Anion Influence on Aggregation Behavior of Imidazolium-Based Ionic

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Anion Influence on Aggregation Behaviour of Imidazolium-Based Ionic Liquid in Aqueous Solutions. Effect on Diverse Chemical Processes Claudia G. Adam, Maria Virginia Bravo, and Alejandro Manuel Granados Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03083 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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Industrial & Engineering Chemistry Research

Anion Influence on Aggregation Behaviour of Imidazolium-Based Ionic Liquid in Aqueous Solutions. Effect on Diverse Chemical Processes Claudia G. Adam†,¶,*, M. Virginia Bravo†, and Alejandro M. Granados‡,¶ †Departamento de Química, Área Química Orgánica, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, Santa Fe-3000. Argentina. ‡INFIQC, Departamento de Química Orgánica. Facultad de Ciencias Químicas – Universidad Nacional de Córdoba; Ciudad Universitaria, Córdoba-5000, Argentina ¶ Researcher from National Council of Scientific and Technical Research (CONICET),

KEYWORDS Surfactant Ionic Liquid, Micelles, Solvatochromism, Design Ionic Liquids, Hydrolysis Reaction

ACS Paragon Plus Environment

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ABSTRACT

Ionic liquids (ILs) have been proven to be a special class of materials with wide applications, including the formation of aggregates of micellar type that are of most industrial interest. In this work, we analysed the effect of 1-alkyl-3-methylimidazolium bromide/tetrafluorborate in aqueous solution ([Cnmim] [Br]/[BF4]+W) upon three chemical processes taken as models. It was possible to obtain information about how these systems are organized, their structure and the function of the counterion in the formation of this structure. We confirmed that the counterion participation is crucial in the micellization process. It is clear that the systems [C8,10,12mim] [Br] are better organized than those with counterion [BF4]. In the last case, it was possible to prove the formation of micelles only for [C12mim] [BF4]. These results show features of interest when designing ILs with amphiphilic characteristics.

ARTICLE TEXT

1. Introduction The search of “new” alternatives to volatile organic solvents placed the ionic liquids (ILs) in the middle of the scene of numerous researches.1 They are entirely composed of cations and anions, and they have been proven to be successful solvents for a variety of reactions. Many of the ILs properties, their low volatility and nonflammability in particular, are looked for in the design of a ‘new material”. We can design ILs for a particular purpose and, in turn, to perform a specific task within a given chemical process. These new materials present a plethora of applications in various domains of the chemistry and physical sciences. Thus, they can act as new environmentally friendly solvents, as electrolytes, as lubricants, as stationary phase for chromatography, in separation technologies, templates for the synthesis of mesoporous and nanoparticules, and so on.2 Indeed, it is possible to modulate their physicochemical properties, such as density; viscosity; ionic mobility; hydrophobicity; acidity and basicity, either by combining different cations and anions, modifying the alkyl chain length in the cation or anion3, or adding different functional groups.4,5 Imidazolium ILs are very popular and the most studied of all ILs. Many of their physicalchemical properties and their behaviour even in molecular solvents and water have been


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determined.6-8 It is known that an increase of alkyl chain length in the imidazolium ring improves their amphiphilic properties. Consequently, since these ILs have the possibility of self-assembly in aqueous solution,9,10 it is possible to design ILs by combining cations and anions with that aim. These organized systems are interesting because the differential properties between the interfacial region and the bulk water play a fundamental role on reaction rates, mass transfer and chemical equilibrium positions.11-13 Specifically, some of these ILs have been used in extraction processes such as those involving metals and phenols.14,15 The aim of this paper was to analyze the effect of IL with amphiphilic properties in aqueous solutions on different chemical processes used as models. The selected processes will generate a criterion when selecting an IL for a specific task at industrial scale. According to the effects obtained upon these processes, it is expected to describe some properties of the ILs studied, particularly regarding their ability to form micelles in an aqueous solution (W) and the interfacial properties of these chemical entities. The selected ILs are composed of 1-alkyl-3-methylimidazolium salts, [Cnmim] [A] (where n is the number of carbon atoms in the alkyl chain of the imidazolium cation and A is the counterion). Lengths of the alkyl substituent in 1-position of the cation were C8, C10 and C12, with two different counterions: bromide [Br], representative of halogen ions with localized charge, and tetrafluoroborate [BF4], representative of spherical inorganic anions, with the equally distributed negative charge on the fluorine atoms. Three different chemical processes - the response of solvatochromic microsensors, a synthetic process and a kinetic process- were chosen to evaluate the behaviour of these ILs as surfactants, analyzing the impact of the anion on the formation of these organized systems.

