Effect of Functional Group on the Monolayer Structures
of Biodegradable Quaternary Ammonium Surfactants
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Abstract
The monolayer structures
and conformational ordering of cationic
surfactants including the biodegradable quaternary ammonium molecules
have been systematically characterized by π–<i>A</i> isotherm, surface potential, atomic force microscopy (AFM), X-ray
photoelectron spectroscopy (XPS), and sum frequency generation (SFG)
vibrational spectroscopy. It was found that the monolayer of the typical
dialkyl dimethylammonium on the water surface was less densely packed
along with many conformational <i>gauche</i> defects. The
packing density and ordering of these monolayers were improved as
halide ions were added to the subphase. A similar condensation effect
was also observed when amide or ester groups are present in the alkyl
tails of the surfactant. These results are discussed on the basis
of the repulsive electrostatic interactions between the terminal ammonium
moieties, the hydrogen bonding between the functional groups in the
alkyl chains, as well as the flexibility of the alkyl chains in these
surfactants. The present study is crucial to understanding the relationship
between the interfacial structures and the functionalities of the
biodegradable quaternary ammonium surfactants