Water vapor molecule adsorption onto 'Ajwa' dates: Analytical investigation via infinite multilayer statistical physics model

A gravimetric technique was used to examine the water activity onto the 'Ajwa' dates. The equilibrium adsorption isotherms of water molecules were carried out at three temperatures (between 303 K and 323 K). A theoretical method was developed using statistical physics treatment to describe the experimental data at the ionic scale. The date's isotherms were analyzed via the infinite multilayer adsorption model (formation of a high number of adsorbed layers) which is established based on the ideal gas law (there are no lateral interactions influences on the adsorption mechanism). The chosen model gave significant interpretation of the adsorption of water on the Ajwa dates based on the physicochemical model's parameters (the density of binding sites (D-m), the number of water molecules per site (n) and the energetic parameters (a(1)) and (a(2))). The physicochemical interpretation of the appropriate model indicated that the adsorption of water on the Ajwa dates occurred via a multi-anchorage process since the n values are lower than 1 for the three tested temperatures. The Ajwa dates adsorption was found typical to an exothermic process by the intermediate of the steric parameter D-m (D-m (303 K) = 0.58 kg/kg> D-m (323 K) = 0.33 kg/kg). Moreover, the energies values vertical bar-epsilon(1)vertical bar and vertical bar-epsilon(2)vertical bar, which varied from 27.8 KJ/mol to 51.2 KJ/mol, confirmed that the 'Ajwa' dates adsorption was a chemical process presenting covalent bonds between the water molecules and the dates' sites. (C) 2021 Published by Elsevier B.V. on behalf of King Saud University.