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Attenuated Total Reflection Fourier Transform Infrared Spectroscopic Investigation of the Solid/Aqueous Interface of Low Surface Area, Water-Soluble Solids in High Ionic Strength, Highly Alkaline, Aqueous Media A. R. Hind,† S. K. Bhargava,*,† and S. C. Grocott‡ Department of Applied Chemistry, RMIT, G.P.O. Box 2476V, Melbourne, Victoria 3001, Australia, and Research and Development Department, Alcoa of Australia Limited, P.O. Box 161, Kwinana, WA 6167, Australia Received February 11, 1997. In Final Form: April 8, 1997X A new method for the investigation of the adsorption of the series of surface active quaternary ammonium (QA) compounds, dodecyltrimethylammonium bromide (C12), tetradecyltrimethylammonium bromide (C14), and hexadecyltrimethylammonium bromide (C16) on the surface of sodium oxalatesa Bayer process solidshas been developed using Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy. The technique involves the use of a finely ground sodium oxalate combined with an appropriate adsorption matrix and, for the first time, permits the in situ investigation of adsorption from high ionic strength, highly alkaline (pH 12), aqueous media onto a water soluble, low surface area solid: sodium oxalate (a compound traditionally treated as ligand or adsorbate). Spectroscopic results show the formation of surfactant aggregate clusters on the surface of sodium oxalate and suggest adsorption in the order C16 > C14 > C12. This new method will allow the acquisition of “dose-response” curves for the C12, C14, and C16 QAs on sodium oxalate (under the aforementioned conditions), while also leading to the in situ investigation of the surface of sodium oxalate in synthetic and process Bayer liquors (high ionic strength, extremely alkaline media). Our results suggest that this method will be well suited to interfacial research in other similar areas involving complex, nonideal industrial systems which also depend upon chemical processes occurring at the solid/aqueous and solid/liquid interfaces.
Introduction The Bayer process is used to refine bauxite ore to smelting grade alumina. Quaternary ammonium compounds (QAs) have the potential for use as surface active agents in this process. In particular they seem suited to the role of sodium oxalate stabilizer.1-3 At present however, due mainly to the complex nature of Bayer liquor, there is a lack of fundamental understanding of the surface chemistry of these (and other) types of compounds under Bayer and Bayer-like conditions. Such an understanding of processes occurring at the solid/aqueous interface under said conditions would be of great scientific and industrial importance. Fourier transform infrared (FTIR) spectroscopy has the ability to characterize the surfaces of solids. It has been used extensively in this manner, in areas as diverse as catalysis4 and polymer characterization.5 Techniques such as transmission FTIR6 and diffuse reflectance FTIR (DRIFT)7 spectroscopy have been widely utilized in the study of solid surfaces. FTIR spectroscopy also has potential for the examination of solid surfaces in solution. Attenuated total reflection (ATR) FTIR provides a means * Corresponding author. † RMIT. ‡ Alcoa of Australia Limited. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Farquharson, J. D.; Kildea, J. D; Gross, A. E.; Grocott, S. C. U.S. Patent 5385586, 1995. (2) Farquharson, J. D.; Gotsis, S.; Kildea, J. D; Grocott, S. C.; Gross, A. E. Light Met. (Warrendale, PA) 1995, 95. (3) Farquharson, J. D.; Gotsis, S.; Kildea, J. D; Grocott, S. C. Alumina Quality Workshop, Proc. 4th Int. Conf. 1996, 447. (4) Gunter, G. C.; Craciun, R.; Miller, D. J. J. Catal. 1996, 164 (1), 207. (5) Theodore, A. N.; Zinbo, M.; Carter, R. O., III. J. Appl. Polym. Sci. 1996, 61 (12), 2065. (6) Ballinger, T. H.; Wong, J. C. S; Yates, J. T., Jr. Langmuir 1992, 8, 1676. (7) Gong, W. Q.; Parentich, A.; Little, L. H.; Warren, L. J. Langmuir 1992, 8, 118.
