Multilayers at the surface of solutions of exogenous lung surfactant: direct observation by neutron reflection.

Pharmacy-grade exogenous lung surfactant preparations of bovine and porcine origin, dispersed in physiological electrolyte solution have been studied. The organization and dynamics at the air/water interface at physiological temperature was analysed by neutron reflection. The results show that a well-defined surface phase is formed, consisting of a multilayer structure of lipid/protein bilayers alternating with aqueous layers, with a repetition period of about 70 A and correlation depths of 3 to >25 bilayers, depending on electrolyte composition and time. The experimental surfactant concentration of 0.15% (w/w) is far below that used in therapeutic application of exogenous surfactants and it is therefore likely that similar multilayer structures are also formed at the alveolar surface in the clinical situation during surfactant substitution therapy. Lung surfactant preparations in dry form swell in aqueous solution towards a limit of about 60% (w/w) of water, forming a lamellar liquid-crystalline phase above about 34 degrees C, which disperses into lamellar bodies at higher water concentrations. The lamellar spacings in the surface multilayers at the air/water interface are smaller than those in the saturated limit even though they are in contact with much greater water concentrations. The surface multilayers are laterally disordered in a way that is consistent with fragments of Lalpha-phase lamellae. The near surface layers of the multilayer structure have a significant protein content (only SP-B and SP-C are present in the preparations). The results demonstrate that a multilayer structure can be formed in exogenous surfactant even at very low concentrations and indicate that multilayers need to be incorporated into present interpretations of in vitro studies of similar lung surfactant preparations, which are largely based on monolayer models.

Formation of supported phospholipid bilayers via co-adsorption with beta-D-dodecyl maltoside.

We have investigated the formation of supported model membranes via the adsorption of phospholipid-surfactant mixtures at the Si-water interface by specular neutron reflection. The adsorption of mixed micelles of the nonionic surfactant beta-D-dodecyl maltoside and DOPC or POPC was determined as a function of bulk concentration, and using d25-beta-D-dodecyl maltoside, the composition of DOPC and POPC bilayers was determined. Bilayer thicknesses of 39+/-3 A for DOPC and 41+/-3 A for POPC agree well with data from bulk lamellar phases for both lipids, and the average area per lipid molecule can be varied from 62 to 115 A2 by varying the bulk concentrations used. The amount of surfactant in the bilayer is very sensitive to the bulk volume-to-surface area ratio, but it can be fully eliminated by ensuring a sufficiently large dilution/rinsing volume of the solution.

Synthesis and aqueous phase behavior of 1-glyceryl monooleyl ether.

Synthesis of 1-glyceryl monooleyl ether (GME) has been accomplished yielding material of high purity (99.6%). The aqueous phase behavior of synthesized lipid has been investigated by using polarized microscopy and small angle X-ray diffraction. As a result, a partial temperature-composition phase diagram has been constructed. GME forms a reversed micellar solution and reversed hexagonal liquid crystalline phase at low and high hydration, respectively. The hexagonal phase coexists with excess water and is stable up to about 63 degrees C. These findings make GME an interesting alternative to glycerol monoesters in various fields of applications.

Formation of model lipid bilayers at the silica-water interface by co-adsorption with non-ionic dodecyl maltoside surfactant.

This present article describes a new and simple method for preparing model lipid bilayers. Stable and reproducible surface layers were produced at silica surfaces by co- adsorbing lipid with surfactant at the silica surface from mixed micellar solutions. The adsorption was followed in situ by use of ellipsometry. The mixed micellar solution consisted of a lipid (L-alpha-dioleoyllecithin) and a non-ionic sugar-based surfactant (n-dodecyl-beta-maltoside). The latter showed, by itself, no affinity for the surface and could, therefore, easily be rinsed off the surface after the adsorption step. By first adsorbing from solutions with high lipid and surfactant concentrations and then, in succession, rinsing and re-adsorbing from solutions with lower lipid-surfactant concentrations, a dense-packed lipid bilayer was produced at the silica surface. The same result can be achieved in a one-step process where the rinsing, after adsorption from the concentrated solution, is performed very slowly. The thickness of the adsorbed lecithin bilayer after this treatment found was to be about 44 +/- 3 A, having a mean refractive index of 1.480 +/- 0.004. The calculated surface excess of lipids on silica was about 4.2 mg m(-2), giving an average area per lipid molecule in the two layers of 62 +/- 3 A2. The physical characteristic of the adsorbed bilayer is in good agreement with previously reported data on bulk and surface supported lipid bilayers. However, in contrast to previous investigations, we found no support for the presence of a thicker multi-molecular water layer located between the lipid layer and the solid substrate.

