Both high pH at 25°C and humidity at 37°C prevent DPL films from attaining zero surface tension, whereas humidity at 25°C and high pH at 37°C do not. At 37°C DPL lowered surface tension to zero when spread from organic solvent or when adsorbed from aqueous 0.15 M Nad in the surface balance in which the surface film was exposed to the room air (dry film). Upon saturation of the atmosphere with water vapor in equilibrium with the aqueous phase at 37°C in a closed chamber, DPL lost the ability to produce zero surface tension, and the γminof the DPL film increased from zero to 22 dyne/cm. Addition of DPL in chloroform to distilled water before dispersion by sonication did not prevent the effect of the humidity. However, when the chloroform solution of DPL was added to 0.15 M Nad before sonication, the adsorbed film produced immediately a stable γminof zero in a saturated atmosphere, 37 °C. In the absence of chloroform, with DPL adsorbed from either distilled water or 0.15 M Nad, the effect of humidity was reversed either by removing the chamber and returning the wet film to room air or by introducing small quantities of dispersing agents such as cholesteryl palmitate. However, whereas the effect of humidifying the air was reversible indefinitely, the effect of cholesteryl palmitate (zero surface tension, wet or dry film) was irreversible. This means that there are substances or conditions that can assist DPL films in maintaining zero surface tension when such films are exposed to humidity-saturated air at 37 °C. © 1976.
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