Ion-selective molecular inclusion of organic dyes into pH-responsive gel assemblies of zwitterionic surfactants

Abstract

Surfactant-based gels have attracted considerable attention for molecular capture applications. Methods to promote selective capture of target molecules based on their molecular discrimination behaviour are one route in the development of gel formulations. In this work, it was demonstrated that highly efficient molecular inclusion and unique molecular discrimination based on the ionicity and/or structure of the target molecules can be realized by adopting a pH-responsive sol–gel transition for molecular capture. The zwitterionic amphiphilic compound C16CA formed pH-responsive gel assemblies, which captured organic dye molecules. It was found that the pH-responsive sol–gel transition contributed to highly efficient molecular capture due to the inclusion of dyes in the hydrophobic inner core of the gel assemblies. It was also found that the amine sites of C16CA discriminated between the ionicity of dye molecules via electrostatic interactions. ¹H NMR analysis revealed that cationic dye molecules were included to a limited extent into the hydrophobic core of the gels, whereas anionic dye molecules were incorporated throughout the hydrophilic and hydrophobic domains. These differences in molecular inclusion capacities highlight the selectivity of molecular capture by the C16CA sol–gel transition. Furthermore, target dye molecules could be selectively recovered and released from the C16CA gels.