Equilibrium Surface Pressure Research Articles

Interactions between Small Inorganic Ions and Uncharged Monolayers on the Water/Air Interface.

The interaction of several simple electrolytes with uncharged insoluble monolayers is studied on the basis of tensiometric and potentiometric data for the surface electrolyte solution|air. The induced adsorption of electrolyte on the monolayer is determined via a combination of data for equilibrium spreading pressure and surface pressure versus area isotherms. We show that the monolayer-induced adsorption of electrolyte is not only strongly ion-specific but also surfactant-specific. The comparison between the ion-specific effects on a carboxylic acid monolayer at low pH and an ester monolayer shows that the anion series follows the same order while the cation series reverses. The effect of the electrolyte on the chemical potential of the monolayer shows attraction between the surfactant and the ions at low monolayer densities, but at high surface densities, repulsion seems to come into play. In nearly all investigated cases, a maximum of monolayer-induced electrolyte adsorption is observed at intermediate monolayer densities. This suggests specific interactions between the surfactant headgroup and the ions. The Volta potential data for the monolayers are analyzed on the basis of the equations of quadrupolar electrostatics. The analysis suggests that the ion-specific effect on the Volta potential is due to the ion-specific decrement of the bulk dielectric constant of the electrolyte solution. Moreover, we present evidence that in most cases the effect of the electrolyte on the orientation of the adsorbed dipoles cannot be neglected. Instead, the change in the ion distribution in the electric double layer seems to have a small effect on the Volta potential.

The influence of xylanase and thermal treatment on the composition and interfacial rheology properties of whole wheat dough liquor

SummaryThe dough liquor (DL) centrifuged from dough played an important role in determining the gas cell stability during dough making, and the adding of xylanase (XYL) in the dough could efficiently increase the stability of DL foam, especially at 60 °C. In this work, we prepared whole wheat dough added with different dosages of XYL and extract the DL. The components especially proteins in DL and their interaction mechanism and interfacial rheology properties at the air–water interface were studied. The proteins formed protein aggregates (>150 kDa) at 60 °C. After XYL treatment, DL solution exhibited higher protein surface hydrophobicity and surface pressure. And the equilibrium surface pressure of DL varied between 23 and 28 mN m−1. At 25 °C, the interfacial dilatational modulus increased by 26.77% and 14.22% compared with control, respectively. And the XYL can improve the interface shear strength both for the unheated and heated DL solutions. These findings showed that XYL might improve the ability of DL constituents to stabilise the air–water interface to contribute to the stability of gas cells in the dough. The obtained results have provided a theoretical basis for improving the gas cell stability by changing the properties of whole wheat DL with the addition of XYL.

A Scaling for Atmospheric Heat Redistribution on Tidally Locked Rocky Planets

Abstract Atmospheric heat redistribution shapes the remote appearance of rocky exoplanets, but there is currently no easy way to predict a planet’s heat redistribution from its physical properties. This paper proposes an analytical scaling theory for the heat redistribution on tidally locked rocky exoplanets. The main parameters of the scaling are a planet’s equilibrium temperature, surface pressure, and broadband longwave optical thickness. The scaling compares favorably against idealized general circulation model simulations of TRAPPIST-1b, GJ1132b, and LHS 3844b. For these planets, heat redistribution generally becomes efficient, and a planet’s observable thermal phase curve and secondary eclipse start to deviate significantly from that of a bare rock, once surface pressure exceeds  ( 1 ) bar. The scaling additionally points to planetary scenarios for which heat transport can be notably more or less efficient, such as H2 and CO atmospheres or hot lava ocean worlds. The results thus bridge the gap between theory and imminent observations with the James Webb Space Telescope. They can also be used to parameterize the effect of 3D atmospheric dynamics in 1D models, thereby improving the self-consistency of such models.

