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.     

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.     

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.     

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.     

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.     

Effect of humidity on the adsorption kinetics of lung surfactant at air-water interfaces

The in vitro adsorption kinetics of lung surfactant at air-water interfaces is affected by both the composition of the surfactant preparations and the conditions under which the assessment is conducted. Relevant experimental conditions are surfactant concentration, temperature, subphase pH, electrolyte concentration, humidity, and gas composition of the atmosphere exposed to the interface. The effect of humidity on the adsorption kinetics of a therapeutic lung surfactant preparation, bovine lipid extract surfactant (BLES), was studied by measuring the dynamic surface tension (DST). Axisymmetric drop shape analysis (ADSA) was used in conjunction with three different experimental methodologies, i.e., captive bubble (CB), pendant drop (PD), and constrained sessile drop (CSD), to measure the DST. The experimental results obtained from these three methodologies show that for 100% relative humidity (RH) at 37 degrees C the rate of adsorption of BLES at an air-water interface is substantially slower than for low humidity. It is also found that there is a difference in the rate of surface tension decrease measured from the PD and CB/CSD methods. These experimental results agree well with an adsorption model that considers the combined effects of entropic force, electrostatic interaction, and gravity. These findings have implications for the development and evaluation of new formulations for surfactant replacement therapy.     

Thermodynamic studies of bovine lung surfactant extract mixing with cholesterol and its palmitate derivative

Langmuir film behavior of bovine lipid extract surfactant (BLES), mixed with cholesterol (CHOL) and cholesterol palmitate (CHOLP), has been studied by surface pressure (pi)-area (A) measurements. Associative interactions, observed for both systems, were less favored at lower BLES content. The presence of unsaturated phospholipids and surfactant proteins in BLES favored the association. Miscibility of BLES was better with CHOLP than with CHOL at all compositions, indicating more compact packing of the BLES-CHOLP than of the BLES-CHOL system. The most stable mixtures were found at 30-40 mol% CHOL and at low pi and at 20-25 mol% CHOLP but at higher pi. These results suggest that BLES-CHOL miscibility is better at low pi and low CHOL concentrations, while BLES-CHOLP miscibility is better at high pi and high CHOLP concentrations.     

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.     

Effect of anionic surfactant and short-chain alcohol mixtures on adsorption at quartz/water and water/air interfaces and the wettability of quartz

Measurements of the advancing contact angles for aqueous solutions of sodium dodecyl sulfate (SDDS) or sodium hexadecyl sulfonate (SHS) in mixtures with methanol, ethanol, or propanol on a quartz surface were carried out. On the basis of the obtained results and Young and Gibbs equations the critical surface tension of quartz wetting, the composition of the surface layer at the quartz-water interface, and the activity coefficients of the anionic surfactants and alcohols in this layer as well as the work of adhesion of aqueous solutions of anionic surfactant and alcohol mixtures to the quartz surface were determined. The analysis of the contact angle data showed that the wettability of quartz changed visibly only in the range of alcohol and anionic surfactant concentration at which these surface-active agents were present in the solution in the monomeric form. The analysis also showed that there was a linear dependence between the adhesion and the surface tension of aqueous solutions of anionic surfactant and alcohol mixtures. This dependence can be described by linear equations for which the constants depend on the anionic surfactant and alcohol concentrations. The slope of all linear dependence between adhesion and surface tension was positive. The critical surface tension of quartz wetting determined from this dependence by extrapolating the adhesion tension to the value equal to the surface tension (for contact angle equal zero) depends on the assumption whether the concentration of anionic surfactant or alcohol was constant. Its average value is equal to 29.95mN/m and it is considerably lower than the quartz surface tension. The positive slope of the adhesion-surface tension curves was explained by the possibility of the presence of liquid vapor film beyond the solution drop which settled on the quartz surface and the adsorption of surface-active agents at the quartz/monolayer water film-water interface. This conclusion was confirmed by the work of adhesion of aqueous solutions of anionic surfactants and short-chain alcohol mixtures to the quartz surface determined on the basis of the contact angle data and molar fraction of anionic surfactants and alcohols and their activity coefficient in the surface layer.     

