Surfactant‐Based Materials*
4.3 Surfactant‐Based Liquid‐Phase Extraction
4.3.1 Cloud‐Point Extraction
Cloud‐point extraction (CPE), introduced in 1976 by Watanabe et al. [5], is based on phase separation of aqueous micellar solutions of surfactants (usually non‐ionic or zwitterionic surfactant) after an increase in temperature. The extraction occurs when
Amphiphilic
monomers Aqueous
micelles
Aqueous phase Phase
separation
Surfactant-rich phase
Non-polar chain
Polar head CTAB
N –Br +
>> cmc
(a)
(b)
Analytes
i.e. organic compounds, metal chelates Interferences
Surfactant molecule
Cloudy solution
Aqueous phase
Aqueous phase Phase
separation
Extractant phase
Extractant phase
< cmc
Analytes
i.e. organic compounds, metal chelates Interferences
Surfactant molecule Extractant
Figure 4.1 Solubilization of analytes in (a) cloud‐point extraction and (b) surfactant‐assisted emulsification microextraction.
Surfactant-Based Materials 107
the temperature rises above the cloud point temperature where the surfactant becomes cloudy, resulting in two‐phase separation. One phase contains a surfactant at concen- tration close to the cmc (or a water‐rich phase) and the other is a surfactant‐rich phase.
This phenomenon occurs due to an increase in micellar size and the dehydration of the hydrated outer micellar layers with the increase in temperature. Surfactant aggregates orientate their hydrocarbon tail towards the center of the formation, creating a non‐
polar core. It seems evident that the hydrophobic core of the micelle is very much like that of the corresponding liquid hydrocarbon. Hydrophobic and covalent target com- pounds initially present in the aqueous solution are favorably partitioned in the non‐
polar microenvironment. The analytes can be solubilized in the micelles aggregates depending on the micelle–analyte binding interaction, and extracted to the small volume of the surfactant‐rich phase, while the hydrophilic matrices move into the bulk aqueous solution. The experimental scheme of CPE is depicted in Figure 4.3. In recent decades, CPE techniques have been extensively utilized as a versatile and simple method for the extraction and preconcentration of a wide variety of organic and inorganic species. Some recent interesting examples of CPE applications will be described in this section.
4.3.1.1 CPE of Trace Elements
The application of CPE for enrichment of trace elements is remarkably simple – a com- plexing agent is usually added to form the hydrophobic chelate, which can be extracted into the hydrophobic core of the micelles in the surfactant‐rich phase. A few milliliters of concentrated surfactant is added and the solution is heated above the cloud point temperature. After phase separation (which usually takes place after centrifugation), a
120 100 80 60
Year
Number of publications
40 20 0
cloud-point extraction
microextraction & surfactant & ultrasound microextraction & surfactant & vortex microextraction & surfactant
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
Figure 4.2 Contributions published in the period 1998–2017 demonstrating the application of surfactants in modern liquid‐phase extraction techniques.
small volume of surfactant‐rich phase is obtained. The highly viscous surfactant‐rich phase is usually diluted by mineral acid (mainly nitric acid in methanolic or ethanolic solution) before subsequent analysis by various analytical techniques.
The CPE efficiency of trace elements depends mainly on the complex formation con- stant, the kinetics of complex formation, the inherent interaction of the metal complex with the surfactant moiety, and the phase transfer in the micellar media [6]. The extrac- tion of metal ions is significantly improved by the formation of insoluble or sparingly water‐soluble chelates. The existence of metal complexes in a surfactant‐rich phase is due to the hydrophobic interaction between the metal chelates and micelles. Therefore, the CPE efficiency depends on the hydrophobicity of the complex. Generally, non‐ionic surfactants, mainly polyoxyethylated alkylphenols, from the Triton and PONPE series, are the most widely employed for CPE in trace element analysis because of their com- mercial availability with high purity grade, relatively low price, stability, non‐volatility, low toxicity, and low flammability. Among them, Triton X‐114 is preferably used due to its low cloud point temperature (23–25 °C) and high density of the surfactant‐rich phase (1.052 g ml−1) [7]. Table 4.2 summarizes the available literatures in the last few years related the application of CPE for trace elements using Triton X‐114.
