posted on 2021-11-18, 12:39authored byAbdullah
O. Ba-Salem, Jean Duhamel
The
cationic gemini surfactant PyO-3-12 was designed to include
two dimethyl ammonium groups, one dodecyl tail, and 1-pyrenemethyl
hexyl ether tail into the structure of the surfactant. The pyrenyl
label ensured that the fluorescence of pyrene could be employed to
probe the behavior of PyO-3-12 at the molecular level. The introduction
of the oxygen atom in the β-position to pyrene was found to
be critical for restoring the sensitivity of the pyrenyl label to
the polarity of its environment. The properties of PyO-3-12 were characterized
in water by surface tension and a fluorescence methodology that involved
the global model-free analysis (MFA) of the pyrene monomer and excimer
fluorescence decays to provide quantitative information about the
state (unassociated-vs-aggregated) of PyO-3-12. The
MFA was combined with a fluorescence quenching study with 2,6-dinitrotoluene
to determine the size of the PyO-3-12 micelles. PyO-3-12 was found
to behave like a typical gemini surfactant, exhibiting a critical
micelle concentration (CMC) of 0.38 (±0.05) mM and an aggregation
number (Nagg) equal to 23 (±2). Besides
allowing PyO-3-12 to probe the polarity of its environment, the oxygen
atom in the β-position next to pyrene brought some pyrenyl labels
closer to the interface between the micellar interior and the aqueous
phase, in a process that increased the effective volume of the hydrophobic
part of PyO-3-12. This led to an increase in the packing parameter
of PyO-3-12 and, consequently, an increase in Nagg compared to the Nagg value
of 14 (±0.2) obtained for Py-3-12, a gemini surfactant, whose
chemical structure was similar to that of PyO-3-12 but without the
oxygen in the β-position to pyrene. The methodology described
in this study to prepare and characterize pyrene-labeled surfactants
is general and can be applied to study any pyrene-labeled surfactant
and its interactions with oppositely charged macromolecules.