Supercritical Fluid Science and Technology - American Chemical

Chapter 11

Pressure Tuning of Reverse Micelles for Adjustable Solvation of Hydrophiles in Supercritical Fluids

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch011

Keith P. Johnston, Greg J. McFann, and Richard M. Lemert Department of Chemical Engineering, The University of Texas, Austin, TX 78712

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for thefirsttime for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. Supercritical fluids (SCFs) such as carbon dioxide have a "hydrocarbon-like" solvent strength at typical conditions, so that they are appropriate solvents for lipophilic substances. The solvent strength may be raised significantly by the addition of small amounts of cosolvents such as ethanol to increase solubilities of moderately polar substances selectivelyQ), sometimes by several hundred percent(2,2,4). The solvent and cosolvent form clusters about solutes, in which the cosolvent concentrations are enhanced significantlyd,6). The present objective is to explore the effects of considerably more powerful solvent additives, that is surfactants. Since very little is known about surfactants in SCFs, spectroscopic probes were used to measure polarities inside the reverse micelles. Polarity is a key indicator of the ability of a reverse micelle to solvate a hydrophile. Using the 0097-6156/89yO406-0140$07.25A) © 1989 American Chemical Society

Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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polarity data, surfactant /co-surfactant systems were designed for the purpose of solubilizing hydrophilic substances. Reverse micelles are thermodynamically stable aggregates of amphiphilic molecules in non-aqueous solvents. The hydrophilic heads form a core, and the lipophilic tails extend into the oil continuous phase, as shown in Figure 1. If enough water is present it collects as a "water pool" in the micelle core. It is difficult to distinguish between the micelle core and the interfacialregioncontaining the surfactant head groups, so we willreferto both as the "polar microdomain." In reverse micelles the driving forces for aggregation are the ionic interactions among the head groups and counterions, as well as dipolar forces and hydrogen bonding (7.8V Aggregation is opposed by the entropy gain in dispersing the surfactant and by steric repulsion between the tails. A change in the solvent can promote or oppose aggregation as discussed below. The forces that promote aggregation in reverse micelles are weaker than the hydrophobic effect, which can cause aggregation numbers of over 100 for normal micelles in a water continuous phase. Thus the aggregation numbers forreversemicelles are small, often no more than 20. The magnitude of solvent effects onreversemicelles is dependent upon the nature of the surfactant It is important to make the distinction between the solvent effect on [l]aggregation number for dryreversemicelles, and [2]the ability of reverse micelles to solubilize water. These are two very different phenomena. AOT is a good example for highlighting this difference, as the solvent effect is small for the former and large for the latter. For AOT, the maximum amount of water solubilization varies over a wide range for a series of hydrocarbon solvents®. This means that the micelle size depends upon the solvent, since size isrelateddirectly to W (molar ratio of water to surfactant) (10). The optimal hydrocarbon solvent for solubilizing water in AOT is octane. A model has been developed to relate the alkane carbon number of the optimal hydrocarbon solvent to the solvent-solvent, solvent-tail, and tail-tail interactions(ll). For systems without added water, very little data are available. The solvent effect on the aggregation number is pronounced for sodium dinonylnaphthalenesulfonate CD, but small for AOT Q2). The cohesive energy density of a SCF solvent may be adjusted by a change in the pressure or temperature. Therefore, it is likely that pressure may be used to adjust the size and/or the polarity for reverse micelles. It is possible to vary the cohesive energy density of a SCF over a wider range than available for a series of hydrocarbon liquid solvents. Previous studies lend support to this hypothesis of significant pressure effects. It is already well-known, for example, that pressure may be used to manipulate chemical potentials of solutes, with partial molar volumes reaching thousands of mL/mole negative (13). Randolph,etal.,(14). found striking changes in the size of small cholesterol aggregates in near-critical CO2. As pressure is increasedfrom81 to 104 bar, the cholesterol suddenly begins to aggregate,reachesa maximum size, then begins to dissociate. Reverse micelles have been investigated in SCF solvents only recently. It was observed qualitatively that Cytochrome-c forms a colored solution for AOT in 0



SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y

SURFACTANT

Figure 1. Structure of reverse micelles of AOT(sodium di-2-ethylhexyl sulfosuccinate) in nonaqueous solvents


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JOHNSTON ET AL.

