Electroactive Self-Assembled Monolayers Detect Micelle Formation

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Electroactive Self-Assembled Monolayers Detect Micelle Formation Eric R. Dionne and Antonella Badia* Département de chimie, FRQNT Centre for Self-Assembled Chemical Structures, and Regroupement québécois sur les matériaux de pointe, Université de Montréal, C.P. 6128 succursale Centre-ville, Montréal, Quebec H3C 3J7, Canada S Supporting Information *

ABSTRACT: The interfacial electrochemistry of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold (FcC12SAu) electrodes is applied to detect the micellization of some common anionic surfactants, sodium nalkyl sulfates, sodium n-alkyl sulfonates, sodium diamyl sulfosuccinate, and sodium dodecanoate, in aqueous solution by cyclic voltammetry. The apparent formal redox potential (E°′SAM) of the FcC12SAu SAM is used to track changes in the concentration of the unaggregated surfactant anions and determine the critical micelle concentration (cmc). The effect of added salt (NaF) on the sodium alkyl sulfate concentration dependence of E°′SAM is also investigated. Weakly hydrated anions, such as ClO4−, pair with the electrogenerated SAMbound ferroceniums to neutralize the excess positive charge created at the SAM/electrolyte solution interface and stabilize the oxidized cations. E°′SAM exhibits a Nernstian-type dependence on the anion activity in solution. Aggregation of the surfactant anions into micelles above the cmc causes the free surfactant anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°′SAM versus the logarithm of the concentration of anionic surfactant. The concentration at which this deviation occurs is in good agreement with literature or experimentally determined values of the cmc. The effects of Ohmic potential drop, liquid junction potential, and surfactant adsorption behavior on E°′SAM are addressed. Ultimately, the E°′SAM response as a function of the anionic surfactant concentration exhibits the same features reported using potentiometry and surfactant ion-selective electrodes, which provide a direct measure of the free surfactant anion activity, thus making FcC12SAu SAM electrodes useful for the detection of surfactant aggregation and micelle formation. KEYWORDS: chemically modified electrode, self-assembled monolayer, ferrocenylalkanethiolate, surface-confined redox reaction, ion pairing, anionic surfactants, critical micelle concentration

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INTRODUCTION

We have investigated the redox-controlled reversible association of anionic surfactants to ferrocenyldodecanethiolate (FcC12SAu) SAMs (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 related molecules via an electrical stimulus.17,18 We previously reported the reversible Faradaic electrochemistry of SAMs of FcC12SAu in aqueous solutions of sodium n-alkyl sulfates (CnSO4Na) as the electrolyte.17 We showed using cyclic voltammetry that the CnSO4− anions pair with the oxidized ferrocenium species, and the apparent formal redox potential of the SAM was found to depend on chain length. Surface plasmon resonance (SPR) was further used to investigate the association of the surfactant anions to the oxidized SAM and compare the ion-pairing abilities of CnSO4Na of different chain lengths via competitive association from binary mixtures.18

Organized monolayer films of redox moieties attached to the metal electrode surface by various lengths of an organothiolate spacer were originally developed for investigations of the interfacial electron transfer between an electronic conductor and a reversible redox couple.1 Due to the attractive electrochemical properties of ferrocene (facile electron transfer, low oxidation potential, and two stable redox states), ωferrocenylalkanethiolates on gold are the most studied electroactive self-assembled monolayer (SAM) system.2−8 Researchers have notably investigated these and related ferrocene-terminated monolayers on silicon for applications in molecular electronics, including diodes, 9 transistors, 10 switches,11 and information storage/transfer12. Moreover, the changes in surface wettability,13 molecular orientation,14 and monolayer volume expansion8 that accompany the oxidation of the SAM-bound ferrocene (neutral) to ferrocenium (cationic) have attracted interest in the use of ferrocene-terminated SAMs as electrochemically switchable surfaces for the control of liquid flow,13 low-power switching of liquid crystals,15 and micromechanical actuation,16 respectively. © XXXX American Chemical Society

Received: November 28, 2016 Accepted: January 18, 2017 Published: January 18, 2017 A

DOI: 10.1021/acsami.6b15290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

oxidized cation.4−6,20 The ability of the anion to pair with the ferrocenium in the typically nonpolar and sterically crowded environment of the SAM determines the electrochemical response: formal redox potential, number of surface-available ferrocenes oxidized, and redox stability upon repeated potential cycling.4−6 Weakly hydrated or “hydrophobic” monovalent anions, such as PF6−, ClO4−, and BF4−, pair more effectively with the ferrocenium than “hydrophilic” ones (e.g., Cl− and F−).5,6 The formal redox potential of the SAM-bound ferrocenes (E°′SAM) at 25 °C is given by the Nernst equation for the electron-transfer and ion-pairing reactions

Scheme 1. Schematic Illustration of the Oxidation-Induced Pairing of Surfactant Anions to a ω-Ferrocenylalkanethiolate SAM on Golda

E o′SAM = E oSAM − (0.05916 V)· log − (0.05916 V)· log a X−

a

Reproduced from ref 18. Copyright 2015 American Chemical Society.

FcSAM ⇌ Fc+SAM + e− (1)

The formation constant K of the 1:1 Fc+X− pair is expressed as

K=

ΓFc+X− ΓFc+a X−

(3)

where E°SAM is the standard redox potential and ΓFc is the surface concentration of ferrocene.4,5,19 The last term in eq 3 suggests a linear dependence of E°′SAM on the logarithm of the anion activity in solution, analogous to the non-Faradaic potential measured by ion-selective electrodes in potentiometry.21 A Nernstian-type relation is indeed observed between E°′SAM and the logarithm of the molar concentration of the electrolyte anion for hydrophobic anions that ion pair strongly with the SAM-bound ferroceniums.4,5,19,20 The interfacial electrochemistry of FcC12SAu SAMs in aqueous solutions of sodium salts of anionic surfactants (Table 1) is investigated here by cyclic voltammetry. Anionic surfactants are the most commonly employed surfactants (i.e., detergents, foaming agents, wetting agents, dispersants, and soaps).22 In sufficiently dilute solutions, anionic surfactants behave like simple electrolytes. They typically form micellar aggregates comprised of tens to hundreds of surfactant anions when the surfactant concentration exceeds the cmc.23 Above the cmc, free (unaggregated) surfactant anions are in equilibrium with micelles. Measurements with membranebased surfactant ion-selective electrodes, whose potential response is specifically proportional to the activity of free surfactant anion in solution, show that micelle formation causes the surfactant anion activity to deviate from the concentration of added salt, as shown in Figure 1 for C12SO4Na.24−27 The surfactant anion activity above the cmc is governed by the chemical equilibria between the free surfactant anions, counterions, and micelles.24

The SAM redox response was also found to be sensitive to the aggregation state of the surfactant in solution.17 The present work builds on this last findingit focuses on using the apparent formal potential of the FcC12SAu SAM to detect micelle formation in aqueous solution. In addition to demonstrating the feasibility of using ferrocenylalkanethiolate SAM-modified electrodes to determine the critical micelle concentration (cmc) of anionic surfactants, we address the factors that influence the measured redox potential. The electrochemical oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc+) proceeds via coupled electrontransfer and ion-pairing reactions:4−6,19

Fc+SAM + X−(aq) ⇌ (Fc+X−)SAM

ΓFcK ΓFc+X−

(2)

where ΓFc+ and ΓFc+X− are the surface concentrations of ferrocenium and anion-paired ferocenium and aX− is the activity of the anion X− in solution.4,5 Pairing of the anion with the ferrocenium neutralizes the excess positive charge generated at the SAM/electrolyte solution interface and stabilizes the

Table 1. Critical Micelle Concentrations of the Anionic Surfactants Investigated cmc (mM) anionic surfactant C12SO4Na C10SO4Na C8SO4Na C6SO4Na C12SO3Na C6SO3Na (C6O2)(C7O2)C1SO3Na C11CO2Na a

solution

lit. (25 °C)

aq soln 0.3 M NaF aq soln 0.3 M NaF aq soln aq soln aq soln aq soln aq soln aq soln

8.0−8.223,45,62,63

exptl 1.1a

33.0−33.245,62,63 10.1a 45,62

130 42023,62 9.063 46063 5364 25.7;65 27.263

440b 65b

E°′SAM 8.1 1.0 33.2 9.9 130 420 9.0 460 67.6 27.5

cmc determined by surface tension measurements. bcmc determined by electrical conductivity measurements. See Experimental Section for details. B


