Turbidity regime

In order to calculate particle size distributions in the adsorption regime and also to determine the relative effects of wavelength on the extinction cross section and imaginary refractive index of the particles, a series of turbidity meas irements were made on the polystyrene standards using a variable wavelength UV detector. More detailed discussions are presented elsewhere (23) > shown here is a brief summary of some of the major results and conclusions. 

Very recently Husain et al.(12) obtained an analytical solution to the integral equation describing peak broadening for a turbidity detector in the Mie scattering regime. 

Therefore, in principle, the parameters of the particle size distribution can be estimated from specific turbidity measurements at different wavelengths. This is not true, however, in the Rayleigh regime (i.e. small particles, (D/Am) less than 0.1). In this case, the extinction coefficient is proportional to (D/Am)4 and... 

In order to elucidate the effect of temperature, the authors of Refs. [310,210] determined experimentally the boundary points x x = 0.08 and XgX = 0.65 of the transparency region for the (ST + HA) system at complete conversion, p = 1, when in the course of synthesis the temperature was increased in a given way from 28 °C to 78 °C. Despite a noticeable difference between such a regime and the isothermal one (see Fig. 24), it was found that the regions, in which at p = 1 turbid copolymers were formed, practically coincide. The same could be said about the calculated values of dispersion cr2(l) at the boundary points of the mentioned regions. This might be associated with a rather weak dependence of the reactivity ratios on temperature. A similar practical independence of the turbidity region... 

Fig. 26. The Gibbs-Rooze-boom triangle for the weight composition of (styrene + methyl acrylate + heptyl acrylate) terpolymer, prepared under Regime II (see Fig. 24) at complete conversion. Open and dark circles denote transparent and turbid terpolymer, respectively, and figures above the circles denote the theoretical value of...

The controversy over whether turbidity is capable of giving a true size distribution has been fully discussed by Kourti and MacGregor, who conclude that in many cases it can, and much of the controversy arises due to unjustified extrapolation from one regime to another [51]. 

Figure 3. Changes in scattering efficiency with aggregate diameter enable the suspension turbidity to increase, remain constant, or decrease depending on the size regime of the aggregate...

Although the coalescence assumption may lead to small errors in the scattering cross section, it does enable the rapid correlation of turbidity data (n) with electronic particle counter data (n and fi), provided the relative refractive index and size regime are known. Because the electronic particle counter measures the aggregate volume, and therefore determines only an equivalent spherical radius, the data from the electronic particle counter relate only to the coalesced-sphere approach. 

Two general regimes of turbid media need to be considered, namely, those of low and high absorption. In the ca.se of dense solutions of particles with low absorption, multiple scattering occurs but considerable progress has been made in recent years. We shall discuss the high-absorption case in the next section. 

For still slightly lower [OH"]o values, the system reaches a complex dynamic regime in which the gel is essentially collapsed with a turbid core surrounded by a thin transparent shell. Some sections of the gel start to swell and shrink back at random positions as shown on Figure 4 for an area located at mid-height of the cylinder. Like in the previous case, bends often form at the location of strong gradients of thickness. 

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