Date of Experiment Apr 2005 Date of Report Nov 2005

Objectives

Solutions of C ₆₀fullerene in nitrogen-containing solvents constitute a specific class of fullerene solutions characterized by the cluster formation. The experiment [1] was devoted to the study of the clusters in mixtures of N-methylpyrrolidone (NMP) with water by small-angle neutron scattering (SANS). NMP dissolves C60 comparatively good (solubility 1240 µМ) and is miscible with water. So, it can be used for fullerene transfer into aqueous media, which is of current interest with respect to employing biological activity of fullerenes in medical applications.

Results

Fulleren C60 was directly dissolved in NMP with stirring during four days at room temperature. Maximal concentrations found by the UV-Vis spectra achieved 1000 µМ. Solutions С60-NMP-Н2О were prepared by water dilution only. The mixture NMP-Н2О itself did not dissolve С60. Slow changes in the UV-Vis spectra for pure С60-NMP solutions (solvatochromic effect) during one month after preparation showed the development of initial clusters. The solvatochromic effect at the addition of water depended on the time between preparation of the С60-NMP solution and the addition of Н2О. The most pronounced changes in the UV-Vis spectrum took place if water was added into the fresh С60-NMP solution (within one day after preparation). Two types of С60-NMP solutions, type I and type II, taken after one and thirty days, respectively, were used to prepare two probes С60-NMP-Н2О for SANS experiments. SANS shows highly small, but concentration dependent, signal for the pure С60-NMP solutions, which can be connected with very large clusters (size ~ 500 nm) observed well by DLS.

A sharp increase in the scattering takes place if a sufficient amount of water (above 40 vol. % ) is added into the system. The scattering curves for the two types of initial С60-NMP solutions after addition of water are shown in Fig.1. One can see that for the fresh initial С60-NMP solution (type I) new clusters are of smaller size than in the case of the old С60-NMP solution (type II), which is correlated with the fact that in the old С60-NMP solution the number of clusters with the size more than 100 nm is higher than in the fresh one. Taking into account the DLS data, which shows a tendency towards a lower size of large clusters at the addition of water, the conclusion is that the large clusters are partially destroyed in the binary solution.

References

1. V.L.Aksenov, M.V.Avdeev, T.V.Tropin, M.V.Korobov, N.V.Kozhemyakina, N.V.Avramenko, L.Rosta, Physica B (2006), in press