# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 3 Characterization of Hydrophobic Catalyst
### Contact Angle Measurements  
As already mentioned in the introduction, contact angle (CA) measurements are probably the most common way to evaluate surface wettability. This kind of analysis is performed by measuring the tangent angle that a liquid drop forms on a solid surface. The relationship between solid surface tension and the CA formation was firstly recognized by [PERSON] [43]. The angle is, in fact, defined by the mechanical equilibrium to which the drop is subjected by the action of three interfacial tensions (Figure 5):  
1. Solid-liquid;
2. Solid-vapor;
3. Liquid-vapor. The equilibrium is described by [PERSON]'s equation:  
\[\gamma_{\mathrm{Iv}}\cos\theta_{\mathrm{Y}}=\gamma_{\mathrm{sv}}-\gamma_{ \mathrm{sl}} \tag{1}\]  
where \(\theta_{\mathrm{Y}}=\mathrm{Young}\) contact angle; \(\gamma=\mathrm{interfacial}\) tension (lv = liquid-vapor, sv = solid vapor, sl = solid-liquid). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Catalyst** &
\begin{tabular}{c} **Sulfur Content a** \\ **(mmol/g)** \\ \end{tabular} & **Contact Angle** \\ \hline H-PDVB-0.05-SO\({}_{3}\)H & 0.31 & 152\({}^{\circ}\) \\ H-PDVB-0.10-SO\({}_{3}\)H & 0.60 & 148\({}^{\circ}\) \\ H-PDVB-0.20-SO\({}_{3}\)H & 0.88 & 143\({}^{\circ}\) \\ H-PDVB-0.33-SO\({}_{3}\)H & 1.31 & 137\({}^{\circ}\) \\ H-PDVB-0.50-SO\({}_{3}\)H & 1.78 & 128\({}^{\circ}\) \\ H-PDVB-1.00-SO\({}_{3}\)H & 1.06 & 120\({}^{\circ}\) \\ H-PDVB-1.50-SO\({}_{3}\)H & 2.36 & 118\({}^{\circ}\) \\ PDVB-SO\({}_{3}\)H & 3.64 & 38\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Hydrophobicity related to sulfur content of sulfonic acid-based resins. a Determined by elemental analysis. Adapted from Reference [42]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Surface/Bulk Composition Evaluation** & **Measures of Interactions** \\ \hline Thermogravimetric Analysis & Adsorption Isotherms \\ IR-spectroscopy & Contact Angle Measurements \\  & Hydrophobic Index \\  & IR-spectroscopy \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Methods of hydrophobic properties evaluations. Although being a very common method to assess hydrophobicity of a surface, its application on heterogeneous surface is not so trivial. In the case of rough surfaces, CA are larger than those observed with the smooth ones (contact angle hysteresis), thus leading to a misleading interpretation of the results. CA also reflects surface topography and not only surface energetics [7; 44]. Moreover, in the case of porous solids, the liquid could be adsorbed in-between the pores making the measurement of the angle impossible to be done [6]. However, while taking these limitations into consideration, contact angle measurements can provide easily an assessment of the hydrophilicity or hydrophobicity (angle major or minor of 90\({}^{\circ}\)), especially if solid catalysts of similar chemical nature are analyzed. CA are typically measured using the so called sessile-drop technique, in which a drop of liquid is let fall on the surface, and then, with the help of a camera, the tangent angle is measured with goniometric methods [7]. Water is the most common liquid used in the assessment of surface hydrophobicity showing precisely how much the surface repels it. However, other liquids can be used; in particular, organic liquids can show directly the affinity of the catalyst for the organic substrates of interest. As already said, CA measurements are easily performed to characterize the hydrophobicity of a catalyst [45; 46] and can be related to catalytic performances of a catalyst. [PERSON] et al. utilized contact angle measurements for the characterization of their hydrophobic sulfonic resins demonstrating also that the higher the contact angle the better the catalytic performance in the dehydration of fructose to HMF. Thus, the catalyst with the highest contact angle does not catalyze the consecutive reaction of HMF hydration to levulinic acid by keeping the water away from the substrates [47].