Effect of Counterions on Micellization of Pyrrolidinium Based Silicone

Apr 15, 2014 - Manoj Kumar Banjare , Ramsingh Kurrey , Toshikee Yadav , Srishti Sinha , Manmohan L. Satnami , Kallol K. Ghosh. Journal of Molecular ...
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Effect of Counterions on Micellization of Pyrrolidinium Based Silicone Ionic Liquids in Aqueous Solutions Jinglin Tan and Shengyu Feng* Key Laboratory of Special Functional Aggregated Materials, Ministry of Education; School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, China ABSTRACT: The micellization of two silicone ionic liquids, 1-methyl-1-[tri(trimethylsiloxy)]- silylpropylpyrrolidinium nitrate (Si4pyNO3) and 1-methyl-1[tri(trimethylsiloxy)]- silylpropylpyrrolidinium acetate (Si4pyAc), with different counterions was systematically investigated by surface tension and electrical conductivity. Surface tension of water can be reduced almost to 20 mN·m−1 with the addition of the silicone ionic liquids, indicating that the silicone ionic liquids exhibit excellent surface activity. The critical micelle concentration (CMC) values of Si4pyNO3 are less than those of Si4pyAc. Electrical conductivity measurements show that the degree of counterion binding (β) for Si4pyNO3 is twice as large as that for Si4pyAc. Thermodynamic parameters (ΔH0m, ΔS0m, and ΔG0m) of micellization derived from electrical conductivities indicate that the micellization for both Si4pyNO3 and Si4pyAc is enthalpy-driven process. The heat capacities, Δc0m,p, are negative for the two silicone ionic liquids relating to the removal of water accessible nonpolar surfaces. The addition of sodium halides in the aqueous solution decreases CMC remarkably; however, the surface tension at CMC stays the same with the salt-free system.

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INTRODUCTION Ionic liquids (ILs) are increasingly gaining interest as a class of environmentally friendly solvents due to their unique properties, such as nonvolatility, nonflammability, and high ionic conductivity.1−3 To date, ILs based on pyrrolidinium, pyridinium, imidazolium, and quaternary ammonium cations with a variety of anions, such as halides, PF6−, NO3−, BF4−, CH3COO−, and CF3SO3−, have been successively synthesized and widely utilized in separation, organic synthesis, fuel cell, catalysis, and preparation of nanomaterials.4−9 Specially, in recent years, hydrocarbon ILs, such as 1-methyl-3-alkylimidazolium ILs, have been extensively studied as novel surfactants, and much research on their aggregation behavior in aqueous solution has been reported.10−13 In addition, more research has focused on the effect of structural change (especially the long hydrocarbon chain and cationic structure) on the aggregation behavior of ILs in aqueous solution.14−20 Meanwhile, silyl and siloxy substituted imidazolium ILs have been reported.21−23 Then, aggregation behavior of trisiloxane-tailed ionic liquids in aqueous solution has also been investigated.24 Owing to the excellent properties of silicone surfactants, such as excellent surface activity, low toxicity, and outstanding association behavior, silicone surfactants have been widely used in the field of colloid and interface science.25−31 The interfacial behaviors of silicone surfactants were strongly influenced by the number of silicon atoms and demonstrated that the siloxane portion oriented at the water surface with an “umbrella” conformation.31 Moreover, the entire hydrophobic effect of the silicone surfactants is due to CH3 and CH2 groups coupled to siloxane chain and the Si−O units are neither hydrophobic nor hydrophilic.32 By comparison (surface activity), it is roughly estimated that CMe2 units can be equated only to two CH2 © 2014 American Chemical Society

