Water dispersions of natural graphene based carbon nanoparticles: ESR spin probe study

Abstract

Dynamic and phase behavior of an aqueous dispersion of shungite carbon (ShC) nanoparticles, one of natural graphene-like nanomaterials structured in a special way in an aqueous medium, is of considerable interest, as in the case of other carbon nanomaterials, due to its effects and presumable technological and biomedical applications. ShC nanoparticles in aqueous dispersions of different composition were studied in the temperature range 7–60°C using electron spin resonance (ESR) technique of stable spin probes. Hydrophobic probe 5-DOXYL stearic acid was introduced into the structure of hydrated ShC nanoparticles. Hydrophilic probe 4-oxo-TEMPO was dissolved in the surrounding bulk solvent. It is assumed that an increase in the immobilization of the hydrophobic probe in the presence of NaCl, urea, HCl, NaOH, sucrose is associated with a change in the state of hydration of ShC nanoparticles. At the same time, an increase in the carbon concentration during the evaporation of water did not cause changes in the dynamics of the probe until the nanoparticles were dehydrated. Changes in hydration of the hydrophobic probe on the ShC nanoparticles at a temperature of about 17–20°C can be associated with a change in the state of nanoparticle dispersion near the critical mixing temperature of the main and metastable phases. The sigmoid type Arrhenius dependence of the mobility of both the hydrophilic and hydrophobic probes has maximum steepness in the region of the supercritical mesophase near 37°C. The structural and dynamic variations of the dispersion associated with the change in the dynamic regime in the supercritical mesophase near the Frenkel line are discussed as a probable explanation. The data obtained are interpreted on the basis of a hypothetical phase diagram of the dispersion of fullerenes and ShC nanostructures.