2 research outputs found
Structure and Lateral Interaction in Mixed Monolayers of Dioctadecyldimethylammonium Chloride (DOAC) and Stearyl Alcohol
π–<i>A</i> isotherms, atomic force microscopy
(AFM), and sum frequency generation (SFG) vibrational spectroscopy
are employed to investigate the molecular structure and lateral interactions
in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC)
and stearyl alcohol (SA) at air/water and air/solid interfaces. To
avoid possible interference between the two molecules in the SFG spectroscopic
measurements, perprotonated DOAC and perdeuterated SA (dSA) were used.
The thermodynamic analyses for the π–<i>A</i> isotherms show that DOAC is miscible with dSA. SFG observations
reveal that DOAC molecules become conformationally ordered as dSA
molecules are introduced into the monolayer. AFM observations demonstrate
coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer
with ratios different from their initial composition in the subphase.
The present study suggests that DOAC molecules in the mixed monolayer
are condensed by mixing with dSA in which the repulsive interactions
between positively charged head groups of the DOAC molecules are largely
reduced along with an increase of van der Waals interactions with
dSA
Effect of Functional Group on the Monolayer Structures of Biodegradable Quaternary Ammonium Surfactants
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