Advances in Colloid and Interface Science (v.105, #1-3)

Honorary notes by Reinhard Miller; John Berg (1).

A review of spontaneous rupture in thin films with tangentially immobile interfaces is presented that emphasizes the theoretical developments of film drainage and corrugation growth through the linearization of lubrication theory in a cylindrical geometry. Spontaneous rupture occurs when corrugations from adjacent interfaces become unstable and grow to a critical thickness. A corrugated interface is composed of a number of waveforms and each waveform becomes unstable at a unique transition thickness. The onset of instability occurs at the maximum transition thickness, and it is shown that only upper and lower bounds of this thickness can be predicted from linear stability analysis. The upper bound is equivalent to the Frenkel criterion and is obtained from the zeroth order approximation of the H 3 term in the evolution equation. This criterion is determined solely by the film radius, interfacial tension and Hamaker constant. The lower bound is obtained from the first order approximation of the H 3 term in the evolution equation and is dependent on the film thinning velocity. A semi-empirical equation, referred to as the MTR equation, is obtained by combining the drainage theory of Manev et al. [J. Dispersion Sci. Technol., 18 (1997) 769] and the experimental measurements of [Radoev et al. J. Colloid Interface Sci. 95 (1983) 254] and is shown to provide accurate predictions of film thinning velocity near the critical thickness of rupture. The MTR equation permits the prediction of the lower bound of the maximum transition thickness based entirely on film radius, Plateau border radius, interfacial tension, temperature and Hamaker constant. The MTR equation extrapolates to Reynolds equation under conditions when the Plateau border pressure is small, which provides a lower bound for the maximum transition thickness that is equivalent to the criterion of Gumerman and Homsy [Chem. Eng. Commun. 2 (1975) 27]. The relative accuracy of either bound is thought to be dependent on the amplitude of the hydrodynamic corrugations, and a semi-empirical correlation is also obtained that permits the amplitude to be calculated as a function of the upper and lower bound of the maximum transition thickness. The relationship between the evolving theoretical developments is demonstrated by three film thickness master curves, which reduce to simple analytical expressions under limiting conditions when the drainage pressure drop is controlled by either the Plateau border capillary pressure or the van der Waals disjoining pressure. The master curves simplify solution of the various theoretical predictions enormously over the entire range of the linear approximation. Finally, it is shown that when the Frenkel criterion is used to assess film stability, recent studies reach conclusions that are contrary to the relevance of spontaneous rupture as a cell-opening mechanism in foams.
Keywords: Draining thin films; Thin film stability; Maximum transition thickness; Critical thickness; Spontaneous rupture; Cell opening mechanisms;

Specific ion effects via ion hydration: I. Surface tension by Marian Manciu; Eli Ruckenstein (63-101).
A simple modality is suggested to include, in the framework of a modified Poisson–Boltzmann approach, specific ion effects via the change in the ion hydration between the bulk and the vicinity of the surface. This approach can account for both the depletion of the interfacial region of structure-making ions as well as for the accumulation of structure-breaking ions near the interface. Expressions for the change in interfacial tension as a function of electrolyte concentrations are derived. On the basis of this theory, one explains the dependence of the surface potential on pH and electrolyte concentration, the existence of a minimum in the surface tension at low electrolyte concentrations and the linear dependence, with a positive or sometimes negative slope, of the surface tension on the electrolyte concentration at sufficiently high ionic strengths.
Keywords: Specific ion effect; Ion hydration; Structure-making ion; Structure-breaking ion;

The procedure proposed by Packer and Rees (J. Colloid Interface Sci. 40 (1972) 206) to interpret pulsed field gradient spin-echo (PGSE) experiments on emulsions is commonly used to resolve for the distribution of droplet sizes via nuclear magnetic resonance (NMR). Nevertheless, such procedure is based on several assumptions that may restrict its applicability in many practical cases. Among such constrains, (a) the amplitude of the spin-echo (signal) must be influenced solely by the drop phase, and not by the continuous phase; and (b) the shape of the drop size distribution must be assumed a priori. This article discusses new theory to interpret results from PGSE experiments and a novel procedure that couples diffusion measurements (PGSE) with transverse relaxation rate experiments (the so-called CPMG sequence) to overcome the above limitations. Results from experiments on emulsions of water dispersed in several crude oils are reported to demonstrate that the combined CPMG–PGSE method renders drop size distributions with arbitrary shape, the water/oil ratio of the emulsion and the rate of decay of magnetization at the interfaces, i.e. the surface relaxivity. It is also shown that the procedure allows screening if the dispersion is oil-in-water (o/w) or water-in-oil (w/o) in a straightforward manner and that it is suitable to evaluate stability of emulsions.
Keywords: Emulsion; Drop size distribution; Nuclear magnetic resonance; CPMG; Pulsed field gradient spin-echo; Surface relaxivity;

In this work, various techniques for solid surface acid-base (AB) characterization are reviewed. Different techniques employ different scales to rank acid–base properties. Based on the results from literature and the authors’ own investigations for mineral oxides, these scales are compared. The comparison shows that Isoelectric Point (IEP), the most commonly used AB scale, is not a description of the absolute basicity or acidity of a surface, but a description of their relative strength. That is, a high IEP surface shows more basic functionality comparing with its acidic functionality, whereas a low IEP surface shows less basic functionality comparing with its acidic functionality. The choice of technique and scale for AB characterization depends on the specific application. For the cases in which the overall AB property is of interest, IEP (by electrokinetic titration) and H0,max (by indicator dye adsorption) are appropriate. For the cases in which the absolute AB property is of interest such as in the study of adhesion, it is more pertinent to use chemical shift (by XPS) and the heat of adsorption of probe gases (by calorimetry or IGC).
Keywords: Acid–base; Surface characteristics; Inverse gas chromatography (IGC); Isoelectric point (IEP);

