Interfacial pressure response to area disturbances in presence of

Appendix 6C Interfacial Pressure response to Area Disturbances in Presence OF Insoluble Monolayers 

In a very recent development the pendent drop technique was used to study the static and dynamic behaviour of insoluble monolayers. Kwok et al. (1994a) were the first who performed measurements of surface pressure - area isotherms of octadecanol monolayer by using a pendent water drop and ADSA (cf Section 5.4) as a film balance. Consistent results with classical Langmuir-Wilhelmy film balance measurements reveal ADSA as a powerful tool also for monolayers. 

The general idea is to spread very small amounts of a spreading solvent, containing the definite amount of insoluble surfactant, onto the surface of a pendent drop, for example a water drop. Then, by changing the volume of the pendent drop the area of the drop surface can be compressed or expanded. ADSA provides surface tension, surface area and drop volume simultaneously at any time an image of the drop is acquired. Thus, exact surface pressures and surface areas are available continuously, and isotherms can be measured. 

The pendent drop technique has also been extended to studies of insoluble monolayers of phospholipids at the water/n-dodecane interface (Li et al. 1994b). In these experiments first a monolayer is produced on a water drop surface as described above. Then, the water drop is gently immersed into the second liquid, for example n-dodecane. Then the change of the drop size enables one to compress and expand the interfacial film. Again, the isotherm obtained with ADSA shows the same type of dynamic behaviour as measured with the classic Langmuir-Blodgett trough technique (Thoma Mohwald 1994). 

In the pendent drop experiments, the surrounding temperature is easily controlled by using a small quartz cuvette. The cuvette can be sealed during the measurements to avoid evaporation effects. This makes the pendent drop technique particularly suited for measurements of very 

---