Influence of Cations, Alkane Chain Length, and Substrate on Molecular Order of Langmuir-Blodgett Films

Atomic force microscope images of Langmuir-Blodgett films show that manganese arachidate (MnA₂) monolayers are short-range ordered and lead stearate (PbSt₂) monolayers are long-range ordered on crystalline mica substrates but disordered on amorphous oxidized silicon substrates. The lattice structures of PbSt₂ and MnA₂ monolayers on mica were previously unknown and have larger lattice parameters and molecular areas than do multilayer films of the same materials, indicating the strong interactions with the larger mica lattice. Multilayer films of PbSt₂, cadmium arachidate (CdA₂), and MnA₂ have centered rectangular “herringbone” lattices on both silicon and mica substrates. After sufficient layers are added, the effect of the mica substrate is eliminated and the lattice parameters and area per molecule of films deposited on mica relax to those of multilayer films on amorphous oxidized silicon. This limiting area per molecule correlates well with the degree of ionic versus covalent bonding as estimated by the Pauling electronegativity, with barium arachidate (BaA₂) > MnA₂ > CdA₂ > PbSt₂. For BaA₂ and MnA₂ the increased molecular area is sufficient to induce a tilt in the molecular packing. The lattice parameters, symmetry, and area per molecule are independent of the length of the alkane chain of the fatty acid for all cations and substrates examined.