9074
`
`J. Phys. Chem. B 1998, 102, 9074—9080
`
`Investigation of Cosolvent Effects on the Solvation of AOT Reverse Micelles in Supercritical
`Ethane
`
`Christopher B. Roberts" and Jason B. Thompson
`
`Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849
`
`Received: May 15, 1998;
`
`In Final Form: September 2, 1998
`
`We present spectroscopic evidence of the preferential solvation and penetration of the cosolvents benzene
`and methylene chloride into the surfactant tail regions of AOT reverse micelles in supercritical ethane. The
`nricropolarities on both sides of the AOT surfactant interface were investigated using the UV—vis absorption
`probes phenol blue and methyl orange. Phenol blue was identified to reside in the surfactant tail region of the
`AOT reverse micelles, and its wavelength of maximum absorption remained sensitive to property changes in
`the ethane continuous phase. Furthermore, the preferential penetration of cosolvents benzene and methylene
`chloride was observed as evidenced by a synergistic effect (enhanced red shift) on the phenol blue absorption
`spectra over the experimental pressure range. Studies with the probe methyl orange, which resides in the
`reverse micelle core, further showed that, while the composition of benzene is enhanced in the surfactant tail
`region, benzene did not penetrate the reverse micelle core.
`
`/
`
`C
`H2
`
`H
`
`2
`
`H2
`/C”3
`\C
`H2
`
`0
`O
`\ /
`
`C A
`
`H
`/ 2
`+ -
`NaoaS'CH\C
`H
`/
`H2
`0/ \O—(:2
`\H C/C\
`\
`H20
`
`2
`
`H2
`c
`fi/ \CHa
`2
`
`\
`
`CH3
`
`<—>
`
`
`
`hydrophilic
`head
`
`lipophilic
`tail
`
`Figure 1. Structure of AOT (sodium bis(2-ethylhexyl) sulfosuccinate)
`surfactant.
`
`separated from the AOT headgroups by what has been referred
`to as the “palisade region” which contains most of the positively
`charged counterions.21 This arrangement permits partitioning
`of ionic and polar molecules between the micelle core and a
`low-polarity medium. The micelle core is characterized by the
`ratio of water-to-surfactant molecules in the solution, W0 (W0
`= [H20]/[surfactant]). In the present paper we report on “dry”
`reverse micelle systems, meaning that no additional water is
`added to the surfactant— solvent system.
`While there has been a tremendous number of studies on
`
`reverse micelles and microemulsions in liquid solvent systems,
`studies of AOT microemulsions in SCFs have only appeared
`in the literature7713>16>22732 since their discovery by Gale et al.
`in 1987.7 The ability to adjust physical properties of SCFs over
`
`Introduction
`
`Supercritical fluids (SCFs) have received much attention in
`the past two decades as solvents for separations,“3 as reaction
`media,4 as a means of fine particle formation}6 and in many
`other applications. This attention stems primarily from the ability
`to dramatically change such bulk physical properties as density,
`solute capacity, diffusivity, and viscosity with small variations
`in temperature and pressure. Unfortunately, many SCF applica-
`tions that involve low critical temperature fluids are limited by
`low solubilities of ionic, highly polar, or high molecular weight
`molecules. An alternative of contemporary interest to increase
`these solubilities involves the formation of so-called water-in-
`
`oil microemulsions or reverse micelles by adding nonionic or
`ionic surfactants,7 such as the anionic surfactant sodium bis(2-
`ethylhexyl) sulfosuccinate (known as AOT). Previous studies
`of reverse micelle systems in SCFs have shown the ability to
`solubilize hydrophilic substances including biomoleculesg’12 and
`dyes,7>13’16 opening the door to many new applications.17
`Figure 1 shows the structure of the anionic surfactant sodium
`bis(2—ethylhexyl) sulfosuccinate, or AOT, which readily forms
`reverse micelles and microemulsions of water in alkane solvents.
`
`In addition, AOT also readily forms small reverse micelles in
`the absence of water in a variety of hydrocarbon solvents with
`aggregation numbers between 20 and 3018>19 with a spherical
`to slightly ellipsoidal shape.19>20 These will be referred to herein
`as “dry” reverse micelles.
