Supercritical Carbon Dioxide - American Chemical Society

Chapter 19

Design and Performance of Surfactants for Carbon Dioxide 1,*

1

1

2,*

Julian Eastoe , Audrey Dupont , Alison Paul , David C. Steytler , and Emily Rumsey 2

Downloaded by FUDAN UNIV on March 5, 2017 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch019

1

School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom

2

Phase stability and aggregation structures of various CO -active surfactants are reported. Evidence for aggregation in carbon dioxide has been obtained using High-pressure Small-Angle Neutron Scattering (SANS). Custom synthesized double-fluorocarbon chain compounds are shown to be effective at stabilizing water-in-CO2 microemulsion droplets. However, branched, methylated hydrocarbon surfactants tend to form reversed micelles, which are alone ineffective at dispersing water. 2

© 2003 American Chemical Society

Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

285

286 Over the past years, concerns have been raised, as the emission of volatile organic compounds (VOC's) and effluent waste streams represent an environmental threat. Fortunately, C 0 is one of the few solvents that is not regulated as a V O C by the US Environmental Protection Agency (EPA). Recently, effort has been made to promote the use of supercritical C 0 (sc-C0 ) and to develop suitable formulations for a whole range of industrial processes. For these applications, C 0 has a number of attractive properties: low cost, environmental friendliness, it is non-flammable and non-toxic for food and pharmaceutical grade uses. Furthermore, as a supercritical fluid, its critical pressure and temperature (respectively 73.8 bar and 31.1 °C) are readily accessible so that solvent quality can be fine-tuned by T-P variation. Therefore .sc-C0 offers great opportunities for applications in a variety of domains including extractions (1) and polymer processing (2). More recent applications include the production of silver nanoparticles (3-6), the preparation of nanocrystals (7) and of carbon nanotubes (8). C0 -active surfactants have been developed to boost solubility levels through formation of micelles and water-in-C0 (w/c) microemulsions. Preliminary studies by Consani and Smith (9) showed that most conventional surfactants are insoluble in C 0 . They studied over 130 commercially available surfactants, among which only a few nonionics exhibited reasonable solubility in C 0 . On the other hand (for sound reasons (10, 11)), certain fluorocarbon (12, 13), and to a lesser extent silicone surfactants (14-17) are known to dissolve in C0 . However, such compounds often require specialized synthesis and therefore remain expensive. Although supercritical conditions are useful in fractionation processes, the cost and engineering requirement for scale-up of practical applications of C 0 are much simplified when liquid C 0 is used at its vapor pressure. The ability to stabilize microemulsions under this condition is therefore an important feature of surfactant design. Being mindful of the high costs, and environmental issues relating to fluoro-surfactants, considerable efforts are also being made to design C0 -active hydrocarbons. This article provides a review of relevant literature (§1), a summary of the behavior of di-fluorocarbon chain sulfosuccinates and phosphates (§2), and finally, section 3 covers aggregation of various hydrocarbon surfactants in C 0 . 2

2

2

2


2

2

2

2

2

2

2

2

2

2

1. C0 -active Surfactants 2

Following the finding by Iezzi et al (10) that fluorocarbons and C 0 are compatible, Hoefling et al. (11-12, 14-15) designed the first effective surfactants for C 0 . These surfactants were made of low cohesive energy density tails; by insertion of groups with low solubility parameters and low polarizability, such as fluoroalkyls (11), fluoroethers (12) and silicones (14, 15). The effect of tail branching was also found to boost solubility and double-tailed surfactants were noted to perform better. To date, amongst others described in section 2 below, 2

2


287 fluoroacrylates (18), fluoroethers (12, 19) and a few fluoroalkyls (11, 20) have been shown to be compatible with C 0 . 2


