Molecular-Level Insights into the Microstructure of a Hydrated and

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Molecular Level Insights into the Microstructure of a Hydrated and Nano-Confined Deep Eutectic Solvent Somenath Panda, Kaushik Kundu, Johannes Kiefer, Siva Umapathy, and Ramesh L. Gardas J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01603 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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The Journal of Physical Chemistry

Molecular Level Insights into the Microstructure of a Hydrated and Nanoconfined Deep Eutectic Solvent

Somenath Panda,1† Kaushik Kundu,2† Johannes Kiefer,3 Siva Umapathy,2 and Ramesh L. Gardas1*

1

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India. 2

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.

3

Technische Thermodynamik, University of Bremen, 28359 Bremen, Germany.

†

S. P and K. K have contributed equally to this work.

Corresponding Author Information: Tel.: +91-44-2257-4248 ; Fax: +91-44-2257-4202 URL: http://www.iitm.ac.in/info/fac/gardas E-mail: [email protected]

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Abstract Despite the recent advancements in the field of deep eutectic solvent (DES), their high viscosity often prevents practical applications. A versatile strategy to overcome this problem is either to add co-solvent or to confine the DES inside a nanoscaled self-organized system. The present work assesses the microstructures of a hydrated and nano-confined DES comprising of benzyltripropylammonium chloride, [BTPA]Cl and ethylene glycol (EG). They act as hydrogen-bond acceptor and donor, respectively. The hydrogen-bonding between [BTPA]Cl and EG in the DES (i.e., BTEG), and the molecular states of water in the hydrated BTEG were studied by Raman spectroscopy. The results show different hydrogen-bonding associations between water-water and water-BTEG or EG molecules. In addition, we investigated the confinement effects of BTEG in a polysorbate-80 (Tween-80)/cyclohexane (Cy) reverse micellar (RM) system. The results are compared with an ionic liquid (IL)-encapsulated RM system. The formation, bonding characteristics and thermal stability of the RM droplets were studied by solubilization, dynamic light scattering, rheology, and Raman spectroscopy experiments. Further, it is shown that hydrogen bonding between the DES and the surfactant leads to a stable RM system. Interestingly, the viscosity of the RM system is significantly lower than that of the neat DES suggesting that DESs have a much wider practical applicability in the form of RMs.


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1. Introduction Conventional organic solvents are widely used in industrial chemical processes, for example, to dissolve the reactants, to separate the reaction mixture, and to extract the final products. These solvents are mostly volatile, flammable, and toxic. Furthermore, despite being hazardous chemicals, they are often released to the environment.1 Green alternatives with reduced toxicity and enhanced biodegradability are therefore desirable.2 In the last decades, ionic liquids (ILs) have been identified as potential surrogates of conventional volatile organic solvents as they exhibit beneficial properties that can be tuned to meet the requirements of specific applications.3 However, recent studies pointed out that most of the common ILs based on imidazolium and pyridinium cations have a wide range of toxicities and poor biodegradability as well.4 Therefore, they cannot generally be labeled as ‘green’. On the other hand, in recent years, deep eutectic solvents (DESs) have gained enormous attention as environmentally benign alternatives to ILs. DESs are usually a low-melting blend of a hydrogen-bond acceptor (HBA) such as quaternary ammonium salts (for example, choline chloride, ChCl) and a hydrogen-bond donor (HBD) such as amides or alcohols (for example, urea, glycerol, etc.).5,6 In spite of comparable physicochemical properties, DESs have notable advantages over ILs concerning their rather simple synthesis, low cost, non-toxicity, and high biodegradability.7 Nevertheless, the high viscosity of both ILs and DESs sparks serious concern regarding their practical applicability. The controlled addition of co-solvents and additives is a promising approach to solve this issue. However, detailed knowledge of the impact on the physiochemical characteristics of the solvents is crucial8–11, and this calls for the systematic experimental and theoretical analysis of potentially interesting mixtures. Eventually, the detailed understanding of the phenomena at the molecular level and their relationships with the macroscopic properties can open up for fascinating applications in fields such as gas separation,12 lipase-catalysed esterification,13 controlled folding of DNA nanostructures,14 protein conformation,15 and membrane processes.16 Extensive work on water in room temperature ionic liquids (RTILs) has been published,17–19 whereas similar studies on DESs are rare. Very recently, a few groups provided experimental and theoretical evidence on the hydration behavior of DESs, mostly based on choline chloride.20–27 However, studies of the molecular states of water confined in DESs with other HBA-HBD combinations represent a gap in the literature. Another way to handle the high viscosity of DESs is to confine them at the nanoscale, e.g., in reverse micelle (RM) systems. Typical RMs represent macroscopically homogeneous mixtures Page 3 of 31 ACS Paragon Plus Environment


