Steric repulsion origin

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start overlapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stabiUty that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stabihty, and its treatment dominated the colloid science Hterature for several decades. 

In filtration, the particle-collector interaction is taken as the sum of the London-van der Waals and double layer interactions, i.e. the Deijagin-Landau-Verwey-Overbeek (DLVO) theory. In most cases, the London-van der Waals force is attractive. The double layer interaction, on the other hand, may be repulsive or attractive depending on whether the surface of the particle and the collector bear like or opposite charges. The range and distance dependence is also different. The DLVO theory was later extended with contributions from the Born repulsion, hydration (structural) forces, hydrophobic interactions and steric hindrance originating from adsorbed macromolecules or polymers. Because no analytical solutions exist for the full convective diffusion equation, a number of approximations were devised (e.g., Smoluchowski-Levich approximation, and the surface force boundary layer approximation) to solve the equations in an approximate way, using analytical methods. 

The filled (Is)2 cores lead to additional steric repulsions with the incoming donor hybrid. These combine with nuclear Coulomb repulsions to oppose high electronic overlap. Figure 3.6 displays the calculated increase in steric repulsion as each filled 2s spin-orbital of Li2 collides with the filled core Is of the opposite atom. The paired one-electron steric repulsions shown in Fig. 3.6 are similar to the ionic two-electron steric repulsions of Fig. 2.11, and the wave-mechanical origin of the steric pressure would be analogous to that described in the discussion surrounding Fig. 2.12. 

The steric repulsions between off-axis lone pairs at Req are stronger in F2 than in Cl2, and the difference increases rapidly at smaller R. This is shown in the plot below, which compares the full potential curves (solid lines) with the pairwise sum of steric repulsions between off-axis lone pairs (dashed lines) for F2 (circles) and Cl2 (squares), both shifted to a common origin at Req (1.4083 and 2.0528 A, respectively) ... 

The t-butylammonium cation forms a relatively weak complex, which was attributed to steric repulsion. The stability of anilinium complexes is about the same as that of alkylammonium complexes. The introduction of substituents at the 2- and 6-positions increases the steric repulsion which results in a drop in binding constant. This effect is enthalpic in origin. These results show that variations in the group R have only a minor effect on the stability of complexes in which the crown ether has a fully exposed cavity. Furthermore, there is no relationship between the p-K.-value of RNHJ and the stability of the complex. 

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

On the other hand, in support of the hypothesis that the polarization of water might play an important role in the hydration force between silica surfaces, one should note that the polarization model predicts an increase in the hydration force at higher ionic strength [30], which can be indeed observed in Fig. 5, by comparing the experiments at c =0.01 M (pH=3) with those at cE= 1 M. While both the gel-induced steric repulsion and the polarization model are consistent with the present experimental data, a final decision about the microscopic origin of the hydration force in the case of silica should be postponed until more accurate data or additional information regarding the nature of the silica surface will become available. 

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