The dispersion stability of graphene oxide in various solvents is a critical factor for its successful application in coatings, composites, and other functional materials. Graphene oxide, with its oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups, exhibits amphiphilic behavior, allowing it to disperse in both aqueous and organic solvents. However, achieving stable dispersions requires careful consideration of factors such as pH, sonication parameters, and surfactant use, while challenges like reaggregation and sedimentation must be addressed.

In aqueous solutions, graphene oxide disperses readily due to its hydrophilic nature. The presence of ionizable carboxyl and hydroxyl groups allows the sheets to become negatively charged in water, creating electrostatic repulsion that prevents aggregation. The pH of the solution plays a crucial role in determining the stability of the dispersion. At high pH (above 7), the deprotonation of carboxyl groups increases the negative charge density on the graphene oxide sheets, enhancing electrostatic repulsion and improving dispersion stability. Conversely, at low pH (below 4), protonation reduces the surface charge, leading to weaker repulsion and increased aggregation. Studies have shown that stable aqueous dispersions can be achieved at pH levels between 8 and 10, where the zeta potential remains sufficiently negative (typically below -30 mV) to prevent flocculation.

Sonication is another critical factor in achieving stable dispersions. Ultrasonic energy helps exfoliate graphene oxide sheets from bulk material and reduces their size, improving homogeneity. However, excessive sonication can fragment the sheets excessively, reducing their lateral dimensions and potentially compromising their mechanical or electrical properties in composites. Optimal sonication times vary depending on concentration and solvent but generally fall within 30 to 60 minutes for aqueous dispersions at moderate power (100–300 W). Prolonged sonication may also introduce defects, altering the material's properties.

In organic solvents, graphene oxide dispersion is more challenging due to its partial hydrophilic character. Polar organic solvents such as N,N-dimethylformamide (DMF) or tetrahydrofuran (THF) can stabilize graphene oxide through polar interactions, but nonpolar solvents require additional stabilization strategies. Surfactants are often employed to improve compatibility. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) are common ionic surfactants that adsorb onto graphene oxide surfaces, providing steric and electrostatic stabilization. Nonionic surfactants like Triton X-100 or Pluronic polymers can also enhance dispersion by forming protective layers around the sheets. The choice of surfactant depends on the solvent system and intended application, as residual surfactants may affect composite properties.

Despite these strategies, challenges such as reaggregation and sedimentation remain. Over time, dispersed graphene oxide sheets may restack due to van der Waals forces, especially at high concentrations or in the absence of sufficient stabilizing agents. Sedimentation is more pronounced in organic solvents where electrostatic stabilization is weaker. To mitigate this, optimizing concentration is essential—typical stable dispersions range from 0.1 to 2 mg/mL, depending on the solvent and additives. Additionally, gentle stirring or periodic sonication can help maintain dispersion homogeneity over time.

Stable graphene oxide dispersions have been successfully utilized in coatings and composites. For example, in anticorrosion coatings, aqueous graphene oxide dispersions are mixed with polymers such as epoxy or polyvinyl alcohol, forming uniform films that enhance barrier properties. The high aspect ratio and oxygen functionalities of graphene oxide improve adhesion and mechanical strength. In composite materials, stable dispersions in DMF or ethanol have been used to fabricate conductive films by integrating graphene oxide with polymers like polyaniline or polypyrrole. These composites benefit from the even distribution of graphene oxide, which facilitates charge transport.

Another example is the use of surfactant-assisted dispersions in organic solvents for creating reinforced polymer composites. Polyethylene or polystyrene matrices blended with well-dispersed graphene oxide exhibit improved tensile strength and thermal stability. The key lies in maintaining dispersion stability during processing to prevent filler aggregation, which could otherwise lead to weak interfacial bonding.

In summary, achieving stable graphene oxide dispersions requires balancing multiple factors. pH adjustment ensures electrostatic stability in water, while sonication parameters must be optimized to exfoliate without damaging the sheets. Surfactants expand the range of compatible solvents but must be selected carefully to avoid unwanted side effects. Despite challenges like reaggregation, well-dispersed graphene oxide has proven valuable in coatings and composites, provided that concentration and processing conditions are carefully controlled. Future advancements may focus on solvent-free dispersion techniques or novel stabilizers to further improve long-term stability.