Coexistence of phases in monolayers of branched-chain phospholipids investigated by scanning force microscopy

The coexistence of phases in multiple-chain phospholipid monolayers is revealed after both horizontal (‘scooping up’ or Langmuir–Schaefer–Kato technique) and vertical (Langmuir–Blodgett technique (LB)) transfer onto mica substrates by means of scanning force microscopy (SFM) in tapping and contact mode experiments. Both methods show similar surface topography features. Brewster angle microscopy (BAM) experiments display the formation of round-shaped and dendritic domains in the coexistence region between liquid-condensed (LC) and liquid-expanded (LE) phases in monolayers at the air/water-interface. Higher resolution SFM on samples transferred from this region indicate the presence of defects inside the larger domains and the formation of smaller aggregates and stripes in the surrounding background. The measurements produce direct proof of a substrate-mediated condensation during/after the transfer. The main difference between the two dipping modes used is the elongation of the small aggregates in the case of vertical transfer. Horizontal deposition appears to better preserve shape and size of the domains.