Heats of immersion related to the extent of water loss and specific surface area: CoCl2- and NiCl2-loaded aluminas as dessicant materials

CoCl₂- and NiCl₂- loaded aluminas were thermally dehrdrated in vacuo at various temperatures ranging from 20 to 500 °C. The water loss, nitrogen surface area and integral heat of water immersion were determined for the thermally treated samples. The samples inder investigation were analysed using several methods. Changes in porosity were detected by means of the V_a-t method. The corrected modelless method was used to determine the surface locations, and the area fraction located in the mesopores was also calculated. This fraction denotes the surface accessibility of various samples for nitrogen adsorbate molecules as well as for the smaller water molecules.

Both the integral heat of immersion and the water loss were normalized to the unit nitrogen Brunauer-Emmett-Teller surface area. The correspondence between the changes in the heat of immersion per unit area and the water loss per unit area when they are plotted as functions of the heat treatment temperature indicated that a reversible dehydration-hydration phenomenon occured at high treatment temperatures.

Since samples which have the highest surface accessibility for water molecules and are characterized by a relatively high heat of immersion can be assumed to be effective dehydrating materials, it can be concluded from our results that CoCl₂-loaded alumina should be heat treated at 400 °C to obtain optimum dessicant properties. However, the NiCl₂-loaded alumina requires heat treatment at 200 °C. Further details are presented and discussed.