Study of Frictional Properties of a Phospholipid Bilayer in a Liquid Environment with Lateral Force Microscopy as a Function of NaCl Concentration

Friction properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-supported planar bilayers deposited on mica were tested in a liquid environment by lateral force microscopy. The presence of these bilayers was detected by imaging and force measurements with atomic force microscopy. To test how the presence of NaCl affects the frictional properties of the phospholipid bilayers, four DMPC bilayers were prepared on mica in saline media ranging from 0 to 0.1 M NaCl. Changes in the lateral vs vertical force curves were recorded as a function of NaCl concentration and related to structural changes induced in the DMPC bilayer by electrolyte ions. Three friction regimes were observed as the vertical force exerted by the tip on the bilayer increased. To relate the friction response to the structure of the DMPC bilayer, topographic images were recorded at the same time as friction data. Ions in solution screened charges present in DMPC polar heads, leading to more compact bilayers. As a consequence, the vertical force at which the bilayer broke during friction experiments increased with NaCl concentration. In addition, the topographic images showed that low-NaCl-concentration bilayers recover more easily due to the low cohesion between phospholipid molecules.