Cleiton Maciel

Note on the free energy of transfer of fullerene C60 simulated by using classical potentials.

Diverse atomistic parameters of C60 have been developed and utilized to simulate fullerene solutions in biological environments. However, no thermodynamic assessment and validation of these parameters have been so far realized. Here, we employ extensive molecular dynamics simulations with the thermodynamic integration method in the isothermal-isobaric ensemble to investigate the transfer of a single fullerene C60 between different solvent environments using different potential models. A detailed analysis is performed on the structure and standard Gibbs free energy of transfer of C60 from benzene to ethanol. All of the interactions concerned in the transfer process are included via atomistic models. We notice that having only structural and dynamical properties is not decisive to validate reliable atomic parameters capable of describing a more realistic thermodynamic process. Thus, we employ the calculated free energy of transfer to validate more accurate atomic parameters for the solvation thermodynamics of fullerenes by direct comparison with the solubility experimental data.

Does the Like Dissolves Like Rule Hold for Fullerene and Ionic Liquids

Over 150 solvents have been probed to dissolve light fullerenes, but with a quite moderate success. We uncover unusual mutual polarizability of C60 fullerene and selected room-temperature ionic liquids, which can be applied in numerous applications, e.g. to significantly promote solubility/miscibility of the highly hydrophobic C60 molecule. We report electron density and molecular dynamics analysis supported by the state-of-the-art hybrid density functional theory and empirical simulations with a specifically refined potential. The analysis suggests the workability of the proposed scheme and opens a new direction to obtain well-dispersed fullerene containing systems. A range of common molecular solvents and novel ionic solvents are compared to 1-butyl-3-methylimidazolium tetrafluoroborate.Graphical Abstract

Solvent polarity considerations are unable to describe fullerene solvation behavior.

Atomistic molecular dynamics simulations were employed to investigate the solvation properties of the fullerene C60 in binary water/dimethyl sulfoxide (DMSO) mixtures. Structural analysis indicates a preferential solvation with the predominance of DMSO molecules in the first solvation shell for the solutions with low concentrations of DMSO. PMF calculations indicate a maximization of the hydrophobic interaction at low concentrations of DMSO. The contact minima indicate a tendency of aggregation of these nanostructures in water/DMSO mixtures and in the both pure solvents. The free energy of solvation suggests that the hydrophobicity of the fullerene increases monotonically with the increase of DMSO concentration. This result is incompatible with the polarity of DMSO, since it was expected that increasing the concentration of DMSO entailed an increase of C60 solubility.

Urea enhances the photodynamic efficiency of methylene blue.

Methylene blue (MB) is a well-known photosensitizer used mostly for antimicrobial photodynamic therapy (APDT). MB tends to aggregate, interfering negatively with its singlet oxygen generation, because MB aggregates lean towards electron transfer reactions, instead of energy transfer with oxygen. In order to avoid MB aggregation we tested the effect of urea, which destabilizes solute-solute interactions. The antimicrobial efficiency of MB (30 μM) either in water or in 2M aqueous urea solution was tested against a fungus (Candida albicans). Samples were kept in the dark and irradiation was performed with a light emitting diode (λ = 645 nm). Without urea, 9 min of irradiation was needed to achieve complete microbial eradication. In urea solution, complete eradication was obtained with 6 min illumination (light energy of 14.4 J). The higher efficiency of MB/urea solution was correlated with a smaller concentration of dimers, even in the presence of the microorganisms. Monomer to dimer concentration ratios were extracted from the absorption spectra of MB solutions measured as a function of MB concentration at different temperatures and at different concentrations of sodium chloride and urea. Dimerization equilibrium decreased by 3 and 6 times in 1 and 2M urea, respectively, and increased by a factor of 6 in 1M sodium chloride. The destabilization of aggregates by urea seems to be applied to other photosensitizers, since urea also destabilized aggregation of Meso-tetra(4-n-methyl-pyridyl)porphyrin, which is a positively charged porphyrin. We showed that urea destabilizes MB aggregates mainly by causing a decrease in the enthalpic gain of dimerization, which was exactly the opposite of the effect of sodium chloride. In order to understand this phenomenon at the molecular level, we computed the free energy for the dimer association process (ΔG(dimer)) in aqueous solution as well as its enthalpic component in aqueous and in aqueous/urea solutions by molecular dynamics simulations. In 2M-urea solution the atomistic picture revealed a preferential solvation of MB by urea compared with MB dimers while changes in ΔH(dimer) values demonstrated a clear shift favoring MB monomers. Therefore, MB monomers are more stable in urea solutions, which have significantly better photophysics and higher antimicrobial activity. This information can be of use for dental and medical professionals that are using MB based APDT protocols.

Prediction of the hydration properties of diamondoids from free energy and potential of mean force calculations.

Molecular dynamics simulations were used to predict the thermodynamical properties of the hydration process of the adamantane, diamantane, and trimantane, the first three members of the series of diamondoids. Free-energy results suggest that the water solubility of these molecules is low. The hydration free energy increases with size of the diamondoid. As for the alkane hydrocarbons, hydration free energy correlates linearly with the surface accessible solvent area; however, here it has been shown that small diamondoids present hydration free energy significantly lower than the n-alkanes of similar molecular weights. The decomposition of the hydration free energy in enthalpic and entropic terms revealed that the hydration process of the small diamondoids is entropic driven. The potential of mean-force calculations indicates that the aggregation of these species in the aqueous medium should occur spontaneously and that the contribution of the solvent is greater the larger the diamondoid.