Correlation of Secondary Particle Number with the Debye-Hückel Parameter for Thickening Mesoporous Silica Shells Formed on Spherical Cores

Abstract

Mesoporous silica shells were formed on nonporous spherical silica cores during the sol-gel reaction to elucidate the mechanism for the generation of secondary particles that disturb the efficient growth of mesoporous shells on the cores. Sodium bromide (NaBr) was used as a typical electrolyte for the sol-gel reaction to increase the ionic strength of the reactant solution, which effectively suppressed the generation of secondary particles during the reaction wherein a uniform mesoporous shell was formed on the spherical core. The number of secondary particles (N _2nd) generated at an ethanol/water weight ratio of 0.53 was plotted against the Debye-Hückel parameter κ to quantitatively understand the Debye screening effect on secondary particle generation. Parameter κa, where a is the average radius of the secondary particles finally obtained in the silica coating, expresses the trend in N _2nd at different concentrations of ammonia and NaBr. N _2nd was much lower than that expected theoretically from the variation of secondary particle sizes at a constant Debye-Hückel parameter. A similar correlation with κa was observed at the high and low ethanol/water weight ratios of 0.63 and 0.53, respectively, with different hydrolysis rate constants. The good correlation between N _2nd and κa revealed that controlling the ionic strength of the silica coating is an effective approach to suppress the generation of secondary particles for designing mesoporous shells with thicknesses appropriate for their application as high-performance liquid chromatography column packing materials.

Figure 1

TEM images of SiO₂@mSiO₂ prepared at different NH₃ concentrations: 10 mM (a), 50 mM (b), and 100 mM (c). An ethanol/water weight ratio of 0.53. The images were taken for the samples obtained after several centrifugations to clearly observe the silica shell formed on the cores.

Figure 2

TEM images of SiO₂@mSiO₂ prepared at different NaBr concentrations: 1 mM (a), 2 mM (b), and 4 mM (c). NH₃ concentration was 10 mM and the ethanol/water weight ratio was 0.53. The images were taken for the samples obtained after several centrifugations to clearly observe the silica shell formed on the cores.

Figure 3

Relationship between the number of secondary particles (N_2nd) and the Debye–Hückel parameter (κ) at an ethanol/water weight ratio of 0.53. Filled circles (●) show N_2nd at different NH₃ concentrations without any electrolyte addition. Filled squares (■) show N_2nd at [NH₃] = 10 mM and different NaBr concentrations.

Figure 4

TEM images of SiO₂@mSiO₂ prepared at an ethanol/water weight ratio of 0.63. The sol–gel reactions without any electrolyte addition were conducted at different NH₃ concentrations: 10 mM (a), 50 mM (b), 100 mM (c). For the addition of electrolyte at the fixed NH₃ concentration of 10 mM, the NaBr concentration was set to 2 mM (d) and 4 mM (e). The images were taken for the samples obtained after several centrifugations to clearly observe the silica shell formed on the cores.

Figure 5

Relationship between κa and the relative number of secondary particles. The red solid line shows the theoretical N_2nd expected from variation in the secondary particle size (a) at the same κ and [Si]_soln. The squares (■, □) show the relative N_2nd generated at different concentrations of NaBr. The circles (●, ○) indicate those generated at different concentrations of NH₃ without any addition of NaBr. Filled and open symbols represent the relative N_2nd formed at the ethanol/water weight ratios of 0.53 and 0.63, respectively.