Article pubs.acs.org/Langmuir
Infrared Studies of the Potential Controlled Adsorption of Sodium Dodecyl Sulfate at the Au(111) Electrode Surface J. Jay Leitch,† John Collins,‡ Andreas Kaspar Friedrich,‡,§ Ulrich Stimming,‡ John R. Dutcher,⊥ and Jacek Lipkowski*,† †
Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Department of Physics, Technical University of Munich, James-Franck-Str. 185748 Garching, Germany § German Aerospace Center Institute of Technical Thermodynamics, Electrochemical Energy Technology, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany ⊥ Department of Physics, University of Guelph, Guelph, Ontario, Canada N1G 2W1 ‡
S Supporting Information *
ABSTRACT: Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the conformation and orientation of sodium dodecyl sulfate (SDS) molecules adsorbed at the single crystal Au(111) surface. The SDS molecules form a hemimicellar/hemicylindrical (phase I) structure for the range of potentials between −200 ≤ E < 450 mV and condensed (phase II) film for electrode potentials ≥500 mV vs Ag/AgCl. The SNIFTIRS measurements indicate that the alkyl chains within the two adsorbed states of SDS film are in the liquid-crystalline state rather than the gel state. However, the sulfate headgroup is in an oriented state in phase I and is disordered in phase II. The newly acquired SNIFTIR spectroscopy measurements were coupled with previous electrochemical, atomic force microscopy, and neutron reflectivity data to improve the current existing models of the SDS film adsorbed on the Au(111) surface. The IR data support the existence of a hemicylindrical film for SDS molecules adsorbed at the Au(111) surface in phase I and suggest that the structure of the condensed film in phase II can be more accurately modeled by a disordered bilayer.
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(AFM), and neutron reflectivity.18,19 A stripe-like structured film with a thickness of 14.6 Å was observed for phase I, and a condensed, liquid-like film with a thickness of 20.5 Å was observed for phase II. Based on the thickness and imaging data, the films for phase I and II were modeled using a hemimicellar/ hemicylindrical and all-trans interdigitated film.18,19 However, other possible models can also be used to fit these data; therefore, molecular information is required to confirm the validity of these models. Infrared and Raman scattering measurements have been employed to examine the structure of the SDS aggregates and films in dried and aqueous states.20,21 The Raman scattering studies by Picquart20 showed that the gel to liquid crystalline state transition occurs at ∼17 °C. This was confirmed by IR experiments by Holler and Callis21 which demonstrated that the alkyl chains of the SDS molecules in the micellar aggregates are melted and disordered at room temperature. Several studies have been published in which IR attenuated total reflection (ATR) spectroscopy has been used to study SDS adsorption on the surface of thin oxide films deposited onto the surface of an
INTRODUCTION Surfactant molecules are capable of aggregating at the solid− liquid interface and find numerous applications as detergents,1,2 corrosion inhibitors, ore flotation,3−9 and capping agents for nanoparticles (e.g., ferrofluids, quantum dots, etc.).10,11 Therefore, understanding the behavior of the adsorption process and structure of the adsorbed film is essential for improving its function. Recent works have shown that surfactants can assemble into a wide variety of structures, which include monolayers, bilayers, interdigitated layers, hemicylinders, hemimicellar, cylinders, and micelles.12−17 The structure of the adsorbed film depends on the size and charge of the headgroup as well as the length of the hydrophobic tail and can also be influenced by the strength of the electric field at the surface.17 Previous work by Burgess et al.18,19 showed that the aggregation of sodium dodecyl sulfate (SDS) molecules at the Au(111)−solution interface can be controlled by altering the potential of the electrode. Charge density measurements showed that the initial adsorption of SDS occurred at −250 mV vs Ag/AgCl followed by two distinct adsorption states where phase I occurred for −200 ≤ E < 450 mV vs Ag/AgCl and phase II occurred for E > 500 mV vs Ag/AgCl.17 The structure of these films was then examined using scanning tunneling microscopy (STM), atomic force microscopy © 2011 American Chemical Society
