A reverse micelle is a self-organized assembly of surfactants located within a nonpolar medium. Their polar heads are pointing inwards, and their hydrophobic chains are in the exterior. In minute systems of water such as reverse micelles, a traditional pH probe cannot be used to determine the acidity. Consequently, NMR techniques have been developed to measure pH. Professor Debbie Crans and coworkers at Colorado State University studied the diffusion of CO2 into reverse micelles. They used V-51 NMR spectroscopy to depict how the signal of the vanadate ion (which is isostructural to the phosphate anion) is affected by pH. This is a viable technique, but this produces a complicated spectrum that requires prior knowledge to interpret. Our research group selected the phosphate anion as an alternate probe for pH of the interior of reverse micelles. First, a series of bulk solutions with a wide range of pH values were prepared, and their respective P-31 NMR spectra were obtained. A correlation between pH and chemical shifts was established. We introduced each of these phosphate solutions into reverse micelles and obtained their P-31 NMR spectra. A correlation, similar to that of the bulk solutions, was observed. A reverse micelle solution was prepared from a pH = 8 phosphate solution, to give a 60:1 ratio of [H2O]:[AOT] (w0 = 60). Next, CO2 was introduced into the flask, either as dry ice or as gas directly from a tank. A comparison of the P-31 NMR spectra before and after exposure to the CO2 confirmed the CO2 was able to penetrate into the reverse micelle and acidify the interior.

P-31 NMR as a pH probe in reverse micelles: An inorganic laboratory exercise

ACS Spring 2022

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