posted on 2019-09-26, 15:34authored byRegina
K. Ciszewski, Brittany P. Gordon, Benjamin N. Muller, Geraldine L. Richmond
Mixed surfactant
systems at the oil–water interface play
a vital role in applications ranging widely from drug delivery to
oil-spill remediation. Synergistic mixtures are superior emulsifiers
and more effective at modifying surface tension than either component
alone. Mixtures of surfactants with dissimilar polar head groups are
of particular interest because of the additional degree of control
they offer. The interplay of hydrophobic and electrostatic effects
in these systems is not well understood, in part because of the difficulty
in examining their behavior at the buried oil–water interface
where they reside. Here, surface-specific vibrational sum frequency
spectroscopy is utilized in combination with surface tensiometry and
computational methods to probe the cooperative molecular interactions
between a cationic surfactant cetyltrimethylammonium bromide (CTAB)
and a nonionic alcohol (1-hexanol) that induce the two initially reluctant
surfactants to coadsorb synergistically at the interface. A careful
deuteration study of CTAB reveals that hexanol cooperates with CTAB
such that both molecules preferentially orient at the interface for
sufficiently large enough concentrations of hexanol. This work’s
methodology is unique and serves as a guide for future explorations
of macroscopic properties in these complex systems. Results from this
work also provide valuable insights into how interfacial ordering
impacts surface tensiometry measurements for nonionic surfactants.