Some results from 50 years' research on surface forces

A review is presented about research on surface forces and surface interactions conducted over the past half-century, with some emphasis on the pioneering contributions of the Department of Surface Phenomena at the Institute of Physical Chemistry of the USSR Academy of Sciences.     

Problems of adhesion

Many discrepancies in the theory of adhesion are connected with the confusion of processes of bond-forming and bond-breaking which have very different character. The former consumes several decimal orders of magnitude more work than would correspond to a reversible process.

The most important cause of this is the work needed to separate the parts of the electric double layer formed at the contact of two bodies. The corresponding electrostatic attraction plays the most important part in adhesion of solid bodies and especially of powder particles. It explains especially the photo controlled adhesion of powders. In contrast to this the separation of particles imbedded in a liquid may be realized nearly reversibly when the rate of process is not very high. The theory of adhesion (coagulation) of lyophobic colloid particles (the Derjaguin-Landau-Verwey-Overbeek theory) takes into consideration the Van der Waals forces and ionic-electrostatic interactions. Recently this theory has included in its scope the phenomena of repeptization.     

Direct measurements of long-range surface forces in gas and liquid media

A modified set-up was applied to carry out direct measurements of the forces of molecular attraction of gold spheres and crossed quartz filaments in air within the region of distances from 10 to 100 nm. Some quantitative deviations from Lifshitz's theory for gold may be attributed to an insufficient reliability of the spectral data used in the calculations. The DLVO theory adequately describes the interaction of glass threads in KCl (10^?3 ÷ 10^?5 N) solutions within the region of 5 to 100 nm. At a distance smaller than 5 nm, the deviations from DLVO theory are attributable to the influence of structural forces.When the contact between crossed hydrophobized quartz threads in water is broken, the attraction forces (which exceed the molecular forces by several orders of magnitude) at a distance of up to 300 nm are detected.     

Analytical calculation of repulsion forces arising when the non-ionic diffuse adsorption layers are overlapped

Summary Calculation of the molecular (Van der Waals) component of disjoining pressure of an solution interlayer is rendered difficult by the non-uniformity of the concentration of a dissolved component in this interlayer. This interferes with the direct application of Lifshits et al.'s calculation method. It is the use of an analogue of Gibbs-Duhem' equation that leads to an expression for the disjoining pressure of the interlayer: one component of this expression is equal to the disjoining pressure of a pure solvent, while the other takes into account the influence of the dissolved component. This influence is determined by an excess (quasi-adsorption) of the dissolved component in the interlayer. For determining this excess, the effective Van der Waals' potential energy of the dissolved molecules in the interlayer is used.By introducing a certain simplification, it is possible to obtain a simple analytical expression for the resultant disjoining pressure of the solution interlayer. As the interlayer becomes thinner, the expression changes its sign, whether from the negative to the positive or from the positive to the negative one, or it retains the negative sign. Such a result is indicative of the influence of the effects of osmotic nature, which are similar to those acting in ionic solutions.

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Zusammenfassung Die Berechnung der Molekular-(Van-der-Waals)-Komponente des Spaltdruckes einer Lösung wird durch Ungleichmäßigkeit der Konzentration der aufgelösten Komponente in der Zwischenschicht schwierig gemacht. Dies verhindert die unmittelbare Anwendung der Berechnungsmethode von Lifshits u. a. Infolge der Anwendung eines Analogons der Gibbschen-Duhemschen Gleichung wird ein Ausdruck für den Spaltdruck der Zwischenschicht abgeleitet. Eine Komponente dieser Gleichung ist dem Spaltdruck des reinen Lösungsmittels gleich, während die zweite Komponente den Einfluß des aufgelösten Bestandteils berücksichtigt. Dieser Einfluß wird durch Überschuß (Quasiadsorption) am letzteren in der Zwischenschicht bestimmt. Zur Ermittlung dieses Überschusses benutzt man die Berechnung der effektiven Van-der Waalschen Potentialenergie der aufgelösten Moleküle in der Zwischenschicht.Unter Zuhilfenahme einer Vereinfachung leitet man einen einfachen analytischen Ausdruck für die Resultierende des Spaltdruckes der Lösungszwischenschicht ab.Im Laufe der Verdünnung der Zwischenschicht ändert dieser Ausdruck sein Vorzeichen vom negativen zum positiven oder vom positiven zum negativen ab, oder schließlich behält er das negative Vorzeichen bei.

     

Investigation on the adhesion of polymer particles to the surface of a semiconductor

In this work the role of electrostatic forces in the adhesion of particles of dielectrics to a solid surface was studied. The experiment consisted in measuring simultaneously the force of adhesion and the charges of the electric double layer arising upon contact. The measurements were made with specially developed units—a pneumatic adhesiometer and a charge-spectrometer. The objects of study were polymer powders used in electrography, whose adhesion to thin layers of selenium samples were varied by varying the illumination and were estimated by their volume resistance in the dark and in the light. The shift in the Fermi level of the selenium caused by the change in the output work is accompanied by a change in adhesion of the particles. Experiments on the separation of polymer particles and films from selenium surface on illumination revealed spectral dependence of adhesion. In accordance with the electronic theory of adhesion, the effects observed may be attributed to the change in charge density of the electric double layer.     

An experimental investigation of polymolecular solvate (adsorbed) films as applied to the development of a mathematical theory of the stability of colloids

After a critical discussion of the current concepts of solvation of surfaces and colloid particles, as well as of the methods of studying solvation, a new quantitative definition of solvation is advanced. This definition connects solvation with the disjoining action of thin layers of liquids, discovered and studied in earlier experiments described in the preceding papers of this series.

The present paper describes a new, more convenient method of measuring both the thickness and disjoining action of thin layers for cases when they separate gas bubbles from solids (wetting films).

Observations of non-equilibrium states of such wetting films are described.

Experimental data are quoted, which are used in establishing an equation of state of a solvate layer. This equation expresses the equilibrium pressure (disjoining action) of a solvate layer for aqueous and non-aqueous media as a function of the thickness of the layer.

The effect of electrolytes on the thickness of aqueous solvate films. as well as the influence of surface-active substances upon vaseline-oil films on steel surfaces, has been investigated.

A theory of interaction of micelles, taking into account the disjoining action of thin layers of dispersion medium between the micelle surfaces, is advanced.

In conclusion, the computations used in the latter theory are employed to elaborate a theory of slow coagulation and stability of colloids and disperse systems.