Monodisperse Colloidal Silica Research Articles

Particle morphology and chemical microstructure of colloidal silica spheres made from alkoxysilanes

Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.

Preparation of uniform silica hydrogel particles by SPG filter emulsification method

The main objective of this article is to demonstrate the ability of the Shirasu-porous-glass (SPG) filter emulsification method to synthesize the monodispersed colloidal silica hydrogel particles of micron size.

The transient settling of stable and flocculated dispersions

An experimental investigation of the sedimentation of monodisperse colloidal silica spheres with grafted octadecyl chains with three different interaction potentials is presented. Small particles (0.27 μm) behaved as hard spheres in cyclohexane, but larger ones (0.60 and 0.94 μm) are weakly flocculated by van der Waals attractions. The smallest particles (0.08 μm) in hexadecane are strongly flocculated by attractions between the octadecyl layers. A medical computer tomography (CT) scanner provided an accurate and absolute density measurement without disrupting the process. For the hard spheres and the weakly flocculated systems, the kinetics of sedimentation for the dispersed phase could readily be predicted utilizing the flux curve. For flocculated networks, we found a power-law relationship between compressive yield stresses and solids fractions comparable with other experimental systems.

Adsorption of synthetic polyelectrolytes on colloidal spheres

The adsorption of synthetic polyelectrolytes on the surfaces of monodisperse polystyrene spheres and colloidal silica spheres is studied by electrophoretic mobility measurements. Electrolytes used are NaCl, CaCl2, LaCl3, Na2SO4, sodium poly(ethylenesulfonate) (NaPES), sodium poly(styrenesulfonate) (NaPSS), polybrene® (PB), poly-4-vinyl-N-ethylpyridinium bromide (C2PVP), poly-4-vinyl-N-benzylpyridinium chloride (BzPVP), and copolymer of 4-vinyl-N-benzylpyridinium chloride (95%) and 4-vinyl-N-n-hexadecylpyridinium bromide (5%) (C16BzPVP). Electrophoretic velocity (u) and the effective charge number (α) of a colloidal sphere increase in the presence of PB, C2PVP, BzPVP, and C16BzPVP, and turn to the positive from the negative values in their absence. Addition of NaPES and NaPSS further decreases u and α values. Adsorption of the polymers on the colloidal spheres are explained by the hydrophobic and/or dipoledipole interactions in addition to the electrostatic forces. Weak adsorption of simple electrolytes on the colloidal spheres is deduced from the electrophoretic measurements.

The charging properties of monodisperse colloidal silica in symmetrical quaternary ammonium ion solutions

The zeta potentials of monodisperse colloidal silica were measured as a function of pH and as a function of the concentration of tetramethyl-, tetrapropyl-, and tetrapentylammonium bromide in aqueous solution. The variation of the potential with pH was explained by a simple, point ion adsorption model from which a dissociation constant and Gibbs free energy of dissociation were obtained. By contrast, the results obtained with tetraalkyl ammonium ions could not be explained using this simple model. However, incorporation of a hydrophobic adsorption term and finite ion size in a more sophisticated model gave good agreement with experimental measurements. The validity of the model is supported by the reasonable dissociation constants which were obtained on fitting the experimental data. Dissociation constants and the corresponding Gibbs free energies were calculated for both hydrophobic and site binding adsorption for each tetraalkyl ammonium ion. The stability of colloidal silica in solutions of these ions can now be satisfactorily explained in terms of the electrostatic repulsion between particles rather than by a hydrophobic/hydrophilic solvation effect previously proposed.

Determination of the effective charge numbers of colloidal spheres by conductance measurements

The effective charge number of colloidal spheres was determined for monodisperse polystyrene spheres and colloidal silica (8–1022 nm in diameter and 0.3–171 μC/cm 2 in charge density) by conductance measurements. The fraction of free macroions and/or gegenions (β) in deionized suspension is close to but smaller than unity for spheres having several and several tens analytical valency (Z). The β decreases with increasing Z. The relation log β = 2.61 − 0.87 log Z, which has been obtained from electrophoretic mobility measurements, holds for spheres of Z > 3000. The charge renormalization theory by Alexander et al. is supported.