Biomedical subjects

A V Akentiev

Publications and source records attributed to A V Akentiev.

6 recordsLinked to original sources

PubMed2005

Ellipsometric study of nonionic polymer solutions.

The thickness and refractive index of adsorption films of poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) were determined by null-ellipsometry at the air-aqueous solution interface. Both parameters, in the same way as the earlier studied dynamic surface elasticity and surface tension, exhibit rather abrupt changes when the concentration approaches the range of semidilute solutions. This behavior can be explained by the worsening of the solvent quality with increasing PEG concentration and by the PVP displacement from the surface by a contamination of high surface activity.

Journal Article↗

PubMed2004

Dynamic surface properties of poly(N-isopropylacrylamide) solutions.

The dynamic surface elasticity of aqueous solutions of poly(N-isopropylacrylamide) (pNIPAM) has been measured by the oscillating barrier and capillary wave methods as a function of time and concentration. While the real and imaginary parts of the surface elasticity almost did not change with the concentration, their kinetic dependencies proved to be nonmonotonic. Simultaneous measurements of the film thickness and adsorbed amount by null-ellipsometry showed that the pNIPAM adsorption can be divided into two steps corresponding to the formation of a concentrated narrow region close to the air phase and a region of tails and loops protruding into the bulk liquid. The local maximum of the elasticity can be observed in the course of the first step when the adsorbed macromolecules do not form long loops and tails. The results are in agreement with recent data on the nonequilibrium surface properties of solutions of other nonionic homopolymers and the theory of dilational surface viscoelasticity.

Journal Article↗

PubMed2003

Dilational surface viscoelasticity of polymer solutions.

A review of recent results on the dilational surface viscoelastic properties of aqueous solutions of non-ionic polymers is given. In the frequency range from 0.001 up to 1000 Hz the methods of transverse and longitudinal surface waves and the oscillating barrier method were applied. Viscoelastic behavior of adsorbed polymer films significantly differs from the behavior of films formed by only conventional surfactants of low molecular weight. For example, the dynamic surface elasticity of the former systems is low and almost constant in a broad concentration range. One can observe the increase of the surface elasticity only at extremely low concentrations and/or in the range of semi-dilute solutions. If the surface stress relaxation in conventional surfactant solutions is usually determined by the diffusional exchange between the surface layer and the bulk phase, the relaxation processes in the polymer systems proceed mainly inside the surface layer. Possible mechanism of the latter relaxation is discussed.

Biophysical Phenomena↗

PubMed2003

Relation between rheological properties and structural changes in monolayers of model lung surfactant under compression.

rSP-C surfactant monolayers spread on a native physiological model substrate show two plateau regions in the pi/A-isotherm. The first corresponds to the main phase transition in the monolayer from a LE to a LC phase. Its course is non-horizontal because of the complex composition of the lung surfactant. The second plateau, which is much more pronounced, cannot be attributed to a change of the phase state. Brewster angle microscopy images taken in this region show a sharp apparent decrease of the aggregation degree from the LE to the LC state. This process can be considered as a change in the monolayer orientation relative to the direction of the propagated light. Such a change can be the result of monolayer folding and formation of a thicker layer, which is supported by results of rheological measurements. The dilatation elasticity obtained from oscillating barrier and longitudinal wave measurements reveals a pure elastic behaviour with a steep increase in the second plateau region. Because of the insolubility of the pure lipid components, a possible explanation is squeezing protein components of rSP-C or its complexes with lipids out of the monolayer into the bulk.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2002

Dynamic Surface Properties of Poly(vinylpyrrolidone) Solutions.

The dynamic surface tension and the complex dynamic surface elasticity of poly(vinylpyrrolidone) (PVP) solutions were measured in the concentration range 10(-5) wt% up to about 1 wt%. The surface tension changed slowly with time at low (<10(-4) wt%) and high concentrations (>0.1 wt%). At low concentrations this is a consequence of the slow transport by diffusion of PVP molecules from the depth of the bulk phase to the surface. At high concentrations the time effect is unexpected and probably the result of PVP contamination of high surface activity. The dynamic surface elasticity of PVP solutions gradually decreases with increasing concentration up to the range of high concentrations (>0.1 wt%) where an abrupt increase in the elasticity caused by the adsorbed impurity is observed. At low and medium concentrations the viscoelastic behavior of PVP adsorbed films is similar to that of the previously investigated poly(ethylene oxide) and poly(ethylene glycol) films and is determined by the number of loops and tails protruding into the bulk phase.

Models, Theoretical↗

PubMed2002

Dynamic properties of poly(styrene)-poly(ethylene oxide) diblock copolymer films at the air-water interface.

The complex dynamic elasticity of monolayers of the diblock copolymer poly(styrene)-poly(ethylene oxide) at the air-water interface in the pancake, quasi-brush, and brush regimes has been studied by means of three experimental techniques--the surface transverse and longitudinal waves and the oscillating barrier method. In the pancake regime the surface viscoelastic properties in the frequency range under investigation (0.01-520 Hz) prove to be indistinguishable from the surface properties of the homopolymer PEO. Transition to the quasi-brush regime is accompanied by rather abrupt changes in both components of the surface viscoelasticity. The surface viscosity in the brush regime exceeds significantly the results calculated from the theory of D. M. A. Buzza et al. (J. Chem. Phys.109, 5008 (1998)), which takes into account the dissipation arising from the flow of solvent through the brush phase. Possible reasons of this discrepancy are discussed.

Journal Article↗