Molecular interactions of cationic and anionic surfactants in mixed monolayers and aggregates

The properties of anionic-rich and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry and surface tension measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface tension reduction effectiveness(gamma(CAC)), surface excess(Gamma(max)), and mean molecular surface area (A(min)) were determined from plots of the surface tension (gamma) as a function of the total surfactant concentration. The compositions of the adsorbed films (Z) and aggregates (chi) were estimated by using regular solution theory, and then the interaction parameters in the aggregates (beta) and the adsorbed film phases (beta(sigma)) were calculated. The results showed that the synergism between the surfactants enhances the formation of mixed aggregates and reduces the surface tension. Further, the nature and strength of the interaction between the surfactants in the mixtures were obtained by calculating the values of the following parameters: the interaction parameter, beta, the size parameter, rho, and the nonrandom mixing parameter, P*. These results indicate that in ionic surfactant mixtures the optimized packing parameter has the highest value and that the size parameter can be used to account for deviations from the predictions of regular solution theory. It was concluded that, for planar air/aqueous interfaces and aggregation systems, this nonideality increases as the temperature increases. This trend is attributed to the increased dehydration of the surfactant head groups that results from increases in temperature. Further, our conductometry measurements show that the counterion binding number of mixed micelles formed in mixtures with a high CTAB content is different to those with a high SDS content. This difference is due to either their different aggregation sizes or the different interactions between the head groups and the counterions.     

Study on the distribution of binary mixed counterions in surfactant adsorbed films by total reflection XAFS measurements

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF₄). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB - dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) - 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF₄) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.     

A comparative study of exogenous surfactant preparations and tracheal aspirate: interfacial tensiometry and properties of foam films

Surfactant replacement therapy has a vital role in the management of respiratory distress syndrome (RDS). Several techniques and models have been largely used to investigate interfacial physico-chemical properties in vitro and to assist clinical efficiency of exogenous surfactant preparations (ESPs) in vivo. Among them are interfacial tensiometry (Langmuir balance coupled with Wilhelmy plate method for surface tension measurement) and black foam film (BFF) method for measuring the capability of ESPs for bilayer foam film formation.

Here, we report some freshly established data from a comparative study of Exosurf, Survanta, Curosurf, Alveofact and clinical samples of tracheal aspirate (TA) of newborns with RDS treated with Curosurf. New observations concerning the properties of foam films of ESPs are also reported and discussed.

Measured interfacial physico-chemical parameters prove “better” properties in vitro of the SP-B and -C containing preparations Curosurf and Alveofact. Their properties are similar, Alveofact showing a higher surface tension lowering capacity under dynamic conditions.

A comparison with measured interfacial parameters of clinical samples shows that after treatment with Curosurf the phospholipid concentration in tracheal aspirates (367 μg/ml) is higher than the minimum phospholipid concentration for stable black film formation (C_t) of all four ESPs studied, while before treatment this concentration (63 μg/ml) is lower than C_t.

Values of measured “dynamic” parameters of clinical samples after treatment with Curosurf approach those of the exogenous surfactant preparation itself.     

Investigation of the hydration of nonfouling material poly(ethylene glycol) by low-field nuclear magnetic resonance

The strong surface hydration layer of nonfouling materials plays a key role in their resistance to nonspecific protein adsorption. Poly(ethylene glycol) (PEG) is an effective example of materials that can resist nonspecific protein adsorption and cell adhesion. Thus, the strong interaction between water molecules and PEG was investigated through each T(2) component in water/PEG mixtures using multiexponential inversion of T(2) relaxation time measured by the Carr-Purcell-Meiboom-Gill (CPMG) sequence of low-field nuclear magnetic resonance (LF-NMR). Results show that about one water molecule is tightly bound with one ethylene glycol (EG) unit, and additional water molecules over 1:1 ratio mainly swell the PEG matrix and are not tightly bound with PEG. This result was also supported by the endothermic behavior of water/PEG mixtures measured by differential scanning calorimetry (DSC). It is believed that the method developed could be also applied to investigate various interactions between macromolecules and other small molecules without using deuterium samples, which might open a novel route to quantitatively measure guest-host interactions in the future.     

