Browsing by Author "Fontanella, J.J."

Now showing 1 - 3 of 3

A Fundamental Study of the Transport Properties of Aqueous Superacid Solutions
(Journal of Physical Chemistry B, 2010) Suarez, S.N.; Jayakody, J.R.P.; Greenbaum, S.G.; Zawodzinski, T.; Fontanella, J.J.
An extensive investigation of the transport properties of aqueous acid solutions was undertaken. The acids studied were trifluoromethanesulfonic (CF3SO3H), bis(trifluoromethanesulfonyl)imide [(CF3SO2)2NH], and para-toluenesulfonic (CH3C6H4SO3H), of which the first two are considered superacids. NMR measurements of self-diffusion coefficients (D), spin?lattice relaxation times (T1), and chemical shifts, in addition to ionic conductivity (?), viscosity (?), and density measurements, were performed at 30 �C over the concentration range of 2?112 water to acid molecules. Results showed broad maxima in ? for all three acids in the concentration range of 12?20 water to acid molecules. This coincided with minima in anion Ds and is attributed to a local molecular ordering, reduced solution dielectric permittivity, and increased ionic interactions. The location of the maxima in ? correlates with what is observed for hydrated sulfonated perfluoropolymers such as Nafion, which gives a maximum in ionic transport when the ratio of water to acid molecules is about 15?20. Of the three acids, bis(trifluoromethanesulfonyl)imide was found to be the least dependent on hydration level. The occurrence of the anticorrelation between the ionic conductivity maximum and the anion self-diffusion minimum supports excess proton mobility in this region and may offer additional information on the strength of hydrogen bonding in aqueous media as well as on the role of high acid concentration in the Grotthuss proton transport mechanism.
High pressure studies of hydrated NAFION membranes: Dielectric relaxation and deuteron NMR
(Solid State Ionics, 1993) McLin, M.G.; Wintersgill, M.C.; Fontanella, J.J.; Chen, R.S.; Jayakody, J.R.P.; Greenbaum, S.G.
Audio frequency electrical conductivity/dielectric relaxation and nuclear magnetic resonance studies of NAFION-117 have been carried out at pressures up to 0.3 GPa (3 kbar). For some samples, the electrical conductivity exhibits dispersion typical of hopping conductivity in an amorphous material, ?=?0?m. The values of m are sample dependent and vary from about 0.55 to 0.75. In addition, the values of m are not strongly pressure sensitive. At a given frequency, the electrical conductivity tends to decrease as pressure increases and yield activation volumes on the order of 5 to 10 cm3/mol. The activation volume determined from the NMR spin-lattice relaxation time measurements are also sample dependent. The data shown correspond to an activation volume of about 8.6 cm3/mol. The results of the high pressure electrical conductivity studies also yield activation volumes on the order of that determined via NMR.
Studies of Water in Nafion Membranes: Using Deuteron and Oxygen?17 Nuclear Magnetic Resonance, and Dielectric Relaxation Techniques
(Journal of Electrochemical Society, 1993) Chen, R.S.; Jayakody, J.R.P.; Greenbaum, S.G.; Pak, Y.S.; Xu, G.; McLin, M.G.; Fontanella, J.J.
Deuteron and oxygen-17 nuclear magnetic resonance measurements and dielectric relaxation studies of Nafion-117 membranes with variable water content (approximately 5-18% by weight) have been carried out. Glassy behavior of the water domains at low temperature, below ca. 200 K, is evidenced by the specific nature of the 2H NMR line shapes. Activation energies extracted from H spin-lattice relaxation data on the high temperature side of the T1 minimum exhibit a steady increase with increasing water content. In spite of a high degree of molecular mobility, angular-dependent spectra in both as-received and stretched samples reflect considerable anisotropy of the host polymer. A dielectric loss peak is observed which previously has been attributed to the glass transition of the aqueous domains.