Thermodynamic Insights into the Binding of Mono- and Dicationic

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Thermodynamic Insights into the Binding of Mono- and Dicationic imidazolium Surfactant Ionic Liquids with Methylcellulose in the Diluted Regime Marcos Antonio Villetti, Francieli Isa Ziembowicz, Caroline Raquel R Bender, Clarissa P. Frizzo, Marcos A. P. Martins, Thiane Deprá de Souza, Carmen Luísa Kloster, and Irene Teresinha Garcia J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03525 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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The Journal of Physical Chemistry

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Thermodynamic Insights into the Binding of Mono- and Dicationic imidazolium Surfactant Ionic Liquids with Methylcellulose in the Diluted Regime

Corresponding author: Address: Department of Physics, Center of Natural and Exact Sciences, Federal University of Santa Maria, Avenida Roraima, 1000 CEP 97105-900 Santa Maria, RS, Brazil. Tel.: + 55 55 3220 8858 Fax +55 55 3220 8337. *

E-mail

addresses:

[email protected];

[email protected]

;

[email protected] (M. A. Villetti)

Author names and affiliations: Francieli Isa Ziembowicz,‡ Caroline Raquel Bender,‡ Clarissa Piccinin Frizzo,‡ Marcos Antonio Pinto Martins,‡ Thiane Deprá de Souza,§ Carmen Luisa Kloster,§ Irene Teresinha Garcia,⊥ Marcos Antonio Villetti,*,§

‡

Department of Chemistry (NUQUIMHE), Federal University of Santa Maria, CEP

97105-900 Santa Maria, RS, Brazil §

Spectroscopy and Polymers Laboratory (LEPOL), Department of Physics, Federal

University of Santa Maria, CEP 97105-900 Santa Maria, RS, Brazil ⊥Institute

of Chemistry, Federal University of Rio Grande do Sul, CEP 91501-970

Porto Alegre, RS, Brazil

ACS Paragon Plus Environment


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ABSTRACT: Alkylimidazolium salts are an important class of ionic liquids (ILs) due to their selfassembly capacity when in solution and due to their potential applications in chemistry and materials science. Thereby, a detailed knowledge of the physicochemical properties of this class of ILs and their mixtures with natural polymers is highly desired. This work describes the interactions between a homologous series of mono- (CnMIMBr) and dicationic imidazolium (Cn(MIM)2Br2) ILs with cellulose ethers in aqueous medium. The effect of the alkyl chain length (n=10, 12, 14 and 16), type, and concentration range of ILs (below and above their cmc) on the binding to methylcellulose (MC) were evaluated. The thermodynamic parameters showed that the interactions are favored by the increase of the ILs hydrocarbon chain length, and that the binding of monocationic ILs to MC is driven by entropy. The monocationic ILs bind more effectively on methoxyl group of MC when compared to dicationic ILs, and this outcome may be rationalized by considering the structural difference between the conventional (CnMIMBr) and the bolaform (Cn(MIM)2Br2) surfactant ILs. The C16MIMBr interacts more strongly with hydroxypropylcellulose when compared to methylcellulose, indicating that the strength of the interaction also depends on the hydrophobicity of the cellulose ethers. Our findings highlight that several parameters should be taken into account when designing new complex formulations.


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INTRODUCTION Mixtures of polymers/surfactants,1–3 and polymers/surfactants/alkali4,5 are a new class of materials used to improve sweep efficiency in the enhanced oil recovery processes by increasing the viscosity of the aqueous phase and reducing the water/oil interfacial tension. The suitable properties of the polymer solutions may be achieved by the addition of a surfactant.6,7 Polymer/surfactant systems are of great interest in colloid science since their mixtures lead to the formation of complex structures that are employed in several industrial products such as cosmetics, pharmaceutical formulations and food additives.8 Furthermore, these structures are also used as model systems for the study of intermolecular interactions and hydrophobic aggregation phenomena.9 The major challenge of recent works in this research field is to understand the nature of the interactions involved in the polymer/surfactant association (hydrophobic, electrostatic, hydrogen bond and Van der Waals); in the organization of surfactant into polymer (monomers or aggregates); in the thermodynamic stability; and in the rheological properties.10,11 In recent years, researches involving surfactant ionic liquids have drawn attention to their self-assembly capacity in aqueous solution.12 The interest in the surfactant ILs also arises from the greater ease of fine-tuning of its surface properties, when compared to the conventional surfactants by varying the alkyl chain lengths, the type of the head-group and the nature and size of the counterions.13 Besides, structural changes affect their polar and dispersive interactions,14 which might modulate the aggregation and the binding dynamic of the ILs on polymer chains. Therefore, the ILs have brought new insights to the research on polymers and surfactants mixed systems.15



