Contact angle and surface tension in studies of lung surfactant

The contact angle (theta) was determined in combination with surface tension measured in a Langmuiur-Willhelmy surface tension balance. Correction of the surface tension by the contact angle factor (1/cos theta) was tried on the monolayer curve of dipalmitoyl phosphatidyl choline (DPPC) and total lipid extracts from the bovine lung surfactant material (TLE). The contact angle was observed with an optical apparatus called a Contangulometer and pictures taken discontinuously, and was determined by drawing tangents at the peak of a junction between the water mensicus and a glass plate. The contact angle was about 15 degrees, and had a narrow range of variation until the surface tension became very low, less than 5 dynes/cm, and then the contact angle jumped to almost 90 degrees both in DPPC and in TLE. It is concluded that the directly recorded surface tension curve underestimates the value by about 3-4% and that when the surface tension becomes very low, less than 5 dynes/cm, the value is no longer valid because the contact angle becomes nearly 90 degrees.