Measurements Ostwald ripening

In Nature, however, we always have a contiiinous distribution of particles. This means that we have all sizes, even those of fractional parentage, i.e.-18.56n, 18.57p, 18.58 p, etc. (supposing that we can measure 0.01 p differences). The reason for this is that the mecheuiisms for particle formation, i.e.- precipitation, embryo and nucleation growth, Ostwald ripening, and sintering, are random processes. Thus, while we may speak of the "statistical variation of diameters", and while we use whole numbers for the particle diameters, the actuality is that the diameters are fractional in nature. Very few particle-size" specialists seem to recognize this fact. Since the processes are random in nature, we can use statistics to describe the... 

Transfer of water molecules (in evaporation control), transport of solvent across monolayers at oil-water interfaces (in Ostwald-ripening of emulsions) and transfer of ions across such interfaces (as models for ion conduction in bilayers and membranes) can often be treated in terms of surface concentration fluctuations. Their magnitudes can be expressed in standard deviations (cr for the standard deviation in the surface concentration), which are measures of the probability that random holes are formed in the layer, allowing material transport. We have presented the formal treatment in sec. 1.3.7. From this section we can immediately obtain = IcTOr / 0 ), for a Gibbs monolayer, with... 

Two main procedures can be applied for the characterisation of suspensions and assessment of their stability (such as flocculation). The first method depends on the measurement of particle size distribution and the rate of flocculation and/or Ostwald ripening after dilution of the suspension with the dispersion medium, while the second procedure depends on measurement of the state of suspension without dilution, using rheological techniques. As both methods are described in detail in Chapters 19 and 20, only a summary will be provided here. 

The assessment of flocculation and Ostwald ripening of a suspension requires measurement of the particle size and shape distribution as a function of time. Several techniques may be apphed for this purpose, and these are summarised below [6]. 

The above measurements should be supplemented by particle size distribution measurements of the diluted dispersion (ensuring that no floes are present after dilution) to assess the extent of Ostwald ripening. Another compHcation may arise from the nature of the flocculation which, if it occurs in an irregular fashion (producing strong and tight floes), may cause i (0) to be increased while cr may show some decrease, and this wiU complicate the analysis of the results. Yet, in spite of these complications, constant stress measurements may provide valuable information on the state of the dispersion on storage. 

If flocculation occurs on storage (without any Ostwald ripening or coalescence), the values of //(o) and may show a gradual increase with increase in storage time. As discussed in the previous section on steady-state measurements, the trend becomes complicated if Ostwald ripening and/or coalescence occur simultaneously, as both have the effect of reducing //(o) and cr. ... 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

Eberl DD, Nuesch R, Sucha V, Tsipursky S (1998) Measurement of fundamental illite particle thicknesses by X-ray diffraction using PVP-10 intercalation. Clays Clay Minerals 46 89-97 Eberl DD, Srodon J (1988) Ostwald ripening and interparticle-diffraction effects from illite crystals. Am Mineral 73 1335-1345... 

---