`AOT reverse micelles are thermodynamically stable ag-
`gregates of the amphiphilic surfactants, resulting in a hydrophilic
`headgroup region with hydrophobic tails that extend into a
`nonpolar continuous phase. A schematic diagram of a fully
`developed AOT reverse micelle in a supercritical fluid is shown
`in Figure 2. This reverse micelle is characterized as having a
`polar core (or “water pool”) which is stabilized by a curved
`monolayer of the surfactant with its hydrophobic tails extending
`into the nonpolar continuous phase. The “water pool” is
`
`* Corresponding author. E-mail crober‘ts@eng.auburn.edu; Fax (334)-
`844-2063.
`
`10.1021/jp982263+ CCC: $15.00 © 1998 American Chemical Society
`Published on Web 10/16/1998
`
`lnnoPharma Exhibit 1107.0001
`
`

`

`AOT Reverse Micelles in Supercritical Ethane
`
`J. Phys. Chem. B, Vol. 102, No. 45, 1998 9075
`
`AOT
`
`olecule
`
`
`
`Ionic
`Headgroup
`
`
`
`
`Hydrocarbon
`
`Tailgroup
`
`
`Water Core (Wo)
`
`
`Palisade Region
`
`
`
`Supercritical
`Alkane
`
`Figure 2. Schematic diagram of the structure of an AOT reverse
`micelle in a supercritical alkane solvent at an arbitrary water content,
`W0 = [water]/[AOT].
`
`a continuum has permitted more detailed investigations into the
`general nature of reverse micelles and microemulsions.
`Gale et al.7 in 1987 reported the first observations of reverse
`micelles and microemulsions in SCFs using as their primary
`experimental method the visual observation of the solubility of
`polar, colored dyes in SCF—AOT—water systems. This initial
`study suggested the ability to create a broad range of organized
`molecular assemblies in dense gas and SCF solvents where the
`readily variable properties of the fluids could be exploited. There
`has been continuous progress toward improving our comprehen-
`sion of these AOT reverse micelle systems in SCFs since this
`first report by employing an assortment of techniques including
`dynamic light scattering, small-angle neutron scattering, FT-
`IR, UV—vis,
`fluorescence, and time-resolved fluorescence
`spectroscopy.7’13>16>21’32 In general, these works have shown
`the ability to produce thermodynamically stable reverse micelles
`for which there is essentially no change in the size and interior
`polarity of AOT reverse micelles with changes in pressure in a
`single-phase system, but that attractive interactions (micelle—
`micelle) may become enhanced at low pressures.
`Solvatochromic Absorption Probes. The size, shape, and
`interior polarity of reverse micelles can be measured with a
`variety of spectroscopic probes, including solvatochromic UV—
`vis absorption probes and dyes. Solvatochromic absorption
`probes are extremely sensitive to their local environment and
`can be used to characterize the various microdomains within a
`
`microemulsion. These probes report this sensitivity through
`changes in the wavelength of maximum absorption, lmax, of
`their absorption band. Due to their ability to report on the
`micropolarities within the aggregate, probes incorporated within
`reverse micelles can be used to deduce meaningful information
`about the distribution of the probe, water, and solvent molecules
`within the aggregate. The present work employs two different
`solvatochromic absorption probes located in different regions
`of dry AOT reverse micelles in SCF ethane.
`For examples, Schelly and co-workers33’35 used methyl
`orange [(CH3)2NC6H4N=NC6H4SO3Na] and QB [l-methyl-S-
`oxyquinolinium betaine) to probe aggregation and solvent
`penetration into aggregates of the nonionic surfactant TX-lOO
`in cyclohexane, benzene, and n-hexane. These hydrophilic
`probes were found to reside in the interior or core of the reverse
`micelles. El-Seoud and co-workers36>37 have used a variety of
`UV—vis indicators, such as thymol blue and malachite green,
`in an attempt to assign pH to reverse micelle water pools. In
`addition to using probes to characterize the core region of
`reverse micelles, certain probes can also be used to deduce
`
`information about the micelle interface. Reichardt’s ET(30)
`probe has been used to study interactions at the surfactant
`interface of reverse micelles38>39 while Fernandez and From-
`
`herz40 have also studied coumarin dyes at the interface of micelle
`systems.
`
`Absorption probes have also been used to gain information
`about reverse micelle systems in supercritical fluid solvents.
