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Assembly of Amphiphilic Hyperbranched Polymeric Ionic Liquids in Aqueous Media at Different pH and Ionic Strength Volodymyr F. Korolovych,† Petr A. Ledin,† Alexandr Stryutsky,‡ Valery V. Shevchenko,‡ Oleh Sobko,‡ Weinan Xu,† Leonid A. Bulavin,§ and Vladimir V. Tsukruk*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Institute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kharkivske Shosse 48, Kyiv 02160, Ukraine § Taras Shevchenko National University of Kyiv, Volodymyrska Str. 64, 01601 Kyiv, Ukraine ‡

S Supporting Information *

ABSTRACT: We demonstrated the assembly of amphiphilic hyperbranched protic ionic liquids (HBP-ILs) based on aliphatic hyperbranched polyester (HBP) in aqueous media in a wide range of pH and ionic conditions. The series of new branched polyionic liquids with different terminal groups, HBP-ILs, was synthesized by neutralization of carboxylic and sulfonic terminal acid groups of hypebranched core with Nmethylimidazole (Im) and 1,2,4-1H-triazole (Tr). HBP-IL compounds with triazole and imidazole counterions form 12− 16 nm core−corona micelles at pH 11.6. We found that the introduction of long hydrophobic terminal groups such as noctadecylurethane tails to initial hydrophobic HBP core has larger effect on the size of micellar assemblies than the introduction of ionic terminals groups. Furthermore, tuning the hydrophilic/hydrophobic balance of HBP-ILs can be achieved by changing the degree of ionization of terminal groups and counterions by reducing pH from 11.6 to 5.2 or ionic strength to 0.1 M. These changes caused the formation of much larger micellar aggregates with the size of 150−200 nm due to reduced ionization of carboxylic groups. At the same time, for sulfonate-containing HBP-ILs the micelle size increased modestly (to 25−40 nm) because of the higher degree of ionization of sulfonate terminal groups. The diverse aggregation behavior of these branched polymeric ionic liquids enables control over their micellar morphologies in solution and bulk states.

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INTRODUCTION The unique molecular structure and high charge densities of ionic liquids (ILs) result in their interesting properties such as high chemical, electrochemical, and thermal stability, low flammability, low vapor pressure, structural variability, and high ionic conductivity.1−5 These properties along with their tendency to remain in the liquid state at room temperature make it possible to utilize ILs as novel solvents1−3 or lubricants,6 as a medium for organic synthesis or catalysis,2,3,7 and in energy-conversion technologies.5,8,9 Furthermore, the unique physical properties of such ionic liquids can be retained under extremely low or high temperatures, and the chemical composition allows control over the viscosity, polarity, electric conductivity, or density.5,10−12 For example, it has been demonstrated that poly(ionic liquid)s (PILs)13−16 can form complex and hierarchically structured assemblies, such as multilamellar nanoparticles in a concentrated ionic environment.17−19 However, a vast majority of PILs reported to date are small molecules or short linear macromolecules with low molecular weight, whereas more complex macromolecular architectures are rarely explored. On the other hand, hydroxy-terminated aliphatic hyperbranched polyesters (HBP) demonstrate unique features such as increased solubility, low viscosity of solutions and melts at © XXXX American Chemical Society

high molecular weight, increased thermal stability, and the ability to form complex compounds of guest−host type.20−24 The availability of a large number of reactive end groups on the outer shell provides ample opportunities for their facile chemical modification and synthesis of uniform and heterogeneous branched materials of Janus type, amphiphilic type, or multifunctional types.25,26 Combining unique properties of highly branched macromolecules and ionic liquids might provide interesting opportunities for developing nanoscale building blocks for novel functional materials with ionic transport properties tailored by molecular and supramolecular organization. So far, the few existing reports on branched ionic liquids pertain mainly to their synthesis or the study of structure− property relationships of select cationic HBP-ILs.12,27−29 For instance, the synthesis of amphiphilic aprotic HBP-ILs with a hydrophobic polyester core and alkylimidazolium shell was described.27 In this instance, triflate, tosylate, bis(trifluoromethane)sulfonimide, tetrafluoroborate, and hexafluorophosphate anions were used as counterions. The substituents Received: July 19, 2016 Revised: September 14, 2016

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DOI: 10.1021/acs.macromol.6b01562 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Hyperbranched Amphiphilic HBP-ILs Compounds with 50% of Hydrophobic Tails and Carboxylate (Left Path) and Sulfonate (Right Path) Terminal Groups

been explored yet. It should be emphasized that the majority of the known HBP-ILs (protic and aprotic) are cationic compounds. Anionic HBP electrolytes described in the literature are acid compounds but not ionic liquids with ionic groups located on the periphery of hydrophobic core. Regulation of general hydrophobicity was carried out by neutralizing the amine groups or N-heterocycles as a part of those using aliphatic, aliphatic−aromatic, and aromatic carboxylic and sulfonic acids and sulfuric acid esters. Herein, we report the synthesis and assembly behavior of amphiphilic hyperbranched anionic protic polymeric ionic liquids, in which the hydrophobic components composed of polyester cores with peripheral n-octadecylurethane groups, and the hydrophilic groups are composed of carboxylate and sulfonate terminal ionic groups (Scheme 1). The hydrophilic protic ionic liquid component is formed by the neutralization of the terminal acid groups (carboxylic and sulfonic) with typical for ionic liquids five-membered nitrogen-containing heterocycles of imidazole and triazole types. The benefit of the proposed HBP-ILs is the ability of n-octadecylurethene fragments to increase hydrophobicity of the polyester core and to support not only hydrophobic interactions of long alkyl “tails” but additionally hydrogen bonding of urethane groups as a driving force of self-assembling and stabilizing the micellar structures.

