Steric stabilization, relationship

Surface energies of sohds, surface and interfadal tensions and the interfacial region, thermodynamics of colloidal systems, improved electrical double layer theory, adsorbed pol)mer layers and steric stabilization, relationships between surface energies and bulk properties... 

Influence of Addition of Electrolyte and Increase of Temperature Addition of electrolyte or increase of temperature at a given electrolyte concentration to a sterically stabilized dispersion may result in its flocculation at a critical concentration or temperature, which in many cases coincides with the theta point for the stabilizing chain. At the theta point the mixing term in the steric interaction is zero and any yield value measured should correspond to the residual van der Waals attraction. The energy arising from van der Waals attraction may be calculated from the following approximate relationship,... 

WDo is the weight (in g/1) of macromonomer in the reactants, NA is Avogadro s number, and MD is the molecular weight of the macromonomer. From Eqs. (19)-(21),one can obtain a universal relationship between the particle radius and the extent of polymerization for sterically stabilized particles ... 

To explore structure-function relationships and optimize their activities, NeuAc-substituted acrylamide polymers were prepared with a variety of appended functional groups (Fig. 22) [99]. These studies generated polymers with very high inhibitory potencies toward HA. For example, an acrylamide copolymer substituted with 10% benzylamine and 20% NeuAc residues had a Ki of 0.6 nM [100]. Both the chelate effect and steric stabilization were mechanisms suggested to contribute to the potent activity of this and related materials (Fig. 5). The NeuAc-substituted acrylamide... 

These relationships provide convenient mathematical expressions for the concentration dependence of the interaction parameter that can be used to incorporate this dependence into the theory for steric stabilization. 

First attempts to quantitatively estimate stabilizing capabilities of polymers date back to Faraday s time." Steric stabilization becomes probable because spatial dimensions of at least comparatively low-molecular compounds are commensurable with the range of London s forces of attraction or even exceed them. If the diameter of a macromolecule of a linear polymer coincides with the root-mean-square (rms) distance between its ends then the relationship between the mean geometric radius of the particle (r versus polymer molecular mass M can be expressed by the following relationship ... 

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

Nitroalkanes show a related relationship between kinetic acidity and thermodynamic acidity. Additional alkyl substituents on nitromethane retard the rate of proton removal although the equilibrium is more favorable for the more highly substituted derivatives. The alkyl groups have a strong stabilizing effect on the nitronate ion, but unfavorable steric effects are dominant at the transition state for proton removal. As a result, kinetic and thermodynamic acidity show opposite responses to alkyl substitution. 

Upon formulating these relationships, phenols with branched alkyl substituents were not included in the data of a-cyclodextrin systems, though they were included in (3-cyclodextrin systems. In all the above equations, the n term was statistically significant at the 99.5 % level of confidence, indicating that the hydrophobic interaction plays a decisive role in the complexation of cyclodextrin with phenols. The Ibrnch term was statistically significant at the 99.5% level of confidence for (3-cyclo-dextrin complexes with m- and p-substituted phenols. The stability of the complexes increases with an increasing number of branches in substituents. This was ascribed to the attractive van der Waals interaction due to the close fitness of the branched substituents to the (3-cyclodextrin cavity. The steric effect of substituents was also observed for a-cyclodextrin complexes with p-substituted phenols (Eq. 22). In this case, the B parameter was used in place of Ibmch, since no phenol with a branched... 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The most frequently encountered hydrolysis reaction in drug instability is that of the ester, but curtain esters can be stable for many years when properly formulated. Substituents can have a dramatic effect on reaction rates. For example, the tert-butyl ester of acetic acid is about 120 times more stable than the methyl ester, which, in turn, is approximately 60 times more stable than the vinyl analog [16]. Structure-reactivity relationships are dealt with in the discipline of physical organic chemistry. Substituent groups may exert electronic (inductive and resonance), steric, and/or hydrogen-bonding effects that can drastically affect the stability of compounds. A detailed treatment of substituent effects can be found in a review by Hansch et al. [17] and in the classical reference text by Hammett [18]. 

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