Triple-chain phospholipid monolayers: a scanning force microscopy and grazing incidence X-ray diffraction study

Two triple-chain and one double-chain phospholipid monolayers have been characterised both on water and on different solid substrates by grazing incidence X-ray diffraction and scanning force microscopy. The formation of holes could be observed on water and on a solid support. SFM in contact and tapping mode has revealed surface defects and pinholes in the submicrometer range of all phospholipid films transferred at room temperature on hydrophobic (glass) and hydrophilic (mica) substrates both horizontally and vertically. The double-chain lipids exhibit only a few defects after the transfer, whereas the triple-chain lipids show more and larger holes in the monolayer. SFM contact mode enables the formation of artificial holes in the monolayer on mica in order to estimate the real thickness of the transferred layer. Surprisingly, for all substances investigated the height of the monolayer is too small compared to the length of the molecules.