Solubility origin at the nanoscale: enthalpic and entropic contributions in polar and nonpolar environments

Nanostructures are known to be poorly soluble, irrespective of their elemental composition, shape, electronic structure, dipole moment, hydrophobicity/hydrophilicity and the employed solvent. The methods of colloid chemistry allow for preparing suspensions – metastable systems, the stabilities of which differ greatly from one another – but not real solutions. A systematic investigation of the solubility origin at the nanoscale is hereby reported in terms of its fundamental constituents: enthalpy and entropy. Slightly different one-dimensional solutes – narrow carbon nanotubes (CNTs) of different lengths – were considered in hydrophilic (water) and hydrophobic (benzene) environments. We decompose the process of solvation into the solid → gas transition (sublimation) and the gas → liquid transition (condensation). Sublimation is a thermodynamically unfavorable process under room conditions, while the condensation transition depends on the solvent–solute interactions (enthalpic contribution). Unlike solvation of small molecules, solvation of the nanostructures results in a significant alteration of entropy. This alteration is proportional to the linear dimensions of the nanostructure. If the solvent exhibits peculiar solvent–solvent interactions (such as hydrogen bonding in water), solvation is entropically forbidden, irrespective of the solute nature and its nanoscale dimensions. In the case of the hydrophobic solvent (benzene), the condensation transition can be both enthalpically and entropically favorable. The free energy of solvation is in direct proportion to the CNT length. While highlighting principal difficulties in solvating nanostructures, this paper discusses an optimal choice of solvents for solutes exhibiting hydrophobic and hydrophilic interactions with their environments. Our results allow us to predict the solvation of an arbitrary nanostructure using its small, about 2 nm, atomistic model.