Sergei I. Kargov

The nature of ion exchange selectivity of phenol-formaldehyde sorbents with respect to cesium and rubidium ions

The structure of aqua complexes of alkali metal ions Me+(H2O)n, n = 1−6, where Me is Li, Na, K, Rb, and Cs, and complexes of 2,6-dimethylphenolate anion (CH3)2PhO− selected as a model of the elementary unit of phenol-formaldehyde ion exchanger with hydrated alkali metal cations Me+(H2O)n, n = 0−5, was studied by the density functional method. The energies of successive hydration of the cations and the energies of binding of alkali metal hydrated cations with (CH3)2PhO− depending on the number of water molecules n were calculated. It was shown that the dimethylphenolate ion did not have specific selectivity with respect to cesium and rubidium ions. The energies of hydration and the energies of binding of alkali metal cations with (CH3)2PhO− decreased in the series Li+ > Na+ > K+ > Rb+ > Cs+ as n increased. The conclusion was drawn that the reason for selectivity of phenol-formaldehyde and other phenol compounds with respect to cesium and rubidium ions was the predomination of the ion dehydration stage in the transfer from an aqueous solution to the phenol phase compared with the stage of binding with ion exchange groups.

Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry, 2. Influence of Temperature, Chain Length and N‐Alkyl Substituents on the Conformation of Exhaustively Alkylated Poly(N‐alkyl‐4‐vinylpyridinium) Cations

The temperature dependencies of the conductivity of salt-free dilute aqeous solutions of the vunylic polycations of poly(N-alkyl-4-vinylpyridinium bromides (PAVD) were investigated. Samples of poly(N-ethyl-4-vinylpyridinium) bromides with differebt degrees of polymerization (DP), DP=10-1300, and samples of PAVP with the same DP=100 but with different N-alkyl substituents were investigated. The obtained data were evaluated using Mannings polyelecyrolyte theory and the mean charge spacing b in the chains was calculated. The determined b values were used to assess the conformational changes of the macromolecules. The decrease of the chain length of the poly(N-ethyl-4-(vinylpyridinium) cation to DP=100 was accompanied by a progressive growth of b indicating a transition from coil-like to rod-like regimes.Once this degree of polymerization was attained, further shortening of the chain did not affect the parameter b.These data suggest that the persitence length of this polycation corresponds to approximately 100 monomer units. However, even the chains with DP ≤100 did not adopt their fully stretched rod conformations at room temperature, since the heating of their solutions resulted in the increase of b. The temperature unfolding was most pronounced for the shortest chains and weakened drastically upon their elongation. The important role of the hydrphobic interactions in the stabilization of the chain conformation was revealed with the use of different PAVP samples.The values of b decreased proportionally with the increase of hydrophobicity of the N-alkyl substituent in the series -C 2 H 4 OH<-C 3 H 6 OH<-C 2 H 5 <-C 3 H 7 <-C 4 H 9 , indicating the transition from rather stretched rod-like chains to coils.

Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry, 1. Influence of the Degree of Polymerization on the Conformation of Flexible Vinylic Polyanions and Rigid Native DNA

The temperature dependencies of the conductivities of dilute salt-free aqueous solutions of flexible vinylic polyelectrolytes - sodium poly(acrylate) and sodium poly(styrene sulfonate) - with different degrees of polymerization (DP) were obtained for solutions of rigid native DNA with different degrees of polymerization (i.e. with a different number of base pairs (bp) in the chain) in 0.001 mol . L -1 NaCI as a supporting electrolyte. On the basis of Mannings polyelectrolyte theory, the mean charge spacing b in the polyion chains was calculated from these data. The determined b values were used to estimate the conformational changes of the charged chains. For all studied polyanions, b increased with the shortenning of the regimes. For both vinylmic polyanions, the transition was observed pratically in the same DP region, i.e. 7 10 2 - 10 3 , whereas rigid native double-stranded DNA molecules underwent the conformational change at substantially higher DP values, namely of the order of 10 3 -10 4 . The obtained results shows that the used conductometric approach is appropriate for the estimation of the conformational changes of polyions in dilute aqueous solutions.

Complex Formation of Divalent Cations with Carboxylic Acid Resins as a Factor Determining Different Stationary States of Their Swelling

ABSTRACT Three samples of carboxylic acid gel-type cross-linked polyelectrolytes were examined by registering the extent and kinetics of their volume change depending on the temperature, concentration, and nature of external electrolyte solutions, as well as the initial state (ionic form, water content) of the bead. It is found that in some systems the swelling of the bead (as a result of lowering the temperature) proceeds much more slowly than the preceding shrinking (as a result of the increase of temperature). Moreover, the initial volume of the bead is not restored, and a new stationary state of swelling is established. By means of optical dilatometry, it is found that in the systems in which the differences in the stationary states of swelling are found, the beads display plasto-elastic properties, whereas in the absence of such differences the beads show elastic properties. It is found that different stationary states of swelling are also established when the beads of copolymer of methacrylic acid cross-linked with divinylbenzene in Na+ form in the dry state and in the NaCl solution-swollen state are placed into a solution of a divalent cation salt. The difference in the stationary states of swelling reaches 12% in 4 N NiCl2 solution. The substantial differences in the swelling behavior and viscoelastic properties of the resins in their Na+, Mg2+, Ca2+, and Ni2+ forms are interpreted in terms of formation of different kinds of complexes of functional groups of the polymer with Ca2+ and Ni2+ ions.

Selectivity of ion exchangers in extracting cesium and rubidium from alkaline solutions

We compare the ion exchange selectivity of phenol-type sorbents based on phenol formaldehyde resins, products of condensation of diatomic phenols with formaldehyde, and crosslinked polymer based on C-phenyl[4]resorcinarene resin, for cesium and rubidium ions. It is shown that phenol formaldehyde sorbents are the ones most selective. The interaction of alkali metal cations with the anion of calix[4]arene is investigated via quantum-chemical modeling. It is shown that the selectivity toward cesium and rubidium ions in ion exchangers of the phenolic type is not due to specific interactions of ions with phenolic groups.

Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry, 3. Influence of Charge Density on the Conformation of Partly Alkylated Poly(N-ethyl-4-vinylpyridinium) Cations and Ionenes

The temperature dependencies of the conductivities of dilute salt-free aqueous solutions of partly alkylated poly(N-ethyl-4-vinylpyridinium) bromides (PEVP-β) and 2,n-ionene bromides with different degrees of polymerization (DP) were obtained. The charge density of the polycations was varied over a wide range by using PEVP-β with different degrees of alkylation β (%) in the range 25 β > 60 indicating folding of the chains upon decrease of their charge density. Folding was supported by the fact that the persistence length of PEVP-95 estimated from the obtained dependency of b on DP far exceeded the estimated persistence lengths of PEVP-60 and 2,4-ionene having similar mean charge densities. Further decrease of β at β < 60 caused noticeable growth of b with a pronounced trend of convergence of the curves of all studied PEVP-β samples. These findings might be attributed to the conformational changes of the chains considered in the context of the scaling theory of polyelectrolytes. The obtained results suggest that the revealed trend in conformational changes of the chains upon variation of their charge density is common, irrespective of whether PEVP-β with a random distribution of the charges along the chains or ionenes with the regular charge distribution are under consideration.