Molecular migration, definition

Molecular diffusion (or self-diffusion) is the process by which molecules show a net migration, most commonly from areas of high to low concentration, as a result of their thermal vibration, or Brownian motion. The majority of reactive transport models are designed to simulate the distribution of reactions in groundwater flows and, as such, the accounting for molecular diffusion is lumped with hydrodynamic dispersion, in the definition of the dispersivity. 

Polymerization products with internal plasticization have the advantage that there is no migration or efflorescence of the plasticizer on storage. On the other hand, the mechanical properties—above all, flexibility at low temperatures—are decreased in comparison with the products with low molecular plasticizers. This is because mobility in the solvatized molecules is diminished. The same refers to tensile strength, which is often decreased with internally plasticized polymers. In this respect they are inferior to externally plasticized products, which also have an advantage in that type and quantity of plasticizer can be varied widely. Internally plasticized products must have a definite ratio of components. These are reasons why combinations of both types of plasticizers are preferred. 

The definition of molecular rearrangement includes changes in which there is a migration of an atom or... 

CE can be used as a qualitative or quantitative technique. An example of a CE instrument is shown in Figure 3.31. Qualitative information allows a substance to be identified on the basis of migration time comparisons between a standard and the sample. Alternatively, and more definitively, the use of a mass spectrometric detector enables the molecular weight of eluting components to be determined and, as long as those molecular weights... 

Finally, we must consider how protons are delivered to complete the reaction. Without definitive evidence, molecular modeling has identified three likely proton-transfer routes (71). One is the interstitial channel, filled with water molecules, that nms between the a- and )3-subunits from the surface of the MoFe protein to the pool of water molecules around the homocitrate of the FeMo-cofactor. This channel could deliver protons rapidly to boimd substrate and might also provide a pathway for N2 and NH4+ to enter and leave the reduction site (see also the section Substrate-Binding Site). Extensive theoretical studies of the hydrogen-related chemistry of the FeMo-cofactor use this same interstitial channel to deliver protons (75). These studies indicate that the delivery of electrons to the FeMo-cofactor causes its sulfur atoms to become more basic which, in turn, makes them attractive sites for protonation by water molecules in the interstitial pool. Once transferred, these protons become reduced to hydrogen atoms that can then migrate across the FeMo-cofactor structure to other Fe and S atoms and become involved in substrate reduction (76). 

An adhesive is a linear or branched amorphous polymer above its Tg. It must be able to flow on a molecular scale to grip surfaces. (This definition is not to be confused with polymerizable adhesive materials, present in monomeric form. These are tacky or sticky only in the partly polymerized state. Frequently they are cross-linked thermoset, finally. Contact with the surface to be adhered must be made before gelation, in order to work.) An example is the postage stamp adhesive, composed of linear poly(vinyl alcohol), which is plasticized by water (or saliva) from below its Tg to above its Tg. On migration of the water away from the adhesive surface, it sticks. ... 

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