FIGURE 7.

Heating isobars for DPPC and POPC. Films containing each compound were spread initially at ∼23°C to <40 mN/m, heated to 26°C, and then subjected either to only a “slow” compression (<0.008 min⁻¹) to 46 mN/m, or, after reaching that surface pressure, to a “fast” compression (19 min⁻¹ for DPPC and 30 min⁻¹ for POPC) that reached 68 mN/m. Both rapidly and slowly compressed films were then expanded slowly (0.005 min⁻¹) to 41 mN/m, and then heated isobarically at 1.0°C/min to 70°C. The film's ability to sustain surface pressures above 50 mN/m was tested by slow compression before and after heating. Surface area (left axis) was expressed relative to the initial value (A_o) at 26°C and 41 mN/m. The isobars for rapidly compressed DPPC in these studies are comparable to curves for films compressed only at standard slow rates (Crane et al., 1999). Each curve is representative of three experiments. Symbols at selected temperatures indicate mean ± SD to provide the magnitude of variation. Error bars that are not evident are less than the size of the symbols.