Low Molecular Weight Fluoroalkyls Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). In 1997, Eastoe et al. launched a study of fluorinated anionic sulfosuccinate surfactants, which are analogous to the hydrocarbon Aerosol-OT (sodium bis-2ethylhexyl sulfosuccinate, AOT) (13,24-26, 27 a). Example molecular structures are shown on Figure 1. Furthermore, fluoroalkylphosphates have been assessed for C0 -activity (27 b, c). A detailed presentation of these systems is given in section 2 below. In 2001, Erkey and co-workers also showed interest in these very promising sulfosuccinate surfactants (28, 29). Small-angle X-ray scattering (SAXS) was used to characterize di-HCF at 0.1 M in C 0 at 27 °C and 345 bar for w between 0 and 20 (29). 2

4

2

Fluoroethers: PFPE-based surfactants In 1993, perfluoropolyether (PFPE) carboxylates, with average molecular weights between 2,500 and 7,500, were reported to be soluble in liquid C 0 (19). However, these high M W polymers were not effective at stabilizing w/c microemulsions. Later, Johnson et al. formed w/c microemulsions with an ammonium carboxylate PFPE ( P F P E - C O O N H ^ surfactant of only 740 M W (30). Success with these surfactants was attributed to the chemical structure itself. PFPE constitutes an extremely C0 -philic tail group, accentuated by the presence of pendant fluoromethyl groups, which tend to increase the volume at the interface on the C 0 side and thus favor curvature around the water. Spectroscopic techniques, such as FTÏR spectroscopy and UY-vis (31), were used to give evidence for formation of reverse micelles with bulk like water in the droplet cores, according to the model proposed by Zinsili (32). Electron paramagnetic resonance (EPR) provided further evidence that PFPE-based surfactants form stable reverse micelles in C 0 , and the presence of microemulsified water (31). More recently, standard N M R ( H and F ) and also 2

2

2

2

l

l9


288

0

NaSQ,

0-(CH2)n-(CF2)m-X

)

A Downloaded by FUDAN UNIV on March 5, 2017 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch019

0

0-(CH2)n-(CF5)m-X

di-HCF4 di-HCF6

n=1 n=1

m=4 m=6

X=H X=H

di-CF3 di-CF4 di-CF6

n=1 n=1 n=1

m=3 m=4 m=6

X=F X=F X=F

di-CF4H di-CF6H

n=2 n=2

m=4 m=6

X=F X=F

0 ^-U-O—CHz

CF2CF2-CF2CF2-H

0

^r~ ~-CH

2

CF2-CF2CF2CF2-H

0 di-HCF4GLU Figure 1. Partially fluorinated sulfosuccinate surfactants for use in C0 . Reproduced with permission from Langmuir, 2002, 18, 3014-3017. Copyright 2002 Am. Chem. Soc. 2


289 rotating frame-imaging (RFI) N M R were used to demonstrate micelle formation in w/c microemulsions with 1.8-wt % PFPE-based surfactant (33). Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et al (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction φ in w/c microemulsions formed with a P F P E - C O O ' N H surfactant ( M W = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO"NH surfactants were also shown to form w/c emulsions consisting of equal amount of C 0 and brine (3840). The use of cationic surfactants would be of practical interest in applications on negatively charged surfaces. Recently a polyperfluoroether trimethylammonium acetate (PFPE-TMAA), with an average M W of 1124, formed w/c microemulsions up to w = 32 (41). The results compared favorably with those for anionic P F P E - C O O N H / (42). +

