Article pubs.acs.org/Langmuir
Perfluoro-Alcohol-Induced Complex Coacervates of Polyelectrolyte− Surfactant Mixtures: Phase Behavior and Analysis Mahboubeh M. Nejati and Morteza G. Khaledi* Department of Chemistry, North Carolina State University, 2620 Yarbrough Street, Raleigh, North Carolina 27695-8204, United States S Supporting Information *
ABSTRACT: Perfluorinated alcohols and acids such as hexafluoroisopropanol (HFIP), trifluoroethanol, trifluoroacetic acid, pentafluoropropionic acid, and heptafluorobutyric acid induce coacervation and phase separation in aqueous solutions of a wide variety of individual and mixed amphiphiles [Khaledi et al. Langmuir 2013, 29, 2458]. This paper focuses on HFIP-induced complex coacervate formation in the mixtures of anionic polyelectrolytes, such as sodium salt of poly(methacrylic acid) (PMA) or poly(acrylic acid) (PAA) and cationic surfactants of alkyltrimethylammonium bromides. In purely aqueous media and over a wide concentration range, mixtures of PMA and CTAB form the catanionic complex (CTA+PM−) that is insoluble in water (white precipitate). Upon addition of a small percentage of HFIP, the mixture goes through phase transition and formation of two distinctly clear liquid phases. The phase diagram for the HFIP−PMA−CTAB coacervate system was studied. The coacervate volume was determined as a function of system variables such as charge ratio as well as total and individual concentrations of the system components. These results, combined with the chemical composition analysis of the separated aqueous top-phase and coacervate bottom-phase, shed light on the coacervation mechanism. The results suggest that exchange of counterions and ion-pair formation play critical roles in the coacervation process. This process facilitated by HFIP through solvation of the head groups and dehydration of the hydrophobic moieties of the catanionic complex. Because of the presence of HFIP, coacervation occurs over a wide range of concentrations and charge ratios of the oppositely charged polyelectrolyte and surfactant.
1. INTRODUCTION Coacervation2 is a phase separation process where molecular assemblies of an amphiphile or mixtures of oppositely charged amphiphiles cause immiscibility with water that results in formation of a separate phase from the bulk aqueous solution. In general, coacervation has been classified into “simple”3−7 and “complex” categories.8 Simple coacervates are composed of a single amphiphile/polymer where a change in solution properties such as pH,4 additives,5 temperature,6 or salt concentration6,7 causes dehydration of surfactant or polymer molecules present in aqueous solution. Complex coacervates are composed of oppositely charged amphiphiles/polymers such that one dense phase contains significant amount of the both amphiphilic species while the other phase is mainly depleted from them.8−30 An excellent review of coacervation of surfactants in aqueous solutions has been published.8 Recently, preparation and formation mechanism of surfactant−polyelectrolyte complex coacervates (SPCC) have attracted significant attention.18−37 SPCC systems are utilized in the food industries,30 formulation of health care products,31−34 encapsulation of many active biomolecules in pharmaceutical industries (e.g., for delivery of antibacterial agents),35,36 and modeling of protocells for undestanding of life origin.37 In such systems, micelles of ionic surfactants are stabilized by oppositely charged polyelectrolytes, and the phase separation © 2015 American Chemical Society
is associative. The suitability of a complex coacervate system for a specific application depends on the phase behavior, chemical compositions, and physical and chemical properties of each phase which can be controlled by monitoring initial compositions. The interaction between oppositely charged surfactant and polymer is highly cooperative at a critical aggregation concentration (cac) which is usually much lower than critical micelle concentration (cmc). Since a dominant interaction is electrostatic, parameters such as ionic strength, the charge and chain length of surfactants, and linear charge density of polyelectrolytes can affect phase behavior of the system. The coacervation of a biopolyelectrolyte, hyaluronan (Hy), and alkyltrimethylammonium bromide (CnTAB) was the first complete report of SPCC systems.18,19 Based on these studies, binding of Hy with an oppositely charged surfactant (CTAB) is cooperative. In addition, the phase diagram of polyacrylate and cationic surfactants was also investigated.20 Picullel et al.21−23 showed that the dilution of the concentrated mixtures of sodium salt of poly(acrylic acid) (NaPA) and CTAB by water induces complex coacervate. In dilute mixtures, the screening of Received: February 11, 2015 Revised: April 27, 2015 Published: April 29, 2015 5580
