Langmuir Films Research Articles

Structure of graphene oxide-phospholipid monolayers: A grazing incidence X-ray diffraction and neutron and X-ray reflectivity study

HypothesisGraphene oxide-based nanotechnology has aroused a great interest due to its applications in the biomedical and optoelectronic fields. The wide use of these materials makes it necessary to study its potential toxicity associated with the inhalation of Graphene Oxide (GO) nanoparticles and its interaction with the lung surfactant. Langmuir monolayers have proven to be an excellent tool for studying the properties of the lung surfactant and the effect of intercalation of nanoparticles on its structure and properties. Therefore, to know the origin of the phospholipids/GO interaction and the structure of the lipid layer with GO, in this work we study the effect of the insertion of GO sheets on a Langmuir film of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC). ExperimentsSurface pressure-area isotherms, Neutron (NR) and X-ray Reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXD) measurements of hydrogenated and deuterated DPPC monolayers with and without GO have been carried out. FindingsThe results outline a strong interaction between the GO and the zwitterionic form of DPPC and prove that GO is in three regions of the DPPC monolayer, the aliphatic chains of DPPC, the head groups and water in the subphase. Comparison between results obtained with hydrogenated and deuterated DPPC allows concluding that both, electrostatic attractions, and dispersion forces are responsible of the interaction GO/DPPC. Results also demonstrated that the insertion of GO into the DPPC aliphatic chains does not induce significant changes on unit cell of DPPC.

Host-Guest Sensing Interactions of Calixarene-Entrapped Para-Aminobenzoic Acid Using the Langmuir Technique

Calixarenes are frequently studied as the host supramolecule in drug carrier applications. The ability to encapsulate and control drug delivery is due to their unique structures with a hydrophobic upper rim and a hydrophilic lower rim. Para-aminobenzoic acid (PABA) is a drug used as the main ingredient in sunscreen products with photoallergic contact dermatitis effects. This study has been motivated by the necessity of developing a PABA sensor even at a deficient concentration. In this work, the uses of two calixarenes, calix[4]arene (C4) and calix[6]arene (C6), in entrapping the PABA drug have been studied using the Langmuir technique. In the Langmuir-Blodgett trough, calixarenes in solution form were prepared and spread on the subphase to form the Langmuir monolayer. The formation of Langmuir films by C4 and C6 has been investigated based on the surface pressure-area ( - ) isotherm and surface potential-area ( - ) isotherm recorded using Langmuir apparatus KSV 2000 System 2. The differences and changes of the isotherms with the presence of para-aminobenzoic acid (PABA) in the subphase during the formation of Langmuir films have been observed and studied. The interactions between calixarenes and PABA have been shown by the effective dipole moment ( ) of the calixarene-PABA complexes calculated based on the results using the Helmholtz equation. Host-guest interaction shown by the sensing of PABA by calixarenes happens most probably at the lower rim of calixarenes due to the amphiphilic property of calixarenes. The findings of this study can be applied to the potential application of drug sensors using C4 and C6.

Utilization of Interfacial Films of Organo‐Modified Carbon Nanotubes as Adsorption Templates for a Number of Biomolecules while Maintaining their Activity

AbstractWe investigated the versatility of a monolayer of organo‐modified single‐walled carbon nanotubes (SWCNTs) on a water surface as a template to maintain activity by adsorbing various biomolecules from the subphase. Organo‐modified SWCNTs with amphiphilic properties formed a Langmuir film on the subphase containing biomolecules, and adsorbed and immobilized biomolecules via electrostatic interactions. In this study, the interactions of trypsin, lysozyme, cytochrome C protein, glucose oxidase sugar chain polymer, deoxyribonucleic acid, and chitosan with SWCNTs at the air/water interface were investigated. The uniqueness of this study is that it shows the possibility of wide‐ranging applications by utilizing an organo‐modified SWCNT interfacial film as an adsorption template for various biomolecules. The expansion behavior of the surface pressure‐area isotherms indicated the existence of interactions between the organo‐modified SWCNTs monolayer on the water surface and the biomolecules. The morphology and spectroscopic properties of the films transferred onto solid substrates clearly indicate the adsorption and immobilization of biomolecular groups. DNA and chitosan were not immobilized on the organo‐modified SWCNTs film template; however, the other four enzyme/protein molecules were adsorbed onto the solid substrate and maintained their fine second‐order structures, even under harsh conditions.

