COMPARISON OF DIFFERENT SPREADING TECHNIQUES FOR MONOLAYERS AT THE AIR-WATER-INTERFACE BY EXTERNAL INFRARED REFLECTION-ABSORPTION SPECTROSCOPY

The single shot method (SSM) and the discontinuous compression method (DCM) are compared for the film-forming substances 1-hexadecanol, hexadecanoic acid methyl ester, and L-alpha-dipalmitoylphosphatidylcholine by external infrared reflection-absorption spectroscopy. In particular, the tilt angles of the molecules, the conformational order, and the headgroup structure are analyzed. It is shown that for 1-hexadecanol in the gaseous region different tilt angles are observed between the monolayers spread by the SSM and those compressed by the DCM. In the case of hexadecanoic acid methyl ester the molecules disseminated on the water surface by the SSM are oriented more vertically, the ester carbonyl group is mainly deprotonated, and the hydrophobic chain is conformationally more ordered in comparison with the monolayer compressed by the DCM. Upon relaxation the monolayer spread by SSM approaches the characteristics of the compressed monolayer, although after 80 min this state was not attained completely. L-alpha-Dipalmitoylphosphatidylcholine monolayers spread by the SSM relaxate by achieving a higher conformational order, smaller tilt angles, and a higher deprotonation grade of the ester carbonyl groups in comparison with the initial state. A tentative explanation of the present results is given on the basis of a kinetic model recently published by Buontempo and Rice.