4

+

4


2

Fluoroacrylates: PFOA Block Copolymers In 1992, the fluorinated acrylate polymer, poly (1,1-dihydroperfluorooctylacrylate) or PFOA, was obtained from homopolymerization in sc-C0 at 59.4 °C and 207 bars, with M W 270,000 (18). SANS investigations of dilute solutions of PFOA in C 0 (43), over a wide range of temperatures and pressures, provided clear evidence for favorable interaction between PFOA and C 0 . PFOA grafted with 5 kg m o l polyethylene oxide (PEO) segment (PFOA-gPEO) was shown to be C0 compatible (44,45). Block copolymers of polystyrene and PFOA, PS-6-PFOA, in C 0 were also extensively studied by SAXS, SANS (46, 47), high-pressure, high-resolution (HPHR) N M R (48) and pulsed magnetic field gradient (PFG) N M R (49). Neutron scattering data (46) showed that the micellar size increases with the PFOA mass, when holding M W of the PS component constant. The effect of C 0 density (0.27 to 0.84 g.crn ) at constant temperature (60 °C), and of temperature (29.5 to 76.2 °C) at constant C 0 density (0.733 g. cm' ) was studied on P F O A / C 0 systems by HPHR N M R (48). Transitions may occur in block copolymer/C0 systems, due to change in temperature and pressure. Subsequently a critical micelle density (CMD), analogous to the critical micellisation concentration (CMC) was found. The existence of a C M D was first argued by McClain et al. (47), who observed a transition from aggregates to unimers for the PS-A-PFOA/C0 system, as solvent density was increased. Further evidence for the C M D was provided by Triolo et al. (50-52) and Zhou and Chu (53, 54). 2

2

2

1

2

2

3

2

3

2

2

2

2


290 Silicones: Emergence of P D M S For applications in C 0 , silicones are generally considered less effective than their fluorinated counterparts. Poly(dimethyl siloxane) (PDMS) solubility in C 0 was first reported in 1996; at a level of 4 %wt, PDMS (Mn - 13k) is soluble in C 0 at 35 °C and 277 bar (55). Block copolymer surfactants consisting of C0 -philic P D M S and C0 -phobic ionizable poly(methacrylic acid) (PMA) or poly(acrylic acid) (PAA) were used to form w/c and c/w emulsions (17). These PDMS-based block copolymers exhibited remarkable "ambidextrous" behavior to stabilize P P M A particles both in C 0 on the one hand, and in water on the other hand (56, 57). Steric stabilization is imparted in C 0 by PDMS, which is significantly more soluble in a C 0 / M M A mixture than in pure C 0 (58). 2

2

2

2

2

2

2


2

2

Development of Hydrocarbon Systems: High Hopes Efforts have been made to develop hydrocarbon systems for C 0 , as they could present significant advantages over high-cost fluorocarbon or siloxane counterparts. Recent advances are covered in section 3 of this article. Solubility of hydrocarbon materials in C 0 may be achieved by the addition of a polar cosolvent to C 0 to improve solvent polarity. For instance, A O T was shown by Ihara et al. to be completely soluble in C 0 with ethanol as a co-solvent (59). Along similar lines, Hutton et al. in 1999, formed w/c microemulsions with 0.03 M A O T and 15 mol % ethanol or 10 mol % pentanol (60,61). Nonionic surfactants are also known to exhibit quite high solubilities in scC 0 . Nonionic polyethers, C E (62) and C f E (63), were extensively studied in C 0 with and without the addition of pentanol as co-solvent. Water solubility up to w = 12 was observed for C E with 10 wt % pentanol as a co-solvent. This C j E alone dispersed no water, however the addition of pentanol enhanced notably its solubility. Related nonionic surfactants were shown to exhibit solubility in C O (64-66). In section 3 of this chapter a new SANS study of this mixture, and other nonionic surfactants in C 0 , are reported. Another approach to use A O T as a surfactant for C 0 microemulsion consists in mixing A O T with a PFPE-based surfactant. This may seem quite disconcerting at first sight, as fluorocarbons and hydrocarbons are notoriously known to be immiscible. However Fulton et al. (67); reported microemulsion formation by mixing 15 m M of A O T and 30 m M of P F P E - P 0 up to w = 12. These systems could be successfully used as micro reactors to synthesize metallic silver (3, 4) and copper nanoparticles (5) and to carry out catalytic hydrogénations (5). Eastoe et al. also showed that hydrocarbon surfactants analogous to A O T with branched tails were C 0 compatible (68). More detail is given in section 3 below. 2