at the nitrogen atom of imidazolium cation were represented by hydrophobic alkyl chains of various lengths. The ability of such HBP-ILs to serve as a surfactant for solubilization of hydrophilic compounds in nonpolar solvents and polymeric systems was demonstrated. Additionally, the synthesis of hydrophobic protic HBP-IL by neutralization of polyamidoamine dendrimers with nitric acid with subsequent ion exchange to bis(trifluoromethane)sulfonamide anion was described. 28 It was shown that such compounds are characterized by high thermal stability, photoluminescence, and relatively high proton conductivity (2.2 mS cm−1 at 24 °C in anhydrous conditions).28 The assembly of HBP-ILs composed of a polyether core and methy(l)imidazolium cations and methyl orange anions in outer shell into vesicles in aqueous media was demonstrated.29 It was also shown that with decreasing pH from 9 to 5 the size of the assemblies increased and was accompanied by the change in color of the colloidal solution (from red to yellow) due to isomerization of methyl orange anions. The replacement of HBP-IL counteranions by bovine serum albumin (BSA) resulted in a significant increase in vesicle size. As known, amphiphilicity can be introduced by adjusting the hydrophilicity of the cores and by incorporation of terminal hydrophilic ionic liquid groups or long-chain hydrophobic tails.12,29 Such a combination might lead to interesting aggregation behavior in solution due to interplay between ionic and hydrophobic interactions, which has not B


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equilibrate for 30 min. Compression of the monolayers was conducted at 5 mm/min. Atomic force microscopy (AFM) images were obtained using a Dimension-3000 (Digital Instruments) microscope in the “light” tapping mode according to the established procedure.33,34 For sample preparation, a drop of HBP-ILs solution was placed onto a silicon wafer and dried in air prior to AFM imaging. All silicon wafers were precleaned by “piranha solution” (caution: strong oxidizer!) according to the typical procedure.35 Transmission electron microscopy (TEM) was performed with a JEOL 100CX-2 electron microscope at 100 kV with samples drop-cast on carbon−Formvar-coated copper grids (Ted Pella).

We have synthesized a novel library of amphiphilic HBP-IL compounds with variable hydrophobic−hydrophilic balance that incorporates large hydrophobic fragments and stimuliresponsive hydrophilic fragments composed of terminal ionic groups (Scheme 1). We observed that HBP-ILs with carboxylate groups and imidazolium counterions show smaller surface molecular area of the corresponding HBP-ILs at the air−water interface that caused the formation of larger micelles in aqueous media at neutral or acidic pH values. Furthermore, we demonstrated the ability to tune the overall hydrophilicity of HBP-ILs by controlling the ionization of terminal groups and counterions by changing the pH or ionic strength of aqueous media in a wide range of pH (2.6−11.6) and ionic conditions (0.1−0.6 M). We showed that reducing pH from 11.6 to 5.2 or increasing ionic strength to 0.1 M for carboxy-containing HBPILs promoted the formation of large micellar aggregates with the size of 150−200 nm (10-fold increase), which is a consequence of the lower ionization degree of carboxylic groups and decreasing ζ-potential. In contrast, sulfonatecontaining HBP-ILs with the amphiphilic balance shifted toward hydrophilicity formed smaller aggregates because of the higher degree of ionization of sulfonate terminal groups.

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RESULTS AND DISCUSSION Synthesis and Chemical Composition of HBP-ILs. The synthesized anionic protic HBP-ILs have a hyperbranched structure with hydrophilic carboxylate or sulfonate groups, bearing imidazole and triazole counterions as well as hydrophobic octadecyl substituents in a ratio of 1:1 to each other (Scheme 1). The introduction of n-octadecylurethane fragments into PIL compounds facilitates the hydrophobic− hydrophobic interactions between long aliphatic “tails” and hydrogen bonding between numerous urethane groups (Scheme 1). The aqueous and surface assembly of four types of amphiphilic HBP-ILs was studied under different pH and ionic strength conditions (Scheme 2).

EXPERIMENTAL SECTION

Materials. N-Methylimidazole (Aldrich, 99%), 1,2,4-1H-triazole (Acros, 99.5%), n-octadecylisocyanate (Aldrich, 98%), and 2sulfobenzoic acid cyclic anhydride (Aldrich, ≥95%) were used as received. Hyperbranched aliphatic oligoether polyol Boltorn H30 (Perstorp, Sweden) with weight-average molecular weight (Mw) of 3500 g/mol (comprising 32 terminal OH groups in outer shell) was purified by precipitation of dimethylformamide (DMF) solution in diethyl ether followed by vacuum drying at 25−30 °C for 6 h (an equivalent Mw measured by hydroxyl groups via acetylation technique is equal to 117 g/equiv). Phthalic anhydride was purified by sublimation. DMF, diethyl ether, acetone, and acetonitrile were dried and distilled before use. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ·cm. The pH of water buffers was adjusted using NaOH to prepare buffers with pH up to 7− 12, and HCl, to prepare buffers with pH 2−7. All HBP-ILs were dissolved in a THF for 16 h at room temperature. The resulting solution was added dropwise to water with different pH under stirring. The THF was evaporated by thorough stirring for 20 h at room temperature. The final concentration of HBP-ILs was 0.2 mg/mL. Characterization. Fourier transform infrared (FTIR) spectra of synthesized compounds were recorded with a TENSOR 37 spectrophotometer operated in 600−4000 cm−1 range. 1H NMR spectra were recorded with a Varian VXR-400 MHz spectrometer using DMSO-d6 and CDCl3 as solvents. Measurement of the hydrodynamic size distribution for HBP-IL assemblies was performed by dynamic light scattering (DLS) at 25 °C on a Zetasizer Nano ZS (Malvern), equipped with a HeNe gas laser operating at a wavelength of 633 nm. The measurements were performed at a 173° scattering angle (noninvasive backscatter (NIBS) technology). The autocorrelation functions of the scattered light were calculated by the Malvern Zetasizer software. Sample solutions were prepared at a concentration of 0.2 mg/mL in water with pH value range from 2.6 to 11.6 or ionic strength up to 0.6 M. The ζ-potential was obtained at ambient conditions by averaging three independent measurements of 35 runs. The critical micelle concentration (CMC) was determined using dynamic light scattering at 25 °C by the known approach.30 Pressure−area isotherms of the HBP-ILs at the air−water interface were recorded on a KSV2000 minitrough at room temperature.31,32 The surface pressure was measured with a platinum Wilhelmy plate. The HBP-ILs solutions (0.2 mg/mL) in chloroform were prepared and spread uniformly at the air−water interface and allowed to

Scheme 2. (a) Assembly Routine of HBP-ILs in Aqueous Media: (1) Dissolving of HBP-ILs in THF, (2) Add Dropwise to Aqueous Media, (3) Evaporate THF with Thorough Stirring; (b) Effect of Ionic Environment on the Aggregation Behavior of HBP-ILs