2. Experimental Section ILs composed of [C8,10,12mim] cations with [Br] and [BF4] as counterions, were prepared according to literature procedure.16,17 To obtain a ionic liquid with spectroscopic grade purity, high quality precursors, purified prior to the synthesis of the ionic liquid, were used. Each step of purification was checked by UV-Vis and fluorescence spectroscopy, the same tools also used to determine optical purity in the ILs. 18,19 Solvatochromic microsensors


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For these systems we used the 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (IA) dye. It was prepared as a deep blue material following the procedure reported in the literature.20 UV-visible spectra were recorded on a Shimadzu UV-1800 spectrophotometer, equipped with a thermostatic cell holder. The absorption spectra of the IA dye (C = 10-5-10-4 M) were collected at 25± 0.1 ºC and were measured in triplets. To obtain the indicator spectrum in the micellar phase without water contribution, Vitha and Carr methodology was applied.21-23 Spectra in micellar system were analyzed using a Matlab program for Windows. This method of curve deconvolution is based on the general approach described by Kubista et al.24,

25

For the

application of the curve deconvolution method, we worked with the best single or double peak Gaussian fit for enclosed portions of the corresponding maxima wavelength (λmax). The micellar ET(30) was obtained from the experimental data of parameter ET(33) applying the corresponding linear correlation equation26; and ET(33) and ETN parameters were calculated as reported elsewhere.27, 28, 29

β–carotene (Merck, for biochemistry) was crystallized from benzene and methanol. The final concentrations were 5x10-5 M. Due to the low solubility of the indicator in water, an indicator stock solution in acetonitrile was prepared and some micro liters were transferred to 1 cm pathlength quartz cuvettes with a calibrated micro syringe. Under these conditions, we can say with a quite high confidence that the acetonitrile amount with respect to the total volume in cuvette was < 1%. n-Dodecyl-dimethylammonium bromide (DTAB), n-cetyl-trimethylammonium bromide (CTAB) and n-cetylpyridinium bromide (CPB) were obtained from Sigma-Aldrich and recrystallized from acetone/methanol. The plots of λmax vs surfactant concentration were all sigmoidal in nature so, the critic micelar concentration (CMC) values were obtained by fitting with the sigmoidal Boltzmann equation. Synthetic process Palladium nanoparticles (Pd-NPs) were obtained by reduction in hydrogen (H2) atmosphere using Palladium Acetate (Pd(OAc)2) (4,2 mgr) as a precursor.30 The synthesis was carried out in a modified Fischer-Porter bottle immersed in a silicone oil bath and connected to a H2 reservoir above CMC. To compare types of behaviour, Pd-NPs were synthesized in pure ILs. The suspension in the reactor was pressurized with 4 bar of H2 and heated for 3 hs. at 40 °C under vigorous stirring in order to ensure the total reduction of the metal precursor. NPs were isolated


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from the IL by the addition of isopropanol and centrifugation. Transmission electron microscopy (TEM) was performed on a JEOL JEM 1200Ex operating at 100 kV (CME-UFRGS, Brasil)31 and was used for catalytic experiments. Kinetic process Hydrolysis reaction kinetics of 4-methoxybenzenesulfonyl chloride (MBSC) was recorded by measuring the absorbance at 270 nm in a Shimadzu UV-1800 spectrophotometer with a thermostatic cell holder at 25.0 ± 0.1 °C. Stock solutions of MBSC were prepared in acetonitrile due to its low solubility in water. The final acetonitrile concentration in the reaction medium was < 1% (v/v). The MBSC concentration was always approximately 1.2 × 10−4 M. Error analysis of experimental data presented ET(30) and ETN parameters: for all cases the R2 obtained of fit was > 0.99. In general, if the value of R2 corresponding to the linear correlation from which these values are obtained is greater than or equal to 0.99, no uncertainties are reported, considering that the inherent error is in the last significant figure. 27, 28, 29 CMC values: all the experimental studies were performed in triplets and their average values are reported. The uncertainties reported in Table 1 correspond to the standard deviation of these three measurements. kobs values: The absorbance−time data of all kinetic experiments were performed in triplicate, were fitted by first-order integrated equations, and the values of the pseudo-first-order rate constants, kobs, were reproducible to within 3%. It is particularly worth mentioning that the uncertainty associated with kw corresponds to the corresponding standard deviation of the three kinetic runs carried out for its determination. Km values: the values corresponding to the systems formed with [Br] as counterion arise from the relationship between the slope of the linear regressions and kw. Therefore the uncertainties associated with the Km values reported for these systems, were calculated by propagation of errors for the multiplication (Km / kw) kw using the error of the slope of the linear regression and the relative uncertainty of kw determined as previously mentioned. The value of Km corresponding to [C12min] [BF4] was obtained from the nonlinear fit of the kobs vs Dn values to equation 3, so the uncertainty associated with this value of Km corresponds to the error given by the fit for that parameter.


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kw/kobs[IL]max values: The uncertainties associated with these values were calculated by propagation of errors of the involved values.

Results and discussion 3.1 UV-Vis spectroscopy: Solvatochromic dipolarity microsensor behaviour in the ([Cnmim] [BF4/Br]+W) systems By means of this model process, the effect of an IL with amphiphilic properties can be analysed in the solvatochromic microsensor partition process. The aggregation process of these ILs has been studied by conductivity, surface tension, fluorescence and so on. Solvatochromic microsensors have been used to check micellization processes, which are confirmed by an abrupt change in the measured properties. When the micellar aggregation occurs, a peculiar change is observed in the polarity of hydrophobic and hydrophilic domains. Micropolarity in the interior of the micelle is very different from the polarity in bulk water; therefore, the microsensor detects these differences as a change in λmax versus the surfactant concentration. IA has been used to check this process because it not only displays higher solubility in pure water than Reichardt´s dye20 but also has the advantage of being strongly lipophilic. As far as we know, IA indicator has not been used in these systems to determine a quantitative value of the micellar property ENT, which allows a direct comparison with traditional surfactants. Taking this into account, in the present study we obtain the ET(30) parameter using the Equation 1. This parameter is the most available in the literature, we can find it for hundreds of molecular solvents, and their binary mixtures, pure ILs and in mixture (ILs + molecular solvent/water) and also micellar systems.32

ET(30) = 0.99 (± 0.03) ET(33) – 8.1 (± 1.7) with r = 0.9953 and n = 20

Equation 1

The λmax shift of indicator IA with the IL concentration confirmed that the systems formed by [C8,10,12mim] [Br] are organized in micelles. As a representative example, Figure 1 shows the spectral shift for microsensor IA with the increase of the [C8mim] [Br] concentration. The insert in Figure 1 shows the λmax tendency for indicator IA with the growth of [C8,10,12mim] [Br] concentration logarithm.


