Surface or interfacial properties

Surface or Interfacial Properties which are Considered Directly Responsible for Performance of Various Unit Operations in Mineral Processing... 

An appropriate description of the energetics and kinetics of the nucleation process requires consideration of the equilibrium properties of the bulk phases involved as well as contributions that arise from the formation and growth of the interface between them. Clusters that constitute the seeds of the phase transformation are usually composed of a small number of atoms or molecules. For such small entities, the surface or interfacial properties... 

There is now a wealth of spectroscopic and other analytical techniques for probing the surface properties of solid materials (Vickerman, 1997 Riviere and Myhra, 1998), which yield a variety of surface properties of those parts of sudi materials that are situated anywhere between 1.0 and more than 10 nm below their surfaces. However, to date only contact angle analysis is capable of yielding the actual surface or interfacial properties at the precise surface of solids, that are germane to their interaction with other condensed-phase materials. Thus, methods pertaining to contact angle measurement ind related techniques are the main emphasis of this chapter. 

While the electrokinetic surface, or -potentials, originate from the surface or interfacial properties of solid materials, they are actually situated about 0.3 to 0.5 nm outside a material s surface and have to be extrapolated inward to the (i/>o) potential at the actual surface, using Eq. 5.54. The electrostatic free energy of interaction, AG, between two surfaces, 1, reaches a value of about -M.O mJ/m at V o 75 mV, in an aqueous medium with a 100 mM salt content of a mono-mono-salt see Table 5.1. Now various clay and other mineral particles can have V o-potentials that are between 50 and 90 mV, in which case AG, while not dominant, is no longer negligible. For instance for a contact between two platey clay particle surfaces over about 100 nm (= 10 ° cm ) an attraction of 1 mJ/m still corresponds to w 2,500 kT. Thus, it is always wise to measure -potentials, from which the actual surface, or V o-potential can be derived. 

Surface-active agent. The descriptive generic term for materials that preferentially adsorb at interfaces as a result of the presence of both lyophilic and lyophobic structural units, the adsorption generally resulting in the alteration of the surface or interfacial properties of the system. 

The van der Waals and other non-covalent interactions are universally present in any adhesive bond, and the contribution of these forces is quantified in terms of two material properties, namely, the surface and interfacial energies. The surface and interfacial energies are macroscopic intrinsic material properties. The surface energy of a material, y, is the energy required to create a unit area of the surface of a material in a thermodynamically reversible manner. As per the definition of Dupre [14], the surface and interfacial properties determine the intrinsic or thermodynamic work of adhesion, W, of an interface. For two identical surfaces in contact ... 

The interface between two fluids is in reality a thin layer, typically a few molecular dimensions thick. The thickness is not well defined since physical properties vary continuously from the values of one bulk phase to that of the other. In practice, however, the interface is generally treated as if it were infinitesimally thin, i.e., as if there were a sharp discontinuity between two bulk phases (LI). Of special importance is the surface or interfacial tension, a, which is best viewed as the surface free energy per unit area at constant temperature. Many workers have used other properties, such as surface viscosity (see Chapter 3) to describe the interface. 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

Particles can either be produced by bottom-up processes (e.g. precipitation) or top-down approaches (e.g. wet milling). In these processes particle-particle interactions become relevant when the particle size is below 1 pm. Engineering macroscopic product properties is then only possible through tailored surface and interfacial properties, no matter whether a bottom-up process like precipitation [11] or a top-down process such as milling in stirred media mills [12] is studied. Aggregation is an important aspect in both processes which are studied in the following. 

For a compound to be qualified as a surfactant, it should also exhibit surface activity. It means that when the compound is added to a liquid at low concentration, it should be able to adsorb on the surface or interface of the system and reduce the surface or interfacial excess free energy. The surface is a boundary between air and liquid and the interface is a boundary between two immiscible phases (liquid-liquid, liquid-solid and solid-solid). Surface activity is achieved when the number of carbon atoms in the hydrophobic tail is higher than 8 [3]. Surfactant activities are at a maximum if the carbon atoms are between 10 and 18 at which level a surfactant has good but limited solubility in water. If the carbon number is less than 8 or more than 18, surfactant properties become minimal. Below 8, a surfactant is very soluble and above 18, it is insoluble. Thus, the solubility and practical surfactant properties are somewhat related [1]. 