The azo dyes deserve special attention in CPE of metal analysis due to their capability to form mostly neutral and hydrophobic chelates with the vast majority of transition metals. The complexes are stable with rather limited solubility in aqueous solution but much greater solubility in organic solvents. These dyes are considered as tridentate ligands and form chelates with metal ions through the oxygen atom of the ortho‐
hydroxyl group, nitrogen atom from pyridine, and one of the nitrogen atoms of the azo group, giving two five‐membered chelate rings. Azo dyes are divided into two groups:
pyridylazo‐dyes with a PAN‐type chelating structure and thiazalylazo with a TAN‐type chelating structure. Pyridylazo derivatives have been widely applied as chelating agent for the determination of trace elements. The most frequently used pyridylazo dyes for metal chelate formation are 1‐(2‐pyridylazo)‐2‐naphthol (PAN), 2‐(5‐bromo‐2‐
pyridylazo)‐5‐diethylaminophenol (5‐Br‐PADAP), and 4‐(2‐pyridylazo)resorcinol (PAR) [26]. Compared to pyridylazo dyes, thiazolylazo reagents are less frequently used despite the fact that these compounds have been demonstrated to be promising for trace element analysis due to their good analytical characteristics.
Surfactant
Surfactant-rich phase
OR
Surfactant-rich phase Surfactant-rich phase
Clouding solution Aqueous sample
Analysis
Analyte Interference
Aqueous phase decantation Heating Centrifugation
Figure 4.3 Schematic diagram of CPE procedure.
Surfactant-Based Materials 109 Table 4.2 CPE of trace elements using Triton X‐114.
Analytes (Reference) Sample matrix CPE conditions/analytical technique Complexing agent: azo dyes
Cd, Co, Cr, Cu, Mn,
Ni, Pb, Zn [8] Calcium‐rich
materials CPE/ICP‐OES
Sample: 20 ml pH 10
Complexing agents: PAN, 5‐Br‐PADAP Surfactant: 0.25% v/v Triton X‐114 Temp.: 50 °C (40 min)
Centrifugation: 12 min (3500 rpm) Ice‐bath: 30 min
SRP diluting solvent: HNO3 (1 ml)
Hg [9] Environmental
samples CPE/spectrophotometry
PAN method pH 9.0
Surfactant: 5.0% w/v (0.4 ml) Triton X‐114 Complexing agent: 2.0 × 10−3 mol l−1 PAN (0.5 ml) TAR method
pH 8.0
Surfactant: 5.0% w/v (0.5 ml) Triton X‐114 Complexing agent: 2.85 × 10−4 mol l−1 TAR (0.7 ml) Temp.: 50 °C (10 min)
Centrifugation: 5 min (3500 rpm) Ice‐bath: 5 min
Cu [10] Food, water,
biological samples
CPE/spectrophotometry Sample: 10 ml
pH 4.5
Complexing agent: 5.0 × 10−4 mol l−1 (6.0 ml) ATAP Surfactant: 1.0%v/v (5.0 ml) Triton X‐114 Temp.: 50 °C (10 min)
Centrifugation: 10 min (4000 rpm)
SRP diluting solvent: ethanol acidified with 1.0 mol l−1 HNO3 (0.4 ml)
Cu [11] Serum Dual‐CPE/FAAS
Sample: 10 ml
Complexing agent: 0.4 × 10−4 mol l−1 (0.5 ml) PAN Surfactant: 0.1–0.2% v/v (2 ml) Triton X‐114 Temp.: 45 °C (10 min)
Centrifugation: 10 min (2500 rpm) Back extraction: 0.1 mol l−1 HNO3
(Continued)
Table 4.2 (Continued)
Analytes (Reference) Sample matrix CPE conditions/analytical technique Cr(III), Cr(VI)
speciation [12] Water DMSPE‐CPE/FAAS
Sample: 45.0 ml pH 5.0
