Solution-diffusion mechanism activated process

When the solution-diffusion mechanism is applicable, permeation can usually be treated as an activated process, as described by Equation 15.3. The activation energy Ep changes as the polymer goes through major transitions such as the glass transition or melting. Pq is a pre-exponential factor which has the same units as P(T). 

This article focuses on transport that proceeds by the solution-diffusion mechanism. Transport by this mechanism requires that the penetrant sorb into the polymer at a high activity interface, diffuse through the poljuner, and then desorb at a low activity interface. In contrast, the pore-flow mechanism transports penetrants hy convective flow through porous pol5uners and will not be described in this article. Detailed models exist for the solution and diffusion processes of the solution-diffusion mechanism. The differences in the sorption and transport properties of rubbery and glassy pol5uners are reviewed and discussed in terms of the fundamental differences between the intrinsic characteristics of these two types of polymers. 

There are various concepts about the aluminum silicates dissolution mechanism. Relatively recently a low rate of their dissolution was explained by inner diffuse regime. Currently more substantiated appears hydrolysis with the formation of activated complexes. According to this theory, the dissolution begins with the exchange of alkaline, alkaline-earth and other metals on the mineral surface of H+ ions from the solution (see Figure 2.26). At that, metals in any conditions are removed in certain sequence. In case of the presence of iron and other metals with variable oxidation degree the process may be accompanied with redox reaction. Hydrolysis is a critical reaction in the dissolution of aluminum silicates. It results in the formation on the surface of a very thin layer of activated complexes in Na, K, Ca, Mg, Al and enriched with H+, H O or H O. The composition and thickness of this weakened layer depend on the solution pH. These activated complexes at disruption of weakened bonds with mineral are torn away and pass into solution. For some minerals (quartz, olivine, etc.) the disruption of one inner bond is sufficient, for some others, two and more. The very formation of activated complexes is reversible but their destruction and removal from the mineral are irreversible. 

The atomic mechanism of diffusion was, for many years, controversial, although for interstitial solutes (e.g., hydrogen, helium, carbon, nitrogen, and oxygen [and possibly boron] in iron), there has never been any doubt that diffusion is by migration from one interstice to another. In fee metals, at least, it seems fairly certain that substitutional alloy atoms diffuse by activated jumps into vacant lattice sites, that is, by the vacancy mechanism. In solid solutions in which one component is interstitial, two noncompeting processes occur, and two independent diffusion coefficients are obtained. 

Papain cleaves BAEE into an acidic product, which dissociates and lowers the local pH in the membrane. Papain s activity exhibits a bell-shaped curve as a function of pH [54]. Activity increases with decreasing pH in the alkaline region, so BAEE cleavage and acid production is autocatalytic. Under proper conditions, local BAEE depletes rapidly as pH drops. The reaction then shuts down, and OH ions diffusing in from the external solution realkahnize the region. Once BAEE is replenished, also by diffusion, the cychc process can start anew. It should be emphasized that, for this mechanism to produce oscillatory behavior, the diffusion and reaction fluxes must play off each other so as to prevent the appearance of a stationary state. When one flux dominates, a steady state is reached without sustained oscillations. 

This process of gas transport through a membrane is called permeation, and the mechanism has been identified as solution-diffusion. Gas species i dissolves at the feed-membrane interface (z = 0) by molecular diffusion, the dissolved gas molecules move through the membrane and are finally desorbed into the product gas phase at the product-membrane interface (z = dm). Under the simplest of conditions, each species in a mixture diffuses independently of the others according to flux expression (3.4.72). The nature of the dependence of Di on the effective diameter of the gas molecules, the temperature and the polymer for an activated diffusion is iliustrated in Section 4.3.3 for an amorphous polymer. 

Mass transfer through dense polymeric membranes is nowadays accepted to be described by the sorption-diffusion mechanism. According to this, the species being transported dissolve (sorb) in the polymer membrane surface on the higher chemical potential side, diffuse through the polymer free volume in a sorbed phase, and pass into the fluid phase downstream of the membrane (lower chemical potential side). In the case of dense polymeric membranes the polymer is an active participant in both the solution and diffusion processes. However, since in many porous membranes the mass transfer takes place mainly in the pores, the membrane material is not an active participant and only its pore structure is important. ... 

---