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contained 0.15% (FcC12S)2 after 1 year. The following reagents were purchased and used without further purification: sodium n-hexyl sulfate (C6SO4Na, 99%, Research Plus), sodium n-hexyl sulfonate (C6SO3Na, ≥98%, Sigma-Aldrich), sodium n-octyl sulfate (C8SO4Na, ≥99%, Sigma-Aldrich), sodium n-decyl sulfate (C10SO4Na, 99%, Research Plus), sodium n-dodecyl sulfate (C12SO4Na, 99%, SigmaAldrich), sodium n-dodecyl sulfonate (C12SO3Na, ≥99%, EMD Millipore), sodium diamyl sulfosuccinate ((C6O2)(C7O2)C1SO3Na, ≥98%, Cytec), sodium dodecanoate (C11CO2Na, 99−100%, SigmaAldrich), sodium perchlorate (ACS reagent grade, Sigma-Aldrich), sodium chloride (ACS reagent grade, Anachemia), and sodium fluoride (ACS reagent grade, J.T. Baker Chemical Co.). The surfactant solutions were prepared immediately before the electrochemical or surface tension measurements with deionized water (resistivity of 18.2 MΩ cm and total organic carbon of ≤5 ppb) that had been purged for at least 30 min with nitrogen gas. Choice of Ferrocenylalkanethiol and Gold Substrate. The gold electrode surface was functionalized using a FcCnSH with a −C12− linker for the following reasons. Alkyl−alkyl chain interactions increase with increasing chain length,9 and these are expected to stabilize the SAM. However, longer chain ferrocenylalkanethiols, such as FcC14SH and FcC16SH, are poorly soluble in absolute ethanol at the millimolar concentration typically used for alkanethiol self-assembly.1,29 The quantities of ferrocenium electrogenerated in these SAMs (i.e., 6.2−6.3 × 10−10 mol cm−2, results not shown) are significantly greater than the ferrocene surface coverage of 4.85 × 10−10 mol cm−2 calculated from the planar cross-sectional area through the center of a ferrocene sphere of 0.66 nm diameter.3 An even (−C12−) versus odd (−C11−)8,16,30,31 linker chain was chosen because of the more favorable chain−chain van der Waals interactions associated with a smaller tilt angle of the ferrocene with respect to the surface normal. Polycrystalline thin films of goldthe most common substrates for SAMs formed from alkanethiolswere used as the working electrode.29 The FcC12SAu SAMs were assembled using standard conditions.1,29 Preparation of FcC12SAu SAMs. B270 glass slides (Esco Products Inc.) were cleaned by immersion in piranha solution (3:1 v/v H2SO4/30% H2O2) for 5 min at room temperature. The glass slides were rinsed copiously with deionized water, sonicated thrice in deionized water to completely remove traces of sulfuric acid, sonicated once in absolute ethanol, and dried under a stream of nitrogen gas. A 2 nm thick primer layer of titanium (99.99%, Alfa Aesar) followed by a 50 nm layer of gold (99.99%, Kitco Metals, Inc.) were deposited onto the clean glass by thermal evaporation at rates of 0.01 and 0.02 nm s−1, respectively, using a VE-90 vacuum evaporator (Thermionics Vacuum Products) equipped with a 1 kVA resistive power supply and a turbomolecular pump. The metal thickness and deposition rate were monitored using a calibrated quartz crystal microbalance. The deposition of metal was initiated once the pressure inside the deposition chamber reached ∼5 × 10−7 Torr. The pressure at the end of the gold evaporation was ∼2 × 10−6 Torr. Radiative heating from the evaporation boats increased the sample temperature to 190 ± 10 °C. The evaporated gold film has an average grain diameter of 100 ± 40 nm and a surface roughness factor of 1.07 ± 0.02 over an area of 1.0 μm2, as determined by atomic force microscopy (Figure S1, Supporting Information). The gold thin film-coated glass slides were used for both cyclic voltammetry and SPR. The gold-coated slides were removed from the deposition chamber of the evaporator and immediately immersed in a 0.2 mM solution of FcC12SH in absolute ethanol for 18−24 h at room temperature.1,29 The incubation vials were kept in the dark. Prior to use, a FcC12SAu SAM-modified substrate was removed from the FcC12SH incubation solution, rinsed copiously with absolute ethanol followed by deionized water, and blown dry with nitrogen. Cyclic Voltammetry. All cyclic voltammograms were acquired in a custom-built, one-compartment three-electrode Teflon cell. The FcC12SAu SAM-modified substrate served as the working electrode (WE). The active area of the WE exposed to the electrolyte solution through an O-ring seal is 0.48 cm2, as determined by chronocoulom-

Figure 1. Concentration dependence of the free ion activities for sodium n-dodecyl sulfate (C12SO4Na), an archetypal anionic surfactant, in water. Ion activities measured by potentiometry using sodium and liquid ion-exchange membrane-based surfactant ionselective electrodes. Data is from ref 25. Dashed line indicates ideal solution behavior for which the ion activity aion is equal to the concentration of anionic surfactant [C12SO4Na].

We demonstrate herein that the apparent formal potential of the FcC12SAu SAM can be used to track changes in the concentration of the free surfactant anion in solution and determine its cmc. We begin by investigating a homologous series of sodium n-alkyl sulfates CnSO4Na (where n = 6, 8, 10, and 12), whose aggregation behavior has been well characterized,24 and then move onto other anionic surfactants: sodium n-alkyl sulfonates (CnSO3Na, where n = 6 and 12), sodium diamyl sulfosuccinate ((C6O2)(C7O2)C1SO3Na), and sodium dodecanoate (C11CO2Na). The cmc’s of ionic surfactants, typically in the 10−3−10−1 M range, can necessitate carrying out Faradaic electrochemistry in weakly supported aqueous media. The effect of uncompensated resistance on the cyclic voltammogram and apparent formal potential is therefore considered. This work dif fers f rom previous investigations involving the use of redox-active molecules as electrochemical probes to determine the cmc of surfactants by cyclic voltammetry.28 It exploits the pairing of surfactant anions with surface-conf ined redox moieties, while the latter rely on partitioning of a f reely dif f using (soluble) electroactive species between the bulk solution and surfactant micelles. Potentiometry (ion-selective electrode) and electrical conductivity are the electrochemical methods most often employed for the quantification of ionic surfactants and/or determination of the cmc.28 The ferrocene-terminated SAM/Au electrode potentially presents advantages over these two methods. Unlike membrane-based surfactant ion-selective electrodes,24−27 there are no wet components, which makes for easier handling and operation. Detecting relatively small changes in the solution electrical conductivity due to the aggregation of ionic surfactants in the presence of high concentrations of background electrolyte is problematic. We demonstrate herein the ease of FcC12SAu SAMs to determine the cmc of CnSO4Na in the presence of added salt.

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EXPERIMENTAL SECTION

Materials. 12-Ferrocenyl-1-dodecanethiol (FcC12SH) was synthesized using procedures adapted from the literature. The synthetic details and compound characterization data are given in the Supporting Information. The FcC12SH was stored at 4 °C and C


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ACS Applied Materials & Interfaces etry using K3FeCN6(aq).32 A coiled platinum wire (99.99%, Kitco Metals, Inc.) and Ag/AgCl electrode (3 M NaCl, BASi) were used as the counter (CE) and reference (RE) electrodes. The CE-WE and REWE separation distances were 1.7 and 1.2 cm, respectively. Experiments were performed at a temperature of 21 ± 1 °C, except for C12SO3Na, for which a thermostated cell was used, and the temperature kept at 35.0 ± 0.1 °C to prevent precipitation of surfactant. Cyclic voltammograms were acquired at a potential scan rate of 10 mV s−1 using a SP-150 potentiostat (BioLogic Science Instruments) equipped with a low-current module and built-in electrochemical impedance spectroscopy analyzer. The cell resistance (RΩ) was determined by impedance measurement using a frequency of 100 kHz and a test potential of 0 V versus the RE. The Ohmic potential (iR) drop between the WE and the RE was compensated in cyclic voltammetry by applying an electronic positive feedback correction at 85% of RΩ. The average of the anodic and cathodic peaks of the cyclic voltammograms was used as the apparent formal redox potential E°′SAM. The surface concentration of electrogenerated ferrocenium ΓFc+ was determined using eq 4

ΓFc+ =

Impedance measurements were carried out at 0 V by applying frequencies from 0.1 MHz to 1 Hz with a sinus amplitude of 20 mV to the WE. The conductivity was determined from the extrapolated solution resistance (Figure S2) and the cell constant obtained using 0.01000 M KCl. Surface Tension Measurements. The surface tensions of surfactant solutions were measured by capillary rise at 21 ± 1 °C using the apparatus and protocol described in ref 34. The capillary tube radius was determined using five pure liquids with different surface tensions (i.e., hexanes 17 mN m−1, methanol 22.5 mN m−1, acetonitrile 28 mN m−1, dimethyl sulfoxide 44 mN m−1, and deionized water 72.5 mN m−1).

