Increase In Surface Pressure Research Articles

Effect of carboxymethylcellulose on the affinity between lysozyme and liposome monolayers:evidence for its bacteriostatic mechanism

Lysozyme (Ly), as an important natural preservative, easily interacts with anionic polysaccharides in food, which results in low and unstable bacteriostatic activity. The interaction of carboxymethylcellulose (CMC) with Ly was used to investigate the mechanism of the bioactivity changes caused by Ly binding polysaccharides. The affinity of Ly or Ly-CMC complexes for a mixed monolayer liposome (Lip) as an in vitro biomembrane model was characterized by using dynamic light scattering, Fourier transform infrared spectroscopy, circular dichroism spectroscopy, fluorescence analysis and interface activity measurements. The results showed that the affinity between Ly and Lip mainly depended on hydrophobic and electrostatic forces. The electrostatic interaction in Ly-CMC weakened the Ly-Lip affinity and altered the spatial conformation of Ly molecules. Furthermore, CMC screened the active sites of Ly and neutralized the charge of Ly, which may reduce its bioactivity. The presence of CMC decreased the increase in surface pressure in the Lip-Ly interface, which means that Ly-CMC has a lower insertion capacity and ability to induce lipid molecule rearrangement than Ly alone, indicating an inferior bacteriostatic ability. Our work explains how the bacteriostatic mechanism of Ly is affected by food biopolymers.

Trends in and closure of the atmospheric angular momentum budget in the 20th century in ERA‐20C

It is well known that global warming in the 20th century has influenced the global circulation of the atmosphere. Atmospheric angular momentum (AAM), a measure of the rotation of the atmosphere around the Earth's axis, is a useful quantity to investigate changes in the global atmospheric circulation. In this study, 20th century trends in the AAM budget are determined using the ERA‐20C reanalysis data of the European Centre for Medium‐Range Weather Forecasts (ECMWF). In addition, the closure of the AAM budget is determined to assess the ability of ERA‐20C to conserve angular momentum. The total AAM has increased in the 20th century, associated mainly with an increasing relative (zonal wind) AAM in most of the stratosphere and the tropical upper troposphere, and a poleward redistribution in the midlatitudes. These trends can be related to the warming in the troposphere and cooling in the lower stratosphere found in this study, likely caused by increasing atmospheric CO2 concentrations. The Ω‐AAM, representing the rotation of the atmosphere along with the Earth, shows no clear trend, but a spurious peak around 1920. This peak is caused by a global increase in surface pressure and is considered an artefact of changes in the amount of assimilated observations. It is also found that the AAM budget is not well closed in ERA‐20C, which is mainly the result of the assimilation of observations during production of the reanalysis. The trends in the AAM budget in ERA‐20C are likely affected by changes in the number of assimilated observations and should be validated with other reanalyses in further research.

PH-dependent structure, pattern and hysteresis behaviour of lipid (DMPA)-protein (BSA) monolayer complex

Understanding lipid-protein interactions in model membranes is a challenging task. Limited information exist to-date regarding the relative influence of hydrophobic and electrostatic forces on the organization of proteins inside model membranes, while these forces determine the structure of lipid-protein complexes. We measured the surface pressure (π) - molecular area (A) isotherm cycles of protein (BSA) – lipid (DMPA) mixed monolayers below and above the isoelectric point of BSA (≈ 4.8). At pH ≈ 4.0, below the isoelectric point, BSA is positively charged, and exposes few hydrophobic groups at its surface, compression-decompression isotherms show a nearly reversible hysteresis. At pH ≈ 7.0, above the isoelectric point, BSA is negatively charged and more hydrophobic. At this pH, compression-decompression isotherms show an irreversible hysteresis. This behaviour indicates that the deformation of BSA molecules under pressure is reversible below the isoelectric point, while it becomes irreversible above it. X-ray reflectivity studies for protein-lipid mixed monolayers show that BSA molecules move from the zone close to water and near the lipid polar heads toward the zone occupied by their hydrocarbon tails s when surface pressure increases. Mostly the surface pressure in combinations with hydrophobic and electrostatic interactions is responsible for such structural modifications.