As a starting material for the synthesis of amphiphilic PILs, we employed a third generation of polyester polyol (HBP-OH) and synthesized four different compounds: amphiphilic hyperbranched carboxylate protic HBP-ILs (C 18 H37 ) 16 -HBP([COO]−[HMim]+)16 (designated as CIm16) and (C18H37)16HBP-([COO]−[HTri]+)16 (designated as CTr16) as well as amphiphilic hyperbranched sulfonate protic PILs (C18H37)16HBP-([SO3]−[HTri]+)16 (designated as STr16) and (C18H37)16HBP-([SO3]−[HMim]+)16 (designated as SIm16) (Scheme 1). C


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Macromolecules Complete consumption of the isocyanate groups in the reaction of n-octadecylisocyanate with polyester polyol and close match between theoretical and experimentally determined acid group content in synthesized HBP derivatives indicates the complete modification and the presence of 16 carboxylic acid or sulfonic groups as expected from the chemical nature of HBP cores (Supporting Information).36 The HBP compounds, composed of a pentaerythritol core and 2,2-bis(hydroxymethyl)propionic acid repeat units, contained around 32 hydroxyl terminal groups.37 The hydrophobic component of these PILs was formed by reacting terminal hydroxyl groups with a half equivalent of n-octadecylisocyanate (Scheme 1). The remaining hydroxyl groups were converted into carboxylic or sulfonic groups by their reaction with phthalic anhydride or 2-sulfobenzoic acid cyclic anhydride respectively. Finally, the acid groups were neutralized with N-methylimidazole or 1,2,4-1H-triazole to give the respective HBP-IL groups (Scheme 1). As-synthesized HBP-ILs are viscous liquids at room temperature, except for the compounds based on (C18H37)16-HBP-(SO3H)16 which are solids with low melting temperatures (see Experimental Section and Supporting Information). As tested, carboxylate PILs are well soluble in DMF, DMSO, ethanol, and THF and are insoluble in benzene. The concentrations of saturated solutions of the HBP-ILs CIm16, CTr16, SIm16, and STr16 in water at room temperature were 0.89, 0.48, 8.54 and 5.16 wt %, respectively. Solubility data show better solubility of both sulfonate against carboxylate and imidazolium against triazolium HBP-ILs due to an increase in solubility (hydrophilicity) with increase in ionicity of ionic liquid groups attached to hyperbranched polyester core (as discussed below). Notably, the carboxylic and sulfonic groups on the periphery of the synthesized PILs possess drastically different pKa values (according to the literature on low molecular weight acids with similar structure to acid fragments in HBP-ILs as well as pKa values calculated by ACD/Laboratories program; see below).38 For instance, the pKa values of sulfonate functionalities on related benzene- and p-toluenesulfonic acids are −2.8 and −1.3, respectively, whereas the pKa values of related carboxylic acids, specifically benzoic and phthalic acids, equal 4.17 and 2.98, respectively (the pKa value of a second carboxylic group of phthalic acid is 5.28).39 Thus, sulfonic acid groups are more acidic and therefore possess stronger proton-donating properties. With respect to nitrogen-containing heterocycles used in this study, the N-methyimidazole is more basic than the 1,2,41H-triazole and is therefore a better proton acceptor. The pKa values of conjugated acids of these heterocycles are 7.05 and 2.27, respectively.40,41 In the ideal case, the proton transfer from the acid to the base is complete, such that the only individual species present are the resulting cations and anions. However, for large counterions this is unlikely since the proton transfer may be less than complete, resulting in the neutral acid and base species being present, and therefore aggregation and association of either ions or neutral species can occur.42,43 This feature should be taken into account when considering the dissociation of ionic groups of the proposed HBP-ILs. The FTIR spectra of all synthesized HBP-ILs are similar to some characteristic features related to different peripheral moieties (Figure 1). Major absorption bands originate from stretching vibration modes of ether and ester (C−O−C, C O) bonds, alkylene (C−H) fragments of the HBP core, methylene groups (C−H bonds) of n-octadecylurethane substituents, and aromatic rings (C−C, C−H bonds of both

Figure 1. FTIR spectra of HBP-ILs: CTr16 (1), CIm16 (2), STr16 (3), and SIm16 (4).

phenyl and heterocyclic fragments), as presented in all compounds.44 In HBP-ILs with sulfonic acid terminal groups an additional SO vibration band (1019 cm−1) was indeed observed that overlapped with C−O−C and C−O bonds of ether and ester fragments (Figure 1, spectra 3 and 4). The 1H NMR spectra of synthesized HBP-ILs show characteristic details related to structure of ionic liquids groups (see representative spectrum in Figure 2 and other spectra in Figures S1−S3). A representative spectrum of (C18H37)16-HBP([COO]−[HMim]+)16 shown in Figure 2 displays signals of CH3− protons (a) and −CH2− protons (c) of n-octadecyl fragments, CH3− groups at quaternary carbon atom (b), −CH2− groups adjacent to the nitrogen atom of urethane fragment (d), −CH2− groups adjacent to oxygen atoms of oligoesters core (e), protons of N−CH3 groups of Nmethylimidazole (g), CH2 groups adjacent to the oxygen atom of peripheral ester groups (f), and aromatic protons (h− n). A common difference of 1H NMR spectra between CIm16, SIm16 and CTr16, STr16 HBP-ILs is the absence of proton signals of −CH3 groups from imidazole cation (3.82 ppm), three proton signals of imidazole (7.35−7.76, 8.50 ppm), and the presence of additional triazole cycle peaks (8.11−8.27 ppm). Assembly of HBP-ILs in Aqueous Medium. The facile assembly of HBP-PILs in aqueous medium was performed by the common solvent method for assembly of traditional linear and branched polymers (Scheme 2).45 It was observed that in aqueous solution at pH 11.6 CIm16, CTr16, SIm16, and STr16 form micelles with nanoscale size, which are stable at room and elevated temperatures (Table S1). HBP-ILs with carboxylate groups (CIm16 and CTr16) form micelles with average hydrodynamic size of 12.8 ± 4 and 13.7 ± 5 nm (Figure 3a,b). At the same time, HBP-ILs with sulfonate groups (SIm16 and STr16) form micelles with an average size of 14.0 ± 5 and 16.4 ± 6 nm (Figure 3c,d). It has been shown that the size of HBP-IL micelles is close to the size of micelles formed by the simplest analogues of our compounds, such as HBP-SO3H (∼7 nm) and HBP-COOH (∼5 nm).46,67 The micelles in this study were slightly larger (∼12−16 nm), which is due to the increased size of HBP-IL molecules (compared to HBP-SO3H or HBP-COOH molecules) with the 50% of longer hydrophobic fragments and large counterions. It should be noted that for the amphiphilic linear polymers the role of length or number of hydrophobic moieties on their assembly in aqueous media is well understood; D


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Figure 2. Representative 1H NMR spectrum of CIm16 HBP-ILs.

