R. Sedev

Relaxation behavior of human albumin adsorbed at the solution air interface

Abstract The adsorption kinetics of human albumin (HA) at the aqueous solution/air interface and the surface relaxation behaviour of HA adsorption layers after transient area disturbances are studied using an axisymmetric drop shape technique (ADSA). Transient relaxation measurements were performed with the ADSA technique by changing the volume of the pendent drop by definite increments. In the concentration interval studied, the relaxation behaviour of HA is not diffusion controlled. Relaxation phenomena observed during the induction time show the presence of structured adsorption layers. The relaxation behaviour of a quasi-equilibrium HA adsorption layer is independent of the HA concentration. The calculated apparent diffusion coefficients represent the order of magnitude of the rate constant of the real mechanism. A comparison of results calculated from adsorption kinetics and relaxation data shows good agreement with an apparent diffusion coefficient in the order of 0.001 cm2s−1.

DLVO AND NON-DLVO SURFACE FORCES IN FOAM FILMS FROM AMPHIPHILIC BLOCK COPOLYMERS

Systematic research on foam films from PEO–PPO–PEO triblock copolymers is carried out. These commercial (Synperonic, ICI, Witton, UK) non-ionic polymeric surfactants are rather pure though polydispersed. The microscopic foam film is used as a tool for investigating the interaction between two aqueous solution/air interfaces. The influence of electrolyte concentration, copolymer concentration, pH, and capillary pressure are monitored with the microinterferometric technique. Stable foam films are obtained at concentrations below CMC. Film stability is either electrostatic or steric in origin. Electrostatic repulsion arises from the charge at the water/air interface due to the preferential adsorption of OH− ions. It can be screened (by increasing the ionic strength), obliterated (by decreasing pH), or outweighed (by increasing the capillary pressure). Electrostatic interaction can be described within the framework of the DLVO theory (weak van der Waals attraction is also detected). In this respect PEO–PPO–PEO polymeric surfactants are analogous to low molecular non-ionic surfactants. In the absence of electrostatic repulsion non-DLVO forces are responsible for film stability. At concentrations between CΓ∞ and CMC the copolymers form a PEO-brush at the water/air interface. However, brush-to-brush contact is established only at higher capillary pressure and then the disjoining pressure isotherm follows de Gennes scaling prediction. At lower pressure a softer steric repulsion occurs. It is governed by the bulk surfactant concentration and hence is fundamentally different from the brush-to-brush repellency. Chain length is of utmost importance for steric interaction. The foam film is an effective model for studying de facto separately and quantitatively DLVO and non-DLVO surface forces.

The critical condition for transition from steady wetting to film entrainment

Abstract Siliconized glass cylinders of different radii, vertically withdrawn from the same glycerol-water mixture, served to model a system of constant physico-chemical properties and variable solid surface curvature. The steady velocity dependence of the dynamic capillary height was studied by photographing the profile of the meridian section of the meniscus. Although this relationship strongly depends on the cylinder radius, film entrainment always begins at dynamic heights equal to the corresponding maximum static values. This fact suggests that the visible meniscus remains quasistatic even at high velocities. Thus, assuming a quasistatic meniscus shape, the critical contact angle at which transition from steady wetting to film entrainment occurs was calculated from the static L/θ relationship. It was found that this angle is zero, in accordance with the a priori critical condition of Derjaguin and Levi (1964).

SURFACE FORCES IN FOAM FILMS FROM AN ABA TRIBLOCK COPOLYME

Abstract Foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (M = 14,000) are studied. A narrow polymer concentration range located below the CMC is investigated at both low and high electrolyte (NaCI) concentrations. The dependence of the surface force on film thickness is monitored by two complementary techniques: the porous plate method and the dynamic method of Scheludko and Exerowa. When the film thickness decreases, the total surface force is initially negative and after a minimum gradually increases to positive values. A transition from electrostatic to polymeric stabilization is induced by increasing the NaCI concentration. Our data can be fitted reasonably well with a combination of the DLVO theory ( van der Waals attraction and double layer repulsion) and de Gennes scaling theory of steric interaction between grafted polymer layers.

Transition from electrostatic to steric stabilization in foam films from ABA triblock copolymers of poly(ethylene oxide) and poly(propylene oxide)

The equilibrium thickness of microscopic foam films obtained from aqueous solutions of two different poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymers (Synperonic PE P85 and F108) is experimentally determined. The amphiphilic triblock copolymers adsorb at the air/solution interface as a brush layer. The foam film may be stabilized by electrostatic and/or steric repulsion. At a given copolymer concentration (slightly lower than the CMC) increasing the electrolyte concentration induces a transition from electrostatic to steric stabilization. The thicker foam films are electrostatically stabilized. The thinnest foam films are sterically stabilized, but their thickness is unexpectedly larger than twice the brush layer thickness. While the thickness h of these films strongly depends on the degree of polymerizationN, its scaling structure is essentially the same: two brush layers (h1 ∼ N) and an aqueous core (h2 ∼ N3/5). The microscopic foam film is a suitable tool for exploring the transition from electrostatic to steric stabilization.

