Are long-chain alkanes hydrophilic?

Abstract

Although short n-alkane chains are classic examples of hydrophobic solutes, mounting evidence points to a hydrophilic crossover for the hydration free energies (DeltaG) of sufficiently long n-alkane chains. Experimental and simulation results for the hydration of n-alkanes from methane (C1) to docosane (C22) are combined with fundamental thermodynamic relations to elucidate intermolecular contributions to DeltaG. Theoretical bounds on the influence of solute conformation on DeltaG are inferred by considering the hydration of idealized linear (all-trans) and globular (spherical) model solutes. More detailed theoretical extrapolations of experimental and simulation results imply that the water-mediated free energy change associated with collapsing an all-trans C100 chain is on the order of -100 kJ/mol and thus that n-alkane chains of this length and longer may be hydrophilic (DeltaG < 0).