Steric tensions

Very hindered secondary allylic alcohols may have a great tendency to suffer oxidative transpositions, even without the help of added acid a fact undoubtedly due to the release of steric tension, resulting from the transposition of the initially formed chromate ester.281... 

However, in molecules where the axial alcohol is subject to very severe steric interactions, the release of steric tension may become the major factor affecting DDQ oxidation velocity. For example, the 3 -acetoxy-6 —hydroxy-5a-cholest-7-ene (92) is oxidized faster than the corresponding 6a isomer (93). 

Steric (tension) and electronic effect from proximal base B... 

Most elastomers that are used for nylon modification contain a small amount of maleic anhydride (0.3 to 2%). In the melt blending process, these elastomers react with the primary amine end groups in nylon, giving rise to nylon grafted elastomers. These grafts reduce the interfacial tension between the phases and provide steric stabili2ation for the dispersed mbber phase. Typically, thermally stable, saturated mbbers such as EPR, EPDM, and styrene—ethylene/butylene—styrene (SEBS) are used. 

Shorter chain analogs of DPPC were also investigated in order to determine if the lack of stereo-differentiation in monolayer properties could be due to DPPC s higher gel point or complicating steric effects. Figure 15 shows the compression/expansion isotherms of DPPC as compared with racemic and enantiomeric dimyristoylphosphatidyl choline (DMPC) and dilauroyl phosphatidyl choline (DLPC). Again no stereodifferentiation in monolayer properties was observed as reflected by 11/A isotherms or dynamic surface tension. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

A number of chapters have been overhauled so thoroughly that they bear only minor resemblance to their counterparts in the first edition. The thermodynamics of polymer solutions is introduced in connection with osmometry and the drainage and spatial extension of polymer coils is discussed in connection with viscosity. The treatment of contact angle is expanded so that it is presented on a more equal footing with surface tension in the presentation of liquid surfaces. Steric stabilization as a protective mechanism against flocculation is discussed along with the classical DLVO theory. 

An interesting consequence of the fast formation of the chromic ester is that, sometimes, chromium-based oxidants counter-intuitively are able to oxidize quicker alcohols possessing a greater steric hindrance, as the initially fonned chromic ester releases greater tension on evolving to a carbonyl. Thus, axial alcohols are oxidized quicker than equatorial ones with chromic acid.6 The reverse—a somehow expected behavior—is observed, for example in oxidations with activated DMSO.7... 

A third mechanism involves charge repulsion between protein and cosolvent. The fact that most organic osmolytes lack a net charge suggests that this mechanism is of less general significance than the effects of surface tension and steric hindrance. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

Because protein conformational changes are changes in geometry, it has been tempting to consider a steric effect as the controlling factor in heme reactivity. This effect, as proposed by Hoard and Perutz (6, 7), involves a tension imposed by the protein on the proximal base, which in turn either pulls on the center of the heme as if it were a pump diaphragm or fits the naturally domed shape of the deoxyheme. Re-... 

Comparisons between R- and T-state hemoglobins on the one hand and a variety of synthetic model compounds on the other have allowed an evaluation of the possible occurrence and importance of electronic, proximal-base tension, and distal-side steric effects on the kinetics of ligation of CO and 02. Although all of these effects could influence the reactivities of hemoproteins, we conclude that hemoglobin reactivity and cooperativity are controlled predominantly by the presence or absence of proximal-base tension. 

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