Perfluorinated Alcohols Induce Complex Coacervation in Mixed

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Perfluorinated Alcohols Induced Complex Coacervation in Mixed Surfactants Samuel I Jenkins, Christopher M Collins, and Morteza G. Khaledi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04701 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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Perfluorinated Alcohols Induced Complex Coacervation in Mixed Surfactants

Samuel I. Jenkins†, Christopher M. Collins†† and Morteza G. Khaledi* North Carolina State University, Department of Chemistry, Raleigh, N.C. 27695 and University of Texas - Arlington, Department of Chemistry and Biochemistry, Arlington, TX, 76019

†

Pfizer, 4300 Oak Park Rd, Sanford, NC 27330, United States.

††

Catalent Pharma Solutions, 160N Pharma Drive, Morrisville, NC 27560;

*[email protected] (corresponding author)

1 ACS Paragon Plus Environment

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Abstract Recently, we reported a unique and nearly ubiquitous phenomenon of inducing simple and complex coacervation in solutions of a broad variety of individual and mixed amphiphiles; and over a wide range of concentrations and mole fractions. This paper describes a novel type of biphasic separation in aqueous solutions of mixed cationicanionic (catanionic) surfactants induced by hexafluoroisopropanol (HFIP).

The test

cases included mixtures of cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) (surfactants with different carbon chain lengths), as well as dodecyltrimethyl ammonium bromide (DTAB) with SDS (surfactants with the same carbon chain lengths). The CTAB-SDS-HFIP coacervate systems can be produced at many different mole ratios of surfactant, but DTAB-SDS-HFIP only formed coacervates at equimolar (1:1) mole ratios of DTAB and SDS. The phase transition behavior of both systems were studied over a wide range of surfactant and HFIP concentrations at the stoichiometric (1:1) mole ratio of cationic:anionic surfactants. The chemical compositions of each of the two phases (aqueous-rich and coacervate phases) were studied with regard to the concentrations of HFIP, water, and individual surfactants. It is revealed that the surfactant-rich phase (coacervate phase) contains a large percentage of fluoroalcohol relative to the aqueous phase and is enriched in both surfactants but contains a small percentage of water. Surprisingly, the concentration of water in the coacervate phase increases as the total HFIP concentration is increased while the concentration of HFIP in the coacervate phase remains relatively constant, which means larger amount of water associated with HFIP molecules is extracted into the coacervate phase that results in the growth of the phase. The volume of the coacervate phase increases with an increase in surfactant concentration and total HFIP%. The coacervate phase is highly enriched in the two amphiphilic ions (DTA+ and DS-) whereas the two counter-ions (Br- and Na+) primarily reside in the aqueous-rich phase. The results suggest formation of a catanionic complex in the coacervate phase through ion-pairing with a concomitant release of the surfactant counterions (Na+ and Br-) to the aqueous – rich 2 ACS Paragon Plus Environment

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phase.

Lastly, the fluorocarbon alcohol systems are contrasted with the effects of

aliphatic alcohols in the mixed catanionic surfactant systems. Isopropanol does not have the same interactions as HFIP with respect to solubilization, aggregation, and phase separation of the oppositely charged surfactants.

Keywords HFIP, fluoroalcohol – induced coacervation, catanionic complex, coacervates, mixed surfactants


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Introduction Coacervation is a unique form of self-assembly where aggregation of amphiphilic molecules results in formation of a separate liquid phase in aqueous media. Coacervates are typically grouped into “simple” and “complex” categories. Simple coacervates involve a single amphiphile or a mixture of similarly charged amphiphiles. Formation of coacervates in the aqueous solution of a single amphiphile typically requires adjustment of certain experimental parameters such as temperature, presence of a salt, pH, or organic additives. Complex coacervates occur in mixtures of oppositely charged amphiphiles, which involves formation of a catanionic complex driven by electrostatic interaction and hydrophobic effect. In general, complex coacervates are more readily formed in aqueous solutions of amphiphilic molecules with multiple charged groups or polyions (e.g., charged synthetic polymers, polyelectrolytes, proteins, polysaccharides) than in mixtures of surfactants. For example, complex coacervates have been reported for stoichiometric mixtures of oppositely charged polymers,1,2 protein-polysaccharides,3 charged polymer – proteins,4 Gemini surfactant – charged polymer,5 and others6. On the other hand, formation of complex coacervates between singly charged surfactants would require a specific combination of molecular structure and composition. For example, coacervate formation is observed in the aqueous mixtures of sodium dodecyl sulfate (SDS) and dodecyltriethylammonium

bromide

(DTeAB)7 at

specific

mole

fractions;

but

coacervation would not occur at any composition in mixtures of SDS with dodecyltrimethylammonium bromide (DTAB) or in mixtures of SDS with cetyl trimethyl ammonium bromide (CTAB). Similarly, CTAB would form coacervate with sodium dodecyl sulfonate (SDSu)8, but not with its analog, SDS. Likewise, DTAB would form coacervate with alkyldiphenyloxide di-sulfonate (DPDS).9 Note that even in these systems coacervation occurs in small region(s) in the phase diagram and over a narrow range of non-stoichiometric mole fraction where one of the components in the catanionic complex is in excess. To the best of our knowledge, complex coacervation between oppositely charged surfactants at the stoichiometric mole ratio (i.e. equimolar surfactant concentration) in purely aqueous media at the ambient temperature has not been reported. Addition of a small percentage of a short chain fluorinated alcohol or acids could


