Boundary lubrication imparted by pleural surfactants and their identification.

Phospholipids have been identified in pleural washings from live dogs and were found to include phosphatidylethanolamines, sphingomyelin, and, predominantly, phosphatidylcholines. The extracts were highly surface active when studied on the Langmuir trough using a Wilhelmy balance and produced surface tension/area loops similar to those for synthetic phosphatidylcholines and phosphatidylethanolamines but differing from the pure dipalmitoyl derivative (DPL). The extracts, the synthetic surfactants and their mixtures, were all found to be good lubricants when tested by a standard method for evaluating textile "sizes." The results are consistent with the classical theory of "boundary" lubrication for which surfactant molecules would have an almost ideal molecular structure for adsorption, film cohesion, and mutual interaction of the hydrophobic ends. This concept is suggested as a mechanism that can explain some of the anomalies in the hydrodynamic theory of sliding of the pleurae and may possibly apply to other surfaces.