Advances in Colloid and Interface Science (v.111, #1-2)

Contents (vii).

Foreword by Reinhard Miller (1).

In Section 1, spreading of small liquid drops over thin dry porous layers is investigated from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397]. Drop motion over a porous layer is caused by an interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in an expanding of the drop base, and (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and an expanding of the wetted region inside the porous layer. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations has been derived to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters, one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate, and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments were used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters are determined. Experiments were carried out on the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactory for the experimental data. According to theory predictions [1]: (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function, and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation. These conclusions again are in good agreement with experimental observations [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397].In Section 2, experimental investigations are reviewed on the spreading of small drops of aqueous SDS solutions over dry thin porous substrates (nitrocellulose membranes) in the case of partial wetting [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]. The time evolution was monitored of the radii of both the drop base and the wetted area inside the porous substrate. The total duration of the spreading process was subdivided into three stages—the first stage: the drop base expands until the maximum value of the drop base is reached; the contact angle rapidly decreases during this stage; the second stage: the radius of the drop base remains constant and the contact angle decreases linearly with time; the third stage: the drop base shrinks and the contact angle remains constant. The wetted area inside the porous substrate expends during the whole spreading process. Appropriate scales were used with a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate, and the dynamic contact angle on the dimensionless time. Experimental data showed [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]: the overall time of the spreading of drops of SDS solution over dry thin porous substrates decreases with the increase of surfactant concentration; the difference between advancing and hydrodynamic receding contact angles decreases with the surfactant concentration increase; the constancy of the contact angle during the third stage of spreading has nothing to do with the hysteresis of contact angle, but determined by the hydrodynamic reasons. It is shown using independent spreading experiments of the same drops on nonporous nitrocellulose substrate that the static receding contact angle is equal to zero, which supports the conclusion on the hydrodynamic nature of the hydrodynamic receding contact angle on porous substrates.In Section 3, a theory is developed to describe a spontaneous imbibition of surfactant solutions into hydrophobic capillaries, which takes into account the micelle disintegration and the concentration decreasing close to the moving meniscus as a result of adsorption, as well as the surface diffusion of surfactant molecules [N.V. Churaev, G.A. Martynov, V.M. Starov, Z.M. Zorin, Colloid Polym. Sci. 259 (1981) 747]. The theory predictions are in good agreement with the experimental investigations on the spontaneous imbibition of the nonionic aqueous surfactant solution, Syntamide-5, into hydrophobized quartz capillaries. A theory of the spontaneous capillary rise of surfactant solutions in hydrophobic capillaries is presented, which connects the experimental observations with the adsorption of surfactant molecules in front of the moving meniscus on the bare hydrophobic interface [V.J. Starov, Colloid Interface Sci. 270 (2003)].In Section 4, capillary imbibition of aqueous surfactant solutions into dry porous substrates is investigated from both theoretical and experimental points of view in the case of partial wetting [V. Straov, S. Zhdanov, M. Velarde, J. Colloid Interface Sci. 273 (2004) 589]. Cylindrical capillaries are used as a model of porous media for theoretical treatment of the problem. It is shown that if an averaged pore size of the porous medium is below a critical value, then the permeability of the porous medium is not influenced by the presence of surfactants at any concentration: the imbibition front moves exactly in the same way as in the case of the imbibition of the pure water. The critical radius is determined by the adsorption of the surfactant molecules on the inner surface of the pores. If an averaged pore size is bigger than the critical value, then the permeability increases with surfactant concentration. These theoretical conclusions are in agreement with experimental observations.In Section 5, the spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, M.G. Velarde, J. Colloid Interface Sci. 227 (2000) 185]. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid–vapor interfacial tension and hydrophilise the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by experimental observations.In Section 6, the process of the spontaneous spreading of a droplet of a polar liquid over solid substrate is analyzed in the case when amphiphilic molecules (or their amphiphilic fragments) of the substrate surface layer are capable of overturning, resulting in a partial hydrophilisation of the surface [V.M. Starov, V.M. Rudoy, V.I. Ivanov, Colloid J. (Russian Academy of Sciences English Transaction) 61 (3) (1999) 374]. Such a situation may take place, for example, during contact of an aqueous droplet with the surface of a polymer whose macromolecules have hydrophilic side groups capable of rotating around the backbone and during the wetting of polymers containing surface-active additives or Langmuir–Blodgett films composed of amphiphilic molecules. It was shown that droplet spreading is possible only if the lateral interaction between neighbouring amphiphilic molecules (or groups) takes place. This interaction results in the tangential transfer of “the overturning state” to some distance in front of the advancing three-phase contact line making it partially hydrophilic. The quantitative theory describing the kinetics of droplet spreading is developed with allowance for this mechanism of self-organization of the surface layer of a substrate in the contact with a droplet.
Keywords: Surfactant solutions; Porous substrates; Imbibition;

