The Langmuir trough has been used to study monolayers of surfactant from beef lung extracts, dog lavage and the principal component (dipalmitoyl lecithin; DPL) in which surface tension has been simultaneously monitored on each film by the Wilhelmy method and the original Langmuir method whose readings are independent of contact angle. Readings on the Wilhelmy balance at 80% film compression reached the near-zero values recorded in previous studies, averaging 2.0 dyn cm-1, but Langmuir readings on the same films averaged 13.5 dyn cm-1 for DPL with no reading below 7.0 dyn cm-1. Similar differences were found for bovine extracts and dog lung lavage and have been attributed to a serious contact-angle artifact in the Wilhelmy method widely used in studying pulmonary surfactant. Higher minimum values of surface tension determined by the Langmuir method are shown to be incompatible with normal physiological function in the smaller mammals on the basis of the traditional bubble model of the alveolus. When Langmuir or Wilhelmy values are determined under more physiological conditions of area change, pH, temperature, humidity etc., values for surface tension are even higher and less compatible with the concept of a continuous liquid lining for the alveolar diameter of any mammal.
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