What are the properties that distinguish particles from bulk materials?
Numerous particles exist in any given system. For heterogeneous systems, each individual particle may have different physical or chemical properties. The collective behavior is macroscopically observable as macroscopic properties represent contributions of individual particles. If the relevant property is the same for all particles in the system, the system is monodisperse. Polydisperse systems have some or all particles with differing values for the property of interest. Another term—pausidisperse—is sometimes used to describe systems with a small number of distinct groups. All particles within a given group have the same value for the concerned property.
The specific surface area (i.e., surface area per unit mass) of small particles is so high that it leads to many significant and unique interfacial events including surface interaction with the surrounding medium and adjacent particles. To illustrate—a spherical particle of 2 g/cm3 will have a specific surface area of 3 cm2/g when the diameter is 1 cm. The specific surface area increases to 3,000,000 cm2/g if the diameter is reduced to 10 nm. This example demonstrates how particle dimension determines surface area which in turn, determines the thermodynamics and kinetic stability of a given particulate system.
Overall, the behavior of a particulate system and many of its physical parameters are highly dependent on the size and number of particles present in the system.