Monomeric bonding

Before reaction, the silica is treated with acid (eg refluxed for a few hours with 0.1 mol dm-3 HC1). This treatment produces a high concentration of reactive silanol groups at the silica surface, and also removes metal contamination and fines from the pores of the material. After drying, the silica is then refluxed with the dimethylchlorosi-lane in a suitable solvent, washed free of unreacted silane and dried. This reaction produces what is called a monomeric bonded phase, as each molecule of the silylating agent can react with only one silanol group. 

More complicated surface structures can be produced by changing the functionality of the silylating agent and the conditions under which the reaction is carried out. The use of di- or trichlorosilanes in the presence of moisture can produce a crosslinked polymeric layer at the silica surface, as shown in Fig. 3.2a (if). Monomeric bonded phases are preferred, as their structure is better defined and they are easier to manufacture reproducibly than the polymeric materials. 

Hydrocarbonaceous bonded phases described in the literature have at vialues ranging frolm 2 to 4. It is believed that among monomeric phases, those having the highest ol values are the best for use in RPC. In Table III the pertinent clttihtcteristics of some monomeric bonded phases are listed. 

It is seen that the trichlorosilane reacts with the silanol groups to form siloxane bridges. Subsequently the residual chlorines are hydrolyzed. Under carefiiUy controlled reaction conditions it is possible to obtain a product in which the hydrocarbonaceous layer at the surface is similar to that in a corresponding monomeric bonded phase. However, the hydrolysis of chlorines that did not react with surface silanbis may result in a silanol concentration at the surface that is higher than that in the silica gel proper used as the starting material for the reaction with alkyltri-chlorosilanes. 

Van t Hoff plots of In k versus the inverse of temperature (generally 1000/T for convenience) are very often linear, especially with monomeric bonded phases. They can exhibit nonlinear behavior, and the transition temperature is often close to the undefined room temperature. Temperature optimization is one trend in LC. A rising temperature increase reduces viscosity and increases the diffusion rate, thereby enhancing mass transfer, which flattens the HETP curve at high velocities (31). Conversely, Sander and Wise (32) investigated the influence of temperature reduction. 

The physical structure of the stationary phase depends on the compatibility of the solvent with the bonded n-alkyl chains. Compatible nonpolar solvents tend to promote extension of the chains, allowing full penetration by the solvent. Conversely, fairly polar solvents tend to promote collapse of the chains upon each other, allowing negligible solvent penetration. The stationary phase therefore has the ability to adjust itself to maintain a relatively nonpolar character (113). Retention on monomeric bonded phases with octyl (C8) or longer chains are dominated by a partitioning-like mechanism (114). 

The stationary phases play an important part in Liquid Chromatography using micellar mobile phases. They interact with both the surfactant and with solutes. To study the interactions with surfactants, adsorption isotherms were determined with two ionic surfactants on five stationary phases an unbonded silica and four monomeric bonded ones. It seems that the surfactant adsorption closely approaches the bonded monolayer (4.5 pmol/m2) whatever the bonded stationary phase-polarity or that of the surfactant. The interaction of the stationary phase and solutes of various polarity has been studied by using the K values of the Armstrong model. The KgW value is the partition coefficient of a solute between the... 

In the studied cases monomeric bonded silica and ionic surfactants, the surfactant adsorption was almost constant with differing... 

In RPLC, the influence of pressure on the chromatographic behavior is related to the hydrophobic interactions involved in the retention mechanism and to the change upon adsorption in the numbers of acetonitrile and water molecules in the solvent shells of the protein molecule and of the bonded layer. The importance of the changes in the retention factor and the saturation capacity with a change in the average column pressure will thus depend on the retention mode used and will vary with the hydrophobicity of the molecule [128]. In RPLC, it is larger with polymeric than with monomeric bonded phases [126]. 

Monofunctional silane reagents yield efficient stationary phases with flexible fur-like or brush-Uke structure of the chains bonded on the silica surface. When bifunctional or trifunctional silanes are used for modification. Cl or alkoxy groups are introduced into the stationary phase, which are subject to hydrolysis and react with excess molecules of reagents to form a polymerized sponge-like bonded phase structure. Stationary phases prepared in this way usually show stronger retention but lower separation efficiency (plate number) than do monomerically bonded stationary phases. 

Fig. 4. Formation of bonded-phase siiicas. (a) Monomeric bonded phases (b) End-capping of residuai siianols. R = aikyi, aminoaikyi, ion-exchange groups.

Pirkle column Named for its inventor, the original Pirkle column was a monomeric bonded phase that had the following structure This bonded phase and many others having similar construction are chiral in nature ( denotes the chiral center) and are used in the separation of enantiomers. 

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