(164a) Stabilization of Water-in-CO2 (W/C) Microemulsions with Hydrocarbon Cosurfactant

Stabilization of
water-in-CO₂ (W/C) microemulsions with hydrocarbon cosurfactant

Lei Bao, Yang Chen,
Dongdong Hu, Ling Zhao, Weikang Yuan, Tao Liu^⁎

Shanghai Key
Laboratory of Multiphase Materials Chemical Engineering, East China University
of Science and Technology, Shanghai 200237, PR China

liutao@ecust.edu.cn;
Phone: (+86) 21 64253470; Fax: (+86) 21 64253528

ABSTRACT

Supercritical
carbon dioxide is a green solvent. Unfortunately, its low polarizability per
volume and low dielectric constant leads to poor solubility of macromolecular
and polar substance in it. W/C microemulsions have been widely accepted as a promising
alternative way to overcome these disadvantages. The W/C microemulsions can be
used for nanoparticles synthesis, extraction, and catalytic reaction due to its
nanoscale micelle diameters and high mass transfer coefficient.^1-3
The surfactant is essential in stabilization of W/C microemulsions. Some
fluorinated surfactants were efficient to form W/C microemulsions, but they
were toxic and expensive.^4,5 Hydrocarbon surfactants were
environmental friendly and had a low cost. However, only a few commercial
nonfluorinated surfactants can stabilize water in supercritical CO₂
under high pressure. Cosurfactants usually were used to assist surfactant to
form microemulsions. Therefore, It was an efficient way to lower the fluorine
content by using some hydrocarbon cosurfactant to help fluorinated surfactants
to form W/C microemulsions.

Two fluorinated AOT
based surfactants, di-CF2 and di-CF4, and three hydrocarbon cosurfactants with ether,
ester and alkane tails were synthesized. The fluorinated AOT based surfactants
assisted by hydrocarbon cosurfactants stabilized methyl orange solution of low
centration in CO₂ successfully. The microemulsions were characterized
by UV−visible absorption spectral measurements and the phase behavior was
analyzed by cloud point measurement. It was shown that the cosurfactant with
ester tails can stabilize much more water in CO₂ than others. The self-assembly
structure of C/W microemulsion with fluorinated AOT based surfactants and hydrocarbon
cosurfactants were simulated by molecular dynamics. The slab structure was used
to characterize the interfacial properties, including the density distribution,
interfacial tension, the average orientation of the surfactant and the transport
properties. The molecular dynamic results showed that the cosurfactant with
ester tails were more CO₂-phlic than those with ether and alkane
tails. The interfacial tension for cosurfactant with ester tails was lower than
others. The simulation results agreed well with the experiment results. In
conclusion, both simulation and experiment results indicated that the hydrocarbon
cosurfactants with ester tails can assist fluorinated AOT based surfactants to
form W/C microemulsions with a high water content. It may help to design some
efficient hydrocarbon cosurfactant in order to lower the fluorine content in
the W/C microemulsion system.

References

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2.Yu W, Zhou D, Yin J, Gao J. Selective solubilization
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3.Du Z, An X, Fan D, Shen W. Calorimetric
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4.Sagisaka M, Ogiwara S, Ono S, et al. New Class of
Amphiphiles Designed for Use in Water-in-Supercritical CO₂
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Fluorocarbon and Hydrocarbon Chain Lengths in Hybrid Surfactants for
Supercritical CO₂. Langmuir. 2015;31(27):7479-7487.

(a)                  
(b)                    
(c)

Figure 1.
stabilization of water in supercritical CO₂.(a) W/C microemulsions
with methyl orange; (b) W/C microemulsions without methyl orange; (c) cloud
point of W/C emulsions.

Figure
2.UV-VIS spectra of W/C mcroemulsions stabilized by di-CF4 and hydrocarbon
cosurfactants with ester tails under 15.0 MPa at 35 ^oC

Figure
3. The self-assembly structure of C/W microemulsion stabilized by di-CF4