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1,0 450 λ (nm)

0,9 0,8

440 430 420

0,7

410 -2,5 -2,0 -1,5 -1,0 -0,5

0,6

Abs

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log [LI] mol L

-1

0,5

[C8mim][Br]

0,4

[C8mim][Br]=0M

0,3

[C8mim][Br]=0,051M [C8mim][Br]=0,101M

0,2 0,1

[C8mim][Br]=0,132M [C8mim][Br]=0,202M [C8mim][Br]=0,304M [C8mim][Br]=0,365M

0,0 380

400

420

440

460

480

500

520

540

λ (nm)

Figure 1. Spectral shift of microsensor IA with the increase in [C8mim] [Br] concentration. Insert: λmax trend for indicator IA with an increase in log ILs concentration: n = 8 (red); n = 10 (blue); n = 12 (black). CMC and micellar ENT values corresponding to the systems analysed here and those found in the literature are shown in Table 1. Additionally, applying the methodology described above, the micellar ENT values for traditional cationic surfactants such as DTAB, CTAB, and CPB, were checked.

Table 1 CMC values calculated in this work with its corresponding standard deviation and CMC values

found in the literature

Ionic Liquid

CMC IA, 10-3 mol L-1

[C8mim] [Br]

149 ± 1

ENT (micelar)

0.79

Reported CMC values

0.141b;

0.16b;

0.19b;


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0.16a [C10mim] [Br]

45 ± 2

0.77

0.04ª;

0.031b;

0.039b;

0.046b; 0.039a; 0.041a [C12mim] [Br]

10 ± 1

0.74

0.011b;

0.009b;

0.01b;

0.0098a 0.0092b

[C12mim] [BF4] DTAB

15 ± 1

0.70c

CTAB

0.92 ± 0.01

0.69 c 0.72 c

CPB a

Data from Ref. 33,

17, 34, 35;b 36, 37, c 38

From these results, considering that the ENT value for water is 1.0039, the above results indicate a polar microenvironment, though different to water; so the probe solubilisation site could be the interfacial region. The comparison of these values indicates that the micelles of ILs in aqueous solution are slightly more polar than the ones from traditional surfactants. More detailed analysis of those values allows us to infer the effect of molecular structure on two properties of these self-assembled systems: interfacial polarity and CMC values. As regards the first one, and taking into account the values ENT (micellar) for these ILs, we noted a variation of only about 5% on the scale polarity. This shows that the interfacial polarity is not strongly affected by the length of the chain. On the other hand, as was expected, the CMC values decrease 15 times when the chain length increases by four –CH2- units, showing a linear behaviour when correlating ln CMC with the number of carbons in the chain.35 Comparing the values of CMC corresponding to the same cation [C12mim] with different counterions [Br]/[BF4], the change of counterion does not seem to greatly affect the property mentioned. As it can be observed in Figure 2, the ln CMC corresponding to [C12mim] [BF4] correlates perfectly with the values of ILs with counterion [Br]. As a comparison, and in accordance with our results, in this chart we include values corresponding to [C9mim] [Br] and [C14mim] [Br] taken from reference 35.


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-1

-2

-3

Ln CMC

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-4

-5

-6

-7 7

8

9

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12

13

14

15

Number of carbon atom

Figure 2. Logarithm of the CMC as a function of the number of carbon atoms in the surfactant hydrocarbon chain for 1-alkyl-3-methylimidazolium bromide ionic liquids (circles, blue: this work, green reference 35) and [C12mim] [BF4] (this work, red square) We can note, that the points obtained in literature by a different methodology fit well with the ones of this work, thus confirming the reliability of the applied methodology. For the systems formed with [BF4] as counterion, it was not possible to carry out the experiments with indicator IA, and consequently, to analyse the counterion participation in micellar process, due to the appearance of a whitish solid, probably because these systems lead to unstable aggregates. Taking into account that our intention was to obtain information about this micellar microenvironment from solvatochromic sensors, we selected a non-polar microsensor such as β –carotene. In order to go further into the characterization of the explored micellar systems by analysing the solute-solvent behaviour, we determined the spectral shifts band of β –carotene dye, which can be useful to carry out a correlation analysis. This chemical probe is a polyene with a allowed π-π* polar transition along the longitudinal axis of the molecule.40 The π2* values were estimated by applying equation 2.41, 42 ∼ ν ( β –carotene) = 24213 cm-1 – 2145 cm-1 x π2* (r = 0.994)

