Lipid Membranes as Solvents for Carbon Nanoparticles

Fullerene is scarcely soluble in most solvents, including alkanes. Yet, it has been shown that ${\mathrm{C}}_{60}$ dissolves in lipid bilayers, whose interior is chemically identical to alkanes. Here, we use molecular simulations to explain why lipid bilayers are better than alkanes at dissolving fullerene clusters. Fullerene aggregation is driven by entropy, but enthalpic contributions determine the difference between alkanes and bilayers. Surprisingly, confinement and chain alignment in the bilayer do not affect fullerene aggregation, while solvent density and the perturbation of solvent-solvent interactions are key factors.

Figure 1

MARTINI representation of the molecules used in this work (top), and a snapshot of fullerenes in a POPC membrane with fullerene/hydrophobic tails mass ratio of 8.4% (bottom). Water is omitted for clarity.

Figure 2

Fraction of monomeric fullerene as a function of concentration (expressed as fullerene/solvent mass ratio) in different solvents. For POPC, only the hydrophobic chains are taken into account. Linear regressions are displayed as visual guides only.

Figure 3

PMF for fullerene dimerization in octane, CG (top panel) and atomistic (middle panel) and in POPC (bottom panel), at 313 K. Dashed and dotted lines represent the free energy breakdown into enthalpy and entropy components, respectively. Statistical uncertainties are less than $0.2\text{kJ/mol}$ in all cases.

Figure 4

Fraction of ${\mathrm{C}}_{60}$ monomers as a function of confinement (defined as the region comprising 98% of fullerene mass density) and as a function of bond order parameter.