EFFECT OF SUBSTRATE ANCHORING ON THE MECHANICAL STRENGTH OF LANGMUIR-BLODGETT BILAYERS

Using the surface-force apparatus (SFA), we have made out-of-contact f orce and in-contact adhesion measurements between composite Langmuir-B lodgett (LB) bilayers. The outer layer is a crystalline monolayer of a novel glycine-containing amphiphile, whose headgroups make a strong, hydrogen-bond-aided adhesion when in normal contact with an identical monolayer. Two different inner layers were studied, with different aff inities for the mica substrate. One, a crystalline monolayer of the ph ospholipid dipalmitoylphosphatidyl-ethanolamine (DPPE), is held onto m ica by comparatively weak interactions. Another monolayer, a covalentl y polymerized network of octadecyltriethoxysilane (OTE), is covalently anchored to plasma-treated mica through the thermal hydrolysis of sil anol groups on both substrate and monolayer. We observe that separatio n of two DPPE/glycine-amphiphile bilayers in hydrogen-bonded contact l ends to fracture at the mica/bilayer interface, with a pull-off force of F/R = -75 mN/m. Bonded OTE/glycine-amphiphile bilayers fracture at the OTE/glycine-amphiphile interface, with a pull-off force of F/R = - 142 mN/m. These results demonstrate that the mechanisms of LB film fra cture cannot be explained considering only the breaking of hydrophobic contacts between neighboring amphiphile tails. (C) 1998 Elsevier Scie nce S.A. All rights reserved.