Aqueous Polymeric Solutions as Environmentally ... - ACS Publications

Ind. Eng. Chem. Res. 1999, 38, 2523-2539

2523

APPLIED CHEMISTRY Aqueous Polymeric Solutions as Environmentally Benign Liquid/ Liquid Extraction Media Jonathan G. Huddleston, Heather D. Willauer, Scott T. Griffin, and Robin D. Rogers* Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487

Several liquid-phase extraction technologies employing environmentally benign phase- or micelleforming polymers in aqueous solution have the potential to replace volatile organic compounds in classical solvent extraction technologies. The examples reviewed here include aqueous biphasic systems, cloud-point extraction, micellar extraction, and thermoseparating polymer systems. The apparent similarities of these systems, their phase-separating properties, and their ability to solubilize a wide variety of solutes ranging from metal ions, organic compounds, and biologicals are discussed. Some comparative data from the literature whereby the solvating power of these systems may be compared to traditional solvents are presented along with new data on the polarity of the phases in typical aqueous biphasic systems. The need for additional comparative data in this area and the need to demonstrate the validity of the approach in operational processes are emphasized. A “toolbox” approach to implementing environmentally benign polymers in clean separations science and technology could lead to new and better solvent extraction systems. Introduction For the last several decades, separations scientists have been employing the solubilizing properties of solutions of polymer molecules in aqueous solution to effect the extraction and fractionation of a wide variety of target solutes. Of particular interest in the context of environmentally benign separations technologies are the systems variously referred to as cloud-point extraction (CPE),1 micellar extraction (ME),2 aqueous biphasic systems (ABS),3 and extractions using thermoseparating polymers (thermoseparating polymer systems, TPS).4 Additionally, there are chromatographic methods such as micellar liquid chromatography (MLC),5 micellar electrokinetic chromatography (MEKC),5 and admicellar chromatography6 to which we do not devote much attention here but which, in our view, have many similarities. There are still other extraction techniques such as microemulsions7 and reverse micelles8 which we hardly consider at all simply because of the involvement of volatile organic compounds (VOCs). While modern textbooks on solvent extraction may devote some small section to a consideration of the properties and applications of ABS because of their importance in the biotechnology industry,9 all of the aforementioned methods are recognizable variants of liquid/liquid extraction. Despite this, it is rare to find these techniques discussed simultaneously at all. There are over 70 volumes in the Surfactant Science Series,10 and yet only one covers the solubilizing properties of a wide range of these separations methods, and even that * To whom correspondence should be addressed. Phone: (205) 348-4323. Fax: (205) 348-9104. E-mail: RDRogers@ Bama.ua.edu.

does not feature ABS.11 Another is devoted to block copolymers of the oxyalkalenes12 which, along with similar random copolymers, represent the major thermoseparating polymers. Yet, these novel micellar systems13 could feature in a discussion of ME systems, CPE systems, or TPS, while at the same time being obviously related to ABS. It seems to us that all of these liquid/liquid extraction methods, each with its own by now quite vast and specialized literature, have enough in common to distinguish them and are quite worthy of common consideration, especially in relation to the design of extraction and separation operations for industrial and environmental application. As evidence of this, any list of common features would include the following: (a) All represent anisotropic or multiphase systems and thus lend themselves to development in solvent extraction technologies. (b) All may be applied to the solubilization of otherwise relatively insoluble hydrophobic species. (c) None require VOCs for use in liquid/liquid extraction. (d) All may be formed by addition of polymers to water. (e) All rely on the structuring properties of liquid water for the formation of heterogeneous multiphase systems and also for their solubilizing power. (f) All seem to depend on the remarkable properties of poly(oxyalkylenes) and similar polymers, such as poly(N-vinylpyrrolidones), to be effective. The first two properties allow these systems to be considered suitable for solvent extraction. The remaining properties listed allow them to be considered as environmentally benign methods by the elimination of the requirement to employ VOCs. Indeed in the case of

10.1021/ie980505m CCC: $18.00 © 1999 American Chemical Society Published on Web 04/22/1999

2524 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 1. Examples of Polymers and Salts Used in Aqueous Biphasic Systems a. Salts Forming Biphasic Systems with PEG59 univalent anions NaOH KOH RbOH CsOH NaF Na(formate)

divalent anions Na2CO3 K2CO3 (NH4)2CO3 Rb2CO3 Cs2CO3 Li2SO4 Na2SO4 (NH4)2SO4 Rb2SO4 Cs2SO4 MgSO4 Al2(SO4)3

FeSO4 CuSO4 ZnSO4 alum Na2SeO4 Na2CrO4 Na2MoO4 Na2WO4 K2HPO4 Na2SO3 Na2S Na2(succinate) Na2(tartrate)

trivalent anions

tetravalent anions

Na3PO4 Na4SiO4 K3PO4 Na4(HEDPA) Na3VO4 Na3(citrate) (NH4)3(citrate)

b. Examples of ABS-Forming Polymers19 polymer 1 poly(ethylene glycol)

Figure 1. Envisioned Solvent replacement “toolbox”, from which the environmentally benign polymeric system best suited to a particular separations need could be quickly isolated.

