Thermodynamics of association of water soluble fullerene derivatives [ $$\hbox {C}_{60}\hbox {(OH)}_{\mathrm{n}}$$ C 60 (OH) n , n = 0, 2, 4, 8 and 12] in aqueous media

Abstract

The thermodynamics of association of fullerene [C 60 ] and water-soluble fullerene derivatives, i.e., fullerols [C 60 (OH) n , where, n = 2, 4, 8, 12] in aqueous solutions have been studied using molecular dynamics simulations. The potentials of mean force (PMFs) bring out the tendency of aggregation of these nanostructures in water. The extent of hydroxylation seems to have a minor effect on the depth of the contact minima (the first minimum in the PMFs). The positions of the subsequent minima and maxima in the PMFs change with the size of the solute molecules. Higher stability of the contact state of highly hydroxylated fullerols is due to the van der Waals interactions whereas intermolecular solute-solvent hydrogen bonding nearly flattens the PMFs beyond the 2nd minima for higher fullerols. The solvent contributions to the PMFs for all the solute particles studied here are positive. Entropic and enthalpic contributions to the association of solute molecules are calculated in the isothermal-isobaric (NPT) ensemble. We find that the contact pair formation is governed by entropy with the enthalpic contributions being highly unfavorable, whereas the solvent assisted and solvent separated configurations show entropy-enthalpy compensation.

Energetics of Solvent-Based Deposition of Fullerene Derivative on the Inorganic–Organic Hybrid Lead Halide Perovskite Surface

“…The simplest description of entropic contribution is that it is a measure of dynamics of the overall system. Solvation effects, such as solvent reorganization, or the release of tightly bound solvent molecules upon particle binding can contribute significantly to the entropic contribution of the free energy. − …”

“…In this study, the changes in entropic contribution and enthalpic contribution of the association were calculated from the temperature derivatives of the PMFs via a finite temperature difference method. , In the NPT ensemble, PMF is equivalent to change in Gibbs free energy . The change in entropy is calculated from the finite difference temperature derivative of the PMF or Δ G (r) as a function of COM distances between PCBM and perovskite surface by applying eqs and . , Here, T is 325 K and Δ T is 25 K. The enthalpic contribution, i.e., Δ H ( r ), is computed using eq .…”

“…In stage 2, the CO–HA association peak around 2.8 Å disappears and rest of the association peaks show weaker intensity due to the desolvation of C60 part, esteric and phenyl chain. Such desolvation introduces reorganization of the solvent molecules, which should lead to significant entropic contributions. − To further analyze the mechanism for PCBM deposition and to understand the underlying thermodynamics, we determined the entropic and enthalpic contributions to the PMF.…”

“…57,59 In the NPT ensemble, PMF is equivalent to change in Gibbs free energy. 57 The change in entropy is calculated from the finite difference temperature derivative of the PMF or ΔG(r) as a function of COM distances between PCBM and perovskite surface by applying eqs 2 and 3. 57,59 S r G r T ( ) ( )…”

Surface Activity and Aggregation Behavior of Polyhydroxylated Fullerenes in Aqueous Solutions

“…These carbon nanoparticles are highly soluble in aqueous solution, with reported solubilities as high as 58.9 g/L. , PHFs are known to behave nonideally at higher concentrations, forming aggregates that can be detected by dynamic light scattering. − PHF behavior at lower concentrations is less well characterized, although most reports generally assume–either explicitly or implicitly–that PHF solutes are monomers in aqueous solutions having concentrations ≤ 0.1 mg/mL. , Despite the large number of hydroxyl groups decorating the PHF surface, not all hydroxyl groups contribute to aqueous solubility. Numerous experimental and modeling studies report that hydroxyl groups tend to cluster on the fullerene surface where they can be stabilized internally through intramolecular hydrogen bonding. − This asymmetry complicates efforts to understand PHF chemistry in aqueous solution and at aqueous interfaces. Simple models describing properties based on functional group additivity will be less effective because not all hydroxyl groups contribute equally to stabilization in solution.…”