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dramatically change phase behavior of aqueous solutions of surfactants and certain other amphiphiles. We have recently reported that water-miscible perfluorinated alcohols and acids can remarkably induce simple and complex coacervation in aqueous solutions of various types of amphiphilic molecules (such as synthetic surfactants, phospholipids, and bile salts; as well as polyelectrolytes) and over a broad range of concentrations and surfactant mole fractions (including equimolar 1:1 mole ratio) in catanionic mixtures.10,11 We have referred to these systems as Perfluoro - Alcohol/Acid – Induced Coacervates (PFAIC). Aqueous solutions of catanionic surfactants have been shown to form vesicles,12,13 mixed micelles,14,15 and other lamellar phases16 in addition to solids at various concentrations and mole ratios of surfactants. The type of structure that forms spontaneously is dependent on the size and charge of the head groups in relation to the chain length and volume of the hydrocarbon tails of the amphiphiles. At the stoichiometric mole ratio of the cationic:anionic surfactants (1:1), solids easily become the most abundant phase formed. The electrostatic attraction of the positively and negatively charged species become so great that water can no longer interact favorably with the surfactant head groups; the molecule would thus become very hydrophobic to the point of being insoluble. Thus, it is quite intriguing that fluorocarbon alcohols or acids lessen the electrostatic attraction that leads to formation of coacervates and phase separation in the aqueous media. The focus of the present report is to characterize complex coacervate formation between an anionic surfactant, sodium dodecyl sulfate with a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB) or dodecyl trimethyl ammonium bromide (DTAB) and induced by hexafluoroisopropanol (HFIP). Materials and Methods Sodium dodecyl sulfate (SDS, C12H25OSO3-Na+) and cetyltrimethyl ammonium bromide (CTAB, CH3(CH2)15N+(CH3)3Br-) were purchased from USB Corporation (Cleveland, OH) as “Ultrapure”. Dodecyltrimethyl ammonium bromide (DTAB) was purchased from TCI America with purity ≥ 98%. 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), ≥ 99.0%, was purchased from Fluka. Previously, we reported the phase diagrams of complex coacervates between sodium alkane sulfates with alkyl trimethyl ammonium 5 ACS Paragon Plus Environment

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bromide induced by trifluoroethanol (TFE) as well as the phase diagram for anionic polyelectrolytes, poly (methacrylic acid) (PMA) or poly (acrylic acid) (PAA) and cationic surfactants of alkyltrimethylammonium bromides with HFIP.10,11 The SDS-CTAB-HFIP and SDS-DTAB-HFIP phase diagrams were determined by the same procedure as that described previously10,11. The coacervate volume was determined as a function of total surfactant concentration, anionic:cationic surfactants mole ratio, and the % HFIP. For the SDS-CTAB-HFIP systems, stock solutions of SDS (375mM) and CTAB (375mM) and for the SDS-DTAB-HFIP systems, stock solutions of SDS (800mM) and DTAB (800mM) were added to 15-mL collecting tubes, which had volume graduations of 0.1mL. They were subsequently diluted with DI water and HFIP to obtain the desired concentrations. Rubber stoppers and Parafilm were used to seal the top before mixing by inversion. The tubes were centrifuged using a bench-top centrifuge and then allowed to equilibrate overnight. The volumes of the coacervate and the total solution were recorded for both systems and the experiments were conducted in triplicate. Temperature Study. During the initial stages of this research we conducted a systematic study of the effect of temperature on phase transition in PFAIC using the SDS-CTAB-TFE model system at stoichiometric (1:1) SDS:CTAB mole ratio with TFE as the coacervator over a wide range of concentrations and temperature. The total surfactant concentration ranged from 167 mM to 250 mM whereas TFE concentration varied from the 0% (i.e. no coacervation) to 33%. The temperature range was 6 oC – 70 o

C. In the absence of TFE, no coacervation or liquid-liquid phase separation was

observed at any temperature. Coacervation and phase separation occured in the presence of TFE. We did not observe any notable phase transition over the range of 25 oC- 50 oC. At lower temperatures (15 oC – 22 oC) and lower TFE concentration (9% and 17%) we observed formation of a few solid particles due to solubility issue in the presence of the two liquid phases. At 6 oC we observed formation of crystals. At higher temperatures (>50 oC), we observed the formation of a third liquid phase. For all the future studies with TFE and HFIP, the coacervates were prepared at 25 oC and experiments were performed at the ambient temperature. It is important to note that the coacervate phases are stable at room temperature for several months.


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Since HFIP is a much more effective coacervator, we anticipate that the precipitation issues at lower temperatures would be resolved. The formation of three liquid phases at higher temperature warrants further systematic investigation of the phase transition and composition, however, we need to first obtain a better understanding of the mechanism and structure of the two – phase systems at ambient temperature. Percent HFIP by ATR-IR. Attenuated Total Reflectance-Fourier transforminfrared (ATR-IR) spectroscopy was used to determine the concentration of HFIP found in each phase. The aqueous and coacervate phases were separated from one another with a Pasteur pipet. For SDS-CTAB-HFIP systems, the samples used for analysis included a series of 250mM, 1:1 SDS:CTAB samples and 250mM, 3:7 SDS:CTAB with various amounts of HFIP. These two surfactant mole ratios were chosen because they yielded the larger coacervate volumes, which were necessary for multiple analyses. A series of 273mM SDS:CTAB samples with 9% HFIP and different mole ratios were also prepared. For the SDS-DTAB-HFIP systems, the samples were prepared exclusively at 1:1 SDS:DTAB as coacervate was not formed at other mole ratios. A series of solutions containing 20mM to 400mM total surfactant was prepared using various amounts of HFIP. In addition, HFIP and various aqueous HFIP solutions were analyzed with the ATR-IR instrument. Surfactant Concentration from evaporation and elemental analysis.

A

gravimetric approach was used to determine the amount of total surfactant in each phase for SDS-CTAB-HFIP systems.

