Specific effects of Ca(2+) ions and molecular structure of ß-lactoglobulin interfacial layers that drive macroscopic foam stability.

ß-Lactoglobulin (BLG) adsorption layers at air-water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca(2+) concentration. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy - from the ubiquitous air-water interface through thin foam films to macroscopic foam. For concentrations <1 mM, a strong decrease in SFG intensity from O-H stretching bands and a slight increase in layer thickness and surface pressure are observed. A further increase in Ca(2+) concentrations above 1 mM causes an apparent change in the polarity of aromatic C-H stretching vibrations from interfacial BLG which we associate to a charge reversal at the interface. Foam film measurements show formation of common black films at Ca(2+) concentrations above 1 mM due to considerable decrease of the stabilizing electrostatic disjoining pressure. These observations also correlate with a minimum in macroscopic foam stability. For concentrations >30 mM Ca(2+), micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca(2+) concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.

Evaluation of therapeutic pulmonary surfactants by thin liquid film methods.

An example of the application of the Black Foam Film (BFF) Method and the Wetting Film Method, using the Microinterferomertric and the Pressure Balance Techniques, for characterization interfacial properties of the animal derived therapeutic pulmonary surfactant preparations (TSP), is presented. BFF thickness, probability of black film formation, and disjoining pressure for foam films from TSP aqueous solutions are measured as well as the wetting properties of TSP solutions on solid surfaces with different hydrophobicity have been studied. Interfacial characteristics such as minimal surfactant concentration to obtain black film (critical concentration) and concentration at which a black film is 100% obtained (threshold concentration) are determined. An evaluation of the four widely used TSP ­ Curosurf, Infasurf, Survanta, and Alveofact ­ by these methods has been carried out. Thus the methods of the thin liquid films are useful tools for studying the interfacial properties of TSP solutions, as well as for their improvement.

Effect of surfactant adsorption time on the observation of Newton black film in foam film.

Observation of Newton black film (NBF) in foam film is possible only with a certain probability W which depends on the concentration C of surfactant in the solution and on the time t(a) during which adsorption of surfactant at the solution/air interface has taken place. In the paper, the W(C,t(a)) dependence is derived and used to analyze the effect of t(a) on the critical surfactant concentration C(c) below which NBF in foam film practically cannot be observed. An expression for the C(c)(t(a)) function is obtained which reveals that C(c) decreases substantially with increasing t(a). This expression is found to describe well experimental C(c)(t(a)) data for foam films obtained from aqueous solution of the therapeutic surfactant INFASURF.

Thin liquid films from aqueous solutions of non-ionic polymeric surfactants.

The conditions of formation and stability of foam, emulsion, and wetting films from aqueous solutions of non-ionic polymeric surfactants have been established. Two types of polymeric surfactants - PEO-PPO-PEO three-block copolymers (A-B-A type) and hydrophobically modified inulin graft polymer (AB(n) type) - have been explored. Information about surface forces and nanoscale phenomena in aqueous films containing polymeric surfactants was obtained using the micro-interferometric technique and the Thin Liquid Film-Pressure Balance Technique. Two types of surface forces, which determine the stability of the foam and emulsion films, have been distinguished, namely: DLVO-forces at low electrolyte concentrations and non-DLVO-forces at high electrolyte concentrations. Non-DLVO-forces are steric surface forces of the brush-to-brush and loop-to-loop interaction type according to De Gennes. A substantial difference in the behavior of these two film types has been established and in the case of O/W emulsion films transitions to Newton black film (NBF) have been observed. These films are very stable and so are the respective emulsions. In contrast the wetting films are relatively thicker compared to emulsion films, and their thickness depends on the concentration of the AB(n) polymeric surfactant. The steric repulsion of the loops and tails of the polymeric surfactant determine the film thickness of wetting films on a hydrophilic solid surface. For solid surfaces with different degrees of hydrophobicity the wetting films are stable only at high polymer concentrations and low degree of hydrophobicity. Otherwise the films are unstable and rupture. Two types of bilayer emulsion films have been distinguished for the first time. One type is related to the brush-to-brush or loop-to-loop interactions according to De Gennes. The other type is a NBF where the forces are also steric between strongly hydrated brush and loops but they are short-range forces acting in a two-dimensional ordered system. They determine the high NBF stability.

A pH-study of n-dodecyl-beta-D-maltoside foam films.

The influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-beta-d-maltoside (beta-C12G2) was studied. Foam film measurements were carried out with the thin film pressure balance (TFPB) technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure Pc and, with the latter, complete disjoining pressure versus thickness curves (Pi-h curves) were measured. Most of the results were obtained for 10(-4) and 10(-5) M beta-C12G2 solutions that contained 10(-3) M electrolyte. Measurements were carried out in a pH range from 3 to 9. The major results are the following: (1) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs approximately cmc. This holds true for both low and high pH values. (2) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (3) The pH of the isoelectric point increases with increasing surfactant concentration. (4) The q0-pH curve of beta-C12G2 shows two pH ranges (3-5.5 and 7-10) in which the surface charge density q0 is pH-insensitive, while a significant change of q0 was observed between pH=5.5 and 7.0. A possible explanation is given.

