Poly(ethylene glycol) enhances the surface activity of a pulmonary surfactant

The primary role of lung surfactant is to reduce surface tension at the air–liquid interface of alveoli during respiration. Axisymmetric drop shape analysis (ADSA) was used to study the effect of poly(ethylene glycol) (PEG) on the rate of surface film formation of a bovine lipid extract surfactant (BLES), a therapeutic lung surfactant preparation. PEG of molecular weights 3350; 8000; 10,000; 35,000; and 300,000 in combination with a BLES mixture of 0.5 mg/mL was studied. The adsorption rate of BLES alone at 0.5 mg/mL was much slower than that of a natural lung surfactant at the same concentration; more than 200 s are required to reach the equilibrium surface tension of 25 mJ/m². PEG, while not surface active itself, enhances the adsorption of BLES to an extent depending on its concentration and molecular weight. These findings suggest that depletion attraction induced by higher molecular weight PEG (in the range of 8000 to 35,000) may be responsible for increasing the adsorption rate of BLES at low concentration. The results provide a basis for using PEG as an additive to BLES to reduce its required concentration in clinical treatment, thus reducing the cost for surfactant replacement therapy.     

Interfacial organizations of gel phospholipid and cholesterol in bovine lung surfactant films

Pulmonary surfactants stabilize the lung by way of reducing surface tension at the air-lung interface of the alveolus. 31P NMR, thin-layer chromatography, and electrospray ionization mass spectroscopy of bovine lipid extract surfactant (BLES) confirmed dipalmitoylphosphatidylcholine (DPPC) to be the major phospholipid species, with significant amounts of palmitoyl-oleoylphosphatidylcholine, palmitoyl-myristoylphosphatidylcholine, and palmitoyl-oleoylphosphatidylglycerol. BLES and DPPC spread at the air-water interface were studied through surface pressure area, fluorescence, and Brewster angle microscopy measurements. Langmuir-Blodgett films of monomolecular films, deposited on mica, were characterized by atomic force microscopy. BLES films displayed shape, size, and vertical height profiles distinct from those of DPPC alone. Calcium ions in the subphase altered BLES film domain structure. The addition of cholesterol (4 mol %) resulted in the destabilization of compressed BLES films at higher surface pressures (>40 mN m-1) and the formation of multilayered structures, apparently consisting of stacked monolayers. The studies suggested potential roles for individual surfactant lipid components in supramolecular arrangements, which could be the contributing factors in pulmonary surfactant to attain low surface tension at the air-water interface.     

表面活性剂溶液动态表面张力及吸附动力学研究

本文简介了动态表面张力的定义、测定方法及吸附动力学,对非离子、阴离子、阳离子及两性表面活性剂溶液动态表面张力的研究情况进行了总结,重点讨论了浓度、温度、添加剂及化学结构因素对动态表面张力的影响.     

On the low surface tension of lung surfactant

Natural lung surfactant contains less than 40% disaturated phospholipids, mainly dipalmitoylphosphatidylcholine (DPPC). The mechanism by which lung surfactant achieves very low near-zero surface tensions, well below its equilibrium value, is not fully understood. To date, the low surface tension of lung surfactant is usually explained by a squeeze-out model which predicts that upon film compression non-DPPC components are gradually excluded from the air-water interface into a surface-associated surfactant reservoir. However, detailed experimental evidence of the squeeze-out within the physiologically relevant high surface pressure range is still lacking. In the present work, we studied four animal-derived clinical surfactant preparations, including Survanta, Curosurf, Infasurf, and BLES. By comparing compression isotherms and lateral structures of these surfactant films obtained by atomic force microscopy within the physiologically relevant high surface pressure range, we have derived an updated squeeze-out model. Our model suggests that the squeeze-out originates from fluid phases of a phase-separated monolayer. The squeeze-out process follows a nucleation-growth model and only occurs within a narrow surface pressure range around the equilibrium spreading pressure of lung surfactant. After the squeeze-out, three-dimensional nuclei stop growing, thereby resulting in a DPPC-enriched interfacial monolayer to reduce the air-water surface tension to very low values.     