of a polar solvent and oil in the presence of amphiphilic compounds. They find applications in fields ranging from organic chemistry to biotechnology.28,29 Interestingly, regarding advanced solvents the concept of RMs was already established in the context of ILs to obtain hightemperature stable systems.30–32 Moreover, they served as the carrier for sparingly soluble drugs.33,34 However, to the best of our knowledge, similar investigations concerning RMs with DESs do not exist, although they have been recently employed in a range of self-assembled systems including micelles, vesicles, and liquid crystals.35–43 The successful promotion of DES-based nanostructured RM concepts would certainly open up new opportunities to further exploit their interfacial properties with exciting prospects in drug delivery, organic reaction, etc. So far, no investigations explore the use of DESs as the polar solvent in self-organized RM assemblies. Several questions arise from these considerations: (i) How does the molecular structure of water look like in the presence of a DES compared to hydrated ILs? (ii) Does the HBA-HBD combination affect the state of water in comparison to the HBA-water and HBD-water cases? (iii) How does the nature of the DES influence the microstructure of RMs? (iv) How does the water affect the DES behavior? (v) Are there analogies between DES-based and IL-based RMs? The present work aims at making an important step towards answering these questions to pave the way for the design of nanostructured DES for future applications. For this purpose, a thorough microstructural analysis of the hydrated and nano-confined DES is necessary. Herein, we report the experimental analysis of the DES benzyltripropylammonium chloride/ethylene glycol in combination with water as well as RM system with cyclohexane as a model oil. Rheometry, dynamic light scattering (DLS), and Raman spectroscopy are employed to obtain a full picture of the macroscopic behavior, the microstructures, and molecular interactions, respectively. To the best of our knowledge, this constitutes the first systematic investigation of a nano-confined DES-in-oil RM system having possible applications as an alternative solvent medium for protein stabilization and self-assembled drug delivery systems.


