Size Exclusion—Sieving

Selective barrier structure. Transport through porous membranes is possible by viscous flow or diffusion, and the selectivity is based on size exclusion (sieving mechanism). This means that permeability and selectivity are mainly influenced by membrane pore size and the (effective) size of the components ofthe feed Molecules... 

Size exclusion HPLC has many other common names, such as gel permeation, gel filtration, steric exclusion, molecular sieve chromatography, or gel chromatography. These names all reflect the theoretical mode of action for this type... 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

Shape Selectivity Due to Molecular Sieving. The simplest types of shape selectivity are related to the impossibility for certain molecules of a reactant mixture to enter the micropores (RSS) or for certain product molecules to exit from these pores (PSS). In practice, RSS and PSS are observed not only when the molecule size is larger than the pore openings (size exclusion) but also when their diffusion rate is significantly lower (by two orders of magnitude) than that of the other molecules. 

Of all the desiccants, molecular sieves 4A or 5A are perhaps the best for all-around use. Two obvious advantages are their high affinity for H20 and good capacity. They are fairly inert and, because of their size exclusion of large molecules, are less susceptible than many adsorbents to competitive adsorption of other polar molecules. Molecular sieves are usually compacted with a clay binder into the form of small rods, which minimizes the restriction of gas flowing... 

Size exclusion/molecular sieve chromatography Electron microscopy Ultracentrifugation Ultra nitration NMR spectroscopy Small-angle x-ray scattering Encapsulation of water-soluble markers... 

Size exclusion/molecular sieve chromatography Ultra Itration/dia lysis Ultracentrifugation Fluorescent probes Spin label EPR NMR probes Calorimetry Microelectrophoresis Zeta potential... 

Various analytical techniques make use of both porous and nonporous (semipermeable) membranes. For porous membranes, components are separated as a result of a sieving effect (size exclusion), that is, the membrane is permeable to molecules with diameters smaller than the membrane pore diameter. The selectivity of such a membrane is thus dependent on its pore diameter. The operation of nonporous membranes is based on differences in solubility and the diffusion coefficients of individual analytes in the membrane material. A porous membrane impregnated with a liquid or a membrane made of a monolithic material, such as silicone rubber, can be used as nonporous membranes. 

The range of operating modes is much greater in LC than in GC because both phases, stationary and mobile, affect the separation and because a wide range of stationary phases can be used in LC. Stationary phases include many types of bonded phases with a wide range of polarities, and also materials with ion exchange and sieving (size exclusion) properties. 

Sample size, maximum, 223 Sensitivity (detector), 95 Separation, definition of, 4 Separation factor, 20-22, 77 Separation number, 18 Sieving, molecular sieves, 45-47. See also Size exclusion chromatography Silica, surface of, 166, 167, 236, 237 Simulated distillation, 150... 

The molecular sieving behaviour of Silicalite-I, as illustrated in Table 11.5 by the low saturation uptakes of neopentane and o-xylene, is primarily dependent on size exclusion. It is of interest that n-nonane has been found to give an isotherm of essentially Type I character at 296 K (Grillet et al., 1993). The initial part of this isotherm was completely reversible, but a small sub-step at p/p0 0.2 was followed by a long plateau and associated narrow, Type H4, hysteresis loop. The plateau was located at N° = 4 molec uc"1. This level of pre-adsorption was sufficient to block the whole of the intracrystalline pore structure. The accessibility to nitrogen was gradually restored by the progressive removal of the nonane. These results confirm the complexity of the nonane pre-adsorption and entrapment in relation to the pore network and indicate that there is no simple relation between the thermal desorption of n-nonane and the adsorbent pore structure. 

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