Anal. Chem. 2005, 77, 6019-6025
Use of Attenuated Total Reflection Infrared Spectroscopy for Analysis of Partitioning of Solutes between Thin Films and Solution Viatcheslav Freger* and Adi Ben-David
Zuckerberg Institute for Water Research and Department of Biotechnology and Environmental Engineering, Ben-Gurion University of the Negev, POB 635, Beer-Sheva 84105, Israel
The paper examines attenuated total reflection (ATR) spectroscopy as a tool for quantifying the partitioning of small molecular species between a solution and a thin film, while the film is directly exposed to the solution for equilibration. For the case of a thin film having a thickness substantially smaller than the decay length of the evanescent wave, we developed suitable linear relationships that relate the measured absorption of the characteristic band to the concentration of the species under study in the film and in solution. In the application of ATR-Fourier transform infrared spectroscopy, the method is particularly suitable for films a few tens to hundreds of nanometers thick and for solutes that preferentially partition into the film. As an example, the partitioning isotherm of 1-pentanol between water and a thin polyamide film separated from a reverse osmosis membrane was determined experimentally, and the limitations of the method are discussed. Attenuated total reflection spectroscopy (ATR), particularly Fourier transform infrared version (ATR-FT-IR), has become a routine technique for the study of thin films, powders, surface layers of bulk materials, and strongly absorbing solutions.1-3 Since IR spectroscopy readily distinguishes between different molecular species by focusing on specific bands, it may be utilized for quantifying relative amounts of different species in different phases. The applications of ATR-FT-IR cover a wide range of subjects, including surface modification4 and degradation,5 self* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Harrick, N. J. Internal Reflection Spectroscopy; Interscience Publishers: New York, 1967. (2) Mirabella, F. M. In Internal Reflection Spectroscopy Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993. (3) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers: Theory and Practice; American Chemical Society: Washington, DC, 1996. (4) Lehocky´a, M.; Drnovska´a, H.; Lapı´kova´, B.; Barros-Timmons, A. M.; Trindade, T.; Zembalac M.; Lapı´k, L. Colloids Surf., A 2003, 222, 125-131. Zhang, J.; Yuan, Y. L.; Wu, K. H.; Shen J.; Lin S. C. Colloids Surf., B 2003, 28, 1-9. Mao, C.; Qiu, Y. Z.; Sang, H. B.; Mei, H.; Zhu, A. P.; Shen, J.; Lin, S. C. Adv. Colloid Interface Sci. 2004, 110, 5-17. Janorkar, A. V.; Metters, A. T.; Hirt, D. E. Macromolecules 2004, 37, 9151-9159. (5) Mitra, S.; Ghanbari-Siahkali, A.; Kingshott, P.; Hvilsted, S.; Almdal, K. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6216-6229. Kiwi, J.; Nadtochenko, V. J. Phys. Chem. B 2004, 108, 17675-17684. Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089-5106. Bokria, J. G.; Schlick, S. Polymer 2002, 43, 3239-3246. 10.1021/ac050689w CCC: $30.25 Published on Web 08/11/2005
© 2005 American Chemical Society
assembled thin films,6 grafted polymer layers,7 adsorption phenomena,8 electrochemistry,9 and biological10 and synthetic11 membranes. Most of the applications apply the method for qualitative and semiquantitative analyses of compositional changes,4-11 conformation,12 orientation,1,3,13 and molecular interactions14 in thin layers attached to an ATR crystal with a high index of refraction. Quantitative studies are not common, even though the potential of ATR-FT-IR as a technique for quantitative analysis of thin films has been successfully demonstrated in a number of studies.1-3,15 The main obstacles to the more widespread use of the technique are the nonuniform distribution of the field intensity inside the film and the adjacent bulk material and the need to account separately for the three differently polarized components of the (6) Baptiste, A.; Gibaud, A.; Bardeau, J. F.; Wen, K.; Maoz, R.; Sagiv, J.; Ocko, B. M. Langmuir 2002, 18, 3916-3922. Gershevitz, O.; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482-483. (7) Korematsu, A.; Takemoto, Y.; Nakaya. T.; Inoue, H. Biomaterials 2002, 23, 263-271. Boyes, S. G.; Brittain, W. J.; Weng, X.; Cheng, S. Z. D. Macromolecules 2002, 35, 4960-4967. Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790-2796. Tomita, T.; Li, Y. J.; Nakaya, T. Chem. Mater 1999, 11, 2155-2162, Sukhishvili, S. A.; Granick, S. Langmuir 1997, 13, 4935-4938. (8) Jacobsen, R. J.; Strand, S. W. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993. Chabal, Y. J. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993. Sethuraman, A.; Belfort G. Biophys. J. 2005, 88, 1322-1333. Al-Hosney, H. A.; Grassian, V. H. Phys. Chem. Chem. Phys. 2005, 7, 1266-1276. Li, H. Y.; Tripp, C. P. J. Phys. Chem. B 2004, 108, 18318-18326. Mueller, M.; Kessler, B.; Adler, H. J.; Lunkwitz, K. Macromol. Symp. 2004, 210, 157-164. (9) Bauhofer, J. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993. Moon, S. M.; Bock, C.; MacDougall, B. J. Electroanal. Chem. 2004, 568, 225-233. Wang, S. F.; Du, D. Anal. Lett. 2004, 37, 361-375. (10) Fringeli, U. P. In Internal Reflection Spectroscopy: Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993. Jiang, C. H.; Gamarnik, A.; Tripp, C. P. J. Phys. Chem. 2005, 109, 4539-4544. Mangoni, M. L.; Papo, N.; Mignogna, G.; Andreu, D.; Shai, Y.; Barra, D.; Simmaco, M. Biochemistry 2003, 42, 14023-14035. (11) Belfer, S.; Purinson, Y.; Kedem, O. Acta Polym. 1998, 49, 574-582. Freger, V.; Gilron, J.; Belfer, S. J. Membr. Sci. 2002, 209, 283-292. Vico, S.; Palys, B.; Buess-Herman, C. Langmuir 2003, 19, 3282-3287. Ulbricht, M.; Richau, K.; Kamusewitz, H. Colloid Surf., A 1998, 138, 353-366. Han, M.; Sethuraman, A.; Kane, R. S.; Belfort, G. Langmuir 2003, 19, 9868-9872. (12) Tretinnikov, O. N.; Ohta, K. Macromolecules 2002, 35, 7343-7353. (13) Shin, E. M.; Koenig, J. L. Appl. Spectrosc. 2002, 56, 1251-1258. (14) Sammon, C.; Deng, C. S.; Yarwood, J. Polymer 2003, 44, 2669-2677. Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2004, 20, 9677-9685. (15) Tompkins, H. G. Appl. Spectrosc. 1974, 28, 335-341. Iwamoto, R.; Ohta, K. Appl. Spectrosc. 1984, 38, 359-365. Mirabella, F. M. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 861-871.
Analytical Chemistry, Vol. 77, No. 18, September 15, 2005 6019
incident IR beam.1,2 (An additional complication comprises the spectral distortions originating from the wavelength dependence of the field intensity and from optical dispersion effects near strong bands, but these may be overcome by applying special correction algorithms3 or simply by choosing specific bands with minimal overlap with other bands in the spectrum.) However, the spatial nonuniformity of the evanescent field may actually be exploited as an advantage and has indeed been utilized in applications such as depth profiling,2,3,16 the determination of diffusivity of solutes in polymers,17 and the analysis of the molecular transport and kinetics of surface binding of probe molecules in thin porous films.18 This paper analyzes a previously unexplored potential application of ATR, namely, determination of the partitioning of small molecules between a thin film and a bulk solution. Unlike previous ATR studies of partitioning and transport in thin films (e.g., refs 17 and 18), in which relative changes of the solute concentrations in the film were measured and employed in subsequent calculations, in this study we looked at how the technique could be applied to directly determine the absolute amount of a solute in a film. Our motivation was the need to quantify partitioning in thin polyamide films that form the selective layer of the modern reverse osmosis membranes used for pressure-driven separation (filtration) of solutes from water. Unexpectedly, the high passage of certain organic solutes through these membranes was inconsistent with the conventional exclusion-by-size picture, and it was assumed that differences in thermodynamic affinity toward polyamide, i.e., partitioning, might be responsible for the phenomenon (e.g., see ref 19 and references therein). Unfortunately, these thin (
