Aqueous dilute sedimentation

W. Brown, R. Johnsen, P. Stilbs, B. Lindman. Size and shape of nonionic amphiphile (Ci2Eg) micelles in dilute aqueous solutions as derived from quasielastic and intensity of light scattering, sedimentation and pulsed-field-gradient nuclear magnetic resonance self-diffusion data. J Phys Chem 87 4548-4553, 1983. 

In a similar procedure [32] the sediment is wet oxidised with dilute sulphuric acid and nitric acids in an apparatus in which the vapour from the digestion is condensed into a reservoir from which it can be collected or returned to the digestion flask as required. The combined oxidised residue and condensate are diluted until the acid concentration is IN and nitrate is removed by addition of hydroxylammonium chloride with boiling. Fat is removed from the cooled solution with carbon tetrachlodithizone in carbon tetrachloride. The extract is shaken with 0.1M hydrochloric acid and sodium nitrite solution and, after treatment of the separated aqueous layer with hydroxylammonium chloride a solution of urea and then EDTA solution are added to prevent subsequent extraction of copper. The liquid is then extracted with a 0.01% solution of dithizone in carbon tetrachloride and mercury estimated in the extract spectrophotometrically at 485nm. 

A dry gel may also be used to concentrate a dilute aqueous solution of a macromolecule. When it is added to the solution, it will swell as it takes up water but will exclude the larger molecules. Hence, after sedimentation the supernatant fluid will contain the macromolecule, leaving the majority of the water in the pores of the gel. Such a technique is quicker and more effective than dialysis. 

A number of adsorption studies have been conducted for the adsorption of transition and heavy metals on carbonates. Mn2+ adsorption has been the most intensely studied mechanism. This is possibly because of its importance to carbonate mineral behavior in marine sediments (see Chapter 4). Studies of Mn2+ interaction with the surface of calcite in dilute aqueous solutions indicated that Mn2+ uptake is approximately balanced by Ca2+ release (McBride, 1979). It was suggested that the calcium release may not be the result of direct displacement, but rather results from the following reactions ... 

Sedimentation, used in conjunction with diffusion measurements, has been used for the determination of the molecular weights of macromolecules. At 20°C in a dilute aqueous solution, the sedimentation coefficient of hemoglobin, density 1.33 g/cm3, is 4.3 x 10"13 s. Its diffusion coefficient is 6.9 x 10 cm2/s. Calculate the molecular weight. How is this result affected by the hydration shell ... 

The original Acoustosizer used a single frequency whereas a later development has a range of 13 frequencies between 0.3 and 13 MHz. This allows the measurement of the dynamic mobility spectrum and the determination of the zeta potential and particle size. In order to invert the mobility spectrum into a size distribution a log-normal distribution of particle size is assumed. A comparison with photon correlation spectroscopy for determining particle size and laser Doppler anemometry for particle charge eonfirmed the results using ACS [266]. These and additional sedimentation measurements confirmed that changes in particle size and zeta potential due to dilution effects are likely to occur in aqueous and non-stabilized systems. 

The formation and survival of unstable or metastable micas and clays in sediments and soils at low temperatures reflects kinetic as well as thermodynamic factors. First, the rates of reactions involving solid-aqueous and especially solid-solid transformations in dilute solutions are very slow at low temperatures (most natural waters are dilute )- The slow kinetics of clay transformations reflects small differences in free energy between stable and metastable clays. Also, the occurrence of specific clays is related to the chemistry and crystal structure of source minerals. Thus, illite often results from the weathering of muscovite, and vermiculite results from the weathering of biotite (cf. Drever 1988), consistent with the similar chemistries and structures of these pairs of T 0 T minerals. 

In a research laboratory, fractal aggregation of emulsion droplets is studied. An O-W emulsion is made with a buffer containing /i-lactoglobulin as the continuous phase. To prevent sedimentation of the droplets, part of the oil is brominated to match the density of the oil phase and the aqueous phase. The droplet diameter obtained is 2 pm, (p = 0.1. To prevent convection currents, the emulsion is stored at precisely 20°C in a thermostat. The emulsion then forms a gel after 2.5 hours. By use of Eq. (13.20), assuming RCI — Rg and Z) = 2, it is calculated that W 10. In order to check the assumptions, it is decided to dilute the emulsion with an equal amount of water it is expected that the gel time will then be 23 times as long, i.e., 20 h. However, this proves incorrect after a month, the emulsion is still liquid. What can be the explanation ... 

---