Langmuir, Vol.29, No.19, 5734-5741, 2013
pH-Responsive Aggregation States of Chiral Polymerizable Amphiphiles from L-Tyrosine and L-Phenyl Alanine in Water
Sodium salts of maleamic acid derivatives of lauryl ester of tyrosine (MTNa) and phenyl alanine (MPNa) in water exhibited strong pH-responsive behaviors of viscosity and specific conductivity that originate from the concentration and pH dependence of their aggregation states. The aggregates were characterized by a novel spin-probe-partitioning electron paramagnetic resonance (SPPEPR) method and dynamic light scattering (DLS). Results of high-precision fitting of the second-harmonic EPR spectra of the small spin probe di-tert-butyl nitroxide (DTBN) in these aggregates together with viscosity, conductivity, and DLS showed that, at pH similar to 7.54, MTNa formed micelles and MPNa vesicles and MTNa exhibited a pH-induced micelle to vesicle transition as pH was lowered towards 6. MTNa, at pH similar to 7.54, formed small micelles at low concentrations that transformed to long worm-like micelles for concentrations >= 0.05 M, accompanied by a 30-fold increase in solution viscosity. The hydrodynamic radii from DLS confirmed-the-presence Of small micellar aggregates of radius similar to 2 nm in MTNa. at pH similar to 7.54 at the lower concentrations, With coexisting micelles (similar to 2 nm) and vesicles (similar to 50 nm) at pH near 6.5, vesicles (radii similar to 70 nm) at pH near 6, and large vesicles (85 nm) in MPNa at pH similar to 7.60. Both MTNa and MPNa precipitated upon reduction of pH below 6 and below 7, respectively. The rate of transfer of DTBN between the aqueous phase and the aggregate was calculated from the high-field Lorentzian linewidths of the electron paramagnetic resonance (EPR) spectra. The activation energy for the transfer determined from the temperature dependence of the rate of transfer is 12.7 kJ/mol for MTNa vesicles (pH similar to 6) and 20.6 +/- 1.3 kJ/mol for MPNa (pH similar to 7.60). The pH-induced transformations were reversible.