DMSPE: 25.0 mg Fe3O4/SiO2, 1 min vortex, 2.5 mol l−1 HCl (0.5 ml) eluent
Complexing agent: 3.99 × 10−2 mol l−1 (672 μl) TAR Surfactant: 3.99 × 10−2 mol l−1 (138 μl) Triton X‐114 Temp.: 90 °C (45 min)
Centrifugation: 10 min (1200 rpm)
SRP diluting solvent: 0.1 mol l−1 HNO3 (600 μl)
Pb [13] Water CPE/FAAS
Sample: 10 ml pH 6.0
Complexing agent: 0.1% (200 μl) PAR
Surfactant: 0.2% w/v both (200 μl each) Triton X‐114, benzyldimethyl hexadecyl‐ammonium chloride Temp.: room temp. (10 min)
Centrifugation: 10 min (4000 rpm) SRP diluting solvent: 1.0 mol l−1 HNO3 in methanol (500 μl)
Rh [14] Water CPE/GFAAS
pH 5.5
Complexing agent: 1 × 10−3 mol l−1 (80 μl) 2‐(5‐iodo‐2‐pyridylazo)‐5‐dimethylaminoaniline Surfactant: 1% (0.8 ml) Triton X‐114
Temp.: 60 °C (10 min)
Co [15] Environmental
water UARS‐CPE/ETAAS
Sample: 10 ml pH 1.5
Complexing agent: 5.0 × 10−5 mol l−1 PAN Surfactant: 0.05% v/v Triton X‐114 Synergic reagent: octanol (0.2 ml) Ultrasonication: 5 min
Centrifugation: 5 min (3000 rpm) Complexing agent: dithiocarbamate
Sb(III), Sb(V)
speciation [16] Food packaging materials
CPE/ETAAS Sample: 6.00 ml pH 5.0
Complexing agent: 2.5% w/v (0.5 ml) APDC Surfactant: 1.0% v/v (1.00 ml) Triton X‐114 Temp.: 50 °C (15 min)
Centrifugation: 5 min (3500 rpm)
SRP diluting solvent: HNO3 in methanol (0.2 ml)
Surfactant-Based Materials 111 Table 4.2 (Continued)
Analytes (Reference) Sample matrix CPE conditions/analytical technique
Sb [17] Bottled water,
natural water CPE/ETAAS Samplel: 6.0 ml pH 2.0
Complexing agent: 3.5% v/v (0.5 ml) APDC Surfactant: 3.0% (v/v) (1.0 ml) Triton X‐114 Temp.: 50 °C (15 min)
Ice‐bath: 10 min
Centrifugation: 10 min (2500 rpm)
V [18] Formulations,
dialysate, parenteral solutions
CPE/ETAAS Sample: 50 ml pH 4
Complexing agent: 1.38 × 10−3 mol l−1 (0.5 ml) Surfactant: 0.3% v/v Triton X‐114
Temp.: 45 °C (20 min)
Centrifugation: 6 min (3500 rpm)
SRP diluting solvent: 1 : 10 v/v HNO3 in ethanol (0.2 ml)
Cd, Co, Ni, Pb, Zn,
Cu [19] Water Dual‐CPE/ICP‐OES
Sample: 50 ml pH 7.0
Complexing agent: 0.2 mmol l−1 Surfactant: 0.05% w/v Triton X‐114 Temp.: 55 °C (25 min)
Centrifugation: 13 min (4000 rpm)
SRP diluting agent: 0.8 mmol l−1 HNO3 (2 ml)
Bi [20] Human serum Sample: 25 ml
Complexing agent: 1 × 10−2 mol l−1 (0.2 ml) Surfactant: 2% v/v (3 ml) Triton X‐114 pH 7
Temp.: 50 °C (20 min)
Centrifugation: 10 min (4000 rpm) Ice–NaCl bath: 15 min
Pb, Cd [21] Sera of
different types of gallstone patients
Um‐CPE/FAAS pH 7
Complexing agent: 0.3%
Surfactant: 0.2% Triton X‐114 Temp.: 45 °C (10 min) Ultrasonication: 1 min
(Continued)
Dithiocarbamates, i.e. diethylammonium‐N,N′‐diethyldithiocarbamate (DDTC) and ammonium pyrrolidine dithiocarbamate (APDC), are the most efficient chelating rea- gents, next to azo dyes, used in CPE for preconcentration of metal ions. Dithiocarbamates are highly versatile ligands toward main group metals. They react with a large number of di‐ and tri‐valent metals, e.g. Cu(II), Pb(II), Cd(II), Ni(II), Zn(II), Fe(II,III), or Cr(III).
