J. D. Lamb J. J. Christensen and R. M. lzatt Departments of Chemistry and Chemical Engineering' Brigham Young University Provo, UT 84602
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Experimenting with Liquid Membranes
Liquid membranes have been studied a s models of hiological membranes a n d as a means of performing chemical separations (1-4). A liquid m e m h r a n e is a liquid or quasi-liquid ohase which seoarates t w o o t h e r liauid uhases i n which t h e memhrane is immiscible. A typical m e m b r a n e consists of a hvdronhohic liouid ohase such a s chloroform sevaratine t w o water phases or a water phase separating two hydrophobic liquid phases. Chemical species m a y pass from o n e phase through t h e memhrane to t h e o t h e r phase if t h e y have some solubility in t h e membrane. T h i s transfer m a y b e accomplished by simple diffusion o r by "carrier-facilitated" transp o r t wherein species a r e ushered across t h e memhrane by selective "carrier" molecules which reside in t h e memhrane. O n e hypothesis for movement of Na+ a n d Kf across nerve membrane involves such carriers althoueh " these have not been identified. C u r r e n t research involving liquid membranes provides a basis for novel experiments which c a n be used to illustrate carrier-facilitated transport a n d which lend t h e m selves t o use in almost any instructional chemistly laboratory. T h e objectives of t h i s paper are t o outline two simple experiments using liquid membranes a n d to describe some of t h e i n t e r e ~ t i n g & ~ & tof s liquid membrane research.
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SALT SOLUTiON
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CONTAINING CARRIER
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(
STIRRER BAR
)
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~
1 2 0 rnm
Figure 1. Bulk liquid membrane cell.
SOURCE --~
PHASE
SALT SOLUTION^
CHLOROFORM MEMBRANE PHASE
RECEIVING PHASE IWATERI
Cation Carriers in a Simple Liquid Membrane
A simple "bulk" liquid membrane used by us (51and others (6-10) to study carrier-facilitated transport of cations is illustrated in Figure 1. In this system, a layer of chloroform containing the carrier compound serves as the liquid memhrane separating an aqueous salt solution from distilled water. The aspect of this type of experiment which is of particular chemical interest is the carrier which resides in the membrane and which makes possible transfer ofcharged species through a hydrophobic layer. Without such a carrier, no transfer of inns across the chloroform membrane was found to occur with twenty-eight salts of potassium, sadium,lithium, and barium (51. Yet with the carrier dihenzu-18-cmwn-6 present in the chloroform memhrane, transfer rates as high as fi X i0W moleslhr are measured for KI (51. Structure (11 shows dibenzo-18-crown-6 bound to a Kt ion.
Cation carriers are ligands which serve to solubilize catinns in non-polar solvents, allowing diffusion of a catian-carrier complex acnrss the membrane. As illustrated in Figure 2.cations partition from the phase of greater concentration (herein called the source phase) into the memhrane where they exist in the form of carrier complexes. The movement of the cation-carrier comolex is due to the concen-
cation concentrations in the two phases. This transport process will continue until a condition of equilibrium is reached where the cation concentrations in the source and receiving phases are equal. Several characteristics are necessary for a ligand to qualify as a membrane cation carrier: (1) It must he soluble in themembranesolvent. (2) It must complex the cation strongly enough to overcome the cation's energy of hydration a t the memhrane-source phase interface and yet not so strongly that it will not release the cation into the receiving phase. (3) It must diffuse rapidly through the memhrane. Certain members of the class uf macrocyclic ligands including crown ethers, such as (I) have been used successfully as liquid membrane carriers (.5-101.
