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Sulfonation of Alkyl Phenyl Ether Self-Assembled Monolayers
Irit Katash, Xianglin Luo, and Chaim N. Sukenik*
Department of Chemistry and Institute of Nanotechnology and Advanced Materials, Bar Ilan University,
Ramat Gan 52900, Israel
Received June 11, 2009. Revised Manuscript Received November 27, 2009
The sulfonation of phenyl ether decorated self-assembled monolayers (SAMs) was studied with an eye toward
creating surfaces with a particularly high negative charge density based on a close-packed array of phenyl rings with
more than one sulfonic acid group per molecule. The product distribution and kinetics of this process were studied by
ultraviolet, infrared, and photoelectron spectroscopies and by monitoring changes in the thickness and wetting
properties of the SAM. The sulfonation chemistry could be effected without undermining monolayer integrity and the
isomer distribution of ortho- and para-monosulfonated material, along with the percentages of mono- and disulfonated
molecules could be established throughout the process. As doubly sulfonated molecules appeared, the reaction slowed
drastically. Ultimately, sulfonation stops completely with approximately 60% of the molecules disulfonated and 20%
each of the two monosulfonated isomers. This striking constraint on monolayer reactivity and the relationship between
the surface chemistry and variations in SAM structure are discussed.
Introduction
SAMs is often achieved using in situ functional group transfor-
mations analogous to those typical of bulk media.5,12-26
Reactions that occur at interfaces often differ from their
solution analogues.27-38 The rates and products of interfacial
reactions, for example, often show significant dependence on
surface structure. The number of examples32-38 that clearly
address these issues is limited, in large part, because of the
difficulty in devising appropriate model systems and in applying
the standard approaches of physical organic chemistry to inter-
facial reactions.
Self-assembled monolayers (SAMs) enable the fabrication of
molecularly tailored interfaces with precisely controlled physical
and chemical properties.1-3 The most commonly studied SAMs
are based on siloxanes4,5 and phosphonates6,7 linked to oxide
surfaces or thiols attached to the surface of coinage metals.1,8,9
The alkyl thiols are well suited for the direct preparation of
functionalized monolayers.1,3,10 The phosphonates offer a similar
advantage.7 However, the siloxane-anchored SAMs (despite
being more robust) are limited in terms of the range of functional
groups that can be installed by direct SAM deposition due to the
reactivity of the halo- and alkoxysilanes from which they are
made.5,11 Thus, the chemical diversity of siloxane-anchored
Much remains to be learned about how the incorporation of
reactive groups in monolayers influences their reactivity. It is
known that the anisotropy and packing of a monolayer can have a
strong influence.13,32,37,39,40 These differences are often referred to
*Corresponding author.
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Langmuir 2010, 26(3), 1765–1775
Published on Web 01/04/2010
DOI: 10.1021/la902093x 1765