Strategies for Stability

The complex interdependencies between colloid instability mechanisms make predicting the timescale and outcome of colloidal instability extremely difficult. When seeking to stabilise a colloidal system, it is important to eliminate as many of the instability mechanisms as possible. 

When neither of the above strategies is practical, a popular approach is to increase the viscosity of the continuous phase. This may be achieved by the addition of high molecular weight pol)maers. These act to increase the viscosity of the continuous phase to such an extent that creaming rates become negligible. Another advantage to this approach is that the additional viscosity of the product is often perceived by customers as increased quality . However, these polymers also act to reduce the stability of colloidal systems to flocculation, which can in turn lead to more rapid creaming. 

An additional useful property of polymeric stabilisers is that, even when they fail to render the products stable, it is frequently found that the concentrated layers that 

Coalescence and flocculation are strongly related to one another. Reducing the rate of drop coalescence is one of the main purposes of surfactants in emulsion formulations. The reduction in the interfacial tension on addition of surfactant itself increases the stability of the emulsion with respect to its component phases. However, the role of surfactants is much greater than this and they act to reduce droplet coalescence in several additional ways. 

Ionic surfactants, as their name suggests, have an ionisable hydrophilic group. When appropriately charged surfactants are adsorbed in large numbers to the interface of emulsion droplets they increase the surface charge on them. Since the charge is the same sign on all the droplets, electrostatic forces are developed that oppose (and thus reduce the rate of) droplet encoimter. 

---

Formulation strategies for stabilization of proteins commonly include additives such as other proteins (e.g., serum albumin), amino acids, and surfactants to minimize adsorption to surfaces. Modification of protein structure to enhance stability by genetic engineering may also be feasible, as well as chemical modification such as formation of a conjugate with polyethylene glycol. 

Figure 2.4, a continuation of Figure 2.3, is a flow chart for a strategy for stability testing. The main aim of these tests is the determination of the following parameters ... 

FIGURE 2.4. Flow Chart for a Strategy for Stability Testing... 

Directed evolution as a tool to probe the basis of protein structure, stability, and function is in its infancy, and many fruitful avenues of research remain to be explored. Studies so far have focused on proteins that unfold irreversibly, making detailed thermodynamic analysis impossible. The application of these methods to reversibly folding proteins could provide a wealth of information on the thermodynamic basis of high temperature stability. A small number of studies on natural thermophilic proteins have identified various thermodynamic strategies for stabilization. Laboratory evolution makes it possible to ask, for example, whether proteins have adopted these different strategies by chance, or whether certain protein architectures favor specific thermodynamic mechanisms. It will also be possible to determine how other selective pressures, such as the requirement for efficient low temperature activity, influence stabilization mechanisms. The combination of directed evolu-... 

Li, S. Shoeneich, C. Borchardt, R.T. Chemical instability of protein pharmaceuticals mechanisms of oxidation and strategies for stabilization. Biotechnol. Bioeng. 1995, 48, 500-590. 

The proposed strategies for stabilization of gas-lifted oil wells are offline methods which are unable to track online dynamic changes of the system. However, system parameters such as flow rate of injected gas and also noise characteristic are not constant with respect to time. An adaptive Linear Quadratic Gaussian (LQG) approach is presented in this paper in which the state estimation is performed using an Adaptive Unscented Kalman Filter (AUKF) to deal with unknown time-varying noise statistics. State-feedback gain is adaptively calculated based on Linear Quadratic Regulator (LQR). Finally, the proposed control scheme is evaluated on a simulation case study. 

In general, isolable zirconium and hafnium alkene complexes are rare, as they typically undergo carbon-carbon coupling reactions with additional olefin to yield metallacyclopentanes. Addition of an exogenous donor ligand is a common strategy for stabilizing alkene complexes. Several classes of these compounds have been prepared those... 

Strategies for Stabilization of the Inorganic Framework and Removal of the Template... 

By comparison, decarboxylation is largely a kinetic problem. Enzymes have developed a variety of strategies for stabilizing the anionic intermediate that is produced in the decarboxylation step. Metal ion stabilization of enolates is a common theme, particularly for decarboxylation of /8-keto acids. The most elegant solutions are perhaps the extensive electron delocalizations seen in pyri-doxal phosphate and thiamin pyrophosphate. 

Edmonds, J. A. and Wise, M. (1999). Exploring a technology strategy for stabilizing atmospheric CO,. In International Environmental Agreements on Climate Change. Kluwer, Dordrecht, Netherlands. 

Figure 2.11 Strategies for stabilization of photoelectrodes against corrosion, (a) Chemical stabilization, (b) Stabilization by specific properties of the band structure as for group VIb transition metal dichalcogenides the five metal d-band contributions, the indirect gap, and the direct excitonic gap are shown also indicated is the thermalization process from the highly absorbing excitonic states to the indirect band edges, (c) Kinetic stabilization via reaction rate relations (1) redox reaction of...

A strategy for stabilizing uncommon or reactive oxidation states is to have the burden shared by more than one... 

Photovoltaic devices based on compound semiconductors like CuInSe2, CdTe, and quantum dots as well as liquid junction devices will require improved strategies for stabilization against environmental corrosion and dissolution. 

Chemical modifications of mesophilic enzymes have met with more success than protein engineering. The strategies for stabilization have been reviewed [e.g. 39, 56] and include immobilization and hydrophilization. Chemical modification of trypsin and chymotrypsin gives artificial hydrophilization of non-polar areas of the protein surface resulting in marked thermostabilization and little loss of enzyme activity [57-59]. This phenomenon has also been demonstrated for Bacillus stearothermophilus lactate dehydrogenase by Wigley et al. [60] who used site-... 

---