Two-Dimensional Physical Gels Can Exist even at Zero Surface Pressure at the Air/Water Interface - Rheology of Self-Assembled Domains of Small Molecules

Films of mesoscopic domains self-assembled from fluorocarbon/hydrocarbon diblocks (FnHm) at the air/water interface were found to display highly elastic behavior. We determined the interfacial viscoelasticity of domain-patterned FnHm Langmuir monolayers by applying periodic shear stresses. Remarkably, we found the formation of two-dimensional gels even at zero surface pressure. These monolayers are predominantly elastic, which is unprecedented for surfactants, exhibiting gelation only at high surface pressures. Systematic variation of the hydrocarbon (n = 8; m = 14, 16, 18, 20) and fluorocarbon (n = 8, 10, 12; m = 16) block lengths demonstrated that subtle changes in the block length ratio significantly alter the mechanics of two-dimensional gels across one order of magnitude. These findings open perspectives for the fabrication of two-dimensional gels with tuneable viscoelasticity via self-assembly of mesoscale, low molecular weight materials.