Surface Activity and Aggregation Behavior of Siloxane-Based Ionic

Article pubs.acs.org/Langmuir

Surface Activity and Aggregation Behavior of Siloxane-Based Ionic Liquids in Aqueous Solution Guoyong Wang,*,† Ping Li,† Zhiping Du,†,‡ Wanxu Wang,† and Guojin Li† †

China Research Institute of Daily Chemical Industry, Taiyuan Shanxi 030001, P.R. China Institute of Resources and Environment Engineering, Shanxi University, Taiyuan Shanxi 030006, P.R. China

‡

Downloaded by NEW YORK UNIV on September 1, 2015 | http://pubs.acs.org Publication Date (Web): July 23, 2015 | doi: 10.1021/acs.langmuir.5b02062

S Supporting Information *

ABSTRACT: Six novel siloxane-based surface-active ionic liquids (SAILs)siloxane ammonium carboxylate [Si(n)N(2)CA(1), (n = 3, 4)]were designed and synthesized. Their melting points, surface activities, and self-aggregation behavior in aqueous solution were studied. The results showed that because of the bulky hydrophobic siloxane chains at the end of the tail, all six siloxane-based SAILs are room-temperature ionic liquids (RT-SAILs). The introduction of the siloxane group can reduce the melting point of ionic liquids to below room temperature and can promote the micellization and aggregation behavior more efficiently. These siloxane-based SAILs can greatly reduce the surface tension of water, as shown by the critical aggregation concentration (γCAC) values of 20 mN·m−1; all six siloxane RT-SAILs can form a vesicle spontaneously in aqueous solution, indicating potential uses as model systems for biomembranes and vehicles for drug delivery.

1. INTRODUCTION

Recently, self-assembling vesicle structures from long-chain imidazolium ILs were discovered in water. Zheng et al.7 discovered that rich lamellar structures are observed by a singletailed amphiphilic IL naphthalenesulfonate ([C12mim][Nsa]) in aqueous solution without any additives. By simply changing the IL concentration, a spontaneous transformation from micelles to vesicles, bilayers, and liquid crystals occurs. Wang et al.8 reported that an imidazolium-based ionic liquid [Cnmim]Br (n = 10, 12, 14) can self-assemble unilamellar vesicles in aqueous solutions. Moreover, Sarkar9 discovered spontaneous transitions of micelle−vesicle−micelle in the mixture of CTAB and anionic surfactant-like IL octyl sulfate, [C4mim]-[C8SO4]. However, most of the present work lies in imidazolium-based and pyrrolidinium-based ILs with long aliphatic substituents. The surface activities of such ILs are low (i.e., the surface tension is above 30 mN/m), and the melting points are high (i.e., above 298 K). Indeed, the melting points of these ILs begin to rise dramatically when the length of the N-alkyl group exceeds seven carbon atoms. Thus, the incorporation of progressively more lipophilic structures is incompatible with maintaining a melting point below room temperature. On the other hand, siloxane surfactants represent another important class of surfactants.10 They display lower critical aggregation concentrations (CACs) and lower surface tension; moreover, they spontaneously form vesicles above their CAC owing to the flexibility and low cohesive energy of the siloxane chain.11 Therefore, considering the low cohesive energy just

