Pressure Isotherms Research Articles

Effects of the ametryn pesticide on biomembrane models based on Langmuir films and giant unilamellar vesicles (GUVs)

The plasma membrane plays a crucial role in regulating various physiological processes within cells, controlling the flow of substances in and out of the cell. It primarily comprises lipids, oligosaccharides, sterols, and proteins, which are responsible for its functionalization. In this study, we utilized 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) to create mono and bilayers as mimetic systems for the lipid structure of cell plasma membranes. This approach constitutes the initial phase of comprehending the impact of contaminants on in vivo systems. Thus, the impact of the pesticide ametryn (AMT) on DPPC monolayers, which were formed using Langmuir films at the air/water interface, as well as on DPPC bilayers formed by giant unilamellar vesicles (GUVs), was investigated. The use of Brewster Angle Microscopy revealed a significant influence of AMT when in the water subphase on the morphology of DPPC aggregates in Langmuir monolayers, whose surface pressure vs. mean molecular area (π-A) isotherms are shifted to larger areas for higher AMT concentrations. Phase modulation infrared reflection-absorption spectroscopy indicated interactions between AMT and DPPC in Langmuir films, specifically involving the phosphate and choline DPPC head groups. Basically, the negatively charged hydrophilic portion of AMT (SCH3) would be oriented towards the DPPC choline group. At the same time, the AMT triazine ring would be positioned closer to the DPPC phosphate group, leading also to a slight disturb of the vertical orientation of the DPPC alkyl chains. In line with the findings from the monolayer experiments, the confocal microscopy images obtained through phase contrast revealed that the presence of AMT resulted in approximately 85 % of the DPPC GUVs losing their phase contrast. This loss can be attributed to the exchange between the inner and outer contents of the DPPC bilayer.

Detailed Study on the Interfacial Interaction between Different Polyoxometalates and Tetronic Block Copolymers Exploring the Langmuir-Blodgett Technique.

Polyoxometalates (POMs) interact with various biologically relevant entities. A basic understanding of this interaction is very important for various applications in the biological field. In this work, the focus is on the study of the interaction between tetronics and Keggin POMs. T701 and T90R4 are the two tetronics considered here; they have different solubilities in water due to different PPO/PEO ratios. The arrangement of PPO and PEO is also different with respect to the central ethylenediamine groups. Three different Keggin-type POMs, phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), with different charge densities are chosen for an elaborate investigation using Langmuir-Blodgett technique. The observation is analyzed thoroughly, which shows both electrostatic interaction and adsorption of POMs on the PPO blocks of the tetronics due to the chaotropic effect, which is responsible for the binding of POMs (in subphase) with the tetronic monolayer. This interaction results in an expanded yet rigid monolayer for POM-tetronic association on the surface. Surface pressure vs mean molecular area isotherm is the key characterization to reach the conclusion. UV-vis spectroscopy, NMR, ITC, ellipsometric studies, FTIR, and SEM also serve as supportive characterization techniques.

Effects induced by η6-p-cymene ruthenium(II) complexes on Langmuir monolayers mimicking cancer and healthy cell membranes do not correlate with their toxicity

The mechanism of chemotherapeutic action of Ru-based drugs involves plasma membrane disruption and valuable insights into this process may be gained using cell membrane models. The interactions of a series of cytotoxic η6-p-cymene ruthenium(II) complexes, [Ru(η6-p-cymene)P(3,5-C(CH3)3-C6H3)3Cl2] (1), [Ru(η6-p-cymene)P(3,5-CH3-C6H3)3Cl2] (2), [Ru(η6-p-cymene)P(4-CH3O-3,5-CH3-C6H2)3Cl2] (3), and [Ru(η6-p-cymene)P(4-CH3O-C6H4)3Cl2] (4), were examined using Langmuir monolayers as simplified healthy and cancerous outer leaflet plasma membrane models. The cancerous membrane (CM1 and CM2) models contained either 40 % 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 30 % cholesterol (Chol), 20 % 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), and 10 % 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS). Meanwhile, the healthy membrane (HM1 and HM2) models were composed of 60 % DPPC or DOPC, 30 % Chol and 10 % DPPE. The complexes affected surface pressure isotherms and decreased compressional moduli of cancerous and healthy membrane models, interacting with the monolayers headgroup and tails according to data from polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). However, the effects did not correlate with the toxicity of the complexes to cancerous and healthy cells. Multidimensional projection technique showed that the complex (1) induced significant changes in the CM1 and HM1 monolayers, though it had the lowest cytotoxicity against cancer cells and is not toxic to healthy cells. Moreover, the most toxic complexes (2) and (4) were those that least affected CM2 and HM2 monolayers. The findings here support that the ruthenium complexes interact with lipids and cholesterol in cell membrane models, and their cytotoxic activities involve a multifaceted mode of action beyond membrane disruption.

