Long Range Hydration Effects in Electrolytic Free Suspended Black Films

The force law within free suspended black films made of negatively charged Aerosol-OT (AOT) with added LiCl or CsCl is studied accurately using X-ray reflectivity (ca. 1{\AA}). We find an electrolyte concentration threshold above which a substantial additional repulsion is detected in the LiCl films, up to distances of 100 {\AA}. We interpret this phenomenon as an augmentation of the Debye screening length, due to the local screening of the condensed hydrophilic counterions by the primary hydration shell.Comment: 4 pages, 4 figures, to be published Phys. Rev. Let

The field theoretic derivation of the contact value theorem in planar geometries and its modification by the Casimir effect

The contact value theorem for Coulomb gases in planar or film-like geometries is derived using a Hamiltonian field theoretic representation of the system. The case where the film is enclosed by a material of different dielectric constant to that of the film is shown to contain an additional Casimir-like term which is generated by fluctuations of the electric potential about its mean-field value.Comment: Link between Sine-Gordon and Coulomb gas pressures via subtraction of self interaction terms included. Discussion of results within Debye-Huckel approximation included. Added reference

Direct Observation of the Dynamics of Latex Particles Confined inside Thinning Water-Air Films

The dynamics of micrometer-size polystyrene latex particles confined in thinning foam films was investigated by microscopic interferometric observation. The behavior of the entrapped particles depends on the mobility of the film surfaces, the particle concentration, hydrophobicity, and rate of film formation. When the films were stabilized by sodium dodecyl sulfate, no entrapment of particles between the surfaces was possible. When protein was used as a stabilizer, a limited number of particles were caught inside the film area due to the decreased mobility of the interfaces. In this case, extraordinary long-ranged (>100 Ìm) capillary attraction leads to two-dimensional (2D) particle aggregation. A major change occurs when the microspheres are partially hydrophobized by the presence of cationic surfactant. After the foam films are opened and closed a few times, a layer of particles simultaneously adsorbed to the two interfaces is formed, which sterically inhibits any further film opening and thinning. The particles within this layer show an excellent 2D hexagonal ordering. The experimental data are relevant to the dynamics of defects in coating films, Pickering emulsions, and particle assembly into 2D arrays

Free thin liquid films (foam films) from rhamnolipids: type of the film and stability

WOS: 000183004800014For the first time the foam film model has been used to study the interaction behavior of microbial type biosurfactants adsorbed at the air/solution interface. The studies were carried out with microscopic foam films formed from solutions of anionic type rhamnolipid biosurfactants at a constant concentration of 10(-4) M. Measurements of the equilibrium film thickness (h) as a function of the electrolyte (NaCl) concentration (CI) show that with the increase of C-el from 5 x 10(-4) to 1 M, h gradually decreases from about 100 to 5 nm. Formation of common films (h > 30 nm), common black films (CBF) with h similar to 6-20 nm and 5 nm thick Newton black foam films (NBF) was found. The experimentally determined critical electrolyte concentration of CBF-NBF transition (C-el,C-cr) is 0.8 M NaCl. The direct measurements of disjoining pressure isotherms at different C-el corroborate the type of the common films and demonstrate the existence of an aqueous core in the common and CBF, and the bilayer structure of the NBF. The experimental studies show the role of the Derjaguin-Landau-Verwey-Overbeek (DLVO) surface forces for the stability of the common films. A possible action of additional structural (hydration) and eventual short-range steric; forces and their effect for the stability of the thin CBF and NBF are discussed. For thick common films the surface electric parameters-potential of the diffuse electric layer (phi(o)) and surface charge density (sigma(o)) are assessed by the equilibrium film method using the equations of the DLVO theory. The comparison of the obtained phi(o) = 71 mV with the previous results obtained for films stabilized with the salt of a fatty acid-Na oleate, demonstrates the role of the carboxylic group of the rhamnolipid molecule for the surface charge of the studied foam films. The investigations can contribute to gaining new knowledge not only for the different type of interactions in thin films stabilized with microbial type biosurfactants, but also for the interactions between two surfaces through a thin film. (C) 2003 Elsevier Science B.V. All rights reserved