The properties of C15, C16, C18 and C20-alcohols, fatty acids and fatty acid esters are investigated by external infrared reflection-absorption spectrometry in the range 3000 - 1000 cm-1. Analysis of the methylene stretching vibration shows that an increasing space requirement of the hydrophilic headgroup (fatty acid ester > fatty acid >= alcohol) for the same chain length leads to higher chain disorder (i.e., more gauche conformers). However, for a given headgroup the prolongation of the alkyl-chain generally results in an increased hydrophobic interaction and thus in a higher chain-order, i.e., the molecules attain a more transconformation.
Infrared Reflection-Absorption Spectroscopy (IRAS) is a very useful tool to investigate the conformation of amphiphiles at the air/water interface. Herein, monomolecular films consisting of 1-hexadecanol were studied using different incident angles and states of polarization. It is shown that for s-polarization the bands are negative, and the intensities decrease with increasing angles of incidence. For p-polarization, negative bands are only encountered at small incidence angles, while at large incidence angles the bands become positive.
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