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temperature, type of alcohol/alkyl silicate and concentrations of the various reagents. However, the Stöber process only produces non porous particles. In an attempt to develop porous particles which, have a larger surface area which is a desirable commodity for stationary phases within HPLC for small molecule separations, several research groups began investigating the addition of surfactants or porogens to the Stöber process [4,5]. This, they hoped would combine the properties of monodispersivity of the original Stöber process with the porosity required for a HPLC stationary phase. This addition of porogens has been termed the modified Stöber process or modified Stöber Fink Bohn (mSFB) process.
Modified Stöber Process to yield Porous Particles
The ‘modified’ Stöber system consists of five reagents namely an alkyl silicate, water, an alkaline catalyst, alcohol and a surfactant or porogen. By empirically relating the initial synthesis conditions to the final product, silica particles with varying morphology and particle size have now been produced Thus, particle and pore size (after subsequent removal of the porogen from the silica) may be ‘tailored’ by relating reactant stoichiometry and experimental conditions to the final silica properties rather than trying to design through an understanding the complex physical and chemical processes involved. In general, ‘tailoring’ both the particle and pore size of monodisperse precipitates and porous materials has progressed in this manner. Grün & Unger et al. [4] pioneered work into mSFB reactions. In 1992, Mobil Corporation also worked on a new family of silicate molecular sieves (M41S) which could also be considered a modified Stöber process. However, there are some major short comings of the modified Stöber process and the particles it produces. These shortcomings have hampered its uptake as a stationary phase for HPLC, for example the largest particle size obtainable with this type of process is in the sub 5 µm range. The pore size of as synthesised particles tends to be in the 20 Å range therefore subsequent pore enlargement steps are required. Synthesis is carried out in an alcoholic solvent and hence large scale production might be considered expensive. Also having a stationary phase of highly monodisperse particles (SD < 5%) can cause significant increase in backpressure compared to a packed bed of ‘less’ monodisperse particles. Glantreo has
developed a number of processes which address these issues and have also developed a process that produces particles free from voids or holes within the particle sub structure. Particles that are free from voids in the particle sub structure are expected to reduce band broadening, the more homogenous nature of the partuile may also facilitate the easier packing of these materials.
The concept of Monodensity
Figure 2. Schematic representation analyte diffusion paths within a mon- odense and voided particles and subsequent effect of chromatographic per- formance.
Until now it was assumed that the internal porous structure of silica particle was homogeneous. However detailed work at our laboratories has shown this not to be the case. Whilst the idea of monodispersivity is known and accepted within the silica manufacturing and chromatographic communities, hardly any literature exists on the concept of monodensity (i.e. a uniform or homogeneous distribution of pores within a silica particle) in silica particles. A recent submission to International Symposium on Capillary Chromatography (ISCC) was noted but further details could not be found [6]. On communication to the wider chromatographic community and silica manufacturing community it would appear the concept of Monodensity is known but ‘swept under the carpet’ to some extent. This may be for several reasons;
(a) FIB which allows the dissection and imaging of the interior silica particles has only recently become a main stream analytical tool thereby allowing material scientists to peer into the internal structure of the silica particles.
(b) Manufactures of silica particles may know about the voids within the silica particles but chose not to disclose it to the scientific community. There may be valid reason for this, as realistically with a technique like FIB only a small number of particles can be examined hence it may be hard to draw statistically relevant conclusions.
(c) Whilst it may be relatively easy to separate large particles (>3µm) via air classification. The large scale economics
of being able to separate particle by density via liquid elutriation is not a trivial matter.
Monodensity and its effect on chromatographic performance
Theoretically it is suspected that a more homogeneous pore structure within a silica particle should lead to better mass transfer properties thereby reducing the C term of Van Deemter equation. For a voided particle, it is envisioned that an analyte may become ‘trapped’ within the void of the particle for a significant time and hence lead to band broadening as depicted in Figure 2. Due to the lack of subject matter on the concept of monodensity or even on the pore size distribution effects, no quantifiable data exits. SOLAS™ MonoDense™ is shown to have certain beneficial chromatographic properties owing to this monodense or homogeneous pore structure where in the manufacturing a dual phase surfactant system was employed and have modified several of the traditional processing steps in an attempt to limit and prevent this voiding issue.
Within this paper the aim is to; (A) Introduce the concept of monodensity
(B) Analyse four commercially available silica’s via FIB
(C) Prove the existence and characterise the void structures observed within silica particles
(D) Chromatographically compare a 2 µm SOLAS™ MonoDense™ C18 with other 2 µm C18 columns that are known to contain voided particles.
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