Relaxation phenomena in phospholipid monolayers at the air-water interface

In this work we are concerned with the study of long-term relaxation phenomena in dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers spread at the air-water interface as a function of the surface pressure and the aqueous phase pH (pH 5, 7, and 9). Long-term relaxation phenomena were determined in an automated Langmuir-type film balance at constant temperature (20 degrees C). Two kinds of experiments were performed to analyze relaxation mechanisms. In one, the surface pressure (pi) was kept constant, and the area (A) was measured as a function of time (theta). In the second, the area was kept constant at monolayer collapse and the surface pressure was decreased. This decrease was measured as a function of time. Various relaxation mechanisms, including monolayer molecular loss by dissolution, collapse, and/or organization/reorganization changes, can be fitted to the results derived from these experiments. These relaxation mechanisms are pH and phospholipid dependent. In the discussion, special attention will be given to the effect of the relaxation phenomena on the hysteresis in pi-A isotherms before and after the relaxation experiment. At pi lower than the equilibrium spreading pressure (pi(e)) the relaxation phenomena are mainly due to the loss of DPPC or DOPC molecules by desorption into the bulk aqueous phase. The formation of interfacial macroscopic vesicles, which are dissolved into the bulk phase, makes the phospholipid monolayer molecular loss irreversible. At the collapse point (at pi > pi(e)), the relaxation phenomena may be due either to collapse for DPPC and/or to a complex mechanism including competition between desorption and monolayer collapse for DOPC. (C) 2008 Elsevier B.V. All rights reserved.