Diffusion across interface

Hydrogel-hydrogel interface with PEG as adhesion promoter SNOM Inflnence of incorporated PEG on adhesion, dependence of PEG diffusion across interface on PEG molecnlar weight and contact time 1134... 

Diffusion across Interface Films and Finite Fluid Layers... 

Fluorescence Studies of Polymer Diffusion Across Interface... 

There has been a substantial interest over the past decade in the diffusion of polymer molecules across interfaces. The key features of interest for homopolymer interdiffusion are the influence of the interface on the kinetics of interdiffusion, and the connection between the extent of interdiffusion and the growth of strength of the joint. " One of the reasons for the broad interest in this topic is that polymer diffusion across interfaces represents the essential feature of a number of technologically important processes, such as welding of polymer slabs, sintering or compression molding of polymer powders, and the formation and aging of latex films. 

The use of lower temperatures will also slow the rate of most chemical degradation processes and so should also increase cell and stack hfe. The lower temperature will slow difliisional processes such as grain coarsening and diffusion across interfaces, which can lead to solid state reactions. However, some impurities may not be removed at these lower temperatures building up on the electrodes leading to degradation. It should also be noted that lower performance at lower temperatures may necessitate the use of higher electrochemical loads than would be required for optimal durability. 

This chapter discusses mass transfer coefficients for dilute solutions extensions to concentrated solutions are deferred to Section 9.5. In Section 8.1, we give a basic definition for a mass transfer coefficient and show how this coefficient can be used experimentally. In Section 8.2, we present other common definitions that represent a thicket of prickly alternatives rivaled only by standard states for chemical potentials. These various definitions are why mass transfer often has a reputation with students of being a difficult subject. In Section 8.3, we list existing correlations of mass transfer coefficients and in Section 8.4, we explain how these correlations can be developed with dimensional analysis. Finally, in Section 8.5, we discuss processes involving diffusion across interfaces, a topic that leads to overall mass transfer coefficients found as averages of more local processes. This last idea is commonly called mass transfer resistances in series. 

The overall mass transfer coefficient A often involves diffusion across interfaces. For example, in the lung, oxygen is transported from the air in the alveolus to the alveolus wall, across that wall, and into the blood. These different steps are often described as different mass transfer resistances. Thus, (1/A) is the overall resistance to mass transfer, and equals the sum of the other mass transfer resistances. If this idea is new or not completely clear, then you may wish to review Section 8.5. 

The ripple experiment works as follows In Fig. 6, HDH and DHD are depicted by open and filled circles where the filled circles represent the deuterium labeled portions of the molecule and the open circles are the normal (protonated) portions of the chains. Initially, the average concentration vs. depth of the labeled portions of the molecules is 0.5, as seen along the normal to the interface, unless chain-end segregation exists at / = 0. If the chains reptate, the chain ends diffuse across the interface before the chain centers. This will lead to a ripple or an excess of deuterium on the HDH side and a depletion on the DHD side of the interface as indicated in the concentration profile shown at the right in Fig. 6. However, when the molecules have diffused distances comparable to Rg, the ripple will vanish and a constant concentration profile at 0.5 will again be found. 

Fig. 6. The ripple experiment at the interface between a bilayer of HDH- and DHD-labeled polystyrene, showing the interdifussion behavior of matching chains. The protonated sections of the chain are marked by filled circles. The D concentration profiles are shown on the right. Top the initial interface at / = 0. The D concentration profile is flat, since there is 50% deuteration on each side of the interface. Middle the interface after the chain ends have diffused across (x < / g). The deuterated chains from Que side enrich the deuterated centers on the other side, vice ver.sa for the protonated sections, and the ripple in the depth profile of D results. A ripple of opposite sign occurs for the H profile. Bottom the interface when the molecules have fully diffused across. The D profile becomes flat [20,56].

If diffusion across a barrier layer is rapid compared with the rate of reaction at the advancing interface, then the overall rate is determined by the interface step (Sect. 3.1). 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

Sections 2.1—2.3 give accounts of kinetic and mechanistic features of the three rate-limiting processes (i) diffusion at a surface or in a gas (including the nucleation step), (ii) reaction at an interface, and (iii) diffusion across a barrier phase, [(ii) and (iii) are growth processes.] In any particular reaction, the slowest of these processes will, at any particular instant, control the rate of product formation. (A kinetic analysis of rate measurements must also incorporate an allowance for the geometric factors.)... 

The broken vertical line denotes an area of contact between any two ionic conductors, particularly between liquid ionic conductors (electrolyte-electrolyte interface or liquid junction). Ions can transfer between phases by diffusion across such a boundary hence, circuits containing such an interface are often called circuits or cells with transference. 

One of the features found at interfaces between two electrolytes (a) and ( 3) is the development of a Galvani potential, between the phases. This potential difference is a component of the total OCV of the galvanic cell [see Eq. (2.13)]. In the case of similar electrolytes, it is called the diffusion potential and can be determined, in contrast to potential differences across interfaces between dissimilar electrolytes. 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

Figure 3 Diffusion across a membrane. The solute molecules diffuse from the well-mixed higher concentration cY to the well-mixed lower concentration c2. Equilibrium is assumed at the interfaces of membrane and solutions. The concentrations on both sides of the membrane are kept constant. At steady state, the concentrations cm remain constant at all points in the membrane. The concentration profile inside the membrane is linear, and the flux is constant.

In this section we want to discuss unsteady diffusion across a permeable membrane. In other words, we are interested in how concentration and flux change before reaching the steady state discussed in Section IV.B. The membrane is initially free of solute. At time zero, the concentrations on both sides of the membrane are increased, to C and c2. Equilibrium between the solution and the membrane interface is assumed therefore, the corresponding concentrations on the membrane surfaces are Kc, and Kc2. Fick s second law is still applicable ... 

Considering only the lipid phase as the transport pathway for the peptide, as the solute enters and diffuses across the membrane it will encounter a number of different microenvironments. The first is the aqueous membrane interface (Fig. 23). In this region, the hydrated polar headgroups of the membrane phospholipids separate the aqueous phase from the apolar membrane interior. It has been shown that this region is capable of satisfying up to 70% of the hydrophobic effect... 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

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