Investigation of surface properties of quince seed extract as a novel polymeric surfactant

In recent years, there is a growing trend from both academia and the industry towards the use of “clean-labeled” ingredients obtained from renewable resources. Proteins and polysaccharides, in particular, are becoming increasingly popular as alternatives to already well-established synthetic surfactants. Quince seeds are a relatively novel hydrocolloid source that has recently raised interest among researchers due to their strong surface activity and viscosity-enhancing properties. This study investigates quince seed extract's surface properties (dynamic surface tension and dilatational surface rheology) and how they differ with varying concentrations (between 0.01% and 1%), pH's (3, 7, 9, and 11), and ionic strengths (0.1, 0.3, 0.5 M NaCl). By QSE addition alone, equilibrium surface tension could be lowered to ∼36 mN/m, which is lower than the lowest ST that can be achieved with many other surface active biopolymers. Critical aggregation concentration (CAC) was identified as 0.165% w/v, meaning a relatively low extract concentration was sufficient to provide complete surface coverage. Dynamic surface tension curves revealed almost instantaneous polymer adsorption for concentrations over 0.01% w/v, which demonstrates the strong potential of the gum as a foaming agent. As solution pHs get further from the isoelectric point of QSE proteins, the rate of adsorption of QSE molecules onto the interface and the equilibrium surface pressures increased. Surface properties were also significantly affected by the ionic strength of the medium, with eq. STs decreasing with increasing QSE concentration. pH and ionic strength induced conformational changes in the interfacial layer and also led to local minima and maxima in dilatational elastic and loss modulus within ranges studied. Considering these findings, QSE is a very promising natural alternative to other polymeric surfactants and stabilizers currently used in the food, cosmetic and pharmaceutical industries.

Interfacial Rheology of Foam Stabilized by Nanoparticles and Their Retention in Porous Media

Nanoparticle-stabilized foam has aroused people’s attention due to its wide applications in oil fields. Understanding the dynamic surface properties of nanoparticles and their retention in reservoir formation is crucial for their application. This research presents the impact of mixing SiO2 nanoparticles and sodium dodecyl sulfate (SDS) on foaming and interfacial properties. Also, the retention behaviors of nanoparticles in sandpacks with different permeabilities are discussed by employing the core-flooding apparatus. The experimental results show that nanoparticles and SDS exhibit synergy on foam stability, although the foam volume decreases in the presence of nanoparticles. It is inferred that SDS may determine the equilibrium surface pressure of the mixed system, as the nanoparticles can affect the surface pressure slightly. There exists a competitive behavior between SDS and nanoparticles during adsorption, and the presence of nanoparticles lowers the adsorption rate of the mixing system. The film behaves more elastic for SiO2/SDS dispersion as the dilatational elasticity of the dispersion is enhanced in the presence of nanoparticles, while the dilatational viscosity changes little. In the retention experiments, nanoparticles can flow through the sandpack smoothly due to their nanometer sizes with few changes on the sandpack permeability. The nanoparticle retention decreases with increasing the sandpack permeability, and it is possible to reinject the recovered nanoparticles into the formation by comparing the foam properties before and after sandpack injection. We hope this research may guide the further application of nanoparticle-stabilized foam in oil fields.

Synthesis and physicochemical evaluation of fluorinated lipopeptide precursors of ligands for microbubble targeting.

Ligand-targeted microbubbles are focusing interest for molecular imaging and delivery of chemotherapeutics. Lipid–peptide conjugates (lipopeptides) that feature alternating serine–glycine (SG)n segments rather than classical poly(oxyethylene) linkers between the lipid polar head and a targeting ligand were proposed for the liposome-mediated, selective delivery of anticancer drugs. Here, we report the synthesis of perfluoroalkylated lipopeptides (F-lipopeptides) bearing two hydrophobic chains (CnF2n+1, n = 6, 7, 8, 1–3) grafted through a lysine moiety on a hydrophilic chain composed of a lysine–serine–serine (KSS) sequence followed by 5 SG sequences. These F-lipopeptides are precursors of targeting lipopeptide conjugates. A hydrocarbon counterpart with a C10H21 chain (4) was synthesized for comparison. The capacity for the F-lipopeptides to spontaneously adsorb at the air/water interface and form monolayers when combined with dipalmitoylphosphatidylcholine (DPPC) was investigated. The F-lipopeptides 1–3 demonstrated a markedly enhanced tendency to form monolayers at the air/water interface, with equilibrium surface pressures reaching ≈7–10 mN m−1 versus less than 1 mN m−1 only for their hydrocarbon analog 4. The F-lipopeptides penetrate in the DPPC monolayers in both liquid expanded (LE) and liquid condensed (LC) phases without interfacial film destabilization. By contrast, 4 provokes delipidation of the interfacial film. The incorporation of the F-lipopeptides 1–3 in microbubbles with a shell of DPPC and dipalmitoylphosphatidylethanolamine-PEG2000 decreased their mean diameter and increased their stability, the best results being obtained for the C8F17-bearing lipopeptide 3. By contrast, the hydrocarbon lipopeptide led to microbubbles with a larger mean diameter and a significantly lower stability.

