Salt effects on solute exchange in sodium dodecyl sulfate micelles

We describe the influence of sodium chloride on the rate of solute exchange in aqueous SDS micelles for a water-insoluble solute, a pyrene-containing triglyceride 1. The initially prepared solutions contained a small fraction of micelles containing two molecules of 1 and a large excess of empty micelles. These solutions showed a measurable excimer emission (of intensity I(E)) that was stable for days to weeks in the absence of added salt. Following additions of salt, I(E) decayed exponentially (rate constant, k(obs)) accompanied by an increase in pyrene monomer emission. Values of k(obs) increased strongly with ionic strength (k(obs) similar [Na(+)](4)). There was no contribution of the empty micelle concentration beyond its contribution to the sodium ion concentration. We conclude that the solute exchange involves spontaneous fragmentation of the SDS micelles into two submicelles, each bearing a molecule of 1, which then grow back to normal micelles through condensation of SDS monomers. We propose a model for the fragmentation process in which large amplitude surface fluctuations "pinch off" a subunit that becomes a submicelle. These fluctuations bring sulfate headgroups into close proximity. Fluctuations leading to fission become important only in the presence of sufficient counterion concentration to reduce the electrostatic repulsion between neighboring headgroups.     

Exchange mechanisms for sodium dodecyl sulfate micelles: high salt concentration

Solute exchange experiments for the pyrene-labeled triglyceride TG-Py solubilized in sodium dodecyl sulfate (SDS) micelles in the presence and absence of salt show that the "observed" rate constant k(obs) for solute exchange varies by over 6 orders of magnitude as the free sodium ion concentration [Na(+)](aq) is varied between 10 and 850 mM. There is a sharp break in the log-log plot of k(obs) versus [Na(+)](aq) in the range of [Na(+)](aq) = 200 mM, with the exchange rate showing a weaker dependence on [Na(+)](aq) above this concentration. Up to 100 mM added NaCl, this exchange takes place essentially exclusively by a micelle fission mechanism in which each submicelle carries off one of the solutes. At higher salt concentrations, a bimolecular process becomes increasingly important. This fusion process, which involves formation of a transient supermicelle followed by fission back to two normal micelles, becomes the dominant process at high salt concentrations. The fission rate appears to level off for salt concentrations above 300-400 mM. These fission and fusion processes are related in an intimate way to the changes in the size and shape of the SDS micelles with increasing salt concentration.     

Stratified morphology of a polypropylene/elastomer blend following channel flow

A thermoplastic olefin blend consisting of isotactic polypropylene (PP) and an ethylene‐butene copolymer (EBR) impact modifier (25 wt % EBR) was subjected to a short, high‐shear pulse within the flow channel of a pressure‐driven microextruder following low‐shear channel filling from a reservoir of the melt. The resulting morphology was examined by laser scanning confocal fluorescence microscopy (LSCFM), with contrast provided by a fluorescent tracer in the EBR minor phase. Shear experiments were performed under isothermal conditions with a known wall shear stress for a specified duration, providing a well‐defined thermal and flow history. Low‐shear channel filling produces small droplets across the central region of the channel and large droplets, consistent with steady‐state shear, in the regions near the channel walls. After cooling the molten blend to a crystallization temperature of 153 °C, a brief interval (5 s ～ 1/2000 of the quiescent crystallization time) of high shear (wall shear stress: 0.1 MPa) induces rapid, highly oriented crystallization and a stratified morphology. Ex situ LSCFM reveals a “skin” at the channel walls (～70 μm) in which greatly elongated fiberlike droplets, oriented along the flow direction, are embedded in highly oriented crystalline PP. Further from the walls but directly beside the skin layers are surprising zones in which EBR domains show no deformation or orientation. Several zones of intermediate deformation and orientation at an angle to the flow direction are located closer to the center of the channel. At the center of the channel, EBR droplets are spherical, as expected for channel flow. The various strata are explained by the interplay of droplet deformation, breakup, and coalescence with the shear‐induced crystallization kinetics of the matrix. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2842–2859, 2002     

Surface‐directed morphology evolution in ternary blends of polyethylene,polypropylene, and ethylene–propylene copolymer: A study by laser scanning confocal fluorescence microscopy

With laser scanning confocal fluorescence microscopy, we demonstrate a novel type of morphology evolution in moderately thick films (70–100 μm) of ternary blends of polypropylene (PP), polyethylene (PE), and ethylene–propylene rubber (EPR), in which EPR is labeled with a benzothioxanthene dye (HY‐EPR). The blends are prepared by solution blending, and the phase morphology evolves during the annealing of the blend films in a stainless steel mold. Our results indicate that wetting of the mold surface is a driving force in morphology evolution for the two blend compositions investigated. For 81/14/5 PP/PE/HY‐EPR, phase evolution within the mold results in a laminar structure and hydrodynamic channels, features which have previously been found in thin films of polymer blends as a result of surface‐directed spinodal decomposition. In a blend with a lower weight fraction of the dispersed phase (92/7/1 PP/PE/HY‐EPR), we find that the PE/HY‐EPR domains are larger and more polydisperse closer to the surface because of wetting of the mold wall. We also show that the phase morphology in these films can be controlled by the nature of one or both of the surfaces being varied. When one of the mold surfaces is replaced with a thin film of PP homopolymer, we observe draining of PE/HY‐EPR from the PP to the mold surface, which results in a bilayer structure. A trilayer morphology is likewise obtained by the replacement of both mold surfaces with PP. We also carry out three‐dimensional image reconstruction on a single PE/HY‐EPR particle within the 81/14/5 PP/PE/HY‐EPR blend to obtain detailed information on the interphase structure. We find that HY‐EPR of this composition (30/70 ethylene/propylene) fully coats the PE dispersed phase and partially penetrates the PE droplets. This result falls between the interphase structures found for previously investigated EPR compositions (40/60 and 80/20 ethylene/propylene). © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 637–654, 2003     

Small Variations in the Composition and Properties of Triton X-100

Two samples of Triton X-100 (TX100) were compared. The "old sample," manufactured by Rohm and Haas, has a mean degree of ethoxylation of 10.1. The "new sample," manufactured by Union Carbide, has a mean degree of ethoxylation of 9.6. The cmc values near room temperature (23 degrees C, old sample; 24.6 degrees C, new sample), as determined from surface tension measurements, are similar. Dynamic light scattering measurements on the new sample gave hydrodynamic radii at different temperatures in reasonable accord with those reported a number of years ago. In contrast, time-resolved fluorescence quenching (TRFQ) experiments using a pyrene-labeled triglyceride as a probe gave aggregation numbers N(agg) near room temperature that were similar to those determined previously by light scattering; however, the N(agg) of the new sample was larger, and the difference was outside of experimental error to one standard deviation. More troubling was that the TRFQ experiment failed to give meaningful results on solutions exposed to air, as in the case of the older TX100 sample. To get useful data by TRFQ measurements, one had to first remove oxygen from the solution. Copyright 2000 Academic Press.