Critical Consideration on the Phase Diagrams of Monolayers Drawn from the π–A Isotherms

Abstract

Abstract The shape of π–A isotherms of insoluble monolayers on a water surface was determined by balancing the relaxation times of the molecular processes which occur during the compression of monolayers and the time of observation (tob). Here, tob is defined in terms of the reciprocal of the strain rate of compression; to keep tob constant during compression, monolayers should be compressed under a constant strain rate rather than a constant speed. π–A isotherms of high-purity pentadecanoic acid monolayers were measured under two different tob (300 and 3000 s). Phase diagrams of the pentadecanoic acid in monolayers drawn from the π–A isotherms differ considerably upon changing tob by ten times. Therefore, if one wishes to draw a phase diagram from the results of the temperature dependence of the π–A isotherm, tob of the measurements should be annexed to compare these results with those of other investigators. The so-called solid film region of the monolayers at low temperatures disappeared for isotherms measured at longer tob than 3000 s, in contrast to the general sense of relaxation phenomena. This was explained by the large deviation from πesp of the pentadecanoic acid at the lower end of the solid film region (21 mN m−1) at lower temperatures.