2

2

2

2

8

5

2

4

2

8

2

5

4

a

2

2

4

2


291 In 2000, Beckman was once again the pioneer, setting rules for designing hydrocarbon copolymers in C 0 (69). Different types of monomers were assembled, so that specific interactions with C 0 were favoured via high flexibility, high free volume, low cohesive energy density and Lewis basicity (via carbonyl groups). Poly(ether-carbonate) was synthesized by copolymerisation of C 0 with propylene oxide. (CHO-C0 ) -(EO) -(CHOC 0 ) was soluble in C 0 at low pressures (below 140 bar) and formed a w/c emulsion, which was stable for hours upon stirring. 2

2

2

2

2 5

2

25

7

2

Applications: Towards C 0 - " G r e e n " Chemistry Downloaded by FUDAN UNIV on March 5, 2017 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch019

2

The use of C 0 as a reaction medium for dispersion (18) or inverse emulsion polymerization (70) has been extensively studied. Due to its excellent transport properties (low viscosity and high diffusivity), C 0 is usually a beneficial solvent for reactions. For instance, the polymerization of methyl methacrylate is now well established via various methods (71-75). Based on the polarity difference between C 0 and the interior of the micelles, w/c microemulsions have found many applications as extraction media. Furthermore, by modifying pressure and temperature, solvent quality may be changed and it becomes, therefore, possible to exert a real control over the extraction process; uptake of solutes inside micelles may be varied. This may be of use for separations/extractions involving bio-chemicals and proteins. In conventional solvents their separation from the reaction medium can be quite complicated, involving tedious processes such as fluid-fluid extraction, décantation, chromatography column, filtration, precipitation. Use of supercritical fluid technology with extraction in reverse micelles seems advantageous for proteins (e.g. 19, 76). This process was also used for the extraction of metals (77-79) and more recently of copper from a filter paper surface (1). Finally, w/c and c/w PFPE based emulsions have been used for the synthesis of porous materials, which are the skeletal replica of the emulsions after removal of the internal phase. W/c microemulsions allowed for macroporous polyacrylate monoliths to be produced (80-82). Conversely, c/w emulsions may be used for the preparation of well-defined porous hydrophilic polymers (83). 2

2

2

2. Fluorinated Anionic Surfactants In spite of the various potential applications little is known on the important issue of how to optimize surfactant chemical structure for C 0 dispersions. This is partly due to a lack of suitable, well-characterized compounds, and that one of 2


292 the most studied surfactants is a technical grade product perfluorpolyether (PFPE) consisting of a molecular weight distribution (e.g. 19, 30, 31, 34-36,38). However, fluorinated analogues of Aerosol-OT (sodium bis-2-ethylhexyl sulfosuccinate, A O T ) provide a suitable platform for delineating structureperformance relationships in C 0 (chemical structures and nomenclature given on Figure 1). 2


Air-water Surface Tensions Surface tension measurements were used to check surfactant purity, as well as determine critical micelle concentrations (erne's) and limiting surface tensions at the cmc y 's. The results are listed in Table 1. Methods and techniques described elsewhere (25, 84) were used to obtain representative equilibrium adsorption isotherms. This tension y is characteristic of the efficiency of a given surfactant, and it is strongly linked to the hydrophobic chain chemical structure (85). cmc

mc

Water-in-C0 Microemulsion Stability 2

O f twelve different linear chain fluoro-succinates investigated, nine formed w/c microemulsions with water content w=10 ([water]/[surfactant]); these were di-HCF4, di-HCF6, di-CF3, diCF4, diCF6, diCF4H, diCF6H, di-CF4GLU and the cobalt salt Co-HCF4. As the phase transition pressure (Ptnms) is approached from the high-pressure transparent region, these systems quickly develop opacity around ±5 bars of P ^ . Neither of the shortest chain compounds, di-CF2 and diHCF2, or the long chain di-HCF8 formed stable microemulsions. On the other hand di-CF4, di-CF6 and di-CF4H were very effective, and gave w=10 microemulsions at C 0 bottle pressure (57 bar) at 15°C. This indicates a strong chemical specificity and is consistent with an optimum chain length for w/c formation. Issues of phase boundary reproducibility, effects of (i) surfactant chain length, (ii) fluorination level and (iii) headgroup/counterion type have been dealt with in detail elsewhere (24-28). 2