Study on the Surface Property of Surfactant Ionic Liquids Solutions

The surfactant TX-100 can be dissolved in ionic liquid bmimPF6 and decrease the surface tension of 1-buty1-3-methylimidazolium hexafluorophosphate (bmimPF6) solutions. Here, we confirmed that in this new system, the pure solvents need rearrangement at the air-wate rinterface at the initial stage. The dynamic surface tension (DST) study shows that at the initial adsorption stage, the adsorption model of surfactant accords with the diffusion-controlled adsorption mechanism, and the dilute ionic liquids solutions is further close to the diffusion-controlled adsorption.     

Effect of compressed bovine lipid extract surfactant films on oxygen transfer

In the lungs, oxygen transfer from the inspired air to the capillary blood needs to cross the surfactant lining layer of the alveoli. Therefore, the gas transfer characteristics of lung surfactant film are of fundamental physiological interest. However, previous in vitro studies-most relied on the Langmuir-type balance-fail to cover the low surface tension range (i.e., less than the equilibrium surface tension of approximately 25 mJ/m2) due to film leakage. We have recently developed a novel in vitro experimental strategy, the combination of axisymmetric drop shape analysis and captive bubble technique (ADSA-CB), in studying the effect of surfactant films on interfacial gas transfer (Langmuir 2005, 21, 5446). In the present work, ADSA-CB is used as a micro-film-balance to study the effect of compressed bovine lipid extract surfactant (BLES) films on oxygen transfer. A low surface tension ranging from approximately 25 mJ/m2 to 2 mJ/m2 is studied. The experimental results suggest that lung surfactant films at a low surface tension near 2 mJ/m2 provide resistance to oxygen transfer, as indicated by a decrease of 30-50% in the mass transfer coefficient (kL) of oxygen in BLES suspensions with respect to water. At higher surface tension (i.e., >6 mJ/m2), the resistance to oxygen transfer is only modest, i.e., the decrease in kL is less than 20% compared to water. The experimental results suggest that lung surfactant plays a role in oxygen transfer in the pulmonary system.     

Impairing effect of fibrinogen on the mono-/bi-layer form of bovine lung surfactant

Lung surfactant (LS), a lipid–protein mixture responsible for alveolar stability, is inhibited by serum proteins leaked into the lungs in disease. Interaction of bovine lipid extract surfactant (BLES), a clinical replacement lung surfactant, with serum protein fibrinogen (Fbg) was studied employing various structural and biophysical techniques in adsorbed films and bulk bilayer dispersions. Surface tension area isotherms of the adsorbed films revealed the suppression of interfacial activity of BLES by Fbg (adsorption and surface tension reduction). Fbg, predominantly associated with the fluid phase of BLES films, resulted in the aggregation of the gel lipid domains as evidenced by atomic force microscopy. BLES bilayer dispersion showed phase transition from a diffused gel to liquid–crystalline phase in the temperature range 10–35 °C as studied by differential scanning calorimetry (DSC). Fbg resulted in the shift of peak to a higher transition temperature for the maximal heat flow (T _max) of BLES dispersions. Combined Raman and FTIR spectral studies of the BLES/Fbg dispersions revealed that Fbg altered the –CH₂–, –CH₃, and –PO₄ ^? vibrational modes of the phospholipids present in BLES, suggesting the condensing and dehydrating effect of the protein on surfactant. Studies suggest that Fbg, by directly interacting with the gel lipids in LS in bulk dispersions, alter the packing of the films formed at the interface, and can be used as a specific model for lung disease.     