Enhancing Phototoxicity in Human Colorectal Tumor Cells Through Nanoarchitectonics for Synergistic Photothermal and Photodynamic Therapies.

Phototherapies are promising for noninvasive treatment of aggressive tumors, especially when combining heat induction and oxidative processes. Herein, we show enhanced phototoxicity of gold shell-isolated nanorods conjugated with toluidine blue-O (AuSHINRs@TBO) against human colorectal tumor cells (Caco-2) with synergic effects of photothermal (PTT) and photodynamic therapies (PDT). Mitochondrial metabolic activity tests (MTT) performed on Caco-2 cell cultures indicated a photothermal effect from AuSHINRs owing to enhanced light absorption from the localized surface plasmon resonance (LSPR). The phototoxicity against Caco-2 cells was further increased with AuSHINRs@TBO where oxidative processes, such as hydroperoxidation, were also present, leading to a cell viability reduction from 85.5 to 39.0%. The molecular-level mechanisms responsible for these effects were investigated on bioinspired tumor membranes using Langmuir monolayers of Caco-2 lipid extract. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) revealed that the AuSHINRs@TBO incorporation is due to attractive electrostatic interactions with negatively charged groups of the Caco-2 lipid extract, resulting in the expansion of surface pressure isotherms. Upon irradiation, Caco-2 lipid extract monolayers containing AuSHINRs@TBO (1:1 v/v) exhibited ca. 1.0% increase in surface area. This is attributed to the generation of reactive oxygen species (ROS) and their interaction with Caco-2 lipid extract monolayers, leading to hydroperoxide formation. The oxidative effects are facilitated by AuSHINRs@TBO penetration into the polar groups of the extract, allowing oxidative reactions with carbon chain unsaturations. These mechanisms are consistent with findings from confocal fluorescence microscopy, where the Caco-2 plasma membrane was the primary site of the cell death induction process.

Closely Packed Core-Shell Micelle Structures of Double Hydrophilic Miktoarm Star Copolymers at the Air-Water Interface.

The aggregation behavior of amphiphilic block copolymers at the air-water interface has been extensively studied, but less attention was given to that of star copolymers. In this work, we studied the interfacial aggregation behavior of two double hydrophilic pH- and temperature-responsive miktoarm star copolymers of poly[di(ethylene glycol) methyl ether methacrylate]-poly[2-(dimethylamino)ethyl methacrylate] (PDEGMA3-PDMAEMA3 and PDEGMA4-PDMAEMA7, the subscripts denote arm numbers) with different molecular weights. The effects of subphase pH and temperature on the monolayer isotherms and hysteresis curves of the two star copolymers and the morphologies of their Langmuir-Blodgett (LB) films were studied by the Langmuir film balance technique and atomic force microscopy, respectively. At the air-water interface, the two star copolymers tend to form closely packed micelles. These micelles exhibit a core-shell structure, where the small hydrophobic core consists of cross-linker of ethylene glycol dimethacrylate (EGDMA) and the carbon backbones of PDEGMA and PDMAEMA arms and the short hydrophilic shell is composed of di(ethylene glycol) and tertiary amine side groups. With increasing subphase pH, the surface pressure versus molecular area isotherms shift toward larger mean molecular areas as a result of the enhanced interface adsorption of nonprotonated tertiary amine groups. The isotherm shift of PDEGMA3-PDMAEMA3 monolayers is primarily attributed to high density of tertiary amine groups in the shells, while that of PDEGMA4-PDMAEMA7 is mainly attributed to high density of di(ethylene glycol) groups in the shells. The hysteresis degrees in the monolayers of the two copolymers under alkaline and neutral conditions are greater than those under acidic conditions due to the decreased protonation degree of the tertiary amine groups. At 10 °C, the mobility of the shells is poor and the isotherms are located on the right. Above the lower critical solution temperature, di(ethylene glycol) groups contract, which causes a slight shift of the isotherms toward smaller mean molecular areas.