Figure 3. Micelle size distribution of CIm16 (a), CTr16 (b) and SIm16 (c), STr16 (d) HBP-ILs in aqueous solutions at different pH.

literature data.51 Adding 16 n-octadecylurethane tails (with extended size of ∼2.7 nm) and 16 counterions (size >0.4 nm) to the HBP-OH core caused the size of HBP-ILs to increase by ∼150% (Figure 4 and Figure S4). The initial polyester polyol (HBP-OH) as the basic core of HBP-ILs is hydrophobic and, thus, is quickly precipitated in aqueous media.46 Amphiphilic HBP-ILs should be capable of

however, the behavior of branched HBP-ILs in aqueous media has not been discussed to date.47−50 In order to analyze the difference between the effective size of different cores, we estimated the molecular dimensions on the molecular models generated by Accelrys Materials Studio (Figure 4). The effective size of the HBP core derived from these models is around 2.2 nm, in good agreement with E


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interactions of individual molecules into aqueous media and subsequent formation of sizable micelles of tens and hundreds of a nanometer in sizes (Figure 5). Thus, DLS measurements confirm that the formation of HBP-ILs micelles occurs at low concentrations with CMC values of the CIm16, CTr16, SIm16, and STr16 in aqueous media of 2.4 × 10−4, 5.5 × 10−4, 1.3 × 10−4, and 3.3 × 10−4 mg/mL, respectively (Table 1 and Figure 5). In addition, the CMC values for HBP-ILs with sulfonic groups are lower than that with carboxylic on average by 50% (Table 1). We suggest that the HBP-ILs with higher hydrophilicity (in this case HBP-ILs bearing sulfonate groups) have a more extended conformation in aqueous environment that makes alkyl tails more available for hydrophobic interactions. On the other hand, urethane groups which form strong hydrogen bonding are also become more accessible for intermolecular interactions favorable for stabilization of multimolecular micellar structures that eventually leads to an earlier molecular aggregation.49 It is apparent that the difference in micelle sizes of HBP-ILs with imidazole and triazole counterions is ∼10% for carboxylate groups and ∼20% for sulfonate groups, respectively. The size difference between HBP-ILs with imidazole and triazole counterions can be caused by their proton accepting ability: imidazole is more basic than triazole as can judged from their pKa values (7.0 against 2.3) of conjugated acids.52 This difference causes a higher degree of dissociation of the corresponding terminal acid groups in the case of imidazolium counterions. Furthermore, DLS data indicate that a difference in the average size of HBP-ILs micelles with carboxylate and sulfonate terminal groups is ∼11% for imidazole counterions (Figure 3a,c) and ∼21% for triazole counterions (Figure 3b,d), respectively. We suggest that a different extent of dissociation of carboxylic and sulfonic ionic groups (according to pKa values of corresponding acidic groups), which defines the surface charges and thus their micellar stability in solution. The different size of micelles based on carboxylate and sulfonate HBP-ILs can be related to the difference in ionic strength of acid groups. The higher acidity of the sulfonic acid groups (pKa varies from −1 to −3) in comparison with the carboxyl (pKa varies from 3 to 4) causes a slightly higher surface charge of micelles formed from sulfonate HBP-ILs and results in a greater resistance to their coagulation. The calculated volume of micelles from CIm16, CTr16 and SIm16, STr16 is ∼137, ∼167 nm3 and ∼179, ∼288 nm3, respectively. From these data, the number of HBP-ILs molecules that form a micelle can be calculated by evaluating the aggregation number (Nagg) from eq 1:53

Figure 4. Molecular models of SIm16 (a) and CTr16 (b) HBP-ILs and structure of different HBP-IL fragments, imidazole, triazole, and noctadecylurethane moieties.

assembling in aqueous media in a certain range of solution concentrations. First, we conducted DLS studies of solutions in order to measure CMC values of these compounds (Figure 5).

Nagg = Figure 5. Light scattering intensity vs concentration for various compounds: CIm16 (red squares), CTr16 (dark blue circles), SIm16 (green squares), and STr16 (light blue circles) HBP-ILs.

NAρHBP mcore Vcore = mHBP MHBP

(1)

where mcore and Vcore are mass and volume of the micellar core, mHPB is the mass of a HBP-IL molecule, NA is the Avogadro constant, and MHBP is the molecular weight of HBP-IL. The ρHBP ∼ 1.2 g/mL is taken as the density of the micellar HBP core.54 It should be noted that the density calculated for the hydrophobic part of HBP-ILs (HBP core and n-octadecylurethane tails) is ∼1.1 g/mL, and the hydrophilic component is from 1.2 to 1.5 g/mL depending on the type of ionic-liquid terminal groups. Thereby, the Nagg for CIm16, CTr16 and SIm16, STr16 are ∼10, ∼12 and ∼12, ∼18 per micelle, respectively.

These measurements show that the dependence of light scattering on the HBP-ILs concentration has two different regions: below the CMC, the extremely low scattered light intensity does not depend on concentration, and this regime corresponds to truly molecular solution. At higher concentration, the scattered light intensity increases greatly with increasing of HBP-ILs concentration, indicating the local F


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Macromolecules Table 1. Characteristic of HBP-ILs and Their Assemblies size (nm)b pH 11.6 sample CIm16 CTr16 SIm16 STr16 a

Mn,theory (g/mol) 12384 12176 13312 13104

solubility in water (mg/mL) 0.89 0.48 8.54 5.16

a

MMA (nm2/molecule) 13.8 14.5 16.8 18.6

CMC (mg/mL) 2.4 5.5 1.3 3.3

× × × ×

−4

10 10−4 10−4 10−4

DLS 12.8 13.7 14.0 16.4

± ± ± ±

pH 5.2

AFM 4 5 5 6

15 19 21 24

± ± ± ±

6 8 10 12

DLS 160 197 27 33

± ± ± ±

zeta potential (mV) AFM

50 60 10 10

177 207 30 37

± ± ± ±

30 40 12 14

pH 11.6 −64 −66 −65 −67

± ± ± ±

7 8 7 7

pH 5.2 −56 −55 −57 −58

± ± ± ±

4 4 4 4

Đc 0.16 0.12 0.18 0.21

Mean molecular area (MMA). bAverage size. cPolydispersity from DLS data.