Pressure Tuning ofReverse Micelles

propane, although concentrations were not reporteddS). At 240 bar, AOT in SCF ethane solubilizes three moles of water per mole of surfactant (W = 3) for surfactant concentrations up to 0.15 M (16). Extensive visual experiments have been performed for AOT in ethane, propane, and butane to construct a generalized density versus temperature phase behavior map (12). By using dynamic light scattering, it was shown that the apparent hydrodynamic radius of AOT reverse micelles increases 20% as the pressure is decreased from 340 to 220 bar in ethane(18). The increase was attributed to micelle-micelle interactions, which become more pronounced with the approach to the two-phase region. These studies were performed at high pressures, typically above 200 bar (P = 4) for ethane (to remain in the one-phase region), or in liquid propane at 25 C (T = 0.8). In both cases, the isothermal compressibilities are relatively small compared with ethane at 35 C and 80 bar. Hence the pressure effects were modest, yet larger than in conventional liquid solvents. The present study focuses on the behavior at lower pressures, particularly in the region where the SCF is highly compressible and thus has adjustable properties. Here surfactant and water solubilities are relatively small, so that many of the experiments were performed in the two-phase liquid-SCF region. Although this two-phase region is more complex than the one-phase region, it is interesting for several reasons. The properties of the aggregates are likely to be much more adjustable in this region. In an extraction process, it is likely that pressure adjustments could be used to recover selectively certain products and/or the surfactant. Finally, capital costs would be reduced significantly at these lower pressures. The first objective is to explore the possibility of pressure tuning of the polarity in the reverse micelles(or aggregates) over a wide range in a single SCF. Polarities (solvent strengths) have been measured extensively using solvatochromic probes to predict solvent effects on a wide variety of chemical phenomena (19). including surfactant aggregation in non-aqueous liquid solvents (20.21). PyridineN-oxide was chosen as the probe since it partitions into more hydrophilic regions than most other indicators, because of its large dipole moment - 4.3 Debye. Our second objective is to form highly polar microdomains in ethane even below 100 bar. Co-solvents such as octane and co-surfactants such as octanol play an important role in achieving these pressure reductions. The final objective is to measure quantitatively the solubilities of hydrophiles, that is hydroquinone and tryptophan, in reverse micelles of AOT in SCF ethane. Shield et al. (22) have demonstrated that reverse micelles can be used in organic solvents to recover proteins selectively from aqueous solutions. Protein denaturation can occur, however, during recovery from the organic phase, which requires changes in pH or ionic strength. Supercritical fluid solvents offer the potential advantage that proteins could be recovered simply by changing the pressure. Additional potential applications of surfactants in supercritical fluids 0

r

r


143


144


include separations of other types of hydrophilic substances, enzymatic catalysisQ4), and mobility control in enhanced oil recovery(22).


