Study of the interaction between water and Gelidium sesquipedale (Rhodophyta). Part I: Thermodynamic aspect of the sorption equilibrium

The Moroccan coast shows significant species richness with economic and ecological interests. The Gelidium sesquipedale (Rhodophyta), is a red alga which is exploited for its exceptional richness in a polysaccharide called agar-agar, which is highly acclaimed for its interesting gelling properties. The main purpose in this work is to study the drying process of G. sesquipedale. In order to understand the interaction of water in the alga studied, and to optimize the key parameters of the storage phase, we were interested in studying the water–G. sesquipedale system to determine the influences of water content and storage temperature on hygroscopic equilibrium of the system.The sorption isotherms of G. sesquipedale (Clemente) Thuret were obtained, using the static gravimetric method at 30, 45, and 60 °C. The isotherms obtained have a sigmoidal shape and are classified into the type II IUPAC classification which is a combination of isothermal most commonly encountered in the literature, this classification was originally established by Brunauer, Deming, Deming and Teller (BDDT) [1] in five categories of sorption isotherms. A sixth category concerns isothermal steps and will be addressed separately. These six categories are the classification of the International Union of Pure and Applied Chemistry (IUPAC) [2]. The isotherms vary with temperature and show a slight hysteresis. The thermodynamic analysis of experimental results was used to determine and illustrate the variation of the isosteric heat as a function of equilibrium water content. Two types of water molecules have been demonstrated by this study: 20% of molecules weakly bonded with potential sites for adsorption inside plant cells and 80% of intercellular and wetting water molecules.

► The sorption isotherms of Gelidium sesquipedale were obtained, using the static gravimetric method at 30, 45, and 60 °C. The isotherms obtained have a sigmoidal shape and are classified into the type II IUPAC classification. They vary with temperature and show a slight hysteresis.
► The thermodynamic analysis of experimental results was used to determine and illustrate the variation of the isosteric heat as a function of equilibrium water content.
► Two types of water molecules have been demonstrated by this study: 20% of molecules weakly bonded with potential sites for adsorption inside plant cells and 80% of intercellular and wetting water molecules.
► These two types of water are confirmed by the determination of net esoteric heat of sorption and desorption.
► The results are used to locate discontinuities in heat depending on the relative pressure and depending on the adsorption capacity, so as to locate two successive states of hydration.