Influence of Surface Roughness on Interfacial Properties of Particle Networks

The behavior of colloidal particles near fluid interfaces has attracted significant scientific interest, as particles minimize the contact area between the two fluid phases, stabilizing interfacial systems. This study explores the influence of surface roughness on the properties of particle monolayers at the air–water interface, focusing on colloidal silica particles and fumed silica particles of similar hydrodynamic diameter. This research involves comparing low-surface-area (LSA) and medium-surface-area (MSA) fumed silica particles with spherical colloidal silica particles (250 nm in diameter). Utilizing a Langmuir trough, the interfacial particle networks are compressed and expanded. Analysis of surface pressure isotherms reveals that fumed silica particle monolayers form networks at a lower particle surface coverage compared to spherical particles. The spherical particle monolayer exhibits a higher apparent surface elasticity, indicating greater resistance to the applied compression compared to fumed silica networks. Additionally, monolayers formed by fumed silica particles display hysteresis even after successive compressions and expansions due to irreversible particle interlocking and the formation of multilayered aggregates. These findings provide insights into the impact of surface roughness on the behavior of particle monolayers at fluid interfaces, offering valuable information for designing and optimizing mechanisms involved in emulsion and foam stabilization.

Virialized equation of state for warm and dense stellar plasmas in proto-neutron stars and supernova matter

ABSTRACT We present microscopic molecular dynamics simulations including the efficient Ewald sum procedure to study warm and dense stellar plasmas consisting of finite-size ion charges immerse in a relativistic neutralizing electron gas. For densities typical of Supernova matter and crust in a proto-neutron star, we select a representative single ion composition and obtain the virialized equation of state (vEoS). We scrutinize the finite-size and screening corrections to the Coulomb potential appearing in the virial coefficients B2, B3, and B4 as a function of temperature. In addition, we study the thermal heat capacity at constant volume, CV, and the generalized Mayer’s relation i.e. the difference CP − CV with CP being the heat capacity at constant pressure, obtaining clear features signalling the onset of the liquid–gas phase transition. Our findings show that microscopic simulations reproduce the discontinuity in CV, whose value lies between that of idealized gas and crystallized configurations. We study the pressure isotherms marking the boundary of the metastable region before the gaseous transition takes place. The resulting vEoS displays a behaviour where effective virial coefficients include extra density dependence showing a generalized density–temperature form. As an application we parametrize pressure as a function of density and temperature under the form of an artificial neural network showing the potential of machine learning for future regression analysis in more refined multicomponent approaches. This is of interest to size the importance of these corrections in the liquid–gas phase transition in warm and dense plasma phases contributing to the cooling behaviour of early Supernova phases and proto-neutron stars.

Effect of Water Adsorption on Lubricating Film Stability in Slippery Coatings.