Experimental Spectroscopic and phase behavior studies were conducted using a fixed path length cell. Pressure was increased by adding pure solvent to the cell, so molar concentrations were constant while molefractionsvaried. Additional phase behavior studies and solubility studies were conducted in a variable volume view cell, based on an existing design(24), in which molefractionswere held constant but molarities varied. AOT (Fluka, 98%) was purified according to Kotlarchyk's method (25). The purity of the final product was checked using HPLC. Purified AOT was stored in a desiccator, and AOT solutions were stored with molecular sieves to minimize hydration. The nonionic surfactant C11-14EO5 was provided by Shell Development Co. Brij 56 surfactant wasfromFluka. Pyridine-N-Oxide (Aldrich 13,165-2) was dried prior to use. Purified de-ionized water was used at all times. Reverse micelle solutions of known concentration were prepared in 2 mL constant volume 2.5 inch o.d. by 5/8 inch i.d. stainless steel cellsfittedwith Γ diameter χ 3/8" thick sapphire windows. The path length was 1 cm. The cell was equipped with cartridge heaters and thermostated to ± 0.1 C by means of a platinum resistance thermometer and a temperature controller. A 60 mL syringe pump pressurized the system, and pressure was controlled to ± 0.2 bar. The surfactant and pyridine-N-oxide were introduced into the static cell as solutions in volatile solvents, so that concentrations were known to ±J2 %. The solvent was removed by volatilization and water, if needed, was added with a syringe. Experiments were always performed in the order of increasing pressure so that the overall molarities of surfactant, water, and pyridine-N-oxide were constant. The contents of the cell were equilibrated rapidly using a magnetic stir bar which was small enough to rest below the light beam of the spectrophotometer. Repeated UV measurements over time showed that any suspended drops settled in less than about 10-20 minutes after stopping the stir bar. On one occasion the cell was allowed torestovernight and no change in Xmax was seen. The cell was removed periodicallyfromthe spectrophotometer to ensure that there was no cloudiness in the SCF phase or deposits on the windows. The Xmax was determined byfittingthe absorbance band using a cubic polynomial. The typical uncertainty in X was ±0.2 nm. The magnetically-stirred variable volume view cell apparatus, which was used to measure solubilities, is shown in Figure 2. The 2 in. o.d. χ 5/8 in. i.d. 304 ss cell (28 mL usable volume) contained a 1 in. diameter by 3/8 in. thick sapphire window. A piston with two 90-durometer buna-N o-rings separated the experimental fluid from the pressurizing fluid, CO2. Pressure was controlled by a 175 mL Lee Scientific model 501 computer controlled syringe pump. The view cell m a x


11.

JOHNSTON ET

AL.



COMPUTER

SYRINGE PUMP

SOLVENT

Θ '

Figure 2.

SAMPLE COLLECTION

Variable-volume view cell apparatus with microsampling



146


was placed in a polycarbonate water bath which served also as a safety shield, and the temperature was controlled to within 0.1 °C. Pressures on the sample side of the piston were measured to within ±0.1 bar with a strain gauge transducer. The cell was loaded at atmospheric pressure with the various components to within 0.1 mg. Ethane was condensed into the cell from a 300 cm^ transfer cylinder, which was weighed to within 0.1 g. Samples were obtainedfromthe variable volume view cell by displacing a controlled volume of solution through a Valco (C6U) six-port HPLC sampling valve equipped with a 100 microliter sample loop. The samples were collected by discharging the contents of the loop through a suitable liquid solvent, then rinsing the loop with additional solvent. Instantaneous pressure drops in the cell during the sampling were typically below 10 bar, and can be reduced further in the future by adding more ballast The cell contents were stirred at least twenty minutes to equilibrate at a new pressure. To purge the small dead volume between the cell and the sample loop, the first sample taken at each pressure was discarded. The procedure was tested by measuring the solubility of anthracene in CO2 and was found to give results within 5% of the literature values (1). The samples were analyzed with UV spectroscopy in a Cary 2290 spectrophotometer. Tryptophan was measured at 280 nm. Calibration of the solvatochromic probe in liquid solvents Before using a solvatochromic probe in the SCF state, it is important to perform calibrations in well-defined one-phase liquid systems. A variety of solvatochromic indicators have been used previously to probe AOT reverse micelles at atmospheric pressure. For example El Seoud et al. (26) investigated malachite green and thymol blue, which are hydrophobic. They are insoluble in heptane despite their hydrophobicity. Accordingly it was found that these probes are absorbed at the surfactant/oil interface, but do not partition into the water pool. The Xmax and UV absorbance for thymol blue change significantly for W below 6, but become constant for W from 8.3 to 22.2. UV-VIS and positron annihilation studies showed that even less hydrophobic probes such as nitrophenols are also solubilized in the surfactant/oil interface. Pyridine-N-oxide is one of the smallest and most hydrophilic (μ = 4.3 D) probes that has been studied. Consequently, it is located at the surfactant/water interface or, if sufficient water is present, in the water pool itself (20). We chose to study pyridine-N-oxide since it partitions into more hydrophilic regions of reverse micelles than thymol blue and malachite green. All of these probes are complementary. Pyridine-N-oxide is a "blue shift" indicator in that Xmsx shifts to shorter wavelengths as the solvent polarity increases. The Xmax decreasesfrom281.7 nm for isooctane to 254.4 nm for water (2?) Although it is only sparingly soluble in a hydrocarbon solvent such as cyclohexane, its X can be measured at a 0

0

m a x


11.