Contributions of protein and milled chitin extracted from domestic cricket powder to emulsion stabilization

Interfacial and emulsifying properties of fractionated cricket powder were assessed to identify whether emulsification properties originate from protein or chitin particles. Fractions extracted in alkaline water, containing high protein and mineral contents, increased the surface pressure of heptane-water interfaces with near-saturation equilibrium surface pressure of 31 mN/m. Dynamic surface pressure profiles indicated adsorption of protein clusters to the interface. Emulsification capacity of protein fraction was 50% greater than that of the source cricket flour, although oil-in-water emulsions prepared with 1–2% (w/w) protein fraction formed a cream layer within one day of storage. Emulsified layers persisted for up to 20 days, and light scattering measurements described a stable population with surface-volume-mean diameter of approximately 3 μm. Chitin-rich fractions milled to a particle size of 0.5–200 μm contributed negligible surface pressure, and its emulsification capacity was 5% of the value for the source cricket flour. Emulsions prepared with chitin-rich fractions coexisted with an unstable precipitate layer comprising 60% of the added solid, which was attributed to larger particles with poor emulsifying capability. Stable chitin-stabilized emulsion phases were resistant to creaming, yet volume-mean droplet diameter surpassed 50 μm within 24 h of storage. Both protein and chitin fractions have emulsifying capabilities but would require further processing or secondary additives to achieve desirable storage stability.

Phase Behavior of GM1-Containing DMPC-Cholesterol Monolayer: Experimental and Theoretical Study.

Organization and distribution of lipids in cellular membranes play an important role in a diverse range of biological processes, such as membrane trafficking and signaling. Here, we present the combined experimental and simulated results to elucidate the phase behavioral features of ganglioside monosialo 1 (GM1)-containing mixed monolayer of the lipids 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and cholesterol (CHOL). Two monolayers having compositions DMPC-CHOL and GM1-DMPC-CHOL are investigated at air-water and air-solid interfaces using Langmuir-Blodgett experiments and scanning electron microscopy (SEM), respectively, to ascertain the phase behavior change of the monolayers. Surface pressure isotherms and SEM imaging of domain formation indicate that addition of GM1 to the monolayer at low surface pressure causes a fluidization of the system but once the system attains the surface pressure corresponding to its liquid-condensed phase, the monolayer becomes more ordered than the system devoid of GM1 and interacts among each other more cooperatively. Besides, the condensing effect of cholesterol on the DMPC monolayer was also verified by our experiments. Apart from these, the effects induced by GM1 on the phase behavior of the binary mixture of DMPC-CHOL were studied with and without applying liquid-expanded (LE)-liquid-condensed (LC) equilibrium surface pressure using molecular dynamics (MD) simulation. Our molecular dynamics (MD) simulation results give an atomistic-level explanation of our experimental findings and furnish a similar conclusion.

Impact of formulation pH on physicochemical protein characteristics at the liquid-air interface

Both, formulation parameters and the presence of liquid-air interfaces are known to affect the aggregation of protein drugs. In this study, the impact of pH on the liquid-air interfacial behavior of three proteins, a polyclonal and two monoclonal antibodies (IgG, mAB1 and mAB2) was investigated using different surface sensitive methods. Equilibrium surface pressure values revealed only a minor impact of pH. Infrared Reflectance Absorbance Spectroscopy (IRRAS) proved not only the presence of the proteins at the interface but also showed that the secondary structure was not considerably affected by the adsorption to the interface independent of pH between pH 3 and 9. Additionally, the physical resistance of the film as determined by the interfacial compressibility in a Langmuir trough was not affected by pH. Compression of the interfacial film caused the formation of telescoped areas which were no longer present after decompression at all pH values as investigated by underwater Atomic Force Microscopy (AFM). Brewster Angle Microscopy (BAM) showed some slight changes in the film reflectivity depending on pH, indicating changes in the interfacial film thickness. IRRAS experiments at different angles of incidence as well as section analysis of AFM images proved not only that the film thickness increased upon compression, but also that the interfacial film is thinner at pH 4 than at pH 9. Continuous compression and decompression of the protein film resulted in particle formation with increasing numbers of particles at higher pH value as detected by Light Obscuration (LO) and Micro-Flow Imaging (MFI). The use of different surface sensitive methods provides expedient information on how liquid-air interfacial events are affected by formulation pH. These findings enable a better understanding of not only the events and processes happening at the interface but can also be directly linked to the interface-related formation of particles.