The absorption of water by slippery coatings is a ubiquitous phenomenon that arises due to small but finite water dissolution during the contact of aqueous media with lubricants. In this study, using the concept of surface forces, we have analyzed the influence of trace amounts of water in lubricants on the stability of slippery coatings for both coatings with hydrophilic porous bases, prone to form hydrogen bonds with water, and those with hydrophobic porous bases. To perform such analysis, we have considered for the first time the electrostatic problem of the distribution of the electric potential and electric field strength in stratified films that contain two thin dielectric layers imitating the lubricant and a hydrophobic layer sandwiched between the porous substrate and air or water. Based on the developed approach, the equations for the calculation of the excess free energy and the disjoining pressure, associated with water dissolution in the lubricant film, were derived. An analysis of the excess film energy and the contribution of the image interaction to the disjoining pressure isotherm indicates that the dissolved water can both stabilize and destabilize the oil film, depending on the ratio of the static and dynamic dielectric permittivities of the lubricant and the phases confining the film. The results of calculations and simple experiments carried out here indicate much better water contact stability of silicone films impregnating a hydrophobized porous substrate than films impregnating a hydrophilic porous substrate. The obtained results indicate the preference for using the hydrophobic bases for the fabrication of long-lived slippery coatings characterized by better preservation of slippery functional properties under conditions of lubricant depletion and prolonged contact with water.

Dense Monolayer Network Structures of Double Hydrophilic Hyperbranched Copolymers at the Air/Water Interface.

Influences of subphase pH and temperature on the interfacial aggregation behavior of two double hydrophilic hyperbranched copolymers of poly[oligo(ethylene glycol) methacrylate-co-(2-diisopropylamino)ethyl methacrylate] (P(OEGMA-co-DIPAEMA)) at the air/water interface are studied by the Langmuir film balance technique. Morphologies of their Langmuir-Blodgett (LB) films are characterized by atomic force microscopy (AFM). At the interface, P(OEGMA-co-DIPAEMA) copolymers tend to form a dense network structure of circular micelles composed of branching agent-connected carbon backbone cores and mixed shells of OEGMA and DIPAEMA segments (pendant groups). This network structure containing many honeycomb-like holes with diameters of 6-8nm is identified for the first time and clearly observed in the enlarged AFM images of their LB films. Under acidic conditions, surface pressure versus molecular area isotherms of the two copolymers in the low-pressure region show larger mean molecular area than those under neutral and alkaline conditions due to the lack of impediment from DIPAEMA segments. Upon further compression, each isotherm exhibits a wide pseudo-plateau, which corresponds to OEGMA segments being pressed into the subphase. Furthermore, the isotherms under neutral and alkaline conditions exhibit the lower critical solution temperature behavior of OEGMA segments, and the critical temperature is lower when the hyperbranched copolymer contains higher OEGMA content.

Equation of state of partially ionized hydrogen and deuterium plasma revisited.

We present improved first-principle fermionic path integral Monte Carlo (PIMC) simulation results for a dense partially ionized hydrogen (deuterium) plasma, for temperatures in the range 15000K≤T≤400000K and densities 7×10^{-7}g/cm^{3}≤ρ_{H}≤0.085g/cm^{3} (1.4×10^{-6}g/cm^{3}≤ρ_{D}≤0.17g/cm^{3}), corresponding to 100≥r_{s}≥2, where r_{s}=r[over ¯]/a_{B} is the ratio of the mean interparticle distance to the Bohr radius. These simulations are based on the fermionic propagator PIMC (FP-PIMC) approach in the grand canonical ensemble [Filinov et al., Contrib. Plasma Phys. 61, e202100112 (2021)0863-104210.1002/ctpp.202100112] and fully account for correlation and quantum degeneracy and spin effects. For the application to hydrogen and deuterium, we develop a combination of the fourth-order factorization and the pair product ansatz for the density matrix. Moreover, we avoid the fixed node approximation that may lead to uncontrolled errors in restricted PIMC (RPIMC). Our results allow us to critically reevaluate the accuracy of the RPIMC simulations for hydrogen by Hu etal. [Phys. Rev. B 84, 224109 (2011)1098-012110.1103/PhysRevB.84.224109] and of various chemical models. The deviations are generally found to be small, but for the lowest temperature, T=15640K they reach several percent. We present detailed tables with our first principles results for the pressure and energy isotherms. We expect our updated results will serve as a valuable benchmark for comparison with other methods.

