Tuning Micellar Structures in Supercritical CO

Article Langmuir is published by Tuning Micellar the American Chemical Society. 1155 Sixteenth Structures inWashington, Street N.W., DC 20036 Supercritical CO Publishedby by American of Subscriber access provided University 2

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Using Surfactant and Amphiphile Mixtures Langmuir is published by the American Chemical

Jocelyn Alice Society. Peach, Adam 1155 Sixteenth Street N.W., Washington, Czajka, Gavin Hazell, DC 20036 Published by University American of Christopher Hill,provided Azmi Mohamed, Subscriber access by

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Jonathan C Pegg, Sarah E. Rogers, and Julian Eastoe Langmuir is published by

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Elongated micelles

hydrotrope

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Langmuir water CO2

1 2 3 0 CO42 –philic surfactant 5 ACS Paragon Plus Environment 6 7 co-surfactant

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CO2

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Scattering Intensity, I(Q) / cm

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Na+ literature results Na+ recent results

predicted intrinsic visco

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predicted intrinsic visco

sity ||

1 2 3 40 4 5 35 6 7 30 8 9 25 10 11 20 12 13 15 14 15 10 16 5 17 18 0 19 20 20 15 21 22 10 23 24 5 25 26 27 28 29

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1 2 3 4

Tuning Micellar Structures in Supercritical CO2 Using Surfactant and Amphiphile Mixtures

Jocelyn Peach†, Adam Czajka†, Gavin Hazell†, Christopher Hill†, Azmi Mohamed‡, Jonathan

5

C. Pegg†, Sarah E. Rogers§, Julian Eastoe* ,†

6

Author Address

7

†

School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom

8

‡

University Pendidikan Sultan Idris, Faculty of Science and Mathematics, Department of

9

Chemistry, Tanjong Malim 35900, Perak, Malaysia.

10

§

11

United Kingdom

12

Keywords

13

Supercritical CO2; Viscosity; Surfactants; Hydrotropes; Small-angle Neutron Scattering; self-

14

assembly; reversed micelles

Rutherford Appleton Laboratory, ISIS Spallation Source, Chilton, Oxfordshire, OX11 0QT,

15

1


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16

Abstract

17

For equivalent micellar volume fraction () systems containing anisotropic micelles are

18

generally more viscous than those comprising spherical micelles. Many surfactants used in

19

water-in- CO2 (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl)

20

sulfosuccinate (AOT): here it is proposed that mixtures of CO2-philic surfactants with

21

hydrotropes and co-surfactants may generate elongated micelles in w/c systems at high-

22

pressures (e.g. 100-400 bar). A range of novel w/c microemulsions, stabilised by new custom-

23

synthesized CO2-phillic, partially fluorinated surfactants, were formulated with hydrotropes

24

and co-surfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure

25

and hydrotrope tail length were all investigated. Dispersed water domains were probed using

26

High Pressure Small-angle Neutron Scattering (HP-SANS), which provided evidence for

27

elongated reversed micelles in supercritical CO2. These new micelles have significantly lower

28

fluorination levels than previously reported (6-29 wt % cf. 14-52 wt %), and furthermore, they

29

support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The

30

intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio;

31

from this value a relative viscosity value can be estimated through combination with the

32

micellar volume fraction (). Combining these new results with those for all other reported

33

systems, it has been possible to ‘map’ predicted viscosity increases in CO2 arising from

34

elongated reversed micelles, as a function surfactant fluorination and micellar aspect ratio.

35

2


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36

Introduction

37

In recent years, there has been focus, both in society as a whole and in the scientific community,

38

to work in a more sustainable and environmentally friendly fashion. The philosophy of green

39

chemistry is put into practice in several ways, including reducing or eliminating the use of toxic

40

and hazardous reagents or products, minimizing the energy required for chemical processes

41

and avoiding the use or production of waste, whenever possible1. With this in mind, there is

42

increased interest in the use of supercritical CO2 (scCO2) as a replacement solvent for volatile

43

organic compounds (VOCs)2 due to its abundance, low cost and low toxicity. Self-assembly

44

of additives in scCO2 has been studied for around 30 years and reviewed extensively, most

45

recently in reference 3. Unfortunately, scCO2 exhibits poor solubility for polar and ionic species

46

due to its low dielectric constant and non-polar nature4,5.