Effect of Dengue Fusion Peptide in Langmuir Monolayers

Membrane fusion peptides are part of Dengue virus system, serving as a gateway to the virus access into the cell. Studies on the mechanisms of action of these peptides to improve protocols for the treatment and achievement of new drugs to combat the dissemination of the virus are increasingly necessary. The Langmuir technique is very useful to study peptide-lipid interactions. Indeed, the utilization of lipid monolayers allows controlling several physical parameters such as lipid composition, surface pressure and many other parameters of interest. Here we study the interaction of the Dengue Flavivirus fusion peptide (FLAg) with Langmuir monolayers composed of different types of lipids, including POPG, (palmitoyl-oleoyl-phosphatydyl-glycerol). In order to assess peptide maximum insertion pressure (MIP) into the Langmuir model membrane, kinetic measurements were performed. The kinetics of FLAg binding onto the phospholipid monolayer was monitored until the equilibrium surface pressure (Πe) was reached. The MIP was determined by injecting FLAg at different initial surface pressures (Πi) of the lipid monolayers. To investigate the behavior of FLAg in Langmuir monolayers with different lipid composition, we also used in situ infrared spectroscopy (PM-IRRAS). Preliminary results showed an increase in surface pressure during adsorption kinetics experiments with POPG monolayers, revealing an electrostatic affinity that evidences the role of the membrane charge in the fusion process. Furthermore, surface activity of the peptide solution used as the subphase in the absence of lipid monolayers was also measured. PM-IRRAS measurements reveal how the peptide incorporates at the interface of POPG monolayers, showing changes in the spectra, besides indicating characteristic due to the presence of FLAg. Altogether, these data clearly demonstrate that useful information on the binding FLAg to membranes can be obtained by performing measurements of their monolayer binding parameters.

Predicting the Agitation-Induced Aggregation of Monoclonal Antibodies Using Surface Tensiometry.

Adsorption of antibody therapeutics to air-liquid interfaces can enhance aggregation, particularly when the solution does not contain protective surfactant or when the surfactant is diluted as occurs during preparation of intravenous infusion bags. The ability to predict an antibody's propensity for interfacially mediated aggregation is particularly useful during product development to ensure the quality, potency, and safety of the therapeutic. To develop a predictive tool, we investigated the surface pressure and surface excess of a panel of 16 antibodies as well as determined their aggregation propensity at the air-liquid interface in an agitation stress model. Our data demonstrated that the initial rate of surface pressure increase upon antibody adsorption to the air-liquid interface strongly predicted the extent of agitation-induced aggregation. Other factors, including the hydrophobicity, equilibrium surface pressure, and interfacial concentration of an antibody, were not adequate predictors of its susceptibility to aggregation. In addition to developing a predictive tool, we extended the interfacial characterization to better understand the mechanisms of antibody aggregation at an air-liquid interface during agitation stress. We believe that the kinetics of antibody rearrangement and conformational change after adsorbing to the interface, leading to the development of attractive antibody-antibody interactions, dictated the extent of aggregation. Overall, our results demonstrate how surface pressure measurements can be implemented as a rapid screening tool for the identification of antibodies with a high propensity to aggregate upon adsorption to an air-liquid interface while also furthering our understanding of interfacially mediated protein aggregation.

Interfacial properties of asphaltenes at toluene-water interfaces.

