# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 3 Characterization of Hydrophobic Catalyst
### Adsorption Isotherms  
As already mentioned, hydrophobicity has not a unique definition. In general, hydrophobicity could be defined as the absence of strong adsorption capacity of polar compound in particular water. Apart from liquid-solid interaction evaluation as in the case of CA measurements, adsorption from  
Figure 5: Interfacial tensions acting on the drop-surface system. a gas phase could be a useful tool for hydrophobicity evaluation, especially if CA measurements are not possible due to structural limitations. In fact, it can provide important information on the surface affinity for a certain compound or in general surface-related properties such as dipole moment, polarizability and acidity. A straightforward way to apply this technique is to evaluate the adsorption capacity: A more hydrophobic solid adsorbs less water per unit of area or mass if compared to a more hydrophilic one in the range of all relative pressures (Figure 6) [49]. However, the determination of the adsorption capacity could be challenging if adsorbate-adsorbate interactions take place, especially in the case of mesoporous or microporous solids due to the higher possibility of multilayer adsorption. To overcome this problem, adsorption capacity should be compared at low loading, without exceeding the monolayer adsorption. Nonetheless, water monolayer is not easy to be obtained due to local increase of water concentration caused by hydrogen bonding, acidic sites or highly polar sites. Thus, it is convenient to compare adsorption capacity among solids similar in nature under a well-defined set of conditions [6]. There are some examples of hydrophobic characterization through water adsorption isotherms [49; 50; 51; 52]. A clear example is showed by [PERSON] and co-workers that registered the water adsorption capacity for triethoxyfluorosilane (TEFS) grafted Ti-modified mesoporous silica SBA-15. The relative water adsorption capacity at P/P\({}_{0}\) diminishes with the increase of TEFS loading (Table 6), thus demonstrating the increase of the solid hydrophobicity of the fluorosilane-modified silicas [53]. Figure 6: **(A)** Example of idealized water adsorption isotherms in the case of zeolitic materials. Adapted from Reference [54]. **(B)** Water adsorption capacity for TiSBA-15, F2-TiSBA-15, F5-TiSBA-15, F10-TiSBA-15 and F20-TiSBA-15 measured at 298 K [53]. Water is the most common compound to assess hydrophobicity through adsorption isotherms, but many other adsorptives can be used for this purpose. Hexane [55] and toluene [56] are used as water complementary: the higher the hydrophobicity, the higher the power of organic molecule adsorption capacity. Multicomponent adsorption is also a used technique. This is the case of water and toluene competitive adsorption used to determine the Hydrophobic Index (HI) of zeolites introduced by [PERSON] [57]. This parameter is defined as  
\[\text{HI}=\text{X}_{\text{toluene}}/\text{X}_{\text{water}} \tag{2}\]  
with \(\text{X}_{\text{toluene}}\) and \(\text{X}_{\text{water}}\) as the equilibrium loadings of the adsorbent with toluene or water, respectively, taking in consideration that \(\text{X}_{\text{i}}=\text{m}_{\text{i}}/\text{m}_{\text{zeolite}}\). A more hydrophobic solid will have a higher HI, that means an increase in toluene adsorption compared to water. HI is a useful tool to easily understand hydrophobic properties of a solid together with an insight into the pore structure. The competitive water and toluene adsorption experiments were carried out on SiO\({}_{2}\), MCM-41, titanium grafted SiO\({}_{2}\) and MCM-41 to understand the possible role of titanium in the hydrophobic properties of the solid. Moreover, HI was measured also for silylated titanium grafted SiO\({}_{2}\) and MCM-41 (Table 7). It is noticeable that the presence of titanium does not affect the silica and MCM-41 HI although it increases the water and toluene adsorption capacity in the case of Ti-MCM-41.