Hydrolytic zirconia

Chapter 4 provides a discussion of zirconia and hybrid columns. We will only lightly cover here the hybrid columns and expand on the zirconia discussion. Hybrid columns were developed to reduce the amount of silanol activity as well as to impart better hydrolytic stability, especially at high pH. By incorporating different functional groups into the matrix. 

Hydrolytically stable inorganic carriers are alumina, titania, and zirconia. Their physical and chemical properties are quite different from that of silica. Their application as a base for bonded phases has been described in literature. 

For these three materials, covalent bonding technologies cannot be used. With silanes, mixed anhydrides are formed lacking in hydrolytic stability. Coating with organic polymers [32] is the way to go. A bonded phase based on zirconia has been studied widely [43]. Method development strategies established with silica-based RP cannot be transferred to an RP bonded on zirconia. Selectivity is dependent, e.g., on the type of buffer used. Anions in the mobile phase influence retention. The kinetics of analyte interaction with the different active sites may lead to reduced efficiencies. 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

Two excellent reviews that detail procedures for the preparation of bonded phase supports have recently been published by Leonard - and Buchmeiser. One of the most popular methods of surface chemical modification involves the use of organosilanes. These organosilanes react with the surface metal hydroxyl groups and form a surface, which may be represented as M-O-R, where R represents an alkyl chain and M represents the metal (i.e., silica, zirconia, titania, etc.). One important factor that must be stated, however, is that the order of stability of M-O-R bonds increases in the order of M=Si > Zr > Ti > Improvements in the hydrolytic... 

Given the success of porous silica materials in liquid chromatography other inorganic oxides have been investigated in the hope of providing mechanically strong and more hydrolytically stable particles and bonded phases with a complementary surface chemistry [3,5,40-43]. Alumina (pH 2-12) and titania and zirconia (pH 1-14) are stable... 

For alumina, titania, and zirconia, there exists as yet no covalent bonding chemistry that is equivalent to the silanization technique used for silica. Although attempts have been made to silanize these other oxides, the hydrolytic stability of these phases does not match up to the hydrolytic stability of the support itself. Therefore alternative surface modification tet ques have been developed that do not rely on the attachment of the modifier to the surface. The coating can be simply insoluble in the intended mobile phases, or a crosslinked coating can be formed that stretches like a net around the skeleton of the particle. Both techniques are, in principle, independent of the nature of the substrate and can be applied to all inorganic or polymeric packings. 

CaUlaud F., Smith A., Baumard J.-F. Effect of pH of the solution on the deposition of zinc oxide films by spray pyrolysis. J. Am. Ceram. Soc. 1993 76 998-1002 Clearfield A. Crystalline hydrous zirconia. Inorg. Chem. 1964 3 146-148 Clearfield A. The mechanism of hydrolytic polymerization of zirconyl solutions. J. Mater. Res. 1990 5 161-162... 

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