JOHNSTON ET AL.


-5

147

concentration as low as ΙΟ M because the extinction coefficient is 9000. Thus solvatochromic data can be obtained for a wide variety of solvents, from supercritical ethane to water. In the figures below, the characteristic values of %max in liquid solvents will be indicated to place the polarities in perspective. Figure 3 shows solvatochromic shifts for pyridine-N-oxide in AOT with the solvent cyclohexane, without added water. At extremely low AOT concentrations the probe senses a nonpolar environment that has a solvent strength (polarity) equivalent to that of pure cyclohexane. The pronounced change in slope at 0.001 M is the "apparent critical micelle concentration" where the average size begins to increase significantly. This apparent cmc should not be considered to be a sharp dividing line between surfactant monomer and large micelles, as in the case of aqueous systems (11). The average micelle size increases with surfactant concentration until the largest size that can be supported by the solvent is attained. For AOT this occurs at approximately 0.01 M in agreement with an earlier study(1Q). Here the pyridine-N-oxide senses a polarity equivalent to that of methanol. These results for AOT, which are in accord with those of earlier investigators (10.28.29). further substantiate the reliability of pyridine-N-oxide as a probe. The effect of adding water to the system is shown in Figure 4 for a series of AOT concentrations. As W is increased from 1 to 6, the change in X is large. It becomes less pronounced at higher values of W as the water pools are formed. As W reaches 20, λ χ approaches that in pure water. These results indicate that the probe is useful, as it is sensitive to both AOT concentration and W over a wide range. To further test pyridine-N-oxide as an indicator, the solvatochromic data are correlated versus micelle radius in Figure 5. The Xmax values for pyridine-N-oxide were determined in solutions of .025 M AOT in two solvents, n-bctane and nhexane. For a given W the values are very similar in the two solvents. The micelle radii are from the photon correlation experiments of Zulauf and Eicke (10). at the same AOT concentration, for a similar solvent, isooctane. It is widely accepted that W is a good indicator of the size of reverse micelles, as is evident in the relationship for the two horizontal axes in thefigure.There is a relatively linear relationship between micelle size and X for a W up to 15. The X at this point approaches that of pure water. This is in accord with Eicke and Kvita (28), who indicate that at a W of approximately 15, the water pool has the characteristics of free water. These results supply additional evidence of pyridine-N-oxide's hydrophilic nature and utility as an indicator. In order to make sure that pyridine-N-oxide does not perturb the system, its concentration was varied for AOT in η-octane as shown in Table I. Consider the case for AOT without added water. Assuming an average aggregation number of 25 (12), the number of pyridine-N-oxide molecules per micelle may be estimated. At 0.1 molar AOT, as the pyridine-N-oxide/micelle ratio increases from 0.025 to 0.125 (corresponding to pyridine-N-oxide concentrations of .0001 M and .0005M, 0

m a x

0

0

!η3

0

0

0

max

Q

max

0


148


[P-OJ=.0001 M


—

cyclohexane

280

270

MeOH

260 10
surfactant effects on phase behavior and polarity Below 300 bar, the cohesive energy density of ethane is too low for the formation of AOT reverse micelles which would be sufficiently large to solubilize large amounts of waterQl). Again, octane is the optimal solvent for water solubilization. We have blended the cosolvent octane with ethane to attempt to overcome this limitation. We have also used an amphiphilic cosolvent, octanol, which will be called a co-surfactant. The length of octanol is consistent with AOTs preference for an alkane carbon number of 8. It can intermingle with AOT in the interfacial region and stabilize larger aggregates. The influence of octane on solubilization of water and solvatochromic polarity is pronounced as shown in Table ΠΙ. Two combinations of octane concentration and W were investigated. In both cases, large amounts of water are solubilized at low pressures. The polarities do not change with an increase in pressure as they are already comparable with that of bulk water. At these high concentrations of octane, the fluid is much less compressible than ethane. In the future, we will explore lower concentrations of octane to determine if pressure could be used to tune the polarity in the one phase region. 0


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SUPERCRITICAL FLUID S C I E N C E A N D T E C H N O L O G Y


Table ΙΠ. Addition of octane to SCF ethane to swell reverse micelles of AOT with water at modest pressures at 35 C