47

developments in tackling the poor solvent quality of scCO2, by employing CO2-philic

48

surfactants for the formation of water-in-CO2 (w/c) emulsions and microemulsions (µEs)3,6,7.

49

The majority of CO2-philic surfactants are not commercially available8 and have to be specially

50

synthesised. Substantial breakthroughs have been made in identifying the chemical and

51

physical properties which govern the CO2-philicity of solutes and surfactants, through the

52

inclusion of CO2-philic moieties, such as fluorocarbons (FCs)9. The downsides are that FC-

53

based surfactants are expensive and environmentally unfriendly10,11. The levels fluorination in

54

CO2-philic surfactants have been successfully reduced using ‘hybrid’ di-chained surfactants5,7,

55

such as investigated here and denoted as hybrid CF2:AOT4 and hybrid CF2:SIS1 (Figure 1).

56

These particular hybrid surfactants comprise two generally recognized CO2-philic groups

57

7,12

There have been significant

one a ‘CF2’ (pentafluoropentyl) chain, and the other a separate branched hydrocarbon chain.

58

3


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Chemical Name

F content, wt %

TCF2

Sodium 1,4-bis(4H, 4H, 5H, 5H, 5Hpentafluoropentyl)-3-(4H, 4H, 5H, 5H, 5H-pentafluoropentyloxycarbonyl)1,4-dioxobutane-2-sulfonate

36.9

DCF2

Sodium bis(4H, 4H, 5H, 5H, 5Hpentafluoropentyl)-2-sulfosuccinate

35.2

Hybrid CF2:AOT4

sodium (4H, 4H, 5H, 5H, 5Hpentafluoropentyl-3,5,5-trimethyl-1hexyl)-2-sulfosuccinate

18.8

Hybrid CF2:SIS1

Sodium (4H, 4H, 5H, 5H, 5Hpentafluoropenyl-5,7,7-trimethyl-2(1,3,3-trimethyl-butyl)-octyl)-2sulfosuccinate

15.0

C2benz

Sodium p-ethylbenzoate

0.0

C8benz

Sodium p-octylbenzoate

0.0

DIGSS

Sodium bis(isopropoxy methyl) sulfosuccinate

0.0

Chemical Structure

59

Figure 1 - Surfactants and Additives used in this work. The common ‘CF2’ (pentafluoropentyl) tail group is highlighted.

4


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60

Also investigated here were two other CO2-philic surfactants; DCF2, a di-chained surfactant,

61

with two individual CF2 tails and TCF2, a tri-chained surfactant, with three individual CF2

62

tails (Figure 1). Hence, there is a logical development in chemical structure in the surfactants

63

studied, providing new insight into structure-performance relationships and allowing

64

exploration of the limits of low fluorination.

65

Other applications of scCO2 are for Enhanced Oil Recovery (EOR) and Carbon Capture and

66

Sequestration (CCS)13. However, there are notable issues with handling, storage and viscous

67

fingering in these applications3, and there is a need to develop additives for enhancing the

68

viscosity of scCO214–17. Viscosity increases have been documented in normal organic and

69

aqueous media, arising from elongation of spherical micelles to form ellipsoidal or rod-like

70

micelles18. The formation of anisotropic and rod-like micelles can be induced in a variety of

71

ways: by modifying the surfactant counterions19,20, or through mixing surfactants,

72

hydrotropes21,22 and co-surfactants23 and there has been some success applying these

73

approaches to reversed curvature w/c systems

74

cause changes in surfactant packing parameter Pc 24,25 in reversed curvature systems Pc > 1. Pc

75

= V / (A∙l), where V is surfactant tail volume (Å3), A is the apparent surfactant headgroup area

76

(Å2) and l is the surfactant tail length (Å).

77

The hydrotropes and the co-surfactant investigated are known to associate in mixed micelles

78

with the class of surfactants used here, thereby increasing the effective surfactant headgroup

79

area, and hence decreasing Pc towards 1 for reversed curvature systems21. The result of these

80

changes is to destabilize spherical reversed micelles in favour of ellipsoidal, rod-like or

81

lamellar structures, which have a greater degree of planarity. Following the packing parameter

82

argument, it can also be proposed changing from a short single-chained additive, with a low

83

chain volume (such as C2benz, Figure 1) to an analogue with greater chain volume (e.g.