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RESULTS AND DISCUSSION FcC12SAu SAM Electrochemistry in NaClO4 Solution. To highlight the distinctive electrochemical behavior of the FcC12SAu SAMs in aqueous solutions of anionic surfactant, Figure 2A shows the cyclic voltammograms (CVs) recorded in

Qa nFA

(4)

where Qa is the charge associated with ferrocene oxidation determined by integration of the sigmoidal baseline-corrected anodic voltammetric peak, n is the number of electrons involved in the electron-transfer process (n = 1 for the ferrocene/ferrocenium couple), F is the Faraday constant, and A (= 0.48 cm2) is the surface area of the exposed FcC12SAu substrate electrode.1 Surface Plasmon Resonance (SPR). SPR measurements were carried out in the Kretschmann-type attenuated total reflection configuration with a computer-controlled SR7000 single-channel instrument (Reichert Inc.). The instrument uses stationary optics and a divergent, fan-shaped light beam from a LED source of finite spectral bandwidth (λ = 780 ± 10 nm) to simultaneously interrogate a range of incident angles (Θ = 48−66°). The intensity of the reflected light is measured using a 3696-pixel linear CCD array. The temperature at the gold/liquid interface is controlled to within ±0.015 °C by a Peltier device. A custom-built electrochemical cell fitted with RE (Ag/AgCl, 3 M NaCl electrode inserted in a doublejunction chamber, BASi) and CE (platinum-coiled wire) was mounted onto the FcC12SAu SAM-modified gold-coated surface of the B270 glass slide (WE) in optical contact with the prism. The current and pixel position of minimum reflected light intensity were simultaneously recorded under stationary solution conditions as the applied potential was cycled at a rate of 5 mV s−1. Experiments were carried out at 25.0 °C. The shift in the pixel position of minimum reflected light intensity was converted to a resonance angle change (ΔΘmin) using the pixel-toangle relation of 1 pixel = 0.00506° determined by calibration of the SR7000 instrument using binary mixtures of ethylene glycol and water of different refractive index.33 The surface concentration or coverage (ΓSPR) of adsorbed surfactant was determined from the ΔΘmin value measured at the most anodic potential as detailed previously in ref 18. Liquid Junction Potential Measurements. The variation of the liquid junction potential at the aqueous solution/reference electrode interface with the electrolyte concentration was verified using two Ag/ AgCl (3 M NaCl) reference electrodes and two half cells, one containing 10.0 mL of 3.00 M NaCl and the other 10.0 mL of the electrolyte of interest. The half-cells were contacted by a salt bridge of 3.00 M NaCl in agar. Electrical Conductivity Measurements. The conductivities of aqueous solutions of anionic surfactant were determined by electrochemical impedance spectroscopy with a SP-150 potentiostat. The measurements were carried out in a one-compartment, custom-built, glass cell thermostated at 25.0 ± 0.1 °C. A volume of 10.0 mL of the aqueous surfactant solution was pipetted into the cell. The cell was then capped with a Teflon cover, which also acted as the support for three platinum-coiled wire electrodes. The glass cell, surfactant solution, and electrodes were allowed to reach thermal equilibrium.

Figure 2. Electrochemistry of FcC12SAu SAMs in NaClO4(aq) (A−C) and NaClO4(aq)/1.00 M NaCl(aq) (D−F). Selected CVs as a function of the NaClO4 concentration (A and D). Apparent formal potential E°′SAM vs logarithm of the NaClO4 concentration (B and E). Anodic charge density Qa vs logarithm of the NaClO4 concentration (C and F). Dashed lines are linear regressions in B and E and indicate the average Qa value in C and F.

aqueous solutions containing different concentrations of NaClO4, a nonsurfactant. NaClO4 is a commonly employed electrolyte for this type of SAM.1 Perchlorate is one of the hydrophobic anions with a high propensity to form a single, specific ion pair with the SAM-bound ferrocenium.4−6 The CVs, which resemble those reported by others on evaporated gold thin film electrodes,19,30,35−37 indicate nonideal, reversible electrochemical behavior. It is important to note that ideal or nearly ideal electrochemistry (i.e., Langmuir isotherm conditions) of surface-tethered ferrocenes, characterized by single symmetrical redox peaks with full widths at half-maximum of 90.6 mV and no peak separation,38 is generally only achieved in mixed SAMs in which single isolated (noninteracting) ferrocenylalkanethiolates are surrounded by nonelectroactive alkanethiolates.2,3,39 Two sets of redox peaks are present. At D


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ACS Applied Materials & Interfaces 1.00 M NaClO4, the first pair (of lower peak current densities) occurs at Eo′SAM of 274 ± 16 mV, and the second set (of higher peak current densities) is at Eo′SAM of 348 ± 4 mV (average and standard deviation of six different FcC12SAu SAMs; see Figure S3 for an example of the peak deconvolution). The mean anodic−cathodic peak separation (ΔEp) for the first and second pair of peaks is 20 ± 10 and 12 ± 5 mV, respectively, while the full widths at half height of the anodic peaks (ΔEafwhm) are 91 ± 15 mV for the lower potential peak and 118 ± 25 mV for the higher potential peak. The relative intensities of the two sets of peaks can vary from SAM to SAM prepared under identical conditions.8 Different interpretations of the dual-peak voltammetry exist in the literature: differences in the local packing density or order and intermolecular interactions of the ferrocenylalkanethiolates, ferrocenes buried in the SAM versus electrolyteexposed ferrocenes due to the strain caused by the mismatch between the diameters of the ferrocene units and methylene chains or double-layer effects, which refer to the effects of the interfacial ion spatial distribution on the interfacial redox reaction.1,30,40−42 We propose that the first pair of peaks at lower potential is from ferrocenes located at domain boundaries and defect sites, where structural disorder reduces the degree of ferrocene−ferrocene interaction, and the higher potential peaks originate from the strongly interacting ferrocenes located inside domains, for which the redox reaction is rendered more unfavorable due to electrostatic repulsion between proximal ferroceniums. These peak assignments are based on the effect of the local electrostatics of “isolated” (noninteracting) versus “clustered” (interacting) ferrocenes on the redox potential of binary SAMs reported by Lee et al.7 and on the results of a combined scanning tunneling microscopy and cyclic voltammetry investigation carried out by S. Fujii et al.40 of the place exchange of a CH3(CH2)5S− by a Fc(CH2)11S− in a CH3(CH2)5SAu SAM. Both sets of anodic and cathodic peaks shift to higher potential as the NaClO4 concentration is decreased from 1.00 M to 1.00 mM (Figure 2A). An electronic positive feedback compensation was applied to the CVs to correct for variations in the uncompensated cell resistance with electrolyte concentration (see section that follows). A linear relationship, as predicted by eq 3 and reported previously by other groups,4,5,19,20 is observed between Eo′SAM and the logarithm of the ClO4− concentration (Figure 2B) with slopes of −47 ± 2 (r2 = 0.9921) and −43.8 ± 0.5 mV decade−1 (r2 = 0.9993) for the lower and higher potential peaks, respectively. Slopes that differ from the Nernst value of −59 mV decade−1 and range from −46 to −67 mV decade−1 have been reported for ferrocene- and pentaamminepyridineruthenium-terminated SAMs with inorganic, monovalent electrolyte anions.4,5,19,20,43 The slope deviations from the expected value are attributed to activity effects and varying liquid junction potentials at the aqueous electrolyte solution/reference electrode interface (Table S1). Ju et al. show using a FcCO2C11SAu SAM that these can be reduced and/or eliminated through the use of a high ionic strength of a background electrolyte with poor ionpairing ability.5 Figure 2D and 2E shows the variation of E°′SAM in the presence of 1.00 M NaCl, chosen because Cl− has a low ion-pairing ability compared to ClO4−,5,6 and so that the background electrolyte ions have the same mobilities as the ions present inside the Ag/AgCl reference electrode, which contains 3 M NaCl. The liquid junction potential measured between 3.00 and 1.00 M NaCl is 0 mV and remains 0 mV in

the presence of added NaClO4 in the 35−200 mM concentration range investigated in Figure 2D (Table S1). The CVs exhibit a principal set of redox peaks with shoulders on the negative side of each peak. A slope of −58 ± 2 mV decade−1 (r2 = 0.9948) is obtained, close to the −59 mV decade−1 expected for pairing of the ferrocenium cation with a single ClO4− anion. Integration of the anodic peaks in the CVs recorded for the different NaClO4 concentrations shows that there is no significant variation (within the uncertainty associated with defining the background charging current) in the electrogenerated ferrocenium surface concentration ΓFc+ (Figure 2C and 2F). The average of 4.6 (±0.3) × 10−10 mol cm−2 (Qa = 45 ± 3 μC cm−2) in NaClO4(aq) is close to the theoretical maximum ferrocene surface coverage of 4.85 × 10−10 mol cm−2 calculated from the cross-sectional area of ferrocene.3 FcC12SAu SAM Electrochemistry in CnSO4Na Solution at the cmc. CVs of FcC12SAu SAMs were recorded in aqueous solutions of CnSO4Na homologues, where n = 6, 8, 10, and 12. The CVs were recorded at bulk concentrations equal to the cmc of each surfactant rather than at a fixed CnSO4Na concentration to ensure that the free surfactant anion concentration in solution is at its highest (Figure 1)24 and because maximum surfactant or detergent adsorption onto surfaces is attained at the cmc.44 The ionic strength at the cmc of the CnSO4Na solution decreases with chain length: 420 (n = 6),23 130 (n = 8),45 33.2 (n = 10),45 and 8.1 mM (n = 12).45 Values of the specific solution resistance (ρ), measured by electrical conductivity, and electrochemical cell resistance (RΩ), determined through Ohmic potential (iR) drop measurements on the FcC12SAu SAM-modified electrode, are given in Table S2. RΩ depends on both the electrochemical cell configuration and the solution resistance. RΩ increases from ∼73 (n = 6) to ∼2059 Ω cm2 (n = 12). The consequence of such significant differences in RΩ between the surfactant anion chain lengths (due to differences in the ion concentration/solution resistance at the cmc) is that the CVs are affected or distorted to different extents by Ohmic drop. The effect of uncompensated resistance on the characteristics of a CV (i.e., peak current, peak separation, and peak width) can be understood (and simulated) in terms of the potential scan rate and actual potential that the working electrode feels with respect to the kinetics and thermodynamics of the electron transfer reaction of the redox molecule.46,47 The effect of the cell resistance is to decrease the potential scan rate so that at each time point the actual potential felt by the electrode is lower than the applied potential. The potential ramp seen by the electrode is no longer linear when the uncompensated resistance is non-negligible. The observed effect is a shift of the anodic peak toward more positive potentials, and the cathodic peak moves toward more negative potentials, resulting in an increase in the peak separation. If the scan rate seen by the electrode is lower than the applied scan rate then the peak current will also be lower. An increase in the cell resistance also results in peak broadening. Distortions of the CV created by uncompensated resistance were corrected instrumentally at 85% of RΩ during data acquisition using electronic positive feedback (corrections of 100% lead to overshoots and oscillations in the CV).48 Both noncorrected and corrected CVs are shown in Figure 3. The electrochemical data is summarized in Table 2. While there are notable differences in the peak positions, peak separation, and peak current densities of the Ohmic drop-compensated and E