Surface modification of nanoporous alumina surfaces with poly(ethylene glycol)

Nanoporous alumina surfaces have a variety of applications in biosensors, biofiltration, and targeted drug delivery. However, the fabrication route to create these nanopores in alumina results in surface defects in the crystal lattice. This results in inherent charge on the porous surface causing biofouling, that is, nonspecific adsorption of biomolecules. Poly(ethylene glycol) (PEG) is known to form biocompatible nonfouling films on silicon surfaces. However, its application to alumina surfaces is very limited and has not been well investigated. In this study, we have covalently attached PEG to nanoporous alumina surfaces to improve their nonfouling properties. A PEG-silane coupling technique was used to modify the surface. Different concentrations of PEG for different immobilization times were used to form PEG films of various grafting densities. X-ray photoelectron spectroscopy (XPS) was used to verify the presence of PEG moieties on the alumina surface. High-resolution C1s spectra show that with an increase in concentration and immobilization time, the grafting density of PEG also increases. Further, a standard overlayer model was used to calculate the thickness of PEG films formed using the XPS intensities of the Al2p peaks. The films formed by this technique are less than 2.5 nm thick, suggesting that such films will not clog the pores which are in the range of 70-80 nm.     

Cleaning of Starchy Soils in Clean-in-Place (CIP) Systems: Relationship Between Contact Angle and Detergency

The cleaning of hard surfaces soiled with starch films using nonionic (alkylpolyglucosides and polyoxyethylene alkyl ethers) and zwitterionic (lauramine oxide) surfactant solutions has been investigated. Response surface methodology was applied to determine the effect of the composition of the cleaning solution on detergency, as well as the relationship between the contact angle of surfactant solutions on starch films and the removal efficacy of such films. The surfactants with a higher capacity for the removal of starch films were lauramine oxide and alkylpolyglucosides, and their mixtures. The effect of polyoxyethylene alkyl ethers was also statistically significant; however, their contribution to soil removal was very low. A correlation between contact angle and detergency for starchy soils was found, and the importance of the hydration process of the starch film on its removal was established.     

Effect of the Head Group of Surfactant on the Miscibility of Sodium Chloride and Surfactant in an Adsorbed Film and Micelle

The thermodynamic treatment of a surfactant mixture was applied to the mixture of sodium chloride, NaCl, with octyl methyl sulfoxide (OMS) and that with decyldimethylphosphine oxide (DePO). The surface tension of aqueous solutions of the mixtures was measured as a function of the total concentration and the composition of the mixtures at 298.15 K. The total surface densities of the mixtures and the composition of the adsorbed films and micelles were evaluated by applying thermodynamic equations to the expeimental results. It was found that the adsorbed film and micelle are almost composed of the surfactant and there is slight attractive interaction between the ions of NaCl and the head groups of OMS and DePO molecules in the adsorbed films and micelles. A difference in the miscibility of NaCl and surfactant was observed between the OMS and DePO systems and attributed to the difference in the hydration of the head group between OMS and DePO molecules. The comparison of these results with those of the mixtures of NaCl with tetraethylene glycol monooctyl ether (C(8)E(4)) and dodecylammonium chloride (DAC) indicated that the small difference in the miscibility in an adsorbed film and micelle among these nonionic surfactant systems arises from the difference in hydration and structure of the head groups and the large one between the nonionic surfactant and DAC systems results from electrostatic interactions between dodecylammonium and sodium ions. Copyright 2001 Academic Press.     

Synthesis and characterization of poly(ethylene glycol) methyl ether endcapped poly(ethylene terephthalate)

Linear and branched poly(ethylene terephthalate) (PET) copolymers with polyethylene glycol) (PEG) methyl ether (700 or 2000 g/mol) end groups were synthesized using conventional melt polymerization. DSC analysis demonstrated that low levels of PEG end groups accelerated PET crystallization. The incorporated PEG end groups also decreased the crystallization temperature of PET dramatically, and copolymers with a high content of PEG (>17.6 wt%) were able to crystallize at room temperature. Rheological analysis demonstrated that the presence of PEG end groups effectively decreased the melt viscosities and facilitated melt processing. XPS and ATR-FTIR revealed that the PEG end groups tended to aggregate on the surface, and the surface of compression molded films containing 34.0 wt% PEG were PEG rich (85 wt% PEG). PEG end-capped PET (34.0 wt% PEG) and PET films were immersed into a fibrinogen solution (0.7 mg/mL BSA) for 72 h to investigate the propensity for protein adhesion. XPS demonstrated that the concentration of nitrogen (1.05%) on the surface of PEG endcapped PET film was statistically lower than PET (7.67%). SEM analysis was consistent with XPS results, and revealed the presence of adsorbed protein on the surface of PET films.     

