10 February / March 2017
surface area was determined using the BET (Brunauer-Emmett-Teller) method. The pore volume, pore diameter and pore size distribution were determined using the BJH (Barrett-Joyner-Halenda) method [8].
SOLAS™ MonoDense™ particles are manufactured via a modified Stöber process, the particulates of this process and subsequent bonding and packing protocols are proprietary to the company. The company uses a proprietary two phase surfactant system to yield particles with little or no voids present.
Figure 3. SEM and FIB images of SOLAS monodense particle
LC performance testing was carried out on an Agilent 1200 LC system. Flow rates between 0.025 and 1.2ml/min were employed. Column dimensions used were typically 2.1 x 50 mm. Comparison between the plots of the reduced HETP, h versus the reduced linear velocity, v, of naphtho[2,3-a] pyrene for three columns packed with 100 Å 1.7 µm particles. Plots were fitted after correction from the extra column band broadening
Results Figure 4. SEM and FIB images of manufacturer A particles
(E) Examine several core shell or superficially porous particles (SPP) to understand if the voiding phenomenon is also observed in this particle class.
(F) Outline some of the potential causes of these voids.
Experimental
Scanning electron microscopy (SEM) was carried out on a FEI Inspect F instrument operating at 10 kV. Silica samples were placed on conductive carbon tape prior to analysis. Focussed ion beam (FIB) was performed using a FEI Helios Nanolab
Table 1. Physiochemical properties of FPP studied. Silica Type
Particle size (µm)
SOLAS™
Monodense™ Manufacture A
Manufacture B 1.8 1.9 1.9
Monodispersivity (d90/d10)
1.4 1.2 1.5
600 dual-beam FIB. The electron beam was operated at 5 kV with the ion beam operating at 30 kV for Pt deposition and thinning. The cross sections were prepared using a focussed ion beam method [7].
Nitrogen gas was used to probe the pores of the silica particles. The volume, diameter and size distribution of the pores can be determined by nitrogen sorption measurements. The sorption analyses were performed on a Micromeritics Tristar II surface area and porosity analyser. Prior to analysis, each sample was de-gassed for three hours at 400°C and measurements were performed at -169.15°C (77 K). The
FIB analysis and imaging of ‘Voids’ in FPP particles
Surface area
(m2 g-1 300 245 200 ) Pore
Volume (cm3
g-1 0.60 0.63 0.50 ) Pore
Diameter (Å)
80 93 110
Type of Voids noted
None
Mixture of Large and small voids.
Starburst Crater in centre with homogeneous pore on corona
Manufacture C 1.9 1.5 315 0.7 120
Homogeneous void structure
Figure 3 illustrates SEM and FIB images for SOLAS™ MonoDense™ sub 2µm particles. The SEM images shows a relatively monodisperse particle size distribution (d90/d10 = 1.4 as measure by Elzone data not shown). Analysis of the FIB images shows that SOLAS™ MonoDense™ has a homogenous substructure within the particle. The 10 nm pores are just about visible and are evenly distributed within the particle. This type of particle can be considered monodense. In comparison to this homogeneous internal pore structure, Figures 4, 5 and 6 illustrate silica particles from other silica manufacturers. Figure 4 shows a commercially available 1.9 µm monodisperse (d90/d10 = 1.2) fully porous particle and on inspection of FIB images it is clearly evident that large voids exist are clearly noted throughout the particles, highlighted by red arrows. The centre of the particle appears to contain the majority of the voided space. An elongated voided path through the middle of the particle is noted. In the interest of brevity only one FIB image is shown, however FIB analysis was conducted on approximately 10 particles per batch in an attempt to yield statistically relevant information. All physical properties for silica examined in this study are shown in Table 1.
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