Received: November 12, 2011 Revised: December 22, 2011 Published: December 28, 2011 2455
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ATR element. Bai et al.22 used ATR spectroscopy to examine the adsorption of SDS micelles on hematite particulate films. Dobson et al.23 investigated SDS adsorption on TiO2, ZrO2, Al2O3, and Ta2O5 particles. Sperline et al.24 applied ATR to study SDS adsorption at alumina surfaces. These studies demonstrated that alkyl chains of adsorbed SDS are partially disordered and that the sulfate headgroup are bound to the oxide surface. The objective of this work was to apply subtractively normalized interfacial Fourier transformed infrared reflection spectroscopy (SNIFTIRS) to determine the orientation and conformation of SDS molecules adsorbed at the Au(111) electrode surface. The IR spectra were used to independently determine the conformation and orientation of the anionic sulfate headgroup and that of the hydrophobic tails. The new information concerning the orientation of the SDS molecule with respect to the electrode surface and the conformation of its hydrocarbon tail will be used to improve the currently accepted models for adsorbed SDS films. In addition, SNIFTIRS measures differential spectra (difference between IR spectra of SDS molecules at two selected potentials), and quantitative interpretation of such spectra is difficult. In fact, we made the first attempt to apply SNIFTIRS to study adsorption of SDS at the gold electrode surface about 10 years ago. However, interpretation of the differential spectra acquired at that time was difficult, and we refrained from publication of these data. Here, we apply a new method developed in this laboratory to calculate the IR spectra of SDS molecules adsorbed at the gold electrode surface from the measured differential spectra that are recorded using SNIFTIRS.25 Improvements to the optics, data acquisition, and quantitative data analysis are also described in this work. The new data are presented in the text below, but the comparison of old and new results is described in section SI1 of the Supporting Information. The methodology described in this work is general and can be applied to study adsorption of other surfactants at electrified interfaces.
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electrochemical IR cell. This SDS concentration was selected to ensure that the amount of SDS in the thin layer cavity (a few micrometer thick layer of solution contained between the gold electrode and ZnSe window) was sufficiently high to allow for complete coverage of the electrode surface while maintaining equilibrium between the adsorbed film and SDS in the solution within the thin layer. Note: the critical micelle concentration (cmc) for SDS in this particular electrolyte was determined to be 0.64 mM (data are presented in section SI2 in the Supporting Information). A large single crystal gold (111) electrode (0.91 cm in diameter), prepared according to the procedure presented in ref 25, was used as the working electrode for all SNIFTIRS measurements. The working electrode was flame-annealed with a Bunsen burner, air-dried, and cooled under the plume of the flame before it was mounted into the electrochemical IR cell. A saturated Ag/AgCl (saturated KCl, +197 mV vs SHE) reference electrode was used for all IR measurements. The SDS solution was then deaerated with argon for ∼1 h to remove oxygen from the solution. The mirrors of the IR system were aligned in such a manner to ensure that a collimated beam (±1°) at angle of 30 ± 2° with respect to the surface normal was incident onto the gold electrode. The gold electrode was approached to the ZnSe window to form a thin layer configuration (thin layer cavity) where the solution entrapped between the two elements of the spectro-electrochemical cell was only a few micrometers thick. The thickness of the thin layer cavity was optimized to give the maximum mean-squared electric field strength (MSEFS) using optical constants for a model system consisting of three homogeneous, parallel phases (ZnSe/solvent/Au) and custom software that solved Fresnel equations using the optical matrix method.25 The experimental thin layer cavity thicknesses and corresponding angles of incidence for the 