Surface pressure isotherms classification, theory and scaling laws

1 Surface pressure isotherms classification, theory and scaling laws 

The surface pressure (jt) is the difference in surface tension between that of the pure sub-phase liquid surface and that when the surface is covered by a film. Surface pressure isotherms for materials of low relative molecular mass are values of Jt plotted as a function of the area of surface available per molecule. For spread films of materials of low relative molecular mass, the surface pressure isotherm may display a number of transitions as the area available is decreased. The classification of these isotherms into various types is discussed in various texts on surface and interfacial chemistry (Jaycock and Parfitt 1981, Adamson 1990, MacRitchie 1990). Spread polymer films do not show the same range of isotherm behavioxu as that for compounds of low relative molecular mass. Generally only two types are observed, namely liquid expanded and condensed films. Schematic surface pressure isotherms of these two types are shown in figure 8.1 these are plotted in the manner most common for spread 

Since the surface pressure of a spread film is the two-dimensional equivalent of pressure, attempts have been made to set down an equation of state for the spread film. All derivations make use of the basic thermodynamic relationship between the surface pressure and the surface area (A)  

Hence equation (8.1.3) could be used to analyse surface pressure data to obtain number average relative molecular masses from the intercept at = 0 of a plot of yr/Fs as a function of Fg. The interpretation of the virial coefficients in molecular terms depends on which theory is used Huggins (1965) has collected together the correspondences between the virial coefficients for each of the theories. For the case of the original Singer derivation 

Although numerous surface pressure isotherms have been reported for many different polymers (and pol)mier mixtures) their discussion has been mainly of a qualitative nature and confined to the reporting of limiting surface areas. One aspect that is not always made evident is the tacticity of the polymer, despite its strong influence on the surface pressure isotherm having been noted many 

---