Repulsion mechanism

Fig. 25a and b. A protein resistant surface based on the steric repulsion argument commonly used in the colloid stability field U0). The interaction between a polyethylene oxide grafted surface and a protein solution is shown, a. suggests an excluded volume or steric repulsion mechanism b. the surface dynamics or polymer chain motion mechanism (from Ref., 33))... 

The subsequent discussion will focus on two different repulsion mechanisms which may lift the pseudo-spherical degeneracy of the 2D term ligand delocalization and configuration interaction. 

These methods do not require further purification of PIA before chromatography. However, purging the columns extensively after several runs is necessary, as particularly the PolarPac Wax column tends to bind negatively charged polymers (such as teichoic acids) that need to be removed because they interfere with the repulsion mechanism of positively charged PIA on this column, which is needed for optimal purification of deacetylated PIA (4). 

The steric repulsion mechanism is also difficult tc model in our system. A bitumen/toluene solution itself is a very complex system containing high molecular weight asphaltenes, natural surfactants, and ultrafine particles. These components are very likely to be adsorbed on the water/toluene interface. Due to this complexity, it is hard to model the adsorption layer with a single elastic modulus, as was done for the analysis of poly(ethylene oxide) adsorption layers on latexes (7). However, all steric forces resemble hard-wall interactions. They can be approximately modeled by high-order polynomial functions. We used a simple expression F steric c/h where h is the separation... 

Surface modification with hydrophilic polymers, such as poly(ethylene oxide) (PEO), has been beneficial in improving the blo( compatibility of polymeric biomaterials. Surface-bound PEO is expected to prevent plasma protein adsoiption, platelet adhesion, and bacterial adhesion by the steric repulsion mechanism. PEO-rich surfaces have been prepared either by physical adsorption, or by covalent grafting to the surface. Physically adsorbed PEO homopolymers and copolymers are not very effective since they can be easily displaced from the surface by plasma proteins and cells. Covalent grafting, on the other hand, provides a permanent layer of PEO on the surface. Various methods of PEO grafting to the surface and their effect on plasma protein adsorption, platelet adhesion, and bacterial adhesion is discussed. 

The quantity that we wish to calculate is given by Equation (5.31) where the subscript ap stands for the apolar-polar repulsive free energy of hydration mechanism and cc stands for the charge-charge repulsion mechanism ... 

Figure 5.35. Visualization of the relative efficiencies of the electrostatic charge-charge repulsion and the apolar-polar repulsion mechanisms by comparison of acid-base titration curves for poly(methacrylic acid), which exhibits charge-charge repulsion (negative cooperativity), and for the Model Proteins i and V, which exhibit apolar-posar repulsion (positive cooperativity). The polymers are each compared with the Henderson-Hasselbalch curve as reference. Chemical energy is Ap>An, where Ap = 2.3RTApH for the change in pH to go from one state to the other, and An is the number of moles to go from a degree...

The result is dramatic Tlie efficiency of the charge-charge repulsion mechanism is very limited indeed. 

Availability of the most efficient mechanism for achieving function in an aqueous environment. Comparison of the electrostatic charge-charge repulsion mechanism for chemo-mechanical transduction with that of the apolar-polar repulsive free energy of hydration, AG, shows the latter to be more than an order of magnitude more efficient. This becomes particularly relevant to biomedical applications of controlled release as required in drug delivery, but also whenever a sensitive and responsive (smart) biomaterial is desired. 

Interception. If the suqiended particle radius is greater than the distance between the flow streamline which contains the particle and the collecting media grain, then the su ended particle will contact the target, in the absence of any repulsive mechanisms. 

The major repulsive mechanism between electron pairs around a given atom is a quantum mechanical (QM) effect, which we alluded to in Section 1.R.3 as the exclusion principle. According to this principle, if two electrons occupy the same space, they must possess different spin directions (Scheme 1.R.1). Indeed, all the electron pairs, be they bonds or lone pairs, are net spin-less, involving two opposite-spin electrons. Since there are only two directions of the spin property, this necessarily means that two electron pairs cannot occupy the same space. If they do, there will be two electrons of the one spin type (ft) and two others with the other spin type (f f). This is excluded by the principle the exclusion actually means that this situation raises the energy very much, and hence, molecules avoid the situation as much as possible. The consequence of this is that electron pairs around a given atom will be distanced maximally in space to lower the molecular energy. This is the basis of the VSEPR rules. 

In general, the anionic emulsifiers are extensively preferred in many emulsion polymerization systems. They serve as strong particle generators and stabilize the latex particles via electrostatic repulsion mechanism. But latexes stabilized with this type of emulsifiers are often unstable upon addition of electrol)d es and in freeze-thaw cycles. Furthermore, these emulsifiers have limited stabilizing effectiveness at high solids (e.g.,... 

Zeta potential Related closely with DLVO theory. A measure of the electrical charge on the effective particle surface. A key parameter when relying on the ionic repulsion mechanism for disper-sion/deflocculation. 

To overcome the aggregation and/or coalescence processes, one must overcome the van der Waals attraction by some repulsive mechanism that will give the system kinetic stability with an adequate shelf-life. Normally one requires a shelf-life of 2-3 years under various storage conditions (e.g. temperature variation). 

When two particles or droplets containing electrical double layers with the same charge sign approach each other to a distance of separation whereby the double layers begin to overlap, repulsion occurs since the double layers cannot be fully developed in the confined space between the particles or droplets. This repulsive mechanism will be discussed in detail in subsequent sections. 

The above repulsive mechanisms ensure the colloid stability of suspensions or emulsions. This colloid stability should be distinct from the overall physical stability of the system, which imphes complete homogeneity of the system with no separation on storage. 

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