Force steric

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

Enzyme active sites, 136,148, 225. See also Protein active sites in carbonic anhydrase, 197-199 in chymotrypsin, 173 in lysozyme, 153, 157 nonpolar (hypothetical site), 211-214 SNase, 189-190,190 steric forces in, 155-158, 209-211, 225 in subtilisin, 173 viewed as super solvents, 227 Enzyme cofactors calcium ... 

Rule 3 states that all the atoms covered by delocalized electrons must lie in a plane or nearly so. Many examples are known where resonance is reduced or prevented because the atoms are sterically forced out of planarity. 

Both of the above approaches rely in most cases on classical ideas that picture the atoms and molecules in the system interacting via ordinary electrical and steric forces. These interactions between the species are expressed in terms of force fields, i.e., sets of mathematical equations that describe the attractions and repulsions between the atomic charges, the forces needed to stretch or compress the chemical bonds, repulsions between the atoms due to then-excluded volumes, etc. A variety of different force fields have been developed by different workers to represent the forces present in chemical systems, and although these differ in their details, they generally tend to include the same aspects of the molecular interactions. Some are directed more specifically at the forces important for, say, protein structure, while others focus more on features important in liquids. With time more and more sophisticated force fields are continually being introduced to include additional aspects of the interatomic interactions, e.g., polarizations of the atomic charge clouds and more subtle effects associated with quantum chemical effects. Naturally, inclusion of these additional features requires greater computational effort, so that a compromise between sophistication and practicality is required. 

Polypeptides form various secondary structures (a-heUx, 3-sheet, etc.), depending on solution pHs. We have investigated end-anchored poly(L-glutamic acid) andpoly(L-lysine) in various secondary structures [11,29,35,36], using the analytical method for the steric force... 

Kettle and his co-workers (39—42) used a model rather similar to the AOM to discuss stereochemistry. A perturbation approach led to the proportionality of MO energies (relative to the unperturbed orbitals) to squared overlap integrals, as in the AOM. For systems where the valence shell orbitals are evenly occupied, the total stabilization energy shows no angular dependence, suggesting that steric forces determine the equilibrium geometry. 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

Resonance always results in a different distribution of electron density. For example, if the first term on the left in Figure 5.3 were the actual structure of aniline, the two unshared electrons of nitrogen would reside entirely on that atom. Because the real structure is actually a hybrid that includes contributions from the other canonical forms shown, the electron density of the unshared pair is spread over the ring. This decrease in electron density at one position and the corresponding increase elsewhere is called the resonance effect. When steric inhibition is present, resonance may be reduced or prevented because the atoms are sterically forced out of planarity The resonance effect of a group operates only when the group is directly connected to an unsaturated system. 

Many dispersions are stabilised by polymers. The underlying interaction is often called the steric force. For the understanding of steric interactions it is necessary to know some fundamentals of polymer physics (a good introduction is the book of Strobel [190]). Here we are mainly concerned about linear polymers because these are commonly used for steric stabilization. Fortunately, in many applications we do not need to consider the detailed molecular chemical nature of the polymer such as effects of bond lengths, bond angles, rotation energy, etc. In many discussions we can use simpler models to describe the polymer. 

There is no simple, comprehensive theory and steric forces are complex and difficult to describe. Different components contribute to the force, and depending upon the situation, dominate the total force. The most important interaction is repulsive and of entropic origin. It is caused by the reduced configuration entropy of the polymer chains. If the thermal movement of a polymer chain at a surface is limited by the approach of another surface, then the entropy of the individual polymer chain decreases. In addition, the concentration of monomers in the gap increases. This leads to an increased osmotic pressure. 

Example 6.12. The steric force caused by grafted polystyrene in toluene is repulsive (Fig. 6.17). Toluene is a good solvent for polystyrene. With increasing temperature the... 

Figure 6.17 Steric force between an atomic force microscope tip made of silicon nitride and oxidized silicon onto which polystyrene was grafted [203]. The force was measured in toluene.

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