Figure 6. Composition of individual micelles and assembly of individual HBP-ILs micelles (left) and their micellar aggregation at changed pH (right).

To consider the role of amphiphilic balance and the specific surface area of branched ionic molecules with hydrophobic and hydrophilic fragments in assembly behavior, we found the limiting mean molecular area (MMA) at the air−water interface by dissolving HBP-ILs in THF and spreading solution at the air−water interface to form Langmuir monolayers (Figure 7). A steadily rising surface pressure confirms the formation of stable Langmuir monolayers with liquid and solid 2D phase formation which are characteristics of the conventional amphiphiles.32 We calculated the limiting surface area per molecule by extrapolation of the steep rise in the surface pressure to a zero level in accordance with a typical procedure

Therefore, the aggregation number for HBP-ILs is generally larger than for common micelles of similar linear amphiphilic copolymers (from 0.7 to 8).55 This difference indicates the need of the larger number of individual densely packed branched macromolecules for the formation of the relatively larger micelles and removal of the hydrated water from the core. Another important characteristic of HBP-ILs micelles is the ζ-potential, which determines their colloidal stability in aqueous media.56 The ζ-potential value at the slipping plane of micelles assembled from CIm16, CTr16 and SIm16, STr16 are −64 ± 7, −66 ± 8 and −65 ± 7, −67 ± 7 mV at pH 11.6, respectively (Table 1). The large negative ζ-potential of micelles is caused by the negatively charged carboxylic or sulfonic terminal groups of the hydrophilic part of HBP-ILs (Scheme 1), which form on the surface of micelles that determines the surface charge and ζpotential (Figure 6). The negative charge of HBP-ILs assemblies in region from −50 to −70 mV facilitates their high stability in aqueous media at pH from ≥3.0 to 11.6 (Table 1). As known, the absolute value of ζ-potential defines the stability of polymer assemblies in aqueous media common for colloidal particles.57,58 The polymer assemblies are stable, when the forces causing the mutual repulsion of the particles are dominant with the higher ζ-potential increasing the overall micellar stability (Figure 6). The ζ-potential value between +5 and −5 mV usually indicates the decreased stability of the assemblies and frequently causes uncontrolled aggregation in solution.59 It was noted that there is no strict relationship between value of ζ-potential and nanoparticle stability in aqueous media, but it is considered that assemblies with ζ-potential more than −40 mV are stable in a wide range of conditions.57 This high stability, in fact, was observed experimentally for all compounds synthesized here (Table 1 and see discussion below).

Figure 7. Pressure−area isotherms for various ionic compounds: (a) CIm16 (dark blue), CTr16 (black) and SIm16 (green), and STr16 (light blue) HBP-ILs and sketches of molecular packing of monolayer at low (b) and high (c) surface pressures. G


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Macromolecules (Table 1).60 The analysis of the surface-pressure isotherms shows that their shapes are similar for all HBP-ILs and in good agreement with those observed for HBPs with various terminal groups, such as amino, carboxyl, or alkyl groups.61 At the same time, we observed that the slopes of compression isotherms are different for HBP-ILs with carboxylate and sulfonate terminals groups (Figure 7a). Thus, the addition of carboxylate groups (with lower ionization level) shifts the amphiphilic balance between hydrophobic and hydrophilic fragments of HBP-ILs, making HBP-IL cores more compact. We suggest that this change causes n-octadecylurethane tails (larger hydrophobic fragments) to adapt a more upright orientation during monolayer compression (Figure 7b,c).61 It should be noted that the MMA values can be calculated by this extrapolation only if the molecular weight of compounds is known. Therefore, the MMA is larger when the molecular weight from an idealized molecular structure of HBP-ILs (overestimated molecular weight) is used for calculation. The comparison of the MMA of HBP with alkyl-terminated branches calculated using theoretical molecular weight (from an idealized molecular structure) and found experimentally (from GPC) showed that the difference in MMA can reach up to 30%).32,62 Therefore, the discussion of the HBP-ILs surface area will be mainly limited to the comparison of the MMA for CIm16, CTr16, SIm16, and STr16 HBP-ILs as well as explanation of the differences between them in terms of degree of ionization. The MMA of CTr16 is slightly larger than that of CIm16 and the MMA of STr16 is larger than that of SIm16 by 9% (Table 1). Furthermore, there is a difference in the MMA of HBP-ILs with the same type of a counterion but different terminal ionic groups. In particular, the MMA of CIm16 is smaller than that of SIm16 by 18%, and the MMA of CTr16 is smaller than that of CTr16 by 22%. Overall, we observed that the MMA is larger for HBP-ILs with triazole counterion and sulfonate terminal groups due to differences in ionic nature and hydrophilicity of counterions.63 Carboxylate HBP-ILs have lower ionicity of terminal groups (and, thus, less hydrophilic) compared to sulfonate HBP-ILs, which leads to tighter conformation of polymer chains at the air−water interface with lower surface area. Also, the higher effective charge on sulfonate-bearing HBP-ILs results in higher electrostatic repulsion between adjacent molecules further increasing the MMA values. Furthermore, HBP-ILs with triazolium counterions are characterized by larger surface area at the air−water interface compared with the HBP-ILs with imidazolium counterions. This behavior could be explained by the presence of a larger number of imine groups in the composition of triazole capable of forming hydrogen bonds with water molecules, which, in turn, enhances the hydrophilic properties of the respective compounds. Morphology of Micellar Structures. As is known, HBPs can form various assemblies ranging from macroscopic tubes and wires to vesicles and micelles.64−67 To examine the structure of the HBP-ILs, the AFM was employed to probe the morphology and dimensions of HBP assemblies, and the results were compared to the data from DLS. The size of micelles measured using AFM are in good agreement with hydrodynamics size distribution from DLS method (Table 1). In our AFM studies, we detected the average size of CIm16, CTr16 and SIm16, STr16 micelles, deposited from solution at pH 11.6, are 15 ± 6, 19 ± 8 nm and 21 ± 10, 24 ± 12 nm, respectively (Figure 8 and Table. 1).

