Physicochemical Mechanics

the branch of modern colloid chemistry that studies the dependence of the structural and mechanical properties of disperse systems and materials on the physicochemical phenomena occurring at interfaces (surface phenomena). From its beginnings in the 1930’s and 1940’s, physicochemical mechanics developed into an independent scientific discipline in the 1950’s, chiefly through the work of Soviet scientists, principally P. A. Rebinder. Physicochemical mechanics is closely related to other branches of colloid chemistry (surface phenomena and surface forces, the physical chemistry of adsorption and surfactants, research on the stability of disperse systems, and on the molecular-kinetic, optical, and electrical properties of disperse systems); it is also related to molecular physics, the physics and physical chemistry of real solids, and the physical chemistry of polymers, to rheology and mechanochemistry, and to many branches of the geological and biological sciences.

The objects studied by physicochemical mechanics include natural disperse systems (rocks, soils, tissues of plants and animals), disperse systems occurring in various production processes (powders, pastes, and suspensions, among them drilling muds, emulsions, and cooling lubricants), and various materials of industrial (instrumentation, design, construction) and everyday use. Physicochemical mechanics investigates the heterogeneous macro and nonuniform micro structure characteristic of these systems and universal for the disperse state of a substance. Such systems and materials consist of interconnected particles (globules, grains, fibers) that are extremely varied in size but that significantly exceed the dimensions of individual molecules and retain all of the basic physicochemical (including mechanical) properties of the given substance.

Physicochemical mechanics distinguishes two main types of three-dimensional structures formed by particles under different physicochemical conditions. Coagula are structures in which the interaction of the particles is limited to contact, which is either direct (as in free-flowing structures) or else occurs through residual layers of the dispersion medium (in suspensions and pastes). Here, the adhesive force at the point of contact (strength) usually does not exceed 10^–8–10^–7 newton (N), or 10^–3–10^–2 dyne. Mechanical reversibility, which accounts for, among other phenomena, thixotropy, is characteristic of such structures. Structures with phase contacts constitute the other type. Here, the contacts extend over an area that greatly exceeds molecular dimensions. As a rule, these structures are mechanically irreversible, and the contact strength in them is 10^–7–10^–6 N (10^–2–10^–1 dyne) or more. Phase contacts are reduced in various inorganic and organic crystalline and amorphous disperse systems and materials upon sintering, pressing, and isothermal distillation, as well as during the formation of a new, highly dispersed phase in supersa-turated solutions and melts, for example, in inorganic binders and polymers. Uniform materials, in particular metals and alloys, may be considered the limiting case of the complete intergrowth of grains. Each structure is characterized by a certain degree of dispersion, that is, a certain particle size and, consequently, a certain number of contacts per sq cm of cross section. This number may be 10²–10³ for powders with particles tenths of a mm in size and 10¹¹–10¹² for such highly disperse systems as aluminosilicate gels.

Physicochemical mechanics investigates mechanical (rheological) properties—the most general and important characteristics of all disperse systems and materials—as a function of the structure created by particle interaction. These properties include viscosity, plasticity, and the thixotropic behavior of coagula whose resistance to displacement bears a fixed relationship to the flow rate; also included is the elastoplastic and elastobrittle behavior of solid disperse systems and materials (with phase contacts) that are characterized by a specified strength, durability, and wear resistance. Thus, in the simple case of a globular porous monodisperse structure, the strength may be approximately equal to the product of the number of contacts between particles (per sq cm) and the average value of the adhesive force at an individual point of contact, the product varying over a very broad range (for example, from 10 to 10⁸ N/m²) depending on the type of contact and degree of dispersion.

At the same time, physicochemical mechanics establishes the decisive role of physicochemical phenomena at interfaces (wetting, adhesion, adsorption, changes in the value of interfacial tension, formation of special interfaces) in all processes of particle interaction and structure formation. On this basis, physicochemical mechanics derives its fundamental concepts regarding the possibility and effectiveness of controlling the structural and mechanical properties of disperse systems and materials through an optimal combination of mechanical interactions (such as vibrational, pulsed) and physicochemical factors (chiefly the composition of the medium and the additives of small amounts of surfactants). The surfactants, being concentrated at the interfaces (adsorbed onto the surface of particles), make it possible, when properly selected, to bring about radical changes in the properties of a boundary. Good adhesion can be ensured by strengthening the binding forces, or the forces can be weakened and overcome. Thus, in lyophobic systems (glass particles in a hydrocarbon medium, hydrophobic surfaces in polar liquids), the free energy at the contacts of coagula reaches tens of ergs/cm²; in lyophilic systems (polar particles made hydrophobic by monolayers of surfactant in a hydrocarbon medium), it is only hundredths of an erg/cm².

There are three main divisions of physicochemical mechanics, each of which investigates different phenomena and processes. The first studies the formation and breakdown of all types of three-dimensional structures as the interaction of particles of the disperse phase and dispersion medium. It also investigates various stages in the production of materials (including composite materials) having a specific disperse structure and a specified set of mechanical and physicochemical characteristics. The second area is concerned with the physicochemical effect of a medium and the medium’s surface-active components on the mechanical properties of various types of continuous and porous solids and materials (Rebinder effect). It also seeks to elucidate the conditions under which the Rebinder effect can be used to facilitate the processing of materials and under which any deleterious aspects of the effect can be avoided. The third area analyzes the regularities and the mechanism governing the adhesion of surfaces of solids (contact interactions) under conditions of interfacial friction, wear, lubrication, and the formation of coatings.

Physicochemical mechanics typically concerns itself with all aspects of the structural and rheological (especially nonlinear) characteristics of disperse systems under widely varying conditions; the variations include those of stress, temperature, composition of the medium, and degree of oversaturation. There is also direct experimental study of the interactions of individual particles, and mechanical tests are carried out on solids and materials in reactive media. Physicochemical analysis makes use of mathematical modeling and numerical methods for describing the rheological properties of disperse systems and analyzing the molecular mechanism for the action of the medium.

The general principles of physicochemical mechanics provide the foundation for dispersion techniques and methods of controlling the properties of disperse systems and various materials. These methods and techniques are widely used in, for example heterogeneous industrial chemical processes (production of paper, textiles, paints, varnishes, and fuels and of confectionery and baked goods). They are also used in producing various types of materials, for example, ceramics, catalysts, sorbents, and polymers, in mixing cement mortars, in preparing asphalt concretes and molding sands, in making composite materials in powder metallurgy, and in compacting and stabilizing soils. Techniques based on physicochemical mechanics facilitate processes for crushing and drilling hard rocks and for pulverizing ores before concentration; they are also used in cutting operations and in processes for increasing the stability and durability of structural and other materials in reactive media. In addition, these techniques optimize contact interactions, for example, during the pressure working of metals and the operation of friction units in machines, mechanisms, and devices.