The water and fluoroalcohol were evaporated at a

temperature just below 100°C so that the samples did not boil violently which could result in a sample loss. The solutions were prepared in a similar manner to those used in the coacervate volume analysis above. The coacervate phase was isolated by removing the aqueous phase with a Pasteur pipet. 4-mL sample vials were heated in an oven to ~90°C. They were then removed and allowed to cool to room temperature for 1 hour before being weighed. 1mL aliquots of the aqueous phases were added to separate vials while 200µL aliquots of the coacervate phases were added and the total weight recorded. (For a few solutions with small coacervates 100µL or 75µL aliquots were used.) The solvents were evaporated in the oven for ten days before being removed, cooled for 1 hour, and reweighed. The weight difference is attributed to the surfactants. 7 ACS Paragon Plus Environment

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For SDS-DTAB-HFIP systems, an alternative approach using elemental analysis was employed to determine the amount of total surfactant present in each phase. This includes analysis for the surfactant counter ions, as the counter ions might be expected to partition differently than the related hydrophobic tail and charged head group. All methods used in this section are standardized methods for elemental analysis and were sent out for to an external lab for testing. Information for the methods used can be found in the reference Standard Methods for the Examination of Water and Wastewater.17 Bromide (Br-) analysis was performed using Ion Chromatography Method # 4110. Nitrogen analysis was performed using Combustion Infrared Total Organic Carbon Method # 5310 for total nitrogen in the SDS:DTAB coacervate systems to quantitate DTA- concentration. Sodium ion (Na+) and sulfur concentration for SDS:DTAB complex coacervate system phases were determined using the Inductively Coupled Plasma Method # 3120. Karl Fischer water analysis. The water content for the SDS-CTAB-HFIP and SDS-DTAB-HFIP was determined by the Karl Fischer titration method by using a 701 KF Titrino (Metrohm Ltd.; Herisau, Switzerland). A one-component volumetric system was used where methanol was the solvent and HYDRANAL Composite 5 was the reactant (Fisher Scientific). 25µL samples were titrated in triplicate. The samples were prepared in the same manner as the coacervate volume experiment above.

Results The phase diagrams were created for SDS:CTAB and SDS:DTAB solutions at 1:1 mole ratio and at different total surfactant concentrations and HFIP %(v/v) (Figure 1). The lines on the diagram are not strict transitions as most of the changes are gradual. Note that as little as 1% (v/v) HFIP can induce a coacervate that coexists with solid and a surfactant-lean aqueous phase (phases labeled as L/S+C in Figure 1). The transition to complete liquid-liquid phase (top aqueous-rich and bottom coacervate phase denoted as L/C in Figure 1) required less than 10% (v/v) HFIP for both SDS-CTAB and SDS-DTAB systems and at all surfactants concentrations. The minimum 8 ACS Paragon Plus Environment

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HFIP% required for this phase transition is nearly independent of the surfactant concentration over a wide range from a few mM to several hundred mM.

Figure 1. Phase diagrams of (A.) 1:1 SDS:CTAB and (B) 1:1 SDS:DTAB solutions with HFIP. L is a clear aqueous phase, C is a coacervate, S is an insoluble surfactant phase, and tL is a turbid liquid phase.

For the SDS:DTAB complex coacervate system, the range was as low as 2% v/v for 20mM total surfactant up to 60% v/v for 400mM total surfactant. The complete transition to liquid-liquid phase (L/C) was observed with at least 5%v/v HFIP that remained relatively constant over a wide range of total surfactant concentration (20mM400mM). At lower HFIP concentration (i.e., 1-2% v/v), the surfactant is not completely soluble and some insoluble surfactant is seen in the bottom of the test tube with a single clear liquid phase. A narrow region exists where coacervate is formed, but some insoluble surfactant remains in the coacervate phase for 250-400mM total surfactant concentration, and varies by concentration in the range of 2-4% v/v HFIP (denoted as L/S+C). The upper HFIP concentration, where the coacervate phase is dissolved and the 9 ACS Paragon Plus Environment

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(L/C) transitions to a single phase (L), increases with the total surfactant concentration. As the surfactant concentration increases, the addition of HFIP increases the hydrophobic environment around the SDS:DTAB complex causing it to phase separate from the aqueous phase forming a surfactant- and HFIP-rich coacervate phase. At 20mM total surfactant, the two phases exist up to 19% v/v HFIP, but at 400mM total surfactant this range increases up to 60% v/v HFIP. In other words, at higher concentration of surfactants, stable coacervate phases can exist even at higher HFIP concentrations. This is quite intriguing given that hydrophobic interaction between the oppositely charged surfactant molecules in water should lessen at higher concentrations of an organic cosolvent. The phase diagram clearly shows that the concentration of HFIP is more important in the phase transition than the concentration of surfactant for creating complex coacervates. Coacervate Phase Composition. For a better understanding of the HFIP induced coacervation, the composition of the aqueous-rich and coacervate phases were determined that included concentrations of HFIP, water, and surfactants. HFIP. ATR-IR was used to determine the concentration of HFIP in the aqueous-rich and complex coacervate phases. Figure 2 illustrates the ATR-IR spectra of two representative coacervate phases at different concentrations of HFIP for the SDS-CTAB-HFIP system. Similar spectra were observed for the SDS-DTAB-HFIP system (data not shown). HFIP has a number of distinct peaks that can be useful for quantitation of HFIP contents in each phase.


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Figure 2. (A.) ATR-IR spectra of the complex coacervate of 250mM 1:1 SDS-CTAB with 17% (blue) and 29% v/v (red) HFIP. (B.) ATR-IR spectra of the aqueous phase (blue) and coacervate phase (red) of 273mM 1:1 SDS:CTAB with 9% HFIP. The inset shows the large water peak of the aqueous phase in the higher wavenumber region. The calibration curve (Figure 2 A inset) was created from the spectra of binary solutions of HFIP-water. The peaks at -1 -1 -1 1190cm , 1263cm , and 1380cm gave the most linear responses.

