Macrocyclic carriers

For the alkali metal cations, the stability (14) and permeability (43) sequences for dicyclohexyl-18-crown-6 have been found to be the same (K+ > Rb+ > Cs+ > Na+ > Li+). Thus, a direct relationship exists between the ability of a macrocyclic compound to complex a particular cation (as measured by the log K value for complex formation) and its influence on the biological transport of that cation. Furthermore, it would appear that the biological ion-transport mechanism may in part be due to the complexation properties of the macrocyclic carrier molecules. This subject with respect to cyclic antibiotics has been treated extensively by Si wow and co-workers (2). 

Kinetic analyses allowed the experimental results to be related to the dependence of transport rates and selectivities on carrier properties [6.9, 6.17, 6.18]. Detailed studies of 8 and 9 in vesicles bore on the efficiency, the selectivity and the mechanism of the processes [6.19]. The rates of transport by proton ionizable macrocyclic carriers are pH dependent [6.12]. Diverse other ligands have been used as carriers, such as acyclic polyethers or calixarene derivatives [6.20, A.6]. 

Fig. 13. (left) A competitive divalent/monovalent cation symport coupled to M2+/2H+ and M+/H+ antiport in a pH gradient by a macrocyclic carrier such as 86 the state of the carrier is indicated as diprotonated, [(CC>2H)2], or complexed, [(CC -, M2+] and [(C02H)(C02 ), M+] (right) Rates of K+ and Ca2+ transport by the macrocyclic carrier molecule 86 through a liquid membrane as a function of pH in the starting IN phase the dotted line represents the pH dependence of the transport selectivity log S. 

Schow, A.J., Peterson, R.T. and Lamb, J.D. (1996) Polymer inclusion membranes containing macrocyclic carriers for use in cation separations. Journal of Membrane Science, 111, 291. 

Almost no report is available on the membrane transport studies of trivalent actinide ions with macrocyclic carriers. However, there are some literatures available on trivalent lanthanide ions, which can be used as guidelines for any future studies. Brown et al. [143] and Zhu and Izatt [144] have used bis(l-hydroxyUieptyl)DC18C6 as the membrane carrier and... 

Wieczorek, P. (1997). Factors influencing the transport of tryptophan hydrochloride through supported liquid membranes containing macrocyclic carriers. J. Membr. Sci., 127, 87-92. 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

Christensen, J.J., Izatt, R.M., and Lamb, J.D. Selective transport of Metal Ions Through Liquid Membranes Containing Macrocyclic Carriers Papers presented at A.I.Ch.E. Mtg.. Chicago, IL., Nov. 1980. 

WaUcowiak, W. and Kozlowski, C.A. 2009. Macrocycle carriers for separation of metal ions in liquid membrane processes A review. Desalination 240 186-197. 

Christensen, J. J. Transport of metal ions by liquid membranes containing macrocyclic carriers. In Bioenergetics and thermodynamics Mcdel systems. Braibanti, A. (ed.), pp. Ill to 126. Dordrecht D. Reidel 1980... 

Wlakowiak and Kozlowski " recently reviewed the application of macrocycle carriers including crown ethers, calixarenes, calixcrowns, and CDs in liquid membrane processes. The role of macrocyclic ligands as ion carriers for cations such as alkali and alkaline metals, and heavy metals such as Zn ", Cd +, Hg + and Pb " " was summarized. Mutihac reviewed the calixarenes as membrane... 

Transport Selectivity. For separation purposes, the selectivity of transport by macrocyclic carriers through supported liquid membranes has been investigated extensively. The transport selectivity of a carrier towards two different cations M/ and is obtained by competition experiments in which both M", and were present in the receiving phase. 

Decreased film thiclmess is not the only way to increase diffusion. Structural features of macrocyclic carriers can themselves alter diffusion rates. Several models have been devised to predict the role of various membrane features on membrane flux (32-37). The simplest relationship is the Stokes-Einstein relationship (55) in which the diffusion coefficient, D, is given by ... 

Ionic Additives. Cotransport of anions is the most obvious way to maintain electroneutrality, but alternative means have been explored. In recent years, many studies have been conducted which examine the use of anionic membrane additives for maintenance of electroneutrality (35, 54-58), The anionic additives can be either stationary or mobile and are typically lipophilic carboxylic, phosphoric, or sulfonic acids. Neutral macrocyclic carriers coupled with anionic additives result in a synergistic transport of cations which exceeds that accomplished by each component individually. 

This system offers the advantage of requiring only small quantities of macrocycle and solvent It also is easy to model and yields small standard deviations. However, it possesses significant flaws (64), The macrocyclic carrier and solvent are easily lost to the aqueous phases. As a result, both carrier and solvent must be... 

PIMs have been modelled after the P-diketone-containing membranes used by Sugiura (67-69) and are formed by the polymerization of cellulose triacetate (CTA) to form a thin film. Polymerization takes place in the presence of a macrocyclic carrier, and as the thin sheet forms, carrier molecules are trapped within the CTA matrix. The resultant membrane is then placed between aqueous source and receiving solutions and selectively mediates transport of a desired species from one phase to the other. While PIMs can effectively separate two aqueous phases, they are not dependent upon organic solvents to maintain phase separation and allow transport. Thus they are simpler to use than SLMs and do not suffer from loss of organic solvent nor as much leaching of carrier into the aqueous phases. 

Complex stability for macrocyclic carriers is determined by a number of factors including donor atom type, macrocyclic ring size, chelate ring size, pendant group characteristics, steric considerations, and degree of preorganization. Because complex stability is so crucial to carrier effectiveness, we examine a few of these factors below. 

According Pearson s Hard and Soft Acid and Base Theory, oxygen is a hard donor atom, sulfur is a soft donor atom and nitrogen is intermediate (14,15), To probe the influence of replacing one or two oxygen atoms in lipophilic crown ethers 5, 6 and 8 upon metal perchlorate transport, the lipophilic macrocyclic carriers 9-16 (Figure 3) were investigated. Macrocycles 9 and 10 are lipophilic diaza-15-crown-5 compounds and 11,14 and 15 are diaza-18-crown-6 derivatives. Macrocyle 16 is a lipophilic monothia-15-crown-5 compound and macrocycles 12 and 13 are dithia-15-crown-5 and -18-crown-6 compounds, respectively. 

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