A molecular dynamics study of aggregation phenomena in aqueous n-propanol

Low-frequency Raman studies of various concentrations of aqueous n-propanol at room temperature indicate that both water and n-propanol form single-component aggregates in solution. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) studies also provide evidence of this tendency toward aggregation. Molecular dynamics simulations of 16% aqueous n-propanol, a concentration for which maximum segregation of n-propanol and water is observed, have been carried out in an attempt to elucidate the structure of these aggregates. Kirkwood-Buff integrals calculated from the radial distribution functions of the components show excellent agreement with experimentally derived values. Analysis of the atomic coordinates from the simulations reveal that approximately 50% of the n-propanol molecules are members of homogeneous hydrogen-bonded chains of up to 16 members in length, the majority of which are dimers. The g(r) data also indicate that a strong hydrophobic association exists between the hydrocarbon tails. This hydrophobic association is independent of hydrogen-bonding state, and results in the formation of an approximately 10-member micelle structure centered around the n-propanol chains. Water is excluded from the regions occupied by the n-propanol micelles. The water structure is largely unaffected except for a small amount of disruption at the interface between the bulk solvent and the n-propanol clusters, and the formation of small water clusters at the interface with the bulklike solvent that interact with hydroxyl groups at the ends of the propanol chains.