Structure of polymorphic phases in zinc arachidate LB multilayers

The 3D structure of zinc arachidate (ZnA) LB multilayers transferred at different subphase pH has been studied using X-ray scattering and FT-IR spectroscopy. The molecular packing in ZnA multilayers shows an unusually strong dependence on subphase pH, not observed earlier in LB multilayers of divalent fatty acid salts. In all four polymorphic phases, α, β, γ and δ with characteristic 3D structures were observed and in most cases, two or more phases were found to coexist. The δ-phase, which corresponds to the largest alkyl chain tilt angle of ∼32°, is stable over the complete range of subphase pH investigated and appears as a single phase at a subphase pH of ∼6.5. It corresponds to a rotator phase like 'loose packing' of molecules tilted at an angle of 32° with respect to chain axis, packed in a hexagonal layer cell. The subcell packing changes from hexagonal to orthorhombic and the angle of tilt decreases from 32° (δ-phase) to nearly 0° (α-phase) with increasing pH up to 7.4. The dominant phase at a pH of ∼7.4 (α-phase) has a close packed herringbone structure. However at higher subphase pH, the 'rotator like' δ-phase regains prominence. The interface morphology of different polymorphic phases is found to be unique, independent of the subphase pH at which the monolayers are transferred.