Optical Properties and Band Structure of Boron Nitride Langmuir Films

Purpose. Investigate the structural features of boron nitride films obtained by the Langmuir-Blodgett method. Observe fluorescence spectra and determine the band structure of the resulting coatings using optical methods.Methods. The deposition of Langmuir films was carried out using the KSV NIMA 2002 setup from the colloidal solution of ST BN/CHCl3. The study of optical properties was conducted using the SF 2000 spectrophotometer in spectral range 200 – 1100 nm and the confocal Raman microspectrometer OmegaScope AIST-NT with spectral resolution 3 cm-1. Surface morphology investigation was performed using the scanning probe microscope SmartSPM AIST-NT with standard silicon cantilevers NSA10, tip radius 7 nm. The band structure modeling of stabilized boron nitride nanoparticles was carried out using the MaterialsStudio 2020 software package with the CASTEP module.Results. The spectral characteristics of deposited film structures made of stabilized hexagonal boron nitride nanoparticles have been investigated. The hydrodynamic size of the nanoparticles was determined to be ~100 nm using optical methods, while the lateral size of the nanoparticles in Langmuir films was found to be 84.6 nm, calculated from the spectral peak at 1360 cm-1 with E2g symmetry, and 82.4 nm based on scanning probe microscopy data. Absorption and fluorescence spectra of colloidal particles were obtained, showing an unusually large Stokes shift of 105 nm and a quantum yield of 0.72. The bandgap width of the stabilized nanoparticles was measured using the Tautz method and ab-initio modeling, resulting in values of 5.79 eV and 5.46 eV, respectively. Conclusion. The study examines the surface morphology, optical properties, and band structure of the deposited Langmuir films made of stabilized boron nitride nanoparticles.

Construction of Amphiphilic Indocyanine–Green–Based Langmuir Film and Drop–Casting Film with Photoelectric Conversion Properties

Molecular self–assembly is the automatic formation of functional assemblies of different structural components through weak, reversible, non–covalent interactions on the basis of molecular recognition. Amphiphilic molecules have a natural advantage in self–assembly at the gas/liquid interface. In this work, two amphiphilic molecules with a special molecular structure, indocyanine green (ICG) and a derivative of indocyanine green (CCS), were combined with two dye molecules (tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) and nickel (II) phthalocyanine–tetrasulfonic acid tetrasodium salt (TsNiPc) for self–assembly through the Langmuir–Blodgett (LB) technique. The nanostructure and assembly behavior in ordered self–assembled films are effectively regulated by inducing dye molecules to form different types of aggregates (H– and J–aggregates). In addition, we prepared composite films containing the same functional components using the conventional drop–casting technique and performed a series of comparative experiments with LB films. The degree of hydrophilicity was found to be related to roughness, with LB composite films being flatter and denser, with the lowest roughness and the best hydrophobicity compared to drop–casting films. Notably, the LB films showed better optoelectronic properties under the same conditions, providing new clues for the application of optoelectronic functional ultrathin film devices.