DOI: 10.1021/acs.langmuir.5b00444 Langmuir 2015, 31, 5580−5589
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Langmuir the electrostatic interaction between CTA+ and PA− ions would be reduced (due to the dilution of the corresponding counterions) that results in phase separation. They reported that in the dilute mixtures of NaPA−CTAB the bromide counterions in CTAB were displaced by the carboxylate functional groups in PA−n; thus, polyelectrolytes bridge micelles which causes formation of elongated aggregates of CTA-PA and phase separation.21−23 As will be shown in this paper, the presence of a small concentration of a fluoroalcohol results in coacervation and phase separation even in concentrated solutions. Some additives such as nonionic surfactants can also induce SPCC systems by attenuation of the electrostatic attraction force governing phase separation. Dubin et al.24−28 reported a thorough study on how attenuation of strong electrostatic interaction between a polyelectrolyte and an oppositely charged surfactant leads to coacervation. In their effort to overcome narrow concentration ranges in coacervation of polydiallyldimethylammonium chloride (PDADMAC) and sodium dodecyl sulfate (SDS), they incorporated Triton X-100 in SDS micelles. This procedure helps complex formation above critical micelle concentration (cmc); otherwise, in the absence of nonionic surfactant, the polyelectrolyte−surfactant complex in aqueous solution leads to precipitation. Recently, we reported that perfluorinated alcohols such as hexafluoroisopropanol (HFIP) and trifluoroethanol and perfluorinated acids such as trifluoroacetic acid, pentafluoropropionic acid, and heptafluorobutyric acid induce coacervation in mixtures of oppositely charged surfactants in aqueous media.1 The minimum concentration of the fluoro alcohol or fluoro acid for coacervation is inversely related to the number of the fluorine atoms in the fluoro alcohol. Chen et al. used HFIP induced SDS−DTAB for extraction of polar sulfonamides (SAs) in water.29 This paper is the first detailed account of the HFIP induced complex coacervation in catanionic mixtures of polyelectrolytes and surfactants over a wide range of concentrations and mole ratios.
the stock solutions using three different methods as following: experiment set 1 (1a, 1b, and 1c): constant mole number of CTAB; experiment set 2 (2a and 2b): constant mole number of PMA; experiment set 3 (3a and 3b): variable number of moles for both CTAB and PMA (see S1 in Supporting Information for more details) followed by addition of HFIP for coacervation. HFIP concentration was reported in % (v/v). Thus, charge ratio (CR) in this paper is referred to initial bulk charge ratio of PM− to CTA+. Deionized (DI) water was used for sample preparation. All experiments were performed at room temperature. 2.2.1. Phase Boundary Diagrams. A phase boundary diagram was constructed through visual observations of phase changes upon addition of different amounts of HFIP to the mixed PMA−CTAB samples at different concentrations of PMA and CTAB, keeping the bulk charge ratio constant at 1:1. After 5 min of mixing by vortex vibrator, 5 min centrifuging at 1800 rpm, and 1 h equilibrium, the physical appearance of the samples was visually inspected. The phase diagram of HFIP−sodium methacrylate (NaMA)−CTAB was built with the same concentration range as HFIP−PMA−CTAB system at 1:1 mole charge ratio for comparison purposes. 2.2.2. Coacervate Volume Fraction. Precipitation samples were prepared by mixing aliquots of 10.9 mM PMA and 100 mM CTAB in DI water with charge mole ratios of 1:1, 7:3, and 3:7 of PMA to CTAB (see experiment sets 1, 2, and 3 in Supporting Information). Next, HFIP was added for coacervation. After stirring for 3 min, the samples were centrifuged at 1800 rpm for 10 min at room temperature. A 24 h equilibration time was given for all samples before measuring coacervate phase volume. The volume of each phase was also analyzed at different concentration of PMA and CTAB with 1:1 and variable charge ratios at constant concentration of HFIP (19%). 