Comparison of the Aggregation Behavior of 15 Polymeric and Monomeric Dipeptide Surfactants in Aqueous Solution

The aggregation numbers of 15 dipeptide surfactants were estimated by use of fluorescence steady-state quenching techniques. Polymerization of the surfactants with γ radiation resulted in molecular micelles with a lower number of repeat units than the corresponding monomer aggregation numbers at the concentration of monomer used for polymerization in this study. In addition, the aggregation numbers of the monomers decreased with increasing size of the N-terminal R group of the dipeptide surfactants examined in this study. The aggregation behavior of the dipeptide surfactants was further investigated using proton NMR ( 1 H NMR) spectroscopy. The proton resonances due to NH and Hα were measured above and below the critical micelle concentration of the surfactants. From the differences in proton chemical shifts of the monomeric dipeptide surfactants and aggregation numbers, a model for packing of the monomeric polar head is proposed.