Nonionic Surfactant Adsorption at the Ethylammonium Nitrate Surface: A Neutron Reflectivity and Vibrational Sum Frequency Spectroscopy Study

The adsorbed layers of polyoxyethylene n-alkyl ether surfactants C(12)E(4), C(14)E(4), and C(16)E(4) at the EAN surface have a headgroup layer that is thin and compact (only approximately 30 vol % EAN). The headgroups do not adopt a preferred orientation and are disordered within the ethylene oxide layer. Alkyl tails contain a significant number of gauche defects indicating a high degree of conformational disorder. The thickness of the tail layer increases with increasing alkyl chain length, while the headgroup layer shows little change. Lowering the C(12)E(4) concentration from 1 to 0.1 wt % decreases the adsorbed amount, and the headgroup layer becomes thinner and less solvated, whereas C(14)E(4) and C(16)E(4) adsorbed layers are unaffected by dilution over the same concentration range. The C(16)E(4) layer thickness increases and area per molecule decreases on warming to 60 degrees C, but the adsorbed layer structures of C(12)E(4) and C(14)E(4) are unchanged. Both effects are attributed to surfactant solubility.