S0743-7463(97)00141-8 CCC: $14.00
of producing good quality spectra from in situ investigations of the solid/aqueous interface.8 In recent years, FTIR ATR has become increasingly popular as a research tool of choice in this area. ATR, developed simultaneously and independently by Harrick9 and Fahrenfort,10 is a type of internal reflection spectroscopy (IRS) in which the sample is placed in contact with an internal reflectance element (IRE) of high refractive index. Infrared radiation is focussed onto the edge of the IRE, reflected through the IRE, and then directed to a suitable detector. Although complete internal reflection occurs at the sample/IRE interface, radiation (the evanescent wave) penetrates a short distance into the sample (depending upon the wavelength of the incident light), where it can be absorbed. An absorption spectrum of the sample in contact with the IRE can thus be obtained, the spectrum being dependent upon a number of parameters, including angle of incidence and the IRE material.8 Thus, where other surface analytical techniques require the solid to be removed from solution, washed, and subjected to ultrahigh vacuum prior to analysis (completely changing the surface characteristics of the solid under examination), ATR provides a viable means of investigating the solid/aqueous interface without altering the surface characteristics of the sample. The last 15 years has seen FTIR ATR widely used in the in situ investigation of adsorption from aqueous solution.11-35 The main focus of this research has been adsorption from solution directly onto the IRE, onto a (8) Harrick, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967. (9) Harrick, N. J. J. Phys. Chem. 1960, 64, 1110. (10) Fahrenfort, J. Spectrochim. Acta 1961, 17, 698. (11) Azzopardi, M. J.; Arribart, H. J. Adhesion 1994, 46, 103. (12) Ozanam, F.; Djebri, A.; Chazalviel, J.-N. Electrochim. Acta 1996, 41 (5), 687. (13) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. J. Am. Chem. Soc. 1992, 114, 5436. (14) Couzis, A.; Gulari, E. Macromolecules 1994, 27, 3580. (15) Niu, B.-J.; Urban, M. W. Polym. Mater. Sci. 1995, 73, 368.
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thin film coated onto the IRE, or onto a finely dispersed solid making good contact with the IRE. Silicon,11,12 germanium,13,14 and KRS-515,16 IREs have been regularly used as adsorbates themselves (other materials have also been utilized),17 and IRE coatings have included elemental silicon18 and gold,19 alumina,20 polymers,21 and biopolymers.22 Of particular interest to us has been the growing volume of literature concerning adsorption from aqueous media onto finely dispersed solids. Various metal oxide,23-27 metal sulfide,28 silicon-based,29-32 and other mineral33-35 systems have been investigated, with ligands such as oxalate,24 salicylate,25 benzoate,27 and oleate29,35 studied, along with a number of surfactants including xanthate28,34 and quaternary ammonium31 compounds. These studies have relied upon the use of (relatively) high surface area (10-300 m2/g) insoluble solids while also being performed under relatively mild conditions (pH 4-10).23-25,27,28,31,33-35 Raszka and Strojek29 report investigation of adsorption onto silicon carbide over the pH range 2-11, however the solids were removed from solution and pressed onto the germanium IRE in order to obtain interfacial spectra. Mielczarski et al.28 have recently reported adsorption onto a relatively low surface area solid (0.5-1 m2/g); however, as with the aforementioned studies, the surface of an insoluble solid was the target of the investigation. Sodium oxalate is a relatively low surface area, water soluble solid whose surface chemistry in high ionic strength, highly alkaline aqueous media is of great interest to scientific and industrial researchers alike (especially those in the alumina production industry). For the first time, we present a method for the in situ investigation of the surface of sodium oxalate in high ionic strength, high pH aqueous media. Using this FTIR ATR method, we have been able to detect a series of QAs adsorbed on the surface of a finely ground, high surface area oxalate. Our results suggest that this method will permit the measurement of dose-response curves for these QAs on sodium oxalate and, combined with a method we have recently developed for the extraction and analysis of QAs in Bayer liquor,36 allow a thorough investigation of processes occurring at the oxalate/aqueous interface under Bayer process conditions. (16) Amalvy, J. I.; Soria, D. B. An. CIDEPINT 1995, 59. (17) Fan, Q.; Ng, L. M. J. Electroanal.Chem. 1995, 398, 151. (18) Sperline, R. P.; Jeon, J. S.; Raghavan, S. Appl. Spectrosc. 1995, 49 (8), 1178. (19) Zhuang, G.; Wang, K.; Chottiner, G.; Barbour, R.; Luo, Y.; Bae, I. T.; Tryk, D.; Scherson, D. A. J. Power Sources 1995, 54, 20. (20) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10, 37. (21) Scheuing, D. R. Appl. Spectrosc. 1987, 41 (8), 1343. (22) Chittur, K. K.; Fink, D. J.; Leininger, R. I.; Hutson, T. B. J. Colloid Interface Sci. 1986, 111 (2), 419. (23) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193. (24) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (25) Biber, M. V.; Stumm, W. Environ. Sci. Technol. 