`For example, using the absorption probe pyridine N-oxide
`coupled with the fluorescence probe 8-anilino-l-naphthalene-
`sulfonic acid (ANS), Yazdi et al.16 explored the possibility of
`pressure tuning of the polarity in AOT reverse micelle systems
`in SCF ethane and liquid propane in both one- and two-phase
`regions. Furthermore, the hydrophilic probe methyl orange has
`also been used to investigate the microenvironment in penta-
`ethylene glycol n-octyl ether reverse micelles in SCF C02,21
`large (25 nm) poly(fluoroacrylate)-g-poly(ethylene oxide) re-
`verse micelles in C02,41 and PFPE water-in-C02 microemul-
`sions.17 There are many applications (e.g., emulsion polymer-
`ization) that may be more ideally conducted if possibly done
`in the nonflamable, inexpensive, and relatively nontoxic SCF
`C02. However, Consani and Smith42 showed that most con-
`ventional alkyl-functional
`ionic surfactants (such as AOT)
`exhibit poor solubility in C02. Recently,
`investigators have
`designed, synthesized, and studied model surfactants specifically
`designed for use in C0217>41>43’45 as well as the addition of
`cosurfactants to increase water uptake in reverse micelles in
`C02.14 McFann et al.14 used the solvatochromic probes methyl
`orange and l-methyl-8—oxyquinolinium betaine to investigate
`the effect of pressure on aggregation number and the effect of
`surfactant concentration on aggregation mechanism in SCF C02.
`Ionic dyes were also used to study the pressure effect on size
`and structure near the phase boundary. With these developments
`in reverse micelle formation and surfactant solubility in SCFs,
`many unique applications are on the horizon.17
`Cosolvents and Cosurfactants. The addition of small
`
`amounts of cosolvent with a volatility intermediate to that of
`the solute and SCF can increase the solubility of the solute while
`maintaining the favorable properties of the fluid.2>46 The
`cosolvent addition furnishes the ability to tune solvent properties
`for specific applications;46 Cosolvents and cosurfactants have
`also been used in reverse micelle systems to promote aggrega-
`tion, increase surfactant solubilities, and increase water uptake
`in the cores. For instance, medium chain length alcohols, often
`referred to as cosurfactants, have been added to liquid47 and
`SCF14 reverse micelle solutions. Although there is much debate
`in the literature as to the exact reason for the alcohol effect,
`most studies suggest that the alcohol inserts itself between
`surfactant tails, reducing tail—tail and micelle—micelle interac-
`tions. Some very hydrophilic alcohols such as ethanol can
`partition into the water core of liquid reverse micelles.48 The
`effects of added cosolvents and cosurfactants have also been
`
`exploited in SCF systems. In SC ethane/AOT systems, low
`concentrations of the cosurfactant octanol have resulted in
`
`dramatic increases in W0sat at reduced pressures11 while other
`cosolvents such as octane have also been used to achieve
`
`pressure reductions in SCF systems.21
`In some work very pertinent to this paper, Zhu and Shelly33
`reported a study of the microenvironment and water distribution
`in reverse micelles of the nonionic surfactant Triton X-100 in
`
`mixed liquid solutions of 30% (v/v) benzene and 70% (v/v)
`n-hexane using the absorption probes methyl orange (M0) and
`l-methyl-oxyquinolinium (QB). The results suggested the
`preferential penetration of benzene into the dry Triton X-100
`
`InnoPharma Exhibit 1107.0002
`
`

`

`9076 J. Phys. Chem. B, Vol. 102, No. 45, 1998
`
`Roberts and Thompson
`
`reverse micelle cores, leaving the bulk benzene/n-hexane solvent
`depleted of benzene.
`In this study, we focus on the effects that added cosolvents,
`such as benzene and methylene chloride, have on the solvation
`of “dry” AOT reverse micelles and, in particular, the tail regions
`of these reverse micelles in SCF ethane. Since the AOT
`
`surfactant system has received the most attention in SCFs to
`date, we have chosen it for our investigations concerning the
`effects of added cosolvents on reverse micelle tail solvation.
