Physical microencapsulation

The most important physical methods are physical and ionic adsorption on a water-insoluble matrix, inclusion and gel entrapment, and microencapsulation with a liquid or a solid membrane. The most important chemical methods include covalent attachment to a water-insoluble matrix, cross-hnking with the use of a multifunctional, low-molecular weight reagent, and co-cross-linking with other neutral substances, for example proteins. 

Toxins have also been micropulverized and microencapsulated to facilitate their dispersal and increase their persistency. Color and other physical properties of the toxin may be affected by these modifications. 

Enzymes can be immobilized by matrix entrapment, by microencapsulation, by physical or ionic adsorption, by covalent binding to organic or inorganic polymer-carriers, or by whole cell immobilization (5 ). Particularly impressive is the great number of chemical reactions developed for the covalent binding of enzymes to inorganic carriers such as glass, to natural polymers such as cellulose or Sepharose, and to synthetic polymers such as nylon, polyacrylamide, and other vinyl polymers and... 

Type 3 metal complexes involve the physical interaction of a metal complex, chelates, or metal cluster with an organic polymer or inorganic high molecular weight compound. The preparation of type 3 compounds differs from those of type 1 and type 2, as they are ultimately achieved through the use of adsorption, deposition by evaporation, microencapsulation, and various other methods. 

A problem especially with oxidation catalysts is that the metals in their highest oxidation state tend to be less strongly associated with a support, so that the reaction conditions can lead to leaching of the metal complex from the support. To overcome this problem, microencapsulation, as an immobilization technique for metal complexes, has been introduced by Kobayashi and coworkers. In the microencapsulation method, the metal complex is not attached by covalent bonding but is physically enveloped by a thin film of a polymer, usually polystyrene. With this technique leaching of the metal can be prevented. In 2002, Lattanzi and Leadbeater reported on the use of microencapsulated VO(acac)2 for the epoxidation of allylic alcohols. In the presence of TBHP as oxidant, it was possible to oxidize a variety of substrates with medium to good yields (55-96%) and diastereomeric ratios (60/40 to >98/2) (equation 42). The catalyst is easily prepared and can be reused several times without significant loss in activity. 

Pozzolanic S/S systems use portland cement and pozzolan materials (e.g., fly ash) to produce a strucmrally stronger waste/concrete composite. The waste is contained in the concrete matrix by microencapsulation (physical entrapment). It is a chemical treatment that uses commercially available soluble silicate solutions and various cementious materials such as cement, lime, poz-zolans, and fly ash. By addition of these reagents and rigorous mixing, the waste is fixed or stabilized. Contaminant mobility is reduced through the binding of contaminants within a solid matrix, which reduces permeability and the amount of surface area available for the release of toxic components. 

A large number of metJiods for immobilizing biomolecules on the surface of solid substrate have been proposed in the past few decades, in which the molecules are immobilized on a carrier using covalent bonds ( i, ionic bonds (2), physical adsorption (3), cross-linkage of the biomolecules (4), or by microencapsulation (5). Immobilizing techniques are indispensable to treat biomolecules in an experiment. The provision of an immobilization process is one of the most essential processing steps that are required in order to obtain practical biomolecule carriers such as... 

Physical methods that involve the supporting of a biological element in any way that is not depending on covalent bond formation (e.g., adsorption, entrapment, microencapsulation). These procedures are simple and in most cases the biocatalyst remains unchanged. 

A very interesting technique that has been used widely in the MTO-catalyzed olefin oxidation reaction is the microencapsulation technique. This technique uses poly(4-vinylpyridine) (PVP), either 2% or 25% cross-linked with divinylbenzene (PVP-2% or PVP-25%, Fig. 4), as well as poly(4-vinylpyridine-/V-oxide) (PVPN-2%, Fig. 4). In addition, 2% cross-linked PS (PS-2%, Fig. 4, X = CH2) and a mixture of PS-2% and PVP-2% (5 1, Fig. 4, X = N) have been used as support polymers. This approach is based on the physical envelopment of the Lewis-acidic MTO by the PS polymer, enhanced by interactions of the 7t-electrons of the phenyl rings with MTO. In the case of the pyridine-containing polymers, Lewis acid-Lewis base interactions between the pyridine moiety and MTO obviously play an important role. In the case of the PVP and PVPN polymers, MTO can be incorporated in the support matrix by mixing the polymer and MTO in ethanol to obtain the desired immobilized catalyst. 