Equation 2


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Although in general, β –carotene dye displayed poor wavelength shifts and low absorbance values, the aqueous solutions of [C8,10mim] [Br] above CMC showed a ∆λmax (red shift) of 28 and 26 nm, respectively, with respect to pure water, where the λmax value is 434 nm. These λmax values correspond to a π2* value of 1.20 approximately, similar to those in non-polar molecular solvents such as benzene (1.23) or Cl4C (1.20). We think that this probe senses a non-polar region, possibly the core of the micelle. It is possible to interpret these results taking into account those previously observed for the IA sensor. When the counterion was [Br], the IA sensor detected the presence of micellar systems and this process could be confirmed for β –carotene dye, except for the system [C12mim] [Br]. In this case, the λmax values were coincident with that of water, indicating that the probe was not partitioned in the micelle and remained in the bulk water. The same behaviour was observed for [C12mim] [BF4]. On the other hand, for the systems [C8,10mim] [BF4], poor wavelength shifts were observed with the increase of ILs concentration, the λmax value being 438 nm for both ILs. In these cases, the shift detected with respect to water could be indicating that dye β –carotene could be solubilised in aggregates but not in micellar-like ones. These systems may be formed by monomer molecules in equilibrium with a single type of aggregate, resulting in a non-polar domain that allows the solubilisation of β –carotene sensor. In ILs with amphiphilic molecular structure, which can act as ionic surfactants, the counterion nature must also be taken into account. Counterions participate actively in the delicate balance of the interactions among the headgroups, the micellar structure depending on that balance. On the other hand, the [C8,10mim] [BF4] systems form aggregates which are not organized in micelles because the strong interaction cation-anion prevent the alkyl chain stacking π-π interactions. Then, the effects would be least important when the length of the alkyl chains in the cation is > 10. A similar behaviour was observed in the systems ([C4mim] [BF4/Br] +W).7 To confirm the aggregation process, conductivity values were determined, a small interruption in its linear behaviour being observed at 25 °C. This change was detected for [C8,10mim] [BF4] at 0.003 M and 0.017 M, respectively.43 This phenomenon may possibly arise from variations in the total charge distribution of small aggregates, resulting in coordination strengths of anionic/cationic aggregates.


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3.2 Behaviour of the ([Cnmim] [BF4/Br]+W) systems upon a synthetic process: Synthesis of palladium nanoparticles It is well known that ILs, the imidazolium-based salts in particular, show high selforganization on the molecular scale. They form extended hydrogen-bond networks resulting in pre-organized structures at the liquid state. These structures provide a 3D network of ionic channels and non-polar domains in a balanced coexistence, allowing the differentiated solubility of a solute.2 This fact can be used to generate in situ stabilized M-NPs. Recently a new class of imidazolium-based zwitterionic surfactant have been reported, to be used as stabilizer in the synthesis of Pd- NPs in water.44 We think that a synthetic process demanding a specific order, such as the synthesis of nanostructured materials, is adequate to evaluate, although indirectly, the micellar architecture of self-association systems in aqueous medium. The hypothesis was that the nature of the imidazolium salt and the anion type can have a direct influence on the NPs size and their distribution. In view of this, the Pd-NPs synthesis was selected as a model. We did not pretend to be exhaustive in the characterization of growth mechanism of the Pd-NPs, but rather focus on understanding the structure of the micellar system formed by these ILs through the analysis of the size distribution here obtained. When Pd-NPs were synthesized in the presence of concentrations of ILs lower than the CMC, agglomeration and precipitation of the NPs were observed. When the ILs concentration was above CMC values, a broad size distribution and larger sizes were obtained for the systems ([C10,12mim] [Br] + W) while for ([C12mim] [BF4] + W) the NPs were monodisperse and smaller. It was impossible to obtain stable Pd-NPs in the system ([C8mim] [Br] + W), since precipitation occurred even at concentrations above CMC. As an example, Figure 3 shows, TEM micrographs and histogram distribution for the Pd-NPs synthesized in ([C12mim] [BF4]/Br] + W). Taking into account that the non-polar transition-metal precursors or salts will be dissolved preferentially in the non-polar domains, the size and shape of the M-NPs were modulated by the volume of these IL nanoregions.30 The analysis of results suggest that in the cases where the nonpolar domain is not significant or generated hidrofobic canals are not well defined, the Pd-NPs cannot stabilize due to the water adsorption on the NPs surface, the double layer of IL is not formed around the metal particle, explaining their aggregation and precipitation.


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A) 20

Counts

15

10

5

0 6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22

diameter (nm)

B) 30

25

20

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15

10

5

0 2

3

4

5

6

7

8

diameter (nm)

Figure 3. TEM images of Pd-NPs and their histogram illustrating the particle size distribution of dispersed Pd-NPs synthetized in A) ([C12mim] [Br] + W) and B) ([C12mim] [BF4] + W). The change in anion size may control the volume of the polar domains of imidazolium ILs. In this sense, the difference observed in the size of Pd-NPs synthesized in both micellar systems may be attributed to a different organization of the 3D network of anions and cations usually found in ILs. A factor contributing to explain the observed size distribution is the highest concentration of the metal precursor due to its non-polar nature, in those micelles with better defined non-polar nanoregion or less water penetration. This is the case of the micelles formed by ILs with [Br] as counterion. This factor can be related with the position of the anion. In this sense, previous studies have determined that in the systems [C4mim] [BF4]/[I]45, the [BF4] anion is positioned on top of the imidazolium ring, while the [I] anion lies in the imidazolium ring plane, close to C2-H. In this last case, the interaction could be more specific as compared to


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[BF4]. To confirm some special interaction between the anion and the imidazolium ring, we carried out the attenuated total reflection (ATR) infrared absorption spectra for our pure ILs (Figure 4). The IR spectra of [C4mim] [BF4]/[Br] are shown together for comparison in Figure 4. Just as it was described in the literature, major differences were observed in the bands associated with the C-H stretching for the imidazolium ring which appear in the range of 3000-3200 cm-1, which show no changes with the increase in the length of the alkyl chain, for [BF4] as counterion. Conversely, for ILs with [Br] as counterion, signals appear with a red shift (30503150 cm-1), particularly associated to the C2-H stretching ring. The effect of specific interaction becomes very evident for [C12mim] [Br].