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Figure 2. Facilitated bansport of cation Mt mrough a liquid membrane: (1) Cation Mt forms complex with carrier at first interlace; (2) Complex plus coanion Adiffuseacross membrane: (3)M+ and A- are released into receiving phase; (4) carrier diffuses back across membrane to repeat cycle.
employ oxygen and sometimes nitrogen donor atoms demonstrate an unusually high affinity for alkali metal cations. The cation complexatim process results in the polar donor groups of the ligand being located in the interior of the complex where the cation is entrapped. The hydmphohic carbon exterior of the ligand is exposed to the solvent, making possihle the solubilization of cations in hydrophobic solvents. A striking feature of the chemistry of macrocyclic ligands is the fact that in a given sdvent they selectively hind certain cations more strongly than others ( I l l . For instance, the crown-€ family of macroeycles, of which (I) is a memher, is selective for putassium over other Gmup 1A cations. A selectivity curve (13) such as that found in Figure R illustrates the cation binding characteristics in water solvent of 18-crown-6toward several of the alkali and alkalineearth cations. The cation selectivity of macrocyclic ligands, together with their ability to x,luhilize cations in nonpolar solvents make them good candidates to serve as liquid memhrane carriers.
Transport of Metal Salts Across Liauid Membranes
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Contribution Nn. 151 from the Thermochemical Institute Volume 57. Number 3, March 1980 / 227
Rate of Transporl of Various Salts Through A Chloroform Membrane Containing 7 X 10 M Dibenzo-18-Crown-6 (5. 13)
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Concentration (moles/l)
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Trenspon Rate (moles/hr X lo7) 0.34 98 0.85 6.1 88 620 250 120 220
66 510 0.25 32 570
WATER SOL
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MEMBRANE
WATER SOL"
Na-
OH^
Ns*
Figure 3. Variation of complex stability constant in water for the reaction of 18-crown4 (shown) with alkali and alkaline earth cations (pioned according to ratio of cation size to crown cavily size).
7X M dibenzo-16crownd. Wespeak of transfer of salts because cations cannot be transferred across liauid membranes without conuf the cation or by a flux of cations in the opposite direction. I t has been shown (6,7,14) that in systems such as the membrane illustrated in Figure I , cations bound toneutral carrier molecules are accampanied across the membrane by their co-anions to maintain electrical neutrality. Thus, the eompleved cation is transported as an ion pair with its eo-anion. Measurement of cation flux is accomplished by periodic analysis of the receiving phase for either cation or anion concentration. carrier are some of the parameters whieh may be expected to influence cation transport rate in these liquid membrane systems. For example, we have found (5.13) that altering the anion has a very dramatic effect on transport rate of a given cation as the table illustrates. Many papers have been published detailing the effects of these various parameters (1,3,6.7). Diffusion of chemical species across liquid membranes need not be passive (i.e., down the concentration gradient of the transferring species). Indeed, cations may be made to move against their coneentration gradient fram a solution of low concentration to one of high concentration. This phenomenon has been accomplished (15) for Na+ by use of the naturally-derived ligand monensin, which is a long ligand molecule with a carboxylie aeid group s t one end and an alcohol group a t the other. When the aeid moiety deprotonates, the ligand is able to encapsulate a cation. If a mixture of HCI and NaCl are placed on one side and an NaOH solution on the other side of a membrane containine manensin. Na+ will move toward the acidic side. driven is sufficiently energetic tocauseacountereurrent movement of Nat against its own concentration gradient. T h e movement of a cation against its concentration gradient can be accomplished also by exploiting the principle that the anion accompanies the cation across the membrane. Specifically, KC1 has been caused to move through a diaphragm-type membrane (described below) against the Kt eoncentration gradient (14) by increasing the C1- concentration in the 2 2 8 1 Journal of Chemical Education
OH-
Figure 4. Transport of Nat against its concentration gradient by monensin ( 15): (1) Nat is encapsulated by deprotonated monensin at first interface: (2) Neutral complex diffuses across membrane: (3) Nai is released into receiving phase as monensin is pratonated; (4) Free monensin diffuses back across membrane to repeat cycle.
source phase with addition of LiCI. In this case, movement of the C1ion provides the energy to move Kt into a solution of higher Kt concentration, Experiments in cation transport across a liquid membrane against the cation concentration gradient have been performed by Cussler and co-workers (14J.5) using a supported diaphragm membrane. Such a membrane is constructed hy saakinga porous material suchas filter paper with membrane material (octanol or chloroform containing the cation carrier). The impregnated filter is clamped between two O-ring joints and thus separates two liquid chambers, one containing the
essary t o ~ o b t a i ndiaphragm membranes which give reproducible transfer and which are stable aver more than a few hours (6).