Solidification refers to techniques that encapsulate the waste, forming a solid material, and does not necessarily involve a chemical interaction between the contaminants and the solidifying additives. The product of solidification, often known as the waste form, may be a monolithic block, a clay-like material, a granular particulate, or some other physical form commonly considered solid. Solidification as applied to fine waste particles, typically 2 mm or less, is termed microencapsulation and that which applies to a large block or container of wastes is termed macroencapsulation [29]. 

Physical stabilization refers to improved engineering properties such as bearing capacity, trafficability, and permeability. Alteration of the physical character of the material to form a solid material reduces the accessibility of water to the contaminants within a cemented matrix and entraps or microencapsulates the contaminated solids within a dimensionally stable matrix. Since most of the... 

Wohlgemuth, M., Machtle, W., and Mayer, C. (2000), Improved preparation and physical studies of polybutylcyanoacrylate nanocapsules, / Microencapsul., 17,437. 

Macroencapsulation is used for large objects such as concrete debris that is contaminated, or structural steel that has fixed contamination. The chemical stabilization and microencapsulation work together to immobilize chemical constituents, while the macroencapsulation is used to physically encapsulate large objects. For this reason, we will discuss chemical stabilization and microencapsulation together and address macroencapsulation in a separate section in this chapter. 

A novel type of polymer-supported Lewis acid, a microencapsulated Lewis acid catalyst was investigated by Kobayashi [117]. Sc(OTf)3 was immobilized on to polystyrene by microencapsulation—Sc(OTf)3 is physically enveloped by polystyrene and stabilized by the interaction between the jr-electrons of benzene rings and vacant orbitals of the Lewis acid. This microencapsulated catalyst was used successfully in several Lewis acid-catalyzed carbon-carbon bond-forming reactions (imino aldol, aza Diels-... 

The next chapter describes the basic physical and biotechnological processes which are today available for the production of flavourings and flavour extracts. These range from more traditional methods such as extraction and distillation to more recent developments, e.g. supercritical fluid extraction, spray and freeze drying as well as microencapsulation, and include the rapidly increasing field of biotechnology. 

Lee outlines three different physical methods that are commonly utilized for enzyme immobilization. Enzymes can be adsorbed physically onto a surface-active adsorbent, and adsorption is the simplest and easiest method. They can also be entrapped within a cross-linked polymer matrix. Even though the enzyme is not chemically modified during such entrapment, the enzyme can become deactivated during gel formation and enzyme leakage can be problematic. The microencapsulation technique immobilizes the enzyme within semipermeable membrane microcapsules by interfacial polymerization. All of these methods for immobilization facilitate the reuse of high-value enzymes, but they can also introduce external and internal mass-transfer resistances that must be accounted for in design and economic considerations. 

Both chemical and physical methods may be used to immobilize biocatalysts while retaining or modifying their activity, selectivity, or stability. Among the techniques used for immobilization of enzymes are physical adsorption, covalent bonding, ionic binding, chelation, cross-linking, physical entrapment, microencapsulation, and retention in permselective membrane reactors. The mode of immobilization employed for a particular application depends not only on the specific choice of enzyme and support, but also on the constraints imposed by the microenvironment associated with the application. 

Recently, alternative methods have also been developed to stabilize these complex enzyme systems. The technique of microencapsulation [128] is designed to prevent physically the protease enzyme from interacting with the other enzymes (Figure 8.23). This is accomplished by a composite emulsion polymer system which has a hydrophilic portion attached to a hydrophobic core polymer. The protease is stabilized by trapping it within the network formed by the hydrophobic polymer. 

Groenwald BE, Pereiro F, Purnell TJ and Scher HB Microencapsulated thiocarbamate herbicides a review of their physical, chemical and biological properties, Proceedings, British Crop Protection Conference - Weeds, pp. 185-191 (1980). 

Microencapsulation of flavors is a technology of enclosing flavor compounds (core materials) in a carrier matrix. An amorphous or metastable solid is normally used as a carrier matrix. Microencapsulation is useful for improving the chemical stability of flavor compounds, providing controlled release of flavor compounds from microencapsulated flavor products, providing a free-flowing powder with improved handling properties and physical protection of volatile properties of flavor. 

Various methods have been proposed for whole cell Immobilization Including adsorption and covalent attachment to a preformed carrier, crosslinking, flocculation, microencapsulation, and entrapment. Physical entrapment In a porous matrix Is by far the most flexible and most commonly used technique. Considering the fact that the polymer network has to be formed In the presence of the finally entrapped biological material, the performance criteria of chemical and physical nature are as follows ... 

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