Unveiling the mechanisms underlying photothermal efficiency of gold shell-isolated nanoparticles (AuSHINs) on ductal mammary carcinoma cells (BT-474)

Gold nanoparticles are valuable photothermal agents owing to their efficient photothermal conversion, photobleaching resistance, and potential surface functionalization. Herein, we combined bioinspired membranes with in vitro assays to elicit the molecular mechanisms of gold shell-isolated nanoparticles (AuSHINs) on ductal mammary carcinoma cells (BT-474). Langmuir and Langmuir-Schaefer (LS) films were handled to build biomembranes from BT-474 lipid extract. AuSHINs incorporation led to surface pressure-area (π-A) isotherms expansion, increasing membrane flexibility. Fourier-transform infrared spectroscopy (FTIR) of LS multilayers revealed electrostatic AuSHINs interaction with head portions of BT-474 lipid extract, causing lipid chain disorganization. Limited AuSHINs insertion into monolayer contributed to hydroperoxidation of the unsaturated lipids upon irradiation, consistently with the surface area increments of ca. 2.0%. In fact, membrane disruption of irradiated BT-474 cells containing AuSHINs was confirmed by confocal microscopy and LDH leakage, with greater damage at 2.2 × 1013 AuSHINs/mL. Furthermore, the decrease in nuclei dimensions indicates cell death through photoinduced damage.

Langmuir–Blodgett films of magnetic nanowires

One-dimensional magnetic nanomaterials are scientifically interesting and of great importance due to their morphology-dependent properties. As a consequence of magnetic interactions between magnetic nanowires (MNWs) their bottom-up assembly remains a tough challenge. For the first time, using the Langmuir–Blodgett (LB) technique, we prepared two-dimensional (2D) large scale LB films of MNWs, which were synthesized by template directed electrodeposition method, on glass substrates. Using a plastic pipette, MNWs dispersed in chloroform were spread on the surface of the pure water subphase in a Teflon trough. After the stabilization of the monolayer, the Langmuir film was compressed and the surface pressure-area isotherm was recorded. By pulling up the substrate through the Langmuir film, mono- and bilayers of MNWs were deposited on the substrates at the target surface pressures of 10 mN/m and 15 mN/m. The surface pressure isotherm and field emission scanning electron microscope (FE-SEM) images showed that MNWs were successfully transformed onto the substrates. FESEM micrographs showed that the assembly of MNWs was different from that of metallic nanowires. Furthermore, the results showed that rotating the substrate by 90 degrees, after the deposition of the first layer, and then depositing the second layer on it does not lead to the creation of a hierarchical structure. These results open up new opportunities for the applications of magnetic nanomaterials in physical and chemical sensors, data-storage media and magnetic switching devices. Besides, this study affords fundamental insights into the formation of magnetic monolayers and multilayers.

Spontaneous multilayer formation of an amphiphilic alkylammonium cation with a long hydrocarbon chain at an air-clay dispersion interface

Langmuir film formation behavior of a trimethylammonium cation with a long hydrocarbon chain (CH3(CH2)29N+(CH3)3, C30TMA+) was investigated. To weaken the hydrophobic interactions between the hydrocarbon chains, methanol was added to the subphases. A solution of C30TMA+·I– was spread at air-water and air-clay dispersion interfaces. The surface pressure-molecular area curves of the floating C30TMA+ films showed that the surface pressure at the collapse point decreased with increasing methanol concentration in water and in the dispersion, indicating the weakening of the hydrophobic interaction. For the floating films on clay dispersions, the molecular area at the collapse point decreased with increasing methanol concentration. The decrease in the molecular area indicates multilayer formation of C30TMA+ at the air-dispersion interface at a low surface pressure, which was confirmed by the atomic force microscope images and X-ray diffraction (XRD) patterns of the deposited films. The out-of-plane and in-plane XRD results of the multilayers showed not only layer-by-layer ordering, but also lateral ordering of the closely packed C30TMA+ cations in each layer.

Stimuli-Responsive Langmuir Films Composed of Nanoparticles Decorated with Poly(N-isopropyl acrylamide) (PNIPAM) at the Air/Water Interface.