CPE and ME, application of these techniques is widely reported in the context of environmental remediation. The final point is related to the unusual solubility range of PEO in a variety of solvents and its ability to hydrogen bond to water molecules. It can thus serve as the hydrophilic moiety in a wide range of block and random copolymers as well as nonionic surfactants.14 To attempt to redress this imbalance in the discussion of these systems, to emphasize some of their common features, and hopefully to stimulate new thinking and avenues for research, we discuss the properties and features of some of these emerging liquid/liquid extraction technologies here. In the context of pollution prevention or environmental remediation applications, it may prove useful to be able to consider the similarities, differences, advantages, and limitations of what may be considered a “toolbox” (Figure 1) of environmentally benign polymer separations techniques in order to take advantage of the wide range of opportunities in this area. This paper may also act as an introduction to the more specific papers covering these topics in greater depth in the current volume. We cannot, of course, hope to be complete or exhaustive in this discussion, and there is undoubtedly much of importance which we will have missed and which will provide for fertile future discussions. ABS ABS are by now well-known for their utility in the extraction and fractionation of polymeric macromolecules of biological origin. These applications are the subject of numerous books and review papers.3,15-21 ABS are formed by the addition of two (or more) watersoluble polymers, or a polymer and salt, to aqueous solution above critical concentrations. In this way two (or more) wholly aqueous phases are formed without the involvement of VOCs, which have been important in their development as solvent separation systems in biotechnology because biomolecular tertiary and qua-

poly(propylene glycol)

poly(vinyl alcohol)

poly(vinylpyrrolidone) dextran

polymer 2 poly(vinyl alcohol) poly(vinylpyrrolidone) dextran ficoll (hydroxypropyl)starch methoxy poly(ethylene glycol) poly(ethylene glycol) poly(vinyl alcohol) (hydroxypropyl)dextran dextran methyl cellulose (hydroxypropyl)dextran dextran acrylic/methacrylic acid copolymers methyl cellulose dextran ethyl(hydroxyethyl) cellulose (hydroxypropyl)dextran ficoll

ternary structures are only maintained in aqueous solution. Table 1 shows typical polymers and salts which have been used to form ABS. The basis of phase formation in polymer/polymer ABS is incompletely understood at present but has been described thermodynamically on the basis of unfavorable segment interactions of polymers overcoming the entropy increase involved in phase separation.22 It has also been suggested that the basis lies in the separation of phases having differently ordered water structure brought about by the hydration of the polymers.17,23 It may be observed that PEG/polymer phase separation is enhanced by a decrease in temperature and may thus be related to the upper critical solution temperature (UCST) of the PEG.24 Polymer/salt systems, on the other hand, seem to phase separate on the basis of the competition for hydration between the polymer and the salt, resulting in increasing dehydration of the polymer (PEG) chain25 and phase separation. Phase separation is enhanced by an increase in temperature,26 and the phenomenon is thus related to the lower critical solution temperature (LCST) of the polymer.24 The concentration of polymer and salt required to bring about phase separation is dependent on the molecular weight of the polymer (and type of polymer if polymers other than PEG are used) and the salting-out strength of the salt, or in other words its lyotropic number, or position in the Hofmeister series.27 For the polymers shown in Table 1b, biphasic systems may be achieved by an increase in the salt concentration of cosmotropic salts, by an increase in temperature, or by a combination of both.

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2525

Figure 2. Phase diagrams of several PEG/salt aqueous biphasic systems. Binodal curves are represented for ABS composed of PEG-2000 in the presence of three different salts: K3PO4, K2CO3, and (NH4)2SO4. The binodal in each case separates regions below the curve which form homogeneous monophasic systems from regions above the curve in which mixture compositions of PEG2000 and salt separate into two phases of differing composition. One tie line is represented for the (NH4)2SO4 system where the system composition is given at point B, the light, upper PEG-rich phase composition is represented by point A, and the heavy, lower salt-rich phase composition is represented by point C.

Thus, as will become much clearer later, PEG/salt phase separation phenomena may be seen to involve phenomena identical with those of cloud-point separations and the phase separation of thermoseparating polymers. PEG/salt ABS may thus be characterized as CPE systems or TPS which have been adjusted to show separation at room temperature by the addition of salt. On the other hand, ABS differ from CPE and TPS in that they are not believed to be micellar below the critical point. The phase separation of these polymers is most strongly promoted by anions having large negative Gibbs free energies of hydration (i.e., by anions of high charge and small size). An almost identical series would be observed for the cloud points of other polymers (e.g., PVA),24,27 the phase inversion temperature of micellar systems,29 and the precipitation of proteins.30 The effect of the cation on phase separation is, in general, much less except in particular instances.28 Figure 2 shows typical polymer salt binodal curves exemplified by those of PEG-2000/K3PO4, K2CO3, and (NH4)2SO4. The figure shows the binodal curve which delineates the region of phase separation. Above the mixture compositions represented by the binodal curve, biphasic systems are formed, whereas mixture compositions below the binodal are monophasic. CPE The basis of the CPE technique is the separation of an aqueous solution containing, in the majority of cases, a nonionic surfactant in a micellar form into a concentrated surfactant phase and a dilute aqueous phase upon alteration of conditions such as temperature or salt concentration.1 When the temperature of such a solution is raised above its cloud point, the solution exhibits a lower consolute point, becomes turbid, and may be separated (by centrifugation) into two clear phases: one reported to consist of a micelle-free solution of the surfactant close to its cmc and the other a concentrated

Figure 3. Schematic phase diagrams of two typical cloud-point extraction systems. The binodal curves show the relationship between temperature and concentration in the formation of biphasic systems. (Actual temperatures and concentrations of phase separation will depend on the exact nature of the surfactants employed.) Mixture compositions above the curve (in the region labeled II) separate into a surfactant-rich and a surfactant-poor phase. Regions I below the binodal curve represent homogeneous mixtures which are micellar in nature. (Schematic adaptation from data in ref 1 relating to octyl- (A) and nonylphenyl (B) ethoxylates.)

