Equilibrium Surface Pressure Research Articles

Brewster angle microscopy study of supersaturated monolayers of long chain fatty acid methyl esters before and beyond the collapse pressure

The constant surface pressure relaxation of supersaturated methylarachidate and methylbehenate monolayers between the equilibrium surface pressure and the collapse point is analyzed quantitatively on the basis of the general nucleation-growth-collision theory. In all cases, the calculated nucleation rate constants are quite low corresponding to the conditions for progressive nucleation. The transformation kinetics of monolayer material into three-dimensional (3D) aggregates before and beyond the collapse pressure is visualized by Brewster angle microscopy. Both methylester monolayers evolve compact, irregularly shaped 3D aggregates of microscopic scale within the homogeneously reflecting monolayer. An abrupt increase in growth rate for the 3D nuclei beyond the collapse pressure determines the special morphological features of the 3D aggregates formed in this range.

Inhibition of pulmonary surfactant biophysical activity by cationic polyamino acids.

The purpose of this study is to investigate the interaction of cationic polyamino acids, polylysine and polyarginine, with rat pulmonary surfactant at the air/water interface. Surface pressure measurements of rat pulmonary surfactant in the presence and absence of polyamino acids were carried out in both dynamic and static modes. In dynamic cycle studies, compression and expansion of adsorbed surfactant films in the presence of the cationic polyamino acids resulted in a delayed attainment of the plateau surface pressure. In area studies of spread surfactant films at constant surface pressure, cationic polyamino acids in the subphase resulted in an increase in film area. Increased film area was also observed when a polyamino acid was injected beneath films of dipalmitoyl-phosphatidylcholine/phosphatidylglycerol. In the presence of the cationic polyamino acids, the equilibrium surface pressure (at constant film area) of pulmonary surfactant was elevated in a concentration- and molecular weight-dependent manner. These data indicate that the model cationic peptides interact with surfactant lipid, possibly electrostatically with phosphatidylglycerol. It is concluded that the surface activity of pulmonary surfactant is significantly inhibited by the presence of the polycations, possibly by the formation of a mixed lipid/polyamino acid film.

Interfacial properties of recombinant human cholesterol ester transfer protein.

We investigated the interfacial behavior of recombinant human cholesterol ester transfer protein (rCETP) using monolayer and surface balance techniques. rCETP bound to egg phosphatidylcholine monolayers spread at the air/water interface with a maximum surface pressure of 23 millinewtons (mN)/m at subphase concentrations between 3 and 5 x 10(-5) g/dl; the estimated dissociation constant was 7.5 x 10(-6) g/dl or 1 nM. The binding of rCETP to the lipid interface decreased linearly with increasing initial surface pressure; rCETP was excluded at pressures greater than 31 mN/m. rCETP catalyzed the desorption of [14C]cholesterol oleate from mixed lipid monolayers in a concentration dependent fashion. Similar studies with apolipoproteins A-I and A-IV established that cholesterol ester desorption was not caused by changes in surface pressure or cholesterol ester solubility. The desorption rate was proportional to subphase rCETP concentration, but at all concentrations surface radioactivity remained constant until surface pressure reached a plateau. The calculated binding stochiometry was one molecule of cholesterol ester desorbed for every 1000 molecules of rCETP in the subphase. We conclude that rCETP is surface active, binds to phospholipid monolayers with an affinity equivalent to that of the plasma apolipoproteins, and effects the desorption of cholesterol ester molecules from phospholipid monolayers by a carrier mechanism. Moreover, the relatively low equilibrium surface pressure of rCETP suggests that when bound to lipid the entire rCETP molecule may not penetrate the interface.

Effect of pulmonary surfactant protein A (SP-A) and calcium on the adsorption of cholesterol and film stability

