COMPLEXATION OF SILVER CLUSTERS OF A FEW ATOMS BY A POLYANION IN AQUEOUS-SOLUTION - PH EFFECT CORRELATED TO STRUCTURAL-CHANGES

Gamma irradiation of aqueous solutions containing AgNO3 and polyacrylic acid (PAA) in basic medium (pH=10 to 11) leads to the formation of a blue-coloured silver cluster (a-type) associated to the PAA polyanion, which displays a two-peak spectrum with one band (λmax=292 nm) characteristic for silver and the other (λmax=800 nm), not yet reported, characteristic for the polyanion-silver interaction. This a-type species is indefinitely stable at pH> 10 and can be dried as a composite film. Progressive acidification of a solution of this a-type species results in important changes in the spectrum: these changes indicate the successive formation of two other silver clusters b and c (λmax=325 and 375 nm, respectively) associated with the polymer ligand in a distinct way (ligand bands peaking at 640 and 465 nm) with an increasing aggregation number from a to c. The size of the c-type cluster is estimated to be close to eight. The whole process is discussed in terms of conformation changes in PAA induced by progressive protonation of the carboxylate groups. The shift of the ligand band shows that the metal-ligand interaction depends on the aggregation number and on the ligation power (higher for the anionic form -COO- than for the protonated form -COOH). © 1990.