Submitted on June 9, 2003
Accepted on July 13, 2003
Rapid Compressions in a Captive Bubble Apparatus Are Isothermal
Wenfei Yan1 and Stephen B Hall1*
1 Departments of Biochemistry and Molecular Biology, Medicine, and Physiology and Pharmacology, Oregon Health & Science University, Portland, Oregon, USA
* To whom correspondence should be addressed. E-mail: sbh{at}ohsu.edu.
Captive bubbles are commonly used to determine how interfacial films of pulmonary surfactant respond to changes in surface area, achieved by varying hydrostatic pressure. Although assumed to be isothermal, the gas phase temperature (Tg) would increase by over 100°C during compression from 1 to 3 atm. if the process were adiabatic. To determine the actual change in temperature, we monitored pressure (P) and volume (V) during compressions lasting less than a second for bubbles with and without interfacial films, and used P.V to evaluate Tg. P.V fell during and following the rapid compressions, consistent with reductions in n, the moles of gas phase molecules, because of increasing solubility in the subphase at higher P. As expected for a process with first order kinetics, during one hour after the rapid compression P.V decreased along a simple exponential curve. The temporal variation of n was determined from P.V more than 10 min. after the compression when the two phases should be isothermal. Back extrapolation of n then allowed calculation of Tg from P.V immediately following the compression. Our results indicate that for bubbles with or without interfacial films compressed to >3 atm within a second, the change in Tg is less than 2°C.