Correlation Between Cloud Points and Aqueous Surface Tensions Figure 2 demonstrates an interesting correlation between the limiting surface tension y and the corresponding w/c phase boundary pressure (for w=10 systems at 0.05 mol dm" and 15°C), for eight different related surfactants (27a). As shown, there is a distinction between the behavior of H-CF - tipped surfactants (higher and P ^ ) and that for the class of F - C F compounds, which on the whole perform best in C 0 . cmc

3

2

2

2


293 Table 1. Aqueous Phase Critical Micelle Concentrations (cmc's), Limiting Surface Tensions Y s and Microemulsion Stability Pressures for Fluorinated Surfactants. v

cmc

3


cmc I (mmol dm' )

1

y

cmc

/ (mN m )

Ρtrans I bar

di-HCF4 di-HCF6

12 0.8

26.8 24.1

193 163

di-CF3 di-CF4 di-CF6

12 2 0.1

17.8 17.7 15.5

124 70 77

di-CF4H di-CF6H

0.7 0.05

15.6 22.0

89 139

di-HCF4GLU

11

25.8

181

NOTE: Temperatures; di-HCF4 25°C, di-HCF6 40°C, di-CF3 25°C, di-CF4 30°C, diCF6 40°C, di-CF4H 25°C, di-CF6H 40°C, di-HCF4GLU 25°C. The phase transition pressures P water-in C 0 microemulsions are for systems with [surf] = 0.05 mol dm" and w=10 at 25 °C (w=[water]/[surfactant]). SOURCE: Reproduced with permission from Langmuir, 2002, 18, 3014-3018. Copyright 2002 Am. Chem. Soc.

3

trans

2

Although there have been previous reports of w/c formation with fluorosurfactants of this type (24, 26, 28, 29), new results on the widest range of compounds available to date are reported here. Gratifyingly, for di-HCF4, there is good agreement between the new results presented here and those of Erkey. For the longer chain-length surfactant (di-CF6H, us, or di-HCF7 in 28) the experimental conditions used here are different and so direct comparisons cannot be made. A n important finding is highlighted for the first time: a clear correlation between surface tension of an aqueous solution and the performance of the compound in a w/c microemulsion. This represents a new paradigm for designing highly efficient C0 -philes. 2

Water-in-C0 Microemulsion Structure 2

Microemulsion structures were investigated by high-pressure small-angle neutron scattering (SANS). Engineering considerations limit the maximum pressure of the SANS cell to about 600 bar. Therefore, the experimental conditions were chosen as T= 15°C and Ρ = 500 bar, so as to strike a compromise between this and the need to minimize attractive interactions, which increase on approach to P . The scattering was consistent with the added D 0 c


2

294 being dispersed as microemulsion droplets, and fitted radii for three different surfactants are plotted in Figure 3 as a function of water content w. These surfactants exhibit almost identical behavior, at least within the experimental error on R (±2 Â). Although the phase stability depends strongly on surfactant type, there seems to be no obvious effect on internal droplet structure or droplet radius. The effective molecular area at die water-C0 interface, A may be estimated using eq 1 which is valid for spherical droplets. c

2

h9

Α ^ ^ ψ ^

(1)


it

In eq 1 v is the volume of a water molecule. Hence, A can be estimated as 115 ± 5 Â , and the intercept is consistent with a radius for die polar core of a "dry" micelle of around 11 Â, This head group area A can also be calculated from high Q SANS data with the Porod equation, as described elsewhere (13, 86). These results indicate that fluoro-surfactants at the water-C0 interface adopt a lower packing density than a related hydrocarbon surfactant (AOT) at analogous water-alkane interfaces (86). D 2 0