1994, 28, 763. (26) Tickanen, L. D.; Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1991, 7, 451. (27) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6, 979. (28) Mielczarski, J. A.; Cases, J. M.; Barres, O. J. Colloid Interface Sci. 1996, 178, 740. (29) Raszka, K.; Strojek, J. Pol. J. Appl. Chem. 1995, 39 (1), 147. (30) Tripp, C. P.; Hair, M. L. Langmuir 1993, 9, 3523. (31) Kung, K.-H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (32) Smith-Palmer, T.; Lynch, B. M.; Roberts, C.; Lu, Y. Appl. Spectrosc. 1991, 45 (6), 1022. (33) Shewring, N. I. E.; Jones, T. G. J.; Maitland, G.; Yarwood, J. J. Colloid Interface Sci. 1995, 176, 308. (34) Popov, S. R.; Vucinic, D. R. Int. J. Mineral Process. 1994, 41, 115. (35) Mielczarski, J. A.; Cases, J. M. Langmuir 1993, 9, 3357. (36) Hind, A. R.; Bhargava, S. K.; Grocott, S. C. J. Chromatogr. A. 1997, 765 (2), 287.
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Experimental Section Quaternary Ammonium Compounds. Dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide (≈99%, SigmaAldrich Pty Ltd, Castle Hill, NSW, Australia) were obtained commercially and used as received. The critical micelle concentrations (cmc) of these three compounds in water are approximately 1.5 × 10-2, 3.5 × 10-3, and 9.2 × 10-4 M, respectively.37 All other reagents used were of AR grade, and ultrapure MilliQ water was used in all instances. Surface Area Measurements. All surface area values reported are BET measurements performed with a Micromeritics ASAP 2000 surface area analyzer, using nitrogen gas adsorption. Sodium Oxalate. High surface area (HSA) oxalate was prepared from acicular (needle like) sodium oxalate supplied by Alcoa of Australia. This acicular oxalate was found to have a BET surface area of 1.3 m2/g and was originally prepared by Alcoa from AR sodium oxalate (BET surface area, ≈0.5 m2/g). This acicular oxalate was subjected to a series of treatments involving manual grinding, mechanical grinding, sieving, and ultrasonification in ethanol, until a bulk sample was obtained with a BET surface area of 3.3 ( 0.4 m2/g. This HSA oxalate was found to be suitable for performing ATR analyses and was used for all subsequent ATR studies. Adsorption Matrix. An appropriate adsorption matrix was obviously necessary in order to allow the in situ investigation of a water soluble solid in an aqueous system. Also, in order to mimic Bayer process conditions, the matrix was required to be of high ionic strength and high pH. To this end, all adsorption tests were carried out in 5 M sodium chloride solution, saturated with respect to sodium oxalate. This solution was prepared in bulk prior to the commencement of adsorption studies, with the same solution used for all of the experiments outlined in this paper. Adsorption Experiments. All adsorption experiments were carried out in 20 mL glass culture tubes with Teflon-lined caps, at a temperature of 35 °C in a thermostated water bath equipped with a rotating carousel. Adsorption matrix (10 mL) was added to HSA oxalate (0.1000 ( 0.0005 g), followed by addition of 10 M sodium hydroxide (80 µL) and “sodium humate” (extracted from Bayer process sodium oxalate; 10 µL). The surface of the HSA oxalate seed was then allowed to condition for 1 h at 35 °C, prior to addition of the appropriate QA, with the experiment halted 24 h after QA addition. The solutions were then allowed to cool to room temperature for 1 h, prior to analysis by FTIR ATR. All additions were made by means of an appropriate digital pipet/micropipet. FTIR Spectroscopic Measurements. Infrared spectra were acquired using a Perkin Elmer System 2000 FTIR spectrometer equipped with deuterated triglycine sulfate (DTGS) and liquid nitrogen cooled mercury cadimium telluride (MCT) detectors. Spectra of the samples were obtained using either 256 scans and the DTGS detector (aqueous QA samples) or 1024 scans and the MCT detector (solid/aqueous systems) at a resolution of 4 cm-1 using a strong apodization function. All spectra were recorded using a Graseby Specac horizontal ATR accessory fitted with a ZnSe IRE (45°, six reflections), whose energy throughput was always optimized prior to spectral acquisition. The ATR accessory (and surface of the IRE) were always made level to ensure uniform covering of, and contact with, the IRE. This was specially important when acquiring solid/aqueous spectra (as outlined below). All spectra were recorded with respect to the empty, energy-optimized, ATR accessory and were corrected (for variation of penetration depth with wavelength) using the ATR correction algorithm incorporated in the Perkin Elmer Spectrum software (all other spectral manipulations were also performed using this software package). No smoothing or base line correction algorithms were used on any of the spectra presented. Spectra were, obviously, dominated by the strong infrared absorbance of water. Thus, in order to acquire a useful spectrum (37) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36; U.S. Department of Commerce: Washington, DC, 1971.
