Reactive perfluoropolyoxyalkylene oligomers. I. Isothermal equilibrium on silica gel: Fundamental parameters

The equilibrium of the adsorption/desorption process of reactive perfluoropolyether (PFPE) oligomers onto silica gel is reported. The process is adequately described by the classic Langmuir model. The macromolecular dimensions and the number of functional groups per molecule control the entire phenomenon by ruling the equilibrium constant and the maximum number of adsorbed species per unit of surface. The occupied area of each single molecule increases proportionally to the molecular weight and this steric demand is in good agreement with that calculated by a classic theoretical approach, referred to the end-to-end distance of these macromolecules. The degree of maximum coverage, when expressed on a weight basis, is only marginally dependent on the molecular weight, leading to the assumption that a uniform layer of PFPE adsorbed onto the substrate is always reached. The thickness of this layer, which is lower than expected when considering a bulk spherical conformation (i.e. in solution) and independent of the molecular dimension, suggests that PFPEs flatten out since the chains are more parallel to the substrate surface in this very thin layer. The spherical conformation in solution is therefore lost, being the z axis smaller than the x and y axes for the oligomers bonded to the active sites of the silica gel. This monolayer thickness well corresponds to the diameter of the constitutive repeating units of the PFPE chain (5–7 Å).