Grazing Incidence XAFS of Lead Adsorbed Underneath Fatty Acid Langmuir Monolayers Maxim Boyanov,* Alokmay Datta, + Tomohiro Shibata,* Jan Kmetko + , Bruce A. Bunker,* Pulak Dutta + * University of Notre Dame, Notre Dame, IN 46556 + Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3112 Introduction Langmuir monolayers of long-chain fatty acids with divalent metal cations in the aqueous subphase are of long-standing interest, especially as precursors to technologically useful Langmuir-Blodgett films with a considerable degree of structural order [1]. Grazing Incidence Diffraction (GID) of x-rays from synchrotron sources have been used to study the structure of such monolayers. For Cd ions in the subphase GID data from a monolayer at near zero surface pressure show diffraction peaks from an asymmetrically distorted fatty acid monolayer along with weaker peaks from a supercell of the monolayer lattice which have been assigned to a monolayer of Cd ions [2]. However, direct confirmation of the presence of a metal monolayer is lacking, as well as more detailed information about the immediate environment of the metal ions. Preliminary results from grazing incidence XAFS of Zn ions underneath a fatty acid Langmuir monolayer indicate, among other things, a decrease in the Zn coordination number with respect to the bulk value [3] indicating a considerably different microenvironment. GID studies on heneicosanoic acid (CH 3 (CH 2 ) 19 COOH, C21 acid) on a subphase containing Zn ions show peaks only from the fatty acid monolayer [4] whereas some extra peaks similar to those with Cd ions are seen when Pb ions are present in the subphase [5]. In this study, systematic polarization dependent grazing incidence XAFS studies at the L 3 edge of Pb has been carried out on a monolayer of C21 acid with Pb ions in subphase to detect the interfacial adsorption of Pb by the fatty acid monolayer and to selectively probe the in-plane environment of the metal atoms in this thin layer. Methods and Materials XAFS studies were carried out at the MRCAT 10-ID beamline at the Advanced Photon Source. The MRCAT beamline is equipped with a cryogenic double-crystal Si (111) monochro- mator and tunable undulator which allows XAFS measurements over the energy range from 5 keV to over 30 keV. A harmonic rejection mirror was used to eliminate the third and higher x-ray harmonics from the monochromator. The equipment used has been described elsewhere [6]. The x-ray beam at around 13 keV (near the L 3 absorption edge of Pb) was defined to a vertical width of 20μm and a horizontal width of 15 mm by slits and was made incident on the monolayer-covered water surface in a Langmuir trough at ∼ 0.8 mrad, below the critical angle of water at 13.5 keV to achieve grazing incidence. The trough is an enclosed system with Kapton windows for the incident and scattered x-rays and an inlet and outlet to maintain a slight overpressure of He. This reduces radiation damage of the film. GID data were collected on monolayers as a check on the diffraction pattern and to see the extent of radiation damage suffered by the monolayers. Fluorescence detection mode was used, with 100% Kr in the two detectors which were placed in- plane and vertically above the liquid surface. The two signals are separately collected in slew-scan mode (about 9 min/scan). About 65μL of C21 acid in chloroform (0.9mg/mL) was spread on millipore water (resistivity 18MΩ-cm) with ∼10 -4 M PbCl 2 dissolved in it. Monolayers were compressed by a constant perimeter barrier to a slight positive surface pressure which was taken to be ∼1.0±0.5 dynes/cm (Balance ST9000, Nima Technology). Temperature was maintained at 9°C by circulating water. The pH of the subphase was either unadjusted (∼5) or raised to ∼7 by CsOH. GID checks showed the presence of both C21 acid monolayer peaks and the weak peaks as previously observed [5]. The pattern was found to be stable for about 45 mins under radiation damage. Hence the monolayer was exposed to x-rays for this time during each XAFS scan. Continuous (slew)-scanning mode was used which minimizes the radiation damage to the film. The x-ray penetration depth at the L 3 edge is about 120Å. Thus the x-rays sampled the monolayer and about 100Å of the subphase underneath, and there could be some contribution in the XAFS signal from ions in this solution (“bulk” ions). XAFS data were also collected at grazing incidence for a 10 -4 M PbCl 2 solution without the C21 acid monolayer, to assess the effect of the fatty acid monolayer on the Pb ions. For calibration standards used in the XAFS analysis, various standard lead compound spectra measured previously at NSLS [7] were used, as well as some taken from the XAFS database [8]. Results and Discussion In absence of the C21 acid monolayer no fluorescence signal was observed from the PbCl 2 solution. The clear signal obtained from a solution with the same concentration in presence of the fatty acid monolayer is an unequivocal indication that the fatty acid headgroups are causing formation of a dense layer of Pb atoms or ions at the air-water interface. The Fourier transforms (FTRs) of the data are shown in Fig. 1. These correspond to a 0 2 4 6 8 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 s1a s1slow pbfoil Amplitude, a.u. Radial distance, Å Fig. 1. Fourier transform magnitudes for the PbCl 2 / C21 monolayer sample and for metallic Pb. Data obtained both conventional energy step scans and a continuous slew scan are both shown for the unadjusted pH sample.