The effect of exogenous cholesterol on the stability of surface films at 37°C from various surfactants was studied with the pulsating bubble surfactometer. Addition of cholesterol (5%, w/w) to bovine lipid extract surfactant (bLES) or mixtures of dipalmitoylphosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol/SP-B (7:3:1%) dispersed in 1.5 mM CaCl 2/0.9% NaCl resulted in unstable surface films. Although 10% cholesterol only partially impaired the surface activity of bLES, it virtually abolished that of the reconstituted surfactant. The inhibitory effects of cholesterol were significantly repressed by SP-A (10%, w/w of lipid) and 3 mM CaCl 2 or 5 mM CaCl 2 without SP-A. Adsorption of cholesterol from various surfactants into the air/water interface was examined by measuring the surface radioactivity of [ 14C]cholesterol. Cholesterol alone dispersed in 1.5 mM CaCl 2/0.9% NaCl could not adsorb to the interface, but it adsorbed readily when mixed with bLES. Cholesterol adsorption was markedly suppressed by SP-A in 3 mM CaCl 2/0.9% NaCl or 5 mM CaCl 2/0.9% NaCl without SP-A. Electron microscopy revealed striking ultrastructural differences between bLES/5% cholesterol/10% SP-A in 3 mM CaCl 2/0.9% NaCl and bLES/5% cholesterol in 3 or 5 mM CaCl 2/0.9% NaCl. The former exhibited large multilayer and small unilamellar vesicles, while the latter displayed condensed patches of aggregates. Adsorption studies showed aggregated patches adsorbed more rapidly than vesicles but attained lower equilibrium surface pressures. These results indicate SP-A and calcium limit the adsorption of surfactant cholesterol to the air-water interface.

Surface rheological properties of the monolayer of synthetic lung surfactant

The characteristics of synthetic lung surfactants with or without the surfactant proteins, SP-B, SP-C and SP-BC, were evaluated on the basis of surface modulus and viscosity obtained by applying the Maxwell model to the relaxation of the surface pressure produced by an area strain of 10% on a monolayer on the surface of a buffer solution and were compared with those of the synthetic lung surfactant TA developed earlier. The surface shear modulus and viscosity characteristic of the surface rheology for synthetic lung surfactant TA containing a trace of SP-BC showed a constant value for area strain of 10% under all equilibrium surface pressures and coincided with values of the mixed monolayers obtained by adding SP-BC and SP-B + C to the other components of lung surfactant (except surfactant proteins), but these moduli and viscosities for the systems without surfactant proteins increased with increase in equilibrium surface pressures. These results suggest that the existence of a trace of SP-BC in the synthetic lung surfactant TA improves its effectiveness and may serve as a ‘cushion’ among dipalmitoyl lecithin molecules in the monolayers during surface compression and expansion of these monolayers.

Temperature-dependent studies of the phase behaviour of 1-monostearoyl-rac-glycerol monolayers

The direct visualisation of the temperature-dependent phase behaviour and phase transformation of monolayers of 1-monostearoyl-rac-glycerol has been studied by Brewster angle microscopy. Images of the monolayer on a microscopic scale are related to the characteristic points of the equilibrium surface pressure—area π—A isotherm and are presented and discussed.

On the thermodynamic properties of several two-dimensional Ising lattices

The thermodynamic properties of the triangular, square and honeycomb lattices have been investigated. Numerical results involving the temperature dependence of the equilibrium surface pressure, the configurational free energy, entropy, internal energy and specific heat are presented and discussed.

Chiral molecular recognition in the thermodynamics of spreading and transition for racemic and enantiomeric stearoyltyrosine films

Equilibrium spreading thermodynamics, surface pressure vs area isotherms, and surface shear viscosities were measured for ionized monolayer films of the title compounds on an aqueous buffer subphase at pH 6.86. When allowed to equilibrate with their bulk crystalline phases (equilibrium spreading pressures), the racemic and enantiomeric films spread to significantly different surface pressures and displayed clearly defined transitions in their equilibrium spreading pressure vs temperature-phase diagrams at 30.3 and 26.3 OC, respectively. A thermodynamic analysis of the results demonstrates large differences between the racemic and enantiomeric films in terms of their changes in entropy and internal energy as they undergo the transition. In sharp contrast, racemic and enantiomeric films, spread from solution in the absence of the bulk phase, demonstrate only scant differences in their packing arrangements as reflected by surface pressure vs area isotherms taken in the same temperature range. Surface shear viscosities indicate that both spread films may be characterized as fluid. Taken together with previous results, these data indicate that short-range forces govern enantiomeric discrimination in fluid monolayer films and that the mechanism of detectable chiral molecular recognition between enantiomers lies in the transition to a tightly packed, crystalline surface state.

