Jan Christer Eriksson

The lifetime of a colloid-sized gas bubble in water and the cause of the hydrophobic attraction

When considered to be open in the thermodynamic sense with respect to the enclosed gas, bubbles that are present in a liquid bulk phase are unstable in all respects and tend to dissolve. Conversely, full mechanical (and physicochemical) stability is guaranteed when a bubble is closed with respect to the gaseous component. By assuming the diffusion of dissolved gas molecules away from a spherical gas bubble to determine the shrinkage rate, we calculate the lifetime of a bubble as a function of its size. Gas bubbles in the colloidal size range, with radii between 10 and 100 nm, have surprisingly short lifetimes, between about 1 and 100 μs, whereas a nm-sized bubble can persist for months. Our results (which confirm and partwise extend the old calculations of Epstein and Plesset [5] [J. Chem. Phys., 18 (1950) 1505], indicate that the bridging bubble/cavity mechanism proposed earlier, can hardly provide the proper explanation of the long-ranged attraction force observed between hydrophobic surfaces immersed in water.

Salt effects on the cloud point of the poly(ethylene oxide)+ water system

The cloud point of high-molecular-weight poly(ethylene oxide), PEO, in aqueous salt solution has been determined as a function of the salt concentration for all potassium halides, alkali-metal chlorides and alkali-metal hydroxides. Our theoretical model for the pure PEO+water system (J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2053) has been extended to include the effects of salt on the phase separation. Basic features of the present model are a hydration shell with enhanced structuring of water as well as a zone with decreased salt concentration surround each chain. Overlaps of such regions are involved in polymer–polymer contacts and imply transfer of water molecules and ions from the proximity of the chains to the bulk solution, which gives important contributions to the free energy of interaction. The existence of the salt-deficient zone is explained as a consequence of asymmetric hydration of the ions near the polymer. The effects of the zone are large enough to account for the influence of salts on the clouding. The experimental differences found for the alkali-metal halides have been rationalized mainly in terms of varying degrees of salt penetration into the region around the chain.

A phenomenological theory of long-range hydrophobic attraction forces based on a square-gradient variational approach

Through recent surface force measurements it has been convincingly demonstrated that strong and amazingly long-range, attractive interaction forces act between hydrophobic surfaces immersed in water. Upon separating two such surfaces from molecular contact a vapour/gas cavity normally forms. This is not the case, however, when gradually diminishing the surface separation. The hydrophobic attraction forces have been recorded in this latter, metastable regime.A mean-field theory based on a square-gradient assumption is presented in this paper which is shown to account reasonably well for the surface forces found experimentally for two cylindrically shaped, hydrophobic surfaces interacting in water. The order parameter/ variational approach taken is closely related conceptually to earlier theories of repulsive hydration forces by Marcelja et al. and Cevc et al. The present theory implies that rather minor, hydrogen-bond-propagated molecular ordering effects, in the contact layers of water molecules next to the hydrophobic surfaces and in the core of the thin water film, give rise to the attraction observed. However, it does not fully address the intriguing question as to how it comes about that the hydrophobic attraction forces extend over such a wide range as 70–90 nm. It merely points in the direction that surface-induced structural changes in the core of the this water film (so far not captured by molecular dynamics simulations) which demand minimal free-energy expense may generate an interaction of a long-range nature.

Surface composition studies of the (100) and (110) faces of monocrystalline Fe0·84Cr0·16

Abstract Surface compositions of the (100) and (110) faces of monocrystalline Fe0.84Cr0.16 have been determined using Auger Electron Spectroscopy (AES) and monocrystalline Cr (100), Fe (100), Cr (110) and Fe (110) surfaces as standards. When subjected to different annealing procedures with and without oxygen gas present, drastic changes in Cr enrichment occurred in the investigated surface region, the thickness of which is of the order of 10 A. Under thermal equilibrium conditions the Cr Fe ratio is mainly influenced by differences between Cr and Fe as to surface free energy and affinity to oxygen whereas far from equilibrium, the difference in diffusion rate between Cr and Fe also plays an important role.

An ESCA and AES study of ion-exchange on the basal plane of mica

The densities of Na+, Cs+, and Ca2+ ions adsorbed onto cleaved mica surfaces from aqueous electrolyte solutions are reported. The variation in adsorption density with solution concentration of these adsorbing ions was found in each case to be in reasonable agreement with the predictions of a simple site-binding model used previously to interpret diffuse double-layer potentials. The results suggest that mica has only one type of adsorption site for these cations. However, the adsorption of the oxonium ion could not be explained by the simple model. Comparison of adsorption densities on the isolated surface with those observed on forcing two opposing mica sheets together supports the hypothesis made earlier by one of us that in dilute solutions hydrated ions can be exchanged and replaced by less hydrated ions under pressure, whereas, in more concentrated solutions this displacement is prevented. The results are consistent with the identification of short-range repulsive forces observed between mica surfaces as “hydration forces” due to the work required to dehydrate cations adsorbed at the surface.