The surfactant peaks in the coacervate spectrum (Figure 2B) are much larger, and more distinct than the corresponding peaks in the aqueous-rich phase spectrum due to higher concentrations. Almost all of the surfactant peaks are separate from the HFIP peaks.


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Figure 3 The % HFIP (v/v) was determined for the Coacervate and Aqueous phases by ATR-IR. Figure A represents 250mM, (1:1) SDS:CTAB-HFIP. Figures B and C are for (1:1) SDS:DTAB-HFIP at (B) constant total surfactant concentrations and (C) constant initial HFIP%.


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The results in Figure 3 clearly show that the coacervate phase has a significantly larger % of the HFIP than the aqueous-rich phases for both systems. It is quite remarkable that the coacervate phases comprised between 50% -70% (w/w) HFIP, even when the starting HFIP% is less than 10%. This suggests that the HFIP is extracted into the surfactant-rich coacervate, which could be due to strong interactions between the HFIP and the surfactants. The high HFIP% in the coacervate phase could also be attributed to the much smaller volume of the coacervate phase than the initial solution. As shown below, the coacervate phase volume increases with both total surfactant concentration and HFIP%, which leads to relatively constant HFIP% in the coacervate phase. There is, however, still a significant amount of HFIP in the aqueous phase (10-30% depending on the amount of HFIP added to the solution) (Figure 3). Figure 3C shows that the % w/w HFIP content decreases in both phases of the SDS-DTAB-HFIP system as the total surfactant concentration increases. For the coacervate phase this makes sense due to the rate of increase of the coacervate phase volume with the increase in the total surfactant concentration. For the aqueous phase, this is simply due to a lower volume of HFIP in the aqueous phase resulting from the HFIP extraction into the coacervate phase. Another important observation is the higher concentration of the HFIP in the 1:1 as compared to the 3:7 mole ratio of SDS-CTAB coacervate (data not shown); subsequently, the HFIP concentration in the aqueous-rich phase of 1:1 is lower than that in 3:7 phase. This could be attributed to larger extraction of HFIP into the more hydrophobic 1:1 coacervate phase and/or larger enrichment into the smaller 1:1 coacervate. Surfactants. Tables 1A and 1B list the % w/w and micro-moles of the amphiphilic ions (DTA+ and DS-) and the corresponding counter-ions (Br- and Na+) in each of the two phases for the (1:1) SDS:DTAB-HFIP system at different total surfactant concentrations and HFIP%. The results support the complex coacervation model that involves formation of a catanionic complex between the oppositely charged (DTA+-DS-) that is mostly concentrated in the coacervate phase and concomitant release of the counter-ions that reside in the aqueous-rich phase.


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Table 1A. Surfactant counter ion concentration in the aqueous and coacervate phases of the 1:1 SDS:DTAB-HFIP system.

[total Surf.], mM

% v/v HFIP

%Brin Aq.

%Br- in Coac.

95% CI

µmol Br(Aq.)

95% CI

µmol Br(Coac.)

95% CI

20

10

82.6

17.4

2.3

12.7

2.0

2.7

0.6

100

10

90.0

10.0

0.5

65.3

15.2

7.2

1.0

200

10

89.1

10.9

0.4

132.7

7.1

16.3

1.5

200

5

93.0

7.0

0.7

138.1

7.7

10.2

2.2

[total Surf.], mM

% v/v HFIP

% Na+ in Aq

% Na+ in Coac

95% CI

µmol Na+ (Aq)

95% CI

µmol Na+ (Coac)

95% CI

20

10

91.9

8.1

0.8

13.9

1.8

1.2

0.4

100

10

84.7

15.3

3.5

57.0

2.6

10.3

2.6

200

10

82.6

17.4

1.4

118.3

7.9

24.9

4.3

200

5

86.0

14.0

0.8

128.7

4.7

21.0

4.2


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Table 1B. Surfactant concentration in the aqueous and coacervate phases SDS:DTAB-HFIP complex coacervate system.

µmol

µmol

[DTA+] Coac

[total Surf.],

% v/v

% DTA+

% DTA+

95%

DTA+

95%

DTA+

95%

mM

HFIP

(Aq)

(Coac)

CI

(Aq)

CI

(Coac)

CI

(µmol/µL)

20

10

0.36

99.6

0.1

0.05

0.0

14.1

1.3

0.70

100

10

0.36

99.6

0.1

0.27

0.1

73.0

4.2

0.81

200

10

0.38

99.6

0.1

0.53

0.1

140.7

11.0

0.94

200

5

0.27

99.7

0.0

0.42

0.1

154.0

5.7

1.03

µmol -

-

-

µmol -

[DS-] Coac

[total Surf.],

% v/v

% DS

% DS

95%

DS

95%

DS

95%

mM

HFIP

(Aq)

(Coac)

CI

(Aq)

CI

(Coac)

CI

(µmol/µL)

20

10

0.49

99.5

0.3

0.10

0.0

14.1

0.5

0.71

100

10

0.37

99.6

0.2

0.27

0.1

72.1

14.6

0.80

200

10

0.24

99.8

0.0

0.35

0.1

148.3

5.4

0.99

200

5

0.21

99.8

0.0

0.34

0.1

157.5

6.8

1.05

CI = Confidence Interval The data also suggests that as the total HFIP% increases, the number of moles of the amphiphilic ions in the coacervate phase decrease while the number of moles of the counterions in the phase increase. The gravimetric analysis of the SDS-CTAB system at different surfactant mole fractions indicates a similar trend. In other words, the surfactants are transferred from the coacervate phase to the aqueous-rich phase with the increase in HFIP concentration. Thus, at higher HFIP%, the two phases become more similar in composition and converge into one phase where the transition between L/C to L occurs on the phase diagram (Figure 1). 15 ACS Paragon Plus Environment

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Water Content. The results in Figure 4 A-B show that % water (w/w) increases linearly in the coacervate phase with an increase in the total % HFIP in both mole ratios of 1:1 and 3:7 SDS-CTAB. An opposite trend is observed for the aqueous-rich phase in both systems.

