Christian S. Vassilieff

Effect of droplet deformation on the interactions in microemulsions

The deformability of droplets is accounted for as a new effect in the statistical mechanics of microemulsions. It is assumed that a plane-parallel film is formed between two colliding microemulsion droplets. The collision is accompanied by deformation of the droplets and by an increase of their interfacial area. The increased interfacial energy gives rise to an effective soft repulsion between the two droplets. In addition, the deformed droplets, having the shape of spherical segments, exhibit considerably greater van der Waals attraction than two spheres of the same volume, separated at the same surface-to-surface distance. The superposition of these two effects leads to the appearance of a potential well in the energy of droplet—droplet interaction. The latter depends on the droplet interfacial tension, the Gibbs elasticity, the Hamaker constant, and the thickness of the liquid film separating the droplets. The soft repulsion and the increased attraction predicted by this model lead to pronouncedly lower values of the calculated second virial coefficient in comparison with the hard sphere model of microemulsions. The deformable droplet model proposed in this paper allows the explanation of experimental findings such as the abnormally low measured values of the second virial and diffusion coefficients as well as the dimer formation in microemulsions.

Convective model of cross-flow microfiltration

In membrane microfiltration of colloidal and cellular suspensions tangential feed ensures continuous operation. Neglecting diffusion (Pe much greater than 1) the purely convective approach predicts well observed dependences between technological parameters and solves the so called microfiltration paradox.

Foam film study of albumin inhibited lung surfactant preparations: effect of added hydrophilic polymers

In Adult and Acute Respiratory Distress Syndrome (ARDS) the concentration of albumin in the alveolar fluid reaches 25–100 mg ml−1. Due to its high adsorption rate albumin adsorbs at the air/water interface making it inaccessible for the lung surfactant (LS). LS inactivation can be prevented by hydrophilic polymers due to depletion attraction osmotic pressure or other specific action. Two commercially available lung surfactant preparations, LSP (Curosurf and Survanta), and four hydrophilic polymers (PEG, dextran, PVP, hyaluronic acid-HA) were investigated in control experiments at one (monolayer) and two interacting air/solution interfaces (foam film) under albumin free and albumin inactivated conditions. The established procedure to measure the minimal surface tension in successive compression–expansion cycles of monolayers revealed that minimal surface tension ≤10 mN m−1 for Survanta is achieved with PEG, PVP, and HA, while for Curosurf with dextran, PVP, and HA. This correlates with the observations of Lu et al., 2005. Foam film experiments with the microinterferometric method of Scheludko and Exerowa revealed the relative ability of the polymers to decrease the effect of albumin inhibition in restoring the formation of stable and homogeneous black films (thickness <17 nm). Stable black foam films are formed by Survanta with PEG, PVP, and HA, while by Curosurf with dextran, PVP, and HA. Kinetic data of Curosurf foam film thinning were interpreted to obtain values of the disjoining pressure. An interesting change from repulsion to attraction was observed at a thickness of appr. 100 nm. It may be explained by depletion attraction overcoming the steric repulsion. The magnitude of the depletion attraction was estimated.

The close approach of cells to surfaces.

The attachment of cells to other cells and noncellular surfaces, commonly referred to as aggregation and adhesion, respectively, have come to be recognized as playing an important role in most biological and biotechnical processes. A number of examples can be cited. Tissue-specific adhesion plays a primary role in the generation of form during development.’ Most microbial infections have specific or nonspecific adhesion as a step. Also, cell-cell adhesion is an integral part of the immune response to these infections. It has been shown that altered adhesion is a fundamental property of cancer cells.* Of greatest pertinence in the present context are the facts that adhesion is a major factor in thrombus formation, endothelial repair, and the response of blood to artificial surfaces. Although the importance of adhesion and its prevalence in nature is appreciated, its fundamental physiological aspects are far from well understood. It is now known that the physics of aggregation and adhesion is very complex, involving a number of interacting factors that affect the process of approach in various ranges of ~eparation.’.~ From the point of view of the researcher, this process is difficult fully to comprehend, simulate, and control and measure experimentally. The first theory to be applied to cell interactions was that of lyophobic colloid^.^ Physical theories such as this have been gaining sophistication so as to include characteristics particular to cell adhesion:a although no model seems yet to be both physically and biologically satisfactory. The experimental techniques that make possible detailed studies of cell-cell and cellsurface interactions, in particular methods allowing direct visualization, are relatively new.”“ The purpose of this article is twofold: to review current understanding of adhesion and aggregation, particularly as these processes might apply to blood cells, and to describe a possible experimental arrangement and procedure fcr a kinetic study of the behavior of a cell at certain critical separations from a target SUI face.

Tuning of Surface Properties of Thin Lipid-Protein Films by Hydrophilic Non-Surface Active Polymers

ABSTRACT We study the capability of hydrophilic polymers (HPs) to reverse the inhibitory effect of albumin and to restore the surface activity of lipid-protein lung surfactants preparations (LSPs): Curosurf and Survanta. HPs implied are dextran and Polyethylene glycol 10000 (PEG10000). LSPs surface activity is evaluated in Thin Films (monolayer and foam) at the air/water interface. Albumin displaces LSP from the interface and inhibits their ability to sustain low surface tension in monolayers and to form stable thin foam films (FFs). When HPs are added to monolayers they restore the surface activity of LSPs and recover their capability to maintain low dynamic surface tensions. Similarly the inclusion of HPs in foam films results in neutralization of the effect of albumin and in formation of stable, thin (with thickness <17 nm) films with homogeneous surfaces, as in the case of pure LSPs. Our findings permit to search for the answer of the intriguing problem why PEG recovers better the surface activity of Survanta, while Dextran—of Curosurf. Such results might be of major importance for the treatment of Adult Respiratory Distress Syndrome.

Investigation of the Interaction between Three-Block Copolymers with Phospholipid Monolayers and foam Films

ABSTRACT The interaction of three-block copolymers (poloxamers) with dimiristoyl-phosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) is investigated experimentally at one (monolayer) and two interacting air/water interfaces (foam film). The poloxamers selected are F-88 and F-98. Both of them possess two hydrophilic polyoxyethylene (POE) and one hydrophobic polyoxypropylene moiety but differ in molecular weight. These substances and their interfacial behaviour are interesting for the development of targeted drug delivery systems. Monolayer tensiometry revealed the degree of poloxamer penetration in PC monolayers and the changes in monolayer compression/decompression behaviour. Foam film experiments showed the effect of steric disjoining pressure increase in presence of F-88 and F-98. It resulted in changes of film structure and equivalent water thickness. It was found that the interaction between poloxamers and PC molecules in monolayers and foam films depends both on copolymer size and on the acyl chain length of phosphatidylcholines.

Effect of The Hydrocarbon Chain Length on the Interfacial Tension in n-Alkane-Water-Sodium Dodecyl Sulfate Systems

Abstract The interfacial tension in some normal alkane-water-sodium dodecyl sulfate systems has been measured with the spinning drop technique. The alkane scan of the data shows a deep minimum in the interfacial tension at dodecane as an oil phase. Simple explanation of this phenomenon based on the entropy of random mixing in the interfacial layer is proposed.