2.2.3. Phase Composition Analysis. HFIP Analysis. The ATR-IR spectra were obtained as the average of 64 scans, with 1 cm−1 resolution at room temperature. The samples were prepared at 1:1 charge ratio containing 84.6 mM CTAB, 1.50 mM PMA, and 10% (v/ v) HFIP. After stirring for 5 min and centrifugation at 1800 rpm, and 24 h equilibrium time, samples were analyzed. A 50 μL of each phase was directly used for analysis. The calibration curves (see Figure S1 in Supporting Information) were built at two different wavenumbers using standard solutions of HFIP in DI water over the concentration range of 2%−80% (v/v). The wavenumbers of 1104 and 1190 cm−1 were chosen for calibration due to the higher sensitivity and greater linearity. Neither CTAB nor PMA had an absorbance band at the selected characteristic wavelengths of HFIP. Water Analysis. Water percentage (w/w) was determined by Karl Fischer titration. The aqueous and coacervate phases were separated by 1 mL syringes and analyzed without any sample manipulation. This analysis was performed for experiment set 1 at three different mole ratios of 1:1 (set 1a), 7:3 (set 1b), and 3:7 (set 1c) of PMA:CTAB ratio (see S1 in Supporting Information). Sodium Ion Analysis. This analysis was performed for both aqueous top phase and coacervate bottom phase. Samples were prepared corresponding to experiment set 1 (1a, 1b, and 1c) at charge ratios of 1:1, 7:3, and 3:7. Different concentrations of HFIP (10−30% (v/v)) were used for coacervation. Aliquots (30 μL) of the aqueous-rich top phases were diluted to 100 mL by DI water. An aliquot (200 μL) of the coacervate phase of each sample was diluted in 30% (v/v) 2propanol. Standard solutions of NaCl (0.2−1.0 μg/mL) in 30% (v/v) 2-propanol and DI water containing 0.5% (w/v) KCl (as ionization buffer) were used for construction of calibration plots for the analysis of sodium content in coacervate and aqueous phases, respectively. CTAB Analysis. Two-phase volumetric analysis method was applied for determination of CTAB concentrations in the top aqueous phases of coacervate samples at different HFIP concentrations (experiment set 1, see Supporting Information). A 0.020 M standard solution of sodium dodecyl sulfate (SDS) was used as titrant. Bromide Analysis. Concentration of the bromide ion in the top aqueous phase of the coacervate was determined by volumetric Fajan’s titration for the experiment set 1a samples in which fluorescein was used as the indicator. The accuracy of the method was determined to be 1.3% by titration of standard solutions of KBr and CTAB.
2. EXPERIMENTAL SECTION 2.1. Materials. Cetyltrimethylammonium bromide (CTAB), CH3(CH2)15N+(CH3)3Br−, was purchased from USB Corporation as “ultrapure” and used without further purification. 1,1,1,3,3,3Hexafluoroisopropanol (HFIP), ≥99.9%, was purchased from TCI America. Dimidium bromide (DB), 95%, for the two-phase titration of CTAB was ordered from Alpha Aesar. Sulphan blue (SB), C27H31N2O6S2Na, was purchased from Spectrum. Chloroform was provided from Fisher Scientific. The sodium salt of poly(methacrylic acid) (PMA) with average molecular weight (MW) of 6500 was from Sigma-Aldrich. Sodium analysis was performed by flame atomic emission spectrometry using AAnalyst 100 instrument, PerkinElmer, at λ = 589.6 nm. The concentration of HFIP in each phase of the HFIP− PMA−CTAB coacervates was determined by attenuated total reflection-infrared spectroscopy (ATR-IR). A germanium disk (Pike Technologies Inc. MIRalce single reflection ATR, Madison, WI) attached on Bio-Rad (Hercules, CI) Digilab FTS-3000 was used for this analysis. Water analysis was performed by 701 KF Titrino (Metrohm Ltd.; Herisau, Switzerland). Fluorescence images of separated aqueous phase and coacervate phase were taken by a Nikon Eclipse 80i with a mercury lamp (X-Cite 120 Q) used for excitation. The images were recorded by Andor iXon 897 Camera (16 × 16 μm pixel size, 512 × 512 imaging array). 2.2. Method. Coacervate Sample Preparation. HFIP-induced coacervate systems of PMA−CTAB at three different mole ratios (CR) of 1:1, 7:3, and 3:7 were prepared by mixing appropriate volumes of 5581
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Figure 1. Phase boundary diagram of HFIP-induced PMA−CTAB SPCC system (A) and HFIP-induced of sodium methacrylate (NaMA)−CTAB coacervate (B) at a 1:1 bulk charge ratio. WG = white gel, L = amphiphile-lean aqueous phase, C = coacervate, S = solid (precipitate); the solvent is DI water. The lines are for easy tracking.