20,22

. The different approaches just mentioned

5


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C8benz, Figure 1), Pc should also decrease21,23 towards 1 favouring ellipsoidal/rod-

85

like/lamellar domains over highly curved reversed spherical micelles. Furthermore, it is

86

interesting to see how a double chain additive, such as DIGSS (Figure 1), which is recognized

87

as being CO2-philic 23 will affect preferred curvature compared to the single chain hydrotrope

88

additives.

89

Unfortunately, although there has been some success in the past, all of the previously reported

90

results come with limitations; either high levels of fluorine in the surfactants23,26 (up to 60

91

wt.%), low solubility of water (w5 only, where w = [water] / [surfactant] 27) or high cost and

92

unacceptable environmental effects (use of Nickel and Cobalt counterions)20. It has been shown

93

that certain weakly CO2-philic non-fluorinated surfactants and hydrotropes can be combined

94

to generate ellipsoidal micelles in CO2 20, however only modest aspect ratios (1.00 < Jmic
100 bar) viscosity measurements are

297

experimentally challenging: hence it is of interest to find a way to circumvent these by

298

estimating global effects of anisotropic self-assembly on bulk viscosity.

299

In previous studies, a set of fluorinated CO2-philic surfactants with transition metal counterions

300

were synthesised

301

micelles in w/c systems. Based on SANS and high-pressure falling cylinder viscometry results,

302

a relationship was proposed between micellar aspect ratio (e.g. Jmic, Ra/Rb for ellipsoids) and

303

the resulting viscosity enhancement (Equation 239, Equation 36,40).

. These surfactants self-assembled to form elongated rod-like reversed

[𝜂] ≅ 2.5 + 0.4075(𝐽𝑚𝑖𝑐 − 1)1.508

304 305

6,20

Equation 2

𝜂𝑚𝑖𝑐 ≅ 𝜂𝑟𝑒𝑙 ≅ 1 + [𝜂]𝑝 + 𝐾𝐻 [𝜂]2 𝑝 2 𝜂𝐶𝑂2

306

307

Equation 3

308

In the above expressions [η] is the intrinsic viscosity, Jmic is the micellar aspect ratio as

309

determined by SANS, ηrel is relative viscosity (the ratio of the micellar solution viscosity, ηmic

310

to the viscosity of neat scCO2, ηCO2),  the micellar/microemulsion droplet volume fraction

311

and KH the Huggins coefficient for rods (taken as 0.4075 in these cases)41. Using these

312

equations along with the micellar aspect ratios gleaned from HP-SANS measurements, the

313

predicted changes in intrinsic viscosity for these systems have been estimated, and are

314

presented in Table 2. For ease of comparison between results reported in this manuscript and 18


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315

previous literature results, the predicted intrinsic viscosity values ([η]) are compared, thus

316

removing concentration or volume fraction effects. Plots showing the impacts of micellar

317

aspect ratio on predicted relative viscosity (ηrel), at different fluorination levels can be found in

318

the Supporting Information, for a range of systems with a volume fraction 0.016 <  < 0.063.

319

Although this approach sidesteps the need for challenging high-pressure viscosity

320

measurements, it must be recognised that the viscosity enhancements presented should be

321

considered as estimates for. More information can be found in the Supporting Information.

19


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additive

surf

C8benz DCF2

C2benz

TCF2

Hybrid CF2:AOT4

DIGSS Hybrid CF2:SIS1

w

5

10

15

20

30

%F Jmic [η] ηrel %F Jmic [η] ηrel %F Jmic [η] ηrel %F Jmic [η] ηrel %F Jmic [η] ηrel %F Jmic [η] ηrel

33.6 4.65 5.37 1.19 29.3 5.81 6.85 1.28 32.4 2.12 2.99 1.12 15.5 1.00 9.6 5.49 6.42 1.36 8.5 5.01 5.81 1.37

25.5 5.47 6.40 1.26 25.8 5.47 6.39 1.26 29.4 1.33 2.58 1.13 13.5 1.00 8.8 4.82 5.57 1.34 7.9 5.02 5.82 1.40