Ρ (bar)

(nm)

3

300

260

1.2

20

42

257

2.2

10

41

254

[octane] M

Wo

0

[AOT] = 0.084 M , [pyridine oxide] = 0.0002 M

The influence of octanol is shown in Table IV for a constant AOT concentration of 0.01 M. Pressures are indicated which are required to dissolve the specified amounts of water so that the systems become one phase. The existence of multiple phases is common for these systems, particularly at lower pressures. For 0.5 M octanol, and W = 3, four phases are present at 52 bar. Q

Table IV. Addition of octanol to SCF ethane to swellreversemicelles with water at modest pressures at 35 C

[octanol] M

Wo

Ρ (bar)

^max (nm)

0.

3

300

260

0.01

11

345

255

0.5

19

117

262

[AOT] = 0.01 M , [pyridine oxide] = 0.0002 M


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The addition of 0.01 M octanol increases significantly the solubilization of waterfroma W of 3 to 11. The polarity increases to approach the pure water value of 254 nm, but it was not adjustable with pressure in the one phase region. As the octanol concentration is increased by a factor of 50, a large amount of water is solubilized at only 117 bar. Here the ratio [octanol]/[AOT] = 50 and [octanol]/[water] = 2.5. Compare this case with the one above it. Even though a larger amount of water is solubilized, the polarity has shifted significantly towards that of pure octanol where X x is 266 nm. Similar behavior was obtained in liquid octanol, as the polarity varied only slightly for a water concentration of 0 to 0.4 M. Based on these results, we explored the possibility of using octanol as an amphophilic cosolvent without the need for AOT. With the addition of octanol at a concentration of 0.5 M , it becomes possible to solubilize water at 0.05 M at only 80 bar. The Xmax is close to that of pure octanol, so that it is unlikely that water pools were formed. Hie cosolvent octanol could play an important role in SCF technology by increasing the solubilization of water. 0


ma

Use of AOT and octanol to solubilize hydrophilic substances Hydroquinone The ability of AOT/co-surfactant to solubilize non-ionic hydrophiles in ethane was tested using l,4-dihydroxybenzene(hydroquinone, HQ). To improve aggregation, octane and octanol were utilized based on the above results. Before measuring solubilities, phase boundaries were determined using the variable volume view cell. Octanol was more effective than octane for forming a one-phase system (not counting the solid phase). For W = 10, the system became one-phase at 145 bar for 3.6 mole % octanol and at 195 bar for 6.2 mole % (AOT = 0.62 mole %). At low concentrations, the octanol acts as a co-surfactant and favors water solubilization, but it causes a new liquid phase to form at high concentrations. In cases where only a single fluid phase is present (in addition to the solid), its appearance usually did not change with pressure, but an interesting effect was observed in the ethane - 0.6% AOT - 6.0% octanol system with W = 10. At high pressures well removedfromthe dew point, the fluid was clear. As pressure was lowered, the fluid very slowly became gold in color. Within twenty to thirty bar of the dew point, the intensity of the color increased rapidly, and at the dew point, the fluid was practically opaque. Possible explanations include light scattering due to coalescence of micelles(18). or to critical phenomena. The solubility of HQ is shown in Table V for various octanol concentrations at a pressure of 300 bar. For all of the data listed, there is only one fluid phase in equilibrium with the solid. In this solid-fluid region, solubilities did not change significantly with pressure. Without any AOT present, octanol is an excellent cosolvent. At a concentration of 1 M , it raises the solubility by over two orders of magnitude to reach 17 mM(about 1 wt%). At this octanol concentration, the 0

0


160


addition of 0.62 mole % AOT raises the solubility by a factor of 1.4 with no water added and by 2.2 with a W of 10. For HQ, high solubilities can be achieved using octanol without AOT, but in the next section a system will be discussed where both AOT as well as octanol are required for solubilization. 0

Table V. Effect of AOT and octanol on the solubility of hydroquinone (in mM) in SCF ethane at 35 C and 300 bar


[Octanol]

0%AOT

0.62% AOT* (approx. 0.1M)

-

(mole %)

(M)

0

0

Supercritical Fluid Science and Technology - American Chemical

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