Colloidal systems kinetic properties

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

Most of the electrochemical phenomena occur in size regimes that are very small. The effects of size on diffusion kinetics, electrical double layer at the interface, elementary act of charge transfer and phase formation have recently been reviewed by Petrri and Tsirlina [12]. Mulvaney has given an excellent account of the double layers, optical and electrochemical properties associated with metal colloids [11]. Special emphasis has been given to the stability and charge transfer phenomenon in metal colloid systems. Willner has reviewed the area of nanoparticle-based functionalization of surfaces and their applications [6-8]. This chapter is devoted to electrochemistry with nanoparticles. One of the essential requirements for electrochemical studies is that the material should exhibit good conductivity. 

The participation of colloidal particles in thermal motion (the entropic factor) was taken into consideration, mostly indirectly, in earlier studies dealing with the molecular-kinetic properties of disperse systems. Volmer was the first to realize the importance of the role that the thermal motion of colloidal particles played in controlling the formation and stabilization of disperse systems. However, the attempt to compare the work of surface formation and the entropic factor directly, undertaken by March, was not successful, since it was applied only to systems with high interfacial energy. 

Thermodynamic description presented in Chapter IV allowed us to subdivide all colloidal systems into two large classes thermodynamically stable systems, referred to as lyophilic and those characterized by kinetic stability only, referred to as lyophobic systems. Detailed description of properties and stability of lyophobic systems is presented in chapters that follow, while in the present chapter we will focus on the properties, structure and formation conditions of the lyophilic colloidal systems. 

The limiting case No. 7, does not really belong to Colloid Science because both ions fall below the agreed dimensions nevertheless No. 4, 5, 6 and 7 form a completely allied group. Characteristic properties of 4 are to be found not only in 5 and 6 where at any rate one colloid is present but also in 7 in which there is no longer any colloid at all. The limiting case No. 7 is therefore theoretically important because it shows us clearly that the essentials of the dicomplex colloid systems No. 4, 5 and 6 cannot be sought in the macromolecular structure of the colloid (or the association nature of the kinetic units in the case of the association colloids) but rather in the electrolyte nature of the macromolecule (or associate). 

Two-phase mixtures of polymers differ from classic ll colloid systems mainly by a transition layer that exists between the system components and is of special significance. The formation of such a layer in mixtures of linear polymers is governed by the kinetic factors of the retarded process of phase sep u ation, by the collid-chemic ll mechanism of formation, or by the adsorption interaction as well as by the segmental solubility [103,104], In mixtures of crosslinked polymers its formation is governed also by the conditions of synthesis. Note also that in the thermodynamic llly nonequilibrium mixtures of polymers in the two-phase systems, the processes of segmental solubility usually have time to reach completion while the macromolecules do not move inside the high-viscosity medium, which ensures the stability of the structure and its mechanical properties [103]. 

We have not studied all types of colloidal systems in detail but limited ourselves to suspensions, siufac-tants, emulsions and foams. In terms of properties, the stability and associated concepts (double layer, van der Waals forces, steric effects) as well as the DLVO theory have been presented in detail, while kinetic and especially the optical properties have been discussed more briefly. 

It is, however, quite different with solid or liquid aggregates which are built up from units which can under appropriate circumstances be recognised as large kinetic units such systems can certainly be fruitfully considered as colloid systems, because many of their properties should be explained from the standpoint that they have been built up of those units which are the objects of study in colloid science. For example, cellulose, rubber, charcoal, thermoplastics both in the solid and in the molten state, are reasonably included in this domain. We shall return frequently to this point in this book. 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

In discussing the mechanisms of the formation of monodispersed colloids by precipitation in homogeneous solutions, it is necessary to consider both the chemical and physical aspects of the processes involved. The former require information on the composition of all species in solution, and especially of those that directly lead to the solid phase formation, while the latter deal with the nucleation, particle growth, and/or aggregation stages of the systems under investigation. In both instances, the kinetics of these processes play an essential role in defining the properties of the final products. 

Microstmctures are frequently present in a kinetically trapped nonequilibrium state, and their structures depend on the components and colloidal interactions based on their different chemical and physical properties, as well as on the procedure by which these components have been assembled. These structures are thermodynamically unstable and tend to reduce their free energy (surface area) with time. On the contrary, self-assembly nanostructures are thermodynamically stable, unless the molecules react with the environment or degrade. Most food systems are based on an interplay of kinetically stabilized and thermodynamic equilibrium structures. Some typical examples of structures at different length scales formd in food systems are shown in Figure 11.1. 

---