Electrical surface potential of pulmonary surfactant

Pulmonary surfactant is a mixed lipid protein substance of defined composition that self-assembles at the air-lung interface into a molecular film and thus reduces the interfacial tension to close to zero. A very low surface tension is required for maintaining the alveolar structure. The pulmonary surfactant film is also the first barrier for airborne particles entering the lung upon breathing. We explored by frequency modulation Kelvin probe force microscopy (FM-KPFM) the structure and local electrical surface potential of bovine lipid extract surfactant (BLES) films. BLES is a clinically used surfactant replacement and here served as a realistic model surfactant system. The films were distinguished by a pattern of molecular monolayer areas, separated by patches of lipid bilayer stacks. The stacks were at positive electrical potential with respect to the surrounding monolayer areas. We propose a particular molecular arrangement of the lipids and proteins in the film to explain the topographic and surface potential maps. We also discuss how this locally variable surface potential may influence the retention of charged or polar airborne particles in the lung.     

Surface properties of mixed monolayers of sulfobetaines and ionic surfactants

To study the influence of the head group in the properties of the mixed monolayers adsorbed at the air-water interface, the surface tension and surface potential of binary mixtures of surfactant have been determined as a function of the surfactant composition. Experiments were carried out with anionic-zwitterionic sodium dodecyl sulfate and dodecyl dimethyl ammoniopropane sulfonate (SDS/DDPS), and cationic-zwitterionic dodecyl trimethylammonium bromide and dodecyl dimethyl ammoniopropane sulfonate (DTAB/DDPS), and dodecyl trimethylammonium bromide and tetradecyl dimethyl ammoniopropane sulfonate (DTAB/TDPS). It was shown that mixed monolayers of cationic-zwitterionic surfactant exhibit small negative deviations of ideal behavior, whereas for SDS/DDPS monolayers show strong negative deviation from the ideality. Deviations of ideal behavior are interpreted by regular solution theory. The surface potential values agree very well with the concentration of the ionic component at the interface. The dynamic surface tension values show that the adsorption kinetics on the interface is a diffusion-controlled process. In monolayers with significant deviation of the ideal behavior, anionic-zwitterionic, there is some evidence of intermolecular attractions after diffusion of both surfactants at the interface.     

New methodology to determine the rate-limiting adsorption kinetics mechanism from experimental dynamic surface tension data

We present a new methodology to determine the rate-limiting adsorption kinetics mechanism (diffusion-controlled vs mixed diffusion-barrier controlled), including deducing the kinetics parameters (the diffusion coefficient, D, and the energy-barrier parameter, beta), from the experimental short-time dynamic surface tension (DST) data. The new methodology has the following advantages over the existing procedure used to analyze the experimental DST data: (a) it does not require using a model for the equilibrium adsorption isotherm, and (b) it only requires using the experimental short-time DST data measured at two initial surfactant bulk solution concentrations. We apply the new methodology to analyze the experimental short-time DST data of the following alkyl poly(ethylene oxide), CiEj, nonionic surfactants: C12E4, C12E6, C12E8, and C10E8 measured using the pendant-bubble apparatus. We find that for C12E4 and C12E6, the effect of the energy barrier on the overall rate of surfactant adsorption can be neglected for surfactant bulk solution concentrations below their respective critical micelle concentrations (CMCs), and therefore, that the rate-limiting adsorption kinetics mechanism for C12E4 and C12E6 is diffusion-controlled at any of their premicellar surfactant bulk solution concentrations. On the other hand, for C12E8 and C10E8, we find that their respective CMC values are large enough to observe a significant effect of the energy barrier on the overall rate of surfactant adsorption. In other words, for C12E8 and C10E8, the rate-limiting adsorption kinetics mechanism shifts from diffusion-controlled to mixed diffusion-barrier controlled as their premicellar surfactant bulk solution concentrations increase. We test the new methodology by predicting the short-time DST profiles at other initial surfactant bulk solution concentrations, and then comparing the predicted DST profiles with those measured experimentally. Very good agreement is obtained for the four CiEj nonionic surfactants considered. We also compare the results of implementing the new methodology with those of implementing the existing procedure, and conclude that using a model for the equilibrium adsorption isotherm can lead not only to different values of D and beta, but it can also lead to a completely different determination of the rate-limiting adsorption kinetics mechanism. Since the new methodology proposed here does not require using a model for the equilibrium adsorption isotherm, we conclude that it should provide a more reliable determination of the rate-limiting adsorption kinetics mechanism, including the deduced kinetics parameters, D and beta.     

