Entropic selectivity

Singh A and Koros WJ. Significance of entropic selectivity for advanced gas separation membranes. Ind. Eng. Chem. Res. 1996 35 1231-1234. 

The use of semirigid tridentate binding units restricts the structural issues to [] (L ) ] whereby n = 1-3 characterizes the number of strands wrapped about the central metal. The target Cs-symmetrical triple-helical complex [f (L )3] can be further produced as the major species thanks to a strict control of the conditional information expressed in the stoichiometric ratio f L = l 3 (i.e., entropic selection). 

Singh (1997) and Singh and Koros (1996) have discussed the importance of entropic selectivity in molecular sieving materials such as CMS membranes. These authors analyzed the activation energy of diffusion for oxygen and nitrogen in an upper-bound... 

Temperature can also be used to optimize enantioselectivity in SFC. The selectivity of most CSPs increases as temperature decreases. For this reason, most chiral separations in SFC are performed at ambient or subambient temperatures [50, 74]. Subambient temperatures are particularly useful for compounds having low conformational stability [75]. Stringham and Blackwell explored the concept of entropically driven separations [76]. As temperature increased, enantioselectivity decreased until the enantiomers co-eluted at the isoelution temperature. Further increases in temperature resulted in reversal of elution order of the enantiomers. The temperature limitations of the CSP should be considered before working at elevated temperatures. 

It is necessary to use values of vapour pressure in order to be able to deal with the last stage since boiling point values under reduced pressure are not available. The selected vapour pressure value is the only one that is coherent since the others would provide a value of S very far outside the entropic domain. The calculated value of S is already greater than the maximal value Sg (as in the case of 2-propanol), a point that will be returned to later. 

Such esterifications and acetal formations are achieved through enzyme catalyses. However, such reactions are relatively rare in aqueous conditions chemically. This is because the reversed reactions, hydrolysis, are much more favorable entropically. Kobayashi and co-workers found that the same surfactant (DBSA) that can catalyze the ether formation in water (5.2 above) can also catalyze the esterification and acetal formations reactions in water.52 Thus, various alkanecarboxylic acids can be converted to the esters with alcohols under the DBSA-catalyzed conditions in water (Eq. 5.6). Carboxylic acid with a longer alkyl chain afforded the corresponding ester better than one with a shorter chain at equilibrium. Selective esterification between two carboxylic acids with different alkyl chain lengths is therefore possible. 

Naturally selectivity in a several-component system is primarily influenced by rather strong effects such as the presence or absence of strong H-bonding, but possibly also by much weaker interactions (e.g. of C—H... O type). In this regard, it is interesting to note the similarity between the selectivity exerted by such simple inclusion hosts, e.g. /, and chiral recognition 103). In both cases, weak interactions are of decisive importance in the final outcome of the experiments. Entropic effects have been demonstrated to play a fundamental role in enzymatic reactions 102,107 >. Conceptual similarity of inclusion compounds to more complicated associates is underlined thereby. 

We are attempting to understand the biological significance of the large variations in frequency of putative LDRs, whether between different types of bacteria or archaea, or between pro- and eukaryota. We have carefully studied the literature of more than 90 example proteins selected from our disordered protein databases and found reports on the functions of most of the disordered regions (Dunker et al., 2002). The observed functions and the number of examples in each functional class are given in Table VI. As indicated, four major functional classes were found molecular recognition, molecular assembly or disassembly, protein modification, and entropic chains. 

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

A significant increase in the specific maximum entropic change was achieved by substituting the fumaric acid with the much smaller formic acid, as reported in Table 4.1 where data for a selection of networks based on gadolinium... 

In summary, although the metal-ion selectivity of the cryptands is normally largely enthalpy-controlled, entropic terms may also be quite important. Once again, the factors underlying these respective terms may be quite variable and, as a consequence, a criterion for preferred complexation based solely on a match of the cavity for the cation radius may not always be appropriate. 

The Flory-Huggins interaction parameter, x Is the sum of enthalpic (xH) and entropic (x ) contributions to the polymer-solute interactions (28). xs is an emPitical constant related to the coordination of the polymer subunits (29). Chiou et al. (20) have selected a value of 0.25 for xs of humlc matter. From regular solution theory, xq is given by... 

Chowdhury, M.A.J., Boysen, R.I., Ihara, H., and Hearn, M.T.W., Binding behavior of crystalhne and noncrystalhne phases evaluation of the enthalpic and entropic contributions to the separation selectivity of nonpolar solutes with a novel chromatographic sorbent, J. Phys. Chem. B, 106, 11936, 2002. 

As the aforementioned example demonstrates, for strong-strong separations the selectivity can be dependent on the loading of the membrane. When the size of the adsorbed molecule is similar to the zeolite pore, at loadings near saturation the zeolite framework atoms will adjust to allow for entropically favorable packing. Under these conditions, constituents must compete with one another for adsorption sites and molecule-molecule interactions play a dominant role [33]. 

---