Transfer across membranes physical properties

Absorption describes the rate and extent to which a drug leaves its site of administration and enters the general circulation. Factors that, therefore, affect absorption include the physicochemical properties of the drug that determine transfer across cell membranes as described earlier formulation or physical state of the drug site of absorption concentration of drug circulation at absorption site and area of absorbing surface. 

Notes The flow per area of 0.005 cm/s is typical of that used in absorption or extraction. All physical properties assume oxygen dissolved in water being transferred across a microporous membrane into rapidly flowing, water-saturated nitrogen. 

There are many studies of the transfer of electrons from enzymes to substrates, across biological membranes, to (or from) electrodes from (or to) substrates, between adsorbed molecular dyes and semiconductor particles, within synthetic films and nano-scale arrays, within molecular wires , and so on. Only a few, general comments will be offered on these topics here. The basic physics of molecular electron transfer does not change with the scale of the system, as long as identifiable molecular moieties are present with at least partly localized electronic configurations. The nature of the properties observed, the experimental probes available, and the level of theoretical treatment that is useful may be very different. Different approaches, different limiting models are used for extended arrays (or lattices) of very strongly coupled moieties. 

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