New theoretical framework for designing nonionic surfactant mixtures that exhibit a desired adsorption kinetics behavior

How does one design a surfactant mixture using a set of available surfactants such that it exhibits a desired adsorption kinetics behavior? The traditional approach used to address this design problem involves conducting trial-and-error experiments with specific surfactant mixtures. This approach is typically time-consuming and resource-intensive and becomes increasingly challenging when the number of surfactants that can be mixed increases. In this article, we propose a new theoretical framework to identify a surfactant mixture that most closely meets a desired adsorption kinetics behavior. Specifically, the new theoretical framework involves (a) formulating the surfactant mixture design problem as an optimization problem using an adsorption kinetics model and (b) solving the optimization problem using a commercial optimization package. The proposed framework aims to identify the surfactant mixture that most closely satisfies the desired adsorption kinetics behavior subject to the predictive capabilities of the chosen adsorption kinetics model. Experiments can then be conducted at the identified surfactant mixture condition to validate the predictions. We demonstrate the reliability and effectiveness of the proposed theoretical framework through a realistic case study by identifying a nonionic surfactant mixture consisting of up to four alkyl poly(ethylene oxide) surfactants (C(10)E(4), C(12)E(5), C(12)E(6), and C(10)E(8)) such that it most closely exhibits a desired dynamic surface tension (DST) profile. Specifically, we use the Mulqueen-Stebe-Blankschtein (MSB) adsorption kinetics model (Mulqueen, M.; Stebe, K. J.; Blankschtein, D. Langmuir 2001, 17, 5196-5207) to formulate the optimization problem as well as the SNOPT commercial optimization solver to identify a surfactant mixture consisting of these four surfactants that most closely exhibits the desired DST profile. Finally, we compare the experimental DST profile measured at the surfactant mixture condition identified by the new theoretical framework with the desired DST profile and find good agreement between the two profiles.     

Adsorption properties of novel gemini surfactants with nonidentical head groups

Novel environmentally friendly gemini surfactants, each with two hydrophilic and two hydrophobic groups, have been synthesized and their physicochemical properties investigated. One of the hydrophilic groups is a methyl-capped polyoxyethylene chain with mol wt 350, 550, and 750 g/mol, respectively, and the other is a sulfate group; the hydrophobic part of the surfactant is made from oleylnitrile. This nitrile derivative of the fatty acid is used to achieve good hydrolytic stability. Du Nouy ring and maximum bubble pressure tensiometry were used for equilibrium and dynamic surface tensions, gamma(e) and gamma(t), respectively. The aqueous-phase critical micelle concentrations of the heterogeminis (HGs) have been investigated. The results have been compared with those for mixtures of standard surfactants sodium decylsulfate and octaoxyethyleneglycol mono n-decyl ether under equivalent conditions. The HGs are shown to exhibit improved performance over the mixed system both in terms of micellization and surface tension lowering. Dynamic surface tension (DST) studies were performed to investigate air-water adsorption mechanisms. A diffusion-limited mechanism was confirmed in the initial stages of adsorption. However, closer to the equilibrium the DST data are inconsistent with a diffusion-only mechanism. In particular, the HGs show a larger deviation from diffusion control as compared to the model mixture, which is a signature of slower adsorption kinetics. In addition to air-water interfaces, properties of these HGs have also been investigated at solid silica-solution surfaces by optical reflectometry. These surfaces were either naturally hydrophilic or rendered hydrophobic by chemical modification. On either surface the maximum amount of adsorbed surfactant was found to increase when the polyoxyethylene chain length decreases.     

