Controlling Water Delivery to an Electrochemical Interface with Surfactants

Abstract

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 spectroelectrochemistry, 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.

Surfactants Accelerate Isotope Exchange-Based 18F-Fluorination in Water

“…The 19 F-precursor of the E[c(RGDyK)] 2 -derived peptide tracer (OPF-E[c(RGDyK)] 2 , 13), was obtained via conjugation of perfluorophenyl 4-(((dimethylamino)fluorophosphoryl)methyl)benzoate to E[c(RGDyK)] 2 in 43% yield (Scheme S12). All newly synthesized compounds were characterized by nuclear magnetic resonance spectroscopy ( 1 H NMR, 13 C NMR, 31 P NMR, and 19 F NMR) and mass spectrometry (MS). Chemical purities were measured by NMR, TLC, and HPLC.…”

“…S51), respectively. The CLogP value of the OPF-E[c(RGDyK)] 2 (13) was predicted to be −7.26 by ChemBioDraw Ultra 14.0 (Table S51).…”

“…Increasing k through harsh conditions (high temperatures and organic solvents), which could limit substrate scope and increase failure risk, is excluded in this study. Beyond individual structural changes, a universal strategy to reduce E a and increase k is to use inverse phase transfer catalysts (IPTC), especially surfactants, as catalysts. , Besides, the concomitant changes in dielectric property/polarity of the solvent system can decrease E a and increase the solubility/concentration of insoluble reactants …”

“…Beyond individual structural changes, a universal strategy to reduce E a and increase k is to use inverse phase transfer catalysts (IPTC), especially surfactants, as catalysts. 12,13 Besides, the concomitant changes in dielectric property/polarity of the solvent system can decrease E a and increase the solubility/concentration of insoluble reactants. 14 On the other hand, higher local concentrations of the reactants lead to a higher colliding chance, while lower net concentration of 19 F-substrate benefits higher A m .…”

Non-Covalent Interactions Mimic the Covalent: An Electrode-Orthogonal Self-Assembled Layer

“…The addition of amphiphiles to electrochemical systems is known to modulate key charge-transfer events. ,− It is theorized that these charge-transfer events are driven by the non-covalent electrostatic attraction of the charged amphiphile to the oppositely charged electrode surface, which creates a hydrophobic pocket. − This self-assembly has been posited to be potential-dependent, ,− e.g. , for a cationic amphiphile, the structure is formed negative of the potential of zero free charge (PZFC, the potential at which the interfacial field is the weakest).…”