Eugene D. Shchukin

Ostwald ripening theory: applications to fluorocarbon emulsion stability

Abstract A brief description of the Ostwald ripening theory is given with applications to emulsion stability (mainly to the stability of fluorocarbon emulsions).

PHYSICAL-CHEMICAL MECHANICS IN THE STUDIES OF PETER A. REHBINDER AND HIS SCHOOL

Abstract In connection with the birthday centennial of academician P.A. Rehbinder, a short review is presented of studies in some principal directions of the physical–chemical mechanics initiated by him and developed by his successors namely: the liquid metal embrittlement observation with the method of electrochemical microscratching; discovery of the reciprocal interaction of solid phase and medium in heterogeneous catalysis and catalytically enhanced sintering; direct experimental study of crystal bridging and residual stresses arising in processes of hydration hardening; and explaining the role of the lyophilic structure–mechanical barrier formed by adsorption layer as a factor of strong stabilization in colloid dispersions.

Influence of the nature of non-polar phase on the mechanical stability of adsorption layers of hydrocarbon and fluorocarbon surfactants at the interface between their aqueous solutions and non-polar media☆

Abstract A variety of experimental approaches has been used for companson of the stabilizing effect with respect to droplets coalescence caused by the interfacial adsorption layers (IAL) of a nunber of hydrocarbon and fluorocarbon surfactants at the boundary between their aqueous solutions and various non-polar hydrocarbon and fluorocarbon liquids: (I) compression of two individual droplets in surfactant solution up to their coalescence and consequent tension and rupture of a newly formed drop; (II) evaluation of the free energy of interaction between non-polar surfaces by measuring the contact rupture force for smooth spherical particles; (III) rheological study of IAL by torque pendulum method; (IV) SEM observation of the IAL morphology; (V) study of the stability with respect to the Ostwald ripening. These observations reveal the predominant role of the lyophilic structure-mechanical barrier formed by the IAL as a factor of strong stabilization with respect to coalescence and particular dependence of the mechanical strength of such layer on the nature of the non-polar liquid and on the interaction between this liquid phase and hydrophobic parts of the surfactant molecules.

Effect of cationic polyelectrolyte and surfactant on cohesion and friction in contacts between cellulose fibers

Abstract The methodics and devices are presented for quantitative study of the characteristics of interaction in contact between individual fibers: friction force F in shear test, and cohesion force, i.e. contact strength p in rupture test. In experiments with cellulose fibers in various liquid media, the friction coefficient μ has been estimated, and the molecular component of friction force related only to attraction of fibers, in the absence of any external normal load has been found. The specific free energy of interaction U has been evaluated in measurements using model samples with the nature of surface similar to that of cellulose fibers. The effects of cationic polyelectrolyte and surfactant: polyethyleneimin and tetrabuthylammonium iodid on these parameters have been quantitatively determined. Complicated, non-monotonic (with several extrema) dependence have been estimated between values F , μ , p , U and surfactant concentration C . Comparison of these data with the ζ -potential measurements of cellulose fibers in the same surfactant solutions allows one to propose an explanation of the mechanisms of these polyelectrolyte and surfactant influence on fiber interactions.

Some Colloid-Chemical Aspects of the Small Particles Contact Interactions

Depending on the character of the particles’ interaction, two main types of the particle contacts (and, respectively, two types of disperse structures) can be considered, namely, coagulational and phase contacts [1–3]. In coagulational contacts, disperse phase particle adhesion is limited to simple “touching”, either directly or by means of a residual layer of dispersion msdium, mainly due to molecular van der Waals forces. These contacts are not strong (e.g., 10-9 – 10-7 N) and are mechanically reversible, predetermining the viscoelastic properties of the corresponding thixotropic systems. A phase contact, on the other hand, is the result of bridging of particles (by displacing the dispersion medium from an area much bigger than the sizes of elementary cell) due to the same valence bonding as in the bulk of the given phase. These contacts are relatively strong (e.g., 10-6 N and higher) and, generally speaking, are mechanically irreversible. Elastic-brittle or elastic-plastic behavior is typical for these structures. Phase contacts determine the material’s quality during use. In contrast, properties of coagulational contacts (and corresponding systems) are important during numerous steps of the material production and forming, especially when easy mobility is required. In diluted systems (sols), interactions of particles (energy of adhesion in contact being compared with the kT value) predetermines aggregative stability of a system.