Adsorption of QAs on Sodium Oxalate of the compound of interest, a reference spectrum was always subtracted from the sample spectrum. In the case of the aqueous QA samples, a water spectrum (recorded immediately after acquisition of the QA spectrum) was subtracted from the QA spectrum (subtraction factor ) 1), yielding a spectrum of the aqueous form of the QA. A similar procedure was used to obtain interfacial spectra of adsorbed QAs. As outlined by Biber and Stumm,25 spectral subtraction of a supernatant from a suspension, yields a spectrum of the solid phase and the interfacial region. Further subtraction of a spectrum of the solid phase (without adsorbate) results in a spectrum of the interfacial region only. Interfacial spectra of the adsorbed QAs were thus acquired as follows. (1) After cooling, the sample culture tube was inverted three times in order to redistribute the solid phase oxalate throughout the adsorption matrix. The slurry was then quickly sampled (from the same depth on each occasion) using a 1000 µL digital micropipet, and the sample carefully placed in the ATR trough plate making sure that the IRE was completely covered. The surface of the ATR plate and IRE were kept level to ensure an even covering of solid oxalate, thus maximizing sensitivity (see Results and Discussion). (2) The solid oxalate was then allowed to settle onto the IRE for a period of 10 min, after which spectral acquisition was begun (1024 scans co-added, taking approximately 15 min). During this time the remaining sample was centrifuged (15 min, 5000 rpm). (3) Upon acquisition of the slurry spectrum, the ATR plate was carefully removed from the sample compartment of the FTIR, and the slurry was removed with a pipet. The surface of the IRE was then thoroughly cleaned with a jet of water, the process being repeated a number of times, and was dried under a stream of air before being carefully returned to its original position in the sample compartment (using guide pins attached to the ATR accessory). A supernatant spectrum was subsequently acquired by pipetting 1000 µL of the supernatant onto the clean IRE surface, using the same number of scans as outlined above. (4) After ATR correction, subtraction of the supernatant spectrum from the slurry spectrum yielded a surface spectrum. Further subtraction of an untreated surface from a treated surface resulted in a spectrum of the interfacial region, showing adsorbed QAs. All subtraction factors used were in the range 1.00 ( 0.05. Factors outside this range suggested poor or unreproducible IRE contact or inadequate ATR plate realignment and were rejected. In such cases, the spectral acquisition was repeated. Other workers26,31 have reported subtraction factors differing quite markedly from unity, but, with the long-term goal of quantitative measurement in mind, such practice was deemed unacceptable. Spectral manipulation, as outlined by Biber and Stumm,25 was thus used to obtain interfacial spectra of the QA compounds adsorbed on the surface of HSA oxalate.
