C O MMU N I C A T I O N S
in biosurfactants, will not. Future work will focus on exploring the
pH and the temperature dependence of Cr(VI) binding to the
functionalized surfaces. Furthermore, we plan to use functionalized
siloxanes as scaffolds for ligating a wide range of bio- and
2
4
geochemically relevant species such as metalloregulatory proteins,
5
as well as polyphenols and R-hydroxy carboxylic acids, and to
study their chemical binding and transport properties in the presence
of metal ions.
Acknowledgment. Financial support for this work was provided
by the NSF (CAREER Award CHE-0348873), the ACS-PRF (Grant
38960-G5S), the Northwestern Institute for Environmental Catalysis
(CHE-9810378 and DE-FG02-03-ER15457), the Northwestern
Nanoscale Science and Engineering Center (EEC-0118025), and a
NASA Fellowship in Earth System Sciences to A.B.V. We thank
Professors Scheidt (Northwestern University), Walker (University
of Maryland), and Chen (University of Michigan) for stimulating
discussions.
Figure 2. Cr(VI) adsorption isotherm measured at pH 7 on the acid-
bottom) and the ester-functionalized (top, offset for clarity) surfaces shown
(
in Figure 1, indicating monolayer coverage in each case. For each interface,
the data represent the composite of two runs on two freshly prepared
samples. The experiments were carried out as described previously.1
5,16
Supporting Information Available: Silane synthesis and charac-
terization and SFG spectra (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
alized silica surfaces is slower than that from the ester-functionalized
surfaces based on the shape of the desorption SHG time traces
(
Figure 1). This observation indicates that silica surfaces and those
References
containing acid-functionalized organic adlayers can decrease
Cr(VI) mobility to a larger extent than ester-functionalized organic
adlayers.
The thermodynamics of Cr(VI) binding to the acid- and the ester-
functionalized surfaces was investigated by recording the Cr(VI)
adsorption isotherms at pH 7 on both surfaces (see Figure 2). At
pH 7, the acid-functionalized surface contains both protonated and
deprotonated carboxyl groups.20 Cr(VI) binding is expected to occur
predominantly at protonated carboxyl groups, while deprotonated
carboxyl groups and CrO
repulsion.21 By applying the Langmuir model17 to the isotherm data
shown in Figure 2, we obtained free adsorption energies of 37.4 (
0
surface, respectively. The finding that the two different surface
functionalities afford the same free adsorption energies is consistent
with hydrogen bonding or ion-dipole interactions between Cr(VI)
and the carboxyl groups on the surface.
The results presented in this work suggest that a high surface
concentration of carboxyl groups on silicate surfaces can favor
Cr(VI) adsorption and decrease the mobility of these metal ions in
soil. This is consistent with the “surface chelate effect” recently
reported by Major and Zhu,22 who used ex situ XPS to demonstrate
that copper ions bind to acid-terminated thiols on gold but not to
alkanethiols on gold. In addition, our results shed light on the initial
phase of interaction between Cr(VI), a cytotoxin, and living cells:
the high mobility of Cr(VI) across cell membranes requires
Cr(VI)-membrane interaction, which is expected to be facilitated
by polar groups located at the membrane surface.
In conclusion, our work demonstrates, for the first time, how
the interaction of metal ions with organic adlayers at solid/liquid
interfaces can be studied in real time using nonlinear optical
spectroscopy. Our direct, surface-specific measurements show that
Cr(VI) can bind to ester- and acid-functionalized surfaces and
indicate that organic adlayers rich in carboxyl groups can decrease
Cr(VI) mobility in soils, whereas alkyl groups, commonly present
(
1) Brown, G. E. Science 2001, 294, 67-69.
(
2) Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston,
C.; Felmy, A.; Goodman, D. W.; Graetzel, M.; Maciel, G.; McCarthy,
M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M.
Chem. ReV. 1999, 99, 77-174.
(3) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. ReV. 2003, 103, 4883-
4940.
(
4) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower
Atmosphere; Academic Press: New York, 2000.
(5) Schwarzenbach, R.; Geschwend, P. M.; Imboden, D. M. EnVironmental
Organic Chemistry; John Wiley & Sons: New York, 1993.
(6) Langmuir, D. Aqueous EnVironmental Geochemistry; Prentice-Hall: New
2-
Jersey, 1997.
4
should not interact due to Coulomb
(7) Torrens, J. L.; Herman, D. C.; Miller-Maier, R. M. EnViron. Sci. Technol.
1998, 32, 776-781.
(8) Newman, D. K.; Bandfield, J. F. Science 2002, 296, 1071-1077.
(
9) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic
Chemistry; Wiley-Interscience: New York, 1993.
.5 and 37.1 ( 0.6 kJ/mol for the acid- and the ester-functionalized
(10) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; Wiley-Inter-
science: New York, 1996.
(
11) Al-Abadleh, H. A.; Grassian, V. H. Surf. Sci. Rep. 2003, 52, 63-162.
(12) Gustafsson, J. P.; Pechova, P. EnViron. Sci. Technol. 2003, 37, 2767-
2774.
(
13) Katz, S. A.; Salem, H. The Biological and EnVironmental Chemistry of
Chromium; VCH: New York, 1994.
(
14) Richter, L. J.; Petralli-Mallow, T. P.; Stephenson, J. C. Opt. Lett. 1998,
23, 1594-1596.
(15) Mifflin, A. L.; Gerth, K. A.; Geiger, F. M. J. Phys. Chem. A 2003, 107,
9620-9627.
(
16) Mifflin, A. L.; Gerth, K. A.; Weiss, B. M.; Geiger, F. M. J. Phys. Chem.
A 2003, 107, 6212-6217.
(
17) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: 1998.
Applications of more complex isotherm models will be presented in
forthcoming work.
(
18) Chlistunoff, J. B.; Johnston, K. P. J. Phys. Chem. B. 1998, 102, 3993-
4003.
2
3
(
19) Boily, J.-F.; Nilsson, N.; Persson, P.; Sjoberg, S. Langmuir 2000, 16,
5719-5729.
(
(
20) Gershevitz, O.; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482-483.
21) Preliminary experiments have indicated that at pH 10, Cr(VI) binding
does not occur on the acid-functionalized surface. This is consistent with
previous studies of the silica/water interface where no binding of Cr(VI)
is observed above pH 10 (ref 15 and 16).
(22) Major, R. C.; Zhu, X.-Y. J. Am. Chem. Soc. 2003, 125, 8454-8455.
(
23) Nieboer, E.; Jusys, A. A. In Chromium in the Natural and Human
EnVironment; Nieboer, E., Ed.; John Wiley & Sons: New York, 1988.
(24) O’Halloran, T. V. In Metal Ions in Biological Systems; Sigel, H., Ed.;
Marcel Dekker: New York, 1989; Vol. 25, pp 105-146.
JA048063V
J. AM. CHEM. SOC.
9
VOL. 126, NO. 36, 2004 11127