`
`By using UV—vis absorption probes that reside in different areas
`of the micelle structure (core, palisade, tails, etc.), we can report
`on the preferential solvation effects on the micelle tails by the
`cosolvents and on the integrity of the micelles under these
`conditions. The present study was designed, in part, to explore
`short-range interactions between solvent and the surfactant tails
`and cosolvent and surfactant tails and to explore the possibility
`of penetration of the cosolvents into the micelle core. Specif-
`ically, the probe phenol blue was used to explore the solvation
`of the micelle tail region and the effects of preferential
`penetration of the tails by cosolvents such as benzene and
`methylene chloride. The probe methyl orange was also used to
`explore the micelle core region and to examine micelle integrity
`and degree of cosolvent penetration.
`
`Experimental Section
`
`Materials. The anionic surfactant sodium bis(2-ethylhexyl)
`sulfosuccinate (AOT) was purchased from Fluka (Purum grade,
`>98%) and used as received. The surfactant retains a small
`amount of residual water, and this “as-received” condition for
`the AOT is designated as WO = 1. Care was taken to ensure
`that the AOT received from the supplier absorbed no additional
`water and was verified using baseline spectra of the absorption
`probes described within. Benzene and methylene chloride were
`obtained from Aldrich and used as received. CP grade ethane
`(To = 32.2 0C, PC = 48.8 bar) was purchased from BOC Gases.
`The probes phenol blue (N,N-dimethylindoaniline) and methyl
`orange were obtained from Eastman and Aldrich, respectively,
`and used without further purification.
`Spectroscopic Investigations. The UV—vis spectroscopic
`studies were conducted in a custom 128 mL constant volume
`
`optical cell with a fixed path length of 7 cm mounted on an
`adjustable stage to provide precise alignment and reproducible
`spectra. The stainless steel cell (measuring 2 in. i.d. by 4.5 in.
`deep and 27/8 in. 0d. by 5.5 in. high) was fitted with 0.75 in.
`diameter by 0.3 in. thick UV grade windows. The large path
`length resulted in a measurable absorbance with only a minimal
`amount of probe in the cell (<1 x 10’6 M), thus perturbing
`the micelle system as little as possible. Reverse micelle solutions
`of known concentration were prepared by loading AOT and
`the appropriate solvatochromic probe directly to the cell. The
`cell was then sealed and pressurized with a 266 mL Isco 260D
`syringe pump. For experiments with ethane—cosolvent mixtures,
`a desired amount of ethane was first condensed into the Isco
`
`syringe pump. An Eldex model B-100-S metering pump was
`then used to charge a known quantity of cosolvent. The mixture
`was allowed to equilibrate for at least 16 h and then charged to
`the optical cell. Spectroscopic experiments were always per-
`formed in the order of increasing pressure such that the molar
`concentration of the AOT remained constant. Pressure was
`
`measured to ::0.2 bar with a digital Heise pressure gauge. A
`heating tape, platinum RTD, and an Omega temperature
`controller were used to maintain the cell temperature to within
`
`::0.1 0C. The solution was agitated directly in the sample
`compartment of the UV with a 0.5 in. long magnetic stir bar
`and a micro-electromagnetic stirrer.
`
` solvent 1m
`
`TABLE 1: Wavelength 0f Absorption Maximum (1m) 0f
`
`Phenol Blue in Various Systems
`
`cyclohexane
`5 50
`benzene
`575
`methanol
`608
`
`Absorption spectra were measured with a Varian Cary 3E
`UV—vis spectrophotometer. Absorption spectra were obtained
`by averaging a minimum of 10 scans and by subtracting an
`average baseline spectrum collected at the same conditions in
`the absence of the probe. The wavelength of maximum
`absorption, lmax, was determined by fitting the spectra with a
`
`polynomial. Typical uncertainty in lmax was ::0.2 nm. For
`experiments in which a SCF phase was in equilibrium with a
`surfactant-rich phase, the beam of the spectrophotometer passed
`through the fluid phase alone and did not contact the surfactant-
`rich phase on the bottom of the cell.
`
`Results and Discussion
`
`A solvatochromic probe, when incorporated within a reverse
`micelle, can yield structural information about the micelle and
`its environment. These probes can be extremely sensitive to their
`immediate environment, and it is necessary to obtain specific
`information about a probe in order to properly interpret data
`from these solvatochromic probes. It is important to verify that
`the probe does not perturb the system being studied, to determine
`the location of the probe within the reverse micelle aggregate,
`and to determine the specific features of its surroundings the
`probe is sensitive to.21
`Determination of the actual location of the probe within the
`surfactant aggregate is often very difficult. In the context of
`our simplified model of an AOT reverse micelle given in Figure
`2, a probe can be located in the water pool, in the palisade
`region, or among the surfactant tails. Many probes contain both
`polar and nonpolar moieties and are, therefore, attracted to some
`degree to the surfactant interface.
