Table of Content Dedication ii Abstract iv Acknowledgements v Table of Content vii List of Figures xi List of Tables xvii List of Abbreviations xviii Chapter 1 Fundamentals of Mon
Silica-Based Monolithic Columns in Liquid Chromatography 1
Investigations into new separation media continue to be a major aspect in separation sciences This is due to the variety of new challenges in separating very complex sample mixtures Emerging technology is enabling very high separation efficiency and speed of analysis, surpassing the conventional particle-packed columns in high performance liquid chromatography (HPLC) These included capillary electrochromatography, 1 ultrahigh pressure HPLC (UHPLC), 2, 3 and the use of monolithic silica columns 4 Monolithic columns attracted attention because of their potential high performance under common operating conditions that rivals that of packed columns without high pressure requirements
Conventional HPLC columns are packed with particles of micrometer size Packing a high-efficiency column requires skills as well as the appropriate packing material with suitable properties Columns having one-piece network structures are thought to be desirable to avoid difficulties in packing columns, which can change as new surface modifications are introduced 5 Monolithic columns were reported first with organic polymers, 6, 7 followed by silica columns prepared in capillaries 8 There have been numerous reports on polymer- and silica-based monolithic columns for HPLC 9-11
One of the great features of monolithic silica columns is their rod-like structure consisting of a skeleton and through-pores The columns can offer a variable external porosity and through-pore size/skeleton size ratios that are impossible to achieve with particle-packed columns These characteristics give monolithic columns very high permeability that allows their operation at pressures much lower than traditional HPLC The preparation of silica-based monoliths make use of sol-gel processing under controlled conditions allowing desirable characteristics
The most common approach to fabricate silica-based monolithic columns is the acid-catalyzed sol-gel process.10, 12,13,14, 15 This process consists of hydrolysis and condensation reactions of metal alkoxides under acid conditions (Figure 1-1) 16 The commonly used metal alkoxides for silica-based monoliths are alkoxysilanes such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) Polycondensation then occurs with the linkage of silanol groups to form cyclic oligomers and eventually cast a silicate network
The pore size and the mechanical properties of gels can be varied with the addition of polyethylene glycol (PEG) to the sol PEG is a porogen which acts as a through-pore template and solubilizer of the silane reagent This has been done by Nakanishi 17 , Judenstein 18 and Martin 19 who claimed that high concentrations of PEG weaken the solid
Figure 1-1: Schematic representation of the silica reactions involved in the sol-gel process M: Silica, R: alkyl group Reaction is under acid conditions
4 matrix whereas a small concentration of PEG strengthens the matrix The pore size of macroporous silica aerogel can be controlled by varying the concentration of water soluble polymer Narrow and more uniform pore size distribution is observed with the addition of glycerol which acts as a drying additive since it prevents further reaction of water Mesopores are formed in the silica skeletons by a treatment with ammonia, introduced after the formation of a network structure of silica skeletons Ammonia can also be generated by the hydrolysis of urea, which can be added in an initial reaction mixture
Once the gel forms with a desired shape, rinsing in H2O/EtOH will increase the permeability of the solid portion of the silica gel by a dissolution–reprecipitation process Aging in a siloxane solution increases the stiffness and strength of the alcogel by adding new monomers to the silica network and by improving the degree of siloxane crosslinking; conversely this step will reduce the permeability 20 Einarsrud et al 21, 22 have reported strengthening of the silica gels aged in TEOS, water, and ethanol solutions Gels are washed with a 20% water–ethanol solution for 24 h at 50-60 ºC, then an aging solution (70%TEOS/ethanol, v/v) is used for 6–72 h at 50-70 ºC followed by washing with ethanol and heptane Data from small angle neutron scattering shows only a slight increase in the volume fractal dimension of the porous gel network The same group demonstrated that washing in a water solution increases the permeability of the gels by dissolution–reprecipitation 23 Silylation removes Si–OH surface groups by promoting silica polycondensation resulting in a decrease of pore size
Drying of the gel is a critical step Drying is governed by capillary pressure During drying, shrinkage of the gel occurs due to capillary stress It is the gradient in capillary pressure within the pores that leads to mechanical damage; the capillary tension developed during the drying may reach up to 15,000-30,000 psi (1000-2000 bar) 24 with consequent shrinkage and cracking Silica gel may decrease in volume by as much as a factor of 10 as it dries
The extent of shrinkage is governed by the balance between capillary pressure Pc , and modulus of the solid matrix: h
= (1) where γ(LV) is the surface tension of the pore liquid at the liquid vapor interface, θ is the contact angle of the liquid, and rh is the hydraulic pore radius
Three phases are present: liquid, solid and gas Figure 2 explains shrinkage of the solid matrix γ(SG), γ(SL), γ(LG) a are surface tension for solid/gas, solid/liquid and liquid/gas When liquid evaporates from the pores of a gel and when the contact angle, θ, between the liquid and the network is γ(SL) the liquid ascends the pore by capillary rise, the solid / gas interface is replaced by a solid / liquid interface, and a concave meniscus is observed If θ=0º, or → 0, the capillary liquid is stressed, and the solid network is compressed When γ(SL) >γ(SG), 90º < θ