Formation of a stable, highly concentrated O/W emulsion modeled by means of foam films

Abstract A stable, highly concentrated heptane-in-water emulsion stabilized with a polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) triblock copolymer (Synperonic PE F108, M =14u2008000) is prepared. The stability of the emulsion is characterized by monitoring the droplet size distribution with the NMR self-diffusion method. The results are compared with data from investigations of a single foam film. The surface force versus foam film thickness isotherms at different electrolyte concentrations (0–0.05xa0M NaCl) are measured directly by the thin liquid film pressure balance technique. By comparing the highly concentrated emulsion to single foam films under similar conditions, it is concluded that the emulsion is stabilized mainly by steric surface forces. It is demonstrated that the stability of highly concentrated O/W emulsions can be successfully modeled if the modeling is based on studies of the interaction forces in thin liquid foam and emulsion films.

Surface forces in foam films from ABA block copolymer: a dynamic method study

The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius ≈ 60–70 μm) from 10−5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 Å down to 460 Å. The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers.

PEO-BRUSH AT THE LIQUID/GAS INTERFACE

Abstract Water-soluble amphiphilic polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock copolymers (pluronics and synperonics) are used to build a PEO-brush at the aqueous solution/air interface. Microscopic foam films are formed from these solutions and their equilibrium thickness h is measured. Under certain conditions (high electrolyte concentration and low capillary pressure) h can be taken as a measure of the PEO-brush thickness L0: h≈2L0. The results are in general agreement with the simple brush theory. However an additional influence of polymeric surfactants’ asymmetry is observed. Qualitatively this effect can be attributed to the self-assembly character of adsorption at the fluid interface, i.e. the competition between adsorption of PPO blocks and stretching of PEO chains. Results for low molecular EO-containing surfactants are also in line with the above findings.

Effect of geometry on steady wetting kinetics and critical velocity of film entrainment

Abstract The velocity dependence of receding dynamic contact, angles θ r / U for siliconized cylinders of different, radii withdrawn from a glycerol-water mixture [19] show an independence on geometry and substantial influence of the material properties of the solid surface. These data are compared with the results of Ngan and Dussan [16a,b] for advancing angles θ α / U (silicon oil displacing air), which suggest a considerable effect of geometry. A similar asymmetry of the effects of geometry and material properties on the critical velocities of liquid and air film entrainment follows from the Juxtaposition of our previous results for u cr R [19,20] with literature data on U cr A onto different solid substrates. The experimental data are interpreted on the basis of the equations of Cox-Voinov [21,22], describing the data of Ngan and Dussan quantitatively. The data for the receding meniscus can be represented quantitatively only by a combined Blake-Haynes-Voinov equation taking into account the dissipation in the three-phase contact zone and in the bulk liquid.

Surface force measurement in foam films from mixtures of protein and polymeric surfactants

Abstract The disjoining pressure in foam films from Bovine Serum Albumin (BSA) and its modification upon addition of PEO-PPO-PEO polymeric surfactants (Pluronics L62 and F68) is monitored with the Thin Liquid Film–Pressure Balance Technique. The disjoining pressure Π in foam films from an aqueous solution of 4xa0gxa0L −1 BSA is always positive and monotonously increases upon decreasing film thickness h . Experiments are conducted close to BSAs isoelectric point and steric interaction seems to be of chief importance. Indeed de Gennes equation Π ≅ kT h −3 , for the steric repulsion between two adsorbed polymer layers at constant surface coverage —gives a reasonable description of our data. The addition of 4xa0μM L62 has no marked influence on isotherm shape and location but induces film rupture at about 5×10 4 xa0dynxa0cm −2 . At 40xa0μM L62 foam films are so unstable that no isotherm can be measured. The addition of 40xa0μM F68 shifts the isotherm to the right. Given the asymmetry of the two surfactants (L62: EO 9 PO 30 EO 9 and F68: EO 76 PO 30 EO 76 ) a qualitative explanation is suggested. Both Pluronics adsorb onto the BSA molecule with their PPO block. The dimension of the resulting protein/surfactant complex is practically unchanged by L62 (short PEO chains) but sensibly increased by F68 (long PEO chains) with respect to the BSA molecule. Thus the isotherms obtained with the biopolymer/surfactant mixtures remain similar to that of the pure BSA but only F68 shifts the curve toward larger thickness.