Self-Assembly of Alcohols Adsorbed on Graphene

Self-assembly in two dimensions (2D) can generate structures with no counterparts in three dimensions (3D); therefore, apart from its technological applications, this phenomenon is also theoretically intriguing. We report results from molecular dynamics simulations on the spontaneous crystallization and self-organization of primary and secondary alcohols adsorbed on a graphene sheet. For the 1-alcohol systems, ethanol to 1-decanol, we find the freezing temperatures of the adsorbed alcohols in 2D to be higher than the corresponding values in the 3D bulk. The structures are characterized by linear chains of alcohols held together by a zigzag pattern of hydrogen bonding between neighboring hydroxyl groups, as well as, by dispersive interactions between the alkyl tails. Reminiscent to alkanes in 3D, alcohols with even number of carbons are packed more efficiently than those with odd numbers. Furthermore, alcohols shorter than 1-hexanol exhibit a preference for antiparallel alignment of the dipole moments of neighboring chains, whereas the preferred alignment in longer alcohols is parallel. Although in both cases, herringbone ordering is obtained, in the latter, the 120 degrees kinks are twice as frequent. In contrast to the chain formation of primary alcohols, secondary alcohols form small-sized ring clusters, tetramers for 2-propanol and dimers for 3-pentanol and 4-heptanol. This is a result of steric repulsions between the branched tails that are also manifested in larger distortions of the hydrogen bonds and out-of-plane configurations of the adsorbed alcohols.