Ionic liquids (ILs) are a kind of organic salt, and their melting points are below 100 °C. As a result of their striking advantages, such as nonflammability, high thermal stability, negligible vapor pressure, impressive recyclability, and excellent solvation abilities,1 they have potential applications in many fields; for example, they can be used as electrolytes in battery cells, as solvents and catalysts in chemical reactions, and in metal ion isolation and extraction processes. Their structure and properties can be tuned by changing the type or alkyl chain length of the cation or anion. Accordingly, ionic liquids have been extensively studied in recent years.2 Surface-active ionic liquids (SAILs), which are structurally similar to traditional ionic surfactants composed of distinct hydrophobic and hydrophilic moieties, have attracted significant attention recently.3 Many SAILs have been synthesized, and their surface activity and micellar behavior have been investigated. For example, Kang et al.4 have reported on some amide-functionalized SAILs. The thermal stability of the amidefunctionalized SAILs was decreased because of the amide moiety in the alkyl chain. However, amide-functionalized SAILs exhibit better surface-active properties than nonfunctionalized SAILs with a simple alkyl chain. Zheng5 showed that the transition from wormlike to spherical micelles of pyrrolidinium based ionic liquid (C16MPBr) and sodium (4-phenylazophenoxy)-acetate (AzoNa) is induced by the trans−cis photoisomerization functionality of AzoNa. Additionally, Kumar6 has developed an amino acid-based IL that forms micellar aggregates in aqueous media with an average diameter of 5 nm. © 2015 American Chemical Society

Received: March 30, 2015 Revised: July 7, 2015 Published: July 14, 2015 8235

DOI: 10.1021/acs.langmuir.5b02062 Langmuir 2015, 31, 8235−8242

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Scheme 1. Synthesis Routes and Acronyms of Siloxane Ammonium Carboxylate [Si(n)N(2)-CA(1), (n = 3, 4)]

accelerating voltage of 100 kV. The SAIL drops were stained with 2% (w/w) phosphotungstic acid on a carbon-coated copper grid. 2.6. Entrapment of Water-Soluble Dye. Solutions were prepared by dissolving SAIL in 5 × 10−4 mol·L−1 bromophenol blue and aged for 6 h at room temperature. Then 0.5 mL of the solution was separated by Sephadex G-25 gel (medium grade) column chromatography. The vesicle solution containing the entrapped water-soluble dye should be separated early. The amount of the dye in the effluent was analyzed using an ultraviolet−visible (UV−vis) spectrophotometer (at 590 nm). The encapsulation efficiency (EF) was obtained, which is defined as

mentioned, it can be assumed that the incorporation of a siloxane chain may reduce the melting temperature of ILs and improve their surface activity and ability to form vesicle-like aggregates.3b,12 Herein we report on a new class of SAILs composed of siloxane cations and the well-known homologous series of carboxylate anions [Si(n)N(2)-CA(1) (n = 3, 4), see Scheme 1] by employing simple acid−base reactions. Their melting points, micellization, and aggregation behavior in aqueous solutions are systemically investigated to gain insight into the effects of the siloxane group and counterions in aqueous solutions.

EF = Cdye/C t,dye

2. EXPERIMENTAL SECTION

in which Cdye and Ct,dye are the entraped and total dye concentrations, respectively.

2.1. Materials. 3-Chloropropyl trichlorosilane and 3-chloropropyl dichloromethylsilane, along with hexamethyldisiloxane, were purchased from Aldrich. Dichloromethane, acetic acid, propionic acid, and n-butyric acid were provided by the Beijing Chemical Reagent Company. All materials were A.R. grade. Ultrapure water was used throughout the experiments. 2.2. Surface Tension Measurements. Surface tension measurements were performed using a Wilhelmy plate tensiometer (Kruss K12) with the single-point method at 298.15 ± 0.1 K. 2.3. Electrical Conductivity Measurements. The electrical conductivity of solutions was determined by a conductivity analyzer (model DDS-6700, Shanghai, China.). The temperature was maintained at 298.15 ± 0.1 K using a constant-temperature bath. The CMC value can be obtained from the break point appearing in the plot of conductivity versus concentration. The degree of counterion dissociation (β) can be obtained from the ratio between the plots above and below the CMC. 2.4. Dynamic Light Scattering. Dynamic light scattering (DLS) at a scattering angle of 90° was determined by using a Brookhaven particle size analyzer (USA) to study the size distribution and mean diameter of the SAIL aggregates in aqueous solution. All of the solutions were aged at 298 K for 8 h. All measurements lasted for 3 min and were made at 298 K. 2.5. Transmission Electron Microscopy. The morphology of aggregates formed by the SAIL in solution was determined by a JEM1011 transmission electron microscopy (JEOL, Japan) with an

