8 February / March 2017
An Introduction to the Concept of Monodensity in Silica Particles and its
Effect on Chromatographic Performance
by John P Hanrahan, Tom O’ Mahony, Ricky Curley, John J Hogan & Joseph M Tobin Glantreo Ltd, ERI Building, Lee Road, Cork T23 XE10, Ireland. Tel: +353 21 4901965. Email:
j.hanrahan@glantreo.com
Have you ever imagined what the inside of fully porous or a superficially porous silica particle looks like? Traditionally the internal structure of a silica particle has been considered to be a homogeneous arrangement of pores with little or no imperfections. However this is not the case. On examination of several different silica particle types from a range of silica manufactures we have found that the majority of porous silica particles currently on the market contain significant internal voids or holes. These voids are clearly visible under a Scanning Electron Microscopy (SEM) after the particle has been manipulated using Focused Ion Beam (FIB) milling technology. These voids can be up to an order of magnitude larger than the claimed pore size of the silica. The presence of these voids or holes suggest that the particles are not monodense, i.e. there is a significant density distribution across particles. Theoretically these holes or voids could cause mass transfer issues which could lead to peak broadening, peak tailing and ultimately losses in chromatographic efficiency. Within this article we aim to introduce the concept of ‘monodensity’ which for years has been spoken about within the silica community but to date has not been published on. This article will also study the effect of monodensity by chromatographically comparing Glantreos SOLAS™ MonoDense™ particles with commercial particles known to contain voids.
Introduction
Traditionally, silica manufacture has been based on the hydrolysis of an alkyl silicate in the presence of an acid or base in solution or gas phase. Several detailed reviews have been written on the different manufacturing processes of silica particles. These manufacturing approaches can be broken into different sub sets as illustrated in Figure 1. An in-depth discussion on the pros and cons of each process is well beyond the scope of this article, although several very good review articles exist for the interested reader [1,2]. Glantreo has concentrated on liquid sol gel routes to the production of monodisperse and more recently monodense particles, in particular we have utilised the modified Stöber process as our platform manufacturing process for the production of fully porous and superficially porous silica particles. In order to understand the rational for basing our process around this modified Stöber process a brief background is beneficial.
Non Porous Stöber Particles
In 1968, Werner Stöber et al. [3] reported a process for the controlled growth of
Figure 1. Schematic representation of industrial silica manufacturing routes.
spherical non-porous silica particles of uniform size. The particles obtained via this method ranged in size from 50 nm to 2000 nm in diameter. The relatively simple procedure involves mixing a silica precursor, in the form of an alkyl silicate, with ammonia in a water/alcohol solution. The alkyl silicate is hydrolysed to give
silicic acid, which subsequently condenses to give the monodisperse silica particles. Ammonia is used as a base catalyst and influences the morphology of the particles in a manner which produces spherical particles. A key aspect of the Stöber process is the ability to produce different particle sizes by altering the reaction conditions, i.e.,
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