Comparison of Aggregation Behaviors of a Phytosterol Ethoxylate Surfactant in Protic and Aprotic Ionic Liquids

Two different room-temperature ionic liquids (ILs), the protic ethylammonium nitrate (EAN) and the aprotic 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF(4)), have been employed to investigate the solvent protonation effect on aggregation behaviors of a phytosterol ethoxylate surfactant (BPS-10). The calculated thermodynamic parameters based on surface tension measurements present a stronger solvophobic interaction in EAN than that in [Bmim]BF(4) and disclose different driving forces for micelle formation. In addition, the polarized optical microscopy and small-angle X-ray scattering techniques are used to characterize the phase structures formed in both systems at 25 °C. Due to the H-bonding networks in protic EAN, BPS-10 exhibits a lyotropic liquid-crystalline behavior different from that in [Bmim]BF(4). Results obtained from the rheological measurements reflect a more viscoelastic nature of lyotropic liquid-crystalline phases in EAN. The obtained results indicate that the protic EAN behaves more effective than [Bmim]BF(4) to promote the nonionic BPS-10 aggregation.