11
calcination/ porogen removal may cause mini explosions within the particles. Particularly at the centre of powders.
(F) Collapse of thin walls during calcination/ porogen removal
Chromatographic Analysis Figure 5. SEM and FIB images of manufacturer B particles Figure 6. SEM and FIB images of manufacturer C particles
Figure 5 illustrates SEM and FIB images from a ‘traditional’ sub 2 µm particle with a relatively large particle size distribution (d90/ d10 = 1.5). A large starburst void is noted within the particle substructure. The void is concentrated in the centre of the particle (highlighted by red arrows). A more uniform homogeneous structure of the pore is noted on the outside corona (blue arrows) of the particle. On examination of the physical property of this samples this sample is seen to have a relatively low surface area (200 m2
g-1 ) and average pore volume (0.5 cm3 g-1 ).
Figure 6 shows another SEM and FIB image for a traditional polydispersed (d90/d10 > 1.5) 2 µm silica particle. In this case an almost homogeneous network of voids is observed throughout the particle. The physical properties of this this silica are as follows, surface area 315 m2 = 0.7 cm3
g-1 g-1 and pore diameter 120 Å. This
material contains voids which seem to be homogeneous and consistent throughout
Table 2. Van Deemter Coefficients
Columns Solas™ Monodense™ 1.8um C18 Manufacturer A 1.9 C18 Manufacturer B 1.9 C18 A Term B Term C Term
1.87 4.85
0.005
1.89 7.89
0.091
0.17 5.33
0.147
the particle. Indeed, it could be interpreted that whilst the monodispersity of these particle isn’t the best the homogeneous nature of the voids may counter act any losses in efficiency.
Without knowing the specifics of different manufacturing method to produce the above particle types it is difficult to postulate a reason for these voids. Several different reasons may be hypothesised at this stage;
(A) Presence of impurities within reaction solvents e.g. dust
(B) Presence of emulsion droplets with reaction mixture
(C) Variation in hydrolysis rate of silica precursors
, pore volume
(D) Impure porogens or surfactant/ polymers with large molecular weight distributions leading to the presence of micoremulsion droplets with the liquid
(E) Localised heating effects during
Figure 7A is the van Deemter plot of the reduced HETP vs the reduced linear velocity for the SOLAS™ MonoDense™ C18 column and two commercial brands of C18 packing in a 2.1 ID x 50 mm column after subtraction of extra column broadening due to the instrument. The experimental coefficients of the HETP plots obtained from the SOLAS™ MonoDense™ C18 and the two commercially C18 packed columns are given in Table 2. The parameters that contributed to band broadening according to the van Deemter equation revealed some interesting results when comparing the mass transfer properties of the SOLAS™ MonoDense™ C18 columns to the commercial brand of columns Figure 7 illustrates the chromatographic performance of the SOLAS™ MonoDense™ C18 against manufactures A and B. Two observations are quite quickly noted from the Van Deemter plots and back pressure profiles. These are namely (a) The C term in the van Deemter for SOLAS™ MonoDense™ tracks downwards at higher flow rates suggesting better mass transfer properties for this type of particle. (b) The van Deemter for Manufacturer B which contains large star burst voids shows a large increase in the C term. This phenomenon is well known which offers some proof to the theory that a large void space within the particle can cause dispersion issues.
A review of Table 2, shows that the experimental A-term for the column from manufacturer B is the smallest (0.17), suggesting that it is the best packed column for this study. This assumption was made by taking into account the generality of the intrinsic surface roughness of the particle. Having, a rough surface tends to improve the packing quality. This can be promoted by the large coefficient of shear friction that takes place between the rough particles of the silica during the slurry packing process, requiring a large amount of stress to be applied to the growing packed bed. Because of the roughness on the surface of the particles, less slippage is encountered on depressurisation of the column after packing, thus keeping a homogeneous packing and ultimately contributing to a reduced A-term. Conversely, the particles
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