Adsorption of mercaptoacetic acid on a colloidal silver surface as investigated by Raman spectroscopy.
The vibration nu(SH) has never been observed in the surface-enhanced Raman scattering of mercaptoacetic acid recorded in a wide range of pH. This behavior enables us to deduce that the -SH group is deprotonated and links to the metal forming an Ag-S bond as 1-alkanethiols do. On the contrary, the carboxylic or carboxylate groups do not link to the metal and the carboxylic group is preserved even at pH values under which it should be deprotonated. This fact enables the stabilization of the adsorbed monolayer by removing the electrostatic repulsions between -COO(-) groups and by the formation instead of hydrogen bonds between carboxylic groups. Only under rather basic conditions (pH > 8) does the carboxylic groups dissociate, but the nu(s)(OCO) band is neither enhanced nor shifted toward low frequencies.
Castro
J L
JL
Department of Physical Chemistry, University of Málaga, E-29071 Malaga, Spain.
López-Ramírez
M R
MR
Centeno
S P
SP
Otero
J C
JC
D000327:Adsorption; D002264:Carboxylic Acids / Q000378:metabolism; D003102:Colloids / Q000737:chemistry*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D013059:Spectrum Analysis, Raman / Q000379:methods*; D055672:Static Electricity; D013864:Thioglycolates / Q000737:chemistry*
D002264:Carboxylic Acids; D003102:Colloids; D013864:Thioglycolates; C017487:2-mercaptoacetate