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when the CnSO4Na concentration > ∼50 mM. Of the CnSO4Na homologues investigated, C10SO4Na and C12SO4Na have cmc’s < 50 mM. The residual Ohmic drop estimated using the noncorrected anodic peak current is 0.55 mV for 33.2 mM C10SO4Na(aq) and 1.7 mV for 8.1 mM C12SO4Na(aq) (Figure S4). We thus assume that the Ohmic-drop-corrected CV parameters given in Table 2 reasonably reflect the electrochemical behavior of the SAM. Single anodic and cathodic peaks with tailing on the negative potential side are observed at the cmc for each of the four chain lengths in contrast to the double peaks observed for the same SAM in NaClO4(aq). The CVs do not show significant variations in the peak heights or shape over the three successive scans (Figure 3). We have gone up to six consecutive cycles without observing any changes.17 Epa, Epc, and E°′SAM decrease linearly with increasing chain length (Figure 3, inset), suggesting that the SAM redox potential value at the cmc is dominated by the ion-pair formation constant K or ion-pairing ability of the CnSO4− anion (second term of eq 3) rather than the anion activity (third term of eq 3). If the ion-pair formation constants of the CnSO4− anions were of similar magnitude, one would expect an increase in the SAM redox potential with chain length since the cmc of C6SO4Na in water, and therefore the free anion activity, is 50-fold greater than that of C12SO4Na.24 The relative ion-pair formation constants of the longer chain C12SO4−, C10SO4−, and C8SO4− versus the short-chain C6SO4− as the reference anion were calculated from E°′SAM using eq 5:5

Figure 3. Electrochemistry of FcC12SAu SAMs recorded in aqueous solutions of sodium n-alkyl sulfates (CnSO4Na) at concentrations equal to the cmc: 420 mM C6SO4Na, 130 mM C8SO4Na, 33.2 mM C10SO4Na, and 8.1 mM C12SO4Na. CVs acquired at a scan rate of 10 mV s−1 with (solid line) and without (dotted line) Ohmic drop compensation are shown for each surfactant. Three consecutive oxidation−reduction sweeps are overlaid to show the reversibility of the redox process. (Inset) Plot and linear regressions of the anodic peak potential (Epa, r2 = 0.9908), cathodic peak potential (Epc, r2 = 0.9995), and the apparent formal potential of the FcC12SAu SAM (E°′SAM, r2 = 0.9981) vs the number of carbons n in the alkyl sulfate chain.

K CnSO4− K C6SO4−

uncompensated CVs recorded in C12SO4Na(aq) (8.1 mM) and to a lesser extent in C10SO4Na(aq) (33.2 mM), the compensated CVs in C8SO4Na(aq) (130 mM) and C6SO4Na(aq) (420 mM) overlap those without compensation. Ohmic drop compensation has the expected effect on the CVs of the FcC12SAu SAM recorded in C12SO4Na(aq) and C10SO4Na(aq): decreased anodic peak potential (Epa), increased cathodic (Epc) peak potential, reduced ΔEp, and reduced ΔEafwhm (Table 2). The CVs are sufficiently symmetrical that the errors in the anodic and cathodic peak potentials compensate for one another and thus do not significantly change the apparent formal redox potential E°′SAM, estimated as the midpoint of the peak potentials. Measurements carried out as a function of the CnSO4Na concentration (and presented in the next section) indicate that CV distortion due to the solution resistance becomes negligible

o′ ⎡ F (E o ′ ⎤ C6SO4 − − E CnSO4 −) ⎢ ⎥ = exp ⎥⎦ aCnSO4− ⎢⎣ RT

aC6SO4−

(5)

The values, given in Table 3, suggest that there is a difference of ∼1800 in the ion-pairing abilities of C6SO4− and C12SO4−. The average anodic charge density (surface concentration of electrogenerated ferrocenium) increases from 25 μC cm−2 (ΓFc+ = 2.6 × 10−10 mol cm−2) in C6SO4Na(aq) to 35 μC cm−2 (ΓFc+ = 3.6 × 10−10 mol cm−2) in C12SO4Na(aq) (Table 2), consistent with the greater surface activity exhibited by longer chain surfactants leading to higher amounts of surfactant adsorbed at the solid/aqueous solution interface.44 The anodic charge densities generated by the oxidation of the FcC12SAu SAM in the CnSO4Na(aq) solutions are lower than the charge density of 45 ± 3 μC cm−2 (ΓFc+ = 4.6 (±0.3) × 10−10 mol cm−2) obtained in the presence of perchlorate anions, which enable

Table 2. Electrochemical Data for FcC12SAu SAMs in Aqueous Solutions of Anionic Surfactant at the cmca Qa (μC cm−2)

C (mM)

Epa (mV)

Epc (mV)

ΔEoSAM ′ (mVb)

ΔEp (mV)

ΔEafwhm (mV)

C12SO4Na

8.1

370 ± 9 (384 ± 9)

355 ± 5 (351 ± 8)

35 ± 2

33.2 130 420

9 ± 6 (11 ± 7)

45 ± 6 (53 ± 3) 57 ± 12 (59 ± 12) 60 ± 11 (64 ± 7)

30 ± 3 30 ± 2

C6SO4Na

395 ± 5 (399 ± 5) 435 ± 18 (438 ± 11) 464 ± 8 (469 ± 8)

31 ± 12 (65 ± 21) 12 ± 3 (23 ± 3) 13 ± 2 (16 ± 3)

48 ± 4 (60 ± 6)

C10SO4Na C8SO4Na

C12SO3Na C6SO3Na (C6O2)(C7O2)C1SO3Na

9.0 460 67.6

418 ± 5 (423 ± 6) (515) (443 ± 3)

338 ± 10 (315 ± 17) 383 ± 7 (376 ± 7) 421 ± 17 (421 ± 11) 455 ± 11 (457 ± 13) 411 ± 4 (402 ± 7) (493) (426 ± 3)

7 ± 2 (21 ± 7) (22) (17 ± 4)

36 ± 6 (46 ± 13) (104) (56 ± 6)

20 ± 1 (4.5) (32 ± 2)

surfactant

390 ± 5 (390 ± 3) 427 ± 12 (429 ± 11) 459 ± 9 (461 ± 11) 415 ± 4 (413 ± 6) (504) (435 ± 3)

25 ± 3

a Mean values with standard deviation for at least three different FcC12SAu SAMs. bE°′SAM is the average of the anodic and cathodic peak potentials. Epa, Epc, E°′SAM, ΔEp, ΔEafwhm, and Qa values without parentheses are from CVs recorded with Ohmic drop compensation. Values in parentheses are without Ohmic drop compensation.

F


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Table 3. Parameters That Characterize the Redox-Induced Association of Anionic Surfactants with FcC12SAu SAMs surfactant

Kanion/KC6SO4− a

C12SO4Na C12SO4Na (0.3 M NaF) C10SO4Na C10SO4Na (0.3 M NaF) C8SO4Na C6SO4Na C12SO3Na C6SO3Na (C6O2)(C7O2)C1SO3Na

1806 140 11 1 259 0.16 16

Kdb (mM) 0.14 0.023 1.7 0.36 13 65

± ± ± ± ± ±

0.02 0.008 0.2 0.07 2 7

b −10 Γsat ) (mol cm−2) Fc+ (×10

ΓSPRc (×10−10) (mol cm−2)

± ± ± ± ± ±

5.3 ± 0.6

3.62 2.86 3.18 2.46 2.58 2.95

0.02 0.04 0.05 0.03 0.06 0.08

4.7 ± 0.7 4.6 ± 0.6 4.5 ± 0.7

a Relative ion-pair formation constants calculated using Eo′SAM measured at the cmc (Table 2) and eq 5. bValues obtained from fitting the ΓFc+ − CnSO4Na concentration data to eq 6. Uncertainties are those associated with the fits to the data. cAdsorbed surfactant anion coverages measured by SPR at the cmc. Average and standard deviation of 5−10 different FcC12SAu SAMs.