0,25

[C4mim][Br] [C8mim][Br]

0,20

[C10mim][Br] [C12mim][Br]

0,15 0,10 0,05 0,00

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,20

[C4mim][BF4] [C8mim][BF4]

0,15

[C10mim][BF4] [C12mim][BF4]

0,10

0,05

0,00 2800

2850

2900

2950

3000

3050

3100

3150

3200

-1

ν (cm )

Figure 4. ATR absorption spectra for pure ILs: (a) corresponding to [Br] as counterion and (b) corresponding to [BF4] anion. This could indicate that the conformation of the micelle polar domain depends on the counterion in a way similar to that proposed for [C4mim] [BF4]/[I], i.e., in this case, counterion [Br] would have a closer interaction with the C2-H of the imidazolium cation than [BF4] anion.


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3.4 Behaviour of the ([Cnmim] [BF4/Br]+W) systems upon hydrolysis reaction kinetics of 4methoxybenzenesulfonyl chloride. The influence of the surfactant concentration on the MBSC solvolytic rate constant (Scheme 1), has been studied in a range of concentrations that includes both the regions prior to the CMC, where the molecules of the surfactant appear as monomers dispersed in the solution and the regions above the CMC where the surfactant molecules are associated to form micelles. This reactive process involves not only a partition process similar to the solvatochromic microsensors but also a kinetic process. In this way, we can deepen even more in the understanding of these micellar aggregates.

Scheme 1: Schematic representation of a brief reaction mechanism for the nucleophilic reaction of H2O with MBSC Figure 5 shows the influence of the surfactant concentration on the first-order rate constant, kobs, for the solvolysis of the MBSC. As can be seen, the first-order rate constant remains unchanged with the increase of the surfactant concentration up to the CMC. At surfactant concentrations higher than the CMC, a clear decrease in kobs can be observed due to the presence of micellar aggregates.


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-3

7,0x10

-3

6,0x10

-3

-1

5,0x10

kobs,s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

4,0x10

-3

3,0x10

-3

2,0x10

-3

1,0x10

0,0 1E-3

0,01

0,1

1

[Surfactant],M

Figure 5. Influence of the surfactant concentration on the observed rate constant for the hydrolysis of MBSC at 25.0 °C. [C12mim] [BF4] (), [C12mim] [Br] (), [C10mim] [Br] (), [C8mim] [Br] () These inhibitions are attributed to the substrate incorporation into the micelles, where the rate of the solvolytic reaction is smaller than in bulk water because of a less water content in the micellar pseudo-phase. The formalism of the micellar pseudophase was applied to obtain a quantitative interpretation of the experimental results. Two well differentiated environments were considered: water and a micellar pseudophase between which the MBSC is distributed (Scheme 2).


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Scheme 2: Schematic representation of the reaction steps of MBSC solvolytic reaction in terms of pseudophase model of micelles

Considering that the solvolysis can simultaneously take place in water, kw, and at the micellar pseudophase, km, it is possible to derive the following rate equation, which relates the observed rate constant with the surfactant concentration (Equation 3). =

Equation 3

Where Km is the distribution constant of MBSC between water and the micellar pseudophases, Km = [MBSC]m/ ([MBSC]w [Dn]); [Dn] is the concentration of the micellized surfactant, [Dn] = [surfactant]T – CMC; and [surfactant]T is the total concentration of the surfactant. Values of CMC are required to fit the equation 3 to the experimental results. Although these values can be kinetically obtained as the minimal surfactant concentration necessary to observe an appreciable change in kobs, we used CMC values shown in IA (Table 1) to give the kinetic results greater independence. From the analysis of equation 3, if kw is >>> km Km [Dn], the equation can be transformed into equation 4, which predicts a linear relationship between 1/kosb and Dn. As seen in Figure 6, this is true for [C8,10,12min] [Br]. This indicates that a negligible percentage of solvolytic reaction takes place in the micelle for these systems micellar. This behaviour only allows to obtain the value of Km for these systems and assumes that km is negligible compared to kw.

= +

Equation 4


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3

3,0x10

3

2,5x10

3

-1

2,0x10

1/kobs,s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

1,5x10

3

1,0x10

2

5,0x10

0,0 0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

[Dn],M

Figure 6. Correlation of 1/kobs vs Dn: [C12min] [Br] (Black), [C10min] [Br] (red), [C8min] [Br] (blue)

The relationship between the slope and intercept of these straight lines allows us to estimate Km for the [C8,10,12min] [Br] series, being (0.30 ± 0.05) x102 M-1, (1.1 ±0.1) x102 M-1 and (13.7 ± 0.2) x102 M-1 respectively. These values imply that even in the case of [C8min] [Br] with a value of Dn = 0.1 the ratio [MBSC]m/[MBSC]w is 3.3, i.e., under these conditions 75% of MBSC is incorporated into the micelle, reaching a value of 90% for Dn = 0.3. Given these estimates, the hydrolysis reaction inhibition could be explained assuming a very small amount or absence of water at the site where MBSC is housed in the micelle. Conversely, [C12mim] [BF4] behaviour is very different, as there is a deviation from linearity in the relationship 1/kobs vs [Dn], as can be seen in the insert of Figure 7.


