Structure and Self Assembly of Pluronic Amphiphiles in Ethylammonium Nitrate and at the Silica Surface

The self-assembled structures formed by three Pluronic surfactants (P65, L81, L121) in ethylammonium nitrate (EAN, a protic room temperature ionic liquid) in bulk solution and at the silica-EAN interface have been investigated using polarizing optical microscopy, small-angle neutron scattering (SANS), and atomic force microscopy (AFM) to assess how surface active Pluronics are in ionic liquids. Polarizing microscopy revealed optical textures for P65 only, allowing a detailed phase diagram to be determined with features similar to those determined for Pluronics in water and formamide. Small-angle neutron scattering experiments were conducted at 1 and 10 wt % Pluronic at 25 and 63 degrees C to ascertain whether critical micelle temperatures existed in EAN. SANS experiments using 1 wt % solutions at room temperature reveal that the three Pluronics adopt a random flight chain conformation. For 10 wt % at room temperature, P65 and L81 are dissolved as random coils, but L121 forms lamellar vesicles. When the temperature is increased, the solubility of the Pluronics in EAN decreases, mostly on account of the PPO block. At 63 degrees C, P65 forms micelles, 1 wt % L81 forms lamellar stacks, 10 wt % L81 forms unilamellar vesicles, and L121 forms bilayer stacks at both concentrations. AFM images of the P65-sitica-EAN at room temperature revealed an amorphous layer of surface aggregates for concentrations above 3 wt %. To our knowledge, this is the first report of aggregates adsorbed to a charged surface in an ionic liquid. For L81 and L121 concentrations between 1 and 10 wt %, AFM images do not reveal structure, but the force profiles recorded are consistent with an adsorbed brush layer. The approach force profiles for the three Pluronics have been modeled using the Alexander de Gennes and Milner-Witten-Cates theories, with the Alexander de Gennes theory generally providing better fits to the data. The L81 retraction force data has been modeled using the wormlike chain theory. The fitted Kuhn lengths are in accordance with those determined for aqueous good solvent polymer systems, but the contour lengths are too long to be due to a single L81 chain, suggesting that L81 aggregates upon confinement between the AFM tip and the surface.