Asphaltenes are "n-alkane insoluble" species in crude oil that stabilize water-in-oil emulsions. To understand asphaltene adsorption mechanisms at oil-water interfaces and coalescence blockage, we first studied the behavior in aliphatic oil-water systems in which asphaltenes are almost insoluble. They adsorbed as monomers, giving a unique master curve relating interfacial tension (IFT) to interfacial coverage through a Langmuir equation of state (EoS). The long-time surface coverage was independent of asphaltene bulk concentration and asymptotically approached the 2-D packing limit for polydisperse disks. On coalescence, the surface coverage exceeded the 2-D limit and the asphaltene film appeared to become solidlike, apparently undergoing a transition to a soft glassy material and blocking further coalescence. However, real systems consist of mixtures of aliphatic and aromatic components in which asphaltenes may be quite soluble. To understand solubility effects, we focus here on how the increased bulk solubility of asphaltenes affects their interfacial properties in comparison to aliphatic oil-water systems. Unlike the "almost irreversible" adsorption of asphaltenes where the asymptotic interfacial coverage was independent of the bulk concentration, an equilibrium surface pressure, dependent on bulk concentration, was obtained for toluene-water systems because of adsorption being balanced by desorption. The equilibrium surface coverage could be obtained from the short- and long-term Ward-Tordai approximations. The behavior of the equilibrium surface pressure with the equilibrium surface coverage was then derived. These data for various asphaltene concentrations were used to determine the EoS, which for toluene-water could also be fitted by the Langmuir EoS with Γ∞ = 3.3 molecule/nm(2), the same value as that found for these asphaltenes in aliphatic media. Asphaltene solubility in the bulk phase only appears to affect the adsorption isotherm but not the EoS. Further support for these observations is provided by dilatational rheology experiments for the EoS and contraction experiments in which desorption to the equilibrium surface pressure was observed.

Effect of Premixed Asphaltenes and Demulsifier on Oil-Water Interfacial Properties

During crude oil recovery, asphaltene-stabilized water-in-oil emulsions are formed. Asphaltenes is the component of crude oil, which is insoluble in n-alkane and is considered to be largely responsible for the stability of water-in-crude oil emulsions. To avoid wide range of problems, water has to be removed from these emulsions by using chemical ‘demulsifiers’. How demulsifiers act is still poorly understood. As the real crude oil system is extremely complex, system consisting of model oil with 100 ppm asphaltenes concentration is used as this concentration corresponds to nanoaggregation in the bulk fluid as shown in previous work. The surface pressure values appear to be a result of both asphaltenes and demulsifier competing to adsorb at the interface. The fractional interfacial coverage achieved by each appears to depend on their respective bulk concentrations. Thus demulsifier-asphaltene systems exhibit an equilibrium surface pressure, reached at higher demulsifier concentrations possibly due to adsorption balancing desorption in the demulsifier rich regions, the presence of which block the ‘almost insoluble’ asphaltenes from further coverage of interface. The behavior of the surface pressure on expansion and contraction of droplets using pendant drop to emulate a Langmuir trough supports the notion of concentration-dependent fractional coverage by each species.

Adsorption and thermodynamics of biosurfactant, surfactin, monolayers at the air-buffered liquid interface

Adsorption of surfactin, a powerful lipopeptide biosurfactant, at the air-liquid interface has been investigated in this article. The adsorption took place from buffered solutions containing relatively high concentrations of surfactin co- and counterions. Dynamic surface tension measurements were used to follow the self-assembly of surfactin at the interface until equilibrium surface pressure Π e is reached at a given surfactin concentration (C s). Gibbs adsorption equation in conjunction with the Langmuir adsorption isotherm was used to predict surfactin surface excess as a function of the biosurfactant concentration up to the critical micelle concentration (CMC). The predicted surface excess at saturation (Γ ∞ ) is 1.05 ± 0.05 μmol m−2, corresponding to an area per molecule (A ∞ ) of 159 ± 8 A2. The adsorption equilibrium constant (K = (1.5 ± 0.6) × 106 M‐ 1) was also estimated from the nonlinear regression of Π e − C s data in region B of the Π e − ln C s plot. The value of K suggests that surfactin has strong affinity for the interface, which is in line with its known high surface activity. Gibbs elasticity (E G) of the interfacial surfactin monolayers, which is an important thermodynamic property, was also predicted at different surfactin concentrations. The limiting value (at the CMC) of E G was found to be 183 mN m−1, which is comparable to those reported in the literature for similar systems. The findings reported in this work reveal that the surface tension measurements coupled with appropriate theoretical analysis could provide useful information comparable to those obtained using highly sophisticated techniques.

