| Surfactants, having their special amphiphilic characteristics, are able to aggregate to form a wide variety of structures at interfaces. The crystallizing phenomena of two kinds of novel cationic gemini surfactants, bis(quaternary ammonium) salt, N, N’-([1,1’-biphenyl]-4,4’-diylbis(methylene) bis(N,N-dimethyldodecan-l-aminium)(referred as12-2Ar) and ([1,1’-biphenyl]-4,4’-diylbis(methylene) bis(N,N-dimethyloctane-l-aminium)(referred as8-2Ar), which can spontaneously generate hierarchical crystals, containing a biphenyl group, were investigated. This work focuses on their interfacial crystallization, crystallizing conditions and crystallizing mechanism at air/water interface by many methods, such as polarizing microscope, TGA-DSC, X-ray powder diffraction (XRD) and so on.XRD and X-ray crystallography reveal that12-2Ar crystals belong to monoclinic system that surfactant molecules exhibit an almost fully extended conformation with partly interdigitating alkyl chains, together with a dihedral angle of two benzene rings of～40°or32°in a biphenyl. The crystallographic structure refinement as well as DFT calculation suggests that it is multiple weak intermolecular interactions that are responsible for the formation of the interfacial crystal, involving van der Waals interaction, π-π stacking, and cationic-π interactions, which are coupled and have a synergetic effect on the interfacial self-assembly of geminis.8-2Ar crystals are thinner and less than12-2Ar crystals, which reveals the interfacial crystallization can be controlled by precisely tailoring weak intermolecular interactions. Finally, the formation of the hierarchical crystal with well-defined sword strips at the air/liquid interface is proposed to undergo four steps:surface enrichment, initial nucleation, minicrystal coalescence and subsequent growth. |
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