INFLUENCE OF CATIONS, ALKANE CHAIN-LENGTH, AND SUBSTRATE ON MOLECULARORDER OF LANGMUIR-BLODGETT-FILMS

Atomic force microscope images of Langmuir-Blodgett films show that ma nganese 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. T he lattice structures of PbSt2 and MnA2 monolayers on mica were previo usly unknown and have larger lattice parameters and molecular areas th an do multilayer films of the same materials, indicating the strong in teractions with the larger mica lattice. Multilayer films of PbSt2, ca dmium arachidate (CdA2), and MnA2 have centered rectangular ''herringb one'' lattices on both silicon and mica substrates. After sufficient l ayers are added, the effect of the mica substrate is eliminated and th e 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 v ersus 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 mo lecular packing. The lattice parameters, symmetry, and area per molecu le are independent of the length of the alkane chain of the fatty acid for all cations and substrates examined.