Figure 8. AFM topographical images of the CIm16 (a), CTr16 (b) and SIm16 (e) and STr16 (f) HBP-ILs solution drops deposited from pH 11.6 solutions on silicon and corresponding height profiles (c, d, g, h) along selected lines. Z-scale is 10 nm for (a) and 12 nm for (b, e, f).

The AFM data indicate the difference between the micelle sizes with different terminal groups and counterions. First, the average size of micelles based HBP-ILs with carboxylic groups is smaller than of that with sulfonic groups by ∼25% for the same type of counterion. Second, the average size of micelles based HBP-ILs with triazole counterions is larger than that with imidazole by ∼11% (for analogies type of terminal groups). Thus, changing the terminal groups or counterions in HBP-IL structure can result in up to 25% change in size. At the same time, the addition of n-octadecylurethane tails to HBP-COOH or HBP-SO3 compounds increases the size of HBP-ILs micelles by more than 50% (Figure 4).68 This indicates that during the micelle formation the introduction of large hydrophobic fragments plays a major role and that the micellar assembly of three-dimensional HBP-ILs is more stable to the changing ionic environment. Additionally, all collapsed micelles in a dry state have flattened shape with average height ∼4 nm for CIm16, ∼5 nm for CTr16, ∼6 nm for SIm16, and ∼8 nm for SIm16 (Figure 8c,d,g,h). The area occupied by micelles is π(d/2)2 ∼ 177 nm2 for CIm16, ∼283 nm2 for CTr16, ∼346 nm2 for SIm16, and ∼452 nm2 for STr16 assuming a circular shape of the surface structures. We hypothesized that the flattened shape and decrease in height of HBP-ILs assemblies in dry state is due to the formation of a more loose structure (with lower density) H


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CIm16, CTr16 and SIm16, STr16 form micelles with size 12.8 ± 4, 13.7 ± 5 nm and 14.0 ± 5, 16.4 ± 6 nm at pH 11.6 by DLS method, respectively (Figure 3). However, with decrease of pH to 5.2 the size of CIm16 assemblies increased manifold to ∼160 nm (Figure 9b) and size of CTr16 assemblies to ∼197 nm (Figure 9c). Further decrease of pH to 2.6 resulted in sedimentation of CIm16 and CTr16 HBP-ILs (Figure 9a). It should be noted that current data on assembly of HBP-ILs in aqueous media at different pH or ionic strength are virtually absent. It is known that the behavior of linear polymer assemblies in aqueous media can be very diverse. For example, with decreasing pH of the aqueous environment the size of assemblies based on linear polymers may decrease, increase, or show a nonlinear behavior.12,69 Furthermore, the majority of publications in this area are dedicated to investigating the effect of environmental pH on size of already formed assemblies rather than assembly at different pH.64 The change in size of CIm16 and CTr16 assemblies was accompanied by the decrease in ζ-potential from around −65 mV at pH 11.6 to around −45 mV at pH 5.2, which is caused by a change in the degree of ionization of terminal ionic groups (Table 1). This indicates the decrease of the stability of HBP-IL assemblies and formation of micelle aggregates with size distribution from ∼80 to ∼600 nm during the transition from basic to acidic media (see Scheme 3). At the same time, SIm16 and STr16 solutions form stable assemblies at pH from 11.6 to 2.6 with an average size of about 27 nm and ζ-potential of −60 mV for SIm16 and an average size of about 33 nm and ζ-potential of −55 mV for STr16 (Figures 3 and 9a,d,e). The different size distributions of HBP-ILs assemblies at pH 5.2 and pH 11.6 suggest that HBP-ILs with sulfonate groups mainly form micelles and HBP-ILs with carboxylate groups forms micelle aggregates (see Scheme 1). This hypothesis is in good agreement with DLS data (Figures 3 and 9) and AFM data (Figures 8 and 10). Consequently, the aggregation number can be estimated by eq 1, where for Vcore we use the volume of the aggregate. Then, the aggregation number for the CIm16, CTr16 and SIm16, STr16 HBP-ILs assemblies at pH 5.2 are 1.4 × 105, 2.6 × 105 molecules and 6.2 × 102, 1.1 × 103 molecules, respectively. Therefore, the pH-induced structural rearrangement of micelles (pH 11.6) to micelles aggregates (pH 5.2) was

around the hydrophobic core of the micelle in aqueous medium. Notably, the hydrophobic nature of the core components can enhance the internal component separation of the core and micelle corona composed of the hydrophilic fragments.53 In addition, the size of HBP-ILs assemblies in aqueous media also depends on the pH (Figures 3 and 9). For example, the

Figure 9. HBP-ILs solution at different pH (a). Effective size (black) and ζ-potential (blue) of the CIm16 (b), CTr16 (c), SIm16 (d), and STr16 (e) HBP-ILs assemblies in aqueous medium at different pH. The red regions (b, c) indicate the sedimentation.

Scheme 3. Effect of pH on Hyperbranched Amphiphilic Carboxylate and Sulfonate HBP-ILs

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trend is similar to that obtained from DLS and AFM, but absolute values are systematically lower due to full drying in a high vacuum. To further understand the influence of ionic environment on HBP-ILs assembly, the effect of ionic strength on the effective size and ζ-potential of the CIm16, CTr16, SIm16, and STr16 HBP-ILs assemblies in aqueous media were investigated at temperature 25 °C, concentration 0.2 mg/mL, and pH 7 (Figure 11). Notably, the latest reviews of the assembly of

Figure 10. AFM topographical images of the CIm16 (a), CTr16 (b) and SIm16 (e), and STr16 (f) HBP-ILs solution drops deposited from pH 5.2 solutions on silicon and the height profiles (c, d, g, h) along the lines for the samples. Z-scale is 150, 150, 30, and 28 nm for (a), (b), (e), and (f), respectively. TEM images of the CTr16 HBP-ILs assemblies (i) deposited from pH 5.2 solutions on carbon−Formvarcoated copper grids. Scale bar is 200 nm.