The superscript s denotes the properties of a surface or interfacial layer. In the presence of adsorption it may be replaced by the superscript a. 

The physics of condensed phases is commonly formulated as of infinite extent. However, solid and liquid objects in the laboratory are of finite size and terminate discontinuously in a surface (in vacuum) or an interface, under all other conditions. Atoms or molecules at the surface or interface of the condensed object find themselves in a completely different environment, compared to those in the interior of the body. They are less confined in at least one direction, which means that the wave function looks different in this direction - it is less classical. It is implied that surface or interfacial species show more quantum-mechanical behaviour, compared to the bulk. This is the basic reason for the special properties of surfaces and the origin of all interfacial phenomena. Surface chemistry should therefore be formulated strictly in terms of quantum theory, but this has never been attempted. In its present state of development it still is an empirical science, although many physico-chemical concepts are introduced to rationalize the behaviour of interfaces. 

Ability to reduce the surface or interfacial tension of water and oU is one of the properties required of an emulsifier. Different chain length fatty acids in monoglycerides were found to have little effect on the interfacial tension between palm oil and water, in contrast C18 2 (linoleic acid) monoglyceride (143) reduces the interfacial tension to greater than 50% (Table 55). 

Surfactant Any substance that lowers the surface or interfacial tension of the medium in which it is dissolved. Soaps (fatty acid salts) are surfactants. Detergents are surfactants or surfactant mixtures whose solutions have cleaning properties. 

The measurement of rheological properties at the surface of a solution or the interface between a solution and, for example, a biological film is called surface or interfacial rheology. In this technique also, experiments are performed either in tension, compression or shear, and phenomena observed in bulk rheology such as flow and viscoelasticity are also observable. An introduction to the techniques available and some key findings are discussed by Warburton. ... 

Carbon black is produced industrially in the form of different products (e.g., furnace black, thermal black, channel black, lampblack, acetylene black) with specific properties. In addition to the relevance of carbon black for basic research on adsorption, or as a reference sohd, appUcations of this material in fields such as elastomer reinforcement, as modifier of certain properties of plastics (UV protection, electrical conductance, color), or as xerographic toners make its surface and interfacial properties extremely important. Soot is a randomly formed particulate material similar in nature to carbon black. The main (pragmatic, rather than conceptual) difference between these two carbon forms is that soot is generally formed as an unwanted by-product of incomplete combustion of pyrolysis, whereas carbon black is produced under strictly controlled conditions. Bansal and Donnet [78] have reviewed various possible mechanisms for the formation of soot and carbon black. Soot can retain a number of tars and resins on its surface. There is therefore some interest in studying the adsorption of polyaromatic hydrocarbons in soots, especially those of environmental significance such as diesel soot. 

As a rule, the fluid dispersions (emulsions, foams) are stabilized by adsorption layers of amphiphile molecules. These can be ionic and nonionic surfactants, lipids, proteins, etc. All have the property to lower the value of the surface (or interfacial) tension, o, in accordance with the Gibbs adsorption... 

A surfactant (a contraction of the term surface-active agent) is a substance that, when present at low concentration in a system, has the property of adsorbing onto the surfaces or interfaces of the system and of altering to a marked degree the surface or interfacial free energies of those surfaces (or interfaces). The term interface indicates a boundary between any two immiscible phases the term surface denotes an interface where one phase is a gas, usually air. 

The concentration at which this phenomenon occurs is called the critical micelle concentration (CMC). Similar breaks in almost every measurable physical property that depends on size or number of particles in solution, including micellar solubilization of solvent-insoluble material (Chapter 4) and reduction of surface or interfacial tension (Chapter 5), are shown by all types of surfactants—nonionic, anionic, cationic, and zwitterionic in aquecus media. 

Reduction of surface or interfacial tension is one of the most commonly measured properties of surfactants in solution. Since it depends directly on the replacement of molecules of solvent at the interface by molecules of surfactant, and therefore on the surface (or interfacial) excess concentration of the surfactant, as shown by the Gibbs equation... 

---