The nanotechnology shift from static toward stimuli-responsive systems is gaining momentum. We study adaptive and responsive Langmuir films at the air/water interface to facilitate the creation of two-dimensional (2D) complex systems. We verify the possibility of controlling the assembly of relatively large entities, i.e., nanoparticles with diameter around 90 nm, by inducing conformational changes within an about 5 nm poly(N-isopropyl acrylamide) (PNIPAM) capping layer. The system performs reversible switching between uniform and nonuniform states. The densely packed and uniform state is observed at a higher temperature, i.e., opposite to most phase transitions, where more ordered phases appear at lower temperatures. The induced nanoparticles' conformational changes result in different properties of the interfacial monolayer, including various types of aggregation. The analysis of surface pressure at different temperatures and upon temperature changes, surface potential measurements, surface rheology experiments, Brewster angle microscopy (BAM), and scanning electron microscopy (SEM) observations are accompanied by calculations to discuss the principles of the nanoparticles' self-assembly. Those findings provide guidelines for designing other adaptive 2D systems, such as programable membranes or optical interfacial devices.

Spontaneous clustering of exfoliated two-dimensional materials at the air-water interface

HypothesisCoating approaches which trap nanoparticles at an interface have become popular for depositing single-layer films from nanoparticle dispersions. Past efforts concluded that concentration and aspect ratio dominate the impact on aggregation state of nanospheres and nanorods at an interface. Although few works have explored the clustering behaviour of atomically thin, two-dimensional materials, we hypothesize that nanosheet concentration is the dominant factor leading to a particular cluster structure and that this local structure impacts the quality of densified Langmuir films. ExperimentsWe systematically studied cluster structures and Langmuir film morphologies of three different nanosheets, namely chemically exfoliated molybdenum disulfide, graphene oxide and reduced graphene oxide. FindingsWe observe cluster structure transitions from island-like domains to more linear networks in all materials as dispersion concentration is reduced. Despite differences in material properties and morphologies, we obtained the same overall correlation between sheet number density (A/V) in the spreading dispersion and cluster fractal structure (df) is observed, with reduced graphene oxide sheets showing a slight delay upon transitioning into a lower-density cluster. Regardless of assembly method, we found that cluster structure impacts the attainable density of transferred Langmuir films. A two-stage clustering mechanism is supported by by considering the spreading profile of solvents and an analysis of interparticle forces at the air–water interface.

Nanoarchitectonic approaches for measuring the catalytic behavior of a membrane anchored enzyme. From Langmuir-Blodgett to a novel Langmuir-Schaefer based nanofilm building device

Self-organized lipid monolayers at the air-water interface (Langmuir films, LF) are commonly used for measuring the catalytic properties of membrane-bound enzymes. This methodology allows to provide a consistent flat topography molecular density, packing defects and thickness. The aim of the present work was to show the methodological advantages of using the horizontal transfer method (Langmuir-Schaefer) with respect to the vertical transfer method (Langmuir-Blodgett) when mounting a device to measure catalytic activity of membrane enzymes. Based on the results obtained we can conclude that it is possible to prepare stable Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films from Bovine Erythrocyte Membranes (BEM) preserving the catalytic activity of its native Acetylcholinesterase (BEA). In comparison, the LS films showed Vmax values more similar to the enzyme present in the vesicles of natural membranes. In addition, it was much easier to produce large amounts of transferred areas with the horizontal transfer methodology. It was possible to decrease the time required to mount an assay with numerous activity points, such as building activity curves as a function of substrate concentration. The present results show that LSBEM provides a proof of concept for the development of biosensors based on transferred purified membrane for the screening of new products acting on an enzyme embedded on its natural milieu. In the case of BEA, the application of these enzymatic sensors could have medical interest, providing drug screening tools for the treatment of Alzheimer's disease.

