R. M. Iyer

Methanol and water uptake, densities, equivalental volumes and thicknesses of several uni- and divalent ionic perfluorosulphonate exchange membranes (Nafion-117) and their methanol-water fractionation behaviour at 298 K

Abstract Maximum methanol and water uptake by H + , Li + , Na + , K + , Rb + , Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ forms of perfluorosulphonate ion-exchange (Nafion-117) membranes, as well as their densities, equivalent volumes and thicknesses in their dry and methanol-swollen and water-swollen states have been determined and the extent of solvent electrostriction evaluated. Compared with a water sorption of 6–20 wt. %, the membranes show an expanded range of 6–40 wt.% methanol sorption, the corresponding volume changes being 14–40% (water) and 19–132% (methanol). The swollen-membrane densities exhibited a linear relationship with solvent uptake. Membrane thicknesses, found to be independent of the cationic form in the dry state and showing a small increase on water sorption, showed significant variations on methanol-wetting. Solvent fractionation studies carried out in 50 mol% methanol-water revealed a larger solvent sorption and selective enrichment of water in the membrane phase. Unusually large swelling of the H + , Li + and Na + form membranes in methanol and other observations have been interpreted in terms of cluster sizes computed from the data, free energies of hydration of the counter ions involved, solvent-matrix and solvent-sulphonate ion interaction, molar volumes of solvents, swelling pressures in the membranes and the known hydration/solvation effects in polystyrene divinylbenzene sulphonate type of exchangers.

Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and polystyrene–divinylbenzene sulphonate (Dowex 50W) ion-exchange resins at 298 ± 1 K

Water sorption as a function of activity (aH2O) by H+-, Li+-, N(CH3)4+- and Cs+-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique. All exchangers show the sequence of water sorption as H+ > Li+ > N(CH3)4+ > Cs+. However, the H+ and Li+ forms of Nafion-117 absorb significantly larger amounts of water, while the Cs+ form sorbs much less water at all water activities compared to the corresponding ionic forms of Dowex 50W × 8, the N(CH3)+4 isotherms of the two exchangers being quite close to each other. Sorption of water by the 4 and 8% crosslinked Dowex 50W resins is found to be the same at low aH2O( Dowex 50W × 8 > Dowex 50W × 4 (any ionic form). The results are discussed in relation to the relative degrees of hydration of the (respective) ions, including the sulphonate anion, and the differences between the two types of ion exchangers.

Photochemical behaviour of rhodamine 6G in Nafion membrane

Sorption of laser dye rhodamine 6G in perfluorosulphonate cation-exchange membrane (Nafion) from aqueous solution has been investigated by optical absorption and emission studies. In the H+ form, the optical absorption spectra changes with water content in the membrane and is attributed to the change in H+ ion concentration in dry and wet (with water) conditions of the membrane. The dye is present in the protonated form in the dry membrane and in the cationic form in the wet membrane. In the Na+ form, rhodamine 6G is present in the cationic form and the optical absorption spectra do not show changes in dry and water-wet conditions of the membrane. The fluorescence lifetime of rhodamine 6G is 3.97 ns and decay is single exponential. In the Na+ and H+ forms of the membrane, the fluorescence decay is biexponential. The lifetime and the relative ratio vary for the Na+ and H+ forms and also with the water content in the membrane. Owing to the large size of the dye low values for the diffusion coefficient are observed (3.2 × 10–11 and 2.0 × 10–8 cm2 s–1 for Na+ and H+ form of Nafion, respectively). As compared to aqueous solutions, rhodamine 6G in Nafion membrane is observed to be photostable.

Identity of intermediates formed on photolysis of CH2I2 and CHI3 in 3-methylpentane at 77 K

Abstract Transient intermediates formed on photolysis of CH2I2 and CHI3 in 3MP glass at 77 K have been characterized from their optical absorption and stable products analysis. Variation of stable products yields and intensity of absorption bands in presence of benzene, which is known to lessen solute clustering by molecular complex formation, has indirectly indicated solute aggregation at 77 K and formation of solute cations by energy transfer from excited solute molecules.

γ-radiolysis of CH2I2 in 3-methyl pentane

Abstract γ-Radiation induced decomposition of CH2I2 dissolved in 3-methyl pentane was examined at 77 K and 300 K using optical absorption spectroscopy and product analysis in order to understand the general features of radiation chemical behaviour of polyhalides. Two new bands absorbing at 385 nm and 570 nm were found following irradiation at 77 K. The 385 nm band was identified as due to CH2I2 +. Effects caused by γ-dose rate, irradiation temperature, ionic and free radical scavengers on the yield of the products CH2I, I2 and RI were estimated. A mechanism to explain these observations has been suggested.

