THE CONFORMATIONAL ORDER AND HEADGROUP STRUCTURE OF LONG-CHAIN ALKANOIC ACID ESTER MONOLAYERS AT THE AIR-WATER-INTERFACE

The structural properties of long-chain alkanoic acid ester monolayers are investigated at the air/water interface by infrared reflection-absorption spectroscopy. It is shown that the conformational order of alkanoic acid methyl ester monolayers increases with increasing acyl alkyl-chain length and/or decreasing temperature. For expanded monolayers the alkanoic ester headgroup is E-configured, where the alcoholic alkyl chain substantially is pointing out of the water, while for compressed states (or condensed domains) the headgroup is Z-configured with an alcoholic alkyl component directed into the water In the compressed states (Z-form) the ester-carbonyl group is found to be unprotonated, presumably as a result of a shielding by the alcoholic alkyl component of the ester. For alkanoic acid methyl ester monolayers this shielding depends on the conformational order and the tilt angle of the molecules, while for the respective ethyl- and n-propyl esters an unprotonated carbonyl group was also found for states with a low conformational order. By comparing the IR-spectra of 1-alkanol, fatty acid and alkanoic acid ester monolayers evidence is presented that fatty acid monolayers are stabilized increasingly with decreasing chain lengths by forming hydrogen-bridges between adjacent molecules.