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Alkylimidazolium salts are a very important class of ILs that usually consists of imidazolium cations combined with either organic or inorganic anions. The 1alkyl-3-methylimidazolium salts, such as CnMIMBr, where n is the alkyl chain length, are monocationic ionic liquids because they have one cationic head group linked to an alkyl chain, and one counterion that provides them amphiphilicity similar to conventional ionic surfactants.16 On the other hand, the dicationic ILs, such as, the Cn(MIM)2Br2, are composed by two imidazolium rings covalently attached to a spacer group and two counterions, representing a new generation of bolaform amphiphilic molecules.17 The bolaform compounds have demonstrated potential application in materials science, as coatings on solid surfaces where one head group is attached to the surface of electrodes, polymers, or nanoparticles, while the other free head group is used for solubilization in water,18 or to bind into colloids, transition-metal ions, or electrophilic organic molecules.19,20 Besides, due to the presence of two imidazolium head groups on dicationic ILs, these have a superior ability to disperse multi-walled carbon nanotubes when compared to monocationic ILs,21 and are more easily deposited on the surface of TiO2 nanoparticles.14 Moreover, they improve the long-term stability of transition-metal nanoparticles.22 Recently, we have evaluated the effect of the spacer chain length23, type of anions12,24 and temperature25 on the aggregation behavior of dicationic imidazolium ionic liquids in solution. After better understanding the aggregation process for several types of dicationic ILs, we started looking towards more advanced studies seeking a deeper knowledge about the interactions between the cationic ILs and polymers. In this work, methylcellulose (MC) was chosen since it is a cellulose ether generally recognized as safe by the United States Food and Drug Administration (US FDA). Also, it is widely used in several industrial products,26,27 and its


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properties have been previously studied by our group in aqueous solution28 and in presence of cationic surfactants.29 The associations of ionic liquids with cellulose derivatives was not widely studied so far13,30,31 and the thermodynamic aspects of the interaction should therefore be evaluated. Ray et al.13 studied the aggregation of the 1-decyl-3methylimidazolium

chloride

(C10MIMCl)

on

sodium

carboxymethylcellulose

(NaCMC) in aqueous solution. The authors showed that both the critical aggregation concentration (cac) and the polymer saturation concentration increase with increasing temperature and that the micellization of C10MIMCl in presence of NaCMC is an entropy-driven process. Liu et al.31 studied the interactions of the ionic liquids CnMIMBr (n=8, 10, 12, 14 and 16) and N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n=12, 14 and 16) with NaCMC in aqueous solution. The authors showed that the complexes are formed due to the interplay of electrostatic and hydrophobic interactions. In another study, the same authors point out that the addition of NaBr markedly affects the formation of NaCMC /C12MIMBr complexes.30 The aim of this work was to investigate the aggregation phenomena between cationic ionic liquids and methylcellulose in aqueous solution as a function of the alkyl chain length (n= 10, 12, 14 and 16), type (see Scheme 1) and concentration range of ILs, using isothermal titration calorimetry, electrical conductivity, surface tension techniques and 1H-NMR. The knowledge of the thermodynamic aggregation parameters allows us to understand the nature of the intermolecular interactions and the stability of the MC/ILs systems. Thus, the development of mixtures MC/ILs is expected to open up interesting possibilities for the development of novel materials that might be useful in many industrial applications. Insert Scheme 1



