Comparatively Speaking: Hysteresis vs. Syneresis
By: Anthony J. O'Lenick Jr., Siltech LLC; and Thomas O'Lenick, SurfaTech Corp.
Posted: April 12, 2011
When designing a cosmetic gel or wax, the terms hysteresis and syneresis must be understood, as both affect the physical properties, aesthetics and appearance of a product.
Hysteresis
Hysteresis is a term coined by Sir James Alfred Ewing from an ancient Greek word meaning “deficiency” or “lagging behind.”1 Hysteresis applies to a system’s physical properties, i.e. cloud point, melt point, etc. These properties, specifically in polymeric or impure systems, can be path-dependent, meaning the properties will have different values based on whether the system is heated or cooled. An example of this is the cloud point of a polymeric solution. If the polymer solution is heated from 0°C to 100°C, the cloud point can be a couple of degrees higher than the same polymer solution cooled from 100°C to 0°C.
For example, in a hydrophilic polymer with a cloud point in water at low temperatures, the polymer chains can hydrogen-bond with the surrounding water, making the polymer soluble in water. As the temperature is increased, the polymer chains begin to move around more rapidly, and the hydrogen-bonds start to break down. Once these hydrogen bonds are broken between the water molecules and the polymer chain, the polymer collapses upon itself and a could point is observed.
When the same polymer is used in hot water and slowly cooled, the hydrogen bonds are reformed, and the polymer becomes soluble again. In both the heating and cooling processes, a cloud point is observed, but typically these cloud points will occur at different temperatures due to the difference in making/breaking hydrogen bonds.
Many physical systems naturally exhibit hysteresis. A piece of iron that is brought into a magnetic field retains some magnetization, even after the external magnetic field is removed. Once magnetized, the iron will stay magnetized indefinitely. Demagnetizing the iron would require a magnetic field in the opposite direction. This is the effect that provides the element of memory in a hard disk drive.