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Kevin Braeckmans <[hidden email]> wrote:
Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal Dear all,
Can someone tell me what is the surface charge of a typical microscope cover glass?
And a related question: can someone explain why negative
nanospheres (carboxylated polystyrene), negative pDNA molecules, as well as positive liposomes or lipoplexes all strongly adhere to the cover glass?
Thanks,
Kevin
Dear Kevin,
The surface charge of glass depends on the pH and ionic strength of the solution with which it's in contact. A detailed and illuminating description can be found in
The charge on glass and silica surfaces, S. H. Behrens and D. G. Grier,
Journal of Chemical Physics 115, 6716-6721 (2001),
available at:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000115000014006716000001 and (free) at
http://arxiv.org/abs/cond-mat/0105149 For example, at pH 7 and 1 uM ionic strength, the effective charge density of silica is about 1000 charges / um^2, while at 10 uM it's about 4000.
As for your second question: I assume you're observing this adhesion in some sort of buffer or salt solution, rather than plain water. There are two important reasons negative objects will adhere to negative glass. (i) Interactions between objects in solution are determined by two forces: van der Waals interactions, which are "always" attractive but typically short range, and electrostatic interactions, which can be attractive or
repulsive. In an electolyte, electric fields are screened. In 100 mM NaCl, for example, fields decay within a few nanometers. Therefore negative objects can come quite close to the negative glass, a few nm, without being repelled; at this point the attractive van der Waals forces make them stick. (ii) If you have multivalent cations in your electrolyte, e.g. Calcium, this can "bridge" single negative charges, binding negatively charged objects to one another . Imagine [-1] -- [+2] -- [-1].
This is a quick and incomplete discussion (especially item (i) above, which ignores the repulsive forces due to the "gas" of counterions between objects). If you'd like more, I recommend Israelachvili's excellent book, "Intermolecular and Surface Forces."
best wishes,
Raghu
--
Raghuveer Parthasarathy
[hidden email]Assistant Professor
Department of Physics
1274
University of Oregon
Eugene, OR 97403-1274
http://physics.uoregon.edu/~raghu/
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