Pattern formation in Langmuir–Blodgett films of tricycloquinazoline based discotic liquid crystal molecules

The assembly of molecules on the surface has drawn considerable attention because of its potential to tailor the physicochemical, electronic, and magnetic properties of materials. The structure of aggregates of molecules can be influenced by the molecular interactions and the external parameters like temperature, pressure and ion contents. We have found a stable Langmuir monolayer of amphiphilic tricycloquinazoline based discotic liquid crystal (AmTCQ) molecules at air–water (A–W) interface. The monolayer exhibits gas phase, low density liquid phase and high density liquid phase. The structural evolution in Langmuir–Blodgett (LB) films of the AmTCQ molecules as a function of target surface pressure is studied using an atomic force microscope. We found aggregation of the discotic molecules leading to nucleation sites at a lower surface pressure. With the increasing surface pressure, each nucleation site grows to form a stable triangular structure. The shape and size of the triangular structure remain stable till the target surface pressure for LB films deposition exceeds the equilibrium surface pressure of the AmTCQ molecules. The number of triangles in the film increases with increasing target surface pressure of LB deposition. The target surface pressure of LB deposition acts as a controlling parameter for obtaining desired number of triangular domains in the film.

Emulsification Capacity of Microgels Assembled from β-Lactoglobulin and Pectin

Microgels formed from beta-lactoglobulin were used to prepare oil-in-water emulsions in order to examine their emulsifying capacity. Corn oil emulsions prepared with microgels of pure beta-lactoglobulin at pH 5.8 were initially stable, but a fraction of the droplets quickly flocculated to form a creamed layer that could not be dispersed by shear, which was attributed to hydrophobic attractions between the microgels on adjoining droplets. Emulsions prepared from microgels of beta-lactoglobulin and pectin at pH 4.75 possessed greater droplet sizes at lower concentrations, yet all emulsions were relatively stable to irreversible flocculation. Increased stability of emulsions stabilized by BP-gels was attributed to the presence of pectin on the surface of microgels, which increased repulsions between adjoining droplets. Stable corn oil emulsions were still prepared from microgels that were previously dialyzed to remove non-aggregated protein, which verified that the microgels were responsible for stabilizing emulsion droplets. Equilibrium surface pressure of corn oil droplets was similar between microgels and the unheated beta-lactoglobulin and pectin, yet the dynamic surface pressure was reduced at intermediate times and indicated a slow relaxation and deformation of the microgels at the interface. Microgels formed with pectin stabilized emulsions containing 90 % limonene for up to 5 days of room temperature storage, demonstrating the capacity of such protein microgels to stabilize flavor oil emulsions.

Application of the Lippmann electrocapillary equation to adsorption potential shift at the air/solution interface – Analysis for undissociated n-decanoic acid

The equilibrium surface pressure (Πe) and electric surface potential (Δχe) vs. concentration (c) isotherms of n-decanoic acid (DA) in 5×10−3mol/dm3 HCl were concurrently measured at the air/solution interface – the Π with a platinum Wilhelmy plate, the Δχ with the vibrating plate method (named also the dynamic condenser). The Πe–Δχe dependence was used by us for calculating the surface charge’ density basing on the Lippmann first electrocapillary equation. It was applied under the assumption that: (i) the surface charge is solely formed by orientation of the adsorbate, and (ii) the changing DA bulk concentration does not deprive the Πe–Δχe dependence of consistency with the Lippmann equations. The charge (Qe) expressed per the adsorbate molecule (basing on the Γ–c dependence predicted by the best fit of Frumkin model to the experimental Πe–c results) increases with the adsorption coverage (Θ) within a fraction of electron per the adsorbate molecule – the reasonable result for a non ionic surfactant. The Qe vs. Θ course shows a great analogy to that of quotient of the effective dipole moment to the interface’s permittivity, μ⊥εsvs.Θ, derived from the Δχe–Γ dependence. The two dependencies: Qevs.Θ and μ⊥εsvs.Θ, show a plateau within the coverage range of 20<Θ<80% and a steep increase above it. Recalculation of the Qevs.Θ dependence for the equivalent dipole moment, under the assumption that the dipole length is comparable to length of a polar bond of the head group (ca. 1.2Å) implies the bond’s inclination to the interface of 12°. Such low inclination is reasonable in view of an angular deviation of molecular dipole moment from amphiphile molecule’ long axis.

