Molecular and Crystal Structure of the Lipid-Model Amphiphile, Dioctadecyldimethylammonium Bromide Monohydrate

Abstract

Dioctadecyldimethylammonium bromide monohydrate (2C18N+2C1Br−·H2O), C38H80NBr·H2O, was crystallized from its solution in chloroform and hexane. The crystals are triclinic with the space group P\bar1, Z=2, a (normal to the bilayer surface)=3.811(2), b=0.7890(2), c=0.7418(2) nm, α=104.37(3), β=103.06(4), and γ=74.93(3)°. The structure was solved by the direct method and refined by the block-diagonal least-squares procedure: R=0.08 for 3121 observed reflections. The crystal structure consists of bimolecular layers stacking regularly along the a-direction. The two amphiphile molecules in the unit cell are related by centrosymmetry and pack tail to tail in a bilayer structure with tilting hydrocarbon chains by about 45° to the bilayer surface. The packing cross section per molecule in the layer plane is 0.567 nm2. The hydrocarbon chains pack with an triclinic (T//) subcell with dimensions of as=0.42, bs=0.49, cs=0.26 nm, αs=88, βs=104, and γs=111°. One of the two alkyl chains bends near the nitrogen atom and conforms a part of a head region. Two water oxygen atoms with half occupancies were found in an asymmetric unit and accomodated statistically in a unit cell. These water molecules are linked with bromide anions by hydrogen bonds with the distance of about 0.34 nm. Bromide anions and ammonium cations are on the same plane parallel to the bilayer surface.