Hydrogen-bond-induced hysteresis in the compression/relaxation of monolayer films

Two stereoisomers of surfactants were synthesized in which a diketopiperazine ring was inserted between a hydrocarbon chain (of variable length) and an anionic headgroup. It was found (by HPLC, conductivity, surface tension, and diffusion NMR) that these compounds have low solubilities in water, remarkably high Krafft temperatures, and low critical micelle concentrations. Of particular interest were the pressure/area isotherms of insoluble monolayers of nonionic analogues of the amphiphiles. These isotherms showed an unprecedented hysteresis but only during the first compression/relaxation cycle. Additional cycles were normal in that they lacked the original hysteresis. These results were attributed to diketopiperazine rings that initially lie flat on the water surface at the air/water interface. Compression flips the diketopiperazine rings so that they now reside roughly perpendicular to the interface in a stable hydrogen-bonded orientation where the rings occupy far less space.