h

2

h

2

Microemulsion Formation with Fluorinated Phosphate Surfactants Based on the success of these fluoro-sulfosuccinates described above di-fluorocarbon phosphates have also been investigated. In terms of synthesis and raw materials costs these surfactants have significant advantages over the sulfosuccinates. Surfactants of this kind have also been studied by DeSimone et al (27 c), and the synthesis and purification are described elsewhere (27b, c). Detailed SANS experiments are described in these papers (27b, c), and it is clear that surfactants of this kind stabilize aqueous nano-droplets. Hence, anionics other than sulfosuccinates may be employed in water-in-C0 microemulsions. Significantly, one of these compounds (di-HCF6-P, see ref 27 b) stabilizes microemulsions in liquid C 0 at vapor pressure; a potentially useful result that may be of importance in facilitating applications. 2

2

3. Aggregation of Hydrocarbon Surfactants in CO2 Owing to the high cost of fluorous chemicals efforts have been made to obtain C0 -soluble hydrocarbon polymers and surfactants (68,69). With readily available hydrocarbon surfactants there is some indirect spectroscopic evidence for aggregation in C 0 , but mostly in the presence of high levels of shortmedium chain alcohols (60-62). The next section summarizes recent SANS 2

2



295

10

15

20

25

30

1

Y /(mN m- ) cmc

Figure 2. Correlation between P in w/c systems and y at the air-water interface. Phase transition pressures were measured at 25 °Cfor w = 10 microemulsions and 0.05 mol dm' surfactant concentration. Line is a guide to the eye. di-CF6 (A) di-CF4H (*), diCF3 (·), di-CF4 (Φ), di-HCF6 (Δ), diCF6H (x),di-HCF4 (0), di-HCF4GLU (*). Reproduced with permission from Langmuir, 2002, 18, 3014-3017. Copyright 2002 Am. Chem. Soc. trans

cmc

3

τ

5

1

1

1

1

1

1

10

15

20

25

30

35

w

Figure 3. Variation of water-in-C0 microemulsion droplet radius with water content for three surfactants atT= 15°CandP = 500 bar, and [surf] = 0.05 mol dm . Reproduced from reference 13 by permission of the Royal Society of Chemistry. 2

3


296 evidence for micellisation of custom-made, and commercially available, hydrocarbon surfactants in C 0 . 2

Aerosol-OT analogues It is known that y decreases with the extent of methylation in the hydrophobic chain tip region (86, 87). The highly methyl-branched compounds A and Β shown on Figure 4a showed the low y ; 27 and 28 mN m' respectively compared with 31 mN m" for AOT (86). O f course, the general structure of these hydrocarbons is closely related to that of the fluorocarbons, which were so effective. The idea was to use this sulfosuccinate backbone, and to substitute hydrocarbon chains, which are as C0 -philic as possible, in place of the fluorocarbon tails. These two compounds both dissolve in stirred sc-C0 : at 33°C and 250 bar at up to 0.1-wt%. The solid was initially insoluble, however after around 10 minutes surfactant began to melt, and after 20 minutes a singlephase transparent solution was formed. Under identical conditions A O T did not dissolve at all. cmc

1

cmc


1

2

2

SANS curves obtained after subtraction of the cell+C0 background are shown in Figure 4b. Data fitting with the model in ref. 37 was consistent with "dry" reversed micelles, as observed with these surfactants in η-heptane solvent (86). UV-vis dye solubilisation measurements shown in Figure 4c also provide evidence for aggregation. If reversed micelles were present the positively charged chromophore, dimidium bromide ( λ ^ - 550 nm), should be incorporated owing to favorable interactions with the surfactant anionic head groups. No dimidium bromide absorbance could be detected in pure C 0 , consistent with an insoluble dye. Further, no uptake was seen with added Aerosol-OT (0.025 mol dm" ). However, with the C0 -soluble surfactant Β as dispersant a spectrum characteristic of dimidium bromide in reversed micelles was obtained (Figure 4c). Although the common anionic hydrocarbon surfactant Aerosol-OT does not aggregate in carbon dioxide, two related compounds, each possessing a high degree of chain tip methylation, do form reversed micelles in sc-C0 . These principles may be helpful for designing a wider range of low-cost " C O p h i l i c " hydrocarbon surfactants, especially for stabilizing bio- and food-compatible water-in-C0 emulsions and microemulsions. 2