solution of the surfactant. The solution is micellar close to the critical temperature, but at temperatures greater than this, the depleted phase becomes more dilute and the surfactant in the surfactant-rich phase may no longer be in micellar form.1 The temperature of phase separation is concentrationdependent, and a consolute curve may be determined in terms of concentration and temperature, above which the solution consists of two phases in equilibrium and below which a single homogeneous phase exists (Figure 3).1 Cloud points vary widely with temperature from one surfactant to another. Such curves exhibit a minimum (the critical point) in terms of temperature (the critical temperature) and in terms of concentration (the critical concentration). Table 2 shows a number of surfactants which may be used for cloud-point extraction.1 Clouding has been said to be due to an increase in micellar size and intermicellar attraction and the dehydration of the hydrated outer micellar layers with an increase in temperature. The fact that the dielectric constant of water decreases rapidly with temperature, and that it becomes a poorer solvent for the hydrophilic part of the molecule, has been used as an explanation of the observed behavior.31 Small amounts of hydrophobic substances lower the cloud point by penetrating and swelling the micelle. Salts may promote or inhibit dehydration, thus either decreasing or increasing the cloud point. The cloud point depends on the molecular structure of the nonionic surfactant. For homologous series of nonionic surfactants (having a hydrophobic headgroup and a hydrophilic tail), the cloud point increases with decreasing length of the hydrocarbon chain and increasing length of the oxyethylene chain.1 This must depend on the structure, because the cloud point of pure PEO is dependent on increasing chain length.24 An empirical relationship has been proposed for the prediction of the cloud point of pure nonionic surfactants of the alkyl ethoxylate class.32 For linear alkyl, branched alkyl, cyclic alkyl, and alkylphenyl ethoxylates, the cloud point can be estimated within 6.3 °C using the

2526 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 2. Polymers Commonly Used in Cloud-Point Extraction1 polymeric surfactant

chemical formula

trade name

poly(oxyethylene glycol) monoethers

CH3(CH2)xO(CH2CH2O)yH

Brij Emulgen

poly(oxyethylene) methyl-n-alkyl ethers

CH3(OCH2CH2)xO(CH2)yH

(tert-octylphenoxy)poly(oxyethylene) ethers

C8H17C6H4(OCH2CH2)xOH

poly(oxyethylene) nonylphenyl ethers

C9H19C6H4O(CH2CH2O)xH

poly(oxyethylene) sorbitan fatty acid esters poly(oxyethylene)-(polyoxypropylene)copolymers

log of the number of ethylene oxide residues and three topological descriptors of the variation in the hydrophobic domain (eq 1).32 A and B are empirical constants,

CP ) A log(EO#) - 5.5C# - B

(1)

and EO# and C# are the number of ethoxide units and the number of carbon atoms in the alkyl chain, respectively. This relationship extended the applicability of an earlier empirical relationship for the cloud point of linear alkyl ethoxylates.33 As is the case with ABS, the cloud point is sensitive to the addition of other solutes and in particular to the presence of electrolytes. Again, this is dependent on the lyotropic number of the salt with cosmotropes and chaotropes, causing a decrease and an increase in the temperature, respectively, for phase separation in proportion to their concentration. It has recently been shown that the cloud-point temperature and the cmc are intimately related through the lowering or raising of these temperatures by the addition of different salts (eq 2),34 where ∆CMT and ∆CP are the changes in

∆CMT ) ∆CP ) CMT(no salt) - CMT(salt) ) CP(no salt) - CP(salt) (2) critical micellization temperature and cloud point, respectively, and CMT and CP are the micellization and cloud-point temperatures measured in the absence and presence of salt. The effect of organic solutes on clouding is dependent largely on the nature of the solubilization effected. Nonpolar hydrocarbons solubilized in the interior of the micelle generally raise the cloud point, whereas more polar species, which are more likely to be solubilized away from the micellar core, generally depress the cloud point.32 A useful practical aspect of the mixing of nonionic surfactants is that this produces an intermediate cloud point.1 It seems that CPE systems could be viewed as ABS, whose cloud points lie conveniently close to room temperature, without the addition of high concentrations of salt. Whether this difference is important from a practical point of view, in terms of solubilizing ability or solute capacity, is not clear at the present time, although some studies may be under way.35 TPS Particularly in the field of biomolecule separation, thermoseparating polymers have been investigated as an alternative to the traditional polymer/polymer and polymer/salt ABS36-38 which could provide a means to reduce the amount of polymer (particularly the rela-

Triton Igepal Triton Igepal Tergitol Tween UCON Pluronic

Table 3. Typical Thermoseparating Polymers13 name

description

Pluronic UCON Poloxamer Sunbright Synperonic PEG EHEC PVA poly(N-vinylcaprolactam)

PEO-PPO block copolymers PEO-PPO random copolymers PEO-PPO block copolymers PEO-PPO random copolymers PEO-PPO block copolymers poly(ethylene glycol) ethyl(hydroxyethyl) cellulose poly(vinyl alcohol)

tively costly dextran) required to bring about phase separation. However, similar thermoseparating polymers have also been investigated for the solubilization of small organic molecules in the context of pharmaceutical and environmental applications.13,39,40 The behavior of these polymers is, in many essentials, identical with those used for CPE in that the polymers exhibit reduced solubility with increased temperature and thus display a lower critical solution temperature or cloud point.36-38 The cloud-point temperature again depends on the polymer structure, its molecular weight and concentration, and the presence of other solutes and salts.36-38 Above the cloud point, the solution consists of two phases, of which the less dense upper phase normally contains most of the solvent and the lower phase most of the polymer. A number of polymers are listed in Table 3 which are often encountered in the context of thermoseparating polymers for biomolecule processing36-38 or in the context of solubilization using amphiphilic copolymers.13,39,40 Some of these polymers are conventionally cited as ABS-forming polymers (e.g., PEG), but otherwise the largest group and the polymers most widely applied in this context are copolymers of PEO and PPG. These copolymers are of two types, either random copolymers, in which each monomeric unit may be either PEO or PPO, to the final overall proportions or block copolymers, in which the polymer is composed of polymerized segments of either PPO or PEO in general of two or three blocks (di- or triblock). The diblock and triblock copolymers have been shown to form micellar structures with an increase in temperature, because of the marked change in solubility of the poorly watersoluble PPO block which comes to dominate the core, while the highly soluble PEO blocks dominate the corona of the micellar structure.13,39-41 A correlation for the estimation of cmc concentrations and temperatures of PEO-PPO triblock copolymers has been published.42 In common with the surfactants employed in CPE and the major polymer, PEG, employed in ABS, PEO-PPO random and block copolymers have wide industrial application and are produced on a bulk scale, thus making them economically attractive for consideration

