Some Other Factors Affecting Stability

Some Other Factors Affecting Emulsion Stability 

A discussion of emulsion stability should include not only possible mechanisms of stabilization but also some comments concerning the timeframe of the stability requirements, because there exist external and internal factors unrelated to interfacial and colloidal phenomena that work unrelentingly to destroy the most stable system. The rates of colloidal degradation of emulsions vary immensely, so that it is not possible to define a single number that can be used as a measure of acceptable or unacceptable persistence. In any emulsion, especially one that is only very poorly stabilized, the breaking process will involve the coalescence of droplets brought together by the action of Brownian motion, convection currents, and other random disturbances. Their stability may be measured on the order of seconds or minutes. 

Emulsions that contain more effective stabilizing additives such as one of those described above may be stable for hours, days, months, or even years. In such systems the action of random or induced motion and droplet collision will continue, but the rheological properties of the continuous phase will slow down such processes and/or interfacial layers will posses sufficient strength and rigidity so that coalescence will occur on a relatively long timescale. 

In the present context, it is often possible to greatly reduce the rate of droplet growth due to Ostwald ripening by employing emulsifiers and stabilizers that form a barrier to the passage of dispersed phase molecules into the continuous phase. This process can be especially important in multiple emulsion systems discussed later. 

Bacterial action can be of importance in areas such as food, pharmaceutical, and cosmetic emulsions, or other systems which contain components subject to biological degradation such as proteins and natural gums. Such systems are obviously of great economic importance, so that a great deal of research has been devoted to the problem. In cases where biological stability is important. 

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Some Other Factors Affecting Emulsion Stability... 

Other factors (pH, temperature, foam dispersity, foam column height, rate of gas and liquid feed, etc.) also affect the accumulation effectiveness (parameter //). Some of these, such as pH, temperature, type and concentration of the collector, are changing the adsorption, others, like dispersity and foam column height, are changing the drainage rate that determines foam stability and expansion ratio. The book of Rusanov et al. [23] summarises the results on the effect of these factors on foam accumulation of surfactants. 

The analysis of pK values to make comparisons of carbanion stability is a standard approach. Yet, as discussed earlier with regard to the use of BDEs and HIAs to analyze radical and carbocation stabilities, other factors can affect the values. This is a more important caveat when evaluating pfCa values than with BDEs or HIAs. As will be discussed in Chapter 5, solvation effects can dramatically change pfCa values, and sometimes in different solvents some pKa values actually reverse in order. Hence, when using pK values to compare carbanion stabilities, it is very important to compare the values under as similar a set of conditions as possible. 

The discussion on reproducibility for the three sources will take a similar tack to the discussion on sensitivity starting with a discussion of the limits imposed by the instrumentation and the ionization process on ion current stability. Secondly, we will describe the most important methodological factor affecting reproducibility, which is under some control of the analyst through the use of sample preparation and chromatography. This is ion suppression and will be discussed in some detail. Thirdly, some real-world examples of assay precision and accuracy will be provided. In all of these categories there is less data available from MALDl than from the other sources because its implementation as a quantitative tool is relatively recent. 

Termination positions are defined by the thermodynamic stability of the polymerase complex, whereas the efficiencies of termination are determined by a kinetic competition between RNA elongation and release at each position (94). The termination efficiency is affected mostly by the strength of the stem—loop, the presence of the U-tract, and also by the concentration of NTP substrates. The efficiency of transcription termination also depends on other factors such as the size and shape of the hairpin, not just its stability, most likely involving interactions with the polymerase. Different RNA hairpins exert varying effects on transcription termination, while a cruciform structure may have no or some attenuating effect on transcription. Furthermore, certain changes in the stem sequence designed to maintain or raise the stability of the stem (AG = —18 kcal/mol) lead to a decrease... 

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