## 3 Results and Discussion
### Function of Groups and Coordination Feature  
It is known that small modifications in the structures of calixarenes can generate a significant change in distribution coefficients, and lead to distinctly different extraction behaviors [36]. [PERSON] [37] reported that the sulfonyl oxygen and adjacent phenol moieties of \(p\)-tetramethyl-butylsulfonylclax[4]arene take part in coordination to metal ions. SC4A has a structure similar to that of \(p\)-tetramethyl-butylsulfonylclax[4]arene with four OH groups and SO\({}_{2}\) groups. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Conditions** & \multicolumn{3}{c}{**Cs(I)**} & \multicolumn{3}{c}{**Sr(II)**} \\ \hline \multirow{2}{*}{**Temperature**} & \(\Delta H^{\circ}\) & \(\Delta S^{\circ}\) & \(\Delta G^{\circ}\) & \(\Delta H^{\circ}\) & \(\Delta S^{\circ}\) & \(\Delta G^{\circ}\) \\  & **(kJ\(\cdot\) mol\({}^{-1}\))** & **(kJ\(\cdot\) mol\({}^{-1}\) K\({}^{-1}\))** & **(kJ\(\cdot\) mol\({}^{-1}\))** & **(kJ\(\cdot\) mol\({}^{-1}\))** & **(kJ\(\cdot\) mol\({}^{-1}\) K\({}^{-1}\))** & **(kJ\(\cdot\) mol\({}^{-1}\))** \\ \hline
288 K & & & \(-3.49\) & & & \(-6.44\) \\
293 K & & & \(-3.03\) & & & \(-4.85\) \\
298 K & \(-29.29\) & \(-0.09\) & \(-2.56\) & \(-80.38\) & \(-0.26\) & \(-3.51\) \\
303 K & & & \(-2.09\) & & & \(-2.08\) \\
308 K & & & \(-1.62\) & & & \(-0.69\) \\ \hline \multicolumn{7}{c}{Conditions: \([\rm SC4A]_{(o)}=1\times 10^{-3}\) M, \([\rm Metal]_{(a)}=1\times 10^{-4}\) M, pH = 6, contact time = 60 min, shaking speed:} \\ \multicolumn{7}{c}{150 rpm.} \\ \end{tabular}
\end{table} 
Table 2: Thermodynamic parameters of the solvent extraction. To examine the effects of OH groups and SO\({}_{2}\) groups on extraction ability toward Cs(I) and Sr(II), TC4A containing sulfur atoms instead of SO\({}_{2}\) groups, and SC4 AOEt containing OCH\({}_{2}\)COOC\({}_{2}\)H\({}_{5}\) groups rather than OH groups (Figure 1) were also employed to the extraction of Cs(I) and Sr(II) at various pH values. The results are listed in Table 3.  
According to Table 3, TC4A can hardly extract Cs(I) or Sr(II), indicating interaction between soft sulfur atoms of TC4A and Cs(I) or Sr(II) is not favorable. SC4A displayed better extraction ability, since the lone pair electrons of the sulfonyl oxygen can coordinate to Cs(I) and Sr(II). Meanwhile, it can be seen that SC4 AOEt displayed weak extraction ability. The observed gap in extraction ability of SC4A and SC4 AEt are likely due to the ionization potential of OH groups of SC4A. To well understand the binding location of Cs(I) and Sr(II), SC4A-Cs(I) and SC4A-Sr(II) complexes were obtained by drying the organic phases in air after extraction. Their IR spectra are presented in Figure 7. It can be seen that the characteristic absorptions at 3412 cm\({}^{-1}\) in the IR spectrum of SC4A are attributed to OH stretching, while that of the SC4A-Cs(I) and SC4A-Sr(II) complexes were observed at 3346 and 3352 cm\({}^{-1}\), respectively. The OH stretching bands of the extracted species were shifted to lower frequencies, which implied that hydroxyl groups were coordinated with metal ions. Previously examined complexes of _p-tert_-butylclax[5]arene hexacarboxylic acid ligand with Cu(II) ion [38] showing that C=O stretching band of ligand-Cu(II) changed intensely and positively. In this experiment, the absorption bands at 1313 and 1156 cm\({}^{-1}\) correspond to asymmetrical and symmetrical sulfonyl stretching vibrations of SC4A, respectively. Obviously, in IR spectrum of SC4A-Cs(I) and SC4A-Sr(II) complexes, and the increased relative intensities (see green indicatrix) of asymmetrical sulfonyl vibration stretching to symmetrical sulfonyl vibration stretching, indicated that SO\({}_{2}\) groups were coordinated with the metal ion. O=S=O stretching bands were shifted to higher frequencies compared to the free SC4A, denoting the complexation of the SC4A with metal ion as well. The changes in peak position of sulfonyl vibrations of SC4A-metal complexes are not spectacular may be due to metal concentration was too small.