2

3

2

2

r

2

Commercial non-ionics

Micellisation of various hydrocarbon surfactants in s c - C 0 was also investigated by high pressure SANS. It can be emphasized that this study widely contrasts with other works where an indirect spectroscopic method is used to provide evidence for aggregation (e.g. 62-66). These surfactants, shown in Figure 5, were selected because of their low cost and C0 -compatible chain tips, 2

2


297 which are highly methylated as for the AOT analogues described above. Triton surfactants are commercially available, and are extensively used in industrial (88, 89) and pharmaceutical formulations (90, 91) and in biochemical research (92). On Figure 5 example high-pressure SANS data confirm the presence of spheroidal aggregates in sc-C0 . This provides further evidence for C 0 compatibility of these branched-chained, /-butyl-tipped nonionic surfactants. 2

2


Table 2. Fitted Form Factor and Guinier Radii for Triton Surfactants. Triton Surfactant TX-45 TX-100R TX-100

Fitted 16 18 14

R±1/(A)

GuinierR±11(A) 13 16 11

N O T E : SANS data were recorded a 40°C and 500 bar.

Water Uptake It would be of great interest if water could be dispersed in the polar cores of the systems described above. Water was added to the binary systems (W=[H 0]/[TX]=5), with an additional amount to account for the small water solubility in C 0 . In some cases added water caused the precipitation of the surfactant. Near-Infrared (NIR) spectra obtained for the Triton surfactants, and interpreted as in ref. 13, indicated no clear evidence of water solubilisation in the micelles and the signals were consistent with water mainly located in the bulk C 0 . Further attempts with brine (0.05 and 0.10 mol dm" ) proved to be no more successful. 2

2

3

2

Surfactant-Alcohol- Water Mixtures There have been reports (e.g. 60-63) claiming that reversed microemulsions can be induced to form by employing medium chain length alcohols as a "cosurfactant". The physical evidence comes mainly from indirect spectroscopic methods ( λ , ^ shifts of probe dyes etc.). To improve understanding about these potentially important systems, SANS experiments were made. Figure 6 shows a Guinier plot (37) for I(Q) data obtained with a system suggested by Howdle et .al (62). Note the added alcohol is soluble in both C 0 and water (weakly) hence, the main role is somewhat unclear since pentanol can act both as a cosolvent and co-surfactant. Since D 0 was used the effective contrast should be at the C 0 - D 0 interface. It is encouraging to see that the calculated radius of ~ 29Â is somewhat larger than that found for a water-free, C0 -soluble reversed 2

2

2

2

2


298

a AOT (Aerosol-OT)

Surfactant A

Surfactant Β

l(Q)/cm* Downloaded by FUDAN UNIV on March 5, 2017 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch019

1

0.05

0.10

0.15 Q/Â-

0.20

0.25

1

Figure 4. (a) Surfactants AOT (Aerosol-OT) Sodium bis (2-ethyl-lhexyl)sulfosuccinate, surfactant A (sodium bis (2,4,4-trimethyl-lpentyl)sulfosuccinate) and surfactantΒ (sodium bis (3,5,5-trimethyl-I-hexyl) sulfosuccinate (b) SANS data obtained after subtracting the cell+sc-C0 backgroundfor surfactant A as a function of concentration at 0.15 (Φ) and 0.10 ( O) mol dm' . Γ = 33 °C and 500 bar. Thefitsare to a polydisperse sphere model with R = 14 ± 1Â and a / R = 0.20 (footnote 37). (c) UV-vis spectrum of dimidium bromide dispersed in sc-C0 with reversed micelles of surfactant Β at 40 °C and 500 bar. The surfactant concentration is 0.025 mol dm' . Reproduced with permission from Journal of the American Society, 2001, 123, 988-989. Copyright 2001 Am. Chem. Soc. 2

3

a v

a v

2

3



299

Figure 4. Continued.



