Postcollapse dynamic surface pressure relaxation in binary surface films containing dipalmitoyl phosphatidylcholine

Abstract

The time-dependent decay of dynamic surface pressure (π) is investigated for binary films of dipalmitoyl phosphatidylcholine: dioleoyl phosphatidylcholine (DPL:DOL) and of dipalmitoyl phosphatidylcholine:cholesterol (DPL:CHOL) after dynamic compression past monolayer collapse. The results show that the addition of either DOL or CHOL to DPL films acts to increase the degree of relaxation of post collapse surface pressure. Thus, while pure DPL films exhibit a highly stable postcollapse dynamic surface pressure that is more than 20 dyn/cm in excess of the static spread value, films of 50:50 DPL:DOL show dynamic π relaxation to values within a few dynes/centimeter of the static collapse pressure in times of the order of 100 sec at 25°C. Films of 50:50 DPL:CHOL also relax to within a dyne/centimeter of the static spread value at 100 sec, although the actual extent of relaxation is quite small for this specific binary because it is able to generate initial dynamic collapse pressures only a few dynes/centimeter in excess of the static case. Films of both 90:10 DPL:DOL and 90:10 DPL:CHOL show postcollapse dynamic π values with a large extent of relaxation, but the relaxation process is relatively slow and these films show surface pressures still above static spread measurements at the same surface area after 1000 sec. The increased relaxation imparted to DPL films by the addition of DOL or CHOL as a second monolayer component correlates with the effect of these other surfactants in increasing the dynamic respreading in DPL films compressed past collapse on successive cycles of the surface.