A Simple "Bulk" Liquid Membrane Experiment The "bulk" membrane shown in Figure 1 provides a simple experiment illustrating carrier-facilitated transport for demonstration or student laboratury applications. We recommend use of potassium picrate as the salt to be transported because of its rapid transfer rate and because pierate concentrations can be monitored visually or calorimetrically using a spectrophotometer at wavelength 400 nm. Potassium permanganate may also be used effectively for demonstration purposes, although formation of Mn02 can be troublesome. As carrier compound, we suggest dibenzo-18-crown-6, a relatively inexpensive and effective carrier which along with many other macrueyclic ligands may be obtained in good purity fram chemical companies which specialize in organic chemicals (Parish Chemical Company, Aldrich Chemical Company). Using the salt and carrier concentrations outlined below, the yellow color of picrate will become visible in the receiving water phase after only a few hours. Quantitative monitoring of the transfer uio colorimetry will produce a linear plot of moles transferred versus time, the slope of which indicates transport rate. The experimental procedure is as follows. Place a 350-ml sample
Water
Water
I water
la1
Figure
5.
Ibl
-Oil
(cl
Figure 6. Formation of simple emulsion membrane droplets. (a) Oil layer on water, (b) NaOHilndicator solution added dropwise. (c) Final membrane configuration.
Emulsion liquid membranes.
of the liquid membrane, chloroform containing 1W3 M dibenzo18-crown-fi,in the bottom of a 120-mm evaporating dish containing a 40-mm magnetic stirring bar. Insert a fi8-mm glass cylinder open a t both ends part way into the chloroform layer and clamp it into position as illustrated in Figure 1. Carefully add 40 ml of 2.0 X lo-" M water solution of potassium picrate to the inner cylinder atop the chloroform layer. In like manner add 170 ml distilled water to the outer ring. Cover and stir a t approximately 100 rpm, avoiding creation of a deep vortex which throws drops of salt solution into the distilled water phase. Withdraw samples from the receiving pbase a t l-hr intervals and record the absorbance or percent transmittance a t 400 nm. The same experiment performed without carrier in the membrane should show virtually no movement of salt across the membrane.
The drops of NaOH solution became coated with oil as they drop through the nil phase, from which they emerge as colored aqueous drops surrounded by a thin oil layer (Fig. 6b). The oil film or membrane prevents mixing of the two aqueous phases as indicated by the observation that no dye color is ohserved in the continuous outer aqwous phase. The drops form a foamy layer of oil and water in the bottom of the beaker with the oil separating the NaOH phase from the aqueous phase above it (Fig. 64. The pH of the aqueous phase can be monitored and will be found not tu change during the experiment. Conclusion The similarity of liquid membranes to biological membranes makes them attractive simple models for studying and describing biological transport mechanisms. The efficiency of biological membranes in effecting both separations and controlled movement of specific chemical species indicates that chemical systems which mimic these processes could be very beneficial to man. Study of liquid membranes may conceivably lead to the successful exploitation of nature's processes for these purposes.
Emulsion Type Liquid Membranes Experiments such as the one described in this paper lay the groundwork for understanding mare exotic liquid membrane systems such as those developed by Li and co-workers (16). Such membranes consist of tiny droplets of solvent surrounded by a thin layer of another "membrane" solvent floating in a body of the first (or conceivably a third) solvent. Waterloillwater membrane systems of this type (Fig. 5) are made by blending a small amount of water with oil and then loosely mixing this emulsion with more water. As described nreviouslv. the membrane which senarates the two ohases mav con, n cnrrwr wmpcuncl whwh allow* P 3 i W d F u i i p r r l l w h ~ m . c ? I spcc~rcacrms t h r m r m l r r s n c . 'l'11c.c c m u l * i m t \ , p c mrmlmn
