Structure of a monolayer of poly(ethylene glycol) end-capped with a fluoroalkyl group and its relationship with proteinadsorption at the aqueous interface

Abstract

Poly(ethylene glycol) (PEG, 6 kg mol⁻¹) terminated at both ends by a hydrophobic fluoroalkyl segment [-(CH₂)₂C₁₀F₂₁] (R_f-PEG) has been shown to self-assemble into an essentially water insoluble hydrogel. Here, we characterize the monolayer, formed by R_f-PEG and the subsequent adsorption of a protein, as a function of packing density at the aqueous interface by using thermodynamic methods, neutron reflectivity, and fluorescence microscopy. The π–A isotherms showed two transitions. The first transition was attributed to the start of a conformational change of PEG chains from a pancake-like shape to a stretched brush-like shape due to intermicellar packing. The second transition was assigned to a collapse of the R_f-PEG monolayer, submerging some of the micellar R_f-PEG underneath the monolayer and forming a multilayered state. BSA (bovine serum albumin), a model protein, adsorption onto the R_f-PEG monolayer from the subphase was also correlated with the transition state of the monolayer. At a lower packing density below the first transition, the R_f-PEG did not suppress BSA adsorption at all. Conversely, an almost complete prevention of BSA adsorption by the R_f-PEG was observed once the R_f-PEG had formed a fully-covered monolayer state.