VARIATION IN INTERMOLECULAR SPACING WITH DIPPING PRESSURE FOR ARACHIDIC ACID LB FILMS

Atomic force microscopy (AFM) has been used to investigate the structu re of arachidic acid Langmuir-Blodgett (LB) films deposited at differe nt surface pressures. The observed molecular arrangements are consiste nt with a distorted orthorhombic subcell packing containing two molecu les per unit cell. For films built-up al high surface deposition press ures (50 mN m(-1)), the unit cell dimensions (a(0) = 0.52 +/- 0.06 nm, b(0) = 0.77 +/- 0.02 nm) are similar to those found for LB layers of fatty acid salts, suggesting that the arachidic acid molecules are ali gned with their long axes approximately parallel to the substrate norm al. As the deposition pressure is reduced, an increase in both the a(0 ) and b(0) unit cell dimensions is found. This implies a progressive t ilt of the molecules with decreasing deposition pressure.