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2527

in a range of other applications. PEO-PPO block copolymers have major applications in detergency, dispersion stabilization, and foaming.39 The aggregation number of these micelles increases with temperature and the cmc, in common with other amphiphile systems, decreases with an increase in temperature, although this is complicated by the existence of polydispersity in these polymeric systems.40 Polyoxalkylene alkyl ethers exhibit similar behavior, but the variation of cmc and micellar size is not as pronounced with temperature.39 Block copolymers with a larger PPO domain form micelles at lower concentration (having lower cmc’s); Pluronics of higher molecular weight form micelles more readily.39 In some PEO-PPO block copolymer solutions of high concentration, thermoreversible gelation may occur. In particular, Polaxamer 407 shows a reversible gelation with temperature (around 25 °C) to form stiff clear gels, and thus its use as a tool for controlled release of pharmaceuticals has been investigated, as has its use in surgical procedures.43 It may be observed from the foregoing that there are numerous similarities between these different separatory systems. ABS are not believed to be micellar but form biphasic systems above critical temperatures, as do CPE systems and thermoseparating polymer systems. Triblock and diblock copolymers of PEO-PPO form micellar systems, but it seems unclear whether similar random copolymers do so; however, both may be used to form biphasic systems on an increase in temperature or salt concentration. Micellar-Enhanced Ultrafiltration (MEUF) In view of the micellar nature of some of the abovementioned systems (TPS, CPE), it has been suggested that a filtration approach to processing could be adopted, as was previously proposed for conventional micellar systems in the form of MEUF. Ordinarily, low molecular weight organic species (500 mM CPC concentration, which is above the concentration at which rejection begins to deteriorate. Data for the rejection of several organic pollutants using CPC during MEUF seem to be approximately related to their log P 1-octanol/water partition coefficients.2 Ultrafiltration and microfiltration of surfactant dispersions have been reviewed.108 MEUF has also been examined in the context of nitrate removal from groundwater using cellulose membranes and the cationic surfactant cetyltrimethylammonium bromide (CTAB).106 It has been suggested that MEUF could be applied to the collection of radioactive uranium and plutonium present in acid wastes or during nuclear plant decommissioning.109 Studies were conducted using a number of acyclic, chiral ligands containing phenylalanine as the complexing agent and trioctylphosphine oxide as an auxiliary ligand in a micellar system of CTAB.109 Enantiomeric separations have also been suggested for application of MEUF processes.110 MEUF is frequently proposed for environmental clean-up operations and the removal of DNAPLs (dense nonaqueous phase liquids) using micellar extraction.2 It has been proposed that micellar extraction could be combined with pervaporation for the removal of organic species and with ultrafiltration for the recycling of surfactants in these situations.111 However, the same properties that give surfactants good solubilizing power act against the efficiency of pervaporation through reduction in the vapor pressure of the solutes.111 An alternative approach to avoid the leakage of micelleforming polymers has been suggested in the form of the incorporation of block copolymers within the interstices of polymeric hydrogel beads allowing the solubilization of organic pollutants within the immobilized micelles.112

One of the most useful properties of micelles, and apparently also of high molecular weight hydrophilic polymers, in free solution or bound to solid-phase supports is their ability to enhance the solubility or capture of hydrophobic substances. Solutes are incorporated into the hydrophobic environment of the micellar core41 or become intercalated in the polymer surface. PEO-PPO and PVP-PS block copolymer solubilization have been compared,13,45 and high solubilization capacities were found for aromatic species over aliphatics compared to conventional low molecular weight micellar systems. A theoretical comparison based on mean-field theory suggests that triblock copolymer behavior should be practically the same as diblock copolymer behavior having the same EO/PO composition but half its molecular weight.41 In the case of CPE, as has been mentioned earlier, because the phases are not completely immiscible, the distribution values are much lower than those measured in traditional organic/water solvent extraction systems.49 For the UCON-TPS systems, the partitioning of amino acids in these systems was shown to be essentially in accord with similar partitioning in aqueous PEG/MgSO4 ABS and to ethanol or dioxane/water partitioning and thus, by extension, to 1-octanol/water partitioning; however, the amino acids in all cases showed a preference for the aqueous phase.4 In a recent study of MEKC, enthalpic and entropic contributions to the observed water micelle partition coefficients of a wide range of organic species were evaluated and contrasted with distributions in 1-octanol/water systems.68 It is obviously of some importance to be able to compare the observed solubilization and distribution effects between different micellar and biphasic systems and indeed between these systems and the bulk solvent phases of traditional solvent extraction systems. Such comparison aids understanding of the processes involved in these systems, both trivially and also by illuminating the molecular forces involved. It also, of course, aids comparison of different systems in the design of specific applications where, for instance, it may be desirable to examine the replacement of a solvent-based process with one based on solubilization using aqueous polymeric solutions. One way (among many113-117) in which such a comparison may be made is by making use of a solvatochromic comparison method.113 For example, the Py scale of solvent polarities has been used to examine the solventlike environment of Triton X-114 micelles.31 The Py scale represents a simple empirical means of ranking solvents in terms of their polarities114 based on the ratio of the first and third vibronic bands of the emission spectrum of a fluorescent probe, pyrene. Using this scale, the Triton X-114 environment (of the probe) seemed to be equivalent to the bulk phase of the solvent 1,2 dichloroethane (1.46) at 5 °C. An increase in temperature to 25 °C close to the critical point led to an apparent decrease in polarity equivalent to that of 1,5-pentanediol (1.36).31 We have examined the polarity of the coexisting phases of a polymer/salt ABS using Reichardt’s carboxylated pyridinium N-phenoxybetaine.118 Reichardt’s ET(30) scale of solvent polarity119 is based on the measured molar transition energy of the intramolecular