Table 4.2 (Continued)
Analytes (Reference) Sample matrix CPE conditions/analytical technique
Cu, Hg [22] Water CPE/ICP‐OES
Sample: 10 ml pH 8.0
Complexing agent: 1.5 × 10–5 mol l−1 3‐NBT Surfactant: 0.3% v/v Triton X‐114
Temp.: 55 °C (30 min)
Centrifugation: 8 min (3500 rpm)
SRP diluting solvent: acedic 80 : 20 methanol–water mixture (1 mol l−1 HNO3) (0.25 ml)
Cu [23] River water CPE/FAAS
pH 2
Salt: 4 g NaSO4
Complexing agent: 4 μmol l−1 dithizone Surfactant: 20% Triton X‐114 Temp.: 25 °C (1 h)
Cr(III), Cr(VI)
speciation [24] Water, beer,
wine CPE/ETAAS
Sample: 20 ml pH 2
Complexing agent: 0.3 mol l−1 (0.4 ml) EDTA Surfactant: 30% w/v (50 μl) Triton
X‐114 + 0.1 mmol l−1 AgNPs Temp.: 60 °C (10 min)
Centrifugation: 5 min (4000 rpm)
Ni [25] Food, water RS‐CPE/FAAS
Sample: 25 ml pH 9
Complexing agent: 1 × 10−3 mol l−1 2,2′‐furildioxime Surfactant: 0.08% Triton X‐114
CP revulsant: 10 μl octanol Shaking: 1 min
SRP diluting solvent: 1 mol l−1 HNO3 in methanol PAN: 1‐(2‐pyridylazo)‐2‐naphthol; 5‐Br‐PADAP: 2‐(5‐bromo‐2‐pyridylazo)‐5‐diethylaminophenol; SRP:
surfactant‐rich phase; TAR: 4‐(2‐thiazolyazo)resorcinol; ATAP: 2‐amino‐4‐(m‐tolyazo)pyridine‐3‐ol;
Um‐CPE: ultrasonically modified cloud‐point extraction; 3‐NBT: 3‐nitro benzaldehyde thiosemicarbazone;
AgNPs: silver nanoparticles; RS‐CPE: rapidly synergistic cloud‐point extraction; PAR: 4‐(2‐pyridylazo) resorcinol; UARS‐CPE: ultrasound‐assisted rapidly synergistic cloud point extraction; APDC: ammonium pyrrolidine dithiocarbamate.
Surfactant-Based Materials 113
The complex formation is sufficiently rapid. The chelates are sparingly soluble in water but dissolve in organic solvents such as carbon tetrachloride, chloroform, amyl acetate, or acetone and, hence, they can be extracted into the micelles.
8‐Hydroxyquinoline (8‐HQ), or oxine, is one of the most versatile chelating agents widely used in complex formation of various heavy metal ions. 8‐HQ can react with at least 43 metals over a wide pH range, giving the sparingly water‐soluble complexes [26], which allowed the extraction to the hydrophobic core of micelles. The CPE of some metal ions, such as V(V), Pb(II), Cd(II), Bi(III), Co(II), Ni(II), Zn(II), and Cu(II), after complex formation with 8‐HQ has been recently documented (Table 4.2). The optimal complexation reaction was occurred at around pH 7.0. Triton X‐114 non‐ionic sur- factant has been used as micellar media for CPE of such metal chelates. A significant improvement of detection limits was achieved with high enrichment factors.
Diphenylthiocarbazone (H2Dz), or dithizone, is also a well‐known organic reagent used for determination of metal ions, i.e. Pb(II), Zn(II), Cd(II), Ag(I), Pd(II), Hg(II), Cu(II), and Bi(III). The reagent is practically insoluble in water at pH <7 but it dissolves in alkaline aqueous media forming orange‐colored solutions containing the anionic form of HDz−. Dithizone reacts with most heavy metals whose sulfides are sparingly soluble in water. Metal ions react with dithizone to form non‐polar colored complexes whose color differs significantly from that of dithizone. Selectivity of the CPE methods for preconcentration and determination of metals using dithizone can be achieved by controlling the acidity of the medium and using masking agents such as cyanide, EDTA, thiosulfate, and iodide [26].
