Binary Colloidal Nanoparticles with a Large Size Ratio in Analytical Ultracentrifugation

Abstract

Sedimenting colloidal particles may feel a surprisingly strong buoyancy in a mixture with other particles of a considerably larger size. In this paper we investigated the buoyancy of colloidal particles in a concentrated binary suspension in situ in a centrifugal field. After dispersing two different fluorescence‐labeled silica nanoparticles with a large size ratio (90 nm and 30 nm, size ratio: 3) in a refractive index matching solvent, we used a multi‐wavelength analytical ultracentrifuge to measure the concentration gradients of both particles in situ. The concentration of the 90 nm silica nanoparticles was used to calculate the effective solvent density for the 30 nm silica nanoparticles. The exponential Boltzmann equation for the sedimentation‐diffusion equilibrium with locally varying effective solvent density was then used to theoretically predict the concentration gradient of 30 nm silica nanoparticles, which describes the experimental results very well. This finding proves the validity of effective buoyancy in colloidal mixtures and provides a good model to study sedimenting polydisperse colloids.

Recent Advances in Binary Colloidal Crystals for Photonics and Porous Material Fabrication

“…To date, BCCs have been obtained by a variety of assembly methods, using a mixture of colloidal spheres with two different sizes, such as layer-by-layer (LbL) deposition, − template-assisted assembly, ,− electric-field-induced assembly, , confined convective assembly, − centrifugal-field-induced assembly, − spin-coating assembly, − horizontal and vertical deposition, ,,, and coself-assembly approaches. − Their structures mainly depend on the size ratio of large (L) colloidal spheres to small (S) colloidal spheres and the number ratio of both spheres. Many different self-assembly routes, aiming to fabricate BCCs rapidly and inexpensively, with precise manipulation of the defined surface and inner structure, have been recently developed.…”

“…For this approach, spin speed and diameter ratio are the two key parameters for precise control over the resulting BCC structure. Moreover, assembly methods such as electric-field-induced assembly, magnetic-field-induced assembly, or centrifugal-field-induced assembly can also achieve desirable goals in the presence of external forces. − In general, to reasonably design the desirable BCC structures or patterns, the content and the size ratio of colloidal spheres should be precisely controlled. The number of colloidal spheres (n) in unit volume can be calculated by n = 6 w /(πρ D 3 ), where w denotes the weight fraction of colloidal spheres, ρ represents the density, and D is the diameter of the colloidal spheres.…”

Self-association and gel formation during sedimentation of like-charged colloids

“…Silica nanoparticles (SNPs) were chosen as a model colloid because they have a relatively low refractive index (n E 1.46) and can be synthesized by well-established methods. [20][21][22] To be able to access high concentration, SNPs were first covalentlylabeled with fluorescent molecules, such as fluorescein isothiocyanate (FITC) and rhodamine B isothiocyanate (RITC), 23,24 and thereafter dispersed in a refractive-index matching solvent 25 (an 80 vol% glycerol and 20 vol% water mixture). The physical properties of this dispersion are listed in Table 1 (particle size measurement details shown in SI 1, ESI †, surface charge number measurement described in Experimental Procedures and Fig.…”