Perylene nanostructured films optimization for organic electronics: Molecular, structural, and electrochemical characterization

The deposition technique, structure, and thermal treatment can influence the optical and electrical properties of perylene derivatives films. In this work, the nanostructured Langmuir-Schaefer films of bis(n-propylimide) perylene (PTCD-Pr) with thermal treatment were optimized for an electrochemical application. The PTCD-Pr film before and after thermal treatment was characterized by ultraviolet-visible absorption, infrared absorption spectroscopy, micro-Raman scattering, and voltammetry. Brewster angle microscopy images confirmed the higher aggregation degree of molecules on Langmuir film. The PTCD-Pr deposition showed a linear growth, forming H, J, and null aggregates. The infrared absorption spectroscopy indicates a preferential head-on orientation with a slight chromophore deviation. After thermal treatment (200 °C for 2 h), the PTCD-Pr film showed a decrease in the contribution of null aggregates while maintaining the same molecular orientation. The Raman and surface-enhanced Raman spectroscopy measurements also confirmed the presence of PTCD-Pr molecules distributed throughout the substrate surface, and the thermal treatment conditions did not degrade the PTCD-Pr molecule. Moreover, thermal treatment contributes to the increase of stability of the film at the electrode surface for electrochemical measurements. In the presence of active species (paraquat in this case), the thermally treated film showed promising electrocatalytic properties compared to the unmodified electrode.

Insights on the Molecular Orientation of Oligo(p-phenylene vinylene) Derivatives with Alkyl Chains in Langmuir Films

One of the oligo(p-phenylene vinylene) derivatives having alkyl chains, OPV-1, is known to form characteristic rod-shaped nanoaggregates on the water surface, and thin films with aligned nanoaggregates can be obtained by the Langmuir–Blodgett (LB) technique. Although the molecular aggregation and orientation of the “conjugated rings part” in the LB films have already been investigated, analysis of the detailed molecular structure involving the “alkyl chains” is left behind. In the present study, to reveal the role of the alkyl chains in the molecular aggregation in Langmuir films prior to the LB transfer, infrared (IR) external reflection spectroscopic measurements on the water surface are carried out. It has been revealed that not only the conjugated rings of OPV-1 but also the alkyl chains with an ordered conformation are lying on the water surface. The characteristic orientation is found to be kept on a solid substrate even after the LB transfer by IR p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). In addition, pMAIRS was first employed for analyses of the biaxial orientation in the LB films, and the molecular orientations for the conjugated rings and the alkyl chains were quantitatively evaluated from both in-plane and out-of-plane perspectives.

Tyrosine-derived polymeric surfactant nanospheres insert cholesterol in cell membranes

HypothesisThe design of biodegradable tyrosine-derived polymeric surfactants (TyPS) through the use of calculated thermodynamic parameters could lead to phospholipid membrane surface modifiers capable of controlling cellular properties such as viability. Delivery of cholesterol by TyPS nanospheres into membrane phospholipid domains could provide further controlled modulation of membrane physical and biological properties. ExperimentCalculated Hansen solubility parameters (∂T) and hydrophile:lipophile balances (HLB) were applied to design and synthesize a small family of diblock and triblock TyPS with different hydrophobic blocks and PEG hydrophilic blocks. Self-assembled TyPS/cholesterol nanospheres were prepared in aqueous media via co-precipitation. Cholesterol loading and Langmuir film balance surface pressures of phospholipid monolayers were obtained. TyPS and TyPS/cholesterol nanosphere effects on human dermal cell viability were evaluated by cell culture using poly(ethylene glycol) (PEG) and Poloxamer 188 as controls. FindingsStable TyPS nanospheres incorporated between 1% and 5% cholesterol. Triblock TyPS formed nanosphere with dimensions significantly smaller than diblock TyPS nanospheres. In accord calculated thermodynamic parameters, cholesterol binding increased with increasing TyPS hydrophobicity. TyPS inserted into phospholipid monolayer films in a manner consistent with their thermodynamic properties and TyPS/cholesterol nanospheres delivered cholesterol into the films. Triblock TyPS/cholesterol nanospheres increased human dermal cell viability, which was indicative of potentially beneficial TyPS effects on cell membrane surface properties.

Role of Sn(II) in Mediating the Formation and Organization of a Langmuir Monolayer.

