Structure Control of Synthetic Bilayer Membranes from Single-Chain Amphiphiles Containing the Schiff Base Segment: II. pH and Temperature Dependence of the Aggregational Behaviors

Abstract

The aggregational behaviors in aqueous solutions of the single-chain ammonium amphiphiles with a rigid Schiff base segment, the diphenylazomethine chromophore, have been investigated by electronic absorption spectroscopy under various chemical (pH) and physical (temperature) environments. pH dependence is not observed for amphiphiles with the sum of tail and spacer not larger than C14. As for bilayer-forming amphiphiles, different pH-dependent spectral features take place at temperatures below and above the phase transition temperature (Tc) of the corresponding bilayer membranes at neutral environment. Basic environment favors the planar conformation of the chromophores in the whole temperature range investigated, and its influence diminishes with increasing alkyl chain length. On the opposite site of pH, i.e., under the strongly acidic condition with a pH of 1.2, a new absorption band attributed to the protonated species appears below room temperature, while it does not show itself at temperature aboveTc. Temperature dependence of the absorption spectra under the strongly acidic condition (pH < 3) reveals that a deprotonation on the amphiphiles takes place at a temperature belowTc. In basic solution (pH > 10.6), planar conformation increases with raising temperature, so leading to the appearance of a strong 330-nm absorption. Under intermediate pHs (3 < pH < 10.6) the bilayer aggregational behaviors are relatively stable. The observed spectral changes with pH were fully reversible.