Figure 4. Percentage water (w/w relative to its phase) found in the aqueous layer (red) and the coacervate layer (blue) for solutions of 273mM SDS:CTAB with HFIP and surfactant mole ratio (A) 1:1 and (B) 3:7. Percentage water (w/w relative to its phase) found in the aqueous layer (red) and the coacervate layer (blue) for solutions of 1:1 SDS:DTAB with HFIP at concentrations of (C) 50mM and (D) 300mM.

Naturally, an increase in total volume% of HFIP would subsequently mean a decrease in total volume% of water. Thus, the trend of the change in % water in the top aqueous phase is according to that for the overall solution. Perhaps the most striking observation is the overall small % water in the coacervate phase that ranges as little as ~5% (w/w) for the 1:1 coacervate phase at 10% (v/v) total HFIP and as high as ~35% (w/w) for the 3:7 coacervate phase at 30% (v/v) HFIP. Typically, coacervate phases that are formed in purely aqueous media are highly enriched in water with more than 90% water content. This is not the case for the PFAIC systems. This is rather surprising given that the 16 ACS Paragon Plus Environment

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starting solution contains 70% - 95% water with only 5%-30% HFIP; yet water is the minor component of the coacervate phases in PFAIC. Overall, the water content in the nonstoichiometric, 3:7 coacervate phase is greater than that for the stoichiometric mole ratio of 1:1; which might be due to the more hydrophobic nature of the latter phase. Figures 4C-D show a similar trend for the SDS:DTAB (1:1) complex coacervate system at different total surfactant concentrations (50mM and 300mM). As can be seen, the % H2O (w/w) decreases linearly in the aqueous phase, but increases linearly in the coacervate phase with HFIP concentration. Note that the rate of change is smaller at the higher surfactant concentration (Fig. 4D), meaning the increase is more dramatic at lower total surfactant concentrations. This makes sense for the aqueous phase because adding HFIP to the bulk solution means that there is less water overall in the system. For the coacervate phase, increasing the HFIP concentration in the total solution means more HFIP molecules will be extracted into the coacervate phase, which brings more water from the hydration layer into the coacervate along with it.


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Volume analysis. The percent volume of the coacervate phase (relative to the total volume of both phases) was measured as a function of total surfactant concentration (

Figure 5), SDS:CTAB mole ratio (Figure 6), and HFIP concentration (Figure 7).

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16 14 12 10 8 6 4 2 0 0

50

100

150

200

250

300

Total Surfactant Concentration (mM) Figure 5. The percentage volume of coacervate (relative to the volume of the total solution) increases with the total surfactant concentration for 1:1 SDS:CTAB solutions with 9% HFIP.

The volume of the coacervate increased linearly with the total surfactant concentration (


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Figure 5),

which is a similar trend to the one obtained for the TFE systems.10 This might

suggest that the surfactant is in a bilayered bicontinuous structure that extends throughout the coacervate phase - and not discrete structures like micelles. Increasing the surfactant concentration simply would make the structure larger, as opposed to more densely packed/organized. The percent volume of coacervate was also determined at different mole ratios of SDS:CTAB at a constant % HFIP (Figure 6). The results showed that the CTAB-rich mixtures had much larger volumes than the SDS-rich phases at comparable mole fractions.

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20 18 16 14 12 10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1

Mole Fraction SDS Relative to Total Surfactant Figure 6. Dependence of the percentage volume of coacervate (relative to the volume of the total solution) on the mole fraction of SDS/(SDS+CTAB) changes for solutions of 273mM SDS:CTAB with 9% HFIP.

As shown in Figure 7, the coacervate volume increases with an increase in %HFIP. Above the solid to coacervate phase transition line (see the phase diagram, Figure 1), an increase in the fluoroalcohol concentration also increases the size of coacervate (until just before the L/C L boundary where it starts decreasing). This effect is similar to the one seen in Figure 7 for the 7:3 SDS:CTAB solutions. The 1:1 19 ACS Paragon Plus Environment

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SDS:CTAB solutions would follow a similar trend as the percentage of HFIP crosses 35%. 80

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70 60

3:7 1:1 7:3

50 40 30 20 10 0 0

5

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% HFIP (v/v) Figure 7. Dependence of the percentage volume of coacervate responds on total concentration of HFIP at 250mM total surfactant and 3 different mole ratios of SDS:CTAB (3:7, 1:1, and 7:3).

The overall volume and the rate of increase for the CTAB-rich phase (3:7 mole ratio) is greater than the other two mole ratios (1:1 and 7:3). The 1:1 SDS:CTAB system had coacervate volumes that were smaller than the 3:7 system, initially by 3-5% over a % HFIP range of 9-17%. The difference between the CTAB-rich and equimolar coacervates then grew larger (to ∆ ~24%) at around 29% (v/v) HFIP. The anionic-rich, 7:3 SDS:CTAB system yielded the smallest coacervates at all %’s of HFIP measured. There are a number of contributing factors to consider the differences in coacervate volumes. One should note that CTAB molecules (with C-16 alkyl chain) are larger than SDS molecules (C-12 alkyl chain), but while this might play a role, the difference cannot fully account for the size discrepancies at the various mole ratios. The differences in coacervate volumes could potentially be attributed to variations in chemical compositions (% of HFIP, water, and surfactants) at different surfactant mole ratios. As described above, an increase in total HFIP% results in a decrease in surfactant concentration, with little or small changes in HFIP concentration, but a considerable increase in the concentration of water in the coacervate phase. Another issue that should be considered is what happens to the excess surfactants in non-stoichiometric coacervates (e.g., 3:7 and 7:3). In non-stoichiometric mole ratios (such as 3:7 or 7:3 SDS-CTAB), the excess surfactant could actually dissolve some or the entire catanionic complex. Upon addition 20 ACS Paragon Plus Environment

