NMR spectroscopy was used to investigate the effect of pH on the aggregation of amino acid-terminated surfactants. Measurement were made of micelle diffusion coefficients in order to investigate how increasing pH affected micelle size, amide free exchange, and the fraction of surfactant molecules bound to the micelles. It was also discovered that the resonance from the NH proton of each surfactant’s amino acid headgroup disappeared from the NMR spectra at high pH. This effect was attributed to a base catalyzed exchange reaction between the NH proton and water. As pH was increased the rate of this reaction increased as well. Saturation-transfer NMR experiments were used to measure the rate constant, kOH, for this exchange reaction both above and below each surfactant’s critical micelle concentration (cmc). The surfactants investigated were Undecenoic acid (UND)-Gly, UND-Ala, UND-Phe, UND-Leu, UND-Val, UND-Gly-Leu, UND-Leu-Gly, and UND-Ile . The exchange reaction was found to be faster in the monomeric solutions and slower when the surfactant concentration was above the cmc and micelles were present. For example, in the case of UND-Leu, the kOH value for the micelle solutions was 1.3x104 M-1 s-1, compared to 7.8x104 M-1 s-1 for the surfactant monomers. Similar decreases were observed for the other surfactants. In each case, the decrease was attributed to the amino acid headgroup having reduced solvent exposure when the surfactants were aggregated into micelles.

Investigation of the aggregation and amide proton exchange in monomeric amino-acid-based surfactants

249th National Meeting (2015)

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