Barrier steric

Another important factor affecting the electronic properties is the steric barrier to planarity along the polymer chain. Since polyheterocycles and polyarylenes must adopt a planar geometry in the ionized state to form quinoid-like segments, steric factors that limit the ability of the polymer to adopt geometries which are planar with respect to adjacent rings have a detrimental effect on the electronic properties (181). 

Cram and his coworkers have pioneered the use of bis-binaphthyl crowns as chiral com-plexing agents for ammonium salts and amino acid salts. In these systems, the chiral binaphthyl unit provides a steric barrier within the macrocycle which allows discrimina-... 

The most likely way for pardaxin molecules to insert across the membrane in an antiparallel manner is for them to form antiparallel aggregates on the membrane surface that then insert across the membrane. We developed a "raft"model (data not shown) that is similar to the channel model except that adjacent dimers are related to each other by a linear translation instead of a 60 rotation about a channel axis. All of the large hydrophobic side chains of the C-helices are on one side of the "raft" and all hydrophilic side chains are on the other side. We postulate that these "rafts" displace the lipid molecules on one side of the bilayer. When two or more "rafts" meet they can insert across the membrane to form a channel in a way that never exposes the hydrophilic side chains to the lipid alkyl chains. The conformational change from the "raft" to the channel structure primarily involves a pivoting motion about the "ridge" of side chains formed by Thr-17, Ala-21, Ala-25, and Ser-29. These small side chains present few steric barriers for the postulated conformational change. 

Table I demonstrates that most liquid crystalline polymers lacking a spacer are formed from a flexible polyacrylate backbone. In contrast, the methyl substituent in polymethacrylate backbones both reduce main chain mobility and imposes additional steric barriers to mesophase formation. Therefore, successful liquid crystalline formation of polymethacrylates has been achieved only...

There are two lines of evidence that the reactions of simple tertiary derivatives such as [1]-C1 to give solvent adducts proceed through the corresponding tertiary carbocation intermediates (1) There is a large steric barrier to the... 

It is now well established that the critical flocculation conditions for dispersions with a low particle concentration can be correlated with the 6 conditions of the steric stabilizing moiety in free solution (1). This correlation is only found in systems where desorption of the stabilizer does not occur and where the thickness of the steric barrier is sufficient to completely screen the attractive van der Waals... 

The combinational contribution to AG,n for PMMA particles stabilized by PIB in 2-methylbutane is shown plotted as a function of temperature in Figure 3(a). The values of the parameters used in Equations 2 and 3 were u = 8 x 10- g cm-, a = 300 nm, >2 = 1.09 cm g- and V = 116.4 cnr mole- . The thickness of the steric barrier,L, was taken to be 25 nm and the particle separation, do, was fixed at 30 nm. It can be seen from Figure 3(a) that AGj (comb) is a positive quantity that becomes more positive as the temperature increases, indicating that in the absence of other contributions to AG, the particle would become more stable with increasing temperature. In the above calculation, we have assumed that the S function, Equation 3, remains invariant with temperature, which is incorrect. 

Combined Electrostatic and Steric Stabilization. The combination of the two mechanisms is illustrated in Figure 4, taken from Shaw s textbook, (13) where the repulsion of the steric barrier during a collision falls off so rapidly as the colliding particles bounce apart that the dispersion force attractions hold the particles together in the "secondary minimum". This is exactly what happens in the system investigated in this paper. 

In such systems the requirement of the electrostatic contribution to colloidal stability is quite different than when no steric barrier is present. In the latter case an energy barrier of about 30 kT is desirable, with a Debye length 1/k of not more than 1000 X. This is attainable in non-aqueous systems (5), but not by most dispersants. However when the steric barrier is present, the only requirement for the electrostatic repulsion is to eliminate the secondary minimum and this is easily achieved with zeta-potentials far below those required to operate entirely by the electrostatic mechanism. 

Figure 4. Potential energy diagrams for a pair of particles with on the left, a steric barrier (V ) and dispersion force attraction (V.) and on the right, with electrostatic repulsion (V ) added. Reproduced with permission from Ref. (13).Copyright 1980, Butterworths.

The zeta-potentials of Table III are greater than those in Figure 9 because the concentration of OLOA-1200 in solution was appreciably higher. The results are quite consistent at the same concentration in solution. These findings show that the zeta-potential in organic media is not a function of how much dispersant is adsorbed, but how much dispersant is left in solution. This is in contrast to the steric barriers, which depend on how much is adsorbed and not on how much is left in solution. 

As the 0L0A-1200 contentrof these dispersions increased it should be remembered that the steric barrier is well-developed at 1% and optimum at about 2% OLOA-1200, as evidenced by the conductivity and viscosity measurements of Figures 8 and 13. 

These findings show that the steric barrier provided by the 50 ft adsorbed films was inadequate to deflocculate the dispersion at all, even though it provided a million times more electrical resistance and reduced the viscosity very appreciably. On the other hand, the electrostatic barrier was very effective in deflocculating the system, but it took more dispersant than... 

Viscosities of concentrated suspensions of carbon black in a white mineral oil (Fisher "paraffin" oil of 125/135 Saybolt viscosity) were measured with a Brookfield viscometer as a function of OLOA-1200 content. Figure 13 shows the viscosities of dispersions with 30 w%, 35 w% and 70 w% carbon black. In all cases the viscosity fell rapidly as the 0L0A-1200 content increased from 0 to 1%, then fell more gradually and levelled off as the 0L0A-1200 content approached 2%. In many respects the reduction in viscosity with increasing OLOA-1200 content parallels the conductivity measurements both phenomena are sensing the buildup of the steric barrier, and this steric barrier weakens, softens, and lubricates the interparticle contacts. As evidenced in foregoing sections, the particles are still flocculated but can be easily stirred and separated mechanically. The onset of electrostatic repulsion at OLOA-1200 contents in excess of 2.5% did not affect viscosities. 

The steric barrier developed upon adsorption of 1-2% of the dispersant was evidenced by a million-fold decrease in conductivity, a twenty-fold decrease in viscosity, a two-fold increase in sediment volume, but no deflocculation of any degree. 

In model studies involving Fe(n) species, three broad approaches have been used to mitigate the problem of autoxidation of the iron (Hay, 1984). These are (i) the use of low temperatures so that the rate of oxidation becomes very slow (ii) the synthesis of ligands containing steric barriers such that dimerization of the iron complex is inhibited, and (iii) immobilization of the iron complex on a solid surface such that dimerization once again will not be possible. 

This contrasts with the purely high spin nature of the complex of 2,6-bis(quinolin-2-yl)pyridine [62] 46 and is consistent with the reduced steric barrier to coordination from substitution adjacent to the donor atom within five-membered rings, evident in the diimine systems. 

It is surprising that the quintet state for iron(II) is appreciably populated in the derivatives of the amidine system 65 (Dq(Ni2+)=1170 cm-1), despite the absence, in this instance, of any apparent steric barrier to coordination from substituents and the formation of five-membered chelate rings. 

Figure 2.18 The pair potential calculated for polystyrene particles of radius 500 nm, with a measured (.-potential of —12mV in 0.5moldm 3 electrolyte. A steric barrier of S = 3.5 nm was used as the particles had monolayer coverage of a monodisperse non-ionic surfactant. This was C 2E06 which represents a dodecyl hydrophobic moiety linked to hexaethylene glycol via an ether link...

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