The solubility of fullerenes in organic solvents is a critical factor influencing their processing and application in various fields. These carbon-based nanostructures exhibit distinct solubility behaviors depending on the solvent's chemical nature, with toluene, benzene, and chlorinated solvents being among the most studied. Understanding these interactions is essential for optimizing dispersion and preventing aggregation, which can hinder performance in practical applications.

Fullerenes demonstrate moderate solubility in aromatic solvents such as toluene and benzene. The solubility of C60 in toluene is approximately 2.8 mg/mL at room temperature, while in benzene, it is slightly higher, around 1.5 mg/mL. This difference arises from the subtle variations in solvent polarity and molecular structure. The interaction between fullerenes and aromatic solvents is primarily driven by π-π stacking, where the delocalized electrons in the solvent's aromatic rings interact with the electron-rich surface of the fullerene molecules. This interaction stabilizes the dispersion and prevents rapid aggregation. However, even in these solvents, fullerenes tend to form aggregates over time, especially at higher concentrations or under prolonged storage. The aggregation is influenced by van der Waals forces between fullerene molecules, which become significant as the solvent's ability to solvate the particles diminishes.

Chlorinated solvents, such as carbon tetrachloride (CCl4) and dichlorobenzene, exhibit higher solubility for fullerenes compared to aromatic solvents. For instance, the solubility of C60 in carbon tetrachloride is about 4.2 mg/mL, while in o-dichlorobenzene, it can reach up to 24 mg/mL. The enhanced solubility in chlorinated solvents is attributed to the polarizability of the chlorine atoms, which interact favorably with the electron density of fullerenes. Additionally, the larger molecular size and lower cohesive energy density of chlorinated solvents contribute to better solvation. Despite this, aggregation remains a challenge, particularly when the solvent evaporates or when the system is subjected to temperature changes. The kinetics of aggregation in chlorinated solvents are slower than in aromatic solvents, but the eventual formation of aggregates can still occur.

The tendency of fullerenes to aggregate in organic solvents is a significant limitation for their practical use. Aggregation reduces the effective surface area and can lead to inhomogeneous dispersions, which are undesirable for applications such as thin-film fabrication or composite materials. Several physical methods have been developed to improve dispersion and delay aggregation. Sonication is one of the most effective techniques, where ultrasonic waves are used to break apart aggregates and disperse fullerenes uniformly in the solvent. The energy input from sonication overcomes the van der Waals forces holding the aggregates together, resulting in a more stable dispersion. The duration and power of sonication must be optimized, as excessive energy can degrade the fullerene molecules or cause solvent evaporation.

Another approach to improving dispersion is the use of surfactants or stabilizing agents. These compounds adsorb onto the surface of fullerenes, creating a steric or electrostatic barrier that prevents aggregation. Nonionic surfactants, such as polyvinylpyrrolidone (PVP), are particularly effective in organic solvents. The surfactant molecules form a protective layer around the fullerene particles, reducing their tendency to come into close contact and aggregate. The choice of surfactant depends on the solvent system and the intended application, as some surfactants may interfere with subsequent processing steps or alter the properties of the final product.

The temperature also plays a crucial role in the solubility and aggregation behavior of fullerenes. Generally, solubility increases with temperature due to the enhanced kinetic energy of the molecules, which helps overcome intermolecular forces. For example, the solubility of C60 in toluene nearly doubles when the temperature is raised from 25°C to 50°C. However, upon cooling, the dissolved fullerenes may precipitate or form larger aggregates, a phenomenon that must be carefully managed in applications requiring thermal cycling.

The choice of solvent for fullerene dispersion depends on the specific requirements of the application. Aromatic solvents like toluene and benzene are suitable for processes where moderate solubility is acceptable, and cost is a consideration. Chlorinated solvents, while offering higher solubility, may pose environmental and health risks, limiting their use in large-scale applications. In all cases, the solvent's ability to maintain a stable dispersion over time must be evaluated, and complementary methods such as sonication or surfactants should be employed to enhance performance.

In summary, fullerenes exhibit varying degrees of solubility in organic solvents, with chlorinated solvents generally providing higher solubility than aromatic solvents. Aggregation remains a persistent challenge, driven by intermolecular forces that are not fully counteracted by solvent interactions alone. Physical methods like sonication and the use of surfactants offer practical solutions to improve dispersion and stability. Understanding these factors is essential for leveraging the unique properties of fullerenes in nanotechnology and materials science applications.