groups of the alkyl chain, while a single SiMe2 unit can be compared to four CH2 units.33 Furthermore, systematic and detailed studies of aggregation behavior for silicone surfactants and hydrocarbon ILs in aqueous solution have been explored. Aggregation behavior in aqueous solutions of five pyrrolidinium ILs, N-alkyl-Nmethylpyrrolidinium bromide, CnMPB, with n = 10, 12, 14, 16, and 18 have been explored by Karukstis et al.34 Systematically investigated for aggregation behavior in aqueous solutions of the long-chain 1,3-dialkylimidazolium ILs, CnmimBr (n = 10, 12, 14, and 16) and 1-dodecyl-3methylimidazolium tetrafluoroborate ([C12mim]BF4) have been reported by Zheng et al.35−37 Meanwhile, our group also synthesized a series of cationic silicone surfactants and studied their aggregation behavior in aqueous solution.38 However, to our knowledge, the aggregation behavior of pyrrolidinium based silicone ionic liquids in aqueous solution has not yet been studied. For the inherent amphiphilic nature of their cations for silicone ILs, it would be interesting to investigate the surface activity of silicone ILs in aqueous solution from the practical and academic points.35 In this paper, two pyrrolidinium based silicone ILs with different counterions were synthesized, and their aggregation behaviors were investigated. A series of useful parameters obtained in this paper will be useful in understanding the role of the counterions in affecting the aggregation behavior of silicone ILs. Moreover, the silicone ILs may have potential application in the fields relating to colloid and interface science, such as silicone emulsions, porous materials, and so on.25−27 Received: December 29, 2013 Accepted: April 8, 2014 Published: April 15, 2014 1830

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EXPERIMENTAL SECTION Materials. γ-Chloropropyltrichlorosilane was purchased from Shandong Qiquan Silicon Co., Ltd. Chlorotrimethylsilane, methanol, isopropanol, n-hexane, silver nitrate (AgNO3), and silver acetate (CH3COOAg) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. The reagents were used as received. Triply distilled water was used to prepare all of the solutions. The structures of the silicone ILs used in this work are shown in Figure 1. Their detailed synthesis process is presented in the Supporting Information.

plot, indicating that the micelles are formed. The concentration of the break point was regarded as the critical micelle concentrations (CMC),31 and the values of the CMC for the two silicone ILs are given in Table 1, together with the data for Table 1. CMCs of Si4pyNO3, Si4pyAc, and Si4pyCl in Aqueous Solutions at 298 K CMC/mmol·kg−1

a

surfactants

determined from surface tension

determined from electrical conductivity

Si4pyNO3 Si4pyAc Si4pyCla

3.7 ± 0.1 9.7 ± 0.1 5.7 ± 0.1

4.0 ± 0.1 9.3 ± 0.1 5.5 ± 0.1

Reported in ref 38.

Si4pyCl.38 From Table 1, it is found that the counterions of the silicone ILs have a remarkable effect on their surface activities. The CMC values follow the order Si4pyNO3 < Si4pyCl < Si4pyAc. Namely, the ability of these counterions to promote micellization follows the order NO3− > Cl− > CH3COO−, which is in good accord with the Hofmeister series of the anions for cationic surfactants.39 Generally, the weaker hydration of the counterions, the more readily it is adsorbed on the micellar surface. Therefore, the weakly hydrated counterions can screen the charge at the surface of micelles and reduce the surface potential effectively. Although CH3COO− is highly hydrated, it can not tend to form ion pairs with silicone ILs headgroups because of its kosmotropicity. Those result not only in lower CMC values of weaker hydrated counterions (NO3−) but also in higher values of the degree of counterion binding.40,41 From the surface tension plot, the surface tension at CMC (γCMC), the adsorption efficiency (pC20) and surface pressure at CMC (πCMC) can be obtained and used to evaluate the surface activities of the silicone ILs. The adsorption efficiency (pC20) is defined as eq 1

Figure 1. Chemical structure of the Si4pyNO3 and Si4pyAc ILs.

