WHAT IS THE CHARACTERISTIC TIME OF MEASUREMENT OF PI-A ISOTHERMS - NECESSITY OF A CONSTANT STRAIN RATE OF COMPRESSION OF INSOLUBLE MONOLAYERS FOR PI-A MEASUREMENTS

The shape of π-A isotherms of insoluble monolayers is determined by the balance between the relaxation times of molecular processes in the monolayers during compression and the time of observation. The time of observation is defined by the reciprocal of the strain rate of compression at the measurement. At compression under constant speed, the strain rate varies continuously during the measurement, and the time of observation cannot be kept constant. Therefore, π-A isotherms of insoluble monolayers should be measured under a constant strain rate of compression, π-A isotherms cannot be explained as thermodynamical quantities but as viscoelastic ones when GSPs of the film materials are small at the temperature. © 1990, American Chemical Society. All rights reserved.