Two preparations of linear polyacrylamide with average molecular weights of 0.37 million and 1.14 million Da, and a deuterated preparation with an average molecular weight of 1.71 million Da, were used to study the effects of molecular weight, polydispersity, and concentration on the mesh size of entangled polymers in a DNA sequencing buffer solution and their ability to resolve DNA sequencing reactions by capillary electrophoresis. The polyacrylamide concentrations were above the overlap threshold C*, the concentration above which an entangled polymer network is expected to form. Small angle neutron scattering experiments showed that between 1% and 8% polyacrylamide, the mesh size ( xi ) can be expressed by the relation xi = 2.09C-0.76, where xi is in A and C is the polymer concentration in g/mL. The mesh size depended only on the concentration and was independent of the average molecular weight of the polyacrylamide. Consistent with this result, electrophoretic mobilities of DNA moving through the polymer network depended almost entirely on the polyacrylamide concentration and not on its molecular weight or polydispersity. Although separation was little affected, band sharpness persisted to longer DNAs when the polymer network contained a higher fraction of larger polyacrylamide molecules. We postulate a dispersive effect that depends on the size of the DNA and the resiliency of the polymer network. This interpretation provides a rationale for optimizing the design of polymer solutions to sieve DNA for sequencing by capillary electrophoresis.

A new value for the depolarization ratio of pure water has been measured at 514.5 nm using an argon ion laser light source and photon counting detection. This depolarization ratio is lower than any previous literature value by a substantial amount. Stray light and photometer geometry are shown to be responsible for overestimated values reported previously. The depolarization ratio is also shown to be a function of the portion of the spectrum being investigated. With no filters we find ρv=0.026. The magnitude of error due to finite acceptance angle is calculated and shown to be substantial. Depolarization ratios using 0.46 and 22.5 nm filters (half-peak bandwidth) are reported.

Xanthate is one of the commonly used collectors in froth flotation beneficiation of sulfide ores. It could decomposes and generates toxic compounds such as carbon disulfide (CS 2) which is a concern in the mining industry. A vast body of literature exists for studies on xanthate/mineral interactions, but xanthate decomposition under various conditions (e.g., in solutions or flotation pulps) is not fully understood. We have undertaken detailed studies to fill this knowledge gap, and this paper shows our study of the xanthate decomposition in aqueous solutions in the absence of minerals. This condition has not been appropriately examined by past researchers, while decomposition under this condition is used as control to other complicated ones (e.g., in flotation pulps). A GC-MS based method was developed to directly measure the decomposition products in the gas phase. Decomposition kinetics was then established based on the generation of CS 2. Decomposition followed the first order kinetics, and the rate constant for Sodium iso-Butyl Xanthate (SIBX) at neutral pH level was determined to be 9.3x10-4 h-1 at 25 °C, 1.7x10-2 h-1 at 50 °C and 1.3x10-1 h-1 at 70 °C. The effect of pH on decomposition behavior was examined in the pH range of 1.5 to 12.5. We determined experimentally a maximum in the extent of decomposition extent as a function of pH, which was predicted theoretically in literature. A mechanism involving multiple reactions that occurred in parallel or in sequence sequentially along with the decomposition was proposed to explain the observed change of xanthate decomposition in over the entire pH range. These results offer valuable insights and can serve as the basis to mitigate the detrimental effects of xanthate decomposition in plant

No abstract

No abstract

No abstract

The geometric characteristics of nanogel particles in aqueous solutions were studied by determining their ratios of radius of gyration (mean-square radius; Rg) to hydrodynamic radius (Rh), Rg/Rh, derived from static light scattering and dynamic light scattering experiments, respectively. The various nanogel samples studied included ones composed of lightly cross-linked N-isopropylacrylamide (NIPA) polymer, NIPA-based anionic or cationic copolymers, and amphoteric terpolymers. Polyelectrolyte complexes between anionic or cationic nanogels and oppositely charged polyions or nanogels having opposite charges were also studied. Most NIPA and NIPA-based polyelectrolyte nanogels in a swollen state had Rg/Rh values >0.775, which is the theoretically predicted value for a solid sphere. In a collapsed state, one may expect nanogel particles to be spherical in shape; however, this was not the case for a variety of nanogel samples, either with or without charges. These data were consistent with the idea that the surfaces of these nanogel particles were decorated with attached dangling chains. The Rg/Rh data from polyelectrolyte-nanogel complexes, however, indicated different structures from this. It was found that most of the polyelectrolyte-nanogel complex particles had Rg/Rh approximately 0.775. This suggested that the complexed nanogel particles were spherical in shape and that there were no dangling surface chains.

The structure of real icosahedral quasicrystals of the Al± Pd± Mn system can be described in terms of a hierarchical self-similar packing of overlapping atomic clusters. An in¯ation scale factor ¿ 3 preserves long-range order but generates a hierarchy of holes and a fractal structure. Such holes have actually been observed and they might be of basic importance in the growth mechanism and stability of quasicrystals. § 1. IntroductionQuasicrystals are a form of the solid state which possesses a long-range translational order despite a non-crystallographic point group of symmetries (Janot 1994). The debate concerning structure and stability of quasicrystals very often revolves about the relative roles of energy and entropy (Henley 1991) but in both cases the description of the structure is based on tiles representing the short-range order in the material. Perfect quasiperiodic structures can be obtained via several geometrical methods including in¯ation± de¯ation of prototiles, tiling with at least two di erent prototiles plus matching rules and also a physical cut of a higher-dimensional periodic structure. These perfectly quasiperiodic models are irreplaceable when one needs to approach experimentally the structure of real quasicrystals for which they give at least a very useful average description (Cornier-Quiquandon et al. 1991Boudard et al. 1992), but space tiling requires a choice of a small number of prototiles together with the de® nition of rules to ® ll space without overlap nor holes. This is intuitively simple for crystals; it is actually easy to imagine a single building block arising as a low-energy atomic cluster of the given elements and, then, periodicity is obtained by adding identical clusters again and again. The procedure works quite well as long as the building block has proper crystallographic symmetries. The physical conditions required to emulate, say, a Penrose (1974) tiling appear to be much more complex. The energetics must be delicately balanced to allow two distinct clusters to be almost equally stable so that they intermix with a speci® c ratio of densities and according to matching rules. The entropic view point (Henley 1991, Joseph andElser 1997) allows phason defects to stabilize the structure, relaxes the