23.0 6.79 8.25 1.39 26.9 1.41 2.61 1.15 12.0 1.13 2.52 1.10 8.1 4.92 5.70 1.38 7.3 4.46 5.15 1.38

7.6 5.58 6.54 1.48 6.9 4.61 5.32 1.42

6.6 5.08 5.90 1.50 -

322

Table 2 –Intrinsic Viscosity ([η]) values and Relative Viscosity, ηrel, values calculated from Equation 2 and Equation 3. For

323

hydrotrope containing systems of these compositions; [surfactant] = 0.05 mol dm-3, C8benz and C2benz mole fraction = 0.1,

324

P = 350 bar, T = 45oC. For co-surfactant containing systems; [surfactant] = 0.05 mol dm-3, [DIGSS] = 0.05m mol dm-3, which

325

can be used to calculate φ, 3P = 120 bar, T = 45oC.

326

From the results shown in Table 2, it is evident that even the system with the lowest degree of

327

elongation reported here (DCF2+C8benz) has the potential to increase the intrinsic viscosity

328

of scCO2 over and above the value of [η] = 2.5 for spherical micelles. Systems with a higher

329

degree of micellar elongation give rise to predicted relative viscosity increases of between 10%

330

and 50%. These analyses are presented in graphical form in Figure 6 and Figure 7, alongside

331

predicted intrinsic viscosities of every system reported so far known to generate elongated

332

micelles in scCO26,8,20,22,23,28. The estimates presented in this paper are first compared to other

333

literature surfactants with related structures (sulfosuccinate headgroups and only Na+ 20


Langmuir

334

counterions), Figure 6. Following this, the new results are compared to all previously reported,

335

CO2-philic surfactants forming elongated reversed micelles, Figure 7 scCO2, including

336

transition metal ion systems.

Na+ literature results

8 7

predicted intrinsic viscos

ity | |

Na+ recent results

6 5 4 3 7 6 5 4 3 2

tio

t ra

ec

sp ra

lla ce

mi

, Jm ic

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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337

1 0

10

20

30

nation in

ge fluori percenta

40

micelle

338

Figure 6- Micellar fluorination levels and micellar aspect ratio vs. predicted intrinsic viscosity of all Na+ CO2-philic

339

surfactants reported to generate elongated micelles in scCO2. Systems introduced in this manuscript are shown as orange

340

filled squares, those in previous research shown as blue circles6,22,23,28.

341

From Figure 6 it is apparent that there are only few known systems forming anisotropic

342

micelles in scCO2 with truly fluorine free surfactants / hydrotropes. James et al. reported a

343

marginal increase in predicted relative viscosity, (8%-12%)22, however this system was only

344

stable up to w5. All other surfactants that induce the formation of elongated micelles have

345

fluorine levels of at least 25 wt. % (Figure 6, circles). The new systems reported here (Figure

346

6, filled squares) have lower overall fluorination, between 6 wt. % and 29 wt. %. As previously

347

discussed, surfactants containing low (or no) fluorine are more environmentally friendly, have

348

a lower toxicity and are less expensive than heavily fluorinated counterparts11,42. Predicted

349

viscosity increases from the new systems are higher (10% - 50%) than the majority of other 21


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350

standard Na+ containing surfactants reported (4% - 14%)8,20,22,23,28. It is apparent from Figure

351

6 that neither aspect ratio, nor fluorination level, show a particularly strong correlation with

352

predicted intrinsic viscosity for this family of Na+ sulfosuccinate surfactants, however, Figure

353

6 shows the maximum effects which have been achieved so far.

Ni2+ Co2+ K+

ity | | predicted intrinsic viscos

40

Li+

35

Na+ literature results

30

Na+ recent results

25 20 15 10 5 0 20 15 10

, tio

t ra ec sp

ra lla

ce

mi

J m ic

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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354

5 0

10

20

40

30

nation in ge fluori ta n e rc pe

micelle

355

Figure 7 – Micellar fluorination level and aspect ratio vs. intrinsic viscosity of all reported surfactants producing elongated

356

micelles in w/c microemulsions6,8,20,22,23,28. Systems introduced in this paper are shown as orange filled squares.