confirmed by a dramatic increase of aggregation number for all HBP-ILs compounds. However, a more significant change in aggregation number was observed for the CIm16 and CTr16 assemblies due to the several orders of magnitude difference in size of CIm16, CTr16 and SIm16, STr16 HBP-ILs assemblies at pH 5.2. AFM imaging of HBP-ILs micelles aggregates deposited from solution at pH 5.2 shows that the average size of aggregates based CIm16, CTr16 and SIm16, STr16 is 177 ± 30, 207 ± 40 nm and 30 ± 12, 37 ± 14 nm, respectively (Figure 10). Additionally, micelle aggregates in the dry state have slightly flattened spherical shape with average height ∼100 nm for CIm16, ∼150 nm for CTr16, ∼8 nm for SIm16, and ∼10 nm for SIm16 (Figure 10c,d,g,h). Thus, dimensions determined from AFM measurements of HBP-ILs micelle aggregates deposited from pH 5.2 solutions are in good agreement with average sizes obtained from DLS data (Figures 3 and 9). Moreover, the aggregate size from HBP-ILs with carboxylic groups is an order of magnitude larger than with sulfonic groups. AFM and DLS data are further confirmed with TEM (Figure 10i and Figure S5). The TEM images further confirm that the HBP-ILs form spherical micelle aggregates with size of 173 ± 50 nm for CIm16, 222 ± 50 nm for CTr16, 60 ± 25 nm for SIm16, and 51 ± 20 nm for STr16 (Figure 10i and Figure S5). The overall

Figure 11. HBP-IL solutions at different ionic strengths (a) at pH 7. Effective size (black) and ζ-potential (blue) of CIm16 (b), CTr16 (c), SIm16 (d), and STr16 (e) HBP-ILs assemblies in aqueous medium at different ionic strengths.

HBP-based compounds indicate the lack of research works on the effects of ionic strength on the assembly of HBP-ILs or compounds without hydrophobic components, such as HBPCOOH or HBP-SO3H.12,20−23,26,64,65,67 Overall, as known, the decrease of ζ-potential of the HBP-ILs assemblies leads to the formation of micelle aggregates.70 It should be noted that HBPILs with carboxylic groups are more sensitive (earlier sedimentation) to changes in ionic environment than HBPILs with sulfonic groups due to the different degree of dissociation of terminal groups (Figure 11). The size of HBP-IL aggregates increases and the ζ-potential decreases with increasing ionic strength until ultimately sedimentation of HBP-ILs occurs at high ionic strength. For example, the effective size of CIm16 and CTr16 assemblies increased to ∼180 and ∼210 nm (Figure 11b,c), when the ionic strength was raised to 0.1 M. At ionic strength values above 0.1 M CIm16 and CTr16 HBP-ILs assemblies showed sedimentation J


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terminal groups have ∼15% larger molecular surface area at air−water interface and 5% larger average size of assemblies in aqueous media than with carboxylic groups. At the same time, a lower degree of ionization of triazole counterions than that of imidazole results in higher hydrophobicity and hydrogen-bonding ability of HBP-IL molecules with triazole counterions. Consequently, we observe a larger molecular surface area and larger size of assemblies in aqueous media (Figures 3 and 7). Furthermore, the difference in degree of ionization of the counterions does not affect the morphology of these assemblies. However, the average size of micelles based HBP-ILs with triazole counterions is 15% larger than with imidazole for both types of terminal groups. Thus, unlike the assembly of linear polymeric ionic liquids (PILs), where ionic groups dictate the geometry and dimensions of the assemblies, the micelles from 3D compact HBP-ILs are more stable to the choice of constituent ions.80 Additionally, it should be noted that the CMC values of HBP-ILs are on average 2 orders of magnitude smaller than the CMC of branched and linear polymers. In fact, CMC of HBPO-star-PDMAEMA is ∼0.025 mg/mL; CMC of HBPs with hydroxyl terminals is ∼0.042 mg/mL or dendrimer core (CMC of HPG-2S-SN38 is ∼6.4 × 10−3 mg/mL) (Figure 5).77,78,81 Moreover, the CMC value of compact HBP-ILs can be tuned by the addition of hydrophobic (n-octadecylurethane tails) fragments in the HBP-IL structure. Such difference in CMC values can be related to the fact that the HBP-ILs synthesized in this work are ionic surfactants with strong ionic contributions, whereas most examples studied before are weak polyelectrolytes. We demonstrated that hydrophilic/hydrophobic balance of HBP-ILs can be tuned by changing the degree of ionization of terminal ionic groups due to change of pH or ionic strength of aqueous media. While reducing pH from 11.6 to 5.2 or increasing ionic strength to 0.1 M for carboxy-containing HBPILs, we observed formation of micellar aggregates with the size of about 150−200 nm, which could be explained by a decrease in ionization of carboxylic groups which was accompanied by a decrease in the absolute value of the ζ-potential to −58 mV for pH and to −45 mV for ionic strength (Figures 3, 9, and 11). Thus, we suggest that small HBP-ILs micelles form micelles aggregates which are formed by the unimolecular micelles aggregation mechanism (UMA).82 It has been suggested as the main mechanism of assembling of amphiphilic dendritic compounds, if they possess dendritic cores with moderate core hydrophobicity and hydrophilic arms. At the same time, for sulfonate-containing HBP-ILs at pH 5.2 or at ionic strength of 0.1 M the micelles size increased only slightly (to 25−40 nm) because of the higher degree of ionization of sulfonate groups compared to that of carboxylate. This effect is in accordance with relatively insignificant, compared to carboxylate HBP-ILs, reduction of absolute value of ζ-potential to −67 mV when pH was reduced to 5.2 (or to −35 mV for ionic strength). Thus, while terminal ionic groups do not have a large effect on micelle dimensions, changing the ionization degree (and as a result hydrophilic/ hydrophobic balance) affects the morphology of HBP-IL aggregates to a greater extent. The pH-tuned HBP-ILs assembly behavior to the nature of the terminal groups and dissociation behavior, which determines the ζ-potential of the HBP-ILs, has been summarized in Scheme 3. According to the evaluation with ACD/Laboratories program,38 the pKa value of sulfonic acid group is −1.09 ±

(see Figure 11a). Meanwhile, SIm16 and STr16 HBP-ILs assemblies displayed sedimentation at ionic strength values above 0.6 M (Figure 11). Increasing the ionic strength to 0.4 M results in the increase of aggregate size by more than 40% to ∼50 nm for SIm16 and to ∼70 nm for STr16 (Figure 11d,e).