17


-3

7,0x10

-3

1/kobs, s

-1

6,0x10

-3

5,0x10

-1

1,5x10

3

1,0x10

3

5,0x10

2

0,0

-3

kobs, s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4,0x10

0,00

0,02

0,04

0,06

Dn, M -3

3,0x10

-3

2,0x10

-3

1,0x10

0,0 0,00

0,01

0,02

0,03

0,04

0,05

Dn, M

Figure 7. Influence of [C12min] [BF4] concentration on kobs for the hydrolysis of MBSC at 25.0 °C. Curve fits the experimental data to equation 3. The insert shows the relationship of 1/kobs vs [Dn]

Experimental data for the [C12min] [BF4] micellar system were fit to equation 3. The Km and km values were (1.5± 0.3) x103 M-1 and (8 ± 1) x10-4 s-1, respectively. These values make it reasonable to think that the site that hosts MBSC in this system contains more water than the micellar systems formed by the [C8,10,12min] [Br] series. In this sense, it is interesting to consider the results obtained Biczók et al.35 On the basis of conductivity measurements, they proposed that the interfacial layer is more accessible to water penetration in [Cnmin][Br] micelles than in traditional surfactants such as the tetraalkyl ammonium ion as polar head.46 So, taking into account the results obtained in this study and the data found in literature, it is reasonable to propose that the water content in these three types of micellar systems could have the following order: [Cnmin] [BF4] > [Cnmin] [Br] > [Cn(CH3)3N][Br]. Similarly, the kinetic data obtained from the hydrolysis of MBSC in the presence of [C12(CH3)3NH4] [Br]46 allow us to support this idea by comparing relations kw/kobs[IL]max for the three analogue surfactants [C12min] [BF4],


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[C12min] [Br], [C12(CH3)3NH4] [Br], where kobs[IL]max is the observed rate constant for the maximum concentration of surfactant used. This relationship provides a measure of the decrease in MBSC reactivity when it is incorporated into the micelles of different surfactants.1 The kw/kobs[IL]max values shown in Table 2 indicate that MBSC reactivity decreases in the same sense as water content in the micelle proposed above. This analysis allows us to conclude that for the same chain length, both the nature of the polar head and the counterion affect water content in the site where the MBSC is housed in these self-assembled systems. In this last process, we obtained certain details about the micellar microenvironment and the structure of these systems which were not detected by the solvatochromic process. The kinetic process showed that the polarity in the micellar interface should be greater in the systems with [BF4] as counterion. Howere, does not noticed in the solvatochromic process. Table 2. Partition constants Km and relatioships kw/kobs[IL]max corresponding to [C12min] [BF4], [C12min] [Br], and [C12(CH3)3NH4] [Br]46 Surfactant

kw/kobs[IL]max

Km, 103 M-1

[C12min] [BF4]

7.8 ± 0.1

1.5 ± 0.3

[C12min] [Br]

15.6 ± 0.1

1.37 ± 0.02

[C12(CH3)3NH4] [Br]

22.7

0.27

4. Conclusions Through of the three chemical processes studied, it was possible to obtain key information to design ILs with amphiphilic characteristic, as well as to deeply understand which variables affect the micellar organization process. As for the more detailed structure of the micellar systems formed, reactive processes have allowed us to expand about shaping self-assembled entities. It must be recognized that the counterion plays a fundamental role in the control of the properties such as water content, polarity and permeability of systems formed by self-assembled amphiphilic ILs. The counterion position in the polar head of the aggregates determines the size of polar and non-polar domains, and thus, a smaller size in Pd-NPs when the counterion is [BF4].


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According to this, it seems reasonable to propose that [Br] interacts more directly with the micelle, making it less permeable to water and therefore with a better defined non-polar nanoregion in comparison to [BF4]. This was consistent with the degree of inhibition observed in the hydrolysis reaction of MBSC. Hence, on the basis of the effect that they produce on different chemical processes, we can understand which variables govern the micellation process and therefore, predict whether an IL has amphiphilic properties or not. Besides, since the specific task to be performed by an IL on a reactive system conditions its design, it is clear that the counterion plays a fundamental role.

ACKNOWLEDGMENT

The authors thank the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Universidad Nacional del Litoral (UNL), República Argentina. This work received financial support from the Science and Technology Secretariat, UNL, CAI+D (2013-2016) Nro 325 and from CONICET, PIP (2014-2016) Nro 120. The authors also acknowledge a UFRGS and Dr Jairton Dupont for TEM images.

