Radiation chemistry of aqueous solutions of dicyandiamide

Abstract

Oxygen-free aqueous solutions of dicyandiamide, DCDA, were exposed to /sup 60/Co ..gamma.. rays or a pulsed 10-MeV electron beam. Fast kinetic spectrophotometry was used for the study of free-radical intermediates, and computer simulation of the reaction mechanism for evaluation of experimental data. The hydrated electron reacts by addition, k(e/sub aq//sup -/ + DCDA) = 1.1 x 10/sup 10/M/sup -1/s/sup -1/; the anion radical (NH/sub 2/C(=NH)NHCN)/sup -/ absorbs in the UV region with lambda/sub max/<255nm and an epsilon (255) of 1150 M/sup -1/cm/sup -1/. It disappears with 2k = 1 x 10/sup 9/M/sup -1/s/sup -1/ by forming a product which absorbs with lambda/sub max/<255nm and an epsilon (255) of 2400 M/sup -1/cm/sup -1/. The hydrogen atom also reacts by addition, k(H + DCDA) = 2.7 x 10/sup 6/ M/sup -1/s/sup -1/, and the radical intermediate has a lambda/sub max/ of 350nm and an epsilon (350) of 1250 M/sup -1/cm/sup -1/; it disappears with 2k greater than or equal to 10/sup 9/M/sup -1/s/sup -1/. Hydroxyl radicals react both by addition and by abstraction, k(OH + DCDA) = 5.6 x 10/sup 6/M/sup -1/s/sup -1/, and two transients with lambda/sub max/ at 450 and 355nm appear. It is estimated that about 90% of OHmore » radicals react by abstraction. It has also been found that the radical-anion SO/sub 4//sup -/ reacts efficiently with DCDA and the product of this reaction, NH/sub 2//sup +/C(=O)NHCN, absorbs light with a lambda/sub max/ of 355nm and epsilon (355) of 1150 M/sup -1/cm/sup -1/. The OH adduct of DCDA has a lambda/sub max/ of 450nm and epsilon (450) of 1700 M/sup -1/cm/sup -1/; it disappears by a second-order process with 2k less than or equal to 4 x 10/sup 9/M/sup -1/s/sup -1/. The search for stable radiolytic products in the kilorad-megarad dose range has shown the presence of only a few smaller molecules and in low yields.« less