Since the total concentration of metal ions does not provide information with which to estimate its toxicity and bioavailability, it is necessary to evaluate the speciation of a metal. CPE can be used for metal speciation with the presence of the chelating agent for a species of interest, which forms a complex with hydrophobic properties. Effective species selection of ionic gold species, and gold nanoparticles (Au‐NPs), was achieved using sodium thiosulfate as a complexing agent [27]. CPE with Triton X‐114 as collect- ing phase was applied. The high viscosity of the surfactant‐rich phase made it necessary to dissolve the sample with ethanol prior to introduction to an electrothermal atomic absorption spectrometer (ETAAS) for quantification.
Silver nanoparticles (AgNPs), even at the μg l−1 level, when submitted to CPE interact with Cr(III) and completely transfer this species to the micelles, where the metal can be measured by ETAAS. The CPE of AgNPs by Triton X‐114 allows Cr(III) ions to be transferred to the surfactant‐rich phase. Speciation of Cr(III) and Cr(VI) was achieved by carrying out two CPE experiments. In the first experiment, in absence of the ethylenediamine tetraacetic acid (EDTA) complexing agent, the total concentration of chromium was obtained. The analytical signal in the presence of EDTA allowed the Cr(VI) concentration to be measured, with that of Cr(III) being calculated by difference.
The amount of Triton X‐114 affects the final volume of the surfactant‐rich phase recov- ered. The optimal temperature for the CPE was found to be 60 °C maintained for 10 min.
Since the chromium species transferred to the surfactant‐rich phase with the aid of AgNPs is in the trivalent form, speciation is possible. For this purpose, advantage can be taken of the relatively slow kinetics of Cr(III) complexation by the EDTA anion compared with the kinetics of the retention of this species on AgNPs [24].
Another method reported for chromium speciation is based on sequential precon- centration of Cr(VI) at pH 5.0 onto mesoporous amino‐functionalized Fe3O4/SiO2
nanoparticles followed by CPE of Cr(III) as metal complex with 4‐(2‐thiazolyazo) resorcinol (TAR). The elution step of Cr(VI) adsorbed on magnetic nanoparticles was carried out using hydrochloric acid under stirring in a vortex mixer and the metal was measured with a flame atomic absorption spectrometer (FAAS) after separation of magnetic nanoparticles using a magnet. In the supernatant under controlled pH (5.0) containing Cr(III), Triton X‐114 and TAR were added. The cloud point was attained in a thermostatic bath at 90 °C for 45 min followed by centrifugation to separate two phases. The surfactant‐rich phase was diluted in nitric acid in methanol to decrease the viscosity and introduced to the FAAS nebulizer for the determination of Cr(III).
The method has shown good tolerance towards co‐existing cations and anions and humic acid [12].
The selective CPE of Sb(III) after its complexation with APDC at pH 2.0 was reported.
A surfactant‐rich phase was separated at the cloud point of the non‐ionic surfactant (Triton X‐114) and subsequent determined by ETAAS. Antimony(V) was not extracted at this pH. Total Sb was determined after reducing Sb(V) to Sb(III) by l‐cysteine. Then, the concentration of Sb(V) was calculated by subtracting Sb(III) from the total anti- mony. The validated CPE method was applied in the speciation of Sb in mineral water samples bottled in poly(ethylene terephthalate) (PET) [17].
The development of a new analytical method employing ultrasound assisted‐cloud‐
point extraction (UA‐CPE) for the extraction of CH3Hg+ and Hg2+ species from fish samples has been reported. Detection and quantification of mercury species were per- formed at 550 nm by spectrophotometry. Owing to the 14‐fold higher sensitivity and selectivity of thiophene 2,5‐dicarboxylic acid (H2TDC) to Hg2+ ions over CH3Hg+ in the presence of mixed surfactant, Tween 20 and SDS at pH 5.0, the amounts of free Hg2+
and total Hg were spectrophotometrically established by monitoring Hg2+ in the pre- treated‐ and extracted‐fish samples in an ultrasonic bath to speed up extraction using a diluted acid mixture (1 : 1 : 1, v/v, 4 mol l−1 HNO3, 4 mol l−1 HCl, and 0.5 mol l−1 H2O2), before and after pre‐oxidation with permanganate in acidic media. The amount of CH3Hg+ was calculated from the difference between total amounts of Hg and Hg2+. The proposed method was successfully applied for preconcentration and speciative deter- mination of the Hg species in fish samples with good accuracy, reproducibility, and statistically significant recoveries [28].