cathodic peak areas are comparable for each chain length (Table S3), indicating that the same quantities of ferrocenium and ferrocene are electrogenerated in the anodic and cathodic sweeps, respectively. Both the anodic and the cathodic peak currents exhibit the linear variation with the potential scan rate characteristic of a surface-confined redox reaction.1,38 Figure 4A shows this scan rate dependence in C6SO4Na(aq), chosen because the Ohmic potential drop is negligible. The peak

electrochemical oxidation of all of the surface-available ferrocenes.4,7 The formation of ion pairs is affected by the size of the electrolyte anion.11,37,49 Even though a sufficiently oxidizing potential is applied to the FcC12SAu SAM, not all of the available ferrocenes are oxidized in CnSO4Na(aq) because of physical crowding of the SAM-bound ferrocenes and the steric constraints associated with the oxidation and pairing of every ferrocene with surfactant anions.17,31 Although single peaks are observed in aqueous solutions of CnSO4Na at the cmc, the CVs exhibit several deviations from the ideal shape. The formal widths at half-maximum of the anodic peaks (45−60 mV) are significantly less than the theoretical value of 90.6 mV, reflecting the existence of nonideal interactions between the redox groups. Voltammetric peaks can be broader or narrower than the expected 90.6/n mV, depending on whether the lateral interactions between redox centers are repulsive or attractive.38,50 The peak halfwidths for the oxidation of the FcC12SAu SAM in aqueous solutions of CnSO4Na (as well as C12SO3Na and (C6O2)(C7O2)C1SO3Na)) suggest that the attractive interactions present between the FcC12SAu molecules in the reduced state (i.e., chain−chain van der Waals and ferrocene−ferrocene interactions)41 are preserved in the oxidized state.30 Peak halfwidths > 90.6 mV (i.e., predominant anodic peak at higher potential) are observed for the same FcC12SAu SAM in NaClO4(aq), as is often the reported case.1 CVs run in NaClO4(aq) before and immediately after cycling the FcC12SAu SAM in CnSO4Na(aq) (five successive oxidation−reduction cycles) and rinsing with water are superimposable, suggesting that the surfactant anions do not alter the SAM structure30 and electroactivity. A possible reason for this difference in the peak half-widths is a difference in neighboring charge effects.7 Oxidation of all of the SAM-bound ferrocene in the presence of perchlorate anions is expected to result in repulsive interactions between ferroceniums and larger peak half-widths. By contrast, approximately 67% (±11%) of the surface-available ferrocenes are oxidized in the presence of CnSO4Na at the cmc, thereby reducing the number of neighboring ferrocenium interactions. The formation of an adsorbed hydrocarbon-rich layer of surfactant anions following ion pairing may also result in attractive interactions and narrower anodic peaks. As the chain length of the CnSO4− anion increases from 6 to 12 carbons, an asymmetry develops between the anodic and the cathodic peaks (Figure 3) such that ΔEfwhm of the cathodic peak is less than that of the anodic peak (Table S3) and the cathodic peak current density is greater than the anodic peak current density. Despite this asymmetry, the anodic and

Figure 4. Cyclic voltammetry data for FcC12SAu SAM in the presence of C6SO4Na(aq) at a bulk concentration of 420 mM. (A) Anodic ja and cathodic jc peak densities as a function of the potential scan rate ν. Linear fit of ja: r2= 0.9998, slope = 0.349. Linear fit of jc: r2 = 0.9967, slope = −0.387. (Inset) Variation of the anodic Epa and cathodic Epc peak potentials as a function of ν. Dashed lines represent the average of each data set. (B) Anodic and cathodic peak areas vs ν. Linear fit of the anodic peak area: r2 = 0.9996, slope = 0.0246. Linear fit of the cathodic peak area: r2 = 0.9957, slope = 0.0246. (Inset) Plot of the anodic Qa and cathodic Qc charge densities vs ν. Dashed line represents the average of the two data sets. G


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ACS Applied Materials & Interfaces current densities increase linearly with scan rate (5−250 mV s−1), while the peak potentials remain essentially unchanged, as expected for a chemically reversible, surface-confined redox reaction at equilibrium.1,38 The peak areas also show a linear dependence with scan rate (Figure 4B). At all scan rates, the anodic peak integration is equal to the cathodic peak integration. The anodic and cathodic charge densities (Qa and Qc) calculated from the peak areas are independent of scan rate. Anodic-to-cathodic peak separations ranging from 10 to 30 mV are observed in NaClO4(aq) and CnSO4Na(aq) (as well as for the other anionic surfactants investigated), even though the potential scan rate is sufficiently slow (10 mV/s) that the interfacial electrochemistry should not be kinetically limited.2 The asymmetry between the anodic and the cathodic peaks mentioned above and nonzero peak splittings suggest that the reduction process does not follow the exact reverse path of the oxidation.1 It has already been demonstrated by Cruañes et al. for FcC11SAu SAMs that the oxidation and reduction processes in NaClO4(aq) are affected to different degrees by pressure due to a monolayer volume expansion (of the order of 10−20 mL mol−1) that is coupled to the electron-transfer and ion-pairing reactions in the oxidation process.8 This volume expansion is necessary to accommodate the sterically constrained pairing of the ClO4− anions with all of the surface-available ferroceniums. Pairing of the surfactant anions to the ferroceniums is expected to induce the aggregation of these amphiphilic anions at the SAM surface.51,52 The presence of an alkane-like layer of adsorbed surfactant (assuming that the surfactant alkyl chains do not penetrate into the SAM and interdigitate with the FcC12SAu chains)18 may alter the kinetics and thermodynamics of the reduction process with respect to the oxidation by destabilizing the charged ferrocenium in favor of the neutral ferrocene.8,53 Effect of the CnSO4Na Concentration on the FcC12SAu SAM Electrochemistry. Selected CVs acquired at different CnSO4Na(aq) concentrations are shown in Figure 5. The positions of the anodic and cathodic peaks remain the same (C6SO4Na and C8SO4Na) or shift to lower potential (C10SO4Na and C12SO4Na) as the surfactant concentration is decreased from above the cmc to the cmc of each CnSO4Na and then move to higher potential as the concentration is further decreased below the cmc. There is also a noticeable broadening of the voltammetric peaks, increase in the peak separation, and decrease in the peak current densities at surfactant concentrations below the cmc. This effect is real and not merely the result of residual Ohmic drop (although at low millimolar concentration, residual Ohmic drop may contribute to the observed changes), as indicated by the CVs of C6SO4Na (Figure 5A) and C8SO4Na (Figure 5B), whose elevated cmc’s (420 and 130 mM, respectively)23,45 enable one to probe concentrations in the premicellar regime down to 50 mM, for which the effect of uncompensated resistance is negligible. Furthermore, similar changes in CV shape are observed in the presence of added salt (Figure 8A and 8C), where the ion concentration is relatively high and quasi-constant and Ohmic drop effects are not an issue. In some cases, a redox peak splitting, analogous to the one exhibited by the FcC12SAu SAM in NaClO4(aq) (Figure 2A), is observed as the surfactant concentration is decreased below the cmc. At concentrations ≥ cmc, there are generally no significant changes in the peak current densities, peak separation, and peak widths. The differences in the peak widths and shape observed between the

Figure 5. Selected CVs as a function of the bulk CnSO4Na concentration: (A) C6SO4Na(aq), cmc = 420 mM, (B) C8SO4Na(aq), cmc = 130 mM, (C) C10SO4Na(aq), cmc = 33.2 mM, and (D) C12SO4Na(aq), cmc = 8.1 mM. Without (top, dash line) and with (bottom, solid line) Ohmic drop compensation.

premicellar and the micellar regimes may arise from differences in the interfacial ion spatial distribution and/or surfactant anion pairing sites due to differences in the quantity of ferrocenium electrogenerated below versus above the cmc.42,53,54 Figure 6A shows the dependence of ΓFc+ calculated from the anodic charge density on the bulk concentration of surfactant. The surface concentration of ferrocenium increases with the CnSO4Na concentration and reaches a limiting value at concentrations ≥ cmc. We observed a similar tendency in our previous work, but a large scatter in the anodic charge density values precluded analysis of the data.17 By contrast, no significant variation in ΓFc+ is observed with concentration for NaClO4 (Figure 2C). The variation in the quantity of electrogenerated ferrocenium with surfactant concentration is therefore reflective of the surface activity of CnSO4Na. If we assume that every ferrocenium generated is paired with one CnSO4− anion then ΓFc+ is a measure of the quantity of surfactant anion that is associated with the oxidized SAM via ion-pairing interactions. The plots resemble the adsorption isotherms reported for the adsorption of C12SO4Na and other surfactants or detergents onto modified surfaces of different chemical properties.44,55,56 H


