About the formation and characterisation of a new phase of polycationic monolayers at the air/water interface

The interfacial behaviour of highly charged cationic polyelectrolytes (PEs) (by alkylation of poly-4-vinylpyridine with C8-, C12- or C16-alkyl halogenides) was investigated at the air/water interface. Great care must be taken with respect to the experimental technique for recording the isotherms. Only a stepwise procedure, applying small pressure increments and checking and recording potential area relaxations, reveals the intrinsic monolayer characteristics. After a true equilibrium range in the isotherm, starting from the onset of first film pressure to the almost closed packed layer, it could be demonstrated that at certain states of compression, a spontaneous kinetically controlled area relaxation is induced, which coincides with a mono- to triple-layer state transition. This region is visible as large shifts in area if the film is kept at constant pressure. The presence and coexistence of well-defined triple-layer states could be proved by Langmuir–Blodgett (LB) transfer and ellipsometric thickness measurements as well as by atomic force microscopy (AFM). Typical features of all area relaxations are induction periods and sigmoidal courses of the plots. According to these relaxations and the characterisation by AFM, the new phase formation is characterised by progressive nucleation and one-dimensional growth. Based on relaxation measurements, a saturation pressure can be derived, constituting the reference for defining supersaturation for nucleation and growth. All experimental findings together can be understood, if one assumes a stretched, stiff and rod-like structure as the most plausible and simple model for the PE in the presence of hard counterions. For the case of soft and polarisable counterions, for electrostatic reasons, the polymer behaviour changes to a worm-like structure. The rate of nucleation of the well-defined phase formation became highly irreproducible in this case.