Surface and foaming properties of polyoxyethylene glycerol ester surfactants

Polyoxyethylene glycerol esters derived from cocoa oil are non-ionic surfactants obtained from a renewable source which fulfil the environmental and toxicological requirements to be used as ecofriendly foaming and/or emulsifying agents. This paper reports a study on the equilibrium adsorption, surface rheology and foaming properties of two commercial polyoxytheylene glycerol ester surfactants which differ in the number of ethylene oxide (EO) groups. Dynamic and equilibrium surface pressure values were obtained with a drop profile tensiometer. The oscillating drop technique was used to study the dilatational rheology of adsorbed surfactant layers. The foaming properties of aqueous solutions of these surfactants were characterized in a commercial foam scan column. Dynamic surface pressure measurements showed two adsorption processes in which a slower condensation of the adsorbed film followed a fast diffussion step. Surface rheology and equilibrium surface pressure data fitted an extended Frumkim model which takes into account the compressibility of the adsorbed layer. The polyoxyethylene glycerol ester with the highest number of EO groups turned out to be more surface active, leading to lower cmc and higher adsorption constant. Both surfactants exhibited similar overall foam capacity at a given concentration. However, the time evolution of the liquid volume in the foam and the size of the air bubbles pointed to the formation of a more homogeneous and stable foam with smaller bubbles for the most hydrophilic polyoxyethylene glycerol ester studied. This is considered to be related to its surface dilatational rheology properties.

Surface behavior of α-Synuclein and its interaction with phospholipids using the Langmuir monolayer technique: A comparison between monomeric and fibrillar α-Synuclein

Due to the involvement of α-Synuclein (α-Syn) in lipid transport and its role in the normal function and in the pathology of Parkinson disease, it is important to study first the surface properties of the protein at the air/water interface and second its behavior related to biological membranes. For this purpose, the monomolecular film technique was used as membrane model to compare the interactions with various phospholipids of monomeric and fibrillar forms of α-Syn. We have determined the equilibrium surface pressure of the two forms of α-Syn (monomeric and fibrillar form) at the air/water interface. The surface pressures reached by monomeric α-Syn were shown to be higher than the ones of fibrillar α-Syn and similar to the value obtained by mellitin, a lytic peptide of bee venom, which has been described as “protein detergent”. The monomeric α-Syn adsorbed more rapidly at the air/water interface with a maximal adsorption rate at least 60-times higher than the fibrillar form. In the presence of a phospholipid monolayer, the surface activities of two α-Syn forms are much greater than observed at the air/water interface. Also we can show that the fibrillar form of α-Syn have a higher value of critical pressure than the monomeric one for the cow brain extract and the Phospatidyl Glycerol (an anionic phospholipid) which confirm its higher affinity for the anionic phospholipid than the monomeric form. According these results, we can suggest that this aggregate form have important implications for the pathological activity and, therefore, for the associated neurotoxicity which can results in layer disruption and cell leakage.

Salt effects on the air/solution interfacial properties of PEO-containing copolymers: Equilibrium, adsorption kinetics and surface rheological behavior

Lithium cations are known to form complexes with the oxygen atoms of poly(oxyethylene) chains. The effect of Li+ on the surface properties of three block-copolymers containing poly(oxyethylene) (PEO) have been studied. Two types of copolymers have been studied, a water soluble one of the pluronic family, PEO–b-PPO–b-PEO, PPO being poly(propyleneoxyde), and two water insoluble ones: PEO–b-PS and PEO–b-PS–b-PEO, PS being polystyrene. In the case of the pluronic the adsorption kinetics, the equilibrium surface tension isotherm and the aqueous/air surface rheology have been measured, while for the two insoluble copolymers only the surface pressure and the surface rheology have been studied. In all the cases two different Li+ concentrations have been used. As in the absence of lithium ions, the adsorption kinetics of pluronic solutions shows two processes, and becomes faster as [Li+] increases. The kinetics is not diffusion controlled. For a given pluronic concentration the equilibrium surface pressure increases with [Li+], and the isotherms show two surface phase transitions, though less marked than for [Li+]=0. A similar behavior was found for the equilibrium isotherms of PEO–b-PS and PEO–b-PS–b-PEO. The surface elasticity of these two copolymers was found to increase with [Li+] over the whole surface concentration and frequency ranges studied. A smaller effect was found in the case of the pluronic solutions. The results of the pluronic solutions were modeled using a recent theory that takes into account that the molecules can be adsorbed at the surface in two different states. The theory gives a good fit for the adsorption kinetics and a reasonably good prediction of the equilibrium isotherms for low and intermediate concentrations of pluronic. However, the theory is not able to reproduce the isotherm for [Li+]=0. Only a semi-quantitative prediction of the surface elasticity is obtained for [pluronic]⩽1×10−3mM.