3. RESULTS AND DISCUSSION 3.1. Synthesis. The six siloxane ammonium carboxylate SAILs [Si(n)N(2)-CA(1), (n = 3, 4)] were synthesized by a multistep reaction, as shown in Scheme 1. The products were characterized by FT-IR and 1H and 13C NMR (details in Supporting Information). Figure 1 depicts images of the SAILs at 298 K. From Figure 1 we can see that all of the siloxane ammonium carboxylate SAILs are liquid at 298 K. In other words, all six siloxane-based SAILs are RT-SAILs. For comparison, n-undecyl ammonium acetate IL [CH3(CH2)10NH3][OOCCH3], a white solid, was also synthesized and characterized using a similar method. In this way, we are able to confirm that the incorporation of a siloxane chain can reduce the melting point of the IL. Because the surfactant molecule is rendered more asymmetric by introducing the bulk siloxane chain and because a larger asymmetry disrupts the crystal packing, the cohesive energy is minimized, which results in a lower melting point.13 3.2. Surface Activity. The surface tension was investigated to evaluate the surface activity of the RT-SAILs in aqueous solution. Figure 2 shows surface tension (γ) as a function of the log of concentration (C) for aqueous solutions of [Si(n)N(2)CA(1), (n = 3, 4)] at 298 K. It can clearly be seen that, for each 8236


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CAC indicates the high purity of these RT-SAILs. All six RTSAILs show high surface activities and can reduce the surface tension of water significantly. The values of γCAC were found to be around 20 mN·m−1 for [Si(n)N(2)-CA(1), (n = 3, 4)]. It is evident that the γCAC values of [Si(n)N(2)-CA(1), (n = 3, 4)] are essentially the same because there is only a small difference in the carboxylate counteranion of the molecules and they are homologues. By contrast, the γCAC values of hydrocarbon ILs are around 40 mN·m−1.15 This could be interpreted to indicate siloxane surfactants with a favorable orientation of the lowenergy methyl groups of siloxane at the interface. More specifically, the values for the CAC are 6.13 × 10−3, 5.20 × 10−3, and 4.80 × 10−3 mol·L−1 for Si(4)N(2)-AC(1), Si(4)N(2)-PC(1), and Si(4)N(2)-BC(1), respectively; they are 14.40 × 10−3, 9.96 × 10−3, and 8.28 × 10−3 mol·L−1 for Si(3)N(2)-AC(1), Si(3)N(2)-PC(1), and Si(3)N(2)-BC(1). The CAC for Si(3)N(2)-CA(1) is greater than that for Si(4)N(2)-CA(1) because the tetrasiloxane group is more hydrophobic than the trisiloxane group. The CAC decreases in the order Si(n)N(2)-AC(1) > Si(n)N(2)-PC(1) > Si(n)N(2)BC(1) in conformity with increased hydrophobicity due to the extension of the hydrocarbon chain of the carboxylate counteranion in the molecules. A similar phenomenon was seen in the other surfactants, such as methylenammonium norbornene dodecanoate (CAC = 7.6 × 10−4 mol·L−1), reported by Perez et al.16 Clearly, the effect of the hydrocarbon chain length in the carboxylate counteranion of the molecules on the CAC is greater than its effect on γCAC. The surface excess maximum concentration, Γmax, and the area occupied by a surfactant at the air/water interface, Amin, were given for the six RT-SAILs according to the Gibbs adsorption equations (eqs 1 and 2).17 The standard free energy of aggregation and adsorption can be obtained from eqs 3 and 4.