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GENERAL DISCUSSION AND CONCLUSIONS The assembly of amphiphiles in aqueous media is entropically driven by the repulsion of water molecules from the hydrophobic fragments of amphiphiles and stabilized or destabilized by the enthalpic interactions between water molecules and hydrophilic fragments.71 The assembly behavior of amphiphilic HBP compounds is first determined by the ratio of their hydrophobic and hydrophilic components. Commonly, spherical micellar aggregates are formed at the 1:1 ratio as is also observed in this study despite additional hydrophobicity caused by the presence of the hyperbranched polyester core. Moreover, we observed that the HBP-ILs micelles are much more compact than typical HBP assemblies such as vesicles or large micelles. For example, the average sizes of vesicles and micelles based on traditional HBP are usually within 100−200 nm (Table 2). In contrast, the sizes of HBP-ILs assemblies detected in this study are on average 10 times smaller for molecular weights in the similar range (Table 2). Table 2. HBP Micelles Dimensions from Literature sample

micelle size

vesicles and their aggregates HBPO core with poly(ethylene glycol) and 1∼320 nm methylimidazole arms HBPO core with hydrophilic poly(ethylene ≥300 nm oxide) arms carboxy-terminated HBP core 200 nm−100 μm HBPO core with hydrophilic poly(ethylene 4−112 μm oxide) arms HBPO core with hydrophilic poly(ethylene 10−100 μm oxide) arms HBPO core with N,N-dimethylaminoethyl 180−300 nm methacrylate micelle aggregates (multimolecular micelles) HBPO core with poly[2-(dimeth ylamino)ethyl ∼100 nm methacrylate] poly(amine-ester)-type HBPs 250−400 nm HBPE core with 1-cyanoindolizine-3-carboxylic 150−300 nm acid groups micelles (unimolecular micelles) HBPO core with carboxyl terminals ∼7 nm HBPE core with hydrophilic sulfonic acid ∼5 nm terminals

ref 29 72 73 74 75 76

77 78 79

46 68

On the other hand, the geometry and dimension of HBP-IL micelles studied here are closer to the dimensions of micelles based HBP-SO3H and HBP-COOH (Table 2).46,68 At the same time, a slightly larger size of micelles in this study (∼12−16 nm) in comparison with micelle-based HBP-SO3H or HBPCOOH (5−7 nm) can be related to longer HBP-IL molecules due to the presence of larger hydrophobic fragments as estimated from the molecular models (Figure 4 and Figure S4). The introduction of large hydrophobic fragments into the HBP core has a larger effect (on average by ∼50%) on the molecular dimensions of HBP-ILs. The replacement/adding of the ionic terminal groups and bulky counterions results in minor variation of their dimensions and the changes in molecular surface area. For example, HBP-ILs with sulfonic K


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0.15, which is in good agreement with pKa values of structurally similar benzenesulfonic and p-toluenesulfonic acid (−2.8 and −1.3). The pKa value of carboxylic groups is 3.32 ± 0.10, which is close to both pKa value of benzoic acid (4.17) and phthalic acid (2.98 and pKa2 = 5.28).39,40 By taking these pKa values into consideration, it can be concluded that sulfonic acid groups have higher acidity comparing to carboxylic groups. Low pKa values ensure that the sulfonic groups are ionized at all pH ranges considered here and consequently provide the coagulation resistance for the micelles containing sulfonic IL groups. The carboxylic groups that are a part of HBP-ILs are ionized in basic conditions (pH = 9−12) which provides a high negative charge to the micelle surface and hence the colloidal stability (Scheme 3). At lower pH (5.2−7.0) the equilibrium shifts to weakly dissociated acid groups bound to imidazole as a result of acid−base interactions and hydrogen bonding. The lower acidity of carboxylic groups results in a low surface charge of the carboxylate-bearing HBP-IL micelles, and as a result, their coagulation into aggregates with sizes up to 200 nm. Upon further decrease in pH the undissociated HBP-IL precipitates out of solution whereas a protonated imidazole remains in solution. Notably, the change in morphology of HBP-ILs assemblies from micelles to their aggregates with decreasing pH (or increasing ionic strength) is nonlinear (Figures 9 and 11). Furthermore, the morphology transition of HBP-IL assemblies is accompanied by the change in light transmittance of their solutions from opaque (pH < 7 or high ionic strength up to sedimentation) to transparent (pH > 7 or low ionic strength), which confirms the increase in size of the optical inhomogeneities in solutions with decreasing pH or increasing ionic strength (Figures 9a and 11a). The comparison of aggregation behavior HBP-ILs with HBP-COOH (literature data) at different pH allowed to consider the influence of counterions on the assembly of HBP-ILs.46 In both cases, decreasing pH led to aggregation of HBP-IL micelles due to the loss of ionization of functional groups. However, the introduction of counterions into HBP-COOH compounds led to increasing impact of ionic environment, such as a higher rate of change in size of assemblies and earlier sedimentation.46 Notably, because of the higher effective charge on the sulfonate group, the sedimentation of carboxylate-containing HBP-ILs occurs at lower ionic strength (0.1 M NaCl) than in the case of sulfonate analogues (0.6 M). In conclusion, we demonstrated how the assembly behavior of novel amphiphilic branched compounds is controlled by the hydrophilic/hydrophobic balance and by the ionization behavior of terminal ionic groups. These findings have implications for the development of new branched polymeric materials with ionic liquid properties, in particular for prospective electrochemistry and catalysis applications. Furthermore, we demonstrated that the variable ionicity has been utilized as the mechanism of responsive behavior under the external stimuli and the ability of HBP-ILs system as a whole to form organized assemblies with unique transport properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph 404-894-6081, Fax 404385-3112 (V.V.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Science Foundation DMR 1505234 and CBET 1402712 projects. The authors thank Kesong Hu for help with TEM measurements and Andrew Erwin for technical assistance.

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REFERENCES

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01562. NMR data, molecular models, and TEM images (PDF) L


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