300

Triton n=8 TX-45 n=10 TX-100 n=10+reduced benzene ring TX-IOOR i(Q)x10/cnr

1

0.20 1

0.10

i

0.05

0.10 0.15 Q/Â-

0.20

1

Figure 5. Molecular structures of the Triton surfactants. SANS data obtained after subtraction of the cell and sc-C0 backgroundfor Triton surfactants at 0.05 mol dm' and at 40 °C and 500 bars. Example error bars are shown for TX100 only. The lines are modelfits;fittedparameters are given in Table 2. 2

3


301 micelle with similar non-ionic surfactants (Table 2). It should be noted that a high level of surfactant and alcohol (26%) is needed to dissolve only a modest amount of water (4%). As stated in the previous section non-ionic surfactants alone do not stabilize "wet" reversed micelles, but in the presence of high alcohol levels water solubility appears to be significantly improved. Hence, it may be possible to formulate microemulsions in C0 -rich solvents using commercially available surfactants and alcohols. 2

Conclusions and Outlook The benefits of employing surfactants in C 0 are clear. However, achieving low-cost effective C0 -philes still represents a major challenge, which continues to occupy a number of research groups worldwide. Owing to high C 0 compatibility various fluorocarbon surfactants can now be used to stabilize water-in-C0 microemulsions. O f these custom synthesized molecules, certain ones (di-CF4 and di-HCF6-P) have been shown to be highly efficient, since they are able to stabilize dispersions at relatively low pressure, just above the vapor pressure of C 0 . A valid, question concerns the economics of fluorocarbon surfactants for applications in C 0 . At current market prices and using scientific (not bulk) suppliers the raw costs can be estimated at $0.6/g, $2.4/g and $50/g for di-HCF4, di-CF4H and di-CF4 respectively (not including human resource costs). For any practical purposes di-CF4H (n=2) would be the preferred option, owing to cost, in spite of the -20 bar increase in over the more expensive di-CF4. The use of added electrolyte is another strategy that can be employed to reduce phase stability pressures (93), so those relatively cheap active components such as di-HCF4 and di-HCF6 could be used.


2

2

2

2

2

2

A clear correlation has been observed between limiting surface tension y and surfactant performance in water-in-C0 microemulsions, as measured by the phase transition pressure Ρ . These results have important implications for the rational design of C0 -philic surfactants. Studies of aqueous solutions are relatively easy to carry out, and surface tension measurements can be used to screen target compounds expected to exhibit enhanced activity in C 0 . Therefore, potential surfactant candidates can be identified before making timeconsuming phase stability measurements in high-pressure C 0 . Building on the success of the fluoro-sulfosuccinates, hydrocarbon analogues, bearing f-butyl chain termini, were developed as possible candidates for C0 -active compounds (68). The trends in y within a surfactant series noted for fluoro-succinates were used to as a guide to design these molecules. This strategy switches a C0 -insoluble compound, Aerosol-OT, into one which micellises in carbon dioxide. These ideas were extrapolated to commercially available Triton non-ionics, which also micellise in C 0 . Unfortunately, neither micelles of the hydrocarbon succinates, nor the Tritons, are able to disperse water. However, limited water solubility can be achieved i f medium chain alcohol is mixed with a non-ionic surfactant. It should be noted that a high level of surfactant and alcohol (26%) is needed to dissolve only a modest amount of cmc

2

&3η8

2

2

2

2

cmc

2

2


302 1


In [l(Q)/cnrr]

Q2/A-2 Figure 6. Guinier plot of SANS data for a dispersion of D 0 (4%), nonionic surfactant C E (10%)ι pentanol (16%) and C0 at 60

Supercritical Carbon Dioxide - American Chemical Society

Recommend Documents