Steric elastic interaction

In practice, the disjoining pressure II(/i) must be taken into account only for films of thickness 1CT9 < h < 10 7m. The disjoining pressure comprises some components of different physical nature [383] Van der Waals attraction, electrostatic repulsion, steric elastic interaction, etc. The approximation formulas for these components in various regions of h acquire different forms [116, 122, 383],... 

Ligand-Ligand Interactions. An elastic contribution to the potential due to loss of conformational entropy as the ligands begin to overlap, a steric repulsion, is also known in the literature (57,58). In terms of the scaled variables it can be written as... 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

From a technical standpoint, it is also important to note that colloids display a wide range of rheological behavior. Charged dispersions (even at very low volume fractions) and sterically stabilized colloids show elastic behavior like solids. When the interparticle interactions are not important, they behave like ordinary liquids (i.e., they flow easily when subjected to even small shear forces) this is known as viscous behavior. Very often, the behavior falls somewhere between these two extremes the dispersion is then said to be viscoelastic. Therefore, it becomes important to understand how the interaction forces and fluid mechanics of the dispersions affect the flow behavior of dispersions. 

In the final section, we build on the thermodynamic theories of polymer solutions developed in Chapter 3, Section 3.4, to provide an illustration of how a thermodynamic picture of steric stabilization can be built when excluded-volume and elastic contributions determine the interaction between polymer layers. 

The second contribution to the steric interaction arises from the loss of configurational entropy of the chains on significant overlap. This effect is referred to as entropic, volume restriction, or elastic interaction, Gei. The latter increases very sharply with a decrease in h when the latter is less than 8. A schematic representation of the variation of Gmix, Gei, G, and Gj =G X + Gei + Ga) is given in Fig. 10. The total energy-distance curve shows only one minimum, at h 25, the depth of which depends on 5, R, and A. At a given R and A, G decreases with an increase in 5. With small particles and thick adsorbed layers (5 > 5 nm), G, becomes very small (approaches thermodynamic stability. This shows the importance of steric stabilization in controlling the flocculation of emulsions and suspensions. 

Priss LS (1981) Molecular origin of constants in the theory of rubber-like elasticity considering network chains steric interactions. J Pure Appl Chem 53 1581-1596 Priss LS, Gamlitski YuA (1983) Mechanism of conformation transitions in polymer chains. Polym Sci USSR 25 117-123... 

FIGURE 10.23 The distance dependence of the steric interaction energy of latex particles stabilized by poly(vinyl alcohol) in water (1) 500 nm, (2) 100 nm, and (3) 10 nm. The elastic modulus is 1.4 x 10 nm . Data from Sonntag [45]. 

In the gel network there are zones, where the polymers interact, and large segments, where the macromolecules are randomly extended. The lattice is responsible for the elasticity and the textural strength of the product. In multicomponent gels all constituents may form separate or coupled networks, or else one component, not involved in network formation, may indirectly affect the gelling by steric exclusion of the active molecules. Such exclusion increases the concentration of the active component in the volume of the solution where the gel is formed. In gels made from the mince of squid meat at 1.5% NaCl, the added carrageenan and egg whites form separate networks that support the structure made of squid proteins, while added... 

Entropic, volume restriction or elastic interaction, G p This results from the loss in configurational entropy of the chains on significant overlap. Entropy loss is unfavourable and, therefore, G j is always positive. A combination of G,. with G gives the total energy of interaction Gj (theory of steric stabilisation). 

Plotting the insect toxicity values against the van der Waals volumes, the authors revealed that a sharply defined minimum volume is required. Steric interaction is apparent between the aliphatic and aromatic substituents, suggesting that the molecule must fit into a tridimensional elastic sac to induce toxicity. 

Benjamins et al (24) studied the effects of aging on the elasticity of (i-casein and K-casein films. The dilatational modulus of K-casein was larger than that of -casein and increased by a factor of three with film age, whereas the dilatational modulus of -casein films changed little with time ( ). K-casein unfolds less at the air/water interface since it has less random structure than Q-casein. This can also be interpreted in terms of K-casein having less direct contact with the film surface at any given protein concentration (25). Significant protein-protein interactions i.e. steric/electrostatic repulsion, are believed to occur between segments of polypeptide chains which extend both above and below the plane of the air/water interface in surface protein films (16,26)... 

The preceding division of the region of close approach of two sterically stabilized particles into three domains leads quite naturally to a discussion of the steric interaction in terms of two basic components the mixing free energy and the elastic free energy. 

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