Circulation Factors for the Current Low Water Level within the Lake Baikal Drainage Basin

We examined the relationships between the dynamics of surface inflow into Lake Baikal and the runoff of the main tributaries of the Baikal drainage basin and regional and global circulation parameters to determine the role of certain of the meteorological factors in the current low water level within the Baikal catchment area. A decrease or an increase in the runoff of Baikal’s rivers in the summer months depends on the synoptic situation in the south of East Siberia and in the north of Mongolia where southern cyclones provide the main influx of moisture to the Baikal region. We used different indices as climatic and circulation factors which characterize the specific features in the forms of atmospheric circulation in high and low latitudes of the northern hemisphere (NAO, AO, SCAND, and others) as well as the anomalies of mean monthly values of surface pressure and geopotential heights of isobaric surface AT-500 in the zone of 43–50°N and 90–115°E. It was found that the low water level within the Lake Baikal drainage basin has persisted since 1996 but it has manifested itself particularly clearly due to the lake stage reduction. In the south of Siberia and in the north of Mongolia, since the early 21st century there has been an enhancement in anticyclogenesis processes accompanied by an increase in air temperature, surface pressure and geopotential altitudes at the level of the mid-troposphere (5 km). The more favorable conditions for river runoff formation are created during the development of the ridge of elevated and low pressure over Ural and the associated ridge of low pressure over Siberia, with cyclogenesis processes occurring in front of them over the territory of Mongolia and the south of East Siberia at the Earth’s surface. Elevated precipitation and runoff are observed at the time of blocking anticyclones over Transbaikalia and the Far East, which are responsible for a long-lasting persistence of Mongolian cyclones.

Close-Packed Langmuir Monolayers of Saccharide-Based Carbon Dots at the Air-Subphase Interface.

Carbon dots (CDs) are zero-dimensional carbon-based spherical nanoparticles with diameters less than 10 nm. Here, we report for the first time CDs forming stable Langmuir monolayers at the air-subphase interface. Langmuir monolayers are of great interest both fundamentally to study the interactions at the interfaces and for many applications such as the development of sensors. However, CDs usually do not form Langmuir monolayers because of their highly hydrophilic nature. In this study, amphiphilic CDs were prepared through hydrothermal carbonization using saccharides as the precursors. The surface chemistry behavior and optical properties of CDs at the air-subphase interface were studied. CDs derived from saccharides consistently formed stable Langmuir monolayers which show all essential phases, namely, gas, liquid-expanded, liquid-condensed, and solid phases. The compression-decompression cycle method showed minimum hysteresis (4.3%), confirming theretaining capacity of the CDs as a monolayer. Limiting CD areas from surface pressure-area isotherm at the air-subphase interface were used to calculate the average diameter of the CDs at the air-subphase interface. UV/vis absorption spectra of CDs dispersed in water and in Langmuir monolayers had the same bands in the UV region. The intensity of the UV/vis absorption increases with increasing surface pressure at the air-subphase interface. Interestingly, photoluminescence (PL) of the Langmuir monolayer of CDs was excitation-independent, whereas the same CDs had excitation-dependent PL when dispersed in water.

Oil-detachment from the calcium carbonate surfaces via the actions of surfactant, nanoparticle and low salinity brine: An insight from molecular dynamic simulation

The molecular dynamic simulation of oil detachment from the calcium carbonate solid surface employing nanoparticles, an ionic surfactant and low salinity brine at petroleum reservoir operating conditions is investigated in this work. All molecular, atomic and ionic force fields and fluids-rock surface interactions are rigorously simulated in the present work. The simulation results indicate that both nanoparticle sand surfactant are adsorbed at the interface between oil and aqueous brine solution such that a reduction of interfacial tension and an increase in total surface pressure lead to the oil detachment. Synergetic effect of nanoparticle, surfactant and ionic species on the alteration of rock surface wettability from oil-wet to water condition is also demonstrated by the proposed approach. The proposed molecular simulation is quite useful for assessment of the performance of complex injecting fluids regarding the oil detachment from calcium carbonate surface at reservoir operating conditions.