Apparatus and Procedures. 1H NMR spectra were recorded using a Bruker AV 300 spectrometer in methanol-d (CH3OD). FT-IR was recorded using a Nicolet FT-IR spectrometer. Measurement was performed on samples dispersed in anhydrous KBr pellets. Surface tension measurements were carried out on a model BZY-1 tensiometer (Shanghai Hengping Instrument Co., Ltd., accuracy ± 0.1 mN·m−1) employing the ring method and using a thermostatic bath (DC-0506, Shanghai Hengping Instrument Co., Ltd.). All measurements were taken at 25.0 ± 0.1 °C and repeated until the values were reproducible. Specific conductivity measurements on the aqueous solutions were performed using a low-frequency conductivity analyzer (model DDS-307, Shanghai Precision & Scientific Instrument Co., Ltd.).

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RESULTS AND DISCUSSION Surface Properties and Micellization. The surface tension measurements were evaluated the surface activities of the two silicone ILs, Si4pyNO3 and Si4pyAc, in aqueous solutions. Figure 2 shows the plots of the surface tension (γ) versus mole concentration (C) for the aqueous solutions of Si4pyNO3, and Si4pyAc at 25 o C. As shown in Figure 2, the surface tension decreases initially with increasing the concentration of the two silicone ILs, suggesting the silicone IL molecules are adsorbed at the air/ water interface. Then a plateau appears in the surface tension

pC20 = −log C20

(1)

where C20 represents the concentration required to reduce the surface tension of pure solvent by 20 mN·m−1. The larger the pC20 value, the higher the adsorption efficiency of the silicone ILs is.42 From Table 2, it can be seen that the values of pC20 increase in the order Si4pyAc < Si4pyCl < Si4pyNO3. The surface pressure at CMC, πCMC, is defined as eq 243 ΠCMC = γ0 − γCMC

(2)

where γ0 is the surface tension of pure water and γCMC is the surface tension at CMC. So, the values of πCMC indicate the maximum reduction of surface tension, and it becomes a measure for the effectiveness of the silicone ILs to lower the surface tension of the water.36 The values of γCMC, pC20, and πCMC were obtained and listed in Table 2. It can be found that the γCMC values of Si4pyAc and Si4pyNO3 are (21.0 and 20.0) mN·m−1, respectively, and lower γCMC values than those of hydrocarbon ILs, e.g., 1-octyl-4-methylpyridinium chloride11 (31.0 mN·m−1), gemini pyrrolidine based ionic liquids43,44 [(40 to 44) mN·m−1], which is attributed to both the superiority of highly surface active methyl substituent and three Si(CH3)3 units. In addition, their πCMC values are larger than those conventional cationic surfactants, e.g., alkyl trimethylammonium bromides/chlorides, alkyl imidazolium bromides/BF4, suggesting that the surface activity of the two silicone ILs

Figure 2. Surface tensions of Si4pyNO3 and Si4pyAc aqueous solutions as a function of their concentrations, ■, Si4pyNO3 and ▲, Si4pyAc at 25 °C. 1831

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Table 2. Adsorption Parameters of Si4pyNO3, Si4pyAc, and Si4pyCl in Aqueous Solutions at 298 K

a

surfactants

γcmc/mN·m−1

Πcmc/mN·m−1

pC20

Γmax/μmol·m−2

Amin/nm2

Si4pyNO3 Si4pyAc Si4pyCla

20.0 ± 0.1 21.0 ± 0.1 20.0 ± 0.1

52.5 ± 0.1 51.5 ± 0.1 52.5 ± 0.1

4.49 ± 0.101 3.90 ± 0.121 4.18 ± 0.109

1.28 ± 0.088 1.33 ± 0.072 1.24 ± 0.080

1.29 ± 0.060 1.24 ± 0.081 1.34 ± 0.078

Reported in ref 38.

surfactants is higher. This is attributed to the superiority of highly surface active methyl substituent.27,31 According to the Gibbs adsorption isotherm, the maximum excess surface concentration (Γmax) and the area occupied by a single silicone IL molecule at the air/water interface (Amin) can be obtained. The two parameters can reflect the surface arrangement of surfactants at the air−liquid interface.45 A greater value of Γmax or smaller values of Amin means a denser arrangement of surfactant molecules at the surface of the solution.3 The Γmax is calculated by Gibbs adsorption isotherm equation (eq 3).46 Γmax = −

