Protonic charge conduction and H2O-induced relaxations in hydrated sodium alginate and in calcium alginate hydrogels and xerogels

Abstract

We employed dielectric spectroscopy to analyze charge conduction, glass transition temperature, and relaxation dynamics in six alginate (Alg) samples at low temperature. All underwent vitrification of a hydrated amorphous fraction: the as-stored Na-Alg powder at 220 K, the temperature-quenched Ca-Alg hydrogel and xerogel at 212 K, the frozen Na-Alg solution and the Na-Alg exposed to water-vapor saturated atmosphere at 194 K, and the slowly cooled hydrogel at 170 K. All samples displayed a Johari–Goldstein relaxation, and three of them displayed a structural relaxation; both processes originated from coupled Alg–H2O motions in the hydrated amorphous fraction. Charge conduction in all samples was dominated by Grotthuss proton shuttling (either in bulk water/ice or in hydration layers). H2O-rich samples displayed the H-bond network relaxation of ice, with an activation energy between 25 and 31 kJ mol−1, close to the energy of formation of one H-bond per H2O molecule. This relaxation was faster in the quenched hydrogel than in the slowly crystallized hydrogel and the frozen solution, likely due to a smaller average size of ice domains in the former; the latter samples display two separate ice relaxations, likely due to ionic impurities in the ice domains in contact with the Alg chains. The Na-Alg powder equilibrated in a water-vapor saturated atmosphere also displayed the local relaxation of bulk-like amorphous water with an activation energy of 51 kJ mol−1, equivalent to two H-bonds per H2O molecule. Most samples displayed spectral changes between 260 and 270 K due to ice melting. In the fast-quenched hydrogel, a premelting transition was observed at 220 K.