Prior reports that the coexistence of the liquid-expanded (LE) and liquid-condensed (LC) phases in phospholipid monolayers terminates in a critical point have been compromised by experimental difficulties with Langmuir troughs at high surface pressures and temperatures. The studies reported here used the continuous interface of a captive bubble to minimize these problems during measurements of the phase behavior for monolayers containing the phosphatidylcholines with the four different possible combinations of palmitoyl and/or myristoyl acyl residues. Isothermal compression produced surface pressure-area curves for dipalmitoyl phosphatidylcholine (DPPC) that were indistinguishable from previously published data obtained with Langmuir troughs. During isobaric heating, a steep increase in molecular area corresponding to the main LC-LE phase transition persisted for all four compounds to 45 mN/m, at which collapse of the LE phase first occurred. No other discontinuities to suggest other phase transitions were apparent. Isobars for DPPC at higher pressures were complicated by collapse of the monolayer, but continued to show evidence up to 65 mN/m for at least the onset of the LC-LE transition. The persistence of the main phase transition to high surface pressures suggests that a critical point for these monolayers of disaturated phospholipids is either nonexistent or inaccessible at an air-water interface.

In conclusion, the present study shows that gaps in the ligamenta flava are frequent at cervical and high thoracic levels but become rare at the T3/T4 level and below, such that one cannot always rely on the ligamentum flavum as a perceptible barrier to epidural needle placement at these levels.

Pulmonary surfactant is secreted by alveolar type II cells as lipid-rich, densely packed lamellar body-like particles (LBPs). The particulate nature of released LBPs might be the result of structural and/or thermodynamic forces. Thus mechanisms must exist that promote their transformation into functional units. To further define these mechanisms, we developed methods to follow LBPs from their release by cultured cells to insertion in an air-liquid interface. When released, LBPs underwent structural transformation, but did not disperse, and typically preserved a spherical appearance for days. Nevertheless, they were able to modify surface tension and exhibited high surface activity when measured with a capillary surfactometer. When LBPs inserted in an air-liquid interface were analyzed by fluorescence imaging microscopy, they showed remarkable structural transformations. These events were instantaneous but came to a halt when the interface was already occupied by previously transformed material or when surface tension was already low. These results suggest that the driving force for LBP transformation is determined by cohesive and tensile forces acting on these particles. They further suggest that transformation of LBPs is a self-regulated interfacial process that most likely does not require structural intermediates or enzymatic activation.

The determinants for the formation of multilayers upon compression of surfactant monolayers were investigated by compressing films, beyond the squeeze-out plateau, to a surface tension of 22 millinewtons/m.

We compared surface tension measures of surfactants with various surface activities by using a pulsating bubble surfactometer (PBS) and a captive bubble surfactometer (CBS). Rabbit lung lavage surfactant (60,000 x average g for 60 min), bovine surfactant extract (Survanta), and a synthetic lipid surfactant mixture (dipalmitoylphosphatidylcholine-egg phosphatidylglycerol-palmitic acid) were studied at 1.25 mg phospholipid/ml. The PBS was used either unmodified according to manufacturer's instructions or with the sample chamber capillary kept dry and the sample adsorbing at maximum bubble size (5 min). The CBS was used in a manner that imitated the unmodified PBS. We found that all three techniques indicated low surface tension on the first cycle for 60K. For Survanta, the CBS and the modified PBS reported low surface tension on the first cycle, whereas the unmodified PBS did not achieve this within 10 cycles. For the synthetic lipid surfactant mixture, only the CBS measured low surface tension within 10 cycles. Video observations indicate that the modified PBS performs better than the unmodified PBS because keeping the capillary dry prevents surface film from occupying this large surface during cycling, thereby allowing larger area compressions.