Results and Discussion HSA Oxalate. Initial attempts to acquire interfacial spectra of compounds adsorbed on the surface of sodium oxalate proved unsuccessful. The type of oxalate used initially was simply AR grade, which was found to have a surface area of 14), of high ionic strength (≈6 M [Na+]), and contain a wide variety of organic compounds, including monobasic and dibasic acids, polyhydroxy acids, alcohols, phenols, and carbohydrates.36 Sodium chloride was subsequently used a source of sodium ions, with the solution saturated with respect to oxalate to guard against the dissolution of solid oxalate. Prior to the addition of QAs, the sodium chloride/oxalate-saturated solution was made alkaline (pH ) 12) by the addition of sodium hydroxide, and, importantly, a Bayer process humic acid fraction (sodium humate) was also added. Humates are known to interact with the surface of sodium oxalate present in the Bayer process.38 We suggest that these humates make the surface of the oxalate hydrophobic and that they must be present for other surface active agents, such as QAs, to adsorb. Little is currently known regarding this phenomenon (hence the research in the area); however, preliminary research carried out using FTIR ATR suggests this to be the case. Adsorption tests performed with and without this humic fraction present only resulted in interfacial evidence of QA adsorption when the humic fraction was added prior to QA addition (as outlined in the Experimental Section). Sodium humate, an alkaline solution of the extracted humic fraction, was thus added prior to QA addition in all the tests performed. The humic fraction was added such that it was present in the tests at the same concentration as found in Bayer liquor. Aqueous Spectra. Aqueous spectra of three QAs studied were acquired at concentrations above and below their respective cmc values in order to ascertain whether the QAs were detectable and quantifiable and to determine the nature of cmc effects (if any) on the analysis. Spectra of each of the QAs were thus acquired at 25%, 75%, and 125% of their reported37 cmc values, with peak positions and peak areas determined after water subtraction. The FTIR ATR aqueous spectra of the C-H stretching region of the C12, C14, and C16 QAs can be seen in Figure 1. The two strong bands around 2925 and 2855 cm-1, in the C12, C14, and C16 spectra, were assigned to the asymmetric and symmetric CH2 stretching vibrations of the methylene chain, respectively, while the two weaker bands evident around 2955 and 2875 cm-1 were assigned to the R-CH3 asymmetric and symmetric stretching vibrations, respectively. These assignments are in good agreement with the literature.31,39 No other bands were discernible in the ATR spectra obtained. It is generally accepted that CH2 stretching absorption energies provide a measure of the degree of order/disorder, compactness, and crystallinity of the methylene chains in surfactant aggregate structures.31 Thus, in moving from less-ordered sub-cmc solutions to more-ordered micellar solutions, a shift to lower wavenumber (5-10 cm-1) is expected.31 This phenomenon, however, was not observed in this case. The CH2 stretching frequencies were found to be constant at values indicative of micellar structures (see Table 1). This may be explained by the formation of micelle-like aggregates at the ZnSe IRE surface even at sub-cmc QA concentrations (see Figure 2). Previous studies employed short path length (18 µm) cells utilizing (38) Personal communication, S. C. Grocott, R&D, Alcoa of Australia Ltd, WA, Australia. (39) Weers, J. G.; Scheuing, D. R. Micellar sphere to rod transitions. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991.
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Figure 1. FTIR ATR spectra of C12, C14, and C16 QAs in the aqueous state.
Figure 2. Schematic diagram representing the formation of micelle-like QA aggregates at the ZnSe IRE surface. Table 1. Methylene Chain Stretching Frequencies for C12, C14, and C16 QAs (in cm-1) QA
experimental
literature (micelle)31
C12 C14 C16
2926, 2855 2925, 2854 2924, 2853
2923, 2853
CaF2 windows for sub-cmc measurements, as they were unable to detect sub-cmc QA concentrations using the ATR technique, reporting that both ZnSe and Ge IREs were insufficiently sensitive to obtain reproducible spectra.31 If the anticipated QA dose-response work was to be in some way quantifiable, reliable quantification of each QA was necessary. Calibration curves could then be used to estimate the amount of QA adsorbed on the surface of sodium oxalate in each case, providing a more complete picture of the adsorption process. Examination of the peak areas of the CH2 stretching vibrations for each of the QAs revealed acceptable linearity over the concentration range investigated (see Figure 3), with a tendency to nonlinearity noticeable above the cmc of each QA (as might be expected). However, with the sub-cmc section of the calibration curves exhibiting good linearity, it was concluded that the calibration was adequate in the context of the work proposed (provided that only the linear portions of the calibration curves below the cmc were used for quantitation). Interfacial Spectra. As outlined in the Experimental Section, interfacial spectra can be obtained by a process of spectral subtraction: slurry minus supernatant yields a solid spectrum; treated solid minus untreated solid yields an interfacial spectrum. Using this technique, interfacial spectra of the C12, C14, and C16 QAs adsorbed on sodium oxalate were acquired. Figure 4 shows FTIR ATR interfacial spectra of the adsorbed QAs. The interfacial