OPTICAL ABSORPTION AND EMISSION STUDIES ON LASER DYE INCORPORATED IN NAFION MEMBRANE

Abstract— Incorporation of the laser dye 7‐amino‐4‐methyl coumarin (C‐120) in a perfluoro sulfonate cation exchange membrane (Nafion) from aqueous solution has been studied by following its light absorption and emission (fluorescence) characteristics. It is shown that in the H+ form of the membrane the dye exists in its monoprotonated cationic form, whereas in different cation exchanged forms of the membrane it is incorporated as the unprotonated neutral molecules. By repeated equilibration with the aqueous solution, high concentrations of the dye could be loaded into the membrane. As compared to an aqueous solution, the polymer matrix was found to confer very good photostability to the dye.

Studies on transients formed on γ radiolysis of bromo aromatic compounds in 3-methylpentane at 77 K

The transient species produced on γ irradiation of mM solutions of bromobenzene, p- dibromobenzene, 4-bromodiphenyl, and 4,4′-dibromodiphenyl in 3-methylpentane glass at 77 K were characterized from a study of their absorption spectra and the changes thereof produced by proton acceptors, electron scavengers, and optical bleaching. It is concluded that the optical absorption bands at 280 and 550 nm are due to C6H5Br+, 320 and 630 nm are due to C6H4Br+2, 385, 410, and 780 nm are due to C12H9Br+ and 400, 430, and 830 nm are due to C12H8Br2+, which are formed by positive charge transfer from the irradiated matrix to the respective solute molecules.

Formation of CS− 2 in the γ radiolysis of CS2−3 MP glassy system at 77°K

Abstract An intense optical absorption with λmax at 275 nm in γ irradiated CS2−3 MP glasses observed at 77°K has been found to be that of CS2− with an estimated molar extinction coefficient of 2.2 × 104 litre mole−1 cm−1. An additional absorption band observed at λmax 372 nm has been assigned to CS2+. The probable electronic transitions involved in CS2− and CS2+ has been discussed. The effect of concentrationof CS2 and γ dose on the 275 and 372 nm bands have been studied and their relevance has been discussed.

Hydrogen isotope effects in the absorption of water by perfluorosulphonate (Nafion-117) and polystyrene divinylbenzene sulphonate (Dowex 50W) ion-exchange resins

Hydrogen/deuterium (H/D) fractionation effects in the exchanger-pore water–outer-equilibrium water have been investigated at 298 ± 1 K using H+, Li+ and Cs+ ionic forms of Nafion-117 and Dowex 50W (4%, 8% DVB) resins and waters containing deuterium from 150 to 11 000 ppm. Single-stage separation factors (αH2OHDO) evaluated show preferential enrichment of deuterium in the outer water phase in the sequences: (i) Nafion-117 > Dowex 50W × 8 > Dowex 50W × 4; (ii) H+ > Li+ > Cs+. Whereas αH2OHDO values were nearly unity in the natural D-concentration region, they were appreciably higher in deuterium-rich water systems. A linear relationship was found between log αH2OHDO and inverse deuterium concentration, which has been interpreted in terms of a Born-type equation dealing with the solvation of ions. The selective enrichment of H2O over HDO in resin-pore water conforms with similar observations reported with binary water–organic-solvent systems and with the lesser hydrogen-bonded structure of resin-pore water compared with normal water. Thermodynamic analysis of the present systems has revealed large contributions of swelling pressure terms of αH2OHDO which readily explains the sequences listed above. The role of cationic and sulphonate anion hydration has also been assessed in these fractionation effects. H+– 117, Li+– 117 and H+× 8 exchangers have been suggested as being most suited for deuterium enrichment using the simple technique of equilibrium contacting of these exchangers with waters of high D-concentrations.

Radiation chemical behaviour of iodoform in cyclohexane

Abstract Degassed solution of iodoform [CHI3] in cyclohexane on γ-irradiation produced CH2I2 and cyclo-hexyl iodide in comparable yields equal to G(-CHI) The G values of the two products were observed to be dose rate dependent and varied from ∼300 at 3.4 × 1015 eV/min to ∼35 at 3.8 × 1017 eV/min indicating a chain reaction. From a study of the effect of radical and electron scavengers it was concluded that chain propagation was through a radical route. Unlike degassed solutions, aerated solutions produced only molecular iodine and cyclohexyl iodide in comparable yields. No CH2I2 could be detected. A reaction mechanism for the decomposition of iodoform in cyclohexane has been discussed.