Fullerenes as new colloidal model systems

Abstract

Water-soluble C₆₀(OH)_22–27 (fullerol) particles with different concentrations in D₂O as solvents were used as colloidal spheres. By means of quasielastic neutron scattering (QENS) we determined the dynamic structure factor, S(Q,ω), for samples with concentrations ranging from 1 to 6 vol%. Recently, it has been shown that C₆₀ in CS₂ is a new hard-sphere colloidal model system. In contrast to this lipophilic system with only low solubility in most solvents, hydrophilic C₆₀(OH)_22-27 can be treated as a new soft-sphere colloidal model system for weakly or even highly charged particles. We show that QENS is well suited for studying the dynamic and static behavior of these fullerolD₂O systems with different concentrations. At 1 bar and 298 K, for scattering vectorsQ = 0.4 Å⁻¹ up to Q = 2.6 Å⁻¹ S(Q,ω) can be well described by a sum of a narrow and a broad Lorentzian line. The first is attributed to the diffusion of C_6o(OH)_22-27 colloidal particles in the solvents, whereas the broad Lorentzian is due to the motion of the solvents molecules themselves. From the width of the narrow line we find the translational diffusion coefficient of the fullerol particles to be about 2.74·10⁻¹⁰ m²V⁻¹. Taking into consideration the hydrodynamic shell and the resulting larger effective hydrodynamic diameter of the particles this value is comparable to those obtained from the StokesEinstein relation. Additionally, we show that the self-diffusion coefficient is significantly influenced by the volume fractions of the fullerol particles in D₂O. This is a direct hint for the beginning of particle-particle self-organization in the sense of a liquid-like-ordered system.