Adsorption steric

FIGURE 8.7 Negative adsorption (steric exclusion) of a solute S from the surface of a protein molecule or particle, (a) Schematic explanation, (b) Relation between nonsolvent water vrns and molecular radius Rs of the solute (mostly sugars) for micellar caseinate a scale of molar mass (M, in Da) is also given. Rw is the radius of a water molecule. 

When several substances are adsorbed from a mixture, their combined adsorption may equal, exceed, or be less than that of the adsorption of any individual ingredient from a pure solution. It is of interest to speculate on factors that might lead to such varied behavior, keeping in mind that there is seldom only a single cause generally what we observe is a resultant of several influences. When the total amount adsorbed from a mixture is approximately equal to the sum of the separate ingredients as measured from pure solutions, this indicates that each solute is attracted to separate areas. More often the data give evidence of competition to occupy the same areas, and this is indicated when the total adsorption from a mixture is less than the sum of the separate individual adsorptions. Steric... 

Define monoclonal antibodies, affinity adsorption, steric structure, and volatile organic compounds. 

G. J. Fleer, J.M.H.M. Scheutjens, and M. A. Cohen Stuart, Theoretical progress in polymer adsorption, steric stabilization and flocculation. Colloid Surf. 31, 1 (1988). 

Gi-op A, Wilke S and Scheffler M 1995 6-dimensional quantum dynamics of adsorption and desorption of H2 at Pd(IOO)-steering and steric effects Phys.Rev. Lett. 75 2718... 

Jin X, Talbot J and Wang N FI L 1994 Analysis of steric hindrance effects on adsorption kinetics and equilibria AlChE J. 40 1685-96... 

Application of Snyder s theory of linear chromatographic adsorption (171) gives the variation in adsorption energy of the thiazole nitrogen atom as a function of this steric hindrance for silica and alumina (see Table III-22). These results show that alumina is more sensitive toward steric effects while silica shows a higher selectivity in the case of polar effects. 

This division is somewhat arbitrary siace it is really the pore size relative to the size of the sorbate molecule rather than the absolute pore size that governs the behavior. Nevertheless, the general concept is useful. In micropores (pores which are only slightly larger than the sorbate molecule) the molecule never escapes from the force field of the pore wall, even when ia the center of the pore. Such pores generally make a dominant contribution to the adsorptive capacity for molecules small enough to penetrate. Transport within these pores can be severely limited by steric effects, leading to molecular sieve behavior. 

Surface Modification. Reaction or adsorption at the soHd surface can alter its properties and lead to a surface charge or steric stabilization... 

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

As a result the polymer chain growth seems to stop due to the impossibility of the monomer coordination on the titanium ion. The propagation centers are nonuniform and the adsorption of the aluminumorganic compound takes place mostly at sterically more accessible nonstereospecific active centers. 

Nonionic functional groups provide steric repulsion and improved particulate adsorption. Each of the various polymers, phosphonates, and other organics employed have combinations of functions. The overall molecules may act as ... 

Other data support the above picture. Hexanol adsorbs very weakly on Ag(l 10), more weakly than expected, and in any case less than on the (100) face.440 Such a poor adsorption on (110) faces has been explained in terms of steric hindrance caused by the superficial rails of atoms. Consistently, adsorption on the (110) face of Cu is vanishing small.587 Predictions based on a linear regression analysis of the data for pentanol (nine metals) give a value of-12 kJ mol 1 for Cu(l 10) and about -16 kJ mol 1 for Au(110). No data are available for polycrystalline Au, but Au(l 11) is placed in the correct position in the adsorption of hexanol.910 Thus, these data confirm the hydrophilicity sequence Hg < Au < Ag and the crystal face sequence for fee metals (111) < (100) < (110). 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

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