ABSTRACT
Solid particles dispersed in water are found in many examples in daily life, such as wastewater treatment. In this, chapter, the essential aspects in the very large industrial application of colloid chemistry will be described. Surprisingly, mankind has been aware of colloids for many thousands of years. Civilizations such as ancient Egypt and Maya civilizations used their knowledge about adhesion (between blocks of stones) when building pyramids thousands of years ago. This was long before modern-day cement was invented. Even the mud houses man has built were based upon the behavior of colloid aspects of materials used in such processes, such as clay and cow dung. In everyday life, one comes across solid particles of different sizes, ranging from stones on a beach, sand particles, or dust ¨oating in the air. A special relation between particle size (surface area) and their characteristics exists. The rather small particles which range in size from 50 Å to 50 μm are called colloids. The simplest difference is when sand particles versus dust particles are considered. It is almost fascinating to observe how dust or other †ne particles remain in suspension in air. On occasion, it has been observed that a particle gets a collision-like thrust. Already in the 19th century (Brown), it was observed under the microscope that small microscopic particles suspended in water made some erratic movements (as if hit by some other neighboring molecules) (Adamson and Gast, 1997). This has since then been called Brownian motion. The erratic motion arises from the kinetic movement of the surrounding water molecules. Thus, colloidal particles remain suspended in solutions through Brownian motion, only if gravity forces do not drag these to the bottom (or top). If one throws some sand into air, the particles fall to the earth rather quickly. On the other hand, in the case of talcum particles, these continue to ¨oat in the air for a long time.
