Effects of metal cations on the dynamic characteristics of a phospholipid thin film at an air/water interface

We measured the time-dependent surface pressure of a dioleoylphosphatidylcholine film at an air/water interface. Our results indicate that: (i) the surface pressure of the thin film traces a closed hysteresis loop accompanied by repetitive cycles of compression and expansion, and (ii) the area of the hysteresis loop markedly increases in the presence of sodium ions. We propose a kinetic model for this phenomenon which includes cooperative aggregation/dissociation at the air/water interface. The experimental findings have been successfully reproduced with the aid of a computer simulation based on this theoretical model.