Annexin A5 stabilizes matrix vesicle-biomimetic lipid membranes: unravelling a new role of annexins in calcification

Matrix vesicles are a special class of extracellular vesicles thought to actively contribute to both physiologic and pathologic mineralization. Proteomic studies have shown that matrix vesicles possess high amounts of annexin A5, suggesting that the protein might have multiple roles at the sites of calcification. Currently, Annexin A5 is thought to promote the nucleation of apatitic minerals close to the inner leaflet of the matrix vesicles’ membrane enriched in phosphatidylserine and Ca2+. Herein, we aimed at unravelling a possible additional role of annexin A5 by investigating the ability of annexin A5 to adsorb on matrix-vesicle biomimetic liposomes and Langmuir monolayers made of dipalmitoylphosphatidylserine (DPPS) and dipalmitoylphosphatidylcholine (DPPC) in the absence and in the presence of Ca2+. Differential scanning calorimetry and dynamic light scattering measurements showed that Ca2+ at concentrations in the 0.5–2.0 mM range induced the aggregation of liposomes probably due to the formation of DPPS-enriched domains. However, annexin A5 avoided the aggregation of liposomes at Ca2+ concentrations lower than 1.0 mM. Surface pressure versus surface area isotherms showed that the adsorption of annexin A5 on the monolayers made of a mixture of DPPC and DPPS led to a reduction in the area of excess compared to the theoretical values, which confirmed that the protein favored attractive interactions among the membrane lipids. The stabilization of the lipid membranes by annexin A5 was also validated by recording the changes with time of the surface pressure. Finally, fluorescence microscopy images of lipid monolayers revealed the formation of spherical lipid-condensed domains that became unshaped and larger in the presence of annexin A5. Our data support the model that annexin A5 in matrix vesicles is recruited at the membrane sites enriched in phosphatidylserine and Ca2+ not only to contribute to the intraluminal mineral formation but also to stabilize the vesicles’ membrane and prevent its premature rupture.

Synthesis, structure and biological evaluation as antibacterial agents of Ru(II)-p-cymene-aryldicyclohexylphosphine complexes

The syntheses of two neutral complexes with formula [Ru(η6-p-cymene)(P(Cy)2Ar)Cl2], where P(Cy)2Ar = dicyclohexylphosphine, Ar = Ph (1) and 4-(N,N-dimethylamino)phenyl (4-dma-ph) (2) were performed. Both complexes were fully characterized by molar conductivity, elemental analysis and spectroscopic methods. The molecular structures of 1 and 2 that were elucidated by single crystal X-ray diffraction show that both structures adopt the pseudo-octahedral geometry characteristic of organoruthenium species. Absorption spectra and electrochemical behavior of the complexes were also evaluated and their results were supported by DFT analysis. Both complexes were active against tested bacterial strains: Gram-positive (Staphylococcus aureus) and three species Gram-negative (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa). 2 shows higher bacterial growth inhibition activity compared to 1, which might be due to its molecular structure that could facilitate the interaction among this complex and the biological environment. These results agreed with the Langmuir monolayer and molecular docking studies. 1 and 2 change the pressure isotherms of 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol, a phospholipid used as a bacterial membrane model. Molecular Docking studies performed with S. aureus and E. Coli bacteria using the Topoisomerase IV (4URN) and DNA Gyrase as receptor (1KZN), showed that both complexes have potential antimicrobial effects compared to the ciprofloxacin.

Comprehensive study of conductivity in the series of monoclinic oxymolybdates: Ln2MoO6 (Ln = Sm, Gd, Dy)

The conductivity in the monoclinic polymorphs of Ln2MoO6 (Ln = Sm, Gd, Dy) oxymolybdates was investigated by theoretical and experimental methods. A theoretical approach consisted of geometrical-topological analysis, bond valence site energy, kinetic Monte-Carlo (KMC) modeling and density functional theory (DFT) calculations. The theoretical results have shown that oxygen ionic conductivity is possible in all oxymolybdates with DFT migration energy <1.39 eV. We also calculated oxygen ionic conductivity at various temperatures (500–800 °C) using KMC modeling and found values higher than 10–3.5 S/cm at 800 °C, which was in agreement with the experimental measurements. The total conductivity achieved ~10−5 S/cm for Sm2MoO6, ~10−4 S/cm for Gd2MoO6 and ~10−3 S/cm for Dy2MoO6 at 800 °C according to impedance spectroscopy data. The oxygen pressure isotherms in the Ln2MoO6 (Ln = Sm, Gd, Dy) indicated electronic conductivity contribution in the temperature range of 500–800 °C. The experimental electromotive force (EMF) method also showed an increase in the contribution of electronic conductivity with the temperature growth. The activation energy of oxygen ionic diffusion according to EMF data was in the range of 0.78–1.27 eV.