1 ⎛ ∂γ ⎞ ⎜ ⎟ nRT ⎝ ∂ln C ⎠

(3)

Figure 3. Specific conductivity as a function of concentration at different temperatures for Si4pyNO3, ★, 35 °C; ⧫, 30 °C; ▲, 25 °C; ●, 20 °C; and ■, 15 °C.

where γ is the surface tension, R is the ideal gas constant, T is the absolute temperature, (dγ/d ln C) is the slope of the linear fit of the data before the CMC in the surface tension plots, and n is the number of ionic species, respectively. For 1:1 ionic surfactants in the absence of any other solutes, n is equal to 2. Then, Amin is obtained by eq 446 A min =

1016 NA Γmax

From Table 3, it can see that the β values of Si4pyNO3 are larger than that of Si4pyAc, resulting from the different hydration for the counterions (as mention above). According to the mass action model, the standard Gibbs energy change (ΔG0m), is obtained form eq 6.3,38,47 ΔGm0 = (1 + β)RT ln χCMC

(4)

where NA is Avogadro’s number. Then, the values of Γmax and Amin were obtained and given in Table 2. The Amin values are (1.29 and 1.24) nm2 for Si4pyNO3, and Si4pyAc, respectively, and larger than those of ILs, e.g., 1-(2,4,6trimethylphenyl)-3-alkylimidazolium bromide, 1-alkyl-3-methylimidazolium ILs, CnmimBr (n = 10, 12, 14, 16), indicating a looser arrangement of the silicone ILs molecules at the air/ water interface.35,45 The reason may be that the “umbrella” conformation oriented parallel to the water surface resulting from the bulky trimethylsilyoxyl groups causes a greater distance between the silicone ILs molecules.47,48 Thermodynamic Analysis of Micellization. The electrical conductivity of the silicone ILs concentration at different temperatures can describe the thermodynamics of micelle formation. Figure 3, as a characteristic plot, shows the electrical conductivity, κ, as a function of Si4pyNO3 concentration at different temperatures which exhibits a progressive increase as temperature increases. The CMC values were obtained from the intersection of the two straight lines in κ−C plots and listed in Table 1. Meanwhile, the CMC values at 25 o C are in good accordance with those estimated from surface tension measurements (see Table 1). The degree of counterion binding can also be estimated form conductivity measurements by eq 5.46 α β=1− 1 α2 (5)

(6)

where the χCMC is the mole fraction of silicone ILs at CMC, T is the absolute temperature, and R is the ideal gas constant. After that, the standard enthalpy change (ΔH0m) can be calculated by using the Gibbs−Helmboltz equation (eq 7) ⎡ ∂(ΔG 0 /T ) ⎤ m ⎥ ΔHm0 = ⎢ ⎣ ∂(1/T ) ⎦

and, the entropy of micelle formation 8

(7)

(ΔS0m)

is obtained by eq

ΔHm0 − ΔGm0 (8) T 0 Finally, the heat capacity of micelle formation (Δcm,p) is a linear function of temperature, estimated as the slope of the ΔH0m versus temperature curve (eq 9).40 ΔSm0 =

⎛ ∂ΔH 0 ⎞ 0 m ⎟ Δcm,p =⎜ ⎝ ∂T ⎠ p

(9)

The thermodynamic parameters for Si4pyNO3 and Si4pyAc are listed in Table 3, which may be used to understand the effect of counterions on the micellization of the two silicone ILs surfactants. In the whole investigated range, all of the ΔG0m values for the two silicone ILs are negative, indicating the micellization of the two silicone ILs is spontaneous. Meanwhile, the ΔG0m values of Si4pyAc are smaller than those of Si4pyNO3, suggesting Si4pyAc may form comparatively weak micelles.42 Meanwhile, the ΔH0m values for Si4pyNO3 and Si4pyAc are negative, suggesting that the micellization of Si4pyNO3 and Si4pyAc is an exothermic process resulting from the transfer of

where α1 and α2 are the slopes of the straight lines before and after the CMC in the conductivity plots, respectively. The β values for Si4pyNO3, and Si4pyAc obtained from conductivity at different temperatures are listed in Table 3. 1832