2532 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 4. ET(30), ET(carboxy betaine), and Py Values of Solvent Polarity for the Bulk Phases of Selected Solvents solvent water phenol formamide 1,2-dichloroethane 1,5-pentanediol methanol ethanol 1-propanol 1-octanol acetone MIBK chloroform toluene

ET(30)a (kcal/mol)

ETb (kcal/mol)

Pyc

63.1 61.4 56.6 41.3

61.2

1.87

56.1

55.4 51.9 50.7 48.3 42.2 39.4 39.1 33.9

55.1 52.0 50.6

1.57 1.46 1.36 1.35 1.18 1.09 0.92

42.6 1.25 1.04

a E (30) values from ref 113. b E values from ref 118. c Py T T values from ref 114.

charge-transfer band of the betaine dye. The values obtained using the carboxylated betaine dye differ slightly from those obtained using the noncarboxylated dye; however, only the carboxylated form is soluble in aqueous solutions containing salts. Values of ET(30) for both forms of the dye along with Py values of the bulk phases of selected solvents are shown in Table 4. Discrepancies between these two scales may largely be accounted for by the difference in response of the betaine dye in protic and aprotic solvents when hydrogenbonding interactions may be imposed on polarity and polarizability effects.113,114 The values obtained using these scales may be compared to ET(30) values obtained for various micellar systems,120 for some PEO-PPO copolymer solutions,40 and for values obtained in our laboratories for the phases of a polymer/salt ABS (PEG2000/K3PO4) and for Triton X-100 and Pluronic F127 aqueous solutions (Table 5).121 It is clear from comparison of Tables 4 and 5 that the solvent polarities of the presently used aqueous polymeric media do represent solvents of a generally higher polarity than most of the solvents in common use for the development of solvent extraction strategies. Few, if any, seem to represent an environment as weakly polar as, for example, that represented by 1-octanol. Considering the Pluronics, the polarity of the environment about which the probe reports seems to be to some

extent reflected in the relative proportions of the EO and PO groups present. This may reflect solubilization in the proximity of the PO groups in the micellar core. This environment appears to be less polar than the environment of typical cloud-point extractants such as the octyl or nonylphenyl ethers or the poly(oxyalkylene) monoethers, and it also appears to be very much less polar than the environment of the PEG-rich phase of a PEG-2000/K3PO4 ABS. Such interpretations of these data need to be advanced cautiously, however. One reason is that such probes of solvent polarity do not fully describe the range of solubilizing features of particular solvents.113,117 Another is that the betaine probe seems unable to distinguish the polarity of the equilibrium phase employed in actual extraction experiments to the same degree that it does for the extracting phase. In, for example, a 1-octanol/water partitioning experiment, the polarity of the phase in equilibrium with the 1-octanol extractant would likely not depart significantly from that of pure water. However, in practice, the polarity of the equilibrium phase of a PEG/salt ABS departs significantly from that of pure water, being composed of relatively high concentrations of inorganic salts (see the phase diagram in Figure 2). Table 6 shows the free energy of transfer of a methylene group for various biphasic systems including a number of ABS. While the free energy of transfer may be very small, particularly for polymer/polymer ABS (in other words, such systems may be exquisitely sensitive to minor differences in surface properties of partitioned solutes), it is also possible to tune ABS, by proper choice of polymers, salts, and operating conditions, to have a selectivity which approaches that of a system like benzene/water. Also implied is that the observed partition is very much dependent on the solubility in the aqueous phase, as has been noted by others.13 In attempting to characterize these phase systems by a solvatochromic comparison method using the Reichardt dye, we also noticed that, in the presence of salt solution, the dye appears to respond to the environment of the polymer in solution even at very low polymer concentrations. The betaine dye is largely insensitive to the presence of relatively high concentrations of salt in aqueous solution and thus shows essentially no

Table 5. ET(carboxy betaine) Values for Aqueous Micellar and Polymeric Systems polymeric or micellar system

ET(30) (kcal/mol)

MWa

Triton X-100 Pluronic L64c Pluronic F127d Pluronic L64e Pluronic F68e Pluronic P103e Pluronic P105e Pluronic F108e Pluronic F127e PEG-2000/K3PO4 PEG-rich phasef Triton X-100 C12EO8g Brij35h SDSi CTABj DTABk

53.5 52.0 55.3 52.4 55.8 51.1 51.6 52.2 52.4 55.7 53.0 52.8 52.8 57.5 53.4 53.7

646 2900 12000 2900 8350 4950 6500 14000 12000 2000 646 540 1200 314 364 308

NPOa 31 64 31 30 62 58 50 64

NEOa

% EO

ref

10 2 × 13 2 × 95 2 × 13 2 × 76 2 × 17 2 × 37 2 × 127 2 × 95 45 10 8 23

70 40 70 40 80 30 50 80 70 100

121b 121 121 40 40 40 40 40 40 121 120 120 120 120 120 120

a Data for Pluronics-MW, NPO (number of propylene oxide units), and NEO (number of ethylene oxide units), taken from ref 40. Determined using the carboxylated betaine dye; all other data refer to the original uncarboxylated form of Reichardt’s dye. c 38% w/w solution. d 10% w/w solution. e Apparently at the cmc; ET(30) calculated from data in ref 40. f ABS composition: 17.3% w/w PEG-2000, 10.0% w/w K3PO4. TLL: 37% w/w. g Poly(oxyethylene)[8] lauryl ether. h (Polyoxyethylene)[23] lauryl ether. i Sodium lauryl sulfate. j Cetyltrimethylammonium bromide. k Dodecyltrimethylammonium bromide. b

