Diffusion lateral surface

Fig. 3.20. A two-dimensional nucleus which may grow either by the addition of a new unit onto the lateral surface or by sliding diffusion within the nucleus (from [146] by permission of the publishers, Butter-worth-Heinemann Ltd. )...

Adsorbed molecules may diffuse laterally at the interface. Although surface diffusion is well-known in classical surface chemistry 33), data on adsorbed macromolecules is sparse. Burghardt and Axelrod3+) and Michaeli et al. 35) have both demonstrated rapid interface diffusion of adsorbed albumin. 

The solution diffusion properties of FITC-labelled BSA were measured by FRAP [12], The results showed that the protein diffused freely in solution with a diffusion coefficient of approximately 3xl0 7 cm2/s. This was in reasonable agreement with previously published values [36]. FRAP measurements were also made on thin films stabilized by FITC-BSA. The films were allowed to drain to equilibrium thickness before measurements were initiated. Thin films covering a range of different thicknesses were studied by careful adjustment of solution conditions. BSA stabilized films that had thicknesses up to 40 nm showed no evidence of surface diffusion as there was no return of fluorescence after the bleach pulse in the recovery part of the FRAP curve (Figure 14(c)). In contrast, experiments performed with thin films that were > 80 nm thick showed partial recovery (55%) of the prebleach level of fluorescence (Figure 14(b)). This suggested the presence of two classes of protein in the film one fraction in an environment where it was unable to diffuse laterally, as seen with the films of thicknesses < 45 nm, and a second fraction that was able to diffuse with a calculated diffusion coefficient of lxlO 7 cm2/s. This latter diffusion coefficient was 3 times slower than that... 

This applies, however, only within limits. Many solids are somewhat mobile and can flow very slowly. In that case methods and models of capillarity can be applied. One case where capillarity plays an important role is sintering. In sintering a powder is heated. At a temperature of roughly 2/3 of the melting point of the material the surface molecules become mobile and can diffuse laterally. Thereby the contact areas of neighbouring particles melt and menisci are formed. When cooling, the material solidifies in this new shape and forms a continuous solid. 

The thickness of the mucus layer in the human stomach is about 570 pm [101], whereas in the human colon the thickness is about 100-150 pm [102], Ryu and Grim [103] suggested that there is little mixing between the villi and the unstirred layer over the tip of the villi of the canine jejunum, which is about 500-1000 pm thick. Therefore, for a solute to reach the lateral surfaces of the villi, an additional barrier of as much as 800 pm needs to be traversed [103]. It was shown that native mucus gel from rat small intestine reduced the diffusion coefficients of 3H-water, urea, benzoic acid, antipyrine, aminopyrine, alpha-methyl-glucoside, L-phenylalanine by 37%-53% compared with buffer solution [104], For high-molecular-weight substances, such as proteins or microparticulates, mucin can offer considerable resistance to diffusion [105],... 

Fig. 1.10 Schematic view of the electrical double layer in agreement with the Gouy-Chapman-Stem-Grahame models. The metallic electrode has a negative net charge and the solvated cations define the inner limit of the diffuse later at the Helmholtz outer plane (OHP). There are anions adsorbed at the electrode which are located at the inner Helmholtz plane (IHP). The presence of such anions is stabilized by the corresponding images at the electrode in such a way that each adsorbed ion establishes the presence of a surface dipole at the interface...

A majority of oral solid dosage forms are tablets and caplets. Tablets can be handled as slabs or cylinders. More accurately, the tablet shape has both slab and cylinder features. Drug release occurs through radial and lateral surfaces, and diffusion of a solute in a tablet is expressed by ... 

Figure 3.28. Lateral surface diffusion coefficient for the head groups of the monolayer depicted in fig. 3.27. (Redrawn from Karabomi and Toxvaerd loc cit.)...

Deactivation of the catalyst pellet has been measured in a laboratory continuous stirred tank reactor The catalyst pellet has been placed in the reactor in such a way that the reacting gas has been in contact only with the lateral surface of the cylinder. The gaseous phase of volume 100 ml has been extensively mixed in order to enable us to assume utiiforw concentrations through the reactor and to neglect external diffusion. The temperature in the jacket of the reactor has been regulated to AO C. The temperatures of the... 

Importantly, any process that results in lateral coverage gradients of either the additive or metal adatom will be countered by surface diffusion. Such surface transport is known to be strongly influenced by both potential and electrolyte composition through associated impact on the structure and composition of the surface. For example, anions are known to lead to substantial enhancement of metal adatom transport with diffusion coefficients ranging from 2 1CT16 up to 8 10-13 cm2 s 1 [137, 138],... 

A simple experiment demonstrates the impermeability of certain tight junctions to many water-soluble substances. In this experiment, lanthanum hydroxide (an electron-dense colloid of high molecular weight) is injected into the pancreatic blood vessel of an experimental animal a few minutes later, the pancreatic acinar cells, which are specialized epithelial cells, are fixed and prepared for microscopy. As shown In Figure 6-10, the lanthanum hydroxide diffuses from the blood Into the space that separates the lateral surfaces of adjacent acinar cells, but cannot penetrate past the tight junction. 

Aggressive ions are released by active pits proportional to the current. These ions can diffuse laterally within the thin diffusion boundary layer on top of the metal surface and into the bulk electrolyte. Two coupled partial differential equations describe the spatiotemporal evolution of s and c ... 

Transient monooxygenase complexes are formed on the membrane surface as a result of collisions between P450s and CPR as each diffuses laterally within the membrane of the endoplasmic reticulum. The role of the membrane in mediating these interactions is poorly imderstood. However, early work has shown that the phospholipid component of the membrane can affect intermolecular interactions of the monooxygenase complex and influence subsfrate binding and may therefore be important in maintaining efficient electron transfer from CPR to P450. 

---