Surface properties of muscle protein extracts

The interfacial properties of proteins extracted from muscle using salt solutions of different concentration and for different extraction times were determined by the drop volume method. At a bulk phase concentration of 10 −2 wt%, it appears that the sarcoplasmic-rich fraction of muscle is more surface active than the salt-soluble fraction. The average equilibrium surface pressure at the air-liquid interface was 21·5 mN m −1 for an aqueous extract and 20·1 mN m −1 for a 1 m KCl-extract. The equilibrium surface pressure decreased from 21·7 mN m −1 to 19·50mN m −1 as the extraction time with Weber-Edsall solution increased from 15 min to 48 h.

Membrane surface pressure can account for differential activities of membrane-penetrating molecules.

It is a common observation that cis-unsaturated and branched chain fatty acids, which are usually liquid, affect membrane function differently from saturated and trans-unsaturated fatty acids, which are usually solid. We also found that the former are much more potent than the latter in inhibiting viral hemolytic activity. A search for the origin of this difference revealed a correlation between inhibition and equilibrium surface pressure (the surface pressure at the air/water interface of a solution of the substance in question). Using a simple but rigorous thermodynamic analysis, we show that penetration of a lipid bilayer is correlated with equilibrium surface pressure of the penetrating molecule. We therefore conclude that an important reason for the difference in effects of liquid and solid fatty acids on membranes is the greater penetrability of the former relative to the latter. We suggest that attributing such effects to fluidity changes in the membrane should await demonstration of actual intramonolayer residence of the fatty acid in the membrane. The thermodynamic analysis is readily generalized and, in the absence of specific interactions between penetration and bilayer molecules, provides a convenient method for predicting membrane penetration by virtually any type of exogenous molecule.

Langmuir-blodgett multilayers of polymer-merocyanine-dye mixtures

Deposition studies of mixed monolayers of poly(methylmethacrylate) (PMMA) and merocyanine chromophore (M 22) were carried out by the Langmuir-Blodgett technique. The equilibrium surface pressure vs. specific area of the monolayers at the air-water interface was measured for several mixtures. The specific area vs. film composition at a fixed surface pressure deviated from linear behavior. The deposition of the monolayer onto a glass substrate was Z type for mixtures with high PMMA content, while Y type was preferred for mixtures with high dye content. The former structure exhibited second harmonic generation and should be of considerable interest in exploring the non-linear optical effects in Langmuir-Blodgett films. When prepared under a class 100 modular laminar flow hood and observed with reflection microscopy, the films showed no evidence of aggregate formation, suggesting that the PMMA-M 22 mixtures were miscible.

Estimation of the equilibrium lateral pressure in 1-palmitoyl-2-[6(pyren-1-yl)]hexanoyl-glycerophospholipid liposomes

Compression isotherms for 1-palmitoyl-2-[6(pyren-1-yl)]hexanoyl- snglycero-3-phosphocholine (PPHPC), -ethanolamine (PPHPE), -glycerol (PPHPG), -serine (PPHPS) and -phosphatidic acid monomethylester (PPHPM) were recorded at an argon/water interface. Thereafter, the ratio of pyrene excimer to monomer flourscence emission intensities (Ie/Im) were determined for liposomes of these lipids and were found to be 20.15, 12.30, 11.80, 10.15 and 6.95 for the ethanolamine, choline, monomethylester, glycerol and serine derivatives, respectively.Assuming (i) Ie/Im to depend on the reciprocal of the mean molecular area of the pyrenelipids in liposomes, (ii) equilibrium surface pressure to be the same in lipids regardless of the head group structure and (iii) neglecting any possible influence due to differences in the orientation of the pyrene moiety, we sought for that surface pressure value in the compression isotherms where the correlation of the reciprocal of mean molecular area in monolayer to Ie/Im values observed in liposomes was maximal. This treatment results in a value of approximately 1 2 mN m −1 for the equilibrium surface pressure in 1-palmitoyl-2-[6(pyren-1-yl)]hexanoyl-glycerophospholipid liposomes.

