Thermodynamic origin of multi-layer structures in Langmuir polymer films

The emergence of polymer-free water surface in a Langmuir polymer film at conditions where a homogeneous coverage has been expected previously is explained on the basis of the surface tensions of polymer and water, γpv and γwv, respectively, as well as the interfacial tension between the two materials, γpw. The polymer molecules considered are 22-residue poly(γ-benzyl-l-glutamate) (PBLG) peptides in α-helical conformation. Values for γpv and γpw derived from MD simulations are consistent with values inferred from experiments considering the emergence of polymer-free surface area for ultrathin films studied using the surface forces apparatus in earlier work. Based on these surface properties, the behavior of individual PBLG peptides at the air–water interface, the dimerization of PBLG peptides, the equilibrium height and width of fibers with given cross section, and the lateral fusion of fibers are described. We show that a prerequisite for the emergence of multilayer structures, which appear locally in domains of sizes of tens to hundreds of micrometers in the considered Langmuir polymer film, is that the condition γpv + γpw – γwv > 0 holds true.