Surface Modification and Contact Interaction of Particles

Abstract The effects of particle surface modification by ambient media and surfactant adsorption on the cohesive forces in the immediate contacts between individual particles have been studied with the CF (cohesive force) apparatus. The values of the free energy of interaction in direct coagulation contacts between particles of various types in liquids of different polarity and in the presence of various surfactants have been measured. They cover a broad range of several orders of magnitude; these interactions define the rheological properties of concentrated thixotropic systems and their stability with respect to peptization. A similar experimental technique has been used for studying active media influences on various physico‐chemical processes of particle bridging and formation of the phase contacts responsible for the mechanical properties of related solid structures and their resistance to fracture. The effect of the adsorption induced decrease in strength and durability of such porous structures with phase contacts and compact solids is considered.

Conditions of Spontaneous Dispersion and Formation of Thermodynamically Stable Colloid Systems

Abstract The macrophase dispersion is thermodynamically favorable if the free energy change due to dispersion (isolation of n particles with radius r, at sufficiently low interfacial energy σ) is negative, i.e., ΔF = n4πr 2σ − TΔS < 0, where ΔS(C) is the entropy gain and C is the concentration. If there is a factor opposing dispersion to molecular dimension b, a negative minimum of ΔF at r > b may occur, i.e., formation of a thermodynamically stable colloid system takes place. In this article, the analysis of the ΔF = ΔF(r, σ, n, C ) function behavior for three different conditions is proposed: (i) C = constant, with a virtual maximum; (ii) r = constant, with a negative minimum; and (iii) n = constant, when this function is monotonic, in all cases, for monodisperse systems, with broad variation of σ. In all the three cases, the equation ΔF = 0 serves as a necessary condition for spontaneous dispersion and formation of a thermodynamically stable, lyophilic colloid system. Under normal temperatures and low concentrations, this needs small r∼10−6 cm and low σ∼10−2–10−1 mJ/m2. These conditions become “easier” for dispersion of an aggregate (e.g., σ on the order of units), and “more difficult” for highly concentrated systems (with σ decreasing to 10−3 mJ/m2). Essential changes and complications can be connected with polydispersity accounting. A special attention is paid to real physical systems corresponding to considered versions of the ΔF behavior.

Microscratching in electrochemical cells: the effect of gallium on surface deformation in aluminum

Abstract The microscratching method has been combined with the electrochemical reduction of an active component at the tested surface for studying initial surface damages in metals and effects caused by active media. This allows the modeling of the active liquid metal influence in the absence of the liquid metal phase, at room temperature. Surface plasticizing of aluminum caused by gallium ions reduction has been observed and discussed in terms of microscopic (dislocation) and macroscopic (finite elements) approaches.

Liquid metal embrittlement in the absence of liquid metal phase: in studies of surface damageability and in hard materials machining

Abstract The liquid metal embrittlement (LME) effect has been studied by a new approach, in the absence of the liquid metal phase. A melt was replaced by the ions of selected surface-active metals induced by electrochemical reduction on treated surface. New methods were elaborated based on this idea. The electrochemical microscratching method has been developed to study the stability and damageability of solid metal surfaces. Electro–chemo–mechanical treatment (ECMT) tests were performed to search for new ways in practical applications of the LME effect in optimizing processes of mechanical machining, friction

The Lyophilic Structure-Mechanical Barrier as a Factor of Dispersion Strong Stabilization

The lyophilic structure-mechanical barrier formed by the interfacial adsorption layer is considered as a factor of strong stabilization of disperse systems with respect to high concentrations of disperse phase and electrolyte. Such barrier must possess two principal features: intrinsic mechanical strength of the layer — preventing coalescence, and high affinity of the external side of the layer and dispersion medium — opposing coagulation; independent experimental approaches for their quantitative characterization are presented.