Thermodynamic Origin of Multilayer 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, γ_pv and γ_wv, respectively, as well as the interfacial tension between the two materials, γ_pw. The polymer molecules considered are 22-residue poly(γ-benzyl-l-glutamate) (PBLG) peptides in α-helical conformation. Values for γ_pv and γ_pw 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 γ_pv + γ_pw - γ_wv > 0 holds true.