Chemical bond liquid-solid

The initial set of experiments and the first few textbook chapters lay down a foundation for the course. The elements of scientific activity are immediately displayed, including the role of uncertainty. The atomic theory, the nature of matter in its various phases, and the mole concept are developed. Then an extended section of the course is devoted to the extraction of important chemical principles from relevant laboratory experience. The principles considered include energy, rate and equilibrium characteristics of chemical reactions, chemical periodicity, and chemical bonding in gases, liquids, and solids. The course concludes with several chapters of descriptive chemistry in which the applicability and worth of the chemical principles developed earlier are seen again and again. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

The vast majority of modem liquid chromatography systems involve the use of silica gel or a derivative of silica gel, such as a bonded phase, as a stationary phase. Thus, it would appear that most LC separations are carried out by liquid-solid chromatography. Owing to the adsorption of solvent on the surface of both silica and bonded phases, however, the physical chemical characteristics of the separation are more akin to a liquid-liquid distribution system than that of a liquid-solid system. As a consequence, although most modern stationary phases are in fact solids, solute distribution is usually treated theoretically as a liquid-liquid system. 

Network solids such as diamond, graphite, or silica cannot dissolve without breaking covalent chemical bonds. Because intermolecular forces of attraction are always much weaker than covalent bonds, solvent-solute interactions are never strong enough to offset the energy cost of breaking bonds. Covalent solids are insoluble in all solvents. Although they may react with specific liquids or vapors, covalent solids will not dissolve in solvents. 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

Water taken up by solid materials is generally classified as water bound by physical forces or water bound by chemical bonds. Physically bound water includes adsorbed water, trapped or liquid-inclusion water, and absorbed water. The physical adsorption of water occurs when water condenses or is held on the surface the surface includes the cracks, crevices, etc. of real materials. Liquid inclusion occurs during the crystallization process when bubbles of water are trapped. 

Liquid-solid chromatography (LSC), sometimes referred to as normal phase or straight phase chromatography, is characterized by the use of an inorganic adsorbent or chemically bonded stationary phase with polar functional groups and a nonaqueous mobile phase... 

The problems of stationary phase erosion can be largely overcome by solvent-generated LLC, where the stationary liquid phase is generated dynamically by the mobile phase, in this approach, one of the phases of an equilibrated liquid-liquid system is applied as a mobile phase to a solid support which is better wetted by the other phase of the liquid-liquid system. The support is usually silica when the stationary phase is aqueous or a polar solvent and a reversed-phase chemically bonded support when the stationary phase is a nonpolar solvent. Under these Conditions a multimolecular layer is formed on the surface of the solid support which has the properties of the liquid phase in... 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

It may be difficult to imagine a liquid mobile phase used with a liquid stationary phase. What experimental setup allows one liquid to move through another liquid (immiscible in the first) and how can one expect partitioning of the mixture components to occur The stationary phase actually consists of a thin liquid film chemically bonded to the surface of finely divided solid particles, as shown in Figure 11.8. It is often referred to as bonded phase chromatography (BPC). Such a stationary phase cannot be removed from the solid substrate by heat, reaction, or dissolving in the mobile phase. 

FIGURE 11.8 An illustration of partition chromatography. A thin liquid film chemically bonded to the surface of finely divided solid particles is the stationary phase. 

In summary, partition chromatography is a type of chromatography in which the stationary phase is a liquid chemically bonded to the surface of a solid substrate, while the mobile phase is either a liquid or gas. The mixture components dissolve in and out of the mobile and stationary phases as the mobile phase moves through the stationary phase, and separation occurs as a result. Examples of mobile and stationary phases will be discussed in Chapters 12 and 13. 

To maximise separation efficiency requires low H and high N values. In general terms this requires that the process of repeated partitioning and equilibration of the migrating solute is accomplished rapidly. The mobile and stationary phases must be mutually well-dispersed. This is achieved by packing the column with fine, porous particles providing a large surface area between the phases (0.5-4 m2/g in GC, 200-800 m2/g in LC). Liquid stationary phases are either coated as a very thin film (0.05-1 pm) on the surface of a porous solid support (GC) or chemically bonded to the support surface as a mono-molecular layer (LC). 

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