Interfacial energetics dispersions

As discussed in Chapters 1-7, diffusion, Brownian motion, sedimentation, electrophoresis, osmosis, rheology, mechanics, interfacial energetics, and optical and electrical properties are among the general physical properties and phenomena that are primarily important in colloidal systems [12,13,26,57,58], Chemical reactivity and adsorption often play important, if not dominant, roles. Any physical chemical feature may ultimately govern a specific industrial process and determine final product characteristics, and any colloidal dispersions involved may be deemed either desirable or undesirable based on their unique physical chemical properties. Chapters 9-16 will provide some examples. 

Very small dispersed particles are highly energetic. In order to approach a stable state, they tend to regroup themselves in order to reduce the surface free energy of the system. An equilibrium will be reached when AG = 0. This condition may be accomplished either by a reduction of the interfacial tension or by a decrease of the total surface area. 

Contact angle measurements provide information on the wettability of the sample, the surface energetics of the solid, and the interfacial properties of the solid-liquid interface. The samples were immersed in water and captive air and octane bubbles were determined by measuring the bubble dimensions. By measurement of both air and octane contact angles the surface free energy (.y) of the solid-vapor ( > ) interface may be calculated by use of Young s equation and the narmonic mean hypothesis for separation of the dispersive and polar components of the work of adhesion. This method for determination of surface and interfacial proper-... 

It can be derived from Antonow s rule, 15.7.4], applying it to partial wetting but accounting for the adhesion between solid and liquid, assuming it to be dominated by the Van der Waals, or dispersion, parts of the surface tensions, y and y. Various studies have shown that [5.7.5] is quite effective for materials that mainly interact through dispersion forces and that it remains a reasonable approximation for systems in which other interactions also operate. The root in the r.h.s. of [5.7.5] stems from the assumption that Berthelot s principle may be applied. In sec. 2.11b we argued that this principle may be applied only to the energetic part of the interfacial tensions and that a more correct form is... 

A more vital application is to discern how reinforcement surface treatments improve adhesion to thermoplastic matrices. Since the nonreactive nature of thermoplastics normally precludes interfacial covalent bond formation, secondary bonding forces, such as London dispersion interactions and Lewis add/base interactions, may play a major role in these drcumstances. These secondary binding forces are subject to surface energetics analysis. 

The physically active sites of carbon black should produce an increase in interfacial adhesion and hence an increase in reinforcement. Furthermore, this increase in reinforcement should be non-specific for different non-polar polymers, since it still reflects a dispersion force interaction. While its existence cannot be questioned, it clearly cannot be assumed to be responsible for the entire observed increase in reinforcement over and above that displayed by graphitized carbon black if other, more energetic bonds are also formed. 

A known effect contributing to an increased saturation solubility is the formation of highly energetic surfaces by the milling process. The breaking of crystals lead to the exposure of inner parts of the crystal to the outer dispersion medium, implying that energetically less favorable surfaces are now in contact with water. The Ostwald-Freundlich equation also describes saturation solubility as a function of the tnterfacial tension between solid—liquid interphase. An increase in the interfacial tension y leads to an increase in the saturation solubility. 

Water and oil do not mix with each other because of high energetic cost associated with replacing water-water and oil-oil contacts by water-oil ones. This energetic cost is responsible for high interfacial tension between bulk oil and water (30-50 mN/m) and for sharpness of the interfacial region. The interfacial tension between immiscible liquids is determined by contributions of dispersion interactions and non-dispersion ones. 

The essential aspect here is that a compound with carbon black is controlled by the dynamics of the interfacial energies, namely adhesion (polymer-carbon black) and cohesion (carbon black-carbon black). First the carbon black is completely dispersed, i.e., the entire agglomerate structure is totally destroyed, and in the process adhesion interfaces are created. Above the critical concentration, however, it becomes energetically advantageous (from a thermodynamic point of view) to form more and more carbon black/carbon black cohesion interfaces a network with a predominantly linear structure is built up. The formation of these structures is thermodynamically induced and kinetically controlled. 

An emulsion may be defined as a heterogeneous system, consisting of at least two immiscible liquids or phases, one of which is dispersed in the form of droplets in the other. Emulsions are generally unstable with respect to their component bulk phases. Rearrangement from the droplet form to the two bulk liquids will occur with a net reduction in interfacial area and this is energetically favourable. However, it is relatively simple to erect kinetic barriers to this process to achieve metastable states that are for all practical purposes completely stable. 

A liquid foam is a dispersion comprising a gas phase and a liquid phase shaving cream and the froth in the head of a beer are two familiar examples. The gas phase forms bubbles that are separated by thin liquid films. The gas is thus called the dispersed phase, while the liquid is the continuous phase. There is an energetic cost associated with any interface between phases and an associated interfacial tension. This tension is, ultimately, the reason a material made up of two fluids can display solid-like responses it allows the thin liquid films to resist deformation. 

All materials have both a polar (yi) and nonpolar (yl) or dispersion contribution to the total surface energy. There are two main models for determining the interfacial energy that consider both contributions to surface energetics for each material—the harmonic mean model and the geometric mean model (9). 

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