Cation exchange entropy

Studies by Deathrage ef a/.137 139 revealed that most of dipeptides were hydrolyzed 100 times faster with cation exchange resins (Dowex-50) than with HC1. Deathrage etal.139 also found that the entropy of activation was significantly less than in the case of hydrolysis of the same compounds by HC1, while the enthalpies of activation for the two cases were practically the same. While the entropy changes associated with catalysis by the cationic exchange resins remain obscure, presumably the mechanism of the catalysis follows that for homogeneous acids as described here later. 

Enthalpies and entropies of complexation have been measured for complexes of 12,13 (105), 135) and 29, 30 (127) Cation exchange rates have been determined for complexes of ligands 15 (137,138) and 30 (106, 127). 

The structural parameters of cation-exchanged montmorillonites prepared from calcium-montmorillonite (Istenmezeje) are listed in Table 2.3. As seen in Table 2.3, the basal pacing of monovalent montmorillonite is approximately 1.25 nm, and the water content is approximately 1%. It means that there is one layer of water in the interlayer space. For bivalent montmorillonite, both basal spacing (>1.5 nm) and water content (>10%) are higher, showing two layers of water molecules in the interlayer space. The basal spacing of Pb-montmorillonite is 1.254 nm, which is similar to the value characteristic of monovalent montmorillonite (1.241 nm). However, it does not mean that lead is sorbed on the surface of montmorillonite as monovalent cation since the other parameters that are determined by the distance between the layers (hydration entropy, charge/ion radius value, water content in the interlayer space) lie between the values for bivalent and monovalent cations (Foldvari et al. 1998). 

Figure 2. Activation energies and activation entropies of internal diffusion in cation exchange on type A zeolite...

If selectivity were governed solely by interactions of a purely electrostatic (coulombic) nature one could anticipate negative enthalpies and entropies of exchange due to dominant contributions by A and A5ii respectively. This is indeed found for alkali metal cation exchange on styrenic sulfonate resins as illustrated by Table 5.3a. 

If experimentation is extended over a range of temperature then enthalpy and entropy changes can be obtained to help in the further interpretation of the cation exchange mechanisms in zeolite frameworks. 

The layered silicate nanoparticles are usually hydrophilic and their interactions with nonpolar polymers are not favorable. Thus, whereas hydrophilic polymers are likely to intercalate within Na-activated montmorillonite clays [24-29], hydrophobic polymers can lead to intercalated [23,30-32] or exfoliated [33] structures only with organophilized clays, i.e., with materials where the hydrated Na+ within the galleries has been replaced by proper cationic surfactants (e.g., alkylammonium) by a cation exchange reaction. The thermodynamics of intercalation or exfoliation have been discussed [34-37] in terms of both enthalpic and entropic contributions to the free energy. It has been recognized that the entropy loss because of chain confinement is compensated by the entropy gain associated with the increased conformational freedom of the surfactant tails as the interlayer distance increases with polymer intercalation [34,38], whereas the favorable enthalpic interactions are extremely critical in determining the nanocomposite structure [39]. 

The exchange of BTM-n cations is mainly governed by enthalpic effects in contrast to alkylammonium exchange, the behaviour of which is primarily determined by entropy changes (All being essentially zero). This suggests that electrostatic interactions, which are enhanced due to better delocalization, are also important. dgp spacings of 1.45 nm confirm a flat orientation of these ions. 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

Examinations of possible correlations between the volume of activation and the entropy of activation for series of similar reactions have been reported for reactions of transition metal coordination compounds, such as solvent exchange, ligand substitution, or isomerization.163 167 A limiting factor in a potential correlation may be the lack of precision that often attends experimental determination of the entropy of activation. Attention has been drawn specifically to the qualitative nature of the correlations between the two parameters for solvent exchange at some 3 + cations, and at square planar Pd2+ and Pt2+ ions.168... 

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