Hydration of Linear Alkanes is Governed by the Small Length-Scale Hydrophobic Effect.

Length-scale dependence of the hydrophobic effect is well understood for apolar spherical solutes: for small solutes (diameter, ≲ 0.8 nm), the hydration free energy is entropically driven, while for larger solutes ( ≳ 2 nm), it is enthalpically driven. The nature of the hydrophobic effect in the case of anisotropic molecules such as linear alkanes is not understood yet. In this work, we have calculated the hydration free energy of linear alkanes going from methane to octadecane and of a spherical decane droplet of ≈ 3 nm using molecular simulations. We show that the hydration free energies of alkanes, irrespective of their size, are governed by the small length-scale hydrophobic effect. That is, unlike the case of large spherical solutes, the hydration free energies of linear alkanes are entropically driven.