| Popis: |
In the present paper, we report on the rate of exchange of 3H-labelled polymers in the adsorbed state with non-labelled polymers in the solution. The method consisted in carrying out preliminary adsorption with radioactive polymers, and subsequently exposing the surface to a solution of non-labelled polymers, and in continuously analyzing the radioactivity of the solution. in that way, chemical composition at thermodynamic equilibrium was maintained constant; only the isotope concentration ratios at the surface and in solution were different. — Investigations were carried out on two systems: — non hydrolyzed polyacrylamide adsorbed from aqueous solutions onto well defined alumino-silicate surfaces with (SiO)2AlOH and SiOH surface groups statistically distributed; and — polystyrene adsorbed from carbon tetrachloride onto silica beads with SiOH surface sites. — For both systems, we found that the rate of exchange of isotopes is very small, which means that polymers have a unusual long “lifetime” in the adsorbed state. The rate of exchange is also a function of polymer concentration and in a certain time interval the kinetics obeys a simple second-order rate reaction characterized by one relaxation time. This is consistent with interfacial equilibrium and kinetically indistinguishable molecules. However, the radioactive polymers desorbing first cannot be characterized by one relaxation time. For polystyrene, the isotherms display a plateau region followed by a region were adsorption increases. The kinetic properties are very different in both regions. Especially at large solution concentration the interfacial exchange cannot be described in terms of one rate constant; instead, a whole spectrum of relaxation times must be considered. |