Enhanced luminescence and electrical conductivity of polymer ultrathin films doped with TiO2 nanoparticles

The chemical polymerization method was used to prepare nanocomposite materials of polyaniline (PANi) and Titanium dioxide (TiO2) nanoparticles. Ultrathin films were deposited by using the Langmuir Blodgett (LB) Technique. Surface Pressure vs mean molecular area isotherms of nanocomposite materials showed an increase in maximum attained surface pressure with variation in nanoparticle doping concentration. Photoluminescence (PL) spectra were taken for LB multilayers deposited on a quartz substrate, an increase in the intensity of PL peaks with increasing doping concentration of TiO2 nanoparticles. The topography of LB monolayer film under atomic force microscopy (AFM) revealed that films were uniformly and densely deposited on the substrate. Phase imaging ensured the presence of nanoparticles in the polymer matrix. Conductive atomic force microscopy (C-AFM) was used to investigate the local electrical properties of nanocomposite LB films. Current-voltage characteristics showed variation in current with changing doping concentrations of TiO2 nanocomposite in PANi matrix. The work presents new insights in analyzing uniformly deposited ultrathin films of the composites which can be probed by C-AFM to reveal local electric properties by understanding topography-current correlation at the nanoscale. The conductive data analysis could be useful for understanding the current flow through the nanostructure surface.

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.

Cholesterol-Controlled Interaction of Ionic Liquids with Model Cellular Membranes.

While ionic liquids (ILs) are considered as prospective ingredients of new antimicrobial agents, it is important to understand the adverse effects of these molecules on human cells. Since cholesterol is the essential component of a human cell membrane, in the present study, the effect of an imidazolium-based IL has been investigated on the model membrane in the presence of cholesterol. The area per sphingomyelin lipid is found to reduce in the presence of the IL, which is quantified by the area-surface pressure isotherm of the lipid monolayer formed at the air-water interface. The effect is considerably diminished in the cholesterol-containing monolayer. Further, the IL is observed to decrease the rigidity of the cholesterol-free monolayer. Interestingly, the presence of cholesterol does not allow any change in this property of the layer at lower surface pressure. However, at a higher surface pressure, the IL increases the elasticity in the cholesterol-induced condensed phase of the lipid layer. The X-ray reflectivity measurement on a stack of cholesterol-free lipid bilayers proved the formation of IL-induced phase-separated domains in the matrix of a pure lipid phase. These domains are found to be formed by interdigitating the chains of the lipids, producing a thinner membrane. Such a phase is less intense in the cholesterol-containing membrane. All of these results indicate that the IL molecules may deform the cholesterol-free membrane of a bacterial cell, but the same may not be harmful to human beings as cholesterol could restrict the insertion in the cellular membrane of a human cell.

Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations.

Surfactants play essential roles in many commonplace applications and industrial processes. Although significant progress has been made over the past decades with regard to model-based predictions of the behavior of surfactants, important challenges have remained. Notably, the characteristic time scales of surfactant exchange among micelles, interfaces, and the bulk solution typically exceed the time scales currently accessible with atomistic molecular dynamics (MD) simulations. Here, we circumvent this problem by introducing a framework that combines the general thermodynamic principles of self-assembly and interfacial adsorption with atomistic MD simulations. This approach provides a full thermodynamic description based on equal chemical potentials and connects the surfactant bulk concentration, the experimental control parameter, with the surfactant surface density, the suitable control parameter in MD simulations. Self-consistency is demonstrated for the nonionic surfactant C12EO6 (hexaethylene glycol monododecyl ether) at an alkane/water interface, for which the adsorption and pressure isotherms are computed. The agreement between the simulation results and experiments is semiquantitative. A detailed analysis reveals that the used atomistic model captures well the interactions between surfactants at the interface but less so their adsorption affinities to the interface and incorporation into micelles. Based on a comparison with other recent studies that pursued similar modeling challenges, we conclude that the current atomistic models systematically overestimate the surfactant affinities to aggregates, which calls for improved models in the future.

