Abstract:

Measurements of the autocorrelation function of scattered light intensity from aqueous tetradecylpyridinium bromide (TPB) solutions werec arried out to investigate the growth of TPB micelles in high salt (NaBr) concentration media. The mean hydrodynamic radius (R_h) of TPB micelles has been deduced as a function of temperature (15-85 ℃), NaBr concentration (0.5-2.0 M), and detergent concentration(2-21 gdm-3). In general, the size of the micelles increases with increasing salt or detergent concentration and decreasing temperature. A plot of measured R_h vs. relative scattered intensity, compared to theoretical curves corresponding to different model shapes, indicates that the enlarged TPB micelles have a shape of prolate spherocylinders. Based on these R_h measurements, values of the thermodynamic parameter K governing the sphere-to-rod transition are determined. Numerical values of enthalpy and entropy change for this transition on per monomer basis, deduced from the linear plot of lnK vs. 1/T in 0.5 and 1.0 M NaBr, are rather consistent with those of CPB micelles, indicating that these energy parameters are independent of the chain length. In respect to temperature-dependent micellar growth an anomalous behavior that a finite size of about 26.0 nm is asymtemperature has been observed under conditions of higher NaBr concentration (2.0 M). In this case the thermodynamic model of the sphere-to-rod transition is invlid and the possible causes are discussed.