A ligandless CPE methodology has also been developed for the preconcentration of trace amounts of nickel [29]. Poly(ethyleneglycol)mono‐p‐nonyl phenyl ether (or poly- oxyethylene 7.5 nonylphenyl ether, PONPE 7.5) was applied as both chelating agent and extractant to preconcentrate nickel. PONPE 7.5 may form a cationic complex with Ni(OH)+ at pH 9.4 through the polyoxyethylene groups and thereby can be extracted in a surfactant‐rich phase. The cloud point of this system is near room temperature (20 °C).
Therefore, the phase separation can be made without heating the micellar solutions.
Hence, the micellar solution is immediately turbid at room temperature (25 °C).
Moreover, after centrifugation, the two phases are easily separated without cooling in an ice bath.
Spectroscopic techniques, i.e. FAAS, ETAAS, inductively coupled plasma‐optical emission spectrometry (ICP‐OES), and inductively coupled plasma‐mass spectrometry (ICP‐MS), are the commonly used analytical techniques for determination of trace ele- ments. Due to its common availability in many laboratories, simplicity of procedure, speed, precision, and accuracy, FAAS is an attractive for the determination of many
Surfactant-Based Materials 115
elements after their preconcentration by CPE. By FAAS detection, the addition of a diluting solution in the surfactant‐rich phase is indispensable to obtain a clear and homogenous solution of low viscosity compatible with the requirements of a flame nebulizer. In hydride generation/atomic absorption detection, the presence of sur- factant can concentrate reactants at a molecular level, modify thermodynamic and kinetic behavior, and solubilize, in a selective manner, analytes and reactants in the aggregates. The use of ETAAS after CPE can be regarded as an appropriate combina- tion because of the elimination of acidified organic solvents (used to dilute the sur- factant‐rich phase) during the gradual increase of temperature prior to the atomization of the analytes. Final extraction volumes are small (hundreds of microliters); the very small volumes needed for injection into the graphite furnace (mainly 20 μl) are another benefit of this method (repeated injections can be done). Surfactants are also compati- ble with ETAAS. The contact angle of water with the carbon surface of the graphite used in ETAAS is 85.7°; the surfactant can diminish the contact angle of an aqueous solution with graphite, so they can provide a possible solution to this inconvenience.
Consequently, aqueous samples deposited on graphite can benefit from the presence of the surfactant in order to spread a liquid sample drop evenly on the graphite surface before analysis. Therefore, no serious difficulties are anticipated in implementing ETAAS with CPE for metal analysis. In the case of ICP‐OES, dilution of the surfactant‐
rich phase before its injection into plasma is needed. In the past, the role of organic solvents in the plasma as signal modifiers for most elements has been reported [6, 7].
4.3.1.2 CPE of Organic Analytes
CPE has also proved its applicability for the extraction of a wide range of organic com- pounds, e.g. carbamate pesticides [30] and phenolic compounds [31]. The solubilization of non‐polar organic molecules in the hydrophobic micellar core is an inherent property of all surfactant systems, widely exploited for the design of new preconcentration proce- dures. The efficiency of these procedures relies on the degree of analyte solubilization into the micelle (non‐polar core and polar micelle–water interface), analyte polarity, and solution composition. Therefore, any experimental approach should focus on the com- bination that ensures maximum extraction recovery. Recent studies on analyte partition- ing in surfactant aggregates have shown that there is a sharp correlation between the octanol–water partition coefficient (Kow) of a given organic compound and its partition into the surfactant‐rich phase. Theoretically, extremely hydrophobic analytes show very favorable distribution constants between the micellar and the aqueous phases, resem- bling those observed with organic solvents. It is therefore estimated that the maximum preconcentration factors that can be achieved coincide numerically with the phase ratio.
In practice, the hydrated nature of the surfactant‐rich phase leads to a smaller partition coefficient than those reported for organic solvents. With regard to surfactant structure, it has been recognized that solubilization of organic solutes increases on increasing the length of the hydrophobic tail and decreasing the size of the polar head. It is therefore conceivable that solubilization of organic analytes into the surfactant micelles can be amended by minimizing the non‐hydrophobic contributions [32].
The CPE method usually has limited capabilities for the extraction of thermally‐labile compounds if the surfactant that provides maximum extraction efficiency has a high cloud point temperature or if much higher temperatures than the cloud point tempera- ture must be maintained for a long time to allow maximum extraction. This problem