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underlying surface, the ion-paired amount depends on the surface activity of the anion. The Kd values follow the relative ion-pair formation constants. As the ion-pair formation constant increases (with chain length), the effective dissociation constant for the surfactant from the SAM surface decreases. The quantity of surfactant anion associated with the oxidized form of the FcC12SAu SAM was also measured by SPR as a function of the bulk CnSO4Na concentration (Figure 6B). The concentrations investigated by SPR are limited in comparison to those examined in cyclic voltammetry; nevertheless, the trend is the same. The surface coverage increases with concentration and plateaus at concentrations ≥ cmc. The values measured at the cmc (ΓSPR, Table 3) are comparable to those reported previously.17,18,31 As stated in an earlier paper, there is a noticeable difference between the coverages measured 18 by electrochemistry Γsat The Fc+ versus SPR ΓSPR (Table 3). resonance angle change measured in SPR should reflect the total surface concentration of associated or adsorbed surfactant, while the anodic charge density reflects the quantity of ionpaired surfactant. That ΓSPR > Γsat Fc+ indicates that not all of the associated surfactant anions are paired with SAM-bound ferroceniums. The saturated surface coverage of C12SO4− measured by SPR of 5 × 10−10 mol cm−2 and effective thickness of 1.1−1.4 nm17,18,31 are not consistent with an adsorbed bilayer of surfactant anions for which the reported coverage is 7−8 × 10−10 mol cm−2 and thickness is 2.0−2.1 nm.52,57,58 As discussed in an earlier report,18 we propose that the associated surfactant anions adopt an interdigitated monolayer configuration (i.e., a thermodynamically favorable monolayer configuration at solid/aqueous interfaces) in which an interfacial layer is formed by pairing of CnSO4− (heads down) with SAM-bound ferroceniums. The alkyl chains of additional CnSO4− ions insert themselves (heads up) between the ferrocenium-paired CnSO4−. A theoretical configuration in which one-half of the surfactant anions are ion paired to the underlying ferroceniums and one-half have their tails interdigitated between the paired surfactant anions is expected to yield a total surfactant anion coverage (ΓSPR) of 4.85 × 10−10 mol cm−2 (equal to the maximum theoretical ferrocene coverage) and ΓFc+ of 2.4 × 10−10 mol cm−2. The negative charge on the interdigitated (nonpaired) CnSO4− could be compensated by the electronic charge of the underlying gold metal and by Na+ counterions. At the lower concentrations of ferrocenium generated in the premicellar regime, the surfactant anions could adopt a less densely packed, interdigitated monolayer structure or another type of arrangement.51 Figure 7 presents plots of Epa, Epc, and Eo′SAM as a function of the logarithm of the ratio of the CnSO4Na molar concentration ([CnSO4Na]) to the cmc. Epa and Epc obtained from CVs acquired with and without Ohmic drop compensation are shown (Figure 7A−D). For the CVs exhibiting peak splitting (premicellar regime), the average (midway point) of the two peaks was used for Epa and/or Epc. The following observations can be made. First and foremost, the individual peak potentials and Eo′SAM (Figure 7E) all show a break or minimum at the commonly accepted value of the cmc23,45 of each CnSO4Na (i.e., at log ([CnSO4Na]/cmc) = 0). Second, the potential separation between the anodic and the cathodic peaks generally increases with increasing chain length. C12SO4Na, the chain length with the lowest cmc (8.1 mM), shows a pronounced trend with concentration (Figure 7A). This larger peak separation cannot solely be explained by a residual Ohmic

Figure 6. (A) ΓFc+ vs bulk CnSO4Na concentration. Open square symbols represent the coverages obtained from CVs corrected for the Ohmic drop, and filled circles are the values obtained from noncorrected CVs. (Insets) ΓFc+ in the presence of 0.300 M NaF. Symbols are the experimental data, and solid lines are the fits to a Langmuir isotherm model (eq 6). (B) Surfactant anion coverages determined by SPR ΓSPR. Dashed vertical lines indicate the cmc.

We found that the ΓFc+−CnSO4Na concentration data can be described well by the Langmuir equation ΓFc+ = Γ sat Fc+

[CnSO4 Na] Kd + [CnSO4 Na]

(6)

where Kd is the effective dissociation constant for the surfactant from the SAM surface and Γsat Fc+ is the limiting value of electrogenerated ferrocenium. As pointed out by Sigal et al. in their article on the use of SPR to investigate the association of detergents to SAM surfaces, it is unlikely that the adsorption of surfactant to a surface is a Langmuir process, but nonetheless, this model provides a useful empirical way of comparing data obtained for different surfactants.55 Table 3 gives the values of Kd and Γsat Fc+ obtained from fits of the electrochemical data. The saturation coverage of ion-paired surfactant Γsat Fc+ increases with chain length. It is well established that the hydrocarbon chain length affects the surfactant activity (Traube’s rule) and amount of surfactant adsorbed at solid/ aqueous solution interfaces.44 The amount adsorbed at saturation increases with increasing chain length due to increasing hydrophobic chain−chain interactions. Even though the association of CnSO4− to the FcC12SAu SAM is driven by redox and ion-pairing reactions, rather than conventional electrostatic51,52 or hydrophobic55,56 interactions with the I


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Figure 7. (A−D) Anodic Epa and cathodic Epc peak potentials vs logarithm of the CnSO4Na molar concentration/cmc ratio, where log([CnSO4Na]/ cmc) = 0 at the cmc. Filled and open symbols are without and with Ohmic drop compensation. Red symbols indicate 4 and 8 mM CnSO4Na. Each concentration data point corresponds to a different FcC12SAu SAM. (E) Apparent formal potential E°′SAM vs logarithm of the CnSO4Na molar concentration/cmc ratio. (Inset) Surfactant ion-selective electrode potential EISE data from ref 24. E°′SAM−log([CnSO4Na]/cmc): cmc’s from the literature.23,45 EISE−log([CnSO4Na]/cmc): cmc’s measured experimentally in ref 24

Like the surfactant ion-selective electrode data, E°′SAM varies linearly (or near linearly) with the logarithm of the CnSO4Na concentration below the cmc. Linear regressions of the semilogarithmic plots (Figure S5) give the following slopes (mV decade−1): −52 ± 2 (C12SO4Na), −80 ± 3 (C10SO4Na), −73 ± 4 (C8SO4Na), and −72 ± 3 (C6SO4Na). We verified using C12SO4Na and C10SO4Na whether the slope deviations in the premicellar regime are due to variations in the liquid junction potential59 or residual Ohmic potential drop with the surfactant concentration. The residual Ohmic drop following compensation at 85% of the cell resistance is estimated to be ∼50 mM and increases steeply at concentrations < ∼10 mM (Figure S4). At concentrations of 1 mM, the residual Ohmic drop is ∼2.5− 6.5 mV. The estimated residual drop is small compared to the magnitudes of the redox potential variation across the premicellar regime (Figure S5), and correcting the E°′SAM values for the residual Ohmic potential drop did not improve the slopes. The liquid junction potentials do not vary significantly over the range of concentrations involved (Table

drop (Figure S4). For instance, a comparison of the Ohmicdrop-corrected Epa and Epc values at 4 and 8 mM CnSO4Na (indicated as red symbols in Figure 7A−D), concentrations where all four surfactant anions are present as monomers in solution and for which the solution resistances are comparable (Table S2), indicates that the peak separations follow the order NaC12SO4 > NaC10SO4 > NaC8SO4 > NaC6SO4. The influence of the CnSO4Na chain length on the peak separation may be related to the differences in Kd and/or the molecular density of the adsorbed surfactant anion layer. Third, E°′SAM and the surfactant ion-selective electrode potential (EISE)24 data as shown as an inset in Figure 7E exhibit strikingly similar concentration dependencies. Nernstian (or near-Nernstian) changes of EISE are obtained up to the cmc (54−59 mV decade−1), signifying that there is no premicellar aggregation.24,26 The activity of free CnSO4− is maximal around the cmc (Figure 1). A sharp departure from ideal solution behavior is observed at concentrations > cmc due to the association of the surfactant anions into micelles and resulting decrease in the anion activity.24,26 J


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to some competition between CnSO4− and F− for the ferrocenium sites. The decrease in the apparent dissociation constant of C10SO4− or C12SO4− from the FcC12SAu SAM is probably due to shielding of the charged head groups by the salt in solution.55 Figure 8B and 8D compares the variation of E°′SAM with the logarithm of the CnSO4Na concentration in the absence and presence of added salt. The data obtained in the presence of 0.300 M NaF shows a sharp break between two distinct linear regimes. The break is shifted to lower concentration with respect to the minimum observed for the surfactant in deionized water. The C10SO4Na or C12SO4Na concentration associated with the break corresponds to the cmc. Due to the relatively high conductivity of the NaF background (κ = 0.022 Ω−1 cm−1), the cmc’s of the anionic surfactants could not be determined by electrical conductivity and were determined by surface tension measurements. Within experimental uncertainty, the surfactant concentration dependence of E°′SAM gives the same cmc as surface tension measurements for C10SO4Na and C12SO4Na in the presence of NaF (Table 1). The results indicate that the CnSO4− anions preferentially pair with the SAM-bound ferroceniums in the presence of F− so that E°′SAM responds to the activity of the surfactant anion in solution. A noteworthy difference between the E°′ SAM −log ([CnSO4Na] plots in the absence and presence of NaF is the variation of E°′SAM at CnSO4Na concentrations above the cmc. It is more pronounced in water than in the NaF solution. The same behavior has been observed for C12SO4Na in 0.1 M NaCl using a surfactant ion-selective electrode (data is shown as an inset in Figure 8B) and has been attributed to a decrease in electrostatic repulsion between the micelles due to an increased screening from the added salt.24,25 Unlike the E°′SAM response in NaClO4(aq) (Figure 2), the addition of background electrolyte to the CnSO4Na solution did not improve the premicellar slope. The precmc slopes in the presence of 0.300 M NaF (Figure S6) are −53 ± 5 mV decade−1 for C12SO4Na and −82 ± 3 mV decade−1 for C10SO4Na. These slope deviations from the Nernst value cannot be attributed to activity effects due to the quasi-constant solution ionic strength or changing liquid junction potentials (Table S1). FcC12SAu SAM Electrode Response. The CnSO4Na homologues were specifically chosen to evaluate the analytical response of the FcC12SAu SAM as these are strong electrolytes that exhibit ideal behavior and a Nernstian response with membrane surfactant-ion selective electrodes below the cmc in water and in the presence of added salt.24−26 As already mentioned, the non-Nernstian linear behavior exhibited by the FcC12SAu SAM electrode in the premicellar regime cannot be explained by varying liquid junction potentials, activity effects, or variations in the residual Ohmic drop. For the Nernst relation (eq 3) to hold, the quantity of the ion-paired electrogenerated ferrocenium ΓFc+X− must remain constant as a function of the anion concentration. The anodic peak areas (Figure 6A) show that this is not the case in the premicellar regime. We therefore attribute the deviation from the Nernst value predominantly to variations of the surface concentrations of ferrocene ΓFc and ferrocenium ΓFc+X− with the surfactant anion concentration. The anion activity and ferrocenium surface concentration influence E°′SAM in opposing directions. E°′SAM decreases with increasing anion activity (third term of eq 3) and increases with the surface concentration of electrogenerated ferrocenium (second term of eq 3). Both of

