Analysis of the aggregation of an anionic porphyrin with a cationic surfactant at the supercritical carbon dioxide-water interface using UV-visible external reflection spectrometry.

Abstract

An external reflection (ER) spectrometric device was developed to directly measure adsorbates at the supercritical carbon dioxide (SC-CO2)-water interface. The aggregation of diprotonated species of 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps(2-)) at the positively charged SC-CO2-water interface, prepared by adsorption by the cetyltrimethylammonium ion (CTA(+)), was studied using this device. Orientations of the H4tpps(2-) monomers and J-aggregates at the SC-CO2-water interface were assessed using s- and p-polarized external reflection (ER) spectra. It appeared that the porphyrin plane of the H4tpps(2-) monomer was nearly parallel to the SC-CO2-water interface, and that the long axis of the rod-like H4tpps(2-) J-aggregate was also nearly parallel to the interface. Dependence of the ER spectra on CTA(+) concentration and CO2 pressure were investigated, and the interfacial CTA(+) concentration was found to cause changes in the interfacial H4tpps(2-) species present. Increasing the CO2 pressure changed the interfacial species from the H4tpps(2-) monomer to the H4tpps(2-) J-aggregate because the interfacial CTA(+) concentration increased as the pressure increased. This suggests that the interfacial chemical species can be changed by controlling the pressure and temperature of the SC-CO2. This is the first report of direct measurements of the chemical species at the SC-CO2-water interface, as far as we know.