AUTHOR INFORMATION Corresponding Author * Dr. Adam, Claudia Departamento de Química, Área Química Orgánica, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, (3000) Santa Fe. Argentina. [email protected]

REFERENCES (1) Hallett, J. P.; Welton,T. Room-Temperature Ionic Liquids: Solvents for synthesis and catalysis. 2. Chem. Rev. 2011, 111, 3508–3576 (2) Dupont, J. On the solid, liquid and solution structural organization of imidazolium ionic liquids. J. Braz. Chem. Soc. 2004, 15, 341-350


20

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Greaves, T. L. C.; Drummond, J. Ionic liquids as amphiphile self-assembly media. Chem. Soc. Rev. 2008, 37, 1709–1726 (4) Chiappe, C.; Rajamani, S. Structural effects on the physico‐chemical and catalytic properties of acidic ionic liquids: An overview. Eur. J. Org. Chem. 2011, 5517–5539 (5) Giernoth, R. Task-Specific ionic liquids. Angew. Chem. Int. Ed. 2010, 49, 2834 – 2839 (6) Fortunato, G.; Mancini, P.; Bravo, M. V.; Adam, C. G. New solvents designed on the basis of the molecular-microscopic properties of binary mixtures of the type (Protic Molecular Solvent + 1-Butyl-3-methylimidazolium-Based Ionic Liquid). J. Phys. Chem. B, 2010, 114, 11804–11819 (7) Adam, C. G.; Bravo, M. V.; Mancini, P. M. E.; Fortunato, G. G. Solvatochromic dipolarity micro-sensor behaviour in binary solvent systems of the (water + ionic liquid) type: Application of preferential solvation model and linear solvation energy relationships. J. Phys. Org. Chem. 2014, 27, 841–849 (8) Anderson, J. L; Ding, J.; Welton, T.; Armostrong, D. W. Characterizing ionic liquids on the basis of multiple solvation interactions. J. Am. Chem. Soc. 2002, 124, 14247-14254 (9) Fletcher, K A; Pandey, S. Surfactant aggregation within room-temperature ionic liquid 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Langmuir. 2004, 20, 33-36 (10) Liu, X.; Zhou, G.; He, H.; Zhang, X.; Wang, J.; Zhang, S. Rodlike Micelle Structure and Formation of Ionic liquid in Aqueous Solution by Molecular Simulation. Ind. Eng. Chem. Res. 2015, 54, 1681−1688 (11) Bunton, C. A.; Savelli, G. Organic reactivity in aqueous micelles and similar assemblies. Adv. Phys. Org. Chem. 1986, 22, 213-309 (12) Bunton, C.A.; Nome, F.; Quina, F. H.; Romsted, L. S. Ion binding and reactivity at charged aqueous interfaces. Acc. Chem. Res. 1991, 24, 357-363 (13) Bunton, C.A. Reactivity in aqueous association colloids: Descriptive utility of pseudophase model. J. Mol. Liq. 1997 72, 231-249 (14) Fan, Y.; Li, Y.; Dong, X.; Hu, G.; Hua, S.; Miao, J.; Zhou, D.; Extraction of Phenols from Water with Functionalized Ionic Liquids. Ind. Eng. Chem. Res. 2014, 53, 20024−20031 (15) Tong, Y.; Han, L.; Yang, Y. Microemulsion Extraction of Gold (III) from Hydrochloric Acid Medium Using Ionic Liquid as Surfactant and Extractant. Ind. Eng. Chem. Res. 2012, 51, 16438−16443


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

(16) Suarez, P.A.Z.; Einloft, S.; Dullius, J.E.L; De Souza, R. F.; Dupont, J. Synthesis and physical-chemical properties of ionic liquids based on 1-N-butyl-3-methylimidazolium cation. J. Chim. Phys. 1998, 95, 1626-1639 (17) Sirieix-Plènet, J.; Gaillon, L.; Letellier, P. Behaviour of a binary solvent mixture constituted by an amphiphilic ionic liquid, 1-decyl-3-methylimidazolium bromide and water. Potentiometric and conductimetric studies. Talanta. 2004 63, 979-986 (18) Burrell, A. K.; Del Sesto, R. E.; Baker, S. N.; McCleskeya, T. M.; Baker, G. A. The large scale synthesis of pure imidazolium and pyrrolidinium ionic liquids. Green Chem. 2007, 9, 449– 454 (19) Annegret Stark, P. Behrend, O. Braun, A. Müller, J. Ranke, B. Ondruschka, B. Jastorff. Purity specification methods for ionic liquids. Green Chem. 2008, 10, 1152–1161 (20) Kessler, M. A.; Wolfbeis, O. S. ET(33), a solvatochromic polatity and micellar probe for neutral aqueous solution. Chem. Phys. Lipids 1989, 50, 51-56 (21) Vitha, M. F.; Weckwerth, J. D.; Odland, K.; Dema, V.; Carr, P. W. Study of the polarity and hydrogen-bond ability of sodium dodecyl-sulfate micelles by the Kamlet-Taft solvatochromic comparison method. J. Phys. Chem. 1996, 100, 18823-18828 (22) Vitha, M. F.; Carr, P. W. Study of the polarity and hydrogen-bond ability of dodecyltrimethylammonium bromide micelles by the Kamlet-Taft-solvatochromic-comparisonmethod. J. Phys. Chem. B. 1998, 102, 1888-1895 (23) Vitha, M. F.; Weckwerth, J. D.; Odland, K.; Dema, V.; Carr, P. W. An adaptation of kubista method for spectral curve deconvolution. Anal. Chem. 1997, 69, 2268-2274 (24) Kubista, K.; Sjoback, R.; Albinsson, B. Determination of equilibrium constants by chemometric analysis of spectroscopic data. Anal. Chem. 1993, 65, 994-998. (25) Scarminio, I.; Kubista, M. Analysis of correlated spectral data. Anal. Chem. 1993, 65, 409416 (26) Sarkar, A.; Pandey, S. Solvatochromic Absorbance Probe Behavior and Preferential Solvation in Aqueous 1-Butyl-3-methylimidazolium Tetrafluoroborate. J. Chem. Eng. Data. 2006, 51, 2051-2055 (27) Reichardt, C.; Welton,T. Solvent Effects in Organic Chemitry, fourth ed, Wiley-VCH: weinheim, Germay, 2011.