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2533 Table 6. Free Energy of Transfer of a Methylene Group from Polar to Apolar Phases of Various Aqueous/Organic and ABS Systems -∆CH2 (kcal/mol)

ref

ABSa

-∆CH2 (kcal/mol)

ref

hexane

1.01

122

0.35

121

chloroform

0.85

122

0.82

121

benzene

0.84

122

0.46

121

octanol

0.73

122

0.85

121

diethyl ether

0.73

122

0.19

121

MIBK

0.72

122

0.35

121

xylene

0.64

122

0.14

121

octane

0.77

122

0.40

121

n-butanol

0.54

122

0.22

123

MEK

0.43

122

PEG-2000 40% w/w 1.0 M K3PO4 PEG-2000 40% w/w 3.5 M K3PO4 PEG-2000 40% w/w 1.5 M K2CO3 PEG-2000 40% w/w 4.5 M K2CO3 PEG-2000 40% w/w 1.7 M (NH4)2SO4 PEG-2000 40% w/w 3.7 M (NH4)2SO4 PEG-2000 40% w/w 4 M NaOH PEG-2000 40% w/w 6 M NaOH 19.4% PEG-1500 16.1% MgSO4‚7H2O 7.81% PEG-8000 14.1% K2HPO4‚3H2O

0.16

123

organic/water

a ABS from the authors’ laboratories were prepared by mixing equal volumes of PEG and salt stock solutions at the concentrations quoted. ABS in ref 123 refer to the final total system composition.

Figure 5. Molar transition energy change of Reichardt’s betaine dye in response to an increase in the PEG concentration at two different concentrations of K3PO4. Plot A (b) shows the change in the molar transition energy of the dye in response to increasing concentrations of PEG-2000 in 14% w/w K3PO4. The final mixture compositions are close to the binodal curve at this high concentration of phosphate. Plot B (O) shows the response of the solvatochromic dye to an increase in the PEG-2000 concentration in 0.5% w/w K3PO4 when much more PEG may be added before the binodal curve (see Figure 2) begins to be approached.

change in the molar transition energy up to concentrations as high as 16% w/w K3PO4. Above this concentration, more profound changes occur because of selfassociation of dye molecules and precipitation.121 Fortunately, those changes due to precipitation do not occur within the region of the phase diagram which would allow them to interfere with determinations on the separated phases of the PEG-2000/K3PO4 ABS. Figure 5 shows the change in the molar transition energy of the betaine dye in two PEG-2000/K3PO4 mixtures which are both confined to the monophasic region of the phase diagram (see Figure 2). In 0.5% w/w phosphate, increasing additions of PEG-2000 cause a bathochromic shift in the wavelength of maximum absorption compared to that of the betaine dye in pure water. At the highest concentrations of added PEG, the composition of this solution approaches that of typical top phases of the PEG-2000/K3PO4 ABS.

Figure 5 also shows the response of the betaine dye in solutions of 14% w/w phosphate containing increasing (but now relatively much smaller) additions of PEG2000. It may be observed that the bathochromic shift now occurs much more rapidly with an increase in the PEG concentration, eventually approaching values similar to those obtained with high concentrations of PEG and the much lower concentrations (0.5%) of phosphate discussed earlier. Because the absorption spectrum of the betaine dye is unchanged by these concentrations of phosphate alone and the spectrum is also constant in aqueous solution, these results imply solvation of the dye in the vicinity of the PEG chains or in the PEGstructured solvent around those chains.124 Under these circumstances, therefore, the dye appears to have “partitioned” to a discreet and observable region of the solution and, thus, the solution is apparently not homogeneous with respect to the solutes that it contains. There is thus apparently no requirement for phase separation of the PEG and salt in these solutions in order to demonstrate a partitioning or unequal distribution of the solute between the PEG environment and the bulk phase. This phenomenon of partitioning to the PEG, or to the environment of the PEG-ordered solvent, below the critical temperature and concentration for phase separation (i.e., below the binodal curve and outside the region in which separation into two different coexisting phases takes place) may also apparently be observed with ABEC adsorbents where the PEG chains are covalently linked to a solid-phase support. Figure 6 shows the binding of a water-soluble dye, quinoline yellow, to ABEC resin in batch conditions in which the equilibrium concentration of (NH4)2SO4 was varied.121 Significant binding of the dye to the PEG-derivatized solid phase takes place well below the concentration of salt required to bring about phase separation of the PEG polymer in free solution (see Figure 2). This shows that phase-separting and micelle-forming polymers immobilized to a solid phase may show significant solubilizing properties normally associated with their phase-separating and micelle-forming behavior in free solution. It is also interesting to note the apparent change in slope of the relationship between the solvatochromic

2534 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

Figure 6. Binding of quinoline yellow to ABEC-5000 [PEGderivatized poly(styrene-divinylbenzene) beads] with increasing concentrations of (NH4)2SO4. The figure shows the equilibrium concentration bound to different amounts of adsorbent from solutions containing 5 mg/mL of quinoline yellow, a sulfonated azo dye whose structure is also indicated in the figure. Significant absorption takes place at concentrations of salt which are below those required to bring about biphase formation in the presence of PEG with this salt.

shift of the dye with an increase in PEG concentrations. It is tempting to suggest that this may be ascribed to a change in the molecular conformation of polyether moieties such as has been observed when PEG-terminated molecules are assembled as surface monolayers.125 In a study on the different packing densities of adsorbed oligoethylene glycol-terminated self-assembled monolayers on gold and silver surfaces, differences in protein adsorption were found. These were ascribed to differences in conformation, where densely packed ethylene glycol monolayers had substantial populations in the trans form around the C-C bond, whereas more sparse monolayers could adopt the stable helical, more highly solvated form which was resistant to protein binding. It was suggested that this was comparable to the dehydration of oligoethylene glycols at higher temperatures.125 If so, this would mark a very definite physical resemblance between cloud-point separating systems and ABS formed in high concentrations of certain salts. Figure 7 shows the way in which the measured polarity of the coexisting phases of this PEG-2000/ phosphate aqueous biphasic system diverges as the phases become increasingly dominated by one of the components (PEG or salt). Figure 7a shows the change in the molar transition energy of the betaine dye in relation to the tie-line length of the ABS. Here the tieline length is a measure of the difference in the phase compositions of the coexisting phases away from the critical point (where the phase compositions are theoretically identical) and is defined as in eq 4, where