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of HFIP, the complex is dissolved and phase separation occurs as a result of coacervation. The much larger volume for the cationic-rich coacervate system (3:7 SDS-CTAB) could be due to the presence of excess CTAB in the coacervate phase; while the smaller volume of the anionic-rich system can be attributed to the presence of the excess SDS in the top aqueous-rich phase. For the SDS-rich phase, the actual concentration of the catanionic surfactants that participate in coacervation would be smaller than that at the stoichiometric ratio that results in smaller volumes. In a separate study, we observed that potassium dichromate (K2Cr2O6) partitioned into a cationic-rich coacervate phase that suggested the presence of excess of cationic surfactant in the coacervate phase. On the other hand, copper nitrate (Cu(NO3)2) did not interact with the anionic-rich coacervate system and remained in the top-aqueous phase. These colored inorganic salts are suitable probes for studying electrostatic effects on partitioning into coacervates due to the absence of hydrophobic interaction.18 Note that simple coacervation would occur in mixtures of CTAB aqueous solutions and HFIP at nearly neutral or basic pH values. This is not the case for the SDS-HFIP mixtures. In other words, there is a possibility that the 3:7 SDS-CTAB system is composed of both complex and simple coacervates. Discussion. A number of factors should be considered in understanding the underlying driving forces and mechanisms for coacervation as well as microstructures of the coacervate phases in PFAIC. In purely aqueous media, formation of catanionic complex between oppositely charged surfactants such as SDS and CTAB is driven by a combination of electrostatic and hydrophobic interactions. The catanionic complex is insoluble in water mainly due to charge neutralization of the oppositely charged surfactants head groups that also leads to the release of the corresponding counterions (Na+ and Br-) in the solution. The induced coacervation by HFIP would then involve at least three processes: solubilization of the catanionic complex, formation of a molecular assembly between surfactants and HFIP that is immiscible with water, and subsequent phase separation from bulk aqueous phase.


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Due to its strong hydrogen bond donating property, HFIP favorably solvates

Figure 8. Proposed ball and stick structures of SDS and CTAB interacting with HFIP with (A) the oxygen interacting preferably and (B) the CF3 groups. Color code: oxygen - red, fluorine – light blue, nitrogen – dark blue, sulfur yellow, carbon – dark gray, hydrogen – light gray.

anions, but is a poor solvating reagent for cations.19 Thus, the HFIP molecules would likely displace some of the localized water molecules of the hydration layer in the head group region, solvating the sulfate group of SDS in the catanionic complex (SD…CTA+). In addition, the two bulky CF3 groups in HFIP could interact with the terminal methyl groups of the alkyl chains, increasing the distance between the alkyl chains of the two oppositely charged surfactants, and consequently reduce the hydrophobic interaction between alkyl groups closer to the head group in the chains (Figure 8). The interaction between the two hydrocarbon groups is stronger than that between a hydrocarbon and a fluorocarbon; in spite of the fact that a fluorocarbon group is more hydrophobic by nature. A combination of the solvation of the anionic head group and localization of CF3 groups between the surfactant alkyl chains leads to weakening of the electrostatic and hydrophobic interaction between the oppositely charged surfactants and subsequently leads to dissolution of the complex. Perfluorinated alcohols and acids with longer chains (or larger number of fluorocarbon groups) are more effective coacervators. They induce coacervation and phase separation at lower concentrations in solutions of amphiphilic molecules. The role of hydroxyl (or carboxyl) group is solvation (or interaction with) a charged (or polar)


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head group of the amphiphilic molecules; while the fluorocarbon groups facilitate the dehydration of the alkyl chain (or hydrophobic tail). This results in the formation of molecular assemblies of the amphiphiles that are immiscible with water, thus leading to phase separation. HFIP is a highly polar and strong hydrogen bond donor solvent and is fully miscible in water; yet the presence of six fluorine groups makes it hydrophobic (with octanol – water partition coefficient of about 46)20. In spite of miscibility with water, solutions of HFIP in water contain different microstructures in different mole ratios. For example, at lower mole fractions (i.e., water–rich solutions), HFIP mainly exists as monomers, hydrated by water molecules. At higher concentrations (e.g., 20%~30% v/v), the HFIP molecules cluster and form micelle-like micro-assemblies with the fluorine groups aggregating toward the center of the cluster while OH groups are oriented at the surface of micro-structures, surrounded by a hydration layer.21 In HFIP-rich mixtures, the HFIP molecules interact with one another through hydrogen bonding. As shown in the phase diagram (Figure 1), coacervation begins at concentrations as low as 1-2% HFIP and is completed at ~3%-8% HFIP, above which two immiscible liquid phases exist until HFIP% reaches upward of 30%. One can then envision that coacervation occurs in aqueous-rich mixtures where HFIP is present as monomers, associated with water molecules through hydrogen bonding in a hydration layer. Upon coacervate formation, HFIP is extracted (along with the associated water molecules in the hydration layer) in the coacervate phase. Due to enrichment, the effective concentrations of HFIP and surfactants in the coacervate phase (40% – 60% (w/w)) are then several folds higher than those in the aqueous – rich phase (5%-30% (w/w)). Remarkably, as the total HFIP% is increased, more water is extracted into the coacervate phase; apparently replacing some of the surfactants that are transferred into the top-aqueous phase. As a result, the rate of increase in water concentration in the coacervate phase with an increase in total HFIP is actually greater than that for HFIP itself. Consequently, the coacervate phase volume increases with an increase in HFIP%. As mentioned earlier, the presence of a strong hydrogen bond donor/acidic group (like hydroxyl or carboxyl) and the fluorocarbon groups is essential for inducing 23 ACS Paragon Plus Environment