Figure 2. (A) ATR-IR spectra of HFIP in separated aqueous (black trace) and coacervate (red trace) phases of the 10% HFIP induced PMA−CTAB coacervate at 1:1 charge ratio. (B) ATR-FTIR spectra of the aqueous and coacervate phases of PMA−CTAB induced coacervate by HFIP over a range of 10%−30% (v/v) HFIP. The PMA−CTAB charge ratio of 1:1, experiment 1.1a. The brown labels are for HFIP peaks of aqueous phases, and the green label is for HFIP peaks of the coacervate phases which have overlapped.
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Langmuir PMA Analysis. The PMA content of the coacervate phase (1:1 charge ratio, experiment set 1a) was quantified by volumetric acid− base titration with NaOH using pH meter for detection of equivalence point. For better determination of the equivalence point, larger volumes of the coacervate samples were prepared. The solvent of the coacervate phase was evaporated, and the remaining precipitate was dissolved in excess HCl and titrated by 0.126 M NaOH. 2.2.4. Fluorescence Microscopy. The images were taken for the separated aqueous and coacervate phases of 15% HFIP−PMA−CTAB at charge ratio of 1:1 containing 1 μM of Nile Red.
coacervation in PMA−CTAB is different due to formation of the catanionic PMA−CTA complex (i.e., precipitate formation in purely aqueous mixtures of PMA and CTAB). There are some similarities between phase diagrams of HFIP−NaMA− CTAB with that of HFIP−PMA−CTAB where the minimum HFIP concentration to induce the two-phase system (L/C) is nearly independent of the CTAB and NaMA concentrations and the upper HFIP concentration, where L/C transition to a single phase (L) takes place, increases with the amphiphiles concentration. One interesting point is the existence of a small three-phase region at higher concentration (L/L/C) in the phase diagram (Figure 1B). 3.2. Phase Composition. HFIP Analysis. HFIP has several distinct absorbance bands in the range of 1050−1400 cm −1 that can be used for its quantitation in coacervate (see Figure S2 in Supporting Information). Figure 2A shows the ATR-IR spectra of the coacervate (red trace) and amphiphile-lean aqueous top phase (black trace) for the 10% HFIP−PMA− CTAB system at 1:1 charge ratio. The last band in the ATR-IR spectra at 1600 cm−1 belongs to water. The ATR-IR analysis shows that the coacervate phase of this system contains high concentration (50% (v/v)) HFIP (Figure 2A). For example, at 10% (v/v) HFIP (initial concentration), the concentrations of the fluoroalcohol in the aqueous and the corresponding coacervate phases are 10−11% and 51−55% (v/v), respectively (Table 1). The HFIP concentration profile in both the aqueous
3. RESULTS 3.1. Phase Diagram. Coacervation of the oppositely charged surfactants with fluoroalcohols/acids has previously been demonstrated in our lab.1 The water-miscible fluoroalcohols/acids can also induce coavervatetion in oppositely charged anionic polyelectrolyte−cationic surfactant systems. Figure 1A shows the complex coacervate phase diagram for the HFIP−PMA−CTAB system. In aqueous media, a mixture of PMA and CTAB forms a white precipitate. Addition a small amount of HFIP (200 mM CTAB). At the upper HFIP threshold, where the two phase L/C transitions to a single liquid phase (L); however, there is a sudden increase in the coacervate area from ∼30% HFIP to ∼70% HFIP (∼130 mM CTAB and 2.2 mM PMA). Interestingly, HFIP induces coacervate phase in the mixture of CTAB with sodium methacrylate (NaMA, the monomer in PMA) (see Figure 1B). By definition, this is a simple CTAB coacervate system since it involves only one amphiphile. Previously, we have observed that the ionized form of HFIPinduced coacervation in buffered CTAB solution at basic pH values (>7). The molar concentration of HFIP is considerably higher than CTAB (for example, 10% HFIP has a molar concentration of around 950 mM). Therefore, a small percentage of HFIP (pKa = 9.3) produces enough fluorinated anion (FIP−) to form an ion pair with CTA+ and induce coacervation. Adding