Molecular size effect adsorption

These conclusions can only be tentative with the limited data here. However the potential scope for future studies of the kinetics of adsorption and partitioning of mixed sorbates is evident. This is of particular interest with microporous sorbents, such as carbons, especially where molecular size effects may be predominant. Such an aspect is indeed particularly relevant in gas separation processes which may involve membranes and pressure swing adsorption. 

Of the possible emulsifiers, most are what are considered true surfactants, in that they are effective at lowering significantly the interfacial tension between the two hquid phases. Other additives such as polymers and sols function primarily as stabihzers, rather than emulsifiers. Most polymers are not sufficiently effective at lowering interfacial tensions to act in that regard. In addition, because of their molecular size, the adsorption process for polymers is generally very slow relative to the timescale of the emulsification process. The same applies to stabilizing colloids, in which their action requires the wetting of the particles by the two hquid phases to facihtate their location at the interface. The primary function of polymers and sols in emulsions is in the retardation of droplet flocculation and coalescence. 

Adsorption. Although several types of microporous soHds are used as adsorbents for the separation of vapor or Hquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeoHtes, which selectively adsorb or reject molecules based on differences in molecular size, shape, and other properties such as polarity. The sieve effect may be total or partial. 

Particle collision frequency due to Brownian motion was estimated to be less than 1% of the collision frequency due to shear. The effects of Brownian motion could therefore be neglected in the flocculation rate calculations. However, for the smallest molecular size, radius of gyration 14 nm (see Table I), the effect of Brownian motion on the particle-polymer collision efficiency was of the same order of magnitude as the effect of shear. These two contributions were assumed to be additive in the adsorption rate calculations. Additivity is not fundamentally justified (23) but can be used as an interpolating... 

Packings based on silicas for separation by molecular size are also presently available. However, the molecular size range available is more limited and possibility of encountering adsorption effects is more likely. Such columns must be throughly evaluated for each new type of sample for which a separation and/or molecular weight distribution is desired. 

Molecular contact area, 148, 132, 133, 193 Molecular shape, effect on contact area upon bimUng, 218 MolecuW size of the eluite, 217 Molecular surftce area, in adsorption and partition, 226... 

This chapter deals mainly with quantum size effects in CD nanocrystalline films. However, another, quite separate property of such films is related to the large percentage of atoms located on the surface of the nanocrystals of these films, e.g. —50% for a crystal size of a few nm this is the effect of adsorption of molecular and ionic species on the nanocrystal surfaces. This aspect has been dealt with much less than has size quantization therefore, it constitutes only a very small part of this chapter, mainly Section 10.2.3, which discusses the effect of adsorbed water on CD CdSe films. Section 9.2.2.2 deals in somewhat more detail with this particular issue. 

Cations and anions with a strong solvation shell retain their solvation shell and thus interact with the electrode surface only through electrostatic forces. Since the interaction is exclusively electrostatic, the amount of these ions at the interface is defined by the electrostatic bias between the sample and the counter electrodes and independent from the chemical properties of the electrode surface non-specific adsorption. Considering the size effect of their hydration shell, these ions are able to approach the electrode to a distance limited by the size of the solvation shell of the ion. The center of these ions at a distance of closest approach defined by the size of the solvation shell is called the outer Helmholtz layer. The electrode surface and the outer Helmholtz layer have charges of equal magnitude but opposite sign, resulting in the formation of an equivalent of a plate condenser on a scale of a molecular layer. Helmholtz proposed such a plate condenser on such a molecular scale for the first time in the middle of the nineteenth century. 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

Despite claims by Spry and Sawyer (1975) of analytical measurements verifying asphaltene molecular sizes in the 100 A range at ambient conditions, it is unlikely that molecules this bulky exist at reaction conditions. The good predictive capability of the model may therefore result from a compensation effect. Electrostatic and adsorption interactions between solute molecules and the pore walls not explicitly accounted for with the purely geometric partition coefficient may result in the diffusing molecules appearing larger than they are at reaction conditions. 

Only a few studies have been reported on the effect of molecular size on the adsorption of normal paraffins in zeolites. For both small- and large-pore low silica/alumina zeolites, in liquid phase... 

Recently, Pcs organized on surface have been used as molecular probes for the determination of quantum confined effects [207], CoPc molecules form ordered self-assembled monolayers (SAM) on the top of Pb(lll) thin films grown on a Si( 111) substrate with the Pc units lying flat on the surface, as revealed by atomically resolved STM. A close analysis of the STM data revealed that the Pc molecules adsorb and self-assemble on the surface following a thickness-dependent adsorption pattern, which is ultimately related to the quantum size effects of the metal surface. 

In conclusion, the use of amines for the characterization of adsorption and active sites raises the same problems as ammonia. The interaction of the amines, saturated aliphatic amines in particular, with oxide surfaces is certainly still less specific than that of ammonia because of their higher basicity and their larger molecular size. The influence of steric effects on amine adsorption has been discussed (172, 201b). Thus, Medema et al. (172) came to the conclusion that the adsorbed amount of an amine on 7-Al2 03 primarily depends on its molec-... 

It is concluded from the foregoing considerations that pyridine may successfully be applied as a specific poison, provided the possible pitfalls are carefully kept in mind. The lower basicity of pyridine as compared to ammonia renders its chemisorption more selective. However, its basicity is in most cases still much higher than that of the commonly used reactants, so that one is usually able to determine an upper limit for the number of active sites by pyridine poisoning (239). On the other hand, the hardness of reactants or reaction products may be comparable with that of pyridine [e.g., dehydration of alcohols (47)] the poison will then be partially displaced. The molecular size of pyridine may bring about difficulties, since it restricts the accessibility of pyridine to narrow pores or even the approach to an adsorption site (214). In favorable cases, however, steric effects may be utilized to improve the specificity of poisoning (35, 36,241). 

Fast and effective adsorption of harmful compounds of different origin and molecular size is of importance for environmental and human protection. Clearly the structural and adsorption characteristics of adsorbents play an important role for their use in various applications. Certain adsorption characteristics can be optimized for adsorbates depending on the application. For instance, specific surface area and adsorption capacity are important because if they increase then the adsorption effectiveness increases. However,... 

---