SURFACE PRESSURE RELAXATION AND HYSTERESIS IN STEARIC ACID MONOLAYERS AT AIR-WATER INTERFACE

Time‐dependent changes in the surface pressure of stearic acid monolayers were examined using an automated Wilhelmy‐type film balance. Different surface pressure‐area isotherms were obtained for two different rates of compression. Pressure relaxation from preselected surface pressures was examined as a function of time. The results indicated two types of relaxation which, along with the compression rate effect, may be rationalized on the basis of changes in molecular orientation and redistribution, together with expulsion from the monolayer at areas below the limiting area per molecule. Marked hysteresis effects were also noted when stearic acid monolayers were subjected to compression expansion cycles. The effect of repeated cycling and the minimum area of compression on hysteresis were investigated. The onset and extent of hysteresis may also be explained on the basis of expulsion and reentry and orientation and redistribution of molecules at the interface. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company