Effects of Cu(II) on the Formation and Orientation of an Arachidic Acid Langmuir-Blodgett Film.

The influence of the copper(II) ion on the formation, morphology, and organization of an arachidic acid monolayer was investigated using Langmuir-Blodgett (LB) monolayers, Π-A isotherms, and Brewster angle microscopy (BAM). Our findings indicate that a Cu2+-complexed LB film exhibits an order that depends on the subphase pH, analogous to other metal ions. Yazdanian , M. ; et al. Ionic Interactions of Fatty Acid Monolayers at the Air-Water Interface . Langmuir 1990 , 6 , 1093 - 1098 . Kurnaz , M. L. ; et al. Morphology of Microphase Separation in Arachidic Acid-Cadmium Arachidate Langmuir-Blodgett Multilayers . J. Phys. Chem. 1996 , 100 , 11113 - 11119 . The metal ion facilitates the formation of solid-phase films at surface pressures as low as 5 mN/m. The films exhibit a rigid, ordered phase, evidenced by the absence of a collapse point and an increase in surface pressure rather than the typical sharp decrease in surface pressure, indicative of film failure. Amphiphile ionic charge vs pH (i.e., the extent of arachidic acid protonation) plays a role in the observed absence of collapse and the ability of the films to maintain order and cohesion at high surface pressures (ca. 65 mN/m). Additionally, film thickness data suggest that the incorporation of Cu2+ ions induces a change in orientation of the aliphatic chains of the amphiphiles and that amphiphile solubility in the subphase may play a role in the observed film behavior at low surface areas and high pH.

Fabricating and Examining of Langmuir Films of Reduced Graphene Oxide

The effect the transfer pressure has on structural, optical, and electrophysical properties of Langmuir-Blodgett (LB) films of reduced graphene oxide (rGO) is studied. Investigation of physicochemical properties of rGO monolayers on the subphase surface shows that the rGO monolayer on the subphase surface predominantly exists in the liquid state. Increased surface pressure in the film makes rGO particles lie closer to and contact one another, followed by the overlapping and crumpling of some film sheets. Electron microscope studies show that as monolayers are transferred onto solid substrates, rGO forms uniform island-type films with clearly discernible clusters whose density grows along with surface pressure. The synthesized rGO films are highly transparent (87–96%) in the visible spectral region. The LB films transferred onto solid substrates at a lower surface pressure exhibit the best electrophysical and electroconductive properties. The reported findings can be used in developing technology for synthesizing transparent nanosized films from reduced graphene oxide and its derivatives for use in molecular electronics and photovoltaics.

Aerosol Impacts on Meteorological Elements and Surface Energy Budget over an Urban Cluster Region in the Yangtze River Delta

ABSTRACT The Yangtze River Delta (YRD) is a typical example of regions that are dramatically influenced by both human activity and a monsoonal climate. Data on the near-surface micro-meteorology, radiation, and energy fluxes were collected and analyzed at the Lishui field observation site in the suburb of Nanjing to investigate aerosol impacts on the radiation budget and land surface-atmosphere heat, water, and mass exchanges. Cluster analysis, composite analysis, and case study were applied to selected typical polluted/non-polluted days. The results indicate that the mean daily surface pressure is 6.6 hPa lower on polluted than non-polluted days in Nanjing. Northeasterly winds often prevail on the polluted days, with wind speeds being 60% lower than on non-polluted days. Aerosols directly reduce the net radiation flux at the surface, with a maximum reduction of 180 W m–2. During the early stage of air pollution events, the surface pressure is lower, and wind speeds rapidly decrease, whereas during the peak of pollution, low surface pressure and wind speeds linger, effectively preventing the dispersion of air pollutants. Meanwhile, the temperature often decreases, and the relative humidity subsequently increases. As the wind speed and surface pressure increase, the AQI gradually decreases, and the air pollution event ends.

