Thermodynamic distribution functions associated to the isothermal phase transition in Langmuir monolayers

The theoretical basis for statistical mechanical deconvolution of an isothermal two-dimensional, surface pressure-induced surfactant phase transition is presented. It is demonstrated that all the thermodynamic quantities characterizing a Langmuir monolayer phase transition can be obtained from the variation of the average molecular area of the system, as a function of the surface pressure, without the need for any assumption of the particular model or mechanism. The cooperativity of the phase changes becomes clearly evident and it is possible to discriminate the behaviour of each coexisting phase state in terms of the average size and statistical distribution functions of thermodynamically correlated molecules undergoing the liquid-expanded to condensed transition in the film.