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2. Experimental section 2.1. Materials. Benzyltripropylammonium chloride, [BTPA]Cl (≥ 97.0%, CAS No. 5197-875, Sigma-Aldrich), ethylene glycol, EG (≥ 99.0%, CAS No. 107-21-1, Merck), Polysorbate 80, Tween-80 (CAS No. 9005-65-6, Sigma-Aldrich), and cyclohexane, Cy (anhydrous, 99.5%, CAS No. 110-82-7, Sigma-Aldrich) were purchased and used without further purification. 2.2. Synthesis of DES. A fixed amount of [BTPA]Cl and EG (Figure S1, Supporting Information) was mixed in a closed temperature-controlled, double-walled beaker and stirred using a magnetic stirrer. Subsequently, the temperature of the system was increased steadily up to 70 °C. The mixture was stirred continuously for a minimum of 3 hours at the elevated temperature. A homogenous colorless liquid formed as DES (referred to as BTEG for the molar ratio 1:3 of [BTPA]Cl : EG). The DES was kept under vacuum for another 24 hours in order to reduce the water content. To avoid the contact with contaminants, air, and moisture, the synthesized DES was kept in tightly sealed vials and was further stored over silica gel in a desiccator.44 2.3. Experimental methods. A commercial Renishaw (InVia) Raman microspectrometer was employed to record the vibrational signatures of the samples under investigation in the range from 400 to 4000 cm-1. The instrument used a 785 nm laser, and a 100X long working distance objective focused onto a quartz glass cuvette filled with the sample. The signal was dispersed with a 1200 lines/mm grating and detected with a thermoelectrically cooled charge-coupled device (CCD) detector. The exposure time was 10 sec, and two spectra were accumulated to obtain a high signal-to-noise ratio. The Raman system was calibrated with a silicon reference utilizing the 520.5 cm-1 line. The analysis of the Raman spectra was performed using the Renishaw WiRE 4.1 software. For the RM sample, the solution was dried prior to the Raman spectroscopic measurement in order to avoid the peaks from the solvent. DLS measurements were performed with a Malvern Zetasizer (nano-series) equipped with a thermostatic sample chamber. The instrument operated with a 4 mW He-Ne laser (λ = 632.8 nm). To estimate the hydrodynamic diameter (dh) of the reverse micelle samples the scattered photons were collected at 90° scattering angle. The instrumental software was used to process the scattering intensity data to determine the size distribution of the scatterers in each RM sample. According to the Stokes-Einstein equation, the dh of a nano-droplet is estimated as: 𝑘𝐵𝑇

(1)

𝑑ℎ = 3𝜋𝜂𝐷 Page 5 of 31 ACS Paragon Plus Environment


where η, D, T, and kB are the viscosity, diffusion coefficient, temperature, and the Boltzmann constant, respectively. For each sample, the DLS measurement was carried out in triplicate with at least 10 runs per measurement to ensure reproducible data. Prior to the measurement, the samples were filtered with a 0.22 μm pore size membrane (MILLEX(R)-GP) to remove dust particles. The concentration of surfactant solutions in the RM samples was fixed at 0.1 mol dm-3, if not mentioned otherwise. Anton Paar’s Modular Compact Rheometer (MCR 102) with a CP 40 Cone and Plate configuration (radius 40 mm and angle = 1°) was used to perform the dynamic rheological measurements of the RM samples. A small drop of sample was deposited at the center of the fixed smooth plate of the rheometer. The cone geometry moved downward compressing the sample until the set truncation gap of 0.08 mm was reached. A pre-attached P-PTD 200/AIR Plate-Peltier temperature device was used to precisely control the temperature of the lower plate. Before each measurement, all samples were allowed to thermalize for at least 10 minutes, which was more than enough to reach the experimental temperature given the small sample volume.

3. Results and discussion This section is structured systematically in a way that the molecular complexity of the system is increased stepwise. First, we focus on the behavior of the neat and hydrated DES at the molecular level from the DES and water perspectives. Thereafter, the RM system is analyzed.

3.1. Hydrogen-bonding in the neat DES Before the complex aqueous mixture and RM systems can be understood, we begin with analyzing the neat DES. Raman spectroscopy provides information about the intermolecular interactions and hydrogen bonding associations between the constituents of the DES. In the first step, the Raman spectra of neat benzyltripropylammonium chloride, [BTPA]Cl and ethylene glycol, EG and with the synthesized DES (BTEG) were recorded. Their fingerprint and CH-stretching regions are displayed in Figures 1A and B, respectively. The vibrational signatures of the neat EG at 440-500, 810-920, 982-1145, 1400-1510 and 2800-3000 cm-1 refer to the presence of C-C-O bending, C-C stretching, CH2-CH2-OH stretching along with -C-OH bending, -CH2 twisting, -CH2 bending and (-CH2)sym and (-CH2)asym stretching modes, Page 6 of 31 ACS Paragon Plus Environment

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respectively. On the other hand, the modes of neat [BTPