1H NMR spectroscopy to investigate the kinetics and the mechanism of proton charge carriers ionization and transportation in hydrophilic/hydrophobic media: Methyl sulfonic acid as a protonic ion source in water/alcohol binary mixtures

Abstract

Methanesulfonic acid (MSA):water-alcohol (ethanol and isopropanol) systems were studied at temperatures in the range from 295 K to 360 K using high-resolution 1H NMR and ac-impedance analyzer in order to investigate the formation of ionic charge carriers as well as their transport in these ionic mixtures and the dissociation constants. The obtained dissociation constants of MSA:water-alcohol at water-rich and alcohol-rich sides are higher than the same system without MSA due to higher protonic charge carrier concentrations facilitating the structure diffusion of protons. The dissociation constant at the water-rich side (0.63) is comparable to that at the ethanol-rich side (0.58) and slightly higher due to the higher acidity of water. Remarkably, a plateau in the molecular diffusion coefficients and ionic mobility curves is observed in the region between the water-rich side and the alcohol-rich side and the diffusion self-coefficients in the plateau are higher than everywhere else. The temperature dependence of ionic conductivity in MSA:water-alcohol is dominated by the reduction and rearrangement of the hydrogen-bond strength accompanied by a heating induced networking rather than by an ionic formation process. The degree of hydrophobicity plays a more effective rule in the self-diffusion coefficient and in the ionic conductivity compared to the degree of hydrophobicity in ethanol and isopropanol.The data are relevant for the understanding and optimization of the properties of PEM fuel cells and for polymeric ion rejection membranes which are based on sulfonic groups (-SO3H). They provide interesting insight into the water dependence of the viscoelastic and kinematic properties, the charge formation mechanism and the dissociation limit, with the presence of a hydrophobic group mimicking the ionization of water into the sulfonated polymeric membrane used in many applications such as in PEM fuel cells.