Effect of synthetic surfactants and soapwort (Saponaria officinalis L.) extract on skin-mimetic model lipid monolayers

The effect of a saponin-rich extract from rhizomes of Soapwort (Saponaria officinalis L) and four synthetic surfactants: sodium lauryl sulphate (SLS), sodium laureth sulphate (SLES), ammonium lauryl sulphate (ALS) and cocamidopropyl betaine (CAPB) on two model lipid monolayers is analyzed using surface pressure, surface dilatational rheology and fluorescence microscopy. The following monolayers were employed: dipalmitoylphosphatidylcholine/cholesterol mixture in a molar ratio of 7:3 (DPPC/CHOL) and Ceramide [AP]/stearic acid/cholesterol in a molar ratio of 14:14:10 (CER/SA/CHOL). They mimicked a general bilayer structure and an intercellular lipid mixture, respectively. Both lipid mixtures on Milli-Q water were first compressed to the initial surface pressure, Π0 = 30 mN/m and then the subphase was exchanged with the respective (bio)surfactant solution at 1% (w/w). All four synthetic surfactants behaved in a similar way: they increased surface pressure to about 40 mN/m and reduced the storage modulus of surface dilational surface rheology, E′, to the values close to zero. The corresponding fluorescence microscopy pictures confirmed that the lipids mimicking the stratum corneum components were almost completely removed by the synthetic surfactants under the present experimental conditions. The components of the Soapwort extract (SAP) increased surface pressure to significantly higher values than the synthetic surfactants, but even more spectacular increase was observed for the storage modulus of the SAP-penetrated lipid monolayers (up to E′= 715 mN/m).

Effect of saponins from quinoa on a skin-mimetic lipid monolayer containing cholesterol

The study discusses the effect of a quinoa seed coat extract on a cholesterol-based Langmuir monolayer mimicking the intercellular lipid mixture in the skin’s outermost layer - stratum corneum. Besides cholesterol (CHOL), the monolayer contains also stearic acid (SA) and ceramide VI (CER), in a molar ratio of 10:14:14. Three quinoa extracts were tested for their surface activity: a) from the whole seed, b) from the dehulled seed, and c) from the seed coat. The latter shows significantly higher ability to reduce surface tension (increase surface pressure) than the others. Its adsorbed layers display also reasonable surface dilational elasticity (storage) modulus, E′. These observations are in line with the literature reports on the high concentrations of triterpenoid glycosidic biosurfactants – saponins, in quinoa seed, especially in its coat. The saponin-rich extract of quinoa seed coat was thus introduced underneath the pre-formed lipid monolayer compressed to surface pressure, Π = 30 mN/m in a Langmuir trough, in order to register the surface pressure response. The increase of both the surface pressure and surface dilational elasticity modulus suggests that saponins, and possibly other surface-active components of the extract, incorporate into the model lipid monolayer, without solubilizing it. This opens new perspectives for the saponin-rich quinoa seed extract as skin penetration-enhancing active components for cosmetics or pharmaceutical purposes.

Modified F-criterion and its influence on potential crack deflection range

With the development of interface-rich composites, crack penetration/deflection at the interfaces have been the focus of considerable research studies. Because the F-criterion may imperceptibly enlarge the potential crack deflection range, a modification was made and related numerical analyses were then conducted. It was found that the modified F-criterion is as effective as the F-criterion and not affected by the fracture toughness ratio of mode II to mode I, when predicting the crack penetration direction. Furthermore, based on the modified F-criterion, the potential crack deflection range gradually shrinks, as the friction coefficient on crack surfaces and the fracture toughness ratio of mode II to mode I increase. However, as the surface pressure increases, the potential crack deflection range gradually broadens. In general, the result that is based on the modified F-criterion is no more than the one that is based on the F-criterion, when predicting the potential crack deflection range. Additionally, in accordance with the modified F-criterion, crack deflection caused by shear failure occurs only when the mode II fracture toughness is similar to the mode I fracture toughness. Otherwise, tensile failure is the exclusive reason for crack deflection.

