Surface excess properties definition

What is a Gibbs surface What are the definition and meaning of surface excess properties ... 

In this chapter, the fundamental concepts of colloid science have been introduced. The definition of colloidal particles, those with sizes (in all directions) ranging from 1 nm to 10 pm, has been presented, and their relevance in soil science is stated. The importance of surface properties was remarked, introducing several definitions. The specific surface area is the area per unit mass the surface tension (or surface free energy) is defined as the Gibbs free energy per unit area. The surface excess of a given species is the amount (in moles per unit area) which is accumulated... 

The Gibbs equation expresses the equilibrium between the surfactant molecules at the surface or interface and those in the bulk solution. It is a particularly useful equation since it provides a means by which the amount of surfactant adsorbed per unit area of the surface, the surface excess , may be calculated. The direct measurement of the surface excess provides almost insuperable experimental problems and hence the Gibbs equation is widely used as an alternative method of determining this quantity. In the derivation of this equation a definite boundary between the bulk of the solution and the interfacial layer is imagined (see Fig. 1.3). The real system containing the interfacial layer is then compared to this reference system in which it is assumed that the properties of the two bulk phases remain unchanged up to the dividing surface. 

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

The chemical structure and physicochemical properties of asphaltenes and resins are not well understood. The operational definitions of asphaltenes and resins are based on their solubility in different diluents. Asphaltenes are defined as the fraction of crude oil insoluble in excess normal alkanes such as n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fraction of crude oil insoluble in excess liquid propane at room temperature. Resins are adsorbed on silica, alumina, or other surface-active material. Figure 5.8 shows the precipitation when a bitumen oil is mixed with various diluents. The normal alkanes used are n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane. According to the operational definition stated above, the asphaltene content of the bitumen... 

A real problem with specifying the impact properties on a product is to define what is a failure. The designer has to decide, for example, to what degree a surface condition, dent, bend, breakage, or shatter will be defined as a failure. Thus, the failure of a product can take many forms. There are situations, for instance, in which excessive elastic deformation will constitute failure. In turn, the definition of failure has to be related to impact testing s results. 

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