spectra of the adsorbed form of the QAs are identical to those of the aqueous form of QAs discussed previously. The positions of the asymmetric and symmetric CH2 stretching bands are also identical to those of the aqueous form and are indicative of micelle-like aggregates.31 This in turn is indicative of the formation of micelle-like aggregate structures on the surface of sodium oxalate. Exactly how the QAs attach themselves to the surface of sodium oxalate is still unclear, however two possibilities present themselves. The first is direct interaction with the surface of sodium oxalate, with the positively charged head-groups of the QAs interacting with the negatively charged carboxylate groups at the oxalate surface (see Figure 5), leaving the methylene tails of the QAs oriented away from the oxalate surface and free to interact with one another. The second possibility involves indirect interaction with the surface of sodium oxalate. Some evidence currently exists which suggests the surface of oxalate is in fact positively charged under Bayer-like conditions, possibly due to a layer of sodium ions present at the oxalate surface. If so, the direct interaction of the positively charged head-group of the QA with the oxalate surface is unlikely. More probable might be an interaction involving the humic material present. The exact nature of this interaction is unknown, however, and is the subject of on-going research. Electrokinetic studies on quartz at neutral pH have shown that alkylammonium ions associate and form hemimicelles at the quartz surface, with the hemimicelle concentration, or the bulk concentration required to induce association at the interface, increasing with decreasing chain length.40 This suggests that adsorption should take place in the order C12 < C14 < C16, and preliminary results suggest this to be the case. QA “dose-response” work is currently underway, in order to confirm this finding. Conclusion The surface of sodium oxalate (a water soluble, Bayer process solid) has, for the first time, been probed in situ using the FTIR ATR spectroscopic technique. The method involves the use of a finely ground, high surface area sodium oxalate combined with an appropriate adsorption matrix and is suitable for use in high ionic strength, high pH media. Central to the technique is the higher adsorbate surface density and greater, more uniform IRE (40) Wakamatsu, T.; Fuerstenau, D. W. Adv. Chem. Ser. 1967, 79, 161.
Adsorption of QAs on Sodium Oxalate
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Figure 3. Plots of peak area (2925 cm-1) versus QA concentration (g/L) for (a) C12, (b) C14, and (c) C16 QAs. Each point on each plot represents 25%, 75%, and 125% of each QAs cmc value.
Figure 4. FTIR ATR spectra of C12, C14, and C16 QAs adsorbed on the surface of sodium oxalate.
C14, and C16 compounds on sodium oxalate (work which is currently underway), while also enabling the in situ investigation of the surface of sodium oxalate in synthetic and process Bayer liquors (high ionic strength, extremely alkaline media). We also believe this method to be well suited to interfacial research in other similar areas involving complex, nonideal industrial systems which also depend upon chemical processes occurring at the solid/ aqueous interface. Figure 5. Schematic diagram depicting the positively charged head-groups of the QAs interacting with the negatively charged carboxylate groups at the oxalate surface, leaving the methylene tails of the QAs oriented away from the oxalate surface.
contact afforded by the high surface area oxalate. Using this technique, adsorbed QA compounds dodecyltrimethylammonium bromide (C12), tetradecyltrimethylammonium bromide (C14), and hexadecyltrimethylammonium bromide (C16) have been detected on the oxalate surface in the form of micelle-like aggregate structures. Our results suggest that this new method will allow for the acquisition of “dose-response” curves for the C12,
Acknowledgment. S.K.B. and S.C.G. thank the Australian Research Council for financial support, provided in the form of an APA(I) Scholarship for A.R.H. The support, both financial and technical, of the Department of Applied Chemistry, RMIT, and Alcoa of Australia Limited is also greatly appreciated. A.R.H. also thanks Mr. Michael Shaw and Dr. Marisa Ioppolo-Armanios of Alcoa of Australia Limited for useful discussions in the area. LA970141N
