IN-SITU INVESTIGATION OF SATURATED LONG-CHAIN FATTY-ACIDS AT THE AIR-WATER-INTERFACE BY EXTERNAL INFRARED REFLECTION-ABSORPTION SPECTROMETRY

Saturated long-chain fatty acids were investigated at the air/water interface by infrared reflection-absorption spectrometry for the temperatures 288 and 294 K and for pH values of 2,6,8, and 9. In the case of octadecanoic acid the chain order is comparable for pH 2-8, while at pH 9 the order is strongly increased. The chain order decreases with decreasing chain length (C1B > C-16 > C-15), although this effect is not so pronounced for pH 2 as for pH 6. At pH2 and pH6 three peaks are observed at 1739, 1720, and 1704 cm-1, which can be attributed to the stretching vibrations of the unprotonated, monoprotonated, and double protonated carbonyl groups, respectively. Upon compression and/or with decreasing pH the band representing the double protonated carbonyl group is enhanced, which implies that the fatty acid molecules are linked together by hydrogen bridges. This effect decreases with prolongation of the alkyl chain, which implies a decrease in order. However, this decrease in order is smaller than the gain due to hydrophobic interaction which is known to increase with prolongation of the alkyl chain. For pH 9 the carboxylic acid group is strongly deprotonated and both the antisymmetric and the symmetric carboxylate stretching vibration bands are split.