`Phenol blue proved to be a suitable solvatochromic dye to
`study solvent—tail and cosolvent—tail interactions because the
`probe resides in the surfactant tail regions of fully developed
`AOT reverse micelles, as illustrated below. Similar behavior
`was observed for the ET(30) probe in liquid reverse micelle
`solutions.38>39 Phenol blue has a single intense absorption band
`at 8550 nm in cyclohexane, corresponding to a yr —> 71*
`transition. The red shift of the absorption maximum (seen for
`several example solvents in Table 1) exceeds 55 nm in going
`from a nonpolar to a polar protic solvent,
`illustrating its
`sensitivity.
`Figure 3 illustrates the change in lmax for phenol blue in pure
`ethane and in a 0.01 M AOT—ethane mixture at various
`
`pressures at 37 0C. From 61 to 100 bar, lmax in the 0.01 M
`AOT—ethane mixture (upper curve) is shifted relatively little
`compared to its values in pure ethane seen as the lower curve.
`This suggests that the fluid phase contains premicellar aggregates
`such as are seen at low surfactant levels in liquid solvents.
`Above 100 bar, phenol blue senses an environment of increasing
`polarity in the 0.01 M AOT—ethane mixture, and lmax increases
`more dramatically above the pure ethane baseline until, at 148
`bar, all of the loaded AOT is solubilized and the system becomes
`one phase. In this region the concentration of AOT is sufficient
`for reverse micelles to form, and solubilization increases rapidly.
`This transition at 148 bar correlates well with visual observations
`
`of AOT cloud points performed in our laboratory/‘9 Notice that
`
`InnoPharma Exhibit 1107.0003
`
`

`

`AOT Reverse Micelles in Supercritical Ethane
`
`J. Phys. Chem. B, Vol. 102, No. 45, 1998 9077
`
`532
`
`530
`
`528
`
`526
`
`l0.01MAOT/Ethane 524
`
`
`
`
`
`
`
`
`
`AM“(nm)
`
`50
`
`100
`
`150
`
`200
`
`250
`
`Pressure (bar)
`
`Figure 3. Wavelength of maximum absorption, Arm, of phenol blue
`probe in pure ethane (A) and in a 0.01 M AOT—ethane mixture (I) as
`a function of pressure. T = 37 °C.
`
`above 148 bar, although phenol blue senses an environment of
`increased polarity in the system with reverse micelles (upper
`curve),
`it remains sensitive to the solvent strength of the
`continuous phase. In this region, the lmax curve for phenol blue
`in the 0.01 M AOT—ethane mixture follows the same general
`trend as that of phenol blue in pure ethane, with an increase in
`lmax with an increase in pressure. Previous studies with
`solvatochromic probes have described the polarity of the interior
`of dry reverse micelles as methanol-like.15 Phenol blue has an
`absorption maximum of 608 nm in methanol whereas its
`maximum absorption in this 0.01 M AOT reverse micelle system
`is slightly greater than 530 nm. This suggests that the phenol
`blue resides in a more hydrocarbon-like environment. For this
`reason and the fact that lmax remains sensitive to the bulk solvent
`strength, it is very unlikely that phenol blue is localized in the
`reverse micelle interior. Probes that are localized in the micelle
`
`interior, such as pyridine N-oxide,16 are virtually insensitive to
`changes in bulk environment once the AOT reverse micelles
`are formed. A more probable explanation of the phenol blue
`experimental data in Figure 3 would be localization of the probe
`in the surfactant tail region. This model explains both the
`increased polarity “felt” by the probe as it interacts with the
`reverse micelle aggregate compared to the pure solvent, as well
`as its continuing sensitivity to the changing solvent strength of
`the continuous phase as pressure is increased.
