~ Spontaneous Deprotection ~
 
        This was the first project that sort of fell into my lap.  I basically needed something to do the first year of graduate school, while Prof. McDonald made final plans and began building the integrated confocal/atomic force microscope.  The carbamate, or "spontaneous deprotection" project we called it, mushroomed.

        Spontaneous deprotection may sound fishy, like spontaneous generation, or worse yet, spontaneous human combustion, but this phenomenon is documented in a contemporary peer reviewed scientific journal.

"Catalytic Activity of Silanols on Carbamate-Functionalized Surface Assemblies: Monoalkoxy versus Trialkoxy Silanes."  Blackledge, C.; McDonald, J. D. Langmuir, v 15(23),  8119-8125 (1999).
 
        Silanes have silanol group(s) which stick to the substrate by condensing into a Si-O-Si bridge, and some arbitrary dangling functional group that points up.  The functional group is usually something like an alkyl group,  bromo-alkyl, or amine; in the case here, it is a carbamate (just look at the upper left cartoon, ignore the rest for now).  The carbamate is made by protecting an aminosilane with a well-known protective agent, CBZ, making if your organic chemistry is weak, a hybrid amide/ester.
 
        Silanes have been used to alter the surface properties of materials like silica, glass, or even mica.  The resulting phase is commonly referred to as a self-assembled monolayer, or SAM.

        The interesting thing about the carbamate group is that it spontaneously hydrolyzes (A--trialkoxy=rapid, B--monoalkoxy=more slowly) at an appreciable rate when it is part of a surface assembly, which is the reaction shown in the cartoon.  Hydrolysis is extremely slow in solution.  The disparity in rates is due to the catalytic action of silanols which are in abundance on the surface.  This also explains why deprotection is rapid for trialkoxy based silanes, which have more dangling silanols than do monoalkoxies.

        Although many groups have claimed that true monolayers are formed from trialkoxy silanes, it seems there are no experimental methods to absolutely confirm this.  It is possible for there to be a few oligimerized balls of silane to be adsorped on the surface that are not detected by ellipsometry, AFM, X-ray photoelectron spectroscopy, and other experimental methods. It is not possible with mono's.

        Trialkoxies are still favored for most applications.  Trialkoxies have the advantage of forming in a matter of hours, while mono's take days, unless formation is done at an elevated temperature.  The density of silane on the surface is also greater for trialkoxies than mono's.  Moreover, the crosslinking between adjacent trialkoxies allows them to stick to surfaces that mono's can't, like mica which does not have terminal silanol groups for the silanes to attach to directly.

        Since our paper was first submitted, other groups and people have made note of differences between monoalkoxy and trialkoxy silanes, the main concern being the degree of order obtained with an organosilane "SAM."  Here are a couple examples:

"Thoughts on the Structure of Alkylsilane Monolayers." Stevens, Mark J. Langmuir, v15(8), 2773-2778 (1999).
"Characterization of the State of Order of Octadecylsilane Chains on Fumed Silica."  Wang, R.; Guo, J.; Baran, G.; Wunder, S. L.  Langmuir, v16(2), 568-576 (2000).

 Chuck Blackledge  chuckbe@scs.uiuc.edu