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Table I. Comparison of Properties of Carbon and Reversed-Phase Silica Packings Property
Reversed-phase silica
Carbon
Surface functional groups
n-Alkyl, alkylaryl, aryl, residual hydroxyls
Aromatic-type acidic and basic functional groups
Specific surface area
100-200 m 2 /g
5-1000 m 2 /g
Type and extent of particle porosity
Open pores 60-80%
Open and closed pores 30-60%
Mean pore size and pore size distribution
5-25 nm homogeneous mesopores
1-1000 nm heterogeneous micro-, meso-, macropores
troduced a porous glassy carbon made by pyrolysis of a phenol/formaldehyde polymer on a silica template. After carbonization the silica template is dissolved and the residue heated to 2500 Κ and above. The specific sur face area ranges from 50 to 500 m 2 /g, and the products offer a high porosity because of the mesopores originating from the porous silica template. All preparations have in common carbonization at elevated tempera ture, followed by a second treatment to achieve a homogeneous surface. Al though the pore structure parameters and the specific surface area are mea sured in most cases, only limited in vestigations have been made on the surface chemistry of the products; even if the oxygen content of the ma terial is extremely low, polar surface functional groups can still be present in low concentrations. The reversed-phase silicas are made by a two-step procedure (4): 1) The silica is prepared in an aqueous sys tem and dried to remove the water from the pore system formed; 2) sur face modification (i.e., binding of n-alkyl functional groups) is accom plished by a reaction between the sili ca and an appropriate silane at tem peratures in most cases lower than 473 K. It should be noted that thermal decomposition of bonded organic groups starts at about 600 K. Surface Characteristics
Although largely of academic inter est, it is worthwhile to consider briefly the surface characteristics of both types of packings. The simplified scheme of Figure 1 indicates the prin cipal differences in the structure of the packings. Carbon, of a more or less aromatic type of structure, bears a va riety of polar functional groups origi nating from the reaction between ac tive carbon surface atoms and oxygen. Even when the material is hydrogentreated at 1273 K, the probability of oxygen chemisorption at ambient tem
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perature still remains. This reactivity will be proportional to the specific surface area of the carbon. These sur face functional groups can be princi pally identified and their concentra tions measured by chemical and phys ical analytical methods; a much more sensitive method, however, is to ob serve the retention behavior of wellselected test solutes when carbon is examined under chromatographic con ditions. The retention of basic, acidic, and hydrophobic test substances re flects the net interactions of all sur face functional groups with the solute. Porous silica is composed of an oxidic framework of three-dimensionallylinked S1O4 tetrahedra. By coordina tion of surface silicon atoms with water, weakly acidic hydroxyl groups are formed, rendering the product hydrophilic. On silanization, about half of these hydroxyls react with the silane. The surface structure of a re versed-phase silica—if one can use this term—consists of a highly open framework of solvated and mobile n-alkyl chains with hydroxyl groups at the matrix surface. It behaves more like a microphase, exhibiting swelling and aggregation behavior depending on the type of eluent employed. Table I lists some additional prop erties. The surface area of carbon spans a much wider range than that of reversed-phase materials, since micro pores in carbon, when present, con tribute to a large extent to the specific surface area. On the other hand, se vere thermal treatment may drastical ly reduce the surface to a few square meters, which gives such materials a low linear sample capacity. The parti cle porosity of carbon is often found to be lower than it is for silicas. However, in some cases very fine pores are dis tributed within the particles. Typical ly, carbons offer a broad pore volume distribution whereas silica's is com paratively narrow. Thus, carbon and reversed-phase silica have different structures and