22

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Reichardt, C. Solvatochromic dyes as solvent polarity indicators. Chem. Rev. 1994, 94, 2319-2358. (29) Wypych, G. Ed. Handbook of Solvents, ChemTech Publushing: Toronto, Canada, 2001 (30) Dupont, J.; Scholten, J. D. On the structural and surface properties of transition-metal nanoparticles in ionic liquids. Chem. Soc. Rev. 2010, 39, 1780-1804 (31) Venkatesan, R.; Prechtl, M. H. G.; Scholten, J. D. Pezzi,; R. P.; Machado, G.; Dupont, J. Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes. J. Mater. Chem. 2011, 21, 3030-3036 (32) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Micelles, Vesicles, Emulsions, Gels, Surfactants, Colloids-Solvatochromism in cationic micellar solutions: Effects of the Molecular Structures of the Solvatochromic probe and the surfactant. Langmuir 2001, 17, 652-658 (33) Dong, B.; Li, N.; Zhang, L.; Yu, L.; Inoue, T. Surface adsorption and micelle formation of surface active ionic liquids in aqueous solution. Langmuir 2007, 23, 4178-4182 (34) Goodchild, I.; Collier, L.; Millar, S. L.; Prokes, I.; Lord, J. C. D.; Butts, C. P.; Bowers, J.; Webster, J. R. P.; Heenan, R. K. Structural studies of the phase, aggregation and surface behaviour of 1-alkyl-3-methylimidazolium halide + water mixtures. Colloid and Interface Science 2007, 307, 455-468 (35) Vanyúr, R.; Biczók, L.; Miskolczy, Z. Micelle formation of 1-alkyl-3-methylimidazolium bromide ionic liquids in aqueous solution. Colloids and Surfaces A: Physicochem. Eng. Aspects 2007, 299, 256-261 (36) Zhao, Y.; Gao, S.; Wang, J.; Tang, J. Aggregation of Ionic Liquids [Cnmim]Br (n= 4, 6, 8, 10, 12) in D2O: A NMR Study. J. Phys. Chem. B. 2008, 112, 2031-2039 (37) Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. Conductivities, volumes, fluorescence, and aggregation behavior of ionic liquids [C4mim][BF4] and [Cnmim] Br (n= 4, 6, 8, 10, 12) in aqueous solutions. J. Phys. Chem. B. 2007, 111 (22), 6181-6188 (38) Novaki, L.; El Seoud, O. Microscopic polarities of interfacial regions of aqueous cationic micelles: Effects of structures of the solvatochromic probe and the surfactant, Langmuir 2000, 16, 35-41 (39) Reichardt, C.; Harbusch-Görnert, E. Über Pyridinium-N-phenolat-Betaine und ihre Verwendung zur Charakterisierung der Polarität von Lösungsmitteln, X. Erweiterung, Korrektur und Neudefinition der ET-Lösungsmittelpolaritätsskala mit Hilfe eines lipophilen penta-tert-


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Page 24 of 25

butyl-substituierten Pyridinium-N-phenolat-Betainfarbstoffes. Liebigs Ann. Chem. 1983, 721743 (40) Abe, T. Improvements of the Empirical π* Solvent Polarity Scale. Bull. Chem. Soc. Jpn. 1990, 63, 2328-2338 (41) Myers, A. B.; Brige, R. R. The theoretical treatment of solvent effects on molecular electronic oscillator strengths. J. Chem. Phys. 1980, 73, 5314-5321 (42) Abe, T.; Abboud, J-L M.; Belio, F.; Bosch, E.; Garcia, J. I.; Mayoral, J. A; Notario, R.; Ortega, J.; Rosés, M. Empirical-treatment of solvent-solute interactions - medium effects on the electronic absorption-spectrum of beta-carotene. J. Phys. Org. Chem. 1998, 111, 193-200 (43) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C. Aggregation behavior of aqueous solutions of ionic liquids. Langmuir. 2004, 20, 2191-2198 (44) Souza, B. S.; Leopoldino, E. C.; Tondo, D. W.; Dupont, J.; Nome, F. Imidazolium-based zwitterionic surfactant: A new amphiphilic Pd nanoparticle stabilizing agent, Langmuir. 2012, 28, 833-840 (45) Yoonnam, J.; Jaeho,S.; Choongwon,S.; Hyunjin, L.; Hyeonsik, C.; Minhyuck, K.; Bongjin, M.; Yukio, O.; Doseok, K. Structures of ionic liquids with Different Anions Studied by Infrared Vibration Spectroscopy. J. Phys. Chem. B 2008, 112, 4735-4740 (46) Cepeda, M.; Daviña, R.; García-Río, L.; Parajó, M.; Rodríguez-Dafonte, P.; Pessêgo, M. Competition between surfactant micellization and complexation by cyclodextrin, Org. Biomol. Chem. 2013, 11, 1093-1102


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Table of Contents Graphic and Synopsis


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Anion Influence on Aggregation Behavior of Imidazolium-Based Ionic

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