Determination of the standard entropy changes of ion transfer for 1-alkylpyridinium ions by thermal modulation voltammetry with laser heating at a water/1,2-dichloroethane interface.

When laser light impinges upon a liquid-liquid interface between an optically transparent aqueous and opaque organic phase from the aqueous to organic phase, the temperature at the interface and the interfacial region rises. Using this rise in temperature, we have proposed and developed thermal modulation voltammetry at an interface between two immiscible electrolyte solutions (TMV-ITIES) and have determined the standard entropy changes of ion transfer, ΔS(tr)(°,O→W). In this work, we have determined ΔS(tr)(°,O→W) for four 1-alkylpridinium ions, namely 1-methylpyridinium ion (MePy(+)), 1-ethylpyridinium ion (EtPy(+)), 1-propylpyridinium ion (PrPy(+)), and 1-buthylpyridinium ion (BuPy(+)) by TMV-ITIES. As a result, we obtained the ΔS(tr)(°,O→W) values of 108.0 ± 0.5 (n = 3), 75.8 ± 4.4, 55.6 ± 1.2, and 42.7 ± 0.9 J K(-1) mol(-1) for MePy(+), EtPy(+), PrPy(+), and BuPy(+), respectively. From these values of ΔS(tr)(°,O→W), we have suggested that MePy(+), EtPy(+), and PrPy(+) are classified as water structure-breaking ions and BuPy(+) as a weak water structure-making ion. Further, we discuss the relationship between the standard free energy change, ΔG(tr)(°,O→W), and ΔS(tr)(°,O→W) of ion transfer for 1-alkylpridinium ions and tetraalkylammonium ions.