Linear and Cyclic Alkanes at Aqueous Interfaces: Effect of Molecular Topology

Alkane molecules on water interfaces are known to preferentially align parallel to the interface. However, this trend has not been examined in a way that directly connects interfacial ordering with free energy profiles, particularly in distinguishing the role of carbon number from that of molecular topology in accessing interfacial configurations. Here, atomistic molecular dynamics simulations were used to investigate linear (n-C5, n-C6, n-C7, and n-C12) and cyclic (c-C5, c-C6, and c-C7) alkanes at aqueous interfaces. Interfacial density profiles, orientational order parameters, and free energy profiles along the surface-normal coordinate were analyzed using logarithmic mean-force dynamics (LogMFD). Linear alkanes show a pronounced increase in interfacial ordering, whereas cyclic alkanes exhibit the same qualitative tendency but a substantially weaker size dependence. The corresponding transfer free energy difference increases from about 26.7 to 49.9 kJ mol-1 for the linear series, compared to about 25.0 to 29.7 kJ mol-1 for the cyclic series. These results demonstrate clear topology-dependent interfacial ordering. Additional conformational analysis indicates that both series respond through reorientation: the end-to-end extension of the linear alkanes changes by less than 0.2% across the interface, and the ring-distortion metric of the cyclic alkanes changes by less than 1%.