ATOMIC-FORCE MICROSCOPE MEASUREMENTS OF LONG-RANGE FORCES NEAR LIPID-COATED SURFACES IN ELECTROLYTES

The interaction of DMPC yl-1,2-diterradecanoyl-sn-glycero-3-phosphocho line C36H72NO8P) lipid-coated Si3N4 surfaces immersed in an electrolyt e was investigated with an atomic force microscope. A long-range inter action was observed, even when the Si3N4 surfaces were covered with no minally neutral lipid layers. The interaction was attributed to Coulom b interactions of charges located at the lipid surface. The experiment al force curves were compared with solutions for the linearized as wel l as with exact solutions of the Poisson-Boltzmann equation. The compa rison suggested that in 0.5 mM KCl electrolyte the DMPC lipids carried about one unit of charge per 100 lipid molecules. The presence of thi s surface charge made it impossible to observe an effective charge den sity recently predicted for dipole layers near a dielectric when immer sed in an electrolyte. A discrepancy between the theoretical results a nd the data at short separations was interpreted in terms of a decreas e in the surface charge with separation distance.