Aqueous pigment dispersions: The thermodynamics of hierarchical aggregation*

Many industrially important materials aggregate to form nanoscale mass-fractal structures. Unlike sintered aggregates such as fumed silica, aqueous pigment-based inks often consist of weakly bound nanoparticles stabilized by a surfactant that can break apart and re-form balancing mixing energy and the reduction in surface energy with aggregation. Rapid thermal motion of small elemental crystallites lead to dense primary particle clusters with slower thermal motion that aggregate into ramified mass fractals. It is proposed that the hierarchical structure relies on subtle and competitive equilibria between different hierarchical structural levels.¹ In the context of the removal of a subunit from a cluster, the thermodynamics of nanoparticle hierarchical equilibria was explored on surfactant-stabilized pigment dispersions using the Vogtt Theory.² Reversible nanoparticle aggregation could be described solely from the degree of aggregation and the volume fraction. In this case, the hierarchical thermodynamics at each of three structural levels is dominated by solubility of the nonionic dispersing surfactant that decreases with temperature (LCST).

¹K. Vogtt et al., Phys. Rev. Research 1 (2019)
²K. Rishi et al., Langmuir 35, 13100 (2019)