CO
2
-Soluble Oxygenated Hydrocarbon Ionic Surfactants
A R T I C L E S
surfactants has impeded their use in commercial applications.
Less expensive, biodegradable, CO2-soluble ionic surfactants
capable of solubilizing water in the cores of reverse micelles
could hasten the application of technologies that exploit water-
(vinyl acetate) and the acetylated sugars, the high degree of CO2
solubility has been attributed to a favorable two-point interaction
between CO2 and the accessible acetate side chain, a Lewis
acid-Lewis base interaction between the C of the CO2 and the
O of the acetate carbonyl, and a weak, complimentary hydrogen
bond between the O of the CO2 and a proton on the methyl
in-CO2 (w/c) microemulsions.
Recently, Eastoe and Johnston1
4-16
described two branched
2
8,29
hydrocarbon-based ionic surfactants, sodium bis(2,4,4-trimethyl-
-pentyl) sulfosuccinate and sodium bis(3,5,5-trimethyl-1-hexyl)
group of the acetate.
Recently, Stone and Johnston found
1
that the interaction between CO2 and CH2 is about the same as
3
0
sulfosuccinate, that exhibit CO2 solubility. These twin-tailed
sodium succinates are similar in structure to the CO2-insoluble
surfactant AOT, sodium bis(2-ethyl-1-hexyl) sulfosuccinate, but
they contain trimethylpentyl or trimethylhexyl tails and are
referred to as AOT-TMP and AOT-TMH, respectively. After
extensive mixing, both AOT-TMP and AOT-TMH were shown
to be slightly soluble in CO2. This favored solvation of the
branched tail surfactants by CO2 may be attributable to the
surface energy of the pendant methyl groups being much lower
than that of the CH2 groups of linear tails.17 AOT-TMH reported
has 0.1 wt % solubility in CO2 at 40, 50, and 80 °C at 34.5, 31,
and 29 MPa, respectively.16
CO2 and CF2.
A level of 1 wt % surfactant soluble in CO2, which would
typically be needed for microemulsions, requires a moderately
high, yet reasonable, pressure. Clearly, solubility is a key factor
that governs whether a surfactant will lead to water-in-CO2
microemulsions. An additional factor, steric force, which plays
an important role in designing hydrocarbon surfactants for W/C
microemulsions, has been described recently. Stubby tails
enhance the formation of W/C microemulsions, as they raise
surfactant solubility in CO2 by weakening interactions between
tails, weaken interactions between droplets, favor curvature of
the interface bending toward water, and reduce the interfacial
3
0-32
tension.
Ryoo and Johnston achieved about 1 wt % water,
There are several oxygenated hydrocarbon groups that exhibit
more favorable thermodynamic interactions with CO2 than
branched alkanes, however. Acetylated sugars, such as per-
significant protein solubilities, and the presence of microemul-
sions as detected with dynamic light scattering formed by a
methylated branched hydrocarbon nonionic surfactant. Further-
more, this study shows that the surfactant lowers the water-
CO2 interfacial tension significantly, which is an important
1
8
19
20
acetylated glucose and galactose, sorbitol, maltose, and
2
1
cyclodextrins, have been shown to dissolve in CO2 at low
pressures up to 10-50 wt %. Low molecular weight PPO
3
2
1
7
requirement for forming microemulsions.
The objective of this study was to design, synthesize,
(
<2000) is quite CO2-soluble at moderate temperature, and
higher MW PPO (>2000) is also soluble in CO2 at elevated
temperatures;22 therefore, PPO has been used as a CO2-philic
characterize, and evaluate the CO solubility of ionic surfactants
2
with oxygenated hydrocarbon tails composed of acetylated
segment in diblock and triblock nonionic surfactants along with
2
3,24
sugar, PPO, or oligo(vinyl acetate). Additionally, these surfac-
tants were examined for their ability to form stable microemul-
sions with polar microenvironments capable of dissolving polar
hydrophilic blocks of poly(ethylene oxide) (PEO).
The
solubility of the PPO oligomers has been attributed to the Lewis
acid-Lewis base interaction between the ether oxygen in poly-
propylene oxide) and the carbon in CO225 and the lower surface
species in the bulk nonpolar CO solvent. Figure 1 shows the
2
(
structures of ionic surfactants investigated in this study. Ab initio
tension caused by the pendent methyl group on each monomer
unit favoring solvation by CO2.17 The lowering of the interfacial
tension at the water-CO2 interface, emulsion formation, and
quantum mechanical methods were used to complement the
experimental work, adding a molecular-level view of the H2O-
surfactant and CO2-surfactant interactions. Optimized geom-
solubilities of block copolymers containing the PPO segment
1
7,26
2 2
etries for either H O or CO interacting with an isopropyl acetate
were reported.