The influence of the Sn(II) ion on the formation and morphology of an arachidic acid (AA) monolayer was investigated using Langmuir film formation technology, pressure-area (Π-A) isotherm measurements, and Brewster angle microscopy (BAM). Our findings indicate that AA Langmuir monolayers exhibit organization that depends on subphase pH and Sn2+ concentration. There are multiple equilibria that are relevant to the complexation of AA monolayers, and the balance of Sn(OH)n equilibria and Sn(AA)n equilibria gives rise to unusual monolayer structural effects. With Sn2+ in the subphase, the AA monolayer exhibits an isotherm characterized by the absence of a collapse point and with a pH-dependent change in isotherm shape not consistent with the formation of an ordered solid phase. The amphiphile headgroup equilibrium mediates the absence of collapse seen experimentally and the ability of the monolayer to retain organization at a surface pressure of ca. 70 mN/m. BAM images show that the morphology of the monolayer depends on the Sn2+ concentration, consistent with several species of Sn(AA)n, where n = 1, 2, or 3, contributing to the overall monolayer order.

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

Towards predicting shear-banding instabilities in lipid monolayers

Langmuir monolayers are advantageous systems used to investigate how lipid membranes get involved in the physiology of many living structures, such as collapse phenomena in alveolar structures. Much work focuses on characterizing the pressure-bearing capacity of Langmuir films, expressed in the form of isotherm curves. These show that monolayers experience different phases during compression with an according evolution of their mechanical response, incurring into instability events when a critical stress threshold is overcome. Although well-known state equations, which establish an inverse relationship between surface pressure and area change, are able to properly describe monolayer behaviour during liquid expanded phase, the modelling of their nonlinear behaviour in the subsequent condensed region is still an open issue. In this regard, most efforts are addressed to explain out-of-plane collapse by modelling buckling and wrinkling mainly resorting to linearly elastic plate theory. However, some experiments on Langmuir monolayers also show in-plane instability phenomena leading to the formation of the so-called shear bands and, to date, no theoretical description of the onset of shear banding bifurcation in monolayers has been yet provided. For this reason, by adopting a macroscopic description, we here study material stability of the lipid monolayers and exploit an incremental approach to find the conditions that kindle shear bands. In particular, by starting from the widely assumed hypothesis that monolayers behave elastically in the solid-like region, in this work a hyperfoam hyperelastic potential is introduced as a new constitutive strategy to trace back the nonlinear response of monolayer response during densification. In this way, the obtained mechanical properties together with the adopted strain energy are successfully employed to reproduce the onset of shear banding exhibited by some lipid systems under different chemical and thermal conditions.

Use of Silica Nanoparticle Langmuir Films to Determine the Effect of Surface Roughness on the Change in the Forces between Two Silica Surfaces by a Liquid Flow.

We aimed to determine how surface roughness changes the effect of a liquid flow on the forces between two charged surfaces. This is because many applications require liquid to flow through charged confined areas and because all surfaces show a degree of roughness. We prepared films of different roughness by making mixed Langmuir films of silica nanoparticles (NPs) of two different diameters at air-100 mM NaCl aqueous interfaces and then by transferring and sintering these films to silicon wafers. Atomic force microscope (AFM) imaging showed that the film roughness decreased (1) with an NP diameter decrease and (2) an increased ratio of small NPs in a mixed film of small and larger NPs. This decrease was explained by a decreased NP aggregation in the film, due to the increased Brownian velocity that accompanies an NP diameter decrease. Force-separation curves were next measured with an AFM between a microsized silica particle (probe) and a smooth substrate (silicon wafer) or the rough NP films in 1 mM NaCl. In the absence of a liquid flow, the repulsive forces decreased with an increased substrate roughness. This reduction was explained by an increased difference between the real zero separation distance and apparent zero separation distance (the distance between the first point of mechanical contact between the probe and substrate) with an increased surface roughness. In the presence of a liquid flow, the repulsive forces decreased in the case of a smooth substrate. However, the repulsive forces were reduced less by a liquid flow for rough substrates. This result was explained by the difference in the effect of liquid flow on the diffuse layer for the smooth and rough surfaces. Surface roughness is postulated to modify the liquid flow trajectory near the surfaces and to cause ion concentration gradients near the surface. These factors are proposed to lessen the change in the diffuse layer brought about by a liquid flow. This would then reduce the change in the forces.

