence of unidentate coordination in the head group. Morphological evolutions as obtained from one, three, and nine monolayers (MLs) of M-St films are consistent with Frank van der Merwe, Stranski-Krastanov, and Volmer Weber growth modes for M=Mn/Co, Zn, and Cd, respectively, as previously assigned, and are found to vary with number (n) of metal atoms per head group, viz. n=1 (Mn/Co), n=0.75 (Zn), and n=0.5 (Cd). The parameter n is found to decide head-group coordination such that n=1.0 corresponds to bidentate and n=0.5 corresponds to unidentate coordination; the intermediate value in Zn corresponds to a mixture of both. The dependence of the growth mode on head-group structure is explained by the fact that in bidentate head groups, with the in-plane dipole moment being zero, intermolecular forces between adjacent molecules are absent and hence growth proceeds via layering. On the other hand, in unidentate head groups, the existence of a nonzero in-plane dipole moment results in the development of weak in-plane intermolecular forces between adjacent molecules causing in-plane clustering leading to islandlike growth.

Monolayer collapse regulating process of adsorption-desorption of palladium nanoparticles at fatty acid monolayers at the air-water interface

In this paper, we investigate the affinity of palladium nanoparticles, stabilized with glucose oxidase, for fatty acid monolayers at the air-water interface, exploiting the interaction between a planar system and spheroids coming from the aqueous subphase. A decrease of the monolayer collapse pressure in the second cycle of interface compression proved that the presence of the nanoparticles causes destabilization of the monolayer in a mechanism driven by the interpenetration of the enzyme into the bilayer/multilayer structure formed during collapse, which is not immediately reversible after monolayer expansion. Surface pressure and surface potential-area isotherms, as well as infrared spectroscopy [polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS)] and deposition onto solid plates as Langmuir-Blodgett (LB) films, were employed to construct a model in which the nanoparticle has a high affinity for the hydrophobic core of the structure formed after collapse, which provides a slow desorption rate from the interface after monolayer decompression. This may have important consequences on the interaction between the metallic particles and fatty acid monolayers, which implies the regulation of the multifunctional properties of the hybrid material.

Langmuir and Langmuir-Blodgett films of multifunctional, amphiphilic polyethers with cholesterol moieties

Langmuir films of multifunctional, hydrophilic polyethers containing a hydrophobic cholesterol group (Ch) were studied by surface pressure-mean molecular area (π-mmA) measurements and Brewster angle microscopy (BAM). The polyethers were either homopolymers or diblock copolymers of linear poly(glycerol) (lPG), linear poly(glyceryl glycidyl ether) (lPGG), linear poly(ethylene glycol) (lPEG), or hyperbranched poly(glycerol) (hbPG). Surface pressure measurements revealed that the homopolymers lPG and hbPG did not stay at the water surface after spreading and solvent evaporation, in contrast to lPEG. Because of the incorporation of the Ch group in the polymer structure, stable Langmuir films were formed by Ch-lPG(n), Ch-lPGG(n), and Ch-hbPG(n). The Ch-hbPG(n), Ch-lPEG(n), Ch-lPEG(n)-b-lPG(m), Ch-lPEG(n)-b-lPGG(m), and Ch-lPEG(n)-b-hbPG(m) systems showed an extended plateau region assigned to a phase transition involving the Ch groups. Typical hierarchically ordered morphologies of the LB films on hydrophilic substrates were observed for all Ch-initiated polymers. All LB films showed that Ch of the Ch-initiated homopolymers is able to crystallize. This strong tendency of self-aggregation then triggers further dewetting effects of the respective polyether entities. Fingerlike morphologies are observed for Ch-lPEG(69), since the lPEG(69) entity is able to undergo crystallization after transfer onto the silicon substrate.

A modular approach to redox-active multimetallic hydrophobes of discoid topology

A new modular [Fe(II)(Fe(III)L(2))(3)](PF(6))(2) species with discoid (disk-like) topology exhibits redox and surfactant properties and points to a new approach for multimetallic Langmuir film precursors.

Elastic Langmuir layers and membranes subjected to unidirectional compression: wrinkling and collapse

We apply the two-dimensional elastic continuum model to describe the wrinkling of elastic Langmuir layers (membranes) subjected to unidirectional compression. The effects of the dilatational, shear, and bending elasticities are taken into account. Among the numerous solutions of the generalized Laplace equation, corresponding to different membrane tensions, we determine the membrane shape as the profile that minimizes the energy of the system. In the case of small deformations, the problem can be linearized. Its solution predicts a wavelike shape of the compressed membrane. At negligibly small bending elasticity, the energy of the system is minimal for a sinusoidal profile, whose amplitude and wavelength tend to zero. In the opposite limiting case, where the effect of bending elasticity prevails over the effect of gravity, the membrane has a half-wave profile. When the two effects are comparable, the membrane shape exhibits multiple periodic wrinkles (ripples). An expression is derived for calculating the bending elasticity (rigidity) from the wavelength, and reasonable values are obtained from available experimental data. To determine the membrane shape at larger out-of-plane deformations, we solved numerically the respective nonlinear problem. Depending on the values of the physical parameters, the theory predicts various shapes: nonharmonic oscillations, toothed profiles, and profiles with two characteristic wavelengths. The results can be used for determining the bending elastic modulus of Langmuir films (membranes) as well as for the interpretation of buckling and collapse of monolayers.