A theoretical study on surfactant adsorption kinetics: effect of bubble shape on dynamic surface tension

A planar or spherical fluid-liquid interface was commonly assumed on studying the surfactant adsorption kinetics for a pendant bubble in surfactant solutions. However, the shape of a pendant bubble deviates from a sphere unless the bubble's capillary constant is close to zero. Up to date, the literature has no report about the shape effect on the relaxation of surface tension due to the shape difference between a pendant bubble and a sphere. The dynamic surface tension (DST), based on the actual shape of a pendant bubble with a needle, of the diffusion-controlled process is simulated using a time-dependent finite element method in this work. The shape effect and the existence of a needle on DST are investigated. This numerical simulation resolves also the time-dependent bulk surfactant concentration. The depth of solution needed to satisfy the classical Ward-Tordai infinite-solution assumption was also studied. For a diffusion-controlled adsorption process, bubble shape and needle size are two major factors affecting the DST. The existence of a needle accelerates the bulk diffusion for a small bubble; however, the shape of a large pendant bubble decelerates the bulk diffusion. An example using this method on the DST data of C12E4 is illustrated at the end of this work.     

Modeling of the adsorption kinetics of surfactants at the liquid-fluid interface of a pendant drop

This paper presents a theoretical model for simulating the adsorption kinetics of a surfactant at the liquid-fluid interface of a pendant drop. The diffusion equation is solved numerically by applying the semidiscrete Galerkin finite element method to obtain the time-dependent surfactant concentration distributions inside the pendant drop and inside the syringe needle that is used to form the pendant drop. With the obtained bulk surfactant concentration distributions, the adsorption at the interface is determined by using the conservation law of mass. It should be noted that the theoretical model developed in this study considers the actual geometry of the pendant drop, the depletion process of the surfactant inside the pendant drop, and the mass transfer of the surfactant from the syringe needle to the pendant drop. The present pendant-drop model is applied to study the adsorption kinetics of surfactant C10E8 (octaethylene glycol mono n-decyl ether) at the water-air interface of a pendant drop. The numerical results show that the Ward and Tordai equation, which was derived for adsorption from a semi-infinite surfactant solution to a planar interface, is unsuitable for interpreting the dynamic surface or interfacial tension data measured by using the pendant-drop-shape techniques, especially at low initial surfactant concentrations. The spherical-drop model, which assumes the pendant drop to be a perfectly spherical drop with the same drop volume, can be used to interpret the dynamic surface or interfacial tension data for pendant drops either with high initial surfactant concentrations or with low initial surfactant concentrations in short adsorption durations only. For pendant drops with low initial surfactant concentrations in long adsorption durations, the theoretical model developed in this study is strongly recommended.     

Poly(ethylene glycol) (PEG) enhances dynamic surface activity of a bovine lipid extract surfactant (BLES)

Shortage or malfunction of pulmonary surfactant in alveolar space leads to a critical condition termed respiratory distress syndrome (RDS). Surfactant replacement therapy, the major method to treat RDS, is an expensive treatment. In this paper, the effect of poly(ethylene glycol) (PEG) to improve dynamic surface activity of a bovine lipid extract surfactant (BLES) was studied by axisymmetric drop shape analysis (ADSA) and a captive bubble method. The activity of BLES+PEG mixtures was compared to that of a natural surfactant containing surfactant proteins A and D. When PEG was added into BLES mixtures, the surface tension hysteresis of BLES films was minimized when the films were compressed by more than 50%. PEG also helps to quickly restore surfactant films after film collapse. Thus, as far as surface tension effects go, the findings suggest that PEG might be used as a substitute for surfactant-associated protein SP-A in therapeutic surfactant products, and might also be used to reduce the amount of BLES required in clinical applications.     

