Fullerenols have been a subject of intense research in many fields with the claim of possible applications in biomedicine such as free-radical sponges, antioxidants, and photosensitizers. However, its transport characteristics, important in determining the feasibility of many applications, have not been studied yet. In this work, electrochemical impedance of aqueous solutions of two types of fullerenols (C(60)(OH)(22-26) and C(60)(OH)(18-22)(OK)(4)) was measured. Sample conductivity was extracted from impedance data, and a nonlinear concentration-dependent conductivity was found for one of two types (C(60)(OH)(18-22)(OK)(4)). A concentration-dependent mobility that accounts electrophoretic and relaxation effects could explain experimental data. As a result, we obtained some fullerenol parameters, relevant to transport phenomena: its hydrodynamic radius, the number of attached hydroxides, and the number of counterions solvated into solution. In addition, an important result for pharmaceutical applications has been discussed, which is the change of pH in water induced by the different concentrations of fullerenol, indicating it behaves as a weak acid.

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Stability of a uniformly spaced dendritic array against a spatial period doubling instability was studied through UV thermal perturbations of every other dendrite tip. We observed that above a critical pulling speed the array is stable against these perturbations and we measured decreasing decay rates of the perturbed mode as we approached the critical speed. In the linear regime, our measurements are qualitatively consistent with the Warren-Langer linear stability analysis for a dendritic array [J. A. Warren and J. S. Langer, Phys. Rev. A 42, 3518 (1990); Phys. Rev. E 47, 2702], while in the nonlinear regime fitting to a third order amplitude equation shows that the transition is subcritical.[S0031-9007(98)06563-6] PACS numbers: 81.10. Aj, 68.70.+ w, 82.65.Dp In directional solidification experiments a dilute binary alloy with solute concentration C`is put in a temperature gradient G and a flat solid-liquid interface is established. The sample is then pulled with pulling speed V P toward the cold side; i.e., the alloy is solidified. Above a critical pulling speed the flat interface becomes cellular and at higher speeds a dendritic pattern is established. The dendrite coarsening process, sidebranching, and the interdendritic spacing l 1 are critical problems in pattern formation, and much work has been devoted to studying the time evolution, the structure, and the stability of the observed patterns.Detailed studies by Trivedi and co-workers, who started crystal growth from a planar front with constant pulling speed V P , indicated that in steady state the interdendritic spacing l 1 is reproducibly determined by the growth conditions and changes smoothly with pulling speed V P [1] or solute concentration C` [2]. Such observations suggested the existence of a dynamical selection mechanism for l 1 . Warren and Langer (WL) carried out a linear stability analysis of an array of dendrites [3,4] using microscopic solvability theory [5,6] to describe individual tip shapes and calculating the interaction between tips through the solute diffusion field. WL found that no dynamical selection mechanism should exist, but for a given V P a range of interdendritic spacings is allowed. Their calculations predict that for given growth conditions there is a range of pulling speeds in which an array of spacing l 1 is stable with a lower limit value V PD ͑l 1 ͒. Below V PD ͑l 1 ͒ the array is unstable and a period-doubling bifurcation should be observed, with every other dendrite continuing to grow.Subsequently, Huang et al. [7] observed that if directional solidification is initiated at some V 0 P until a steady state array develops and the pulling speed is then changed from V 0 P to V P , different values of l 1 are found for the same final V P , depending on the choice of V 0 P . They found that, for a given V P , there is a range of l 1 values whose lower limit lies close to the WL prediction. In a more recent experiment Han and Trivedi [8] observed that once a dendritic array has been established, V P can be increased consid...

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