2900, 1200, and 1000 cm−1 IR absorbance regions were then determined by fitting the experimental reflectivity spectrum, which is attenuated by the solvent layer between the gold electrode and ZnSe window, to a reflectivity curve that is generated from the optical constants using the experimental parameters as previously described in ref 25. The resulting fit is presented in section SI3 of the Supporting Information. Data Collection and Spectral Processing. All in situ SNIFTIRS experiments were carried out on a Nicolet 20DXC FTIR spectrometer (Nicolet, Madison, WI) equipped with a liquid-nitrogen-cooled MCTB detector. The FTIR sample compartment and interferometer were purged throughout the experiment using CO2- and H2O-free air, which was provided by a Puregas heatless dryer (Whatman, Piscataway, NJ). A gold wire grid polarizer (Harrick, Pleasantville, NY) was used to create a p-polarization state of the incident beam. The spectra were acquired using an Omnic macro and digital-to-analog converter (Omega, Stamford, CT) to control the potential on the PAR 173 potentiostat (EG&G, Princeton, NJ). The spectra were collected at a resolution of 4 cm−1 using the SNIFTIRS procedure described in ref 25. To minimize the effects of instrumental drifts in the acquired spectra, 100 scans (∼45 s) were recorded at a desorption potential of Edes = −700 mV versus the Ag/AgCl reference electrode. The electrode potential was then stepped to the desired adsorption potential (Eads) for 15 s before recording 100 scans at this potential. The electrode potential was then stepped back to the desorption potential and another 15 s elapsed before collecting the desorption spectra. This sequence was performed 40 times at each adsorption potential to ensure that a total of 4000 scans (i.e., 40 × 100 scans) were recorded to improve signal-to-noise. The selected adsorption potentials for these experiments were scanned in the anodic direction starting at −500 mV and finishing at 650 mV. The SNIFTIR spectra were recorded as the change in reflectivity between the desorption potential and the selected adsorption potential, which is proportional to the change in the IR absorbance ΔA due to the adsorption of SDS molecules on the gold surface, as described by the equation25
EXPERIMENTAL SECTION
Reagents. All aqueous electrolyte solutions were prepared using a Milli-Q UV-Plus (Ω ≥ 18.2 MΩ·cm) water system (Millipore, Bedford, MA) or deuterated water (Cambridge Isotope Laboratories, Andover, MA). Suprapur sodium fluoride (EM Industries, Hawthorne, NY) was cleaned in an UV-ozone chamber (Jelight, Irvine, CA) for 25 min prior to use. Sodium dodecyl sulfate (SDS), 99% purity (Fluka, St. Gallen, CH), was obtained commercially and twice filtered and recrystallized from ethanol to remove impurities and degradation products. Solution Preparation and in Situ SNIFTIRS Experimental Setup. All glassware was cleaned in hot mixed acid (1 part HNO3: 3 parts H2SO4) and rinsed thoroughly with ultrapure Milli-Q water. The glass electrochemical IR cell, equipped with a Pt foil counter electrode,25 was then soaked in Milli-Q water overnight and rinsed again on the following day. The cell was then dried in an oven at 130 °C for at least 4 h. A 1 in. diameter ZnSe hemispherical IR prism (Janos Technology, Townshend, VT) was rinsed with methanol and ultrapure Milli-Q water and then cleaned in the ozone chamber for 25 min prior to the assembly of the spectroelectrochemical setup. The electrochemical IR cell, equipped with the ZnSe prism, was mounted onto the IR bench and purged with argon (Linde Canada, Guelph, ON, CA) for at least 1 h in order to remove any residual water moisture before the collection of the dry spectrum. The recrystallized SDS powder was dissolved in a 0.1 M NaF electrolyte in H2O or D2O depending on the IR region of interest to give a final SDS concentration of ∼5.0 mM and added to the empty 2456
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⎛ ΔR ⎞ R − R ads ⎜ ⎟ = des ⎝ R ⎠SNI R des
the C−H stretching region for four separate experiments before and after the normalization of the MSEFS. The relative standard deviation for the intensity of the vs(CH2) peaks, which shows a larger spread of the normalized spectra, was 9.8%, whereas in the case of the sulfate region the relative standard deviation was ∼9%. This corresponds to an uncertainty of