S1) so that the observed deviations are not caused by varying liquid junction potentials. The FcC12SAu SAM response in the premicellar regime is discussed in a subsequent section. Effect of Added Salt on the cmc’s of C10SO4Na and C12SO4Na. The addition of salt lowers the cmc of ionic surfactants and increases the aggregation number by reducing electrostatic repulsions between the charged head groups.23 E°′SAM was measured in aqueous solutions containing 0.300 M NaF and different concentrations of C10SO4Na or C12SO4Na. Due to the low resistance of the 0.300 M NaF solution (45 Ω cm), the CVs were recorded without Ohmic drop compensation. The choice of NaF versus NaCl as the added/background electrolyte was motivated by the poorer ion-pairing ability of F− compared to Cl−,5,6 which should favor the pairing of CnSO4− with the SAM-bound ferrocenium, and the low specific adsorption of F− to gold.60 Selected CVs are shown in Figure 8A and 8C. There is no significant change in the CV at surfactant concentrations ≥ cmc

Figure 8. Selected CVs as a function of the bulk CnSO4Na concentration in the presence of 0.300 M NaF(aq): (A) C12SO4Na and (C) C 10 SO 4 Na. E°′ SAM vs logarithm of the C n SO 4 Na concentration in the absence (filled squares) and presence (open squares) of 0.300 M NaF: (B) C12SO4Na and (D) C10SO4Na. (Inset in B) Surfactant ion-selective electrode potentials EISE reported for C12SO4Na in water (filled squares) and 0.1 M NaCl (open squares) in ref 25. E°′SAM values for the CnSO4Na/0.300 M NaF solutions were obtained from CVs recorded without Ohmic drop compensation due to the low specific resistance (45 Ω cm) of the NaF background.

(Table 1). The peaks shift to higher potential as the concentration is decreased below the cmc. As per CnSO4Na in water, the peaks concurrently broaden, the peak separation increases, and peak current densities decrease. There is no peak splitting at surfactant concentrations below the cmc, suggesting that the multiple peaks observed for CnSO4Na in water (Figure 5) arise from a double-layer effect and that the added salt screens the interfacial charge.1,42 The dependence of ΓFc+ on the concentration of surfactant is shown in Figure 6A (inset). The surface concentration of ferrocenium reaches a limiting value at concentrations ≥ cmc. Γsat Fc+ and Kd in the presence of 0.300 M NaF are less than those obtained in water (Table 3). The decrease in Γsat Fc+ is attributable K


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Figure 9. Apparent formal potential E°′SAM vs logarithm of the anionic surfactant concentration: (A) C6SO3Na(aq), (B) C12SO3Na(aq), and (C) (C6O2)(C7O2)C1SO3Na(aq). (D) Anodic peak potential Epa vs logarithm of the C11COONa(aq) concentration. Data points and error bars represent the average and standard deviation of two different FcC12SAu SAMs, except for A where each data point represents one SAM. Dashed lines are guides to the eye. Potentials were obtained from CVs recorded without Ohmic drop compensation. (Insets) Selected CVs as a function of the bulk surfactant concentration.

these parameters influence E°′SAM in the premicellar regime, leading to a slope deviation from the expected value. According to the surfactant anion concentration range spanned by the premicellar regime, residual Ohmic potential drop and changing liquid junction potentials can also contribute to the deviation of E°′SAM from a Nernstian response. In the micellar regime, the quantity of ion-paired ferrocenium is constant (Figure 6A) so that the observed variations in E°′SAM should be due to variations in the free anion activity. Depending on the concentration of the electrolyte ions and cell resistance, contributions from the liquid junction potential and residual Ohmic potential drop may not be negligible. Other Anionic Surfactants. The FcC12SAu SAM redox electrochemistry was used to determine the cmc’s of linearchain alkyl sulfonates (C6SO3Na and C12SO3Na), a branchedchain alkyl sulfonate ((C6O2)(C7O2)C1SO3Na), and an alkyl carboxylate (C11COONa). Except for sodium dodecanoate, the CVs of the anionic surfactants overlap over three successive cycles. The data is presented in Figure 9 and Tables 2 and 3. The FcC12SAu SAM is reversibly oxidized and reduced in the presence of the alkyl sulfonates (Figure 9A−C, insets). The E°′SAM values at the cmc can be used to compare the ability of the surfactant anion to pair with and stabilize the SAM-bound ferrocenium.5,6 Since the anionic surfactants have different cmc’s, we cannot directly compare the E°′SAM values. We instead calculate relative ion-pair formation constants (eq 5), which account for the surfactant anion concentration, using C6SO4Na as the reference anion. In doing so, we suppose that the differences in the surface concentrations of ion-paired ferrocenium electrogenerated in the presence of the different anionic surfactants (Table 2) do not have a significant effect on the calculated relative ion-pair formation constants. The values, given in Table 3, indicate that the ion-pairing tendency depends on the alkyl chain length, the nature of the headgroup (sulfate versus sulfonate), and the structure of the hydrophobic tail. For the CnSO4Na and CnSO3Na analogues, the tendency of the surfactant anion to pair with the SAM-bound ferrocenium

increases with chain length, as already discussed, consistent with the greater hydrophobicity imparted by the longer alkyl chain. The relative ion-pair formation constants of C6SO3Na and C12SO3Na (sulfonate headgroup) indicate that these are less effective at pairing with the ferrocenium than their CnSO4Na analogues (sulfate headgroup) even though they have similar cmc’s (Table 1). C6SO3Na exhibited the poorest ion-pairing ability, and this is reflected by the large (nonFaradaic) background current in the CV (Figure 9A, inset) and lower anodic charge density (Table 2). The ion-pairing ability of the double-tailed (C6O2)(C7O2)C1SO3Na, which also possesses a sulfonate headgroup, is comparable to that of the single-tailed C8SO4Na. The E°′SAM−log([surfactant]) plots show breaks between two linear regions at the cmc (Table 1). There is some or no variation of E°′SAM with the surfactant concentration above the cmc and an increase with decreasing concentration below the cmc (Figure 9A−C). The CVs exhibit variations in shape with the surfactant concentration that are similar to those observed for the oxidoreduction of the FcC12SAu SAM in CnSO4Na(aq). The FcC12SAu SAM is irreversibly oxidized in C11COONa solution (Figure 9D, inset). An oxidation peak is observed for the first anodic potential sweep, but no reduction peak is detected during the reverse sweep. The anodic peak current density is greater than those measured in solutions of alkyl sulfates and sulfonates. No anodic or cathodic peaks are observed in subsequent sweeps. Oxidation of the FcC12SAu SAM may result in the loss of ferrocene or the formation of a stable and insoluble Fc+C11COO− complex or deposit that blocks the redox process. These two possibilities can be validated by X-ray photoelectron spectroscopy (to check for the loss of iron) and atomic force microscopy (to look for the formation of deposits on the SAM). The irreversible electrochemistry of FcC12SAu SAMs in alkyl carboxylate solution merits further investigation but is outside of the scope of this paper. Despite the irreversible nature of the FcC12SAu SAM oxidation in C11COONa, a plot of the anodic peak potential Epa L


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activities and the detection of premicellar aggregation from values of the apparent redox potential.24,25 The following conditions must be met for FcC12SAu or similar ferrocene-terminated SAMs to be used for the determination of the cmc of anionic amphiphiles in aqueous solution. First, the anion must pair with the electrogenerated ferrocenium to produce defined redox peaks in cyclic voltammetry. We investigated some commonly used types of surfactants: n-alkyl sulfate (foaming agent), n-alkyl sulfonate (detergent), dialkyl sulfosuccinate (wetting and dispersing agent), and n-alkyl carboxylate/fatty acid (soap). All of these pair to different extents with the SAM-bound ferrocenium. The alkyl sulfates have higher pairing abilities than their alkyl sulfonate analogues (Table 3). The dodecanoate pairs with the ferrocenium, but the FcC12SAu SAM is irreversibly oxidized in the presence of this fatty acid. Second, the onset of micelle formation must occur over a narrow concentration range. Ionic amphiphiles that associate gradually (over an extended concentration range) because they do not have separate well-defined hydrophobic and hydrophilic regions or that form small aggregates, as opposed to micelles, do not yield abrupt deviations of the ion-selective electrode potential from Nernstian behavior at concentrations ≥ cmc.23,25 The sodium n-alkyl sulfates were chosen as model surfactants because their rapid association into micelles is characterized by a well-defined cmc.23 The sodium alkyl sulfonate analogues, sodium diamyl sulfosuccinate, and sodium dodecanoate also have well-defined micellization points and hence yield sharp breaks in the E°′SAM−log([surfactant]) plots (Figure 9). Third, since the measurements are based on Faradaic electrochemistry, the cmc in water must be > ∼3 mM so that CVs can be acquired at a few concentrations below the cmc. At concentrations < ∼1 mM, the distortion of the CV is too great for a meaningful determination of E°′SAM. In the presence of added salt, where uncompensated solution resistance is generally not an issue, CVs could be acquired at lower concentrations (e.g., down to at least 0.05 mM C12SO4Na in 0.300 M NaF, Figure 8A). The variation of E°′SAM observed above the cmc for C12SO4Na in water (Figure 7E) indicates that redox potential measurements can be performed up to a surfactant concentration of at least 600 mM. By comparison, the specified analytical range of a commercially available plastic membrane ion-selective electrode (Thermo Scientific Orion 9342BN) for anionic surfactants, including sulfated and sulfonated surfactants, is 0.01−10 mM for C12SO4Na.61 Finally, the application of ferrocenylalkanethiolate SAMmodified gold electrodes to the detection of the micellar aggregation of anionic surfactants offers the advantage of an allsolid electrode construction and the possibility of measurements in the presence of an electrolyte background.

versus log[C11COONa] (Figure 9D) exhibits a clear break at the cmc (Table 1).

