Steric separation effect

Molecular sieve zeolites " are hydrated, crystalline aluminosilicates which give off their crystal water without changing their crystal structure so that the original water sites are free for the adsorption of other compounds. Activation of zeolites is a dehydration process accomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specific pore sizes that impose limits on the size and orientation of molecules that can be adsorbed. Molecules above a specific size cannot enter the pores and therefore cannot be adsorbed (steric separation effect). 

When the kinetics of a sorption process do appear to separate according to very small and very large time scales, the almost universal inference made is that pure adsorption is reflected by the rapid kinetics (16,21,22,26). The slow kinetics are interpreted either in terms of surface precipitation (20) or diffusion of the adsorbate into the adsorbent (16,24). With respect to metal cation sorption, "rapid kinetics" refers to time scales of minutes (16,26), whereas for anion sorption it refers to time scales up to hours TT, 21). The interpretation of these time scales as characteristic of adsorption rests almost entirely on the premise that surface phenomena involve little in the way of molecular rearrangement and steric hindrance effects (16,21). 

Compared to hydrocarbonaceous silica RPC sorbents, not as much commitment has been made to the development of bonded, polar-phase sorbents suitable for the high-performance chromatographic separation of peptides. Due to polar, notably hydrogen bonding, interactions between the peptide and the hydrophilic surface of the sorbent useful selectivity effects can, however, be achieved. In fact, at least two types of separation mechanisms can be identified with bonded polar-phase sorbents. In the first mode, the peptides do not interact per se with the bonded polar-phase sorbent but, rather, are separated on the basis of their ability to permeate into the pores and elute in order of their hydrodynamic volume. In this mode, peptides are separated by steric exclusion effects, with the retention (in terms of elution volume, Ve) of a partial retained peptide, Pb described by the following relationships ... 

The chromatographic separation of positional isomers (26-31), geometrical isomers (27,32-36) and enantiomers (37-49) has been achieved by utilizing the concerted action of inclusion complex formation, additional primary and secondary hydrogen-bond formation and steric hindrance effects between the solutes and the cyclodextrins (11,12,14-23,50). There is an abundant literature on the analytical applications of cyclodextrin-silicas (13-50), but not on their preparative chromatographic use. 

It is assumed that the steric exclusion from the gel pores is the decisive separation mechanism in the ideal gel chromatography, and both the latter mechanisms contribute positively (but within a limited extent only) to the overall separation effect. 

The advantages of ceramic UF and MF membranes discussed earher in the chapter are often offset by their low selectivity, which makes their use economically unfeasible for many applications. These membranes separate solutes from solvents primarily by site exclusion, and to a lesser extent, by interactions with the membrane surface. The selectivity of ceramic membranes can be improved by modification of the membrane surface either by polymer grafting or graft polymerisation. The latter has the advantage of providing a covalently bonded brush layer of high surface coverage with minimal difiusional limitations and steric hindrance effects. 

In general a lateral polar substituent can influence the thermotropic behavior of a polymer by two oppositely operating effects steric separation of neighboring molecules or pol3rmer chains and increased dipolar attraction between them. Increased polarizability. 

Any field force can be exploited to create conditions for effective action of the steric exclusion mechanism. The only condition is, as mentioned above, that the field strength be high enough to compress all retained species to the accumulation wall. In experimental practice, sedimentation FFF, flow FFF, and thermal FFF are the techniques actually applied in steric mode to separate effectively some particulate species. 

The separations of monomeric, dimeric and trimeric oxo acids of phosphorus, based on the steric exclusion effect, was achieved on Sephadex G-25 gel, regardless of the oxidation states of the phosphorus atoms [ref. 44]. This result was applied to the characterization of unknown oxo acids of phosphorus [ref. 94-96]. 

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