Self-assembly of halloysite nanotubes in water modulated via heterogeneous surface charge and transparent exopolymer particles

Halloysite clay nanotubes are a viable model to investigate the colloid behaviour of particles dispersed in water. They demonstrate excellent colloid stability in water due to the strong overall negative surface charge. We performed detailed characterisation of halloysite surface inhomogeneity and in situ observations of diluted halloysite water suspensions behaviour. Using dark-field microscopy, we detected an aggregation between individual halloysite rods, behaving unlike the bulk majority of the non-aggregating nanoparticles. The stochastic distribution of positively charged patches on overall smooth halloysite surfaces facilitates the electrostatic clustering of individual clay nanotubes into stable aggregates having end-to-end and end-to-side configurations. Another pattern of halloysite aggregation through seemingly long-range attraction of like-charged colloids was attributed to trace amounts of transparent exopolymer particles (TEP), ubiquitous microgels of biogenic origin, persisting in purified deionised water and bridging individual halloysites into long-standing coordinated clusters. These aggregates demonstrate the earlier unknown role of water-suspended nanoscale particulate organic matter in counterintuitive long-range aggregation of like-charged colloids. We also found that microbial biopolymer mucospheres attract halloysite rods, forming unusual sea urchin-like motile microstructures, confirming the participation of TEP in natural colloids self-assembly. The persistence of barely detectable amounts of TEP in sterile purified deionised water suggests their important role in colloids interactions and other nano/micro scale processes.