TLL ) x∆PEG2 + ∆SALT2

(4)

∆PEG and ∆SALT are the differences in the polymer and salt concentrations between the phases in weight/ weight percent and the tie-line length has units of % w/w. It is apparent that there is a much greater change in polarity (as reported by the dye) of the salt-rich phases

Figure 7. Solvatochromic studies in PEG/K3PO4 ABS. (a) Molar transition energy of Reichardt’s betaine dye in the separated top and bottom phases of PEG-2000/K3PO4 ABS as a function of the tie-line length. (b) Molar transition energy of Reichardt’s betaine dye in the separated top and bottom phases of PEG-2000/K3PO4 ABS as a function of the PEG concentration in the phase.

which approach that of pure water as the tie-line length increases than for the PEG-rich phases as measured by this method. Figure 7b shows the same data, but in this case the polarity is shown in relation to the PEG concentration of the phases. There is a rapid loss of PEG from the salt-rich phases which results in a rapid change in the molar transition energy of the dye away from the critical point. This is similar to the response of the dye to increase in PEG and salt concentration in the monophasic region of the phase diagram mentioned earlier. For the PEG-rich phases, there is only a small decrease in the measured polarity as the PEG concentration in the phase increases away from the critical point. It is apparent that the dye responds to the presence of the polymer in showing a bathochromic shift and that this is promoted by an increase in salt concentration, but, unfortunately, the dye does not respond to any change in the polarity of its environment because of the presence of high concentrations of salt alone (for example, by display of a hypsochromic shift relative to pure water). It may be observed in Figure 7 that the chromatic shift of this dye in the phases of an ABS could be used to determine the phase composition from the two unique values of the molar transition energy obtained. However, examination of the data in Tables 4 and 5 shows PEG/salt ABS to represent relatively polar systems, as mentioned previously, whereas Table 6 suggests a relatively lower polarity on the basis of the free energy of transfer. This may be ascribed to the effect of the high concentrations of salt on the solubilities of the solutes, and this is not being reflected in the solvatochromic response of the betaine dye. Finally on the subject of the solventlike properties of these aqueous polymeric systems, it would, of course,

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2535

be of considerable interest to have more detailed information on the properties of these systems using, for example, the Kamlet and Taft solvatochromic comparison method.117 At least one such study is available for the SDS micellar system.126 Additional studies would be able to provide a more detailed description of the solubilization mechanisms occurring in these systems and point the way toward a detailed group contribution approach to their prediction in contrast to the simple empirical measures of solvent polarity such as have been discussed here. We hope to return to this subject at a later date. Environmental Aspects It has become somewhat traditional in all of the fields reviewed here to stress how essentially environmentally benign and toxicologically safe many of these polymeric biphasic and micellar systems are compared to traditional solvents. Also often emphasized is the bulk availability and cheapness of these materials. The reasons for the latter are not hard to find. As long ago as 1984, some 17 million metric tons of surfactant were being produced worldwide, of which about two-thirds were anionics and one-third linear alkyl benzenesulfonates.127 Presently, some 500 000 tons of alkylphenol ethoxylates are produced annually.128 These latter are produced mainly for use in detergents, paints, pesticides, textiles, petroleum recovery products, metal finishing fluids, and personal care products.128 Some 55% of these are being used in industry, 30% in institutional cleaners, and the remainder in personal care products.128 It has been said that some 0.7 billion kg of alkyl ethoxylates are produced and released to the environment annually.129 PEGs and their mono- and dicarboxylated forms constitute one of the most abundant classes of contaminants of natural waters.129 A relatively recent compilation of available environmental and human safety aspects of the anionic surfactants is available.127 It appears that modern surfactants are fairly readily biodegradeable and that indications of carcinogenicity, mutagenicity, and teratogenicity are lacking.127 Environmental effects of surfactants such as the linear alkyl benzenesulfonates are related to their physical properties and thus, for instance, may be toxic to fish at or even below 10 mg/mL. Synergistic effects may occur with metals such as zinc, copper, and mercury. Some studies have shown similar effects with pesticides and petroleum products.127 The toxicity and environmental safety of the poly(oxyalkylene) block copolymers have also recently been reviewed. They appear to resemble their homopolymers in being of generally low toxicity but often showing rather poor rates of environmental biodegradability.130,131 On the other hand, production of alkylphenol ethoxylates for household use in Europe is currently restricted on a voluntary basis, and restrictions for industrial applications are expected by the year 2000 because of the toxic nature of the breakdown products of these materials.128 They have additionally been suggested as nascent endocrine disrupters.128 Many of these materials appear to pass only partially treated through public treatment facilities. Increasingly, 1 µg/L seems to be a likely limit on the discharge of these materials in both Europe and America. Environmental aspects and human safety of these materials have recently been reviewed.132 In general, poly(oxyethylenes) and poly(oxypropylenes) and their copolymers have similar toxicity which