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coacervation. We have not observed coacervate formation with aliphatic alcohols (alkanols) or other perfluorinated reagents with polar functional groups (amine, carbonyl). Thus the hydrogen bond donor/acidic group and fluorocarbon play a synergistic role in inducing coacervation. A comparison of the physico-chemical properties of the polar fluorinated alcohols and their analog alkanols would be beneficial in achieving a better understanding of why fluoroalcohols induce coacervation, while the hydrocarbon-based analogs would not; for example TFE as compared to ethanol and HFIP compared to 2-propanol. The comparison was limited to the short chain alkanols, ethanol (EtOH) and 2-propanol (IPA), because they are fully miscible with water, similar to TFE and HFIP. Longer chain alkanols form separate phases in aqueous solutions. HFIP and IPA both have dielectric constants of ~18 while TFE and EtOH both have values ~26.22 Perhaps this could be used to explain why HFIP behaves slightly differently from TFE, but it doesn’t explain the different effects of hydrocarbon and fluorocarbon alcohol on coacervation and phase separation in aqueous media. Furthermore, a low dielectric constant would suggest that electrostatic interactions would be heightened, however we see a decrease in the CTA+ - DS- head-group interaction. HFIP is a strong hydrogen bond donor (even stronger than water) but a weak hydrogen bond acceptor; which results in weak intramolecular interactions as evident by a low B.P. (~59 oC), which is less than its hydrocarbon analog, 2-propanol (B.P. = 82 oC). In general, fluorinated alcohols (TFE and HFIP) are less self-associated than their hydrocarbon analogs (EtOH and PrOH).22 The fluorine atoms have the effect of increasing the H-bond donating ability of the molecule while decreasing its H-bond accepting ability, even though the fluorine atoms also have the ability to weakly accept H-bonds while the corresponding methyl groups in alkanols cannot. Nevertheless, the hydrogen bonding between the C-F··· H-O becomes increasingly important as the concentration of fluoroalcohol increases in aqueous solutions. In aqueous solutions, all alcohols cluster to some degree due to the hydrophobicity of the hydrocarbons or fluorocarbons. TFE and HFIP have been shown to 24 ACS Paragon Plus Environment

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cluster to a much larger degree (through X-ray scattering data) even to the point where the clusters have been described as micelles.22 The maximum degree of clustering for fluoroalcohols occurs at ~30% (v/v). The maximum degree of clustering for ethanol occurs between 36 – 42%,23 but is to a lesser extent owing to the less bulky CH3 compared to CF3. The formation of the larger clusters of fluoro-carbon groups in aqueous solutions could change the microenvironment of the surfactants to favor the dispersion interactions of the chains over the electrostatic interactions of the head-groups. Conclusion We are reporting the formation of coacervates using HFIP in catanionic surfactant mixtures over a wide range of compositions. Coacervates were made from 2mM to 400mM total surfactant concentration and could be made at even higher concentrations. The HFIP-induced coacervates were also formed by systems with % HFIP from 2% up to 60%. The coacervates’ volumes grow linearly with an increase in surfactant concentration for equimolar SDS:CTAB and SDS:DTAB solutions. The coacervate phases are surfactant-rich as is expected, but surprisingly contain a significantly higher concentration of HFIP in spite of HFIP’s high polarity and full miscibility with water. The patterns of phase diagram and composition in the mixed surfactant complex coacervates are strikingly similar to the mixed anionic polyelectrolyte – cationic surfactant that was reported recently.11 We speculate the HFIP first solubilizes the insoluble catanionic complex by interacting favorably with the sulfate group of SDS and then shields it from the positively charged CTAB or DTAB molecules. Once soluble, the surfactants aggregate due to hydrophobic interactions and then phase separate from the bulk aqueous phase. HFIP’s stronger hydrogen bond donating ability (relative to isopropanol) is likely a major contributing factor to its coacervate-inducing ability. Furthermore, its clustering nature due to the hydrophobic effect of the fluorine groups could also play a role. Ongoing work will hopefully reveal more about the specific mechanism responsible for the observed phase separations and formation of surfactant-rich coacervate phases. Additional information about the exact structures of these phases are 25 ACS Paragon Plus Environment