It is the forces between the microscopic constituents of materials which to a large extent determine the macroscopic properties. For example, it is the differences in bonding between the carbon atoms which determines the different physical properties of carbon and graphite. The same is true in colloidal systems. In colloidal systems, there are three common types of long-range interactions between particles: van der Waals forces, electrical double layer forces and steric forces. In this paper, examples as to how these forces can be modified and even manipulated will be given. To convincingly demonstrate these effects, it is necessary to measure these interaction forces. We have achieved this by using the principles of atomic force microscopy (AFM). The principle is simple, a small particle, 5–30 μm, is attached onto a small weak cantilever spring. The interaction between this particle and another particle or a surface is measured by monitoring the deflection of the spring as the two particles are moved together. In this paper, I shall give examples of direct measurements of van der Waals, electrical double layer and steric forces and show how they can be modified and how these modifications affect the properties of bulk suspensions.Similar principles are involved in the interactions of biological materials. However, nature is much cleverer than man such that many of the macromolecules on cell surfaces are able to specifically recognise only one other molecule. An example of this recognition-type interaction, namely, cholera toxin interacting with the glycolipid Gm1, will also be presented. Finally, the adhesion of cells to surfaces of different surface chemistries has been determined; this is of significance in many fields ranging from fouling of filtration membranes on the one hand to the biocompatibility of surgical implants on the other.
Keywords: Manipulation; Forces; Colloid science;

Polyelectrolyte multilayer capsules as vehicles with tunable permeability by Alexei A. Antipov; Gleb B. Sukhorukov (49-61).
This review is devoted to a novel type of polymer micro- and nanocapsules. The shell of the capsule is fabricated by alternate adsorption of oppositely charged polyelectrolytes (PEs) onto the surface of colloidal particles. Cores of different nature (organic or inorganic) with size varied from 0.1 to 10 μm can be used for templating such PE capsules. The shell thickness can be tuned in nanometer range by assembling of defined number of PE layers. The permeability of capsules depends on the pH, ionic strength, solvent, polymer composition, and shell thickness; it can be controlled and varied over wide range of substances regarding their molecular weight and charge. Including functional polymers into capsule wall, such as weak PEs or thermosensitive polymers, makes the capsule permeability sensitive to correspondent external stimuli. Permeability of the capsules is of essential interest in diverse areas related to exploitation of systems with controlled and sustained release properties. The envisaged applications of such capsules/vesicles cover biotechnology, medicine, catalysis, food industry, etc.
Keywords: Colloid; Encapsulation; Layer-by-layer; Controlled release; Nanoengineering;

Phospholipid mesophases at solid interfaces: in-situ X-ray diffraction and spin-label studies by Michael Rappolt; Heinz Amenitsch; Janez Strancar; Cilaine V. Teixeira; Manfred Kriechbaum; Georg Pabst; Monika Majerowicz; Peter Laggner (63-77).
In this work, we report on recent investigations, both on the global and on the local molecular architecture of supported phospholipid model membranes. A brief theoretical introduction explains how global structural information on supramolecular lipid ensembles can be retrieved from surface X-ray diffraction measurements as well as how spin-label electron paramagnetic resonance spectroscopy (EPR) provides complementary information on the local environment of probe molecules.The combination of especially designed X-ray cells with the technique of small- and wide-angle X-ray surface scattering makes it possible to explore various fields of lipid research and its applications. Examples for different physico-chemical conditions are presented: (i) in situ chemistry under excess of water conditions demonstrating how solid-supported lipid films sense salinity, (ii) the 3D electron density reconstruction of a vesicle-fusion intermediate under controlled humidity, and (iii) complementary temperature and pressure effects on oriented phospholipid samples.Further, special attention has been given to the influence of different film preparation techniques with respect to quality and the defect structure manifestation. To resolve the proportions and local properties of defects in a hydrated lipid-deposited surface, spin-label EPR was applied. The results from 9.6 GHz EPR as well as from 1.2 GHz EPR suggest the alignment to be in the range between 30% and 80%. In addition, slow time-dependent EPR measurements point to nano-structural rearrangements due to water flow and reduction of alignment quality.
Keywords: Phospholipid; Mesophase; Surface film; X-ray diffraction; Topography; Spin label; EPR spectroscopy;