Figure 1. Images of the ionic liquids (ILs) at 298 K. (a) Si(4)N(2)AC(1), (b) Si(4)N(2)-PC(1), (c) Si(4)N(2)-BC(1), (d) Si(3)N(2)AC(1), (e) Si(3)N(2)-PC(1), and (f) Si(3)N(2)-BC(1). Left: [CH3 (CH2)10NH3][OOCCH3].

Γmax = −

⎛ ∂γ ⎞ 1 ⎜ ⎟ 2.303nRT ⎝ ∂ log C ⎠

(1)

1016 NA Γmax

(2)

T

A min = −

Figure 2. Surface tension as a function of the log of concentration for room-temperature surface-active ionic liquids (RT-SAILs) at 298 K.

⎛ cac ⎞ 0 ⎟ ΔGmic = RT ln⎜ ⎝ 55.5 ⎠

(3)

⎛ C∏ ⎞ 0 s ΔGads = nRT ln⎜ ⎟ − 6.022ΠA m ⎝ 55.5 ⎠

(4)

R is the gas constant, n = 2 (as expected for 1:1 dissociating ionic surfactants), T is the absolute temperature, ∂γ/∂(log C) is the maximum slope, NA is Avogadro’s number, Π(= γ0 − γ) is the surface pressure in surface saturation, and CΠ is the molar concentration at a surface pressure of Π (in mN/m). From Table 1, we can see that the Amin values for [Si(n)N(2)-CA(1), n = 3, 4] range from 63 to 130 Å2, which are substantially higher than the cross-sectional area of the dicephalic and gemini tetrasiloxaane surfactants (∼50 Å2) reported by our group.16 This can be explained by the fact that dicephalic and gemini tetrasiloxaane surfactants are typical nonionic surfactants, whereas [Si(n)N(2)-CA(1), (n = 3, 4)] are ionic surfactants. In other words, the strong electrostatic repulsions between headgroups with the same charge result in an increased area per headgroup at the air−water interface. The

RT-SAIL, the surface tension begin to decrease initially with increasing concentration, indicating that the RT-SAIL molecules became adsorbed at the air/water interface. A plateau was observed in the γ log C plot, suggesting the formation of aggregates. The CAC values were determined from the breakpoint of this plot. Surprisingly, there are two transition points in the surface tension curves, indicating a distinct aggregation. This unusual two-transition-point behavior has also been reported by other investigators, which may be due to the formation of premicellar aggregations (e.g., dimers, trimers, and oligomers).14 The CAC values obtained from the second transition point and γCAC are listed in Table 1. Obviously, no minimum around 8237


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Table 1. Parameters of Aggregation and Adsorption of [Si(n)N(2)-CA(1), (n = 3, 4)] in Aqueous Solutions at 298 Ka surfactant Si(4)N(2)-AC(1) Si(4)N(2)-PC(1) Si(4)N(2)-BC(1) Si(3)N(2)-AC(1) Si(3)N(2)-PC(1) Si(3)N(2)-BC(1)


[C10mim][Br]c [C12mim][Br]c [C12mim][BF4]c [C16AMorph][Br]d a

γcac mN·m−1 γCAC1 γCAC2 γCAC1 γCAC2 γCAC1 γCAC2 γCAC1 γCAC2 γCAC1 γCAC2 γCAC1 γCAC2 39.7 39.4 38.2 39.7

= = = = = = = = = = = =

24.23 20.00 23.36 20.02 25.37 20.42 31.71 20.41 31.36 20.36 30.77 20.27

CAC mmol·L−1

Amin Å2

ΔG0mic kJ·mol−1

ΔG0ads kJ·mol−1

Γmax mol·cm−2 × 10−10

CAC1 = 0.39 CAC2 = 6.13 (6.57b) CAC1 = 0.53 CAC2 = 5.20 (6.07b) CAC1 = 0.75 CAC2 = 4.80 (4.98b) CAC1 = 0.46 CAC 2 = 14.4 CAC 1 = 0.42 CAC 2 = 9.96 CAC 1 = 0.33 CAC1 = 8.28 29.3 10.9 9.2 0.28

