An interface is a surface across which the phase changes. Interfaces must be present in all heterogeneous systems, such as those discussed above. Interfacial properties necessarily differ from those of the bulk phases since the atomic bonding/structure of an interface represents a compromise between those of the phases on either side of the interface. For example, an atom at a free surface, which is an interface between a condensed phase and a gas (or a vacuum), generally has fewer neighbors with which to bond than it would have if it were in the bulk, condensed phase. In an equilibrium multi-component system, the chemical potential of each species must be the same in all phases, as well as at the interface. Not surprisingly, the chemical composition of the interface will, in general, differ from that of the bulk. For example, molecules in a gas (or solute in a condensed phase) can adsorb (segregate) onto the surface (interface) of a condensed phase. Interfacial processes play important roles in all areas of materials science and in many (most) areas of modern technology. As the trend toward miniaturization in microelectronics continues and interest in nanoscale structures grows, interfacial phenomena will become even more important. Clearly, the ratio of the number of atoms at surfaces and interfaces to those in the bulk grows as system size decreases (70% of the atoms in a nanometer diameter particle are on a surface!). Therefore, the thermodynamic properties of a system become increasingly dominated by interfacial properties as the dimensions of the system shrink. We can distinguish several types of interfaces: solid–liquid, liquid–gas, solid–gas, solid phase α–solid phase β, and grain boundaries. The meaning of the first four types of interface is self-explanatory. Grain boundaries represent a special class of interfaces; interfaces across which the phase does not change. What does change abruptly across this interface is the spatial orientation of the crystallographic axes. Most crystalline materials are polycrystalline, which means that they are composed of a large number of grains, each with a unique crystallographic orientation with respect to some laboratory frame of reference.
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