A comparison of interfacial behaviours of pea (Pisum sativum L.) legumin and vicilin at air/water interface

AbstractThe interfacial behaviours of the two main pea globulins (legumin and vicilin) at water/air interfaces were studied in order to improve the general knowledge concerning the physicochemical basis of the foaming and emulsifying properties of these proteins. The surface tension (γ) was monitored by the WILHELMY plate technique till it reaches a constant value. The final concentration c of the proteins injected in the subphase was comprised between 0.3. 10−6 and 80. 10−6 g/ml. The adsorption kinetics γ = f(t) were analyzed using different mathematical relations given in the literature. Different phases can be distinguished, corresponding to the diffusion, penetration and rearrangement of protein molecules at the interface, each phase being characterized by a rate constant and an energy barrier.The rate constants were generally higher in the case of vicilin; in consequence the equilibrium surface pressure π were reached more quickly. The isotherms πe = f(log c) showed that the concentration needed to reach the plateau value of πe, were respectively about 11.7. 10 6 g/ml and 18.5. 10−6 g/ml for vicilin and legumin air/water interface. The plateau value of πe, was about the same for both proteins at air/water interface (24.5 mN/m). For lower concentrations, vicilin gave higher values for ne. The areas per molecule in the adsorbed monolayer were about the same for vicilin (1.05 nm2) and legumin (1.04 nm2) at air/water interface.By studying the surface properties of vicilin/legumin mixtures it was observed that πe increased with increasing relative concentration of vicilin and that inhibiting interactions occured between the two globulins, legumin adsorption being more disturbed by vicilin than the opposite.According to these results, the differences between legumin and vicilin can be related to their molecular characteristics. Pea globulins and especially vicilin appeared to be efficient proteins as surface active agents.

Lung surfactant replacement in premature lambs with extracted lipids from bovine lung lavage: effects of dose, dispersion technique, and gestational age.

Extracted bovine calf lung lipids (CLL) with minimal protein (approximately 1%) were instilled prior to ventilation in groups of premature lambs of average gestational ages of 127 and 133 days. Aqueous dispersions of CLL were prepared by two techniques prior to instillation: sonication in an ice bath (S) and mechanical vortexing at room temperature (V). A low surfactant dose (15 mg CLL/kg animal weight) and a high dose (100 mg/kg) were investigated for each dispersion technique. Following tracheal instillation of surfactant, lambs were ventilated with 100% oxygen for 2 h with umbilical circulation intact, and for up to an additional 10 h after separation. A clear improvement in blood oxygenation and lung compliance was found over controls for lambs given 15 mg/kg and 100 mg/kg CLL(V), and 100 mg/kg CLL(S). Lambs treated with 15 mg/kg CLL(S) failed to improve over controls. Experimental groups treated with equal doses of CLL(V) and CLL(S) had similar amounts of lung lavage phospholipid, with values progressively declining during ventilation. Analyses of in vitro surface properties showed that both vortexed and sonicated CLL dispersions adsorbed to equilibrium surface pressures of 45-47 dynes/cm in seconds at concentrations greater than or equal to 0.25 mg CLL/ml. Both dispersions also lowered surface tension to less than 1 dyne/cm under dynamic compression at 37 degrees C in 100% humidity, although CLL(V) showed some enhancement over CLL(S) in dynamic surface activity at low subphase concentration (0.5 mg/ml). Moreover, CLL(V) and CLL(S) differed markedly in their effects on pressure-volume mechanics in a surfactant-deficient excised rat lung model. Instilled CLL(V) dispersions improved excised lung pressure-volume mechanics at significantly lower concentrations than CLL(S) dispersions.

Use of the Gibbs equation to calculate adsorption into monolayer-covered surfaces

A formal equation has been derived to describe the change in equilibrium surface pressure when the concentration of a soluble surfactant is varied in a subphase covered by a constant amount of insoluble monolayer on a fixed area of surface. Approximations are suggested which enable the equation to be applied to experimental data for the calculation of the surface excess concentration of surfactant.An example is given and the surface penetration results discussed.