132.85

−45.19

−86.88

1.25

105.77

−45.79

−78.90

1.57

105.10

−46.36

−78.81

1.58

63.14

−40.91

−60.56

2.63

72.83

−42.74

−65.43

2.28

83.87

−43.66

−69.84

1.98

−56.1

1.72 1.91 2.16 1.75

96.7 86.8 76.7 95

CAC: critical aggregation concentration. bFrom electrical conductivity measurements. cFrom ref 18. dFrom ref 23.

surface excess maximum concentration, Γmax, increases as Amin decreases. As for the difference in Γmax values among the three RT-SAIL species, the largest Γmax of Si(4)N(2)-AC(1) could be ascribed to the weak hydration of its counterion, which effectively screens the electrostatic repulsion between the polar headgroups. So the adsorption of RT-SAIL at the air/water interface was enhanced. It is also seen from Table 1 that ΔG0ads values are negative, revealing that the adsorption of RT-SAILs at the air−water interface was spontaneous. Furthermore, the ΔG0ads values are 0 more negative than their corresponding ΔGmic values, suggesting that during the formation of a micelle, work is required to transfer the RT-SAIL monomeric molecules from the surface to the micellar stage in the bulk aqueous medium; thus adsorption is the primary process compared to micellization. This is similar to reports for carbohydrate surfactants11b and siloxane surfactants.11a The electrical conductivity measurement is another technique used to study the micellar behavior of RT-SAILs in aqueous solution. Figure 3 shows the variation of the specific conductance, κ, versus the concentration of the RT-SAIL solutions at a temperature of 298 K. The observed breaks on these curves indicate the CAC points, and their values are the intersection points of the tangent lines drawn before and after the breaks. From electrical conductivity we can see that for Si(4)N(2)-AC(1) two fitted straight lines have different slopes and the slopes of the linear region above the CAC are smaller than those below the CAC. This can be explained by the fact that the conductivity below the CAC is due to a lot of the free ions; on the other hand, above the CAC, aggregates are forming, the mobilities of the aggregates decrease, and therefore the rate increase of conductivity is less important. The CACs obtained from the electrical conductivity method given in Figure 3 for Si(4)N(2)-AC(1), Si(4)N(2)-PC(1), and Si(4)N(2)-BC(1) are 6.57 × 10−3, 6.07 × 10−3, and 4.98 × 10−3 mol·L−1, respectively. The values of CAC estimated from the electrical conductivity plot are also shown in Table 1, together with those derived from the surface tension method. The CAC values obtained from two different methods are identical, considering the fact that in general the values obtained for the CAC vary slightly depending on the method used.