`Having identified a solvatochromic probe localized on the
`solvent side of the micelle interface, this probe can be used to
`directly investigate the effect of added cosolvents on micellar
`phenomena in supercritical fluids. Benzene was chosen as one
`of the cosolvents in this study for a number of reasons. Previous
`work indicated that, for at least several surfactant systems,
`benzene would interact with the AOT surfactant headgroup
`region. Using laser photolysis, Atik and Thomas50 studied
`reverse micelle systems of AOT and showed that various
`additives affect the rate of ion transport that occurs when reverse
`micelles collide. Benzene was found to decrease the effective-
`
`ness of solute penetration, suggesting that it interacts in the
`surfactant headgroup region, producing a more compact structure
`with a more rigid interface. A more recent study by Zhu and
`Schelly33 described the preferential penetration of benzene into
`the cores of reverse micelles of the nonionic surfactant TX-
`
`100, leaving the bulk benzene/n—hexane solvent depleted of
`benzene. Based on these previous studies and given the
`enhanced local environment that can occur in supercritical fluid
`solutions,51 a cosolvent effect by adding benzene to the AOT—
`ethane systems could be anticipated.
`
`533
`
`531
`
`o 0.01MAOT/2.5% Benzene / Ethane
`
`I 0.01 M AOT/ Ethane
`O 2.5% Benzene / Ethane
`
`110
`
`130
`
`150
`
`170
`
`190
`
`210
`
`230
`
`Pressure (bar)
`
`Figure 4. Wavelength of maximum absorption, Arm, of phenol blue
`probe in pure ethane (A) and in a 0.01 M AOT—ethane mixture (I),
`in 2.5 mol % benzene—ethane mixture (0), and in 0.01 M AOT—2.5
`mol % benzene—ethane mixture (0). T = 37 °C.
`
`The dramatic effects of adding 2.5 mol % benzene to a 0.01
`M AOT—ethane system on the lmax of phenol blue are seen in
`Figure 4. As discussed earlier, cosolvents can be used to achieve
`pressure reductions in SCF systems. The addition of 2.5 mol
`% benzene to the 0.01 M AOT—ethane system lowers the cloud
`point to 120 bar, such that only the one phase is shown in Figure
`4 for pressures between 120 and 230 bar. The upper trace does
`indeed show the same general shape as that of the 0.01 M AOT
`in pure ethane curve with the cloud point shifted to 120 bar.
`As in Figure 3, the lowest curve in Figure 4 depicts the behavior
`of lmax of phenol blue in pure ethane. As pressure is increased,
`the solvent strength of the ethane increases and is reflected by
`an increase in the lmax for phenol blue. Addition of 2.5 mol %
`benzene to the ethane solvent further enhances the solvent
`
`strength of the medium and essentially shifts the lmax of phenol
`blue in pure ethane to higher wavelengths of maximum
`absorbance. Interestingly, the environment sensed by phenol blue
`in the surfactant tail groups of a 0.01 M AOT system in ethane
`roughly corresponds to that of a 2.5 mol % benzene—ethane
`mixture, giving some indication of the solvent strength “felt”
`by the probe in the AOT reverse micelles. Addition of 2.5 mol
`% benzene to a 0.01 M AOT—ethane system results in lmax
`values of phenol blue that are yet higher than those in either
`the 0.01 M AOT—ethane system (when the probe is in the
`reverse micelle tails) or the 2.5 mol % benzene—ethane system.
`This is interesting in that solvent strength, or micropolarity, is
`not additive. As an example, one would not expect the probe
`to report a higher lmax in a mixture of a polar and a nonpolar
`solvent than in either of the pure solvents. The synergistic effect
`obtained by adding benzene to the fully developed micelles in
`the AOT—ethane system can be explained by preferential
`penetration of benzene into the surfactant tail groups,
`thus
`resulting in an increased micropolarity.33 Similar behavior was
`observed for other benzene compositions, although the effect
`is not as dramatic. These results might suggest an optimal range
`in benzene composition for this preferential solvation effect.
