Aggregation behavior and micellar dynamics in aqueous solutions of the nonionic surfactant pentaoxyethyleneglycol monooctyl ether: Effect of sodium halides

Abstract

The aggregation behavior of pentaoxyethyleneglycol monooctyl ether (C 8E 5) in aqueous solution and some aspects of the dynamics of the C 8E 5 micelles have been investigated in the absence of added salt and in the presence of sodium halides, as a function of surfactant concentration, C (0.05 to 0.2 M), and temperature, T (2–50°C), by time-resolved fluorescence quenching. Some measurements of cloud temperature, T c , also have been performed. Both in the absence and in the presence of salt it has been found that the micelle aggregation number, N, increases slowly with T at temperatures below T c −40°C, then increases more rapidly at T> T c −40°C. The presence of salt amplifies the increase of N with T. It has been found that N is mainly determined by T c − T . Thus all N values for the 0.1 M C 8E 5 solutions in the presence of salt fall on a single curve when plotted versus T c − T , irrespective of the nature and concentration of the added salt. As for other nonionic surfactants, a rapid intermicellar migration of probe and quencher has been evidenced at T> T c − 40°C. This process has been confirmed to take place through collisions among micelles with temporary merging of the collided micelles. The rate constant K e for this process increases with T and with the concentration of added salt, that is, in both instances, upon decreasing T c − T . In the course of this work a peculiar behavior was noted when NaI was added to C 8E 5 solutions and this behavior was interpreted in terms of the formation of I 3 catalyzed by H +, in close proximity to the polyoxyethylene moieties of the surfactant micelles.