Interactions of quaternary ammonium salt-type gemini surfactants with sodium poly(styrene sulfonate)

The interactions of cationic gemini surfactants, 1,2-bis(alkyldimethylammonio)ethane dibromide (m-2-m: m is hydrocarbon chain length, m = 10 and 12), and an anionic polymer, sodium poly(styrene sulfonate) (PSS), have been characterized by several techniques such as tensiometry, fluorescence spectroscopy, and dynamic light scattering. The surface tension of gemini surfactant/PSS mixed systems decreases with surfactant concentration, reaching break points, which are taken as critical aggregation concentrations (cac). The surface tension at the cac of mixtures is higher than that of single surfactants, and it is found that at concentrations above the cac, the surfactant molecules are associated with the polymer in the bulk. The 12-2-12/PSS mixed system shows higher surface activity than both 10-2-10/PSS and the monomeric surfactant of dodecyltrimethylammonium bromide/PSS systems. Fluorescence measurements of these mixed systems suggest the formation of a complex with a highly hydrophobic environment in the bulk of the solution. Additionally, dynamic light scattering measurements show that the hydrodynamic diameter of the 12-2-12/PSS mixed system is smaller than that of PSS only at low concentration, indicating interactions between surfactant and polymer. These result from the electrostatic attraction between ammonium and sulfate headgroups as well as the hydrophobic interaction between their hydrocarbon chains.     

Nonequilibrium features of the association between poly(vinylamine) and sodium dodecyl sulfate: the validity of the colloid dispersion concept

The effect of different mixing protocols on the bulk and surface properties of the aqueous mixtures of linear poly(vinylamine) (PVAm) and sodium dodecyl sulfate (SDS) has been investigated using pH, electrophoretic mobility, dynamic light scattering, coagulation kinetics, and surface tension measurements. For the preparation of the solutions, two kinds of mixing protocols were applied. The so-called "stop flow mixing" enables a very rapid mixing whereas in the case of "gentle mixing" the mixing of the components is less efficient. At high surfactant concentrations a kinetically stable colloid dispersion of the PVAm/SDS particles is formed via the application of the stop flow mixing method. The mixing protocols have a significant effect on the bulk properties of the PVAm/SDS system, in particular, at the low pH range and at large PVAm concentrations. The effect of mixing can be qualitatively understood in terms of the enhanced local rate of coagulation of the PVAm/SDS complexes as well as of the appearance of polyelectrolyte/surfactant aggregates via the application of a less efficient mixing. The study also reveals that the applied methods of solution preparation do not have a major impact on the bound amount of the surfactant as well as on the surface tension isotherms of the system. This latter finding is attributed to the hindered adsorption of the large polyelectrolyte/surfactant aggregates at the air/water interface.     

Foam, emulsion and wetting films stabilized by polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants

This review explores three (A, B, C) polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants belonging to the group of star-like polymers. They have a similar structure, differing only in the number of polymeric branches (4, 6 and 9 in the mentioned order). The differences in these surfactants' ability to stabilize foam, o/w/o and w/o/w emulsion and wetting films are evaluated by a number of methods summarized in Section 2. Results from the studies indicate that differences in polymeric surfactants' molecular structure affect the properties exhibited at air/water, oil/water and water/solid interfaces, such as the value of surface tension, interfacial tension, critical micelle concentration, degree of hydrophobicity of solid surface, etc. Foam, emulsion and wetting films stabilized by such surfactants also show different behavior regarding some specific parameters, such as critical electrolyte concentration, surfactant concentration for obtaining a stable film, film thickness value, etc. These observations give reasons to believe that model studies can support a comprehensive understanding of how the change in polymeric surfactant structure can impact thin liquid films properties. This may enable a targeted design of the macromolecular architecture depending on the polymeric surfactants application purpose.