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EXPERIMENTAL SECTION Materials. Methylcellulose (MC) and Hydroxypropyl cellulose (HPC) were provided by Sigma-Aldrich (St Louis, MO, USA). The degree of methoxyl substitution (DS) specified by the manufacturer for MC was 1.5-1.9.The reactants 1methylimidazole, 1-bromodecane, 1-bromododecane, 1-bromotetradecane, 1bromohexadecane, 1,10-dibromodecane and 1,12-dibromododecane were also purchased from Sigma–Aldrich (St. Louis, MO, USA). The reactants 1,14dibromotetradecane and 1,16-dibromohexadecane were provided by ChemBo Pharma (Kowloon, HK, CN).The solvents acetonitrile and ethyl ether were purchased from Tedia (Rio de Janeiro, RJ, Brazil). All chemical products were of high-grade purity and were used as received without further purification. Pure water was collected from a Millipore Simplicity apparatus (conductivity of 0.05 µS cm-1) and used to prepare the solutions. Synthesis of ILs. The mono- and dicationic ILs were synthesized in accordance with methodologies described by Shirota et al.32 The scheme and procedure for the synthesis of ILs, and the spectral data are available in the Supporting Information. Preparation of Aqueous Solution of Neutral Polymer with ILs. MC and HPC aqueous solutions were prepared by a standard technique used to dissolve cellulose derivatives.33 A weighed amount of polymer was mixed with approximately 50 % of the total amount of water at 70 ºC. The mixture was left under stirring for 2 h at this temperature. Then, another 50% of water cooled to 5 ºC was added to the system and the mixture was stirred in an ice bath for 12 h to obtain a transparent and homogeneous solution. Polymers with different concentrations were prepared by diluting the stock solution with pure water (milli-Q water). The molecular weight


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(Mw), radius of gyration (Rg) and the second virial coefficient (A2) of the neutral polymers were obtained through static light scattering by using Zimm Plot extrapolation procedure: MC (Mw=3.26 x 105 g mol-1, Rg=67 nm; A2=8.8 x 10-4 mol cm3 g-1) and HPC (Mw=3.34 x 105 g mol-1, Rg=71 nm and A2=7.9 x 10-4mol cm3 g-1). From static light scattering results, it was possible to determine the overlap concentration (C*) of the polymers using the relation C*=1/(A2.Mw), and the values obtained for MC and HPC were 3.5 and 3.8 g L-1, respectively . The interactions between the polymers and the ILs were studied with polymer concentration in the dilute regime (C< C*). Aqueous solutions of the mono- and dicationic ILs were prepared by weighing the amount of IL in a volumetric flask and completed with pure water (MilliQ water). ILs with different concentrations were obtained by successive dilution from the concentrated stock with pure water at room temperature. MC/ILs and HPC/ILs mixtures were prepared by adding polymer solution to mono- and dicationic ILs solution, in order to obtain the desired concentration, and were stirred for 12 h at room temperature before the measurements. The interaction polymer/ILs was examined at different ILs concentrations below and above the critical micellar concentration (cmc). Methods. Isothermal Titration Calorimetry (ITC). Isothermal Titration Calorimetry experiments were performed on a VP-ITC instrument, from MicroCal Inc. (Northampton, MA, USA), at 25 ºC. The calorimetric cell was initially loaded with 1.43 mL of titrant, pure water or polymeric solution (MC or HPC). The concentration of MC solution as titrate was fixed at 3 g L-1, for comparative purpose. In addition, the effect of polymer concentration was studied for some MC/ILs or HPC/ILs systems, where polymer concentrations varied in the range of 0.5 to 3.0 g



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L-1. Titration was performed with injections of 3-10 µL each by an automatic injection syringe containing 270 µL of concentrated solution of IL. For the C10MIMBr, C12MIMBr, C14MIMBr and C16MIMBr the concentration of the solutions used in the titration were 400, 100, 30 and 6 mmol L-1, respectively, which are approximately 10 times higher than the cmc value. The interval between subsequent injections was 300 s, which was sufficiently long for the signal to return to the baseline. The heat involved in the process by each injection during the course of titration was directly measured by a microcalorimeter. Electrical Conductivity. Electrical conductivity was measured by using a Digimed DM-3P conductivity meter (DMC-100M) operating at a cell constant of 10 cm-1. The conductivity meter was calibrated with KCl solution (0.1 mol L-1). The solutions of polymer/ILs were thermostated in the cell at 25 ± 0.1 °C. The polymer concentration was maintained constant at 3 g L-1, which is below the C*. The IL stock solution was progressively added into the polymer solution in order to obtain the desired IL concentration. After each addition of the IL, the mixture was homogenized and the system was left to stand for 10 minutes, in order to stabilize the temperature. The conductivity of the solutions was measured in a large concentration range of IL, which was selected depending on the IL cmc. Surface tension. Surface tension measurements were carried out using a Krüss GmbH K20 Easy Dyne tensiometer (Hamburg, Germany) based on the du Noüy ring method, coupled to a Julabo F12 refrigerated/heating circulator (Seelbach, Germany). The tensiometer was calibrated with ultrapure water (milli-Q water) prior to the measurements. The surface tension of the solutions was measured after a stabilization time of 5 minutes at 25 ± 0.1 °C.