Short-time behavior of mixed diffusion-barrier controlled adsorption

This paper focuses on the short-time adsorption kinetics of nonionic surfactants onto water/air surfaces, analyzed in the context of the mixed diffusion-barrier controlled adsorption modeling framework. Specifically, we reconcile the apparent contradiction between theoretical prediction and experimental observations on the adsorption kinetics mechanism at short times: while the mixed diffusion-barrier controlled model predicts a barrier-controlled adsorption, as well as the impossibility of a diffusion-controlled adsorption at asymptotic short times, the short-time experimental dynamic surface tension (DST) behavior of many nonionic surfactants has been interpreted to result from diffusion-controlled adsorption at asymptotic short times. This is because the short-time experimental DST of these surfactants displays a t variation, which is considered as a fingerprint for the existence of diffusion-controlled adsorption, based on the short-time asymptotic behavior of the diffusion-controlled adsorption model. As a result of this interpretation, the fundamental physical nature of the energy barrier has been proposed to be associated with high surfactant surface concentrations. In this paper, we derive a new nonasymptotic short-time formalism of the mixed diffusion-barrier controlled model to describe surfactant adsorption onto a spherical pendant-bubble surface, including determining the ranges of time and surfactant surface concentration values where the short-time formalism is applicable. Based on this formalism, we find that one can expect to observe an apparent t variation of the DST at short times even for the mixed diffusion-barrier controlled adsorption model. We analyze the consequence of this finding by re-evaluating the existing notions of the energy barrier. We conclude that the energy barrier is associated with the adsorption of a single surfactant molecule onto a clean surface.     

Adsorption kinetics of n-nonyl-beta-D-glucopyranoside at the air-water interface studied by infrared reflection absorption spectroscopy

The adsorption of the surfactant n-nonyl-beta-D-glucopyranoside at the air-water interface after injection of the surfactant into the subphase was studied by infrared reflection absorption spectroscopy. In the first part, we investigated the equilibrium adsorption of n-nonyl-beta-D-glucopyranoside and the Gibbs adsorption isotherm was measured by applying the film balance technique. In the second part, the adsorption kinetics was followed by changes in the surface pressure and in the intensities of the OH band, which is related to the layer thickness, and the CH(2) antisymmetric stretching vibrational band. During an induction period, when the molecules are still highly diluted and the surface pressure is low, they are oriented parallel to the air-water interface. IR band simulations for the CH(2) antisymmetric stretching vibrational band support the idea of horizontally oriented molecules at the air-water interface. Later on, when more molecules are adsorbed to the air-water interface, they suddenly rearrange to an upright orientation as indicated by changes of the OH and the CH(2) bands. The observations are discussed in comparison to results obtained for the adsorption kinetics of n-decyl-beta-D-maltopyranoside, n-dodecyl-beta-D-maltopyranoside, and sodium dodecyl sulfate.     

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.     

The interaction between sodium alkyl sulfate surfactants and the oppositely charged polyelectrolyte, polyDMDAAC, at the air-water interface: the role of alkyl chain length and electrolyte and comparison with theoretical predictions

The effect of alkyl chain length and electrolyte on the adsorption of sodium alkyl sulfate surfactants and the oppositely charged polyelectrolyte, polyDMDAAC, at the air-water interface has been investigated by surface tension and neutron reflectivity. The variations in the patterns of adsorption and surface tension behavior with alkyl chain length and electrolyte are discussed in the context of the competition between the formation of surface active surfactant/polyelectrolyte complexes and polyelectrolyte/surfactant micelle complexes in solution. A theoretical approach based on the law of mass action has been used to predict the surface effects arising from the competition between the formation of polyelectrolyte/surfactant surface and solution complexes and the formation of free surfactant micelles. This relatively straightforward model is shown to reproduce the principal features of the experimental results.     

Photofoams: remote control of foam destabilization by exposure to light using an azobenzene surfactant

We report evidence for photocontrolled stability and breakage of aqueous foams made from solutions of a cationic azobenzene-containing surfactant over a wide range of concentrations. Exposure to UV or visible lights results in shape and polarity switches in the surfactant molecule, which in turn affects several properties including critical micelle concentration, equilibrium surface tension, and the air-water interfacial composition (cis isomers are displaced by trans ones). We demonstrate that the trans isomer stabilizes foams, whereas the cis isomer forms unstable foams, a property that does not correlate with effects of light on surface tension, nor with total surfactant concentration. Achieving in situ breakage of foam is accordingly ascribed to the remote control of the dynamics of adsorption/desorption of the surfactant, accompanied by gradients of concentrations out of equilibrium. Photomodulation of adsorption kinetics and/or diffusion dynamics on interfaces is reached here by a noninvasive clean trigger, bringing a new tool for the study of foams.