Specific ion effects via ion hydration: II. Double layer interaction by Eli Ruckenstein; Marian Manciu (177-200).
A simple modified Poisson–Boltzmann formalism, which accounts also for those interactions between electrolyte ions and colloidal particles not included in the mean potential, is used to calculate the force between two parallel plates. It is shown that the short-range interactions between ions and plates, such as those due to the change in the hydration free energy of a structure-making/breaking ion that approaches the interface, affect the double layer interaction at large separations through the modification of the surface potential and surface charge density. While at short separations (below the range of the short-range ion-hydration forces) the interaction can be attractive, at larger separations the interaction is always repulsive, as in the traditional theory. When the long-range van der Waals interactions between the ions and the system (ion-dispersion interactions) are accounted for in the modified Poisson–Boltzmann approach, an attractive force between plates can be generated. At sufficiently large separations, this attraction can become even stronger than the traditional van der Waals attraction between plates of finite thickness, thus generating a dominant long-range ‘double layer attraction’. At small plate separations, the attraction generated by the ion-dispersion forces combined with the electrostatic repulsion due to the double layers overlap can lead to a variety of interactions, from a weak attraction (which is typically by at least one order of magnitude smaller than the traditional van der Waals attraction between plates) to a strong double layer repulsion (for sufficiently large surface charges). Both types of ion interactions (long-range van der Waals or short-range ionic hydration) strongly affect the magnitude of the double layer interaction, and can account for the specific ion effects observed experimentally. However, they do not affect qualitatively the traditional theory of the colloid stability, which predicts that the colloid is stable when there is a sufficiently large charge on the surface, and coagulates when the van der Waals interactions between two colloidal particles dominate. The only qualitative difference found when the ion-dispersion interactions were incorporated into the traditional double layer theory was the emergence of a ‘double layer attraction’ at very large separations, which, however, does not affect much the stability of colloids.
Keywords: Colloidal particles; Double layer interaction; Ion-hydration forces;

Photon correlation spectroscopy investigations of proteins by Vladimir M. Gun'ko; Alla V. Klyueva; Yuri N. Levchuk; Roman Leboda (201-328).
Physical principles of photon correlation spectroscopy (PCS), mathematical treatment of the PCS data (converting autocorrelation functions to distribution functions or average characteristics), and PCS applications to study proteins and other biomacromolecules in aqueous media are described and analysed. The PCS investigations of conformational changes in protein molecules, their aggregation itself or in consequence of interaction with other molecules or organic (polymers) and inorganic (e.g. fumed silica) fine particles as well as the influence of low molecular compounds (surfactants, drugs, salts, metal ions, etc.) reveal unique capability of the PCS techniques for elucidation of important native functions of proteins and other biomacromolecules (DNA, RNA, etc.) or microorganisms (Escherichia coli, Pseudomonas putida, Dunaliella viridis, etc.). Special attention is paid to the interaction of proteins with fumed oxides and the impact of polymers and fine oxide particles on the motion of living flagellar microorganisms analysed by means of PCS.
Keywords: Photon correlation spectroscopy; Dynamic light scattering; Static light scattering; Autocorrelation function treatment; Protein solutions; Diffusion coefficients of proteins; Protein size distribution; Protein aggregation; Protein interaction with solid particles; Motion of microorganisms;

Electron work functions of several metals (Au, W, Ag, Cu, Mo, Ti, Al) and Si at atmospheric conditions have been measured by the method of the static capacitor with an ionized gap between the electrodes. Results for all samples, excluding Au, Al and Si, conform to reference data with an accuracy of ±1.0%. For samples Au, Al and Si, the electron work function values conform to reference data after short-time heat treatment at atmospheric conditions at 200 °C (for Al) and 600 °C (for Au, Si).
Keywords: Surface; Potential; Work function; Static ionized capacitor;

The morphology and electrical microstructure of different anodic oxide films on aluminum and thermic oxide on surface p-type silica (KDB/100) were studied using atomic force microscopy and scanning capacitance microscopy. It was shown that the small basic element in the texture of both thin (0.05 μm) and thick (0.8 μm) oxide films represents a disklike element (‘grain’) approximately 200×200×30 nm in size. For films with a rough surface relief, the capacitance (and consequently, the surface potential) shows strong fluctuations in the vicinity of coarse (∼5–8 μm) pores. Because of this, the image of the surface obtained using atomic force microscopy does not coincide with that obtained by scanning capacitance microscopy (the opposite contrast effect). The manifestation of the opposite contrast correlates with an increase in the surface potential of the anodic oxide films measured by an independent method. A series of experiments under atmospheric conditions at different distances from the end of the cantilever to the surface of anodic oxide films showed that the influence of the surface field is detectable at long distances (up to 0.7 μm). It was shown that at a test temperature of 120 °C, the opposite contrast disappears: the images obtained in the semicontact (atomic force microscopy) and non- contact (scanning capacitance microscopy) modes coincide with each other. The results obtained suggest a relationship between the formation of electrostatic nanosized irregularities at the surface of oxide films and the sorption of water molecules under atmospheric conditions.
Keywords: Electrical surface; Relief; Atomic force microscopy; Scanning capacitance microscopy;