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needed for achieving a better understanding of the involved mechanisms. This would require TEM and diffraction experiments. The unique composition of the SDS-CTAB-HFIP and SDS-DTAB-HFIP systems means that the solutions have the potential for use as solvents. Studies to using these unusual systems (and other PFAICs) for preconcentration, purification, and as a medium for chemical reactions are also underway. It should be noted that fluoroalcohols, especially HFIP is a hazardous and expensive solvent. However, the initial concentration of HFIP that is needed to prepare coacervate phases is small (~10% v/v). Given the high effective concentrations of surfactants and HFIP due to enrichment effect, the coacervate phases are potent solvents for a wide range of compounds from very hydrophobic organic substances to membrane proteins24. Recently, we reported that organic reactions could be carried out in two-phase aqueous - fluoroalcohols systems with equivalent or higher yields than those in pure fluoroalcohol25. References (1) Chollakup, R.; Smitthipong, W.; Eisenbach, C.D.; Tirrell, M. Phase Behavior and Coacervation of Aqueous Poly(acrylic acid)-Poly(allylamine) Solutions. Macromolecules. 2010, 43, 2518-2528. (2) Spruijt, E.; Stuart, M.A.C.; van der Gucht, J. Linear Viscoelasticity of Polyelectrolyte Complex Coacervates. Macromolecules. 2013, 46, 1633-1641. (3) de Kruif, C.G.; Weinbreck, F.; de Vries, R. Complex Coacervation of Proteins and Anionic Polysaccharides. Curr. Opin. Colloid Interface Sci. 2004, 9, 340-349. (4) Ortony, J.H.; Hwang, D.S.; Franck, J.M.; Waite, J.H.; Han, S. Asymmetric Collapse in Biomimetic Complex Coacervates Revealed by Local Polymer and Water Dynamics. Biomacromolecules. 2013, 14, 1395-1402. (5) Deng, M.; Cao, M.; Wang, Y. Coacervation of Cationic Gemini Surfactant with Weakly Charged Anionic Polyacrylamide. J. Phys. Chem. 2009, 113, 9436-9440. (6) Yan, Y.; Kizilay, E.; Seeman, D.; Flanagan, S.; Dubin, P.L.; Bovetto, L.; Donato, L.; Schmitt, C. Heteroprotein Complex Coacervation: Bovine β-Lactoglobulin and Lactoferrin. Langmuir. 2013, 29, 15614-15623. (7) Xiao, J.-X.; Sivars, U.; Tjerneld, F. Phase Behavior and Protein Partitioning in Aqueous Two-Phase Systems of Cationic-Anionic Surfactant Mixtures. J. Chromatogr. B. 2000, 743, 327-338. (8) Nan, Y.; Liu, H.; Hu, Y. Aqueous Two-Phase Systems of Cetyltrimethylammonium Bromide and Sodium Dodecyl Sulfonate Mixtures Without and With Potassium Chloride Added. Colloid Surface A. 2005, 269, 101-111. 26 ACS Paragon Plus Environment

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(9) Weschayanwiwat, P.; Krutlert, D.; Scamehorn, J.F. Effect of Electrolyte and Temperature on Volatile Organic Compounds Removal from Wastewater Using Aqueous Surfactant Two-Phase System of Cationic and Anionic Surfactant Mixtures. Separ. Sci. Technol. 2009, 44, 2582-2597. (10) Khaledi, M.G.; Jenkins, S.I.; Liang, S. Perfluorinated Alcohols and Acids Induce Coacervation in Aqueous Solutions of Amphiphiles. Langmuir. 2013, 29, 2458-2464. (11) Nejati, M.M.; Khaledi, M.G. Perfluoro-Alcohol-Induced Complex Coacervates of Polyelectrolyte – Surfactant Mixtures: Phase Behavior and Analysis. Langmuir. 2015, 31, 5580-5589. (12) Šegota, S.; Težak, Đ. Spontaneous Formation of Vesicles. Adv. Colloid Interface Sci. 2006, 121, 51-75. (13) Safran, S.A.; Pincus, P.; Andelman, D. Theory of Spontaneous Vesicle Formation in Surfactant Mixtures. Science. 1990, 248, 354-356. (14) Kume, G.; Gallotti, M.; Nunes, G. Review on Anionic/Cationic Surfactant Mixtures. J. Surfact. Deterg. 2008, 11, 1-11. (15) Filipović-Vinceković, N.; Bujan, M.; Dragčević, Đ.; Nekić, N. Phase Behavior in Mixtures of Cationic and Anionic Surfactants in Aqueous Solutions. Colloid Polym. Sci. 1995, 273, 182-188. (16) Caria, A.; Khan, A. Phase Behavior of Catanionic Surfactant Mixtures: Sodium Bis(2-ethylhexyl)sulfosuccinate-Didodecyldimethylammonium bromide-Water System. Langmuir. 1996, 12, 6282-6290. (17) APHA, AWWA, and WEF, 2005. Standard Methods for the Examination of Water and Wastewater, 21st. American Public Health Association, Washington, D.C. (18) S. Liang and M.G. Khaledi; Unpublished Results. (19) Evans, D.F.; Nadas, J.A.; Matesich, M.A. Transport Properties in Hydrogen Bonding Solvents. VI. The Conductance of Electrolytes in 2,2,2-Trifluoroethanol. J. Phys. Chem. 1971, 75, 1708-1713. (20) Abraham, M.H.; Chadha, H.S.; Whiting, G.S.; Mitchell, R.C. Hydrogen Bonding. 32. An Analysis of Water-Octanol and Water-Alkane Partitioning and the ∆log P Parameter of Seiler. J. Pharm. Sci. 1994, 83, 1085-1100. (21) Hong, D.-P.; Hoshino, M.; Kuboi, R.; Goto, Y. Clustering of Fluorine-Substituted Alcohols as a Factor Responsible for Their Marked Effects on Proteins and Peptides. J. Am. Chem. Soc. 1999, 121, 8427-8433. (22) Gente, G.; Mesa, C.L. Water-Trifluoroethanol Mixtures: Some Physicochemical Properties. J. Solution Chem. 2000, 29, 1159-1172. (23) Nishi, N.; Takahashi, S.; Matsumoto, M.; Tanaka, A.; Muraya, K.; Takamuku, T.; Yamaguchi, T. Hydrogen bonding cluster formation and hydrophobic solute association in aqueous solution of ethanol. J. Phys Chem. 1995, 99, 462-468. (24) McCord J.; Muddiman DM., Khaledi MG, Unpublished results.


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(25) Weisner N; Khaledi MG; Organic Synthesis in Fluoroalcohol – Water Two Phase Systems, Green Chemistry, 2016, DOI: 10.1039/C5GC01463H

ACKNOWLEGEMENT. "This material is based upon work supported by the National Science Foundation under CHE - 1412911."

TOC Artwork


Perfluorinated Alcohols Induce Complex Coacervation in Mixed

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