Diminishing biofilm resistance to antimicrobial nanomaterials through electrolyte screening of electrostatic interactions.

The extracellular polymer substances (EPS) generated by biofilms confers resistance to antimicrobial agents through electrostatic and steric interactions that hinder molecular diffusion. This resistance mechanism is particularly evident for antibacterial nanomaterials, which inherently diffuse more slowly compared to small organic antibacterial agents. The aim of this study was to determine if a biofilm's resistance to antibacterial nanomaterial diffusion could be diminished using electrolytes to screen the EPS's electrostatic interactions. Anionic (+) alpha-tocopherol phosphate (α-TP) liposomes were used as the antimicrobial nanomaterials in the study. They self-assembled into 700 nm sized structures with a zeta potential of -20 mV that were capable of killing oral bacteria (S. oralis growth inhibition time of 3.34 ± 0.52 h). In a phosphate (-ve) buffer the -ve α-TP liposomes did not penetrate multispecies oral biofilms, but in a Tris (hydroxymethyl)aminomethane (+ve) buffer they did (depth - 12.4 ± 3.6 μm). The Tris did not modify the surface charge of the α-TP nanomaterials, rather it facilitated the α-TP-biofilm interactions through electrolyte screening (Langmuir modelled surface pressure increase of 2.7 ± 1.8 mN/ m). This data indicated that EPS resistance was mediated through charge repulsion and that this effect could be diminished through the co-administration of cationic electrolytes.

Molecular-Level Structure and Packing in Phase-Separated Arachidic Acid-Perfluorotetradecanoic Acid Monolayer Films.

Synchrotron-based X-ray scattering measurements of phase-separated surfactant monolayers at the air-water interface provide molecular-level structural information about the packing and ordering of film components. In this work, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements were used to collect crystallographic structural information for binary mixed monolayers of arachidic acid (AA, C19H39COOH) with perfluorotetradecanoic acid (PA, C13F27COOH), a system that has previously been investigated using a variety of thermodynamic and micron-scale structural characterization methods. GIXD measurements at surface pressures of π = 5, 15, and 30 mN/m indicated that AA in pure and mixed films forms a rectangular lattice at π = 5 and 15 mN/m but a hexagonal lattice at π = 30 mN/m. PA formed hexagonal lattices under all conditions, with films being highly ordered and crystalline (as determined by Bragg peak width) at even the lowest surface pressures investigated. Phase separation occurred for all mixed monolayer film compositions and surface pressures, manifesting as diffraction peaks characteristic of the individual components appearing at different in-plane scattering vector qxy. For both pure and mixed films, the molecular tilt angle of the AA hydrocarbon chain toward the nearest-neighbor was substantial at low pressures but decreased with increasing pressure. The PA fluorocarbon chain showed negligible molecular tilt under all conditions, and was oriented normal to the subphase surface regardless of surface pressure or the presence of AA in the films. In all cases, the two components in the mixed film behaved entirely independently of film composition, which is exactly the expected result for a fully phase-separated, immiscible system. XR measurements of film thickness at the air-water interface supported these results; overall film thickness approached the calculated ideal surfactant tail lengths with increasing surface pressure, indicating nearly normal oriented surfactants. The overall surfactant packing and crystallographic features of the mixed monolayers are discussed in terms of the lipophobic nature of the perfluorinated surfactant as well as in context of thermodynamic miscibility and domain structure formation reported elsewhere in the literature for these mixed monolayer systems.