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SUMMARY AND CONCLUSIONS We exploited surface-confined redox and ion-pairing reactions at ω-ferrocenylalkanethiolate SAM-modified gold electrodes to detect the micellization of anionic surfactants in aqueous solution by cyclic voltammetry. The micellar aggregation of the dissociated surfactant anions that pair with the SAM-bound ferroceniums affects the cyclic voltammogram: peak potentials, peak widths, and peak currents. The variation of the apparent formal potential with the surfactant concentration can be used to determine the cmc. Micelle formation leads to a sharp departure from the linear variation of E°′SAM with the logarithm of the surfactant concentration for the anionic surfactants investigated. The surfactant concentration at which this deviation occurs coincides with the cmc of the anionic surfactant. The effect of uncompensated resistance on the CV and E°′SAM was addressed using Ohmic drop compensation. Our results indicate that CV distortion due to the solution resistance becomes negligible when the anionic surfactant concentration > ∼50 mM. Due to the fairly symmetric shape of the CVs, the errors in the anodic and cathodic peak potentials compensate for one another and do not significantly change E°′SAM down to a surfactant concentration ≈ 1 mM in water. Most importantly, Ohmic drop effects and liquid junction potentials, both of which vary with the bulk concentration of anionic surfactant, did not impede the determination of the cmcthe cmc’s obtained from the breaks in the E°′SAM data are in good agreement with values determined from surface tension or conductivity measurements or taken from the literature (Table 1). As this is a proof of concept, a different FcC12SAu SAM was purposely employed for each anionic surfactant concentration to validate the observed dependence of E°′SAM on concentration. E°′SAM measurements were repeated at least once over the series of concentrations for most of the surfactant systems. We have on occasion used the same FcC12SAu SAM for different surfactant concentrations, rinsing thoroughly with deionized water between measurements, and have not seen any differences. Rinsing electrodes for repeated use is an issue for all electrodes, including the ion-selective ones used in potentiometry. Alternatively, one could use an electrochemical flow cell to flow through solutions of different concentrations, interspersed with water, as done by Sigal et al.55 in their SPR investigation of surfactant adsorption to a SAM surface. The strong similarity between the variations of the apparent formal potential of the FcC12SAu SAM and the surfactant ionselective electrode potential with the concentration of CnSO4Na, both in water (Figure 7) and in the presence of added salt (Figure 8), establishes that both electrode systems respond to the same speciesunaggregated surfactant anions. However, because of the surface-confined Faradaic reaction being exploited in this work and the adsorption behavior of surfactants, E°′SAM exhibits a linear response whose slope deviates from the expected −59 mV decade−1 in the premicellar regime, primarily due to variations of the electrogenerated ferrocenium and associated surfactant anion with the bulk surfactant concentration (Figure 6). The non-Nernstian response of the FcC12SAu SAM electrode in the premicellar regime complicates the determination of the surfactant anion

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15290. Synthesis of FcC12SH and characterization data, atomic force microscopy images of the gold thin film electrode surface and FcC12SAu SAM, and additional results (PDF) M


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(14) Ye, S.; Sato, Y.; Uosaki, K. Redox-Induced Orientation Change of a Self-Assembled Monolayer of 11-Ferrocenyl-1-Undecanethiol on a Gold Electrode Studied by in Situ FT-IRRAS. Langmuir 1997, 13, 3157−3161. (15) Luk, Y.-Y.; Abbott, N. L. Surface-Driven Switching of Liquid Crystals Using Redox-Active Groups on Electrodes. Science 2003, 301, 623−626. (16) Norman, L. L.; Badia, A. Redox Actuation of a Microcantilever Driven by Self-Assembled Ferrocenylundecanethiolate Monolayer: An Investigation of the Origin of the Micromechanical Motion and Surface Stress. J. Am. Chem. Soc. 2009, 131, 2328−2337. (17) Dionne, E. R.; Sultana, T.; Norman, L. L.; Toader, V.; Badia, A. Redox-Induced Ion Pairing of Anionic Surfactants with FerroceneTerminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface. J. Am. Chem. Soc. 2013, 135, 17457−17468. (18) Nguyen, K.-L.; Dionne, E. R.; Badia, A. Redox-Controlled IonPairing Association of Anionic Surfactant to Ferrocene-Terminated Self-Assembled Monolayers. Langmuir 2015, 31, 6385−6394. (19) Uosaki, K.; Sato, Y.; Kita, H. Electrochemical Characteristics of a Gold Electrode Modified with a Self-Assembled Monolayer of Ferrocenylalkanethiols. Langmuir 1991, 7, 1510−1514. (20) Yokota, Y.; Yamada, T.; Kawai, M. Ion-Pair Formation between Ferrocene-Terminated Self-Assembled Monolayers and Counteranions Studied by Force Measurements. J. Phys. Chem. C 2011, 115, 6775−6781. (21) Harris, D. C.; Lucy, C. A. Quantative Chemical Analysis, 9th ed.; W. H. Freeman: New York, 2015; pp 338−364. (22) Tadros, T. F. Applied Surfactants: Principles and Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2005. (23) Evans, D. F.; Wennerström, H. The Colloidal Domain Where Physics, Chemistry, Biology, and Technology Meet; Wiley-VCH, 1999; pp 157−216. (24) Kale, K. M.; Cussler, E. L.; Evans, D. F. Surfactant Ion Electrode Measurements of Sodium Alkylsulfate and Alkyltrimethylammonium Bromide Micellar Solutions. J. Solution Chem. 1982, 11, 581−592. (25) Kale, K. M.; Cussler, E. L.; Evans, D. F. Characterization of Micellar Solutions Using Surfactant Ion Electrodes. J. Phys. Chem. 1980, 84, 593−598. (26) Cutler, S. G.; Meares, P.; Hall, D. G. Ionic Activities in Sodium Dodecyl Sulphate Solutions from Electromotive Force Measurements. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1758−1767. (27) Sasaki, T.; Hattori, M.; Sasaki, J.; Nukina, K. Studies of Aqueous Sodium Dodecyl Sulfate Solutions by Activity Measurements. Bull. Chem. Soc. Jpn. 1975, 48, 1397−1403. (28) Nesměrák, K.; Němcová, I. Determination of Critical Micelle Concentration by Electrochemical Means. Anal. Lett. 2006, 39, 1023− 1040 and references therein.. (29) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1169. (30) Tian, H.; Dai, Y.; Shao, H.; Yu, H.-Z. Modulated Intermolecular Interactions in Ferrocenylalkanethiolate Self-Assembled Monolayers on Gold. J. Phys. Chem. C 2013, 117, 1006−1012. (31) Norman, L. L.; Badia, A. Electrochemical Surface Plasmon Resonance Investigation of Dodecyl Sulfate Adsorption to Electroactive Self-Assembled Monolayers Via Ion-Pairing Interactions. Langmuir 2007, 23, 10198−10208. (32) Freund, M. S.; Brajter-Toth, A. Semiintegral Analysis in Cyclic Voltammetry: Determination of Surface Excess and Concentration in Presence of Weak Adsorption and Thin Films. J. Phys. Chem. 1992, 96, 9400−9406. (33) Badia, A.; Chen, C.-I.; Norman, L. L. Calibration of a FanShaped Beam Surface Plasmon Resonance Instrument for Quantitative Adsorbed Thin Film Studies-No Metal Film Thickness or Optical Properties Required. Sens. Actuators, B 2013, 176, 736−745. (34) Castro, M. J. L.; Ritacco, H.; Kovensky, J.; Fernández-Cirelli, A. A Simplified Method for the Determination of Critical Micelle Concentration. J. Chem. Educ. 2001, 78, 347−348.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eric R. Dionne: 0000-0002-2509-9110 Antonella Badia: 0000-0002-1026-4136 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by NSERC (Discovery grant and Discovery Accelerator supplement) and the Canada Research Chairs program. We thank Dr. Violeta Toader (FRQNT Centre for Self-Assembled Chemical Structures) for the synthesis of the ferrocenyldodecanethiol used in this study and Eric Godin (Université de Montréal) for surface tension and electrical conductivity measurements of the cmc.

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Electroactive Self-Assembled Monolayers Detect Micelle Formation

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