Nucleation is a first step. It can either be the foundation of a new thermodynamic phase or it can be the first step in self-organization or self-assembly that allows a new structure to be formed.
An example of the former type of nucleation are the changes that occur to water under different environmental conditions. If water is cooled to a temperature below the freezing point, then to tends to become ice. Ice that is exposed to temperatures above freezing will melt.
An example of the latter type of nucleation occurs when sugar becomes supersaturated in water. If you were to place a stick into supersaturated sugar water, the sugar molecules would stick to it, forming large crystals.
The classical nucleation theory helps to describe why these changes occur, when they occur, and why there can be delays in the formation. In some instances, the theory even describes why nucleation does not occur, even though it seems like it should be.
Why Does Nucleation Fail to Occur at Times?
Let’s go back to using water as an example. When temperatures are below freezing and water is left outside, ice will begin to form. Yet, if you put a glass of water outside and the temperature was only 1 degree below freezing, there is a good chance that it could remain outside all day long and never form actual ice within the container.
Why is there a delay in the nucleation of the water? Because there are often impurities that can be found within the liquid. The classical nucleation theory recognizes that nucleation is sensitive to any impurities that are found within any system. Even if the impurities are too small to be seen, they still effect the rate of nucleation.
Liquid water doesn’t freeze as quickly, in any temperature, when there is movement on its surface. If there is salt contained in the water, then the freezing temperature must be colder. If water is supersaturated with salt, the freezing temperature would be about -20C instead of 0C. (-6F instead of 32F).
That is why it is important to distinguish between heterogeneous nucleation and homogeneous nucleation.
What Is Heterogeneous and Homogeneous Nucleation?
Heterogeneous nucleation occurs on the surface of the object in question. The nucleus forms on the surface of the vapor or liquid, along with any bubbles or particles that may be provided with the nucleation site. Nucleation starts on the surface and spreads outward from there. That is why you can have a layer of ice in a bucket left outside in the cold, but still have liquid water underneath.
Homogenous nucleation occurs when there isn’t a preferential nucleation site. It is a more of a random occurrence, often spontaneous, because it does not have a preferential nucleation site. It is rare compared to heterogeneous nucleation because it requires the item in question to be either superheated or supercooled.
Mineral water is a common example of a homogenous nucleation occurring. When mineral water is bottled and sealed, the carbonation contained in the water cannot escape. Now let’s say that bottle of water has been placed into a cooler that has temperatures below freezing.
When you pull the bottle out of the cooler, it will feel cold to the touch. The water inside will be liquefied. All of that changes when you open the bottle to take a drink of water.
Because the water couldn’t crystallize in a heterogeneous nucleation, it remained as a liquid within its stable environment. Once the bottle was opened, the environment became destabilized. The nucleation process could begin. That is why supercooled mineral water becomes slush and the bottle becomes colder after it has been opened. This creates a spontaneous nucleation.
These nucleation options, along with their rates, can be calculated for virtually any object or substance today using classical nucleation theory models. Smaller clusters can be difficult to calculate, but larger clusters of molecules can be calculated with a high rate of predictability. That means if you know the volume and surface of an object in question, it becomes possible to predict the nucleation that will occur from a heterogeneous standpoint.
Nucleation can be predictable, but it can also be unpredictable. It can create change or it may not change anything. There will always be factors that create predictability problems, but thanks to the classical nucleation theory and its observations, those problems can have less influence.