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NMR Measurements. For structural characterization of mono- and dicationic ILs the 1H and 13C NMR spectra were recorded on a Bruker Avance III (1H at 600.13 MHz and

13

C at 150.32 MHz). The samples diluted in D2O were placed in 5 mm

tubes containing a sealed capillary tube with TMS diluted in DMSO-d6 as external reference, at 298.15 K. The NMR peak of DMSO (δ = 2.50) was used as the reference in determining the chemical shifts of 1H in ionic liquids. Chemical shifts were expressed in parts per million. In order to evaluate the self-assembly of ILs (C16MIMBr and C16(MIM)2Br2) and intermolecular interactions between MC/ILs (C16MIMBr C16(MIM)2Br2) the NMR 1H and 13C NMR experiments were performed at different concentrations of ILs in D2O and in the presence of MC (1g L-1).

RESULTS AND DISCUSSION

Isothermal Titration Calorimetric Results. The influence of the alkyl chain length on the aggregation of monocationic imidazolium ionic liquid was investigated in pure water through isothermal titration calorimetry. Microcalorimetric titration curve (open circle) of the concentrated solution of C16MIMBr into pure water is shown in Figure 1. The dilution curve for C16MIMBr has a sigmoidal shape and can be subdivided into two distinct regions (pre micellar and post micellar) with an intermediate region where enthalpy change decreases quickly, and in which the cmc is located. The titration curves for C14MIMBr and C12MIMBr in pure water have a profile that is similar to the former IL (see Figure 2 and 3A). Nevertheless, for C10MIMBr the dilution curve exhibits a distorted sigmoid-shape (see Figure 3B). The enthalpy change (∆Hobs) for C10MIMBr increases with the IL concentration and after the maximum point it decreases gradually. This behavior may be attributed to the



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lower hydrophobicity of the C10MIMBr when compared to ILs with higher alkyl chain length (n= 12, 14, 16), leading to a smaller aggregation number and a lower cooperativity to the formation of micelles.15,34 As it can be seen in Figure 1, the ∆Hobs in the premicellar region, at low C16MIMBr concentration, may be attributed to both the enthalpy of demicellization (∆Hdemic) and the enthalpy of dilution (∆Hdil) of surfactant micelles in water (∆Hobs=∆Hdemic+∆Hdil).35,36 It is noteworthy that the initial dilution of C16MIMBr into water, the first plateau observed at low IL concentration, displays a higher value of enthalpy change (10.5 kJ mol-1) when compared to other ILs (see Figures 2 and 3), and this value is very close to the enthalpy change (9.6 kJ mol-1) for the titration of C16TAB (cetyltrimethylammonium bromide) in pure water, found by Bao et al.37 The second plateau observed at high IL concentration corresponds to the post micellar region, where further addition of concentrated solution of C16MIMBr does not cause more demicellization in water. In this region the C16MIMBr concentration is higher than its cmc and the observed enthalpy change is due only to the dilution of the IL micelles, and its value may be used as a measure of the ∆Hdil. Insert Figures 1, 2 and 3 The cmc values for CnMIMBr (n= 12, 14 and 16) in pure water were determined from the inflection point of curves using a simple sigmoidal equation (see Figures 1, 2 and 3A), as suggested by Kresheck.38 However, for C10MIMBr the dilution curve did not show abrupt change in enthalpy in the transition region and consequently the shape of the curve does not exhibit the pure sigmoidal behavior. For this reason, the cmc value for the C10MIMBr was determined from the point of intersection of the two straight lines identified in the curve (see Figure 3B),


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corresponding to the onset of micelle formation.16,37 As shown in Table 1, the cmc values decrease with an increase in the alkyl chain length of the monocationic imidazolium ionic liquid. The cmc values for all ILs obtained by ITC agree with the values from electrical conductivity (see Table 2) and with the data reported in literature.15,39,40 The standard enthalpy of micellization (∆Hºm), whose magnitude is equal to ∆Hdemic, but with opposite sign, was calculated from the enthalpy difference between the two straight lines extrapolated to the cmc (see Figure 1, 2 and 3).41 The ∆Hºm values displayed in the Table 1 indicate that at room temperature (25ºC), the micellization in water for all monocationic imidazolium ILs is an exothermic process (∆Hºm

Thermodynamic Insights into the Binding of Mono- and Dicationic

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