To boost the implementation of colloidal crystals (CCs)
in separation
science, the effects of the most common chromatographic reversed phases,
that is, butyl and octadecyl, on the assembly of silica particles
into CCs and on the optical properties of CCs are investigated. Interestingly,
particle surface modification can cause phase separation during sedimentation
because the assembly is highly sensitive to minute changes in surface
characteristics. Solvent-induced surface charge generation through
acid–base interactions of acidic residual silanol groups with
the solvent is enough to promote colloidal crystallization of modified
silica particles. In addition, solvation forces at small interparticle
distances are also involved in colloidal assembly. The characterization
of CCs formed during sedimentation or via evaporative assembly revealed
that C4 particles can form CCs more easily than C18 particles because
of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran
when C18 chains with a high bonding density have extra hydroxyl side
groups. These groups can only be hydrolyzed from trifunctional octadecyl
silane but not from a monofunctional one. Moreover, after evaporative
assembly, CCs formed from particles with different surface moieties
exhibit different lattice spacings because their surface hydrophobicity
and chemical heterogeneity can modulate interparticle interactions
during the two main stages of assembly: the wet stage of crystal growth
and the late stage of nano dewetting (evaporation of interparticle
solvent bridges). Finally, short, alkyl-modified CCs were effectively
assembled inside silica capillaries with a 100 μm inner diameter,
laying the foundation for future chromatographic separation using
capillary columns.