`This synergistic effect is also observed at other benzene
`cosolvent concentrations. Figure 5 shows the effect of the
`addition of 1.5 mol % benzene to the 0.01 M AOT—ethane
`
`system. Again, an enhanced lmax for phenol blue is observed
`when 1.5 mol % benzene is added to the 0.01 M AOT—ethane
`
`system compared to the 0.01 M AOT—ethane system and to
`
`InnoPharma Exhibit 1107.0004
`
`

`

`9078 J. Phys. Chem. B, Vol. 102, No. 45, 1998
`
`Roberts and Thompson
`
`533
`
`531
`
`
`
`A Ethane
`I 0.01 M AOT/ Ethane
`o 1.5% Benzene/Ethane
`o 0.01M MDT/1.5% Benzene / Ethane
`
`110
`
`130
`
`150
`
`170
`
`190
`
`210
`
`230
`
`Pressure (bar)
`
`Figure 5. Wavelength of maximum absorption, Arm, of phenol blue
`probe in pure ethane (A) and in a 0.01 M AOT—ethane mixture (I),
`in 1.5 mol % benzene—ethane mixture (0), and in 0.01 M ACT—1.5
`mol % benzene—ethane mixture (0). T = 37 °C.
`
`the 1.5 mol % benzene—ethane system. This again suggests the
`preferential solvation of the reverse micelle tails by benzene
`cosolvent.
`
`To examine the degree of this effect, a plot of lmax of phenol
`blue as a function of benzene composition in ethane—benzene
`mixtures with and without the addition of 0.01 M AOT is shown
`
`in Figure 6. The measurements shown in Figure 6 were
`performed at 37 °C and 159 bar. At this temperature and
`pressure, the lmax of phenol blue in pure ethane is 527.2 nm
`while lmax in the 0.01 M AOT—pure ethane system is 529.8
`nm. This corresponds to a 2.6 nm red shift of phenol blue’s
`lmax upon the formation of the AOT reverse micelles with the
`probe located in the micelle tail region. The corresponding red
`shift in lmax upon the formation of AOT reverse micelles in
`pure benzene is 1.2 nm. Interestingly, in a 2.5 mol % benzene—
`ethane mixture, there is only a 1.4 nm red shift of lmax with the
`addition of 0.01 M AOT to the mixed solvent. This is rapidly
`approaching the pure benzene limit of a 1.2 nm shift with as
`little as 2.5 mol % benzene addition. This suggests that even
`
`with this small loading of benzene to the 0.01 M AOT—ethane
`system, the probe senses an enhanced benzene environment,
`potentially approaching near saturation in the tail region with
`benzene cosolvent. If the ethane—benzene mixtures were to
`
`behave ideally, meaning no preferential solvation effects oc-
`curred, one would expect the lmax of phenol blue to increase in
`the 0.01 M AOT—ethane—benzene mixtures with the same slope
`as the ethane—benzene curve in Figure 6. However, the increase
`of lmax with added benzene in the reverse micelle system (upper
`curve) is less steep than simply in the mixed solvent system
`(lower curve), and the separation of the two curves rapidly
`approaches the 1.2 nm limit at
`low (<4 mol %) benzene
`compositions.
`The cosolvent methylene chloride was also explored in order
`to examine the generality of the benzene cosolvent experiments.
`Figure 7 shows the effect on lmax of the addition of 1.6 mol %
`methylene chloride to pure ethane and to the 0.01 M AOT—
`ethane system. Methylene chloride was chosen as a cosolvent
`as it is a relatively small, polar molecule for which an enhanced
`penetration into the AOT reverse micelle tails might be
`anticipated. Once again, Figure 7 plots lmax of phenol blue as
`a function of pressure in pure ethane and in the 0.01 M AOT—
`ethane system. The addition of 1.6 mol % methylene chloride
`to the ethane solvent again further enhances the solvent strength
`of the medium and shifts the lmax of phenol blue to higher
`wavelengths. In this case, the environment sensed by phenol
`blue in the surfactant tail groups of a 0.01 M AOT reverse
`micelles in pure ethane system roughly corresponds to that in
`a 1.6 mol % methylene chloride—ethane solvent mixture.
`Furthermore, the addition of 1.6 mol % methylene chloride to
`the 0.01 M AOT—ethane system again results in lmax values
`that are further red-shifted than in either the 0.01 M AOT—
`
`ethane system or the 1.6 mol % methylene chloride—ethane
`solvent mixture. This can be explained by the preferential
`solvation and penetration of the polar cosolvent into the tail
`groups of the fully developed AOT reverse micelles, resulting
`in an increased micropolarity due to the locally increased
`composition of the cosolvent. By comparison, similar synergistic
`shifts are observed with the addition of 1.6 mol % methylene
`chloride and with the addition of 2.5 mol % benzene (Figures
`4 and 7). This more dramatic effect with the methylene chloride
`
`AOT/ Benzene
`
`533
`
`532
`
`531
`
`E
`5 530
`
`E
`(<
`
`529
`
`528
`
`527
`
`
`1.2 nm
`
`
`%
`
`Benzene
`
`
`
`
`=573.7nm
`(kmax)benzene
`
`
`
`Aer/Ethane/aenzenefi ‘
`‘
`A
`
`A
`
`.
