Choice of the carrier

The choice of the carrier is a key factor in facilitated transport. High selectivities are obtained if the carrier is very specific to one solute, a measure of this selectivity being given by the ratio of the distribution coefficients. In fact, every specific solute needs its own specific carrier which makes the selection of the carrier very important but also very difficult. Much informadon about carrier selection can be obtained from liquid extraction. It is beyond the scope of this book to mention all the different carrier molecules that have been been described to date but some classes of carrier molecules can be mentioned  

The structures of some of these carrier molecules are depicted iii table VI - 20. 

Tlie number of applications is very large and various classes can again be distinguished [66.70-72], e.g. the separation of cations, anions, gases and organic molecules. 

Both cations and anions can be easily removed via facilitated transport because a wide range of carriers is available. Among the numerous cations that can be recovered by liquid membranes, the following may be mentioned copper (Cu2+), mercury (Hg2+), nickel (Ni ), cadmium (Cd ), zinc (Zn2+) and lead (Pb2+). 

Anions can also be transported by liquid membranes, e.g. nitrate (N03 ). chromate (Cr207- ) and uranyl (002(504)2 ). 

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The choice of the carrier depends on several factors. The texture (size and shape of the pores, the way the pores are connected, etc.) of the support determines, to a large extent,... 

The choice of the carrier is important. The most common carriers are serum albumin of various species (generally quite soluble Erlang-er et al, 1959), keyhole limpet hemocyanin, thyroglobulin, ovalbumin, or fibrinogen. Different carriers can be used for immunization and assays or, alternatively, antibodies to the carrier can be removed by absorption. 

The low activity of the supported Rh catalysts is explained by assuming that only about 1% of Rh is active in this case. The choice of the carrier seems to have some impact on the catalytic activity, and X-zeolites have attracted special interest [25]. Also, higher alcohols such as ethanol, have been carbonylated over these systems [26]. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

The choice of solid carriers spans a wide spectrum (Table 1) from materials most suitable for research purposes (sintered glass beads, laterite stone deposited on a gramophone disk) to industrial materials (pumice, activated carbon, etc.). Key properties that affect the performance of the carrier are porosity (from impervious to controlled-size pores), composition (from ceramics to activated carbon), and hydrophilic behavior. It is difficult to perform a direct comparison of different carriers. Colonization and biofilm growth depend strongly on the nature of bacteria and on their intrinsic propensity to adhere on hydrophilic vs. hydrophobic surfaces. 

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