Normal and lateral forces between lipid covered solids in solution: correlation with layer packing and structure.

We report on the normal and lateral forces between controlled-density mono- and bilayers of phospholipid co-adsorbed onto hydrophobic and hydrophilic solid supports, respectively. Interactions between 1,2-dioleoyl-sn-glycero-3-phosphocholine layers were measured using an atomic force microscope. Notable features of the normal force curves (barrier heights and widths) were found to correlate with the thickness and density of the supported lipid layers. The friction and normal force curves were also found interrelated. Thus, very low friction values were measured as long as the supported layer(s) resisted the normal pressure of the tip. However, as the applied load exceeded the critical value needed for puncturing the layers, the friction jumped to values close to those recorded between bare surfaces. The lipid layers were self-healing between measurements, but a significant hysteresis was observed in the force curves measured on approach and retraction, respectively. The study shows the potential of using atomic force microscopy for lipid layer characterization both with respect to structure and interactions. It further shows the strong lubricating effect of adsorbed lipid layers and how this varies with surface density of lipids. The findings may have important implications for the issue of joint lubrication.

Interfacial Behavior of Triblock Copolymers at Hydrophilic Surfaces.

We report on the adsorption of a series of poly(ethylene oxide)-polytetrahydrofuran-poly(ethylene oxide) copolymers, EOn/2THFmEOn/2, at hydrophilic silica surfaces and relate our findings to the corresponding behavior at hydrophobic surfaces. The adsorption of these copolymers is similar to that of poly(ethylene oxide) homopolymers at low bulk concentrations. However, the copolymer adsorption increases strongly above a certain threshold concentration. This increase, which begins more than 1 order of magnitude below the critical micellar concentration (cmc), is related to the concomitant formation of micellar-like structures at the hydrophilic surfaces. We show in this work that a commercial (ethylene oxide-propylene oxide-ethylene oxide) triblock copolymer, Pluronic F127, exhibits a similar behavior at silica. Due to surface aggregation, much thicker layers are measured on silica than at the hydrophobic surface, where the adsorption results in the formation of a monolayer structure. The adsorbed amount and layer thickness measured on bare silica tend to decrease when the bulk concentration is raised above the cmc. We infer that this is due to changes of the molecular weight distribution and relative block sizes of the copolymers in the surface aggregates, i.e., a polydispersity effect. This study also covers some aspects of the adsorption and desorption kinetics exhibited by the copolymers at silica. As is common for adsorbing polymers, the concentration dependent adsorption process is generally observed to be much faster than the desorption process. The adsorption process is in parts diffusion controlled but overall to a complex to be fully analyzed. During adsorption from solutions with bulk concentrations exceeding the cmc, a clear overshoot of the surface excess is observed after intermediate adsorption times. Again, this is interpreted as being due to polydispersity. Finally, after an initial rapid desorption regime, the surface excess exhibits a logarithmic decay with time during desorption.

Adsorption of Polyelectrolyte-Nanoparticle Systems on Silica: Influence on Interaction Forces.

In this work, we have studied the interfacial properties of cationic polyelectrolyte (PE) and silica nanoparticle (NP) systems at macroscopic silica surfaces by means of ellipsometry. The influence of adsorbed layers on the interactions between silica surfaces was also investigated using the bimorph surface force apparatus. Added nanoparticles were observed to strongly swell the interfacial polyelectrolyte layers, an effect partly related to neutralization of charged polyelectrolyte groups. The effect was more pronounced for low charged than for highly charged polyelectrolytes. Overall, the presence of nanoparticles seemed to increase the repulsive interaction measured between silica surfaces. The force measured on approach was long range and quite strongly repulsive. On separation, an attractive bridging interaction was measured for polyelectrolyte-covered surfaces. For the low charged polyelectrolyte used in the study, the force turned repulsive on addition of nanoparticles. For the highly charged polyelectrolyte used, a change from a very strong attraction (involving a jump of the surfaces out of contact) to a very long-range elastic attractive force was observed on adding nanoparticles. The long-range elastic force indicates that polymer chains and nanoparticles form a transient network in the gap between the surfaces. The observed difference in the outward force curves may explain why the addition of nanoparticles appears to improve, e.g., shear-resistance and reflocculation characteristics of polymeric flocculants. Copyright 2000 Academic Press.