This paper discusses synthetic strategies for fabrication of new organized planar inorganic, polymeric, composite and bio-inorganic nanostructures by methods based on chemical reactions and physical interactions at the gas–liquid interface, Langmuir monolayer technique, interfacial ligand exchange and substitution reactions, self-assembling and self-organization processes, DNA templating and scaffolding.Stable reproducible planar assemblies of ligand-stabilized molecular nanoclusters containing definite number of atoms have been formed on solid substrate surfaces via preparation and deposition of mixed Langmuir monolayers composed by nanocluster and surfactant molecules. A novel approach to synthesis of inorganic nanoparticles and to formation of self-organized planar inorganic nanostructures has been introduced. In that approach, nanoparticles and nanostructures are fabricated via decomposition of insoluble metal-organic precursor compounds in a layer at the gas–liquid interface. The ultimately thin and anisotropic dynamic monomolecular reaction system was realized in that approach with quasi-two-dimensional growth and organization of nanoparticles and nanostructures in the plain of Langmuir monolayer. Photochemical and redox reactions were used to initiate processes of interfacial nucleation and growth of inorganic phase. It has been demonstrated that morphology of resulting inorganic nanostructures can be controlled efficiently by variations of growth conditions via changes in state and composition of interfacial planar reaction media, and by variations of composition of adjacent bulk phases. Planar arrays and chains of iron oxide and ultrasmall noble metal (Au and Pd) nanoparticles, nanowires and new organized planar disk, ring, net-like, labyrinth and very high-surface area nanostructures were obtained by methods based on that approach.Highly organized monomolecular polymeric films on solid substrates were obtained via deposition of Langmuir monolayer formed by water-insoluble amphiphilic polycation molecules. Corresponding nanoscale-ordered planar polymeric nanocomposite films with incorporated ligand-stabilized molecular metallic nanoclusters and interfacially grown nanoparticles were fabricated successfully. Novel planar DNA complexes with amphiphilic polycation monolayer were formed at the gas–aqueous phase interface and then deposited on solid substrates. Toroidal and new net-like conformations were discovered in those complexes. Nanoscale supramolecular organization of the complexes was dependent on cationic amphiphile monolayer state during the DNA binding. These monolayer and multilayer DNA/amphiphilic polycation complex Langmuir–Blodgett films were used as templates and nanoreactors for generation of inorganic nanostructures via metal cation binding with DNA and following inorganic phase growth reactions. As a result, ultrathin polymeric nanocomposite films with integrated DNA building blocks and organized inorganic semiconductor (CdS) and iron oxide quasi-linear nanostructures were formed. It has been demonstrated that interaction of deposited planar DNA/amphiphilic polycation complexes with bulk phase colloid inorganic cationic ligands (CdSe nano-rods) can result in formation of new highly organized hybrid bio-inorganic nanostructures via interfacial ligand exchange and self-organization processes. The methods developed can be useful for investigation of fundamental mechanisms of nanoscale structural organization and transformation processes in various inorganic and molecular systems including bio-molecular and bio-inorganic nanostructures. Also, those methods are relatively simple, environmentally safe and thus could prove to be efficient practical instruments of molecular nanotechnology with potential of design and cost-effective fabrication of new controlled-morphology organized planar inorganic and composite nanostructured materials. Possible applications of obtained nanostructures and future developments are also discussed.
Keywords: Liquid–gas interface; Monolayer; Nanostructures; Self-organization; Molecular nanotechnology;

Flocculation of cellular suspensions by polyelectrolytes by Sandor Barany; Adam Szepesszentgyörgyi (117-129).
The regularities, kinetics and mechanisms of flocculation of Escherichia coli and B. thuringiensis var. israelensis (Bti) cellular suspensions by water-soluble polymers–and first of all cationic polyelectrolytes of different charge density and stiffness of the macromolecule chain have been investigated. The effect of the focculant dose and nature, its charge density, the hydrophobic–hydrophilic balance in macromolecule, the suspension concentration, the mode of adding the reagent, the pH and the medium composition on the degree of aggregation of cells both in perikinetic regime and in a flowing system is considered. It has been shown that the main laws of microorganism's suspension flocculation are the same as the laws of flocculation of inorganic dispersions but at the same time the first process is much more complicated because the cell–flocculant interactions are strongly affected by products of cell metabolism, components of the culture liquor, pH value, electrolyte content as well as by the changing structure of the cell surface. On the basis of complex measurements of polymer adsorption and its effect on the electrokinetic potential and degree of aggregation of cells, a conclusion is made that the aggregation of E. coli cells by flexible polyelectrolytes like polydiethylaminoethylmetacrylate and its copolymers with acrylic acid, acrylamide and vinylpyrrolidone is due to charge neutralization, while the flocculation in the presence of rigid-chain chitosan and its derivatives is caused mainly by “bridging” between cells via adsorbed macromolecules. Extraction of cells from suspension can be enhanced by combination of electroflotation and flocculation by cationic polyelectrolytes. It has been shown that dilute suspensions of Bti bacteria can be effectively flocculated and concentrated using different cationic and anionic polyelectrolytes that is necessary for its formulation and use as anti-mosquito agent.
Keywords: Kinetics; Flocculation; Mechanisms;