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Advances in Colloid and Interface Science (v.157, #1-2)

Editorial Board (pp. iii).

Microscopic description of a drop on a solid surface by Eli Ruckenstein; Gersh O. Berim (pp. 1-33).

Two approaches recently suggested for the treatment of macro- or nanodrops on smooth or rough, planar or curved, solid surfaces, based on fluid–fluid and fluid–solid interaction potentials are reviewed. The first one employs the minimization of the total potential energy of a drop by assuming that the drop has a well defined profile and a constant liquid density in its entire volume with the exception of the monolayer nearest to the surface where the density has a different value. As a result, a differential equation for the drop profile as well as the necessary boundary conditions are derived which involve the parameters of the interaction potentials and do not contain such macroscopic characteristics as the surface tensions. As a consequence, the macroscopic and microscopic contact angles which the drop profile makes with the surface can be calculated. The macroscopic angle is obtained via the extrapolation of the circular part of the drop profile valid at some distance from the surface up to the solid surface. The microscopic angle is formed at the intersection of the real profile (which is not circular near the surface) with the surface. The theory provides a relation between these two angles. The ranges of the microscopic parameters of the interaction potentials for which (i) the drop can have any height (volume), (ii) the drop can have a restricted height but unrestricted volume, and (iii) a drop cannot be formed on the surface were identified. The theory was also extended to the description of a drop on a rough surface. The second approach is based on a nonlocal density functional theory (DFT), which accounts for the inhomogeneity of the liquid density and temperature effects, features which are missing in the first approach. Although the computational difficulties restrict its application to drops of only several nanometers, the theory can be applied indirectly to macrodrops by calculating the surface tensions and using the Young equation to determine the contact angle. Employing the canonical ensemble version of the DFT, nanodrops on smooth and rough solid surfaces could be investigated and their characteristics, such as the drop profile, contact angle, as well as the fluid density distribution inside the drop can be determined as functions of the parameters of the interaction potentials and temperature. It was found that the contact angle of the drop has a simple (quasi)universal dependence on the energy parameter ε_fs of the fluid–solid interaction potential and temperature. The main feature of this dependence is the existence of a fixed value θ₀ of the contact angle θ which separates the solid substrates (characterized by the energy parameter ε_fs of the fluid–solid interaction potential) into two classes with respect to their temperature dependence. For θ> θ₀ the contact angle monotonously increases and for θ< θ₀ monotonously decreases with increasing temperature. For θ= θ₀ the contact angle is independent of temperature. The results obtained via DFT were also applied to check the validity of the macroscopic phenomenological equations (Cassie–Baxter and Wenzel equations) for drops on rough surfaces, and of the equation for the sticking force of a drop on an inclined surface.

Keywords: Wetting; Drop profile equation; Contact angle; Microscopic approach; Density functional theory

Update on current state and problems in the surface tension of condensed matter by V.A. Marichev (pp. 34-60).

The dual concept of surface energy formally allows application of Gibbs thermodynamics to the surface tension of solids and is unlimited using the classical Lippmann equation for solids that is shown to contradict all available in situ experimental data. At present, the generalized Lippmann equation is believed to be the most universal, since the classical Lippmann equation, the Shuttleworth and Gokhshtein equations could be derived from it. Lately it was evaluated in two opposite ways: the first—the experimental verification of the Gokhshtein equation supports correctness of the generalized Lippmann and Shuttleworth equations; the second—the incompatibility of the Shuttleworth equation with Hermann's mathematical structure of thermodynamics makes invalid all its corollaries, including the generalized Lippmann and Gokhshtein equations. Both approaches are shown here to be incorrect, since the Gokhshtein equation cannot be correctly derived from any of the above-mentioned equations.The Frumkin derivation of the first and second Gokhshtein equations follows from one thermodynamic relationship general for the surface tension of both solid and liquid electrodes. The classical Lippmann equation is also derived from this general relationship as a particular case of the second Gokhshtein equations. We have considered the hierarchy of these equations and discussed the straightforward application of the classical Lippmann equation for solids with an account for elasticity of the surface structured layers of liquids.The partial charge transfer during anion adsorption cannot be measured in electrochemical experiments or reliably estimated by quantum-chemical and DFT calculations. However, it is directly involved in the adsorbate charge that is experimentally accessible by in situ contact electric resistance technique. We present the first quantitative evaluation of charge transfer during halides adsorption on silver from aqueous solutions in dependence on the electrode potential.

Keywords: Reversible cleavage; Surface tension of condensed matter; Lippmann equation; Shuttleworth equation; Gokhshtein equations; Elasticity of liquid surface layers

An attempt to explain bimodal behaviour of the sapphire c-plane electrolyte interface by Lutzenkirchen J. Lützenkirchen; R. Zimmermann; T. Preočanin; A. Filby; T. Kupcik; Kuttner D. Küttner; A. Abdelmonem; D. Schild; T. Rabung; M. Plaschke; F. Brandenstein; C. Werner; H. Geckeis (pp. 61-74).