Measuring the Adsorption of Electrolytes on Lipid Monolayers.

The interactions between ions and lipid monolayers have captivated the attention of biologists and chemists alike for almost a century. In the absence of experimentally accessible concentration profiles, the electrolyte adsorption remains the most informative quantitative characteristic of the ion-lipid interactions. However, there is no established procedure to obtain the electrolyte adsorption on spread lipid monolayers. As a result, in the literature, the ion-lipid monolayer interactions are discussed qualitatively, based on the electrolyte effect on more easily accessible variables, e.g., surface tension. In this letter, we demonstrate how the electrolyte adsorption on lipid monolayers can be obtained experimentally. The procedure requires combining surface pressure versus molecular area compression isotherms with spreading pressure data. For the first time, we report an adsorption isotherm of NaCl on a lipid monolayer as a function of the density of the monolayer. The leading interactions seem to be the osmotic effect from the lipid head groups in the surface layer and ion-lipid association.

STABILITY OF SOME SILICONE LUBRICATING INTERLAYERS IN LIQUID-INFUSED COATINGS

One of the most actively developing research areas in materials science relevant to polyfunctional coatings is the creation of slippery liquid-infused porous surfaces (SLIPS) based on porous hydrophobic or hydrophilic materials filled with low-volatility viscous liquids (lubricants). In the present work, we have investigated the possibility of using two organosilicon liquids of different polarities, bis(trifluoromethylsulfonyl)imide dicationic ionic liquid and silicone oil, as lubricants for fabricating slippery coatings that reduce the adhesion of solid and liquid aqueous precipitations to aluminium oxide substrates. To calculate the stability of the films of such lubricants, we have employed the theory of van der Waals forces to study the refractive index dispersions and the dielectric properties of the liquids in the region of microwave relaxation. On the basis of experimentally obtained data, the dielectric permittivity spectra have been calculated as functions of imaginary frequency for the entire spectral range, as well as the contribution of the van der Waals forces to the stability of the disjoining pressure isotherms of the lubricant films on the hydrophobic and hydrophilic aluminium oxide substrates. The disjoining pressure isotherms obtained in this work have indicated that the ionic liquid used to prepare slippery coatings is a more durable lubricant than silicone oil, because its films retain their stability when the vapor phase is replaced by an aqueous medium over a wider range of thickness.

Incorporation of acridine orange and methylene blue in Langmuir monolayers mimicking releasing nanostructures

The efficiency of methylene blue (MB) and acridine orange (AO) for photodynamic therapy (PDT) is increased if encapsulated in liposomes. In this paper we determine the molecular-level interactions between MB or AO and mixed monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DPPG) and cholesterol (CHOL) using surface pressure isotherms and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). To increase liposome stability, the effects from adding the surfactants Span® 80 and sodium cholate were also studied. Both MB and AO induce an expansion in the mixed monolayer, but this expansion is less significant in the presence of either Span® 80 or sodium cholate. The action of AO and MB occurred via coupling with phosphate groups of DPPC or DPPG. However, the levels of chain ordering and hydration of carbonyl and phosphate in headgroups depended on the photosensitizer and on the presence of Span® 80 or sodium cholate. From the PM-IRRAS spectra, we inferred that incorporation of MB and AO increased hydration of the monolayer headgroup, except for the case of the monolayer containing sodium cholate. This variability in behaviour offers an opportunity to tune the incorporation of AO and MB into liposomes which could be exploited in the release necessary for PDT.