sodium methacrylate would effectively increase the pH of the solution and subsequently would partially ionize HFIP. It is noteworthy that the NaMA−CTAB mixture is one homogeneous liquid phase in the absence of HFIP (in contrast to CTAB−PMA mixture that results precipitation in aqueous solution). Unfortunately, the pH of the aqueous and coacervate phases cannot be accurately determined with the pH meter due to the presence of HFIP. The use of pH-indicator dyes, however, suggests that the pH of the media is between 5 and 8 (see S6 in the Supporting Information). The polyelectrolyte, PMA, could also have a simple role of raising the pH; however, the mechanism for
Table 1. Concentration of HFIP (v/v %) in the Aqueous and Coacervate Phases of the 1:1 PMA:CTAB Charge Ratio Induced by 10% Total HFIP Corresponding to Experiment Set 1a phase
(1/λ) max (cm−1)
avg % HFIP
std dev
coacervate
1040 1190 1040 1190
51.5 54.8 11 10
0.6 0.8 1.1 1.5
aqueous
and the coacervate phases as a function of initial % HFIP for coacervation at 1:1 charge ratio (related to experiment set 1a) has been investigated. The results clearly show that the HFIP concentration in the coacervate phase is independent of the bulk HFIP concentration, while it increases in the aqueous phase accordingly (Figure 2B). Water Analysis. The concentration of water increases in the coacervate phase and decreases in the amphiphile-lean top aqueous phase linearly with the increase in total % HFIP at three different charge ratios (1:1, 7:3, and 3:7; see Figure 3 and Figure S1 in the Supporting Information) of PMA−CTAB. The volume of the coacervate phase increases linearly with % HFIP
Figure 3. Water concentration (% w/w) in the coacervate (A) and aqueous (B) phases of HFIP induced PMA−CTAB coacervate as a function of % HFIP (10−27%). 1:1 bulk charge ratio of PMA/CTAB; the samples correspond to experiment set 1 (1a). 5583
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of the catanionic complex (CTA+PMA−) in the coacervate phase and consequently, the release of the counterions in aqueous phase. Surfactant Analysis. The mass of CTAB in the top aqueous and bottom coacervate phases as a function of HFIP in the SPCC systems with three different charge ratios (1:1, 7:3, and 3:7) in experiment set 1 (1a, 1b, and 1c) has been investigated (Figure 5). In both 1:1 (Figure 5A) and 7:3 (Figure 5B) charge
in the 1:1 and 7:3 PMA−CTAB systems. We speculate that the increase in the volume of the coacervate phase may be associated with a higher water concentration in this phase; however, different behavior was observed in the 3:7 PMA− CTAB system and the volume of coacervate phase remains constant at all % HFIP. The coacervate phase is fairly stable with an increase in the total HFIP concentration up to a certain point (∼24%), beyond which the amphiphiles begin to transfer from the coacervate to the top aqueous phase. The transfer of the amphiphiles from the coacervate to the top aqueous phases reduces the hydrophobicity of the coacervate phase and increases the water content. Sodium and Bromide Ions Analysis. The results in Figure 4A show that sodium ions mostly exist in the aqueous phase
Figure 5. CTAB mass in the aqueous (open symbol) and coacervate (filled symbol) phases of 10−30% HFIP induced the PMA−CTAB coacervate system as a function of HFIP. Charge ratio 1:1 (A), 7:3 (B), and 3:7 (C) of PMA/CTAB corresponding to experiment set 1 (1a, 1b, 1c) analyzed by biphasic titration; 0.02 M SDS was used as titrant.
ratios of PMA/CTAB, most of CTA+ ions are in the coacervate phase, and there is no change in the quantity of the surfactant over the range of 10−20% HFIP. However, at 24% HFIP the cationic surfactant mass in the coacervate phases starts to decline, and a concomitant increase in the corresponding mass in the aqueous-rich phases occurs. At 3:7 charge ratio of PMA/ CTAB (Figure 5C), the excess CTAB is in the top phase; the amount of CTAB in both top and bottom phases remain relatively constant or changes little at HFIP
