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 C₁₂E₄, C₁₄E₄, and C₁₆E₄ at the EAN surface have a headgroup layer that is thin and compact (only~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 C₁₂E₄ concentration from 1 to 0.1 wt % decreases the adsorbed amount, and the headgroup layer becomes thinner and less solvated, whereas C₁₄E₄ and C₁₆E₄ adsorbed layers are unaffected by dilution over the same concentration range. The C₁₆E₄ layer thickness increases and area per molecule decreases on warming to 60 °C, but the adsorbed layer structures of C₁₂E₄ and C₁₄E₄ are unchanged. Both effects are attributed to surfactant solubility.