Nucleation

Nucleation is the first step in the self-assembly or self-organization process that results in the creation of a new thermodynamic phase or structure. Nucleation is the mechanism that decides how long an observer must wait for a new step or self-organized structure to appear. When a volume of water is cooled below 0 °C (at ambient pressure), it appears to freeze into ice, but the water that is just a few degrees below 0 °C also remains fully free of ice for long periods of time.

Ice nucleation is either slow or non-existent under these conditions. Ice crystals, on the other hand, appear quickly at lower temperatures. Ice nucleation happens rapidly under these conditions. First-order phase transitions are usually initiated by nucleation, which then initiates the process of forming a new thermodynamic phase. Continuous phase transitions, on the other hand, new phases form almost instantly.

Impurities in the system are also found to be very susceptible to nucleation. These impurities may be too small to see through the naked eye, but they may still affect nucleation rates. As a result, distinguishing between heterogeneous and homogeneous nucleation is frequently essential. Heterogeneous nucleation takes place at nucleation sites on the system's surfaces. Away from a surface, homogeneous nucleation occurs.

As already studied nucleation definition, let’s discuss nucleation and growth in detail.

To Calculate Nucleation Rate

If the system is not evolving with time and nucleation occurs in one stage, such as the forming of ice in the water below 0 °C, the likelihood that nucleation has not occurred should undergo exponential decay. This can be seen in the nucleation of ice in supercooled small water droplets, for example. The nucleation rate is calculated by the exponential decay rate. The classical nucleation theory is a commonly used approximation for estimating these rates and how they change with variables like temperature. When supersaturated, it correctly predicts that the amount of time you have to wait for nucleation decreases dramatically.

Homogeneous and Heterogeneous Nucleation

Nucleation with the nucleus at a surface, known as heterogeneous nucleation, is much more common than homogeneous nucleation. Purifying water to remove all or almost all impurities, for example, in the nucleation of ice from supercooled water droplets, results in water droplets that freeze below about 35 °C, while water that contains impurities which freeze at 5 °C or warmer.

Classical nucleation theory is often used to describe the observation that heterogeneous nucleation can occur when the rate of homogeneous nucleation is essentially zero.  This means that the rate of nucleation slows exponentially as the height of a free energy barrier G* increases. The free energy penalty of forming the surface of the expanding nucleus creates this barrier. 

Nucleation of Crystals

In certain cases, liquids and solutions may be cooled or condensed to the point that they are slightly less thermodynamically stable than crystals, but no crystals form for minutes, hours, weeks, or even months. A substantial barrier then prevents the crystal from nucleating. This has consequences; for example, cold high-altitude clouds can contain large numbers of tiny liquid water droplets with temperatures well below 0 degrees Celsius.

For crystallization in small quantities, such as small droplets, only one nucleation event can be needed. The nucleation time is generally described as the time it takes for the first crystal to appear in these small volumes. Using atomic-resolution real-time video imaging, researchers were able to see the first stages of sodium chloride crystal nucleation. Many nucleation events will occur in greater quantities. The KJMA or Avrami model is a basic crystallization model that combines nucleation and growth in that case.

Cholesterol Nucleation

After aggregation of supersaturated vesicles, cholesterol monohydrate crystals nucleate (operationally defined) and develop rapidly in both model and native biles. Cholesterol for crystal growth also comes from the vesicular pathway, rather than from biliary micelles directly.

Examples of the Nucleation of Crystals

Ice formation is the most natural crystallization process on Earth. Liquid water does not freeze at 0 °C unless there is already ice present; to nucleate ice and therefore for the water to freeze, it must be cooled considerably below 0 °C. Small droplets of very pure water, for example, can remain liquid down to -30 °C, despite the fact that ice is the stable state below 0 °C.

Since many of the materials we manufacture and use are crystalline but are made from liquids, such as crystalline iron made from a liquid iron cast into a mold, the nucleation of crystalline materials is a subject of intense research in industry. It's commonly used in the chemical industry for things like producing metallic ultradispersed powders that can be used as catalysts. Platinum deposited on TiO2 nanoparticles, for example, catalyzes the release of hydrogen from water. The scale of nanoclusters influences the bandgap energy in semiconductors, so it's a big deal in the semiconductor industry.

Example of Nucleation of Liquid

1. When wet air cools (often due to rising air), several tiny water droplets nucleate in the supersaturated air, forming clouds. With lower temperatures, the amount of water vapor that air can hold decreases. Excess vapor starts to nucleate, forming tiny water droplets that eventually form a cloud. The nucleation of liquid water droplets is heterogeneous, occurring on particles known as cloud condensation nuclei. Cloud seeding is the addition of artificial condensation nuclei to hasten cloud formation.

2. If the pressure is reduced to the point that the liquid becomes superheated with respect to the pressure-dependent boiling point, nucleation in boiling can occur in the bulk liquid. Nucleation happens more often on the heating surface, at nucleation sites. Nucleation sites are usually small crevices with a free gas-liquid surface or areas of the heating surface with lower wetting properties. After the liquid has been de-gassed, significant superheating of the liquid can be accomplished if the heating surfaces are clean, smooth, and made of materials that have been thoroughly wet by the liquid.

Did You Know?

Classical nucleation theory makes a variety of assumptions, such as treating a microscopic nucleus as a macroscopic droplet with a well-defined surface whose free energy is calculated using an equilibrium property called the interfacial tension. It's not always obvious that we should treat anything as small as a volume plus a surface for a nucleus that's just ten molecules across. Furthermore, since nucleation is a process that occurs outside of thermodynamic equilibrium, it is not always apparent that its rate can be calculated using equilibrium properties.