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Table 3. CMC, Degree of Counterion Binding (β), and Thermodynamic Parameters of Micelle Formation for Si4pyAc and Si4pyNO3 in Aqueous Solutions at Different Temperatures surfactants Si4pyAc

Si4pyNO3

T/°C 15 20 25 30 35 15 20 25 30 35

CMC/m·mol·kg−1 8.2 ± 0.1 8.5 ± 0.1 9.6 ± 0.1 10.5 ± 0.1 10.7 ± 0.1 3.6 ± 0.1 3.8 ± 0.1 4.0 ± 0.1 4.2 ± 0.1 4.5 ± 0.1

β 0.30 0.28 0.26 0.24 0.22 0.80 0.79 0.78 0.77 0.76

± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

ΔG0m/kJ·mol−1

ΔH0m/kJ·mol−1

−ΔTS0m/kJ·mol−1

Δmc0p/J·mol−1·K−1

−27.36 −27.22 −27.03 −26.84 −26.72 −41.58 −41.91 −42.15 −42.42 −42.58

−34.03 −36.34 −38.57 −40.73 −42.82 −22.07 −24.69 −27.22 −29.66 −32.03

6.67 ± 0.066 8.86 ± 0.070 11.54 ± 0.077 13.89 ± 0.085 16.10 ± 0.095 −19.50 ± 0.096 −17.22 ± 0.090 −14.93 ± 0.085 −12.75 ± 0.079 −10.55 ± 0.077

−469.15 −454.40 −439.64 −424.89 −410.13 −531.10 −514.39 −497.68 −480.98 −464.27

± ± ± ± ± ± ± ± ± ±

0.10 0.11 0.12 0.12 0.14 0.11 0.10 0.13 0.14 0.10

the silicone chain from the aqueous environment into the micelle.48 Moreover, the ΔH0m change of micellization is also caused by electrostatic interactions. During the micellization, the electrostatic repulsion of the headgroups and counterions (exothermic) would be screened by counterions. As comparison, the weakly hydrated counterions (NO3−) can be adsorbed more easily on the micellar surface. Consequently, the hydration water is partially broken down, the role of hydrophobic effect becomes weaker and the entropy increase. The energy required to break up the three-dimensional water structure is lower, and ΔH0m became more exothermic.3,40 In addition, The Δc0m,p values for the silicone ILs in Table 3 are negative, as usually observed for the self-association of amphiphiles, and can be ascribed to the removal of large areas of nonpolar surface from contact with water on micelle formation. As comparison, the Δc0m,p values for Si4pyNO3 are more negative than that for Si4pyAc, which could be that because during the process of micellization, the counterions absorb on the micellar surface would reduces the number of water in their solvation shell and share hydration water with the headgroups. Moreover, the counterions with weaker hydration (NO3−) would absord more strongly on the headgroups resulting in the number of water molecules expelled from the headgroups being less in comparison with the highly hydrated counterion (CH3COO−). Therefore, the contribution from the dehydration of NO3− is less positive and the Δc0m,p is more negative.40,49 Furthermore, it is worth noting that the value of ΔG0m for Si4pyAc and Si4pyNO3 are both mainly contributed by ΔH0m, suggesting that the micellization process for both Si4pyAc and Si4pyNO3 in aqueous solution is enthalpy-driven in the whole investigated temperature range. Effect of Sodium Halides on the Surface Activity of Silicone ILs. The surface tension of the silicone ILs was also measured in the presence of sodium halides (NaCl and NaBr) aqueous solution. Unfortunately, the surface tension of the silicone ILs in the presence of sodium iodide and the Si4pyAc in the presence of sodium halides were not obtained because of salting-out effects. Figure 4, a characterization plot, exhibits the results obtained for Si4pyNO3. The CMC values of the silicone ILs in the presence of NaCl and NaBr are given in Table 4. From Table 4, the CMC of Si4pyNO3 decreased by both the sodium halides, and the effectiveness of halide anions on the depression of the CMC follow the order, Cl− < Br−, which is similar to the observation by Zheng and Varade.36,50 However, the γCMC of the silicone ILs in the presence of sodium halides aqueous solution keeps the same with the salt-free system, which is different wtih the results for hydrocarbon ILs

