Adsorption model and phase transitions of diblock perfluoroalkylated surfactants at the water∣alkane interface

We present a theory of the adsorption behaviour and phase transitions in monolayers of perfluoroalkylated alcohols, n-C_nF_2n+1C_mH_2mOH, at the water∣oil interface, and validate it for a range of temperatures and surfactant structures. The reason for the observed cohesive behaviour is identified as dispersion attraction between the fluorocarbon blocks. The London constant is determined from the increment of the lateral attraction parameter with the size of the fluorocarbon chain. The monolayers exhibit phase transition from liquid expanded state to van der Waals crystal. However, they are supercritical with respect to the gas-liquid transition. For the description of the liquid phase, we use the sticky disc model - fluid monolayer made of hard discs interacting with a short-ranged sticky potential. For the crystalline phase, a two-dimensional cell model is developed using the same interaction potential. This new model coincides with the empirical equation of state of Jura and Harkins, and ascribes physical meaning to its parameters. We extend the theory of Ivanov et al. for the adsorption constant K_a to diblock molecules; it predicts accurately the dependence of K_a and the adsorption heat on the surfactant structure. An invariant phase diagram of the monolayers is constructed.