Stability ultracentrifugation

Hensley P. Defining the structure and stability of macromolecular assemblies in solution the re-emergence of analytical ultracentrifugation as a practical tool. Structure 1996 4 367-373. 

A major concern for the use of these quantum dots in liquid crystal applications is their stability. Weller et al. using NMR spectroscopy, UV-vis spectrophotometry, and analytical ultracentrifugation (AUC) showed that thiol-stabilized cadmium chalcogenide quantum dots are unstable at low particle concentrations (<10 pM in DMF), i.e., even the covalently bound thiols desorb from the quantum dot surface... 

Also Fryar and Kaufman8 studied the solvent effect on the stability of barium dinonylnaphthalene sulfonate in toluene, toluene/methanol, and methanol solutions by ultracentrifugation and viscometry. The aggregation number of the micelles reduced from about 10 in toluene to about 4 when the mole fraction of free methanol in the solvent mixture was approximately 0.03. In pure methanol BaDNNS micelles did not exist. 

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

Effect of Urea on the Distribution of 14C-Methyl-/ -Lactoglobulin or 14C-Methyl-/c-Casein in Heated Milk. Muir and Sweetsur (25) exploited the interesting observations by Robertson and Dixon (26) that the heat stability of milk proteins is related to the urea level of the milk, and that adding relatively small amounts of urea increased the heat stability of milk (27). To examine the urea effect further, we added urea to milk containing either M-/3-L or M-k-C and measured the ultracentrifugal distribution of C-14 after heat treatment. The experiments using... 

It is assumed that the optimum hexadecyltrimethylammonium bromide-cetyl alcohol ratio for the formation of perfect crystals also corresponds to the formation of the highest-stability emulsions. The measurement of emulsion stability by ultracentrifugation (8) showed that emulsions prepared with a 1 3 hexadecyltrimethylammonium bromide-cetyl alcohol molar ratio showed the best stability. Therefore, the rodlike particles formed in this system should have the highest crystallinity, which was confirmed by electron diffraction measurements in the transmission electron microscope (2). The ratio P(2)/P(2)b> where the subscript b denotes the system of highest crystallinity, has values of 0.894, 0.419, and 0.189 for hexadecyltrimethylammonium bromide-cetyl alcohol molar ratios of 1 1, 1 0.50, and 1 0.33, respectively, relative to a value of 1.000 for the 1 3 ratio. 

A more rigorous procedure to study emulsion stability using the ultracentrifuge is to observe the system at various speeds of rotation. At relatively low centrifuge speeds, the expected opaque cream layer may be observed, but at sufficiently high speeds a coalesced oil layer and a cream layer may be observed that are separated by an extra layer of deformed oil droplets. This deformed layer resembles a foam that is, it consists of oil droplets separated by thin aqueous films. 

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