The degree of counterion dissociation (β), which is obtained using the ratio of the linear fragments of below and above the cmc, can provided information about counterions associated with the micelle. The values of β for the investigated ILs are 0.44 for Si(4)N(2)-AC(1), 0.37 for Si(4)N(2)-PC(1), and 0.17 for Si(4)N(2)-BC(1). The present ILs have a much lower value of β compared to [C12mim]Br [β = 0.77].18 The interaction between the hydrophilic group and anion is reduced by the larger size of the siloxane groups and the alkyl chain groups in the headgroup.15a 3.3. Aggregation Behavior. Replacing the hydrocarbontailed group with a siloxane-tailed group can result in great changes in the surface properties and aggregate behavior.11a,b To investigate the self-assembly properties of RT-SAILs in water in detail, TEM was used to visualize their ultrastructure. The micrographs for Si(n)N(2)-CA(1) in Figures 4 and 5 clearly reveal the existence of vesicles. These vesicles shows a spherical closed, stained periphery.19 These vesicles exhibit a size distribution from 30 to 400 nm; the diameters of the smaller vesicles are less than 100 nm, whereas the diameters of the large ones can reach more than 200 nm. Moreover, it can be seen from Figure 4 that the vesicular diameter increases with the increasing chain length of the carboxylate anions. One possible reason for this trend is that the anions with stronger hydrophobicity could exhibit a stronger ability to promote the formation of larger aggregates. The dynamic light scattering (DLS) measurements were made to study the mean hydrodynamic diameters of the aggregates formed at different concentrations for Si(n)N(2)CA(1). The size distributions are depicted in Figures 6 and 7. For Si(4)N(2)-PC(1), two bimodal peaks can be observed with diameters from 120 to 250 nm at 2.28 × 10−2 mol·L−1. For Si(3)N(2)-AC(1), two peaks can also be seen between 100 and 365 nm. It can be seen that the sizes of the vesicles formed by Si(4)N(2)-AC(1) and Si(3)N(2)-AC(1) solutions are in agreement with those observed in the respective TEM images. To confirm the spontaneous formation of vesicles, dye encapsulation experiments were performed.20 Bromophenol blue (BPB) is a widely used dye for vesicle encapsulation assays and was chosen as our vesicle cargo. The water-soluble dye encapsulated inside vesicles can been separated from free dye d 8238


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Figure 4. Negatively stained transmission electron microscopy (TEM) images of aggregates formed in Si(4)N(2)-CA(1). (a, b) Si(4)N(2)AC(1) with concentrations of 1.26 × 10−2 and 2.28 × 10−2 mol·L−1, respectively. (c, d) Si(4)N(2)-PC(1) with concentrations of 1.26 × 10−2 and 2.28 × 10−2 mol·L−1, respectively. (e, f) Si(4)N(2)-BC(1) with concentrations of 1.26 × 10−2 and 3.00 × 10−2 mol·L−1, respectively.

on the headgroup area, chain volume, and critical chain length. Israelachvili21 proposed a model based on the molecular packing parameter, which is defined as

P = ν /α0lC

(5)

in which ν, α0, and lC are the volume, area, and length of the hydrophobic moiety, respectively. Generally speaking, a singlechain molecule with a large head compared to its tail (i.e., a cone shape) usually has a P value of less than 0.5 and leads preferentially to micelles; on the other hand, a double-tailed molecule with a small head compared to its tail (i.e., a truncated cone shape) possesses a P value in the range of 0.5−1.0 and leads preferentially to vesicles.22 In our study, the calculation of P is difficult because the molecular volume and chain length of branched siloxane tails cannot be obtained exactly. However, the branched siloxane tails of the RT-SAILs are more bulky and hydrophobic than a hydrocarbon, rendering vesicle formation favorable.11 On the other hand, the alkyl chain of the carboxylate anion RCOO− moiety increase the hydrophobicity of the whole RT-SAILs, which allows molecules to form larger aggregates than micelles because of stronger hydrophobic interaction and the resultant strong aggregation tendencies.17

Figure 3. Specific conductivity of Si(4)N(2)-CA(1) as a function of concentration in aqueous solutions at 298 K. (a) Si(4)N(2)-AC(1), (b) Si(4)N(2)-PC(1), and (c) Si(4)N(2)-BC(1).

by size exclusion chromatography (SEC). The BPB-bearing vesicles eluted distinctly and could be easily separated from the free dye. The encapsulation efficiency (EF) was 2.70% for Si(4)N(2)-PC(1) and 2.84% for Si(4)N(2)-BC(1). The formation of surfactant aggregates is driven mainly by hydrophobic forces in aqueous solution. Different kinds of aggregates such as spherical micelles, wormlike micelles, and vesicles in surfactant solutions have been reported.17 The aggregate structure of surfactants in aqueous solution depends

4. CONCLUSIONS Six novel siloxane-based RT-SAILs [Si(n)N(2)-CA(1), (n = 3, 4)] were synthesized and characterized by FT-IR and 1H and 13 C NMR. The aggregation and adsorption properties of [Si(n)N(2)-AC(1), (n = 3, 4)] in aqueous solution were investigated with a number of different experimental techniques. It was found that all six siloxane-based ILs are 8239


