Solubilization of fullerene in polyoxyethylene tetradecyl ether type nonionic surfactants

Abstract

Fullerene is insoluble in water owing to its bulky spherical structure composed of 60 carbons. Herein, polyoxyethylene tetradecyl ether-type nonionic surfactants with a varying number of oxyethylene groups, i.e., C14En (n = 5, 6, and 7), were used to solubilize fullerene. In addition, a solubilization experiment was conducted using the aromatic compound naphthalene as a reference substance. Fullerene tended to solubilize in aqueous solutions of surfactants bearing long EO chains. Particularly, the maximum solubilization quantities of fullerene increased with an increasing number of EO chains in 100 mmol L−1 C14En solutions ranging from 43.2 (n = 5) to 76.9 (n = 6) and 115.6 μmol L−1 (n = 7). In contrast, naphthalene was well solubilized in surfactants bearing relatively shorter EO chains and showing an opposite tendency to that of fullerene. The Gibbs free energy change (ΔG0) per EO chain for the solubilization of fullerene and naphthalene was –1.51 and +0.27 kJ mol−1, respectively. These results indicated that the extension of EO chain of nonionic surfactants is thermodynamically advantageous for achieving the solubilization of fullerene. In addition, the solubilization site of fullerene in the micelles was studied by analyzing its absorption spectrum in four types of solvents with different polarities. The peak wavelength of fullerene suggested that the surrounding environment possessed an intermediate polarity between water and organic solvents. According to the 1H NMR measurements, the waveform of the EO groups in the NMR spectra significantly split upon solubilization of fullerene. From these results, it was concluded that fullerene located in the vicinity of the EO chains (shell area) in the micelles.