Solubility and Molecular Conformations of n-Alkane Chains in Water

We employ molecular dynamics simulations to study the solubility and molecular conformations of n-alkane chains in water. We find nearly exponential decrease in solubility with carbon number up to n-eicosane (C-20), and excellent agreement with experiment up to n-dodecane (C-12). We detect no harp break in the dependence of the solubility upon carbon number. A free energy landscape analysis of the chain conformations reveals remarkable similarities between the ideal gas and solvated phase landscapes, suggesting that solvated chain conformations are driven primarily by ideal gas statistics. We find no evidence for hydrophobic collapse of n-alkane chains shorter than n-eicosane (C-20). The primary effect of the solvent is the appearance of a barrier on the order k(B)T, not present in the ideal gas, between the free energy basins corresponding to compact and extended chain conformations, and destabilization of the most extended conformations. Our findings are robust to nontrivial modification of the potential model, suggesting that the absence of strong solvent effects on the free energy landscapes is fundamental to relatively short (<= 20-mer) chains composed of small hydrophobic monomers, and does not depend on the precise nature of the chain interactions.