A tentative picture for the charging of the sapphire basal plane in dilute electrolyte solutions allows reconciliation of the available experimental observations within a dual charging model. It includes the MUltiSIte Complexation (MUSIC) model and auto-protolysis of interfacial water. The semi-empirical MUSIC model predicts protonation and deprotonation constants of individual surface functional groups based on crystal structure and bond-valence principles: on the ideal sapphire c-plane only doubly co-ordinated hydroxyl groups exist which cause quasi zero surface potential (defined as the potential in the plane of the surface hydroxyl groups) from pH 5 to 7 and rather weak charging beyond (compared to typical oxide behaviour). MUSIC predictions concur strikingly with recently published sum frequency data for the pH dependence of the so-called “ice-like” water band (interfacial water) and contact angle titrations. Zeta potential as well as second harmonic generation data reveal a sharp IEP of around 4 and a negative surface charge at the pristine point of zero charge predicted by the MUSIC model. New zeta-potential data corroborate (i) the low IEP and its insensitivity to salt concentration and (ii) the second harmonic results.We thus establish two groups of conflicting results arising from different techniques. A conventional model of the mineral electrolyte interface such as the MUSIC model is at odds with the negative zeta potentials in the pH range 5 to 7. Therefore an additional charging mechanism is invoked to explain all the observations. Enhanced auto-protolysis of interfacial water is the most probable candidate for this additional mechanism, in agreement with net water orientation observed with sum frequency generation and second harmonic generation. Our phenomenological explanation is further corroborated by the similarity of the zeta potential vs. pH curves of the c-plane with those of hydrophobic surfaces. Additional support comes from infrared spectroscopic data on thin water films on sapphire c-plane samples. Most stunningly, theoretical calculations on basal planes of this kind suggest a 2D water bilayer that makes such surfaces hydrophobic towards further adsorption of water. The proposed dual charging mode approach comprises the MUSIC model for protonation/deprotonation of the surface aluminols affecting the surface potential and the currently advocated enhanced auto-protolysis picture for hydrophobic surfaces controlling the zeta-potential and can explain the available information in a qualitative way. The respective contributions from the two components of this dual charging mechanism may be different for different single crystal cuts of alumina. Thus interplay between protonation/deprotonation of surface functional groups and auto-protolysis of interfacial water will cause the observed zeta potentials and isoelectric points. Repercussions of one mechanism on the other will result in the most favourable interfacial water structure, which can be followed by non-linear optic techniques like sum frequency generation.

Keywords: Zeta potential; Sapphire; Alumina; Sum frequency generation; Second harmonic generation; Colloid adhesion; Water structure; Contact angle; MUSIC model

Self-organization in the flow of complex fluids (colloid and polymer systems) by A.Ya. Malkin; A.V. Semakov; V.G Kulichikhin (pp. 75-90).

Different types of regular and irregular self-organized structures observed in deformation of colloid and polymer substances (“complex fluids”) are discussed and classified. This review is focused on experimental evidence of structure formation and self-organization in shear flows, which have many similar features in systems of different types.For single-phase (uniform) polymer systems regular periodic surface structures are observed. Two main types of these structures are possible: small-scale regular screw-like periodic structures along the whole stream (usually called “shark-skin”) and long-period smooth and distorted parts of a stream attributed as a “stick–slip” effect. The origin of surface irregularities of both types is elasticity of a liquid. In the limiting case of high enough Weissenberg numbers, medium loses fluidity and should be treated as a rubbery matter. The liquid-to-rubbery transition at high Weissenberg numbers is considered as the dominating mechanism of instability, leading in particular to the wall slip and rupture of a stream.Secondary flows (“vorticity”) in deformation polymeric substances and complex fluids are also obliged to their elasticity and the observed Couette–Taylor-like cells, though being similar to well-known inertial secondary flows, are completely determined by elasticity of colloid and polymeric systems.In deformation of colloidal systems, suspensions and other dense concentrated heterophase materials, structure formation takes place at rest and the destroying of the structure happens as the yield stress. In opposite to this case, strong deformations can lead to the shear-induced structure formation and jamming. These effects are of general meaning for any complex fluids as well as for dense suspensions and granular media. Strong deformations also lead to separation of a stream into different parts (several “bands”) with various properties of liquids in these parts.So, two principal effects common for any polymers and complex fluids can be pointed at as the physical origin of self-organization in shearing. This is elasticity of a liquid and a possibility of its existence in different phases or relaxation states, while in many cases elasticity of a fluid is considered as the most important provoking factor for transitions between different types of rheological behavior, e.g. the fluid-to-rubbery-like behavior at high deformation rates and the transition from the real laminar flow to wall slip.

Keywords: Self-organization; Micellar colloids; Suspensions; Polymer liquids; Vorticity; Banding; Shark-skin; Wall slip; Elastic liquids

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Advances in Colloid and Interface Science (v.157, #1-2)

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Advances in Colloid and Interface Science (v.157, #1-2)