is, in general and in most studies, very low, although aquatic toxicities in the milligram per liter range have been reported.132 Industrially, alcohol ethoxylates are likely to be viewed as replacements of the alkylphenyl ethoxylates.128 These polymers may be expected to distribute themselves in the environment to a large extent in the aqueous phase but with perhaps as much as one-third in the sedimentary phase,131 which will affect their environmental mobility. Some concern has been voiced as to their ability to transport and thus enhance the toxicity of other toxic materials, perhaps in analogy to the way in which similar materials may act as ionophores in relation to the cell membrane. Such is the current ubiquity of these materials in the environment that it has even been suggested that poly(oxyalkylene) presence may be used as a diagnostic test for the existence of anthropic inputs into water courses.128 In view of the likely increasing constraints on the discharge of ethoxylated materials and the increasing pressure for complete plant closure, practical processes for environmental cleanup and large-scale industrial application will have to recognize the requirement for the right choice of polymeric material at the outset. Innovative designs which limit discharges, prevent losses, and permit recycling will be at a premium. Conclusions All of the liquid/liquid extraction techniques discussed here have many common features which convey challenges to the understanding of the underlying mechanisms and, thus, in their utilization and in the design of practical separations processes. All form heterogeneous or phase-separated solutions which lend themselves to the development of sophisticated separations methodologies but, most importantly, which utilize relatively benign media compared to conventional solvent extraction schemes. CPE utilizing solubilizing polymers and analogous solid phases developed for SPE involving a porous particulate support) and SPME involving a membrane supporting phase) combined with MELC represent opportunities to develop methods which dispense with the use of organic solvents in the analytical laboratory. Also, on the larger scale, each may be exploited to take advantage of subtle differences in solubility of target species which in many cases may be approached by a variety of methodologies. In some cases, polymers may be induced to undergo conformational or associational changes in response to environmental variables, of which the most significant is temperature. The physicochemical interactions of the relatively apolar phase may be manipulated to effect separations based on variable molecular properties of mixtures of molecules. Currently, the greatest challenge lies in the development of wholly closed processes utilizing these materials. Too much reliance has, in the past, been placed on their relatively benign nature. Increasingly, the relative sophistication of modern chemical synthetic procedures is leading to the production of a rapidly growing range of polymeric materials (reviewed in ref 14), having diverse properties and potential for useful application. It seems likely that separations processes based on these new synthetic polymers will develop at a rapid rate soon. To this end, we look forward to seeing much more comparative physical and chemical data on these alternative solvent systems to allow direct comparison with traditional solvent extraction methods. Ultimately, one

2536 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

might envisage the development of a solvent replacement toolbox from which the polymeric system best suited to a particular separations need could be quickly isolated. Experimental Methods Solvatochromic wavelength shifts of Reichardt’s carboxylated betaine dye (structure shown in Figure 5) were recorded with a Cary 3 UV/vis spectrophotometer sampling at 0.25 nm wavelength intervals with the signal-to-noise ratio set at 1000:1. Samples consisting of mixtures of PEG-2000 and K3PO4 were made up on a weight/weight percent basis to a total weight of 5 g, or similar aliquots were withdrawn from the equilibrated phases of PEG-2000/K3PO4 ABS. The phase diagram of this system was determined using the cloudpoint method and the tie lines assigned from the known relationship between the mass ratio of the coexisting phases and the binodal curve using the mathematical methods given by Merchuk.133 To these samples was added 100 µL of the carboxylated betaine dye made up at a concentration of 5 × 10-5 M. The wavelength of the peak maximum was estimated by the 9/10 method of Kamlett and Taft.117 The molar transition energy of the peak maximum was calculated as in eq 5, where h

ET ) hcNAνmax

(5)

is Planck’s constant, c is the velocity of light, NA is Avogadro’s number, and νmax is the wavenumber of the absorption maximum. Acknowledgment This research is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant DE-FG02-96ER14673), and for studies involving metal ion partitioning by the U.S. National Science Foundation (Grant CTS-9522159). The kind gift by Professor Christian Reichardt of the carboxylated betaine dye used in these studies is gratefully acknowledged. Nomenclature Abbreviations ABEC ) aqueous biphasic extraction chromatography ABS ) aqueous biphasic systems cmc ) critical micelle concentration CMT ) critical micellization temperature CP ) cloud point CPC ) cetylpyridinium chloride CPE ) cloud-point extraction CTAB ) cetyltrimethylammonium bromide ∆Ghyd ) Gibbs free energy of hydration DNAPL ) dense nonaqueous phase liquid EO ) ethylene oxide HIC ) hydrophobic interaction chromatography HPLC ) high-performance liquid chromatography Kow ) octanol/water partition coefficient LCST ) lower critical solution temperature LLPC ) liquid-liquid partition chromatography log P ) log octanol/water partition coefficient ME ) micellar extraction MELC ) micellar-enhanced liquid chromatography MEKC ) micellar electrokinetic chromatography MEUF ) micellar-enhanced ultrafiltration MLC ) micellar liquid chromatography

MW ) molecular weight MWCO ) molecular weight cutoff NVPS ) N-vinylpyrrolidone-styrene NPE7.5 ) poly(oxyethylene)(7.5) 4-nonylphenyl ether (PONPE-7.5) OPE9-10 ) poly(oxyethylene)(9.5) 4-tert-octylphenyl ether (Triton X-100) PAH ) polycyclic aromatic hydrocarbon PCDD ) polychlorinated dibenzo-p-dioxin PEO ) poly(ethylene oxide) PEG ) polyethylene glycol PLURONIC ) trade name BASF PEO-PPO block copolymer PO ) propylene oxide PONPE-7.5 ) poly(oxyethylene)(7.5) 4-nonylphenyl ether (NPE7.5) PPG ) polypropylene glycol PPO ) polypropylene oxide PS-MA ) polystyrene-methacrylic acid PS-PEO ) polystyrene-poly(ethylene oxide) PSSS-VN ) poly(sodium styrenesulfonate-co-2-vinylnaphthalene) PSSS-VPA ) poly[sodium styrenesulfonate-co-9-(vinylphenyl)anthracene] PVA ) poly(vinyl alcohol) PVP-PS ) poly(vinylpyrrolidone)-polystyrene RPC ) reversed-phase chromatography SPE ) solid-phase extraction SPME ) solid-phase microextraction TAN ) 1-(2-thiazolylazo)-2-naphthol TLC ) thin layer chromatography TPS ) thermoseparating polymer systems UCST ) upper critical solution temperature UCON ) trade name of Union Carbide random EO-PO copolymers VOC ) volatile organic compound

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Received for review August 3, 1998 Revised manuscript received March 3, 1999 Accepted March 7, 1999 IE980505M