Poly(vinyl acetate) is the most CO2-soluble,
molecule were computed, which is meant to model the repeat
unit in the oligo(vinyl acetate) system. Similar calculations have
been used by several groups to study the intermolecular
interactions between CO2 and model CO2-soluble com-
high molecular weight, oxygenated hydrocarbon-based ho-
mopolymer that has yet been identified.2
2,27
In the cases of poly-
(
14) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733-8740.
(
15) Eastoe, J.; Paul, A.; Nave, S.; Steytler, D. C.; Robinson, B. H.; Rumsey,
E.; Thorpe, M.; Heenan, R. K. J. Am. Chem. Soc. 2001, 123, 988-989.
16) Johnston, K. P.; Cho, D. M.; DaRocha, S. R. P.; Psathas, P. A.; Ryoo, W.;
Webber, S. E.; Eastoe, J.; Dupont, A.; Steytler, D. C. Langmuir 2001, 17,
18,25,28,33-36
pounds.
(
(
Experimental Section
7
191-7193.
Materials. All materials used to synthesize the ionic surfactants were
purchased form Aldrich and used as received, unless otherwise noted.
17) O’Neill, M. L.; Cao, Q.; Fang, R.; Johnston, K. P.; Wilkinson, S. P.; Smith,
C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37,
3
067-3079.
N
2
2
(99.995%) and CO (99.99%, Coleman grade) were purchased from
(
18) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 7274-7275.
19) Potluri, V. K.; Xu, J. H.; Enick, R. M.; Beckman, E. J.; Hamilton, A. D.
Org. Lett. 2002, 4, 2333-2335.
Penn Oxygen.
(
(
(
(
20) Hong, L.; Thies, M. C.; Enick, R. M. J. Supercrit. Fluids 2005, 34, 11-
(28) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 12590-12599.
(29) Blatchford, M. A.; Raveendran, P.; Wallen, S. L. J. Phys. Chem. A 2003,
107, 10311-10323.
1
6.
21) Potluri, V. K.; Hamilton, A. D.; Karanikas, C. F.; Bane, S. E.; Xu, J. H.;
Beckman, E. J.; Enick, R. M. Fluid Phase Equilib. 2003, 211, 211-217.
22) Shen, Z.; McHugh, M. A.; Xu, J.; Belardi, J.; Kilic, S.; Mesiano, A.; Bane,
S.; Karnikas, C.; Beckman, E. J.; Enick, R. M. Polymer 2003, 44, 1491-
(30) Stone, M. T.; da Rocha, S. R. P.; Rossky, P. J.; Johnston, K. P. J. Phys.
Chem. B 2003, 107, 10185-10192.
(31) Stone, M. T.; Smith, P. G.; da Rocha, S. R. P.; Rossky, P. J.; Johnston, K.
P. J. Phys. Chem. B 2004, 108, 1962-1966.
1
498.
(
23) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165-168.
24) Liu, J. C.; Han, B. X.; Wang, Z. W.; Zhang, J. L.; Li, G. Z.; Yang, G. Y.
Langmuir 2002, 18, 3086-3089.
(32) Ryoo, W.; Webber, S. E.; Johnston, K. P. Ind. Eng. Chem. Res. 2003, 42,
6348-6358.
(
(33) Diep, P.; Jordan, K. D.; Johnson, J. K.; Beckman, E. J. J. Phys. Chem. A
1998, 102, 2231-2236.
(
(
(
25) Kilic, S.; Michalik, S.; Wang, Y.; Johnson, J. K.; Enick, R. M.; Beckman,
E. J. Ind. Eng. Chem. Res. 2003, 42, 6415-6424.
(34) Raveendran, P.; Wallen, S. L. J. Phys. Chem. B 2003, 107, 1473-1477.
(35) Nelson, M. R.; Borkman, R. F. J. Phys. Chem. A 1998, 102, 7860-7863.
(36) Baradie, B.; Shoichet, M. S.; Shen, Z. H.; McHugh, M. A.; Hong, L.; Wang,
Y.; Johnson, J. K.; Beckman, E. J.; Enick, R. M. Macromolecules 2004,
37, 7799-7807.
26) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15,
4
19-428.
27) Rindfleisch, F.; DiNoia, T. P.; McHugh, M. A. J. Phys. Chem. 1996, 100,
5581-15587.
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