Thermodynamic properties colloid stability

We have already seen from Example 10.1 that van der Waals forces play a major role in the heat of vaporization of liquids, and it is not surprising, in view of our discussion in Section 10.2 about colloid stability, that they also play a significant part in (or at least influence) a number of macroscopic phenomena such as adhesion, cohesion, self-assembly of surfactants, conformation of biological macromolecules, and formation of biological cells. We see below in this chapter (Section 10.7) some additional examples of the relation between van der Waals forces and macroscopic properties of materials and investigate how, as a consequence, measurements of macroscopic properties could be used to determine the Hamaker constant, a material property that represents the strength of van der Waals attraction (or repulsion see Section 10.8b) between macroscopic bodies. In this section, we present one illustration of the macroscopic implications of van der Waals forces in thermodynamics, namely, the relation between the interaction forces discussed in the previous section and the van der Waals equation of state. In particular, our objective is to relate the molecular van der Waals parameter (e.g., 0n in Equation (33)) to the parameter a that appears in the van der Waals equation of state ... 

The precipitation and colloid formation of different metal oxide hydroxides is known in soils when the concentration of the ions reaches the value of stability products. In this case, the precipitation can be explained by the thermodynamic properties of the bulk solution. In the lead ion/calcium-montmorillonite system, however, the production of lead enrichments cannot be explained by the... 

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. 

In systems with liquid dispersion medium, i.e. in foams, emulsions, sols and suspensions, there is a broad variety of means to control colloid stability. In these systems the nature of colloid stability depends to a great extent on the aggregate state of dispersed phase. Similar to aerosols, foams are lyophobic, but in contrast to them can be effectively stabilized by surfactants. Properties of emulsions, and, to some extent, those of sols may be quite close to the properties of thermodynamically stable lyophilic colloidal systems. In such systems a high degree of stability may be achieved with the help of surfactants. 

Most suspensions are not thermodynamically stable. Rather, they possess some degree of kinetic stability, and it is important to distinguish the degree and the time scale of change. In this discussion of colloid stability, we explore the reasons why colloidal suspensions can have different degrees of kinetic stability and how these are influenced, and can therefore be modified, by solution and surface properties. Encounters between particles in a suspension can occur frequently due to Brownian motion, sedimentation, stirring or a combination of them. The stability of the dispersion depends on how the particles interact when this happens. The main cause of repulsive forces is the electrostatic repulsion between like charged objects. The main attractive forces are the van der Waals forces between objects. 

Adsorption of enteric viruses on mineral surfaces in soil and aquatic environments is well recognized as an important mechanism controlling virus dissemination in natural systems. The adsorption of poliovirus type 1, strain LSc2ab, on oxide surfaces was studied from the standpoint of equilibrium thermodynamics. Mass-action free energies are found to agree with potentials evaluated from the DLVO-Lifshitz theory of colloid stability, the sum of electrodynamic van der Waals potentials and electrostatic double-layer interactions. The effects of pH and ionic strength as well as electrokinetic and dielectric properties of system components are developed from the model in the context of virus adsorption in extra-host systems. 

Surface energies of sohds, surface and interfadal tensions and the interfacial region, thermodynamics of colloidal systems, improved electrical double layer theory, adsorbed pol)mer layers and steric stabilization, relationships between surface energies and bulk properties... 

The solubility of the monomers of bilayer-forming molecules is usually very low, say, in the range of 10 -10 ° M. Crystals of such amphiphiles immersed in water tend to swell. In this way lamellar liquid crystals (multilamellar vesicles) made up of bilayers packed in large stacks, separated by water molecules, are usually formed. They reach dimensions of a few thousands of nanometers. These lamellar structures may appear in different forms that readily interchange in response to small variations in temperature or composition. Unilamellar vesicles having a radius of a few tens up to a few hundreds of nanometers are derived from the lamellar liquid crystals by mechanical rupturing as occurs in ultrasonic treatment, for example. The unilamellar vesicles are thermodynamically unstable, and, hence, the properties of a unilamellar vesicle dispersion depend on how it was prepared. The colloidal stability of such a vesicle system is determined by the rate of fusion between two vesicles. This rate, in turn, is governed by the rules of colloidal stability discussed in Chapter 16. Anyway, the colloidal stability of unilamellar vesicles allows their use for in vitro studies of physical and chemical bilayer and membrane properties. 

Nowadays it is well established that the interactions between different macromolecular ingredients (i.e., protein + protein, polysaccharide + polysaccharide, and protein + polysaccharide) are of great importance in determining the texture and shelf-life of multicomponent food colloids. These interactions affect the structure-forming properties of biopolymers in the bulk and at interfaces thermodynamic activity, self-assembly, sin-face loading, thermodynamic compatibility/incompatibility, phase separation, complexation and rheological behaviour. Therefore, one may infer that a knowledge of the key physico-chemical features of such biopolymer-biopolymer interactions, and their impact on stability properties of food colloids, is essential in order to be able to understand and predict the functional properties of mixed biopolymers in product formulations. 

In considering the impact of thermodynamically favourable interactions between biopolymers on the formation and stabilization of food colloids, a number of regular trends can be identified. One of the most important aspects is the effect of complexation on interfacial properties, including rates of adsorption and surface rheological behaviour. 

Tolstoguzov, V. B. (1993). Thermodynamic incompatibility of food macromolecules. In Food Colloids and Polymers Stability and Mechanical Properties, Dickinson, E., and Walstra, P. (Eds.), pp. 94-102. Royal Chem. Soc., London. 

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. 

Size-dependent structure and properties of Earth materials impact the geological processes they participate in. This topic has not been fully explored to date. Chapters in this volume contain descriptions of the inorganic and biological processes by which nanoparticles form, information about the distribution of nanoparticles in the atmosphere, aqueous environments, and soils, discussion of the impact of size on nanoparticle structure, thermodynamics, and reaction kinetics, consideration of the nature of the smallest nanoparticles and molecular clusters, pathways for crystal growth and colloid formation, analysis of the size-dependence of phase stability and magnetic properties, and descriptions of methods for the study of nanoparticles. These questions are explored through both theoretical and experimental approaches. 

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