How to describe particle dispersion? Low-field NMR technology.

Particle Dispersion refers to the process by which powder particles are separated and dispersed uniformly throughout the liquid phase. Depending on the dispersion method, it can be classified into several types:

Mechanical Stirring Dispersion: This primarily relies on mechanical energy, such as shear force or impact force, to disperse nanoparticles in the medium. By applying mechanical force to the dispersion system, physical and chemical property changes, along with a series of chemical reactions, occur to achieve dispersion. However, during the grinding process, the presence of grinding media introduces new impurities, and there are certain limitations for the formation of ultrafine particles.

Dehydration Using Organic Solvents: Organic solvents with low surface tension are used to replace the moisture adsorbed on the surface of particles, reducing the capillary forces that cause particle agglomeration. Common solvents used are alcohols, which help to wash away the coordination water molecules on the particle surface, replacing hydroxyl groups with alkoxy groups.

Particle size characterization for dispersion: When particles are dispersed in a liquid to form a suspension system, the smaller the particle size and the more stable it remains over time, the better its dispersion and resistance to aggregation. Particle size characterization is commonly used to describe the dispersion before and after surface modification. The better the dispersion, the closer the particle size distribution is to monodispersed particles. Conversely, the worse the dispersion, the more the particle size distribution shifts from monodispersed to larger particles.

Electron Microscopy Characterization of Particle Dispersion: Scanning electron microscopy (SEM) is the most direct method for characterizing the state of particles in a liquid system. After dispersing the particles in the liquid phase, a suitable amount of suspension is dropped onto an SEM sample holder, dried, and then observed under an electron microscope for photography. This method allows comparison of the quality of dispersion.

Low-field nuclear magnetic resonance (NMR) technology can be used to describe particle dispersion. It is a fast, non-destructive testing technique suitable for both scientific research and industrial applications. One of the advantages of low-field NMR technology is that the measurement is non-invasive – measurements can be repeatedly performed on the same sample. This minimizes the amount of experimental material required for stability studies and reduces variations in sample results related to sampling, especially in the early stages of product development. Thermal studies can be directly conducted in NMR tubes, including high-temperature studies and freeze-thaw cycles. Scientists use rheology to monitor the physical stability of concentrated dispersions, but these measurements often require large amounts of sample and are destructive, whereas NMR measurements can be performed on the same sample throughout the stability study process.

The relaxation rate of the solvent in a particle dispersion system is linearly proportional to the available particle surface area. Solvent or polymer loops and solvents within the tails of free polymers show no significant changes in relaxation rates, as they still exhibit high fluidity. When the polymer forms an adsorbed layer on the particle surface, the overall relaxation rate increases due to the increased proportion and/or residence time of water molecules in the near-surface area. The dispersion of particles can be described by the relaxation differences observed through low-field NMR technology.