Collapse pressure of insoluble monolayers is a property determined from surface pressure/area isotherms. Such isotherms are commonly measured by a Langmuir film balance, or a drop shape technique using a pendant drop constellation (ADSA-PD) [Kwok et al, Langmuir 1996, 12, 1851-1859]. Langmuir film balances and ADSA-PD normally suffer from film leakage around or along confining barriers at high surface pressures. High rates of compression can not be easily achieved using a Langmuir film balance. A constrained sessile drop constellation (ADSA-CSD) is a sessile drop formed on top of a small flat pedestal with a circular sharp knife edge preventing spreading. ADSA-CSD has been used to study lung surfactant films. Here, the usage of ADSA-CSD as a film balance is illustrated. It was shown that ADSA-CSD has certain advantages when compared to conventional methods. Only small quantities of liquid and insoluble surfactants are required. Environmental control (humidity, pressure and temperature) studies are a straightforward matter and easy to integrate with the current setup; this is useful for lung surfactant studies. The ability to measure very low surface tension values, easier deposition procedure than a pendant drop and leak-proof design make the constrained sessile drop constellation a better choice than the pendant drop constellation when it comes to lung surfactant studies.

Results of compression isotherms are obtained on three different monolayers: octadecanol, DPPC and DPPG. The collapse pressure values are found to be reproducible and in agreement with previous methods. For example, the collapse pressure of DPPC is found to be 70.2 mJ/m². Such values are not achievable with a pendant drop. The collapse pressure of octadecanol is found to be 61.3 mJ/m², while that of DPPG is 59.0 mJ/m². The physical reasons for these differences will be discussed. The results also show the distinctive difference between the onset of collapse and the ultimate collapse pressure (ultimate strength) of these films. ADSA-CSD allows a detailed study of this collapse region.