Determination of the heterogeneous association constants of metal ions to ω-mercaptoalkanoic acids by using double-layer capacity measurements.

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The binding of metal ions to ligands in homogeneous solns. and that to the same ligands anchored to metallic surfaces through self-assembled monolayers (SAMs) are expected to differ primarily due to the difference in the degree of freedom of the ligands and the surface potential. We studied the heterogeneous binding of CdII ions to ω-mercaptoalkanoic-acid SAMs on Au. This was accomplished by adding metal ions at a const. pH and following the changes in the double-layer capacity. A math. treatment, which is based on calcg. the electrochem.-potential differences at the double layer-soln. interface, has been developed. Our approach follows that proposed by Smith, C.P. and others (1992), White, H.S et al. (1998), Kakiuchi, T. and coworkers (200) who used the acid-base equil. at the monolayer-electrolyte interface as a means of calcg. the pK of ionizable SAMs. Exptl., SAMs of ω-mercaptoalkanoic acids, HS(CH2)nCO2H, with different chain lengths (i.e., n = 2, 5, and 10) in 0.1 M sodium perchlorate were assembled on Au surfaces and studied. The capacity was measured first in the absence of CdII at different pH values, and then at a const. pH while increasing the concn. of CdII in the soln. We found that the interfacial capacity decreased as the concn. (of either protons or CdII) increased. The results matched the model fairly well, which allowed the extn. of the thermodn. equil. const. that is established at the monolayer-electrolyte interface. The suggested math. treatment of this model system is system is simple and yet very useful for estg. the heterogeneous assocn. consts. of metal ions by SAMs. [on SciFinder(R)]


CAPLUS AN 2004:940807(Journal)