Which dispersant is better to use to minimize van der Waals forces between powders?
To minimize the van der Waals effect between powders, the use of sterically hindered dispersants (e.g. polymers or graft copolymers) is a very good choice. They basically interrupt the intermolecular pulling by forming a physical barrier on the particle surface. The application will vary according to the dispersion medium (water or organic solvent) and the powder characteristics.
I. Non-aqueous systems: Use of stericly hindered dispersants is more appropriate.
In organic solvents or nonpolar environments, PEG or PMMA dispersants molecules, through their long-chain ones, can bind to the particle surface with a flexible coating layer. When the particle distance gets to a value of fewer than two times the coating layer thickness (2δ), these polymer chains strictly cause strong steric repulsion, with an energy barrier larger than 25 kT (k is the Boltzmann constant, T is the absolute temperature), which is sufficient to resists van der Waals attraction.
Thus, in the paints industry, by introducing BYK-110 (a modified polyacrylate) it can be shown that the viscosity of TiO2 dispersions is decreased by 40%. The molecular chains of its 3-dimensional protective layer on the particle surface, thus the van der Waals interaction force distance is compressed from 10 nm to below 3 nm, is responsible for this change.
II. Aqueous Systems: Synergistic Effects of Electrostatics and Steric Impediments
In aqueous medium, anionic dispersants (e.g. sodium citrate, polycarboxylate) weaken van der Waals through a dual mechanism: their polar groups (e.g. -COO⁻) adsorb onto the particle surface, forming a charged double layer and generating electrostatic repulsion; at the same time, nonpolar segments (e.g. carbon chains) protrude into the water, creating steric hindrance.
To give an example with calcium phosphate nanoparticles, sodium citrate addition would lower the particle surface zeta potential from -15 mV to -45 mV thus tripling electrostatic repulsion force. Besides, the hydroxyl groups of the citrate form hydrogen bonds with the particle surface, which further amplifies steric hindrance, and finally the aggregate particle size is decreased from 500 nm to 80 nm.
III. Extreme Scenarios: Ultrafine Powder-Assisted Dispersion
For smooth-surfaced Geldart A-type particles (e.g., industrial catalysts), adding ultrafine powders with a particle size <2 μm (e.g., nano-silica) can reduce van der Waals forces via "surface roughening". Those ultrafine particles will be attached to the main particles' surface forming physical gaps and consequently increasing the particle-to-particle distance that previously closely-contacted particles had.
According to the report, only by incorporating 0.015 wt% of ultrafine powder is it possible to improve the fluidization performance of the catalyst bed, avoid the formation of channels and stasis, and sustain this effect for several hundred hours.
IV. Dispersant Performance Comparison and Selection Guide
| Dispersant Types | Core Mechanism of Action | Applicable Scenarios | Advantages | Van der Waals Force Inhibition Effect |
| Stereohedral Polymers | Long molecular chains form a physical barrier | Organic solvent systems, high solids content slurries | Unaffected by electrolytes, highly versatile | Excellent |
| Electrostatic-Stereohedral Composite | Electrostatic repulsion + physical barrier | Aqueous systems, inorganic/organic particle dispersion Synergistic effect | Good dispersion stability | Excellent |
| Polyelectrolytes (e.g., sodium citrate) | Mainly based on electrostatic repulsion | Aqueous systems, low ionic strength environments | High dispersion efficiency, low cost | Good |
| Ultrafine Particle Additives | Physical spacing and surface roughening | Dry powder processing, fluidized bed processes | No chemical pollution, high temperature resistance | Moderate |
In practical applications, it is important to ensure that the dispersant concentration exceeds the critical micelle concentration (CMC). For example, the optimal addition amount of polycarboxylate in cement paste is 0.5-1.0% of the powder mass, at which point the molecular chains are fully extended, maximizing the steric hindrance effect. In addition, for multi-component powder systems, two dispersants (such as sodium citrate + PEG) can be used in combination to further reduce van der Waals forces to less than 1/5 of their original value by utilizing the electrostatic-steric synergistic effect.
