Condensed phases of branched-chain phospholipid monolayers investigated by scanning force microscopy

Branched (triple- and quadruple-chain) phospholipid monolayers were characterized on different solid substrates (mica, silicon wafer, and glass) by scanning force microscopy. SFM in contact and tapping mode has revealed the presence of pinholes on submicrometer and nanometer scales for these lipids transferred by the Langmuir - Blodgett technique (vertical transfer) at room temperature. The transfer ratio was close to 1 in the case of triple-chain lipids and clearly lower in the case of the quadruple-chain lipid. Triple- and quadruple-chain phospholipids show different hole sizes and shapes in the monolayer depending on the deposition pressure and on the solid substrate. The tendency of decreasing their number with increasing surface pressure is maintained upon changing the substrate surface. At low pressure the quadruple-chain phospholipid has shown to have large holes while the triple-chain lipid has revealed condensed domains, which are probably induced by a surface-mediated condensation process, within the holes. In all cases the depth is only half that expected from molecular models.