posted on 2023-01-23, 14:10authored byAnuj K. Pennathur, Cindy Tseng, Noemi Salazar, Jahan M. Dawlaty
Most electrochemical reactions require delivery of protons,
often
from water, to surface-adsorbed species. However, water also acts
as a competitor to many such processes by directly reacting with the
electrode, which necessitates using water in small amounts. Controlling
the water content and structure near the surface is an important frontier
in directing the reactivity and selectivity of electrochemical reactions.
Surfactants accumulate near surfaces, and therefore, they can be used
as agents to control interfacial water. Using mid-IR spectro-electrochemistry,
we show that a modest concentration (1 mM) of the cationic surfactant
CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a
large effect on the interfacial water concentration, changing it by
up to ∼35% in the presence of an applied potential. The major
cause of water content change is displacement due to the accumulation
or depletion of surfactants driven by potential. Two forces drive
the surfactants to the electrode: the applied potential and the hydrophobic
interactions with the water in the bulk. We have quantified their
competition by varying the water content in the bulk. To our knowledge,
for the first time, we have identified the electrochemical equivalent
of the hydrophobic drive. For our system, a change in applied potential
of 1 V has the same effect as adding a 0.55 mole fraction of water
to the bulk. This work illustrates the significance of surfactants
in the partitioning of water between the bulk and the surface and
paves the way toward engineering interfacial water structures for
controlling electrochemical reactions.