Reversible Dehydration-Hydration of Poly(ethylene glycol) in Langmuir Monolayers of a Diblock Copolymer Inferred from Changes in Filament Curvature.

We investigated changes in the hydration state of poly(ethylene glycol) (PEG) through morphological changes in Langmuir monolayers of a PEG-poly(l-lactide) (PlLA) (PEG-b-PlLA) diblock copolymer. When the PEG blocks were hydrated, we observed a remarkable morphology of bundles of ring-like filaments, arranged concentrically, yielding densely packed disk-like objects with a hollow center. We attribute the uniform curvature of these filaments to a strong mismatch between the molecular volumes occupied by PlLA blocks and hydrated PEG blocks. Under the constraint that each hydrated PEG block is attached to a hydrophobic PlLA block anchored to the air-water interface, this mismatch of molecular volumes caused strong repulsion within the PEG layer, in particular when the PlLA blocks packed tightly. Induced by a transition in the ordering of the PlLA blocks, the PEG blocks lost their hydration shell and packed into a dense polymer brush, accompanied by a reduction of the pressure within the PEG layer. During this packing process, the curvature of the filaments was eliminated and the ring-like filaments fractured into small linear pieces. Upon compression, the linear pieces coalesced and formed long filaments aligned in parallel. Importantly, upon expansion of the Langmuir film, these changes in morphology were reversible, and the PEG blocks could be rehydrated and bundles of concentrically arranged ring-like filaments were reformed. We conclude that the change in curvature of the filaments provides a means for distinguishing between the hydrated and dehydrated states of PEG.

Solubilization of free β-sitosterol in milk sphingomyelin and polar lipid vesicles as carriers: Structural characterization of the membranes and sphingosome morphology

High consumption of plant sterols reduces the risk of cardiovascular diseases in humans and provides health benefits. Increasing the amount of plant sterols in the diet is therefore necessary to reach the recommended daily dietary intake. However, food supplementation with free plant sterols is challenging because of their low solubility in fats and water. The objectives of this study were to investigate the capacity of milk-sphingomyelin (milk-SM) and milk polar lipids to solubilise β-sitosterol molecules in bilayer membranes organised as vesicles called sphingosomes. The thermal and structural properties of milk-SM containing bilayers composed of various amounts of β-sitosterol were examined by differential scanning calorimetry (DSC) and temperature-controlled X-ray diffraction (XRD), the molecular interactions were studied using the Langmuir film technique, the morphologies of sphingosomes and β-sitosterol crystals were observed by microscopy. We showed that the milk-SM bilayers devoid of β-sitosterol exhibited a gel to fluid Lα phase transition for Tm = 34.5 °C and formed facetted spherical sphingosomes below Tm. The solubilisation of β-sitosterol within milk-SM bilayers induced a liquid-ordered Lo phaseabove 25 %mol (1.7 %wt) β-sitosterol and a softening of the membranes leading to the formation of elongated sphingosomes. Attractive molecular interactions revealed a condensing effect of β-sitosterol on milk-SM Langmuir monolayers. Above 40 %mol (25.7 %wt) β-sitosterol, partitioning occured with the formation of β-sitosterol microcrystals in the aqueous phase. Similar results were obtained with the solubilization of β-sitosterol within milk polar lipid vesicles. For the first time, this study highlighted the efficient solubilization of free β-sitosterol within milk-SM based vesicles, which opens new market opportunities for the formulation of functional foods enriched in non-crystalline free plant sterols.