Nucleation in monolayers.
Two-dimensional monolayer material of a large number of amphiphiles is transformed into overgrown three-dimensional structures in a state of metastability and supersaturation. This process is described by a theoretical concept and characterised by sensitive experimental techniques. Two theoretical models compatible with each other are based on nucleation and growth of the formed 3D nuclei: (i) the model for limiting cases of nucleation and overgrowth of 3D nuclei with assumed shapes, and (ii) the generalised model under formation of lenticular nuclei. The latter model allows determination of the nucleation rate constant which renders possible the determination of the critical nucleus size and the free energy for the formation of critical nucleus in dependence on the surface pressure. A theoretical model developed on the basis of the classical nucleation theory is applied for this. An effective double-surface pressure-step method is introduced to distinguish between the nucleation and growth processes. Consequently, a critical surface pressure, the limiting surface pressure for the formation of 3D nuclei, and an equilibrium surface pressure, at which the 3D nuclei cease to grow, can be defined and experimentally determined. Despite the different boundary conditions, the constant surface pressure relaxation data can be theoretically described by these two consistent nucleation-growth models. In the most cases, the limiting case of progressive nucleation occurs but also instantaneous nucleation is observed. Direct evidence has been provided by AFM studies that constant surface pressure relaxation of monolayers is caused by nucleation-growth of 3D nuclei from the monolayer material. The quantitative analysis of growth and distribution of the 3D micrograins by AFM supports the mechanism obtained by application of the nucleation-growth theories. New instrumental techniques such as, BAM, AFM, GIXD and X-ray reflectivity provide detailed information on the characteristics of the 3D structures overgrown on the monolayer during constant surface pressure relaxation on microscopic and molecular scales.
Vollhardt
D
D
Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany. dieter.volthardt@mpikg.golm.mpg.de