“…In the same year Y. Shibutani et al, studied six kinds of tetra alkylester type calix[4]arene derivatives, a diethyl-didecyl mixed ester, and three kinds of lower rim bridged which were characterized by electrochemical measurement to elucidate the effect of the length of the alkyl group of alkoxycarbonyl substituents on Na + selectivity [17]. They found that to obtain excellent Na + selective ionophores, introduction of short chain alkyl groups rather than long chain ones, such as a decyl group, and maintenance of sufficient solubility of the calix[4]arene derivatives in the membrane solvent are required concurrently.…”