ANOMALOUS PH-DEPENDENCE OF THE COEXISTENCE PRESSURE OF THE POLYMERIZABLE 2-CHAIN N-LIPID METHYL-BIS(PENTACOSADIINOYL-OXYETHYL)-AMINE

Molecular films from polymeric materials play an important role in bas ic research as well as in technology. We have synthesized the double c hain diacetylenic ammonium lipid N-bis-(10, 12-pentacosadiinoyl)-oxyet hyl)-N-methyl-amine (ONCO). We have characterized monomolecular films at the air/water interface by means of microfluorescence filmbalance t echniques and by atomic force microscopy (AFM). ONCO forms stable mono molecular films that exhibit a fluid-solid phase transition with a tra nsition enthalpy of 90 kJ/mol at 10 degrees C and neutral pH. The coex istence pressure was found to decrease with decreasing protonation, wh ich is in contrast to the commonly found Coulomb mechanism. A change i n the chain packing due to a different nitrogen bond angle is discusse d as a possible mechanism. This model is corroborated by the finding t hat crystals at high and at low pH differ in their polymerization prop erties as measured by microfluorescence.