`
`
`3
`. Q
`
`. Ethane/Benzene
`2.6 nm
`
`T = 37.0°C
`
`
`P = 159 bar
`o
`
`[AOT] = 0.01 M
`
`0
`1
`2
`
`Figure 6. Effect of benzene addition on the wavelength of maximum absorption, Arm, of phenol blue in ethane—benzene mixtures (O) and in 0.01
`M AOT—benzene—ethane mixtures (A). T = 37 °C and P = 159 bar.
`
`Mole % Benzene
`
`InnoPharma Exhibit 1107.0005
`
`

`

`AOT Reverse Micelles in Supercritical Ethane
`
`J. Phys. Chem. B, Vol. 102, No. 45, 1998 9079
`
`and potentially to the interface to some degree, the benzene
`does not penetrate the barrier provided by the more rigid
`headgroup structure of the AOT reverse micelles in SCF ethane.
`
`Conclusions
`
`An appropriate solvatochromic probe has been identified that
`resides in AOT reverse micelle tails in SCF ethane. The
`
`preferential penetration of the cosolvents benzene and methylene
`chloride into the AOT reverse micelle tails as evidenced by a
`synergistic effect on phenol blue absorption spectra was
`observed. Furthermore, investigations with the probe methyl
`orange, which resides in the core of the reverse micelle, have
`shown that while the benzene clusters around the reverse micelle
`
`and preferentially penetrates the tails, it does not penetrate into
`the reverse micelle core. These results illustrate the ability to
`use cosolvents to preferentially solvate reverse micelles in SCF/
`cosolvent systems.
`
`Acknowledgment. This work is based in part upon work
`supported by the National Science Foundation EPSCoR Program
`(Grant OSR-9550480) and the Auburn University Grant-in—Aid
`Program. Their support is gratefully acknowledged. We also
`thank J. Todd Reaves and Sean McGinnis for assistance in the
`
`laboratory .
`
`References and Notes
`
`(1) McHugh, M. A.; Krukonis, V. J. alpercritical Fluid Extraction,
`2nd ed.; Butterworth Heinemann: Boston, MA, 1994.
`(2) Brennecke, J. F.; Eckert, C. A. AIChE J. 1989, 35, 1409.
`(3) NATO Advanced Sudy Institute Sipercritical Fluids—Fundar‘rmtals
`for Application; Kiran, E., Ed.; Kluwer Academic Publishers: Dordrecht,
`The Netherlands, 1994; Vol. 273.
`(4) Savage, P. E.; Gopalan, S.; Mizan, T. 1.; Martino, C. J.; Brock, E.
`E. AIChE J. 1995, 41, 1723.
`(5) Debenedetti, P. G. AIChE J. 1990, 36, 1289.
`(6) Dixon, D. J.; Bodmeier, R. A.; Johnston, K. P. AIChE J. 1993, 39,
`127. Dixon, D. J.; Johnston, K. P. J. Appl. Polym. Sci. 1993, 50, 1929.
`(7) Gale, R. S.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987,
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`(8) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J. Phys Chem.
`1990, 94, 781.
`(9) Lemert, R. M.; Fuller, R. A.; Johnston, K. P. J. Phys Chem. 1990,
`94, 6021.
`(10) Beckman, E. J.; Smith, R. D. J. Phys Chem. 1990, 94, 345.
`(11) Johnston, K. P.; McFann, G. J.; Lemert, R. M. SJpercritical Fluid
`Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS
`Symp. Ser., No. 406; American Chemical Society: Washington, DC, 1989.
`(12) McFann, G. J.; Johnston, K. P. J. Phys Chem. 1991, 95, 4889.
`(13) Fulton, J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J. Phys Chem.
`1989, 93, 4198.
`(14) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40,
`543.
`(15) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys Chem. 1991,
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`(16) Yazdi, P.; McFann, G. J.; Fox, M. A.; Johnston, K. P. J. Phys
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`(17) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.;
`Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Sc