High ability of a blue phase polymer based on a diacetylene alcohol derivative

The effect of prolonged UV irradiation and high temperature on a blue phase polymer film prepared by the Langmuir–Schaefer method based on 11-hydroxyundeca-6,8-diyn-1-yl N-(4-methoxyphenyl)carbamate was evaluated. The high stability of the polymer under extreme external influences was revealed. The transition of a monolayer to a bilayer with increasing surface pressure during the Langmuir layer formation was confirmed by atomic force microscopy.

Zonally Varying ITCZs in a Matsuno‐Gill‐Type Model With an Idealized Bjerknes Feedback

AbstractIn the present climate, tropical rain bands exhibit a bifurcated pattern, continuously forming along single intertropical convergence zones (ITCZs) in some regions, and along double ITCZs that straddle the equator in other regions. This bifurcated ITCZ pattern is studied in a Matsuno‐Gill‐type model forced by relaxation to zonally asymmetric surface pressure. The model includes an idealized Bjerknes feedback which couples surface winds and sea surface temperatures (SSTs) via oceanic Ekman balance. Consistent with observations, solutions in the limit of strong damping are explored. Two ITCZ bifurcation mechanisms are identified. First, in the viscous limit, ITCZs form along negative anomalies of the local Rossby number, which tend to occur near the equator for equatorial low pressure and off the equator for equatorial high pressure; this leads to a single ITCZ in the rising branch of zonal‐overturning circulations and a double ITCZ that straddles the equator in the descending branch. Second, ocean upwelling produces an equatorial cold tongue with increased surface pressure, which reduces vertical winds and can lead to precipitation peaks that straddle the equator in regions of near‐equator ascent. Consistent with observations, the cold tongue intensifies with increasing zonal SST gradients, and its base widens with weakened poleward SST gradients, modulating the zonal orientation of the ITCZs on either side of the cold tongue. Analytic approximate solutions in the viscous limit capture the emergence of the bifurcated ITCZ pattern, as well as the dependence of the bifurcated ITCZ pattern on zonal and poleward SST gradients

The rheology of polyvinylpyrrolidone-coated silica nanoparticles positioned at an air-aqueous interface

Particle-stabilized emulsions and foams are widely encountered, as such there remains a concerted effort to better understand the relationship between the particle network structure surrounding droplets and bubbles, and the rheology of the particle-stabilized interface. Poly(vinylpyrrolidone)-coated silica nanoparticles were used to stabilize foams. The shear rheology of planar particle-laden interfaces were measured using an interfacial shear rheometer and the rheological properties measured as a function of the sub-phase electrolyte concentration and surface pressure. All particle-laden interfaces exhibited a liquid-like to solid-like transition with increasing surface pressure. The surface pressure-dependent interfacial rheology was then correlated to the formed micron-scale structures of the particle-laden interfaces which were imaged using a Brewster angle microscope. With the baseline knowledge established, foams were prepared using the same composite particles and the particle network structure imaged using cryo-SEM. An attempt has been made to correlate the two structures observed at a planar interface and that surrounding a bubble to elucidate the likely rheology of the bubble stabilizing particle network. Independent of the sub-phase electrolyte concentration, the resulting rheology of the bubble stabilizing particle network was strongly elastic and appeared to be in a compression state at the region of the L-S phase transition.

Interaction of Myelin Basic Protein with Myelin-like Lipid Monolayers at Air-Water Interface.

Interaction of myelin basic protein (MBP) and the cytoplasmic leaflets of the oligodendrocyte membrane is essential for the formation and compaction of the myelin sheath of the central nervous system and is altered aberrantly and implicated in the pathogenesis of neurodegenerative diseases like multiple sclerosis. To gain more detailed insights into this interaction, the adsorption of MBP to model lipid monolayers of similar composition to the myelin of the central nervous system was studied at the air-water interface with monolayer adsorption experiments. Measuring the surface pressure and the related maximum insertion pressure of MBP for different myelin-like lipid monolayers provided information about the specific role of each of the single lipids in the myelin. Depending on the ratio of negatively charged lipids to uncharged lipids and the distance between charges, the adsorption process was found to be determined by two counteracting effects: (i) protein incorporation, resulting in an increasing surface pressure and (ii) lipid condensation due to electrostatic interaction between the positively charged protein and negatively charged lipids, resulting in a decreasing surface pressure. Although electrostatic interactions led to high insertion pressures, the associated lipid condensation lowered the fluidity of the myelin-like monolayer.

