Obstruction effect

As we saw in Chapter 10, the stress required to make a crystalline material deform plastically is that needed to make the dislocations in it move. Their movement is resisted by (a) the intrinsic lattice resistance and (b) the obstructing effect of obstacles (e.g. dissolved solute atoms, precipitates formed with undissolved solute atoms, or other dislocations). Diffusion of atoms can unlock dislocations from obstacles in their path, and the movement of these unlocked dislocations under the applied stress is what leads to dislocation creep. 

Johansson, L Elvingson, C Lofroth, JE, Diffusion and Interaction in Gels and solutions. 3. Theoretical Results on the Obstruction Effect, Macromolecules 24, 6024, 1991. 

The Desmopressin diffusion coefficient in the cubic phase at 40 C (D=0.24 x 10-10 m2s-l) is about a factor 9 smaller than in 2H20-solution at 25 C (D=2.25 x 10-10 m s" ), a difference which is larger than what is expected from pure obstruction effects a reduction factor of three is expected from the inclusion of a solute in the water channels of the cubic phase (13). Thus, the results indicate an interaction between the peptide and the lipid matrix and/or membrane surface, especially since the peptide and lipid diffusion coefficients are very similar in the cubic phase (Table... 

Jackson, Shen and McQuarrie (95) use an alternative method for the calculation of the obstructional effect of the molecular volume. Their starting point is the assumption that the obstruction becomes anisotropic as soon as the network is stretched. Rather than using probabilities of 1/3 for the random walk steps in any of the three cartesian directions, they derive for unidirectional strain... 

Some remarks in connection with Blokland s theory are appropriate. In the first place it must be pointed out that an additional parameter is introduced in the theory, which is supposed to reflect the existence of intermolecular obstructional effects ( %). It is questionable whether this parameter is an accurate reflection of a structured network,... 

The treatment of ionic diffusion properties is a more difficult task. The obstruction effect of the polymer matrix can be represented by a number of semi quantitative equations, the most popular of which is 111 ... 

Oradd, G., and G. Lindblom (2004) NMR Studies of lipid lateral diffusion in the DMPC/gramicidin D/water system peptide aggregation and obstruction effects. 

Vitamin A intoxication Prolonged and marked vitamin A intoxication leads to a substantial increase in individual Ito cells. This in turn causes constriction of the sinusoids the accompanying fatty degeneration of the hepatic cells supports this obstructive effect. The Ito cells are responsible for perisinusoidal fibrosis. The liver surface is strikingly smooth despite marked portal hypertension (often with considerable oesophageal varices). (119) (s. figs. 14.2, 14.3)... 

The acute interstititial nephritis after oxalate overload may be due to calcium oxalate crystals inducing obstructive effect, nephrocalcinosis and also by inducing apoptosis of renal epithelial cells [22, 23, 25]. 

Diffusion within cells is even more complex than within simple gels first the cytoplasm is a molecularly crowded zone, a complex gel with structural obstacles such as actin and myosin fibers and strands. There is the additional tortuosity that occurs in gels as the moving particle avoids the regions of the macromolecular chains and the obstruction effects from the impenetrable regions of the cytoplasm. If we designate as the effective... 

The latter theory has been challenged by the measuremoit of diffusion of intracellular water i.e. NMR methods have shown that the small difference in measured diffusion coefficients between intra and extracellular water can be accounted for by obstruction effects ( ). Most authors now agree that the majority of intracellular water is "free" euid account for the reduction in the tissue values by assuming that fast exchange occurs between a small "bound" fraction, and the larger "free" fraction. In the absence of structured water and slow exchange, the complex transverse releucation is more difficult to explain. There is no doubt that the complex behaviour is within the water protons because of the size of resolvably different relaxation processes (.2). 

Other examples are PPS (which shows a double melting behavior due to the obstructive effect of branching or crosslinking of the molecules on crystallization at high temperature [Mai et al, 1994]), and PEEK. Much controversy exists about... 

Fuoss and Accascina corrected the predicted theoretical conductances for this obstructive effect assuming the validity of Walden s rule, i.e., Krjs = constant. Thus,... 

Obstruction Effects on Molecular Translation in the Continuous Medium... 

The molecules of the continuous medium experience an obstruction effect of the droplets that depends simply on droplet volume fraction and shape, and the obstruction effect may convey information concerning the shape and size of the colloidal particles in a broad range of systems. The relevant theory has been worked out by Jonsson et al. [28]. In short, the presence of spherical and rod-shaped aggregates gives rise to minor obstruction effects. 

Although the method outlined above is certainly useful, one should be aware of the limitations inherent in the approach. First, a direct application of Eq. (2) assumes infinite dilution. As surfactant aggregates are self-assembled units, one must be aware of the fact that dilution may change their properties. At higher concentrations, interactions become important [cf Eq. (3)], and these must be taken into account [31,32]. In fact, the obstruction effects can be used to advantage, as information pertaining to the interparticle interactions can be derived from them. 

One should note that the equations above describe only the effect of geometrical obstruction. In many cases, solvation effects (solvent molecules have a lower mobility in the vicinity of the surfactant film) may be of similar or even higher magnitude. The solvation effects increase with the surfactant concentration, and care has to be taken to separate solvation from obstruction effects [34]. 

For a tubular structure where distances between branching points are significantly greater than the tube diameter, we expect D/Do of the solvent in the tubes to be close to 1/3, because diffusion is allowed only along the axes of the tubules. The external solvent should have a D/Dq value not much below 1. (The obstruction effect of cylinders is small.) More plausible is an interconnected rod model for which the diffusion was theoretically analyzed by Anderson and Wennerstrom [33]. 

The two-site model leads to the definition of the degree of counterion binding, p, as the ratio of counterions to surfactant ions in a surfactant self-assembly. This is a useful but incomplete characterization of the counterion distribution. The value of p can be obtained directly from self-diffusion data because the self-diffusion coefficients of free ions are easily obtainable. For free counterion diffusion a correction is made for the obstruction effect. The micellar D value is obtained as described above or estimated as Dm free. an exact Dm value is not critical. 

The method monitors transport over macroscopic distances (typically in the micrometer regime). Therefore, when the method is applied to the field of surface and colloid chemistry, the determined diffusion coefficients reflect aggregate sizes and obstruction effects for colloidal particles. This is the origin of the success the method has had in the study of microstruc-toes of surfactant solutions and also forms the basis of its applications to emulsion systems. We expect that the PFG method will also be increasingly important in the study of emulsion systems and therefore we will discuss the method in some detail, with particular focus on its application to emulsions. 

A decrease in forced vital capacity, expiratory volume in one second and of peak exspiratory flow was observed after exposure to water-based paints. Bronchial obstruction in painters was confirmed by White and Baker, but other studies could not detect changes in lung function parameters in house painters who were exposed to solvent-based and water-based paints. Beving et al. did not find obstructive effects in car painters. An increase... 

Johansson, L., Elvingson, C., and Lofroth, J.E. (1991) Diffusion and interaction in gels and solutions. 3. Theoretical results on the obstruction effect. Macromolecules, 24, 6024-6029. 

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