4.2. The effect on the standard molar Gibbs free energy
Figure 3 shows that the standard molar Gibbs free energy calculated by equation (26) at different temperatures is a function of the reciprocal of the nanoparticle diameter.
Several features can be seen in figure 3. First, the standard molar Gibbs free energy at a given temperature decreases with decreasing nanoparticle diameter, which indicates that smaller diameter nanoparticles are much easier to adsorb. Second, the negative values of the standard molar Gibbs free energy indicates that adsorptions are spontaneous. Furthermore, the negative values of the standard molar adsorption Gibbs free energy is smaller at higher temperatures, which suggests adsorptions become favorable at higher temperatures. In addition, there is a good linear relationship between DaGom and 1/d at a given temperature.
A similar trend is also observed in the adsorption of Ag+ onto nano-TiO2. The experimental results are also in agreement with equation (16).
4.3. The effect on the average standard molar enthalpy and the average standard molar entropy
Figure 4 shows plots of the standard molar Gibbs free energy at different particle sizes and temperatures. Values of DaHom and DaSom calculated from linear fittings and regressions of these plots using equation (27) are listed in table 3. Figures 5 and 6 show values of the average standard molar enthalpy and the average standard molar entropy of adsorptions, respectively, as a function of the reciprocal of the nanoparticle diameter. The data indicates that there is a good linear relationship between them.
Positive values of the standard molar enthalpy indicate that adsorptions are endothermic and are entropy-increasing processes. It can also be deduced that adsorptions are driven by entropy in both cases.
The result shows that size of nano-ZnO has negative effects on the standard molar entropy and the standard molar enthalpy. However, the size of nano-TiO2 has positive effects on the standard molar entropy and the standard molar enthalpy. This can be explained by equations (23) and (24).