± ± ± ± ± ± ± ± ± ±

0.041 0.043 0.046 0.049 0.051 0.027 0.030 0.033 0.035 0.039

± ± ± ± ± ± ± ± ± ±

0.070 0.068 0.066 0.064 0.062 0.080 0.078 0.075 0.073 0.070

Figure 4. Surface tension as a function of concentration of Si4pyNO3 in the absence or presence of sodium halides at 25 °C, in ●, 0.05 mol· kg−1 NaCl; ▲, 0.05 mol·kg−1 NaBr; and ■, water.

Table 4. CMC of Si4pyCl and Si4pyNO3 in Sodium Halide Solutions at 25 o C CMC/mmol·kg−1 salt/0.05 mol·kg NaCl NaBr

−1

γCMC/mN·m−1

Si4pyNO3

Si4pyCl

Si4pyNO3

Si4pyCl

1.3 ± 0.1 0.6 ± 0.1

1.2 ± 0.1 0.4 ± 0.1

20.2 ± 0.1 20.2 ± 0.1

20.1 ± 0.1 20.1 ± 0.1

surfactants. The trend in the CMC values caused by the salt addition may be interpreted by the model proposed by Borwankar et,al.51 Generally, the ILs cations are assembled in the Stern layer whereas the counterions exist in the diffuse part of the electric double layer without penetrating into the Stern layer. So, the electrostatic repulsion among the polar head groups was screened by the added sodium halides and the halogen anions surrounding the micelles compress the electric double layer, consequently, resulting remarkably facilitate micellization and lower CMC compared with the salt-free system. Moreover, the characteristic of the interactions of anions to cationic micelle may be responsible for the difference among anion species.36,52−54 The less hydrated Br− can be absorbed more strongly and readily on the micellar surface of the silicone ILs, and neutralize the charges on the micelle surface. As a result, the CMC in an aqueous solution with NaBr is smaller than that with NaCl. It can be concluded from the above that the structures of the counterions of the silicone ILs have a significant influence up on the driving force of the aggregate formation.

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CONCLUSIONS It is concluded that the two pyrrolidinium based silicone ILs with different counterions have excellent surface activity. The 1833

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structure of counterions could greatly affect the surface activities and micellization behavior of the silicone surfactants: The CMC values increase following the order Si4pyNO3 < Si4pyCl < Si4pyAc, and the β values for Si4pyNO3 are more than twice that for Si4pyAc. Concerning the driving force of the micellization, Si4pyNO3 and Si4pyAc both are enthalpy-driven processes. Furthermore, the surface tension of the silicone ILs in the presence of sodium halides aqueous solution stays the same with the salt-free system. These results suggested that properties of counterions of the silicone ILs can significantly affect their surface activity and micellization process. Understanding the aggregation behaviors of the silicone ILs may help to design silicone ILs with novel structures and find potential applications of silicone ILs in colloid and interface science.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-531-88364866. Fax: +86531-88564464. Funding

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21274080 and 20874057) and the Key Natural Science Foundation of Shandong Province of China (No. ZR2011BZ001). Notes

The authors declare no competing financial interest.

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dx.doi.org/10.1021/je401118k | J. Chem. Eng. Data 2014, 59, 1830−1834