Amphiphilic nanostructures in thin liquid films.

A survey on recent experimental investigations of microscopic foam films containing self-assembled amphiphilic nanostructures is presented. The film characteristics are investigated via microinterferometric method, which operates with the measuring cell of Scheludko-Exerowa. The results show the following: (1) Unstable black patterns (dots and spots) are observed; they have very short lifetimes and the films which contain them rupture quickly. (2) Drainage times of the films display sharp changes within the studied surfactant concentration range. (3) The peculiarities in the film drainage properties are in accordance with the specific run of the adsorption isotherms of the initial surfactant solutions. The data are interpreted based on the assumption that a series of smaller self-assembled aggregates (premicelles) exist in amphiphilic solutions. The results show that the microscopic foam film technique has a serious potential as a prospective instrumentation for the study of amphiphilic self-assemblies in surfactant solutions.

Rheological behavior and parameters of the in vitro model of lung surfactant systems: the role of the main phospholipid component.

The proposed in vitro model for studying the alveolar surface layer of the lungs enables one to investigate the surface intermolecular forces which influence the stability of the alveolus. The general role for the stability of the alveolus belongs to the phospholipids in the alveolar surfactant and predominantly to their main component dipalmitoylphosphatidylcholine (DPPC). The aim of the study was to investigate the rheological behavior of DPPC and exogenous surfactant preparations used in neonatal clinical practice. Data for the rheological behavior of the solutions of the commercially available surfactants, Infasurf, Exosurf and Survanta, as well as of DPPC (their main phospholipid component) at shear rates from 0.024 to 94.5 s(-1) under steady and transient flow conditions at 23 degrees C were obtained. Infasurf and Exosurf showed Newtonian rheological behavior, while Survanta revealed the shear-thinning behavior of a non-Newtonian pseudoplastic fluid. The rheological properties of aqueous solutions of DPPC containing 0.14 M NaCl at concentrations from 100 and 630 microg/ml of phospholipid (chosen from the dependence of the probability for bilayer film formation) were studied. Differences observed in the rheological properties of the exogenous surfactants were interpreted on the basis of their composition, the presence of other phospholipid components, certain additives and surfactant proteins, as well as the bulk structures formed from them. The relevance of the results for the delivery of exogenous surfactants and their spreading in replacement therapy is discussed.

Chain-melting phase transition and short-range molecular interactions in phospholipid foam bilayers.

Occurrence of two-dimensional chain melting phase transition in foam bilayers was established for the first time. Microscopic horizontal foam bilayers [Newton black films (NBF)] were investigated by the microinterferometric method of Scheludko-Exerowa. The foam bilayers were formed from water-ethanol solutions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine (Egg PC) and samples of amniotic fluid (AF) at different temperatures. The influence of temperature on the foam bilayer thickness h(w) and on the critical concentration Cc for formation of foam bilayer was studied. It was shown that in the range of the main phase transition the temperature dependence of h(w) and C(c) changed specifically in the case of DMPC and DPPC foam bilayers. The thickness of the foam bilayers increased with decreasing temperature in the range of the main phase transition due to the melting of hydrocarbon tails of phospholipid molecules. These changes took place at the temperatures of the bulk chain-melting phase transitions, as determined by differential scanning calorimetry (DSC) for both aqueous, and water/ethanol DMPC, DPPC, and DPPC dispersions. An effect of the 'disperse medium' on h(w) was found for foam bilayers from DPPC. The results that foam bilayers could have different thickness at different temperatures disproved the current concept that NBF acquired constant thickness at concentrations higher than C(el,cr). The data for Cc were analysed on the basis of the hole-nucleation theory of bilayer stability of Kashchiev and Exerowa. This theory considered the amphiphile bilayer as a two-dimensional ordered system with short-range molecular interactions between the first neighbour molecules (as in a crystal). The short-range molecular interactions were presented by the parameter binding energy Q of an amphiphile molecule in the bilayer. The binding energy Q of two neighbouring phospholipids was calculated for the gel (30-60 kT) and liquid crystalline state (16-18 kT) of the bilayers from DMPC, DPPC, Egg PC, AF. Concentration/temperature phase diagram of DPPC foam bilayers that defined regions of gaseous (ruptured), gel and liquid crystalline foam bilayers were drawn. The values of Q obtained for various samples were very close and vary from 5.3 x 10(-20) to 9.4 x 10(-20) (approx. 13-22 kT) which indicated that in all cases the foam bilayers were in liquid-crystalline state. This is an important result since the parameter studied-threshold concentration (threshold dilution) is crucial for a very successful assessment of the risk for respiratory distress syndrome (RDS) in newborns and could be employed in medicine for assessment of other respiratory disturbances. It is to be expected that foam bilayers from phospholipids could be used as a model for investigation of short-range forces in biological structures, of interaction between membranes, etc.