ESCA Studies of heparinized and related surfaces: 1. Model surfaces on steel substrates

Abstract Blood-compatible surfaces prepared through deposition onto steel substrates of colloidal particles composed of heparin or dextran sulfate reacted with hexadecylammonium chloride, and chemically related surfaces, have been analyzed by means of ESCA in order to explore the detailed surface chemistry prior to and after exposure to an albumin solution. In particular it has been shown that the stabilization of the ionic heparin and dextran sulfate hexadecylammonium chloride complexes resulting from reaction with glutaraldehyde is associated with a partial chemical transformation of the hexadecylammonium ion to the corresponding Schiffs base compound. Furthermore, the ESCA S2p peaks due to disulfide bonds and sulfate groups provide evidence for distinctly different albumin adsorption characteristics on the heparin-glutar and dextran sulfate-glutar surfaces. Both of these surfaces exhibit minimal platelet adhesion. However, it is only the heparin-glutar surface that promotes the inhibition of thrombin activity at blood contact.

Theory of curved interfaces and membranes: Mechanical and thermodynamical approaches

Abstract The mechanical and thermodynamical approaches to the theory of the general curved interfaces are presented and compared. In the mechanical approach a curved interface or membrane is characterized by the tensors of surface stresses and moments. They are connected by the surface balances of the linear and angular momentum. On the other hand, in the thermodynamical approach the surface is characterized by the scalar dilation and shear tensions as well as by the bending and torsion moments. In this review we investigate the problem about the relationships connecting the mechanical and thermodynamical approaches. We find that these two approaches are in a good agreement, that they are complementary to each other and represent the two parts of a self-consistent theory. The latter can be applied to any system where curved interfaces, thin films or membranes are present: microemulsions, lamellar and sponge phases, lipid vesicles and cell membranes, capillary waves at interfaces, undulation and peristaltic surface forces, lateral capillary forces between particles in thin liquid films, etc.

Platelet and plasma coagulation compatibility of heparinized and sulphated surfaces

Abstract The degrees of platelet adhesion and activation of the plasma coagulation system were measured separately on unmodified poly-ethylene, sulphated polyethylene and different modifications of heparinized surfaces stabilized with glutardialdehyde. Thrombus formation on the unmodified surfaces was shown to involve activation of both the platelets and the coagulation system. Despite minimal platelet adhesion on the sulphated surface and the most stable heparinized one, surface-induced plasma coagulation was demonstrated. Two slightly different modifications of the heparinglutar surface proved to inhibit platelet adhesion and surfaceinduced plasma coagulation.

Preparation and protein adsorption properties of photopolymerized hydrophilic films containing N-vinylpyrrolidone (NVP), acrylic acid (AA) or ethyleneoxide (EO) units as studied by ESCA

Preparation and protein adsorption properties of photopolymerized hydrophilic films containing N-vinylpyrrolidone (NVP), acrylic acid (AA) or ethyleneoxide (EO) units as studied by ESCA

Disjoining pressure in soap film thermodynamics

Abstract A rigorous and essentially complete thermodynamic treatment is presented of a thin soap film formed from an adjacent meniscus solution. The entire soap film system includes the components: water (1), a surfactant (2), a salt (3) and an inert gas (4). Mechanically, the thin soap film is modeled as two weakly interacting surfaces of tension positioned a distance h t (= film thickness) apart. h t is an auxiliary independent variable that can be changed by varying the external excess pressure acting in the normal direction on the surfaces of tension and counteracting the effective disjoining pressure, π f , within the film. The resultant excess in lateral tension within the film is assumed to be acting in the planes of the surfaces of tension. The thermodynamics of the film are based on this mechanical model and are formulated both in terms of the overall film tension γ f and the film surface tension γ fs . Taking due account of the variance of the soap film system, a number of thermodynamic fundamental equations are derived which correspond to different experimental situations. Full congruence with the thermodynamics of curved, fluid interfaces is established. It is noted that γ f and π f as well as γ fs and π D (the Derjaguin disjoining pressure) are conjugate thermodynamic quantities. π D accounts for the long range molecular interactions between the film faces in a clearcut fashion insofar as the surfactant monolayer structures remain essentially unaffected by h t changes. Contact angle measurements are particularly useful when the chief object is to explore the thermodynamic properties of thin soap films. For such measurements h t variations are in general thermodynamically insignificant for NB films but not for CB films.