Nature of the Adsorption of Zwitterionic Surfactants at Hydrophilic Surfaces

This paper describes the adsorption of zwitterionic dodecyl-N,N-dimethylammonio alkanoates with polymethylene intercharge arms of different lengths on silica. The data presented were obtained by in situ ellipsometry, allowing time-resolved studies of the surface excess, the mean thickness, and the refractive index of thin interfacial films. It is shown that the mode of adsorption of zwitterionic surfactants is similar to that observed for ethylene-oxide-based nonionic surfactants. The interaction energy between single zwitterionic surfactants and silica is relatively weak and the adsorption process is best described in terms of surfactant self-assembly, promoted by the presence of the solid surface. The mode of adsorption is only weakly affected by increasing the number of intercharge methylene units. The surface aggregation behavior observed at the silica surface displays many parallels with the corresponding solution phase behavior. Finally, the adsorption of zwitterionic surfactants is relatively independent of the pH. However, as the pH is lowered to the pKa values of the terminal carboxyl group (i.e., as the surfactants become increasingly positively charged) desorption is observed.

Dynamics of Capillary Rise.

An overview and detailed analysis of the classical theory of capillarity is presented. A number of known equations of capillary rise dynamics are shown to be different limiting cases of one rather general equation. Some internal inconsistencies of the classical equations are pointed out. The role of nonlinear dissipation and flow pattern effects in the front zone of the liquid column and near the capillary entrance is discussed. Numerical simulations and experimental data demonstrating some characteristic types of dynamic behavior predicted by the theory are reported. Special attention is paid to the capillary rise of surfactant solutions. As applied to this special case, the existing theory is substantially elaborated by setting up a closed system of equations describing the surfactant transport and relaxation processes in the adsorption layer. A simplified relation for the capillary rise dynamics in the case of strong depletion of the interfacial region is obtained, which is in qualitative agreement with the experimental behavior. Copyright 2000 Academic Press.

Beta-casein adsorption at the hydrophobized silicon oxide-aqueous solution interface and the effect of added electrolyte.

The effect of the presence of NaCl, CaCl(2), or MgCl(2) at the same ionic strength on the structure of beta-casein layers adsorbed on hydrophobic surfaces has been investigated by neutron reflectivity measurements. The data were fitted to a four-layer model. The volume fraction versus distance profiles have a similar shape whether beta-casein is adsorbed from NaCl, CaCl(2), and MgCl(2) of the same ionic strength or whether the protein concentration is lowered 10 times. In particular at larger distances from the surface, the volume fraction values are low and similar. However, close to the hydrophobic surface the volume fraction of protein decreases in the order CaCl(2) > MgCl(2) > NaCl. We have also used a specific proteolytic enzyme, endoproteinase Asp-N, which cleaves off the hydrophilic part of beta-casein, as a tool to reveal the interfacial structure of the protein. For all the different types of added electrolytes, endoproteinase Asp N only affects the outermost beta-casein layer. Subsequent addition of beta-casein in all cases led to large increases in amounts adsorbed and in the thickness of the outer layers.

beta-Casein adsorption at the silicon oxide--aqueous solution interface.

Neutron reflectometry was used to investigate the time-dependent beta-casein adsorption at the silica-aqueous solution interface. The transient and steady-state structural characteristics of the adsorbed layer were determined from reflectivity curves, fitted to three-layer and two-layer models. The results show that the beta-casein adsorption to silica is very slow. The adsorption process involves the formation of an inner dense protein layer with a mean thickness of about 30 A onto which a more hydrated outer layer is self-associated. The surface excess and the total layer thickness of the asymmetric bilayer were, after 5 h adsorption time, estimated to be about 6.5 mg/m2 and 105 A, respectively. The adsorption behavior observed on silica contrasts with that previously reported for hydrophobic substrates, where beta-casein adsorption is much more rapid and the final surface excess is less than half of that observed for silica. Rinsing the silica surface with protein-free buffer resulted in a substantial desorption; much more pronounced than observed for hydrophobic substrates. This behavior suggests a weak adsorption affinity for a fraction of the adsorbed casein molecules; most likely the outer self-associated casein molecules in the adsorbed bilayer. The comparative desorption from hydrophobic surfaces was shown to be marginal. The difference between the layer structures adopted on hydrophobic and hydrophilic surfaces is also mirrored in the effects that the addition of a specific proteolytic enzyme (endoproteinase Asp-N) has on the adsorbed layer properties. The rinsing and endoproteinase cleavage processes result together in more than 80% reduction of the originally adsorbed mass at the silica surface. Only a thin but dense adsorbed layer remains after these treatments. The corresponding reduction reported for the hydrophobic adsorbent system was only about 20%. It is concluded that beta-casein adsorption on silica results in the formation of an asymmetric surface bound bilayer that stands in strong contrast to the monolayer structure formed at hydrophobic surfaces. This finding support the previous results obtained by using ellipsometry. The study also shows that neutron reflection, despite its limitations in time resolution, can be used for studying dynamic interfacial phenomena in protein systems.