Interactions between lipopolysaccharide and phosphatidylethanolamine in molecular monolayers

Lipopolysaccharide and phosphatidylethanolamine are the two major lipid constituents of the membrane of Salmonella typhimurium. Interactions between the purified lipopolysaccharide and phosphatidylethanolamine were studied in molecular monolayers at air-water interfaces. The equilibrium surface pressures of mixed films of lipopolysaccharide and phosphatidylethanolamine were determined as a function of the film composition. The plot of the equilibrium surface pressure vs. the area occupied by phosphatidylethanolamine molecules exhibited two distinct regions. Below a phosphatidylethanolamine surface concentration at which 55% of the surface was occupied by phosphatidylethanolamine molecules, the equilibrium pressure was invariant and had the value of a pure lipopolysaccharide monolayer at maximum compression. At phosphatidylethanolamine surface concentrations in excess of 55% surface area occupation (phosphatidylethanolamine/lipopolysaccharide (mol/mol) > 16), the equilibrium surface pressure was a function of the surface concentration of phosphatidylethanolamine. The results suggest a simple model in which lipopolysaccharide and phosphatidylethanolamine form a complex in which each lipopolysaccharide molecule is surrounded (‘lipidated’) by a shell of approx. 16 phosphatidylethanolamine molecules.

Thermodynamics of multicomponent monolayers. II. Phase transitions in two-component monolayers

The thermodynamic equations describing phase transitions in two-component monolayers can be derived with the aid of the equations in Part I of this series. In order to make a comparison with experimental results, we have taken up the mixed monolayers of myristic and nonadecanoic acids, which show the phase transition from an expanded to a condensed state. It has been clarified that they are miscible in both the expanded and condensed phases. We have also found that two kinds of condensed phase exist at a surface pressure higher than that of the second transition point. By plotting the equilibrium surface pressure against the corresponding composition, the phase diagram of the myristic acid-nonadecanoic acid mixed monolayer is constructed. It has been shown that the thermodynamic function changes associated with the phase transition are calculated numerically by making use of the temperature dependence of the equilibrium surface pressure.

Thermodynamics of multicomponent monolayers. I. General formulation

In view of the fact that monolayers are composed of air, water, and film-forming substances and exist in equilibrium with the adjacent air and water phases, we have derived the fundamental equations to describe the thermodynamic states of multicomponent monolayers. With the aid of the equations, it is clarified that the phase rule in the monolayer system is given in a simple form, which is consistent with modified Defay's equation. With respect to thermodynamic function changes on monolayer mixing, we have obtained expressions different from those of Goodrich; the discrepancy is ascribed to his disregard of air and water in the monolayer phase. Phase transitions in the monolayer system are also taken into consideration. General equations have been presented for the partial derivatives of equilibrium surface pressure with respect to temperature and composition. In order to show how to use the derived equations, we have taken up a one-component monolayer, as an example, and indicated that the expressions describing the phase transitions agree with previously published ones.

Equilibrium adsorption of proteins

The adsorption of bovine serum albumin (BSA) at an air/water interface was shown to be reversible and the equilibrium adsorption isotherm measured for concentrations between 0.001% and 10.0%. The desorption of BSA into a subphase of zero concentration was measured as a function of surface pressure and the kinetics shown to be diffusion controlled. The subsurface layer concentrations of BSA calculated from plots of desorption rate vs. t − 1 2 were considerably less than the activities expected from the adsorption isotherm. This is explained by a very large activation energy barrier to the desorption step. The magnitude of this barrier, expressed in terms of an interfacial resistance, was estimated to be of the order of 10 8 sec cm −1 for BSA at a surface pressure of 25.6 dynes cm −1. Some of the reasons for the widely held belief that adsorption of proteins and many polymers is irreversible are discussed, and the use of equilibrium surface pressures for the calculation of thermodynamic properties of proteins in solution is suggested.

Lipid-protein in teraction at the air-water interface

1. 1.The spreading of a protein solution with a glass rod into the plane of a cholesterol monolayer greatly reduced the time necessary to attain an equilibrium surface pressure in the mixed monolayer. The results obtained with this spreading method coincided with those obtained by injecting the protein solution into the bulk of the subphase supporting the lipid monolayer. 2. 2.On a phosphate buffer solution (pH 7.2) there was a linear relationship between the total surface pressure of the mixed monolayer and the molar fraction of amino acid residues at the air-water interface; this suggests a non-specific dissolution process in which the hydrocarbon chains of the amino acid residues and the cholesterol molecules are the chemical species involved. 3. 3.The free energy of mixing for this process, as well as for the surface dissolution of cetyl alcohol and cholesterol, practically coincided with that corresponding to an ideal system; this suggests that the driving forces for both processes could be of entropic origin and that no specific interactions occur between the components of the surface solution.