Air-Water Interfacial Properties of Chloroform-Spread versus Water-Spread Poly((d,l-lactic acid- co-glycolic acid)- block-ethylene glycol) (PLGA-PEG) Polymers.

Polymers at fluid interfaces are used for a number of applications that include coatings, electronics, separation, energy, cosmetics, and medicines. Here, we present a study on an amphiphilic block copolymer, poly((d,l-lactic acid- co-glycolic acid)- block-ethylene glycol) (PLGA-PEG), at the air-water interface. PLGA-PEG at the air-water interface prepared by using an organic spreading solvent exhibits an extremely high surface pressure without the occurrence of desorption, making it an attractive candidate for a variety of uses in the areas mentioned above. The origin of this high surface pressure increase was shown to be due to the glass transition of the PLGA segments. The temperature at which this glass transition occurs for the PLGA segments of PLGA-PEG at the air-water interface was measured to be about 290 K by thermodynamic analysis based on the two-dimensional Maxwell relations. However, from an applications standpoint, spreading by an organic solvent greatly limits its scope of feasible uses. To explore the possibility of maintaining the excellent surface mechanical properties of the PLGA-PEG at the air-water interface while not using an organic solvent, we investigated the air-water interfacial properties of water-spread PLGA-PEG. When spread with water, it was shown that the initial micelles that form in the aqueous spreading solution remain intact even after being spread onto the air-water interface. Due to this different morphology, the surface pressure and monolayer stability were greatly reduced for the water-spread PLGA-PEG at the air-water interface. We used the Daoud and Cotton's blob scaling model to describe the desorption process of the water-spread PLGA-PEG at the air-water interface. From the scaling concept, it was shown that with higher PEG molecular weight and larger micelle size, the adsorption energy of the water-spread PLGA-PEG to the air-water interface was increased.

Interactions between model cell membranes and the neuroactive drug propofol

Vibrational sum frequency spectroscopy (VSFS) complemented by surface pressure isotherm and neutron reflectometry (NR) experiments were employed to investigate the interactions between propofol, a small amphiphilic molecule that currently is the most common general anaesthetic drug, and phospholipid monolayers. A series of biologically relevant saturated phospholipids of varying chain length from C18 to C14 were spread on either pure water or propofol (2,6-bis(1-methylethyl)phenol) solution in a Langmuir trough, and the change in the molecular structure of the film, induced by the interaction with propofol, was studied with respect to the surface pressure. The results from the surface pressure isotherm experiments revealed that propofol, as long as it remains at the interface, enhances the fluidity of the phospholipid monolayer. The VSF spectra demonstrate that for each phospholipid the amount of propofol in the monolayer region decreases with increasing surface pressure. Such squeeze out is in contrast to the enhanced interactions that can be exhibited by more complex amphiphilic molecules such as peptides. At surface pressures of 22–25 mN m−1, which are relevant for biological cell membranes, most of the propofol has been expelled from the monolayer, especially in the case of the C16 and C18 phospholipids that adopt a liquid condensed phase packing of its alkyl tails. At lower surface pressures of 5 mN m−1, the effect of propofol on the structure of the alkyl tails is enhanced when the phospholipids are present in a liquid expanded phase. Specifically, for the C16 phospholipid, NR data reveal that propofol is located exclusively in the head group region, which is rationalized in the context of previous studies. The results imply a non-homogeneous distribution of propofol in the plane of real cell membranes, which is an inference that requires urgent testing and may help to explain why such low concentration of the drug are required to induce general anaesthesia.