chemical bonds between the dye molecules and functional groups
of the adsorbent surface.
The positive value of DS (Table 5) is an indication of
replacement of the solvent molecules onto the PC surface by
adsorbate molecules and favorable adsorption process. It also
indicates the possibility of structural changes or readjustment of
the adsorbate–adsorbent complex.
Despite the endothermic nature of the adsorption process,
the spontaneity of the adsorption process was ensured by a
negative value of Gibbs free energy. The mean DG values
increases from 10.81 kJ mol1 to 16.25 kJ mol1 and 12.08 kJ
mol1 to 19.03 kJ mol1 for MB and EY, respectively with
increasing temperature from 303 K to 323 K. The negative value
of DG indicates that both the dye adsorption onto the adsorbent is
spontaneous and thermodynamically favorable at all temperatures
tested here