Langmuir 1997, 13, 4829-4836
4829
Quantitative Characterization of Aqueous Suspensions using Variable-Angle ATR-FTIR Spectroscopy: Determination of Optical Constants and Absorption Coefficient Spectra Lane D. Tickanen, M. Isabel Tejedor-Tejedor,* and Marc A. Anderson Water Chemistry Program, University of Wisconsin, 660 North Park Street Madison, Wisconsin 53706 Received March 27, 1996. In Final Form: June 13, 1997X In one step further toward developing ATR-FTIR (attenuated total reflection-Fourier transform infrared) spectroscopy as a quantitative technique to interpret interfacial phenomena in aqueous colloidal suspensions, we have used variable angle ATR spectroscopy to concurrently determine the Kramers-Kronig integration constant (or anchor point, nr) and spectra of the optical constants (refractive index, n, and absorption index, k) for R-FeOOH and TiO2 (anatase) aqueous suspensions. In addition, the absorption coefficient (a) spectra were calculated from the k spectra. The suspension conditions (pH, ionic strength, and concentration) were such that the samples were homogeneous over the sampling depth of the IR radiation, behaving as a thick film for ATR. This study shows that discrepancies in the value of the refractive index of a suspension and its corresponding supernatant generate distortion in the regions of strong water absorption of the ATR difference spectra (suspension spectrum-supernatant spectrum). This distortion reduces the ability of the ATR difference spectra to provide quantitative and in many cases qualitative information on the vibrational characteristics of the particles and associated interface. This information, however, can always be extracted from the a-difference spectra calculated with the methodology proposed in this paper.
I. Introduction For the past several years we have been using Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy as an “in situ” method of examining the chemical nature of interfacial complexes in aqueous suspensions. While this technique provided a great deal of qualitative information about interaction of the surfaces with different ligands, we have, until now, been unable to define these systems quantitatively. In ATR spectroscopy, the probe radiation is an exponentially decaying wave that extends from the internal reflection element (IRE) into the sample. Because the sampling mechanism arises from reflection phenomena, the profiles of the absorption bands in the ATR spectra are influenced by factors other than concentrations and oscillator strengths, such as the thickness and homogeneity of the sample, the optical constants (absorption index and refractive index) of the sample, and the refractive index of the IRE. In order to isolate the vibrational spectra for suspended particles and associated interfacial material quantitatively, it is thus necessary to know the arrangement of particles in the sample and the refractive indices of the samples (suspension) and reference (supernatant). In the first paper in this series,1 we demonstrated the use of variable angle ATR-FTIR spectroscopy to determine the distribution of suspended particles in the sampling region near the IRE. In this paper, we present a more complete methodology for the quantitative study of aqueous suspensions using ATR-FTIR spectroscopy, wherein the development of ATR methods for determining optical constants of suspensions is emphasized. In our previous paper,1 suspensions of goethite (RFeOOH) under different solution and solids concentration conditions were used as study media. The ATR “difference” spectra (spectra of the suspended particles and * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Tickanen, L. D.; Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1991, 7, 451.
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associated interfacial material) of these suspensions showed that particle concentration can be seen by the evanescent wave either as stratified or as homogeneous, depending on the state of aggregation of the suspension. Additionally, we devised a method to determine the distribution of particles in terms of films or layers of suspended particles and supernatant liquid. Three models were necessary to describe different behaviors exhibited by the suspended particles under different solution conditions and states of aggregation. A “thick film” model2 was found to describe the behavior of the colloidal particles in many suspensions in which the charge on the particles was high and the ionic strength of the surrounding solution was low (
