Adsorption chain length

The tendency to form organized monolayers improves with chain length. This is illustrated in a study of adsorption kinetics in alkanoic acid monolayers on alumina by Chen and Frank [36]. They find that the Langmuir kinetic equation, discussed in Section XVII-3, (see Problem XI-6)... 

Using guest fluorophores, they show that the adsorption energy increases linearly with the alkyl chain length as... 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

FIG. 4 Density n z) plotted as a function of distance z from the adsorbing wall and of adsorption strength e/k T for chain length = 16. Vertical dotted lines denote the density of the monomers sticking immediately at the wall for > 2.0 this density changes dramatically [13]. 

FIG. 5 (a) Fraction of adsorbed monomers (i.e., those with z-coordinate less than 6) vs ejk T for four different chain lengths, (b) The same for the second Legendre polynomial P2 cosd). (c) Scaling plot of and P2 cos9) vs distance from the adsorption threshold, using = —1.9 [13]. 

Fig. 3 a-c. Summary of data from different laboratories, obtained by surface force measurement, on the average layer thickness L as a function of tethered chain length for flat, tethered layers constructed by adsorption of amphiphilic polymers on mica. Adapted from Ref. 21. (a) Data of reference 20 on poly-tert-butylstyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (b) Data of references 11 and 12 on polystyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (c) Data of references 13 and 14 on polystyrene chains anchored by adsorbing zwitterionic groups [13] or by small adsorbing blocks of polyethyleneoxide [14]... 

When one of the two acids is used in excess and the pk -values of the two acids differ strongly, the salt deficit method should be used with caution. Formic add, acetic acid, propionic acid, and trifluoroacetic acid have been electrolyzed competitively in mixtures of pairs. Formic acid and trifluoroacetic acid are comparable in case of electrolysis, both are more readily electrolyzed than acetic and propionic adds. Deviations are rationalized on the basis of differences in ionization [147]. It might 1 useful in such cases to neutralize both acids completely. Sometimes one of the two acids, although being the minor component, is more favorably oxidized possibly due to preferential adsorption or its higher acidity [148]. In this case the continuous addition of the more acidic add to an excess of the weaker acid may lead to successful cross-coupling [149], The chain length of the two acids should be chosen in such a... 

Since sulfonate groups have been used in other ground-water tracers, the goal of this work was to synthesize several N-substi-tuted pyridone alkyl sulfonates which might be less susceptible to adsorption as well as more soluble. Alkyl sulfonates of varying chain lengths would be unique in a water system and separately identifiable by HPLC analysis. Two series of compounds were synthesized (Vla-c and Vlla-c). These compounds could be prepared by treating compounds III and IV with the appropriate lu-bromoalkyl sulfonate. 

Since the interaction of linear hydrocarbons is dominated by the van der Waals interaction with the zeohte, the apparent activation energies for cracking decrease hnearly with chain length. In some cases, differences in the overall rate are not dominated by differences in the heat of adsorption but instead are dominated by differences in the enthrones of adsorbed molecules. 

Additional adsorption sites are provided on open metal sites, when available. [Cu3(BTC)2] is performant in the selective adsorption and separation of olefinic compounds. The highly relevant separations of propene from propane and of isobutene from isobutane have been accomplished with separation factors of 2.0 and 2.1, respectively [101, 102]. [Cu3(BTC)2] also selectively takes up pentene isomers from aliphatic solvent in liquid phase, and even discriminates between a series of cis- and trans-olefin isomer mixtures with varying chain length, always preferring a double bond in cis-position. This behavior is ascribed to tt -complexation with the open Cu sites [100]. 

Substantial evidence in a number of existing experimental studies can be easily reconciled with the models discussed in the present contribution. For example segregation of short chains reported during crystal growth [1] may be thought to arise with chains which are too short to form bundles and are thus unable to provide a sufficient amount of simultaneous attractive interactions with the crystal to yield stable adsorption. We recall in this respect that one of us obtained the correct trend of the minimum chain length of PE for crystal inclusion vs. the crystallization temperature, using the bundle approach [8]. 

Fig. 8. Adsorption of BSA on surfaces coated with polyethylene oxide (PEO). Influence of the grafting density of PEO at the surface for different polymer chain lengths (---700,-------- 445, and------148 ethylene oxide monomers) on the...

The physical significance of the parameters (or K ) and K is obviously of considerable interest. In the formulation of the model, these are simply arbitrary parameters which define, respectively, the extent of primary and multiple adsorption. For the particular case evaluated above (x=0.5 and xs=l 0) K is approximately constant while appears to decrease exponentially with chain length n. It is expected that, for a given chain length, the parameters and K will both depend on x and X Further comparisons, similar to that given here, will be required to establish the precise correspondence of the parameters used in the two approaches. 

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