357

Figure 7 shows the upper limits of viscosity enhancements which have been achieved to

358

date..6,8,20,22,23,28 This is the first time that the predicted intrinsic viscosities of all reported

359

viscosifying surfactants have been estimated and compared. It is apparent that the most

360

effective surfactants are those containing divalent transition metal counterions (Ni2+, filled

361

green diamonds and Co2+, filled pink hexagons). These Ni2+ and Co2+ surfactants produce

362

highly elongated rod-like micelles, and therefore give potential rise to viscosity effects orders

363

of magnitude higher than other surfactants, as has also been observed in equivalent water-in-

364

oil systems19. Regrettably, these surfactants have a high fluorination levels (43 wt. % - 46 22


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365

wt. %), and this combined with the presence of a transition metal counterion means they are

366

environmentally unfriendly, toxic and costly.

367

Conclusions

368

Elongated reversed micelles have been formed in supercritical CO2 through mixing partially

369

fluorinated CO2-philic surfactants with additives (hydrotropes and co-surfactants). The

370

micelles reported here have lower fluorination levels (6 wt. % - 29 wt. %) than any equivalent

371

systems in the literature, which are a minimum of 25 wt. % F7,23. This reduction in F level of

372

the surfactant additives is a notable advance in the field, reducing cost, toxicity and

373

environmental impacts11,42. Here, the hydrotropes combined with CO2-philic surfactants

374

include C2benz (sodium p-ethyl benzoate) and C8benz (sodium p-octylbenzoate). Previous

375

literature on both w/o and w/c microemulsions shows the combination of these hydrotropes

376

and sulfosuccinate CO2-philc surfactants generates anisotropic micelles, but that hydrotrope

377

tail length seems to have minimal impact on micellar anisotropy21. However, the degree of

378

elongation observed in the w/c microemulsions is not as pronounced as in analogous w/o

379

systems22. Using another approach, the non-fluorinated CO2-philic co-surfactant, ‘DIGSS’, has

380

been successfully combined with a low-F CO2-philic hybrid surfactant CF2:SIS123. This novel

381

combination of partially fluorinated hybrid surfactant and non-fluorinated co-surfactant leads

382

to the formation of ellipsoidal micelles at relatively low CO2 pressure (density). Increasing

383

pressure to 350 bar, Bragg peaks are apparent in the SANS profiles, which may arise because

384

of the formation of lamellar aggregates. Hence, for these mixtures of CF2:SIS1 and DIGSS an

385

interesting pressure (density)-induced transition is observed by SANS, from ellipsoidal

386

micelles at low pressure to lamellar phase dispersions at high temperature, suggesting a new

387

alternative triggerable approach to viscosity modification of CO2.

23


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388

In a new attempt to rationalise the important surfactant structural and composition parameters,

389

predicted intrinsic viscosities have been calculated for the anisotropic micellar systems

390

introduced here, and compared to other structurally related CO2-philic surfactants from the

391

literature7,23. The new systems show some of the greatest predicted effects on intrinsic and

392

relative viscosity ever observed for micelles in CO2. This being said, it is obvious from Figure

393

7 that CO2-philic surfactants with divalent transition metal counterions still give rise to the

394

largest viscosity effects20.

395

The results and analyses reported here should help to guide the design of new surfactants with

396

only minimal fluorine levels, thereby reducing environmental impacts and costs supercritical

397

CO2 viscosifiers. The ultimate aspiration is to obtain surfactants and colloidal systems for

398

applications in CO2 that are totally fluorine-free.

399

Associated Content

400

Supporting Information

401

Additional details of SANS theory, model fitting and relative viscosity calculations and

402

additional HP-SANS data not presented in this manuscript can be found in the Supporting

403

Information.

404

Author Information

405

Corresponding Author

406

*E-mail [email protected]. Fax +44-117-928-9180

407

Notes

408

The authors declare no competing financial interest.

24


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409

Acknowledgements

410

J.A.P thanks the UK Science and Technology Funding Council (STFC) for a PhD scholarship

411

ST/L502613/1. The authors thank the UK Science and Technology Funding Council (STFC)

412

for allocation of beamtime at ISIS and grants toward consumables and travel. This work has

413

been supported through EPSRC EP/I018301/1 under the G8 research Councils Initiative for

414

Multilateral Research Funding G8-2012 and has benefitted from SasView software, originally

415

developed by the DANSE project under NSF award DMR-0520547.

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Tuning Micellar Structures in Supercritical CO

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