# Physical-Chemical and Thermal Properties of Clays from Porto Santo Island, Portugal
## 4 Results and Discussion
### Thermal Characterization  
The residual soil samples have specific heat capacity values that are in the range of 0.6-0.9 J/g\({}^{\circ}\)C (Table 7). According to [PERSON] et al. [35], the specific heat capacity of the samples should be higher than 0.5 J/g\({}^{\circ}\)C, which is a benefit for the pelotherapy treatments, as it requires less energy to heat. Therefore, it leads to the conclusion that they have acceptable values to be considered as healing clays, although these values are slightly lower compared to other samples from Porto Santo Island [11,35], where the values were 2.46 and 3.55 J/g\({}^{\circ}\)C, respectively, and other healing clays [55]. In terms of cooling kinetics, it is one of the fundamental properties of healing clays concerning applications in pelotherapy treatments; on the other hand, it is not a main characteristic in dermocosmetics. According to [PERSON] et al. [2], for a treatment to be effective, the material must maintain a temperature of over 30 \({}^{\circ}\)C for 20-30 min. From the residual soil samples from Porto Santo Island, Sample 36A is the only one that comes close to this time interval (19.0 min), which means that this sample takes longer to dissipate heat compared to others, like Sample 5, which has the lowest time (14.5 min). Sample 36A has the highest smectite contents (Table 3), which is in line with the thermal properties obtained. Studies conducted by [PERSON] et al. [11] and [PERSON] et al. [13] in Porto Santo Island show cooling time values around 38 min, which means that the values of this research (14.5-19.0 min) are also slightly lower compared to other healing clays [29,55]. Because the measurement of the cooling kinetics was made on dry samples, an increase in cooling time with the wetting of residual soil samples is predicted [4,5,55]. Nevertheless, if we correlate the two thermal properties, specific heat capacity and cooling kinetics, it is possible to conclude that overall samples with longer a cooling time (extra time to dissipate heat) have higher specific heat capacity than those samples that take less cooling time.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Soil Samples**} & \multirow{2}{*}{**pH**} & \multicolumn{2}{c}{**Electrical Conductivity (\(\mu\)S/cm)**} & \multirow{2}{*}{\begin{tabular}{c} **CEC** \\ **(meq/100 g)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Exchange Ions** \\ **(mg/L)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} \\ **(mg/L)** \\ \end{tabular} } \\  & & & Na\({}^{+}\) & K\({}^{+}\) & Mg\({}^{2+}\) & Ca\({}^{2+}\) \\
5 & 9.3 & 710 & 50.4 & 93.5 & 21.6 & 57.3 & 131.2 \\
14 & 9.6 & 730 & 74.0 & 155.6 & 20.5 & 78.5 & 6.5 \\
14A & 9.3 & 646 & 60.2 & 2.9 & 3.9 & 7.6 & 4.3 \\
15A & 9.5 & 972 & 74.6 & 168.5 & 24.1 & 7.6 & 6.4 \\
26 & 8.8 & 242 & 43.8 & 10.9 & 25.9 & 65.6 & 7.4 \\
36A & 9.6 & 772 & 86.8 & 185.4 & 18.3 & 58.6 & 8.0 \\
119 & 8.7 & 940 & 59.8 & 139.7 & 29.7 & 80.9 & 75.8 \\ Smectite \({}^{*}\) & - & - & 60–150 & - & - & - & - \\ Pelotherapy applications \({}^{**}\) & - & - & \textgreater{}10 & - & - & - & - \\ \hline \end{tabular}  
* not defined.  
\end{table}
Table 6: Physico-chemical parameters and concentration of exchangeable cations in residual soil samples from Porto Santo Island.