Redox-Induced Ion Pairing of Anionic Surfactants with Ferrocene-Terminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface

Abstract

Oxidoreduction of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of surface-active sodium n-alkyl sulfates (NaCnSO4) of 6, 8, 10, and 12 carbons is investigated by cyclic voltammetry and surface plasmon resonance. The effects of surfactant micellization and alkyl chain length on the redox response of the surface-tethered ferrocenes are examined. The SAM redox electrochemistry is sensitive to the surfactant aggregation state in solution. The nonideal behavior of the sodium alkyl sulfates at concentrations above the critical micelle concentration leads to a non-Nernstian variation of the SAM redox potential with concentration. The presence of micelles in solution results in decreased anodic-to-cathodic peak separations and anodic peak full widths at half-maximum. A longer alkyl chain length results in an increased ability of the alkyl sulfate anion to ion pair with the SAM-bound ferrocenium, resulting in oxidation of the ferrocene at lower potential. A comparison of the SAM redox potential at a fixed surfactant concentration of ideal behavior suggests a 4.5 × 10(4) difference in the ion-pairing abilities of the shorter-chain C6SO4(-) and longer-chain C12SO4(-). One-half of the available SAM-bound ferrocenes are oxidized in the NaCnSO4 electrolyte. Surfactant anions adsorb and assemble onto the SAM surface by specific ion-pairing interactions between the sulfate headgroups and oxidized ferrocenium species, forming an interdigitated monolayer in which the surfactant anions alternate between a heads-down and heads-up orientation with respect to the SAM. The work presented points to applications of ferrocenylalkanethiolate SAMs as anion-selective membranes, probes of micelle formation, and surfaces for the electrochemically switchable assembly of organosulfates.

Energetically efficient electrochemically tunable affinity separation using multicomponent polymeric nanostructures for water treatment

“…It was found previously that ferrocene moieties embedded in thicker organic films or farther away from electrode surfaces were more difficult to oxidize, resulting in higher E 0 values. 35,36 In our case, with a uniform spatial distribution of ferrocene sites throughout the polymer film (as evidenced by EDS), a thicker film may have led to a larger population of ferrocene moieties that were either buried more deeply in an organic PPY environment or were located farther away from the conductive surfaces (either CC or CNT); hence a thicker polymer film should result in a higher ensemble-averaged E 0 value. The thickness of a polymer film with uniformly distributed redox centers can be inferred from the slope (S iv ) of a ln(CV peak current)-ln(scan rate) plot obtained with a redox polymer electrode (RPE) model to simulate current responses (Section S4,ESI †).…”

Redox-Controlled Ion-Pairing Association of Anionic Surfactant to Ferrocene-Terminated Self-Assembled Monolayers

“…In this regard, we have investigated the redox-controlled association of surface-active sodium n-alkyl sulfates (NaC n SO 4 ) to SAMs of ferrocenyldodecanethiolate on gold (FcC 12 SAu, Scheme 1) and shown the feasibility of using the electrochemically induced pairing of counterions with surface-tethered redox moieties to drive the interfacial aggregation of ionic surfactants and surfactant-like molecules via an electrical stimulus. 6 Oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc + ) involves coupled electron-transfer and ion-pairing reactions: 3 …”

“…An earlier investigation focused on the effects of the micellization of NaC n SO 4 in aqueous solution and surfactant chain length (6,8,10, and 12 carbons) on the redox electrochemistry of FcC 12 SAu SAMs. 6 We demonstrated that alkyl sulfate anions pair with SAM-bound ferroceniums and E°′ shifts to lower potential with increasing chain length.…”

Colloidal chirality in wormlike micellar systems exclusively originated from achiral species: Role of secondary assembly and stimulus responsivity

“…G. W. Gokel has reported the first example of vesicle formation from redox-switched hydrophobic ferrocene derivatives [21], and sonication-induced redox-switched vesicles formed by ferrocene derivatives upon oxidation [22]. With respect to surfactant, control of viscoelasticity using redox reaction [23], and redox-induced ion pairing of anionic surfactants [24] is also investigated. By using ferrocenoyl phenylalanine, Zhang Y. et al present a supramolecular hydrogel that exhibit sharp phase transitions in response to a series of different stimuli, including redox, guest-host interactions, and pH changes [25].…”