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Figure 5. Negatively stained transmission electron microscopy (TEM) images of aggregates formed in Si(3)N(2)-CA(1) solutions. (a, b) Si(3)N(2)AC(1) with concentrations of 2.58 × 10−2 and 4.28 × 10−2 mol·L−1, respectively. (c, d) Si(3)N(2)-PC(1) with concentrations of 2.31 × 10−2 and 4.30 × 10−2 mol·L−1, respectively. (e, f) Si(3)N(2)-BC(1) with concentrations of 2.31 × 10−2 and 4.28 × 10−2 mol·L−1, respectively.

Figure 6. Intensity-weighted size distributions of as-prepared Si(4)N(2)-CA(1) surface-active ionic liquids (SAILs). (a, b) Si(4)N(2)-AC(1) with concentrations of 1.26 × 10−2 and 2.25 × 10−2 mol·L−1, respectively. (c, d) Si(4)N(2)-PC(1), with concentrations of 1.31 × 10−2 and 2.28 × 10−2 mol·L−1, respectively. (e, f) Si(4)N(2)-BC(1) with concentrations of 1.26 × 10−2 and 2.27 × 10−2 mol·L−1, respectively.

room-temperature ILs (RT-ILs). The introduction of the tetrasiloxane group in ILs can reduce the melting point of ionic liquids to below room temperature and promote the

micellization and aggregation behavior more efficiently. These siloxane SAILs can significantly reduce the surface tension of water, as shown by the γCAC values of 20 mN·m−1; all six 8240


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Figure 7. Intensity-weighted size distributions of as-prepared Si(3)N(2)-CA(1) surface-active ionic liquids (SAILs). (a, b) Si(3)N(2)-AC(1) with concentrations of 2.58 × 10−2 and 4.28 × 10−2 mol·L−1, respectively. (c, d) Si(3)N(2)-PC(1) with concentrations of 2.31 × 10−2 and 4.30 × 10−2 mol·L−1, respectively. (e, f) Si(3)N(2)-BC(1) with concentrations of 2.31 × 10−2 and 4.28 × 10−2 mol·L−1, respectively.

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ACKNOWLEDGMENTS This project is funded by the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (Grant No. 2014BAE03B03), National Natural Science Found of China (Grant No. 21103228), and Natural Science Found of Shanxi Province (Grant No. 2014011014-1).

siloxane-based RT-SAILs can self-assemble into vesicles in aqueous solution. Furthermore, the resultant vesicles may have potentially wide-ranging applications as model systems for biomembranes and as vehicles for drug delivery.

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ASSOCIATED CONTENT

* Supporting Information S

Structural characterization of SAILs. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02062.

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REFERENCES

(1) (a) Zheng, Z. P.; Fan, W. H.; Roy, S.; Mazur, K.; Nazet, A.; Buchner, R.; Bonn, M.; Hunger, J. Ionic Liquids: Not only Structurally but also Dynamically Heterogeneous. Angew. Chem., Int. Ed. 2015, 54 (2), 687−690. (b) Rogers, R. D.; Seddon, K. R. Ionic liquids–solvents of the future? Science 2003, 302 (5646), 792−793. (2) Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J.; Masel, R. I. Ionic liquid−mediated selective conversion of CO2 to CO at low overpotentials. Science 2011, 334 (6056), 643−644. (3) (a) Kuchlyan, J.; Banerjee, C.; Ghosh, S.; Kundu, N.; Banik, D.; Sarkar, N. Effect of room temperature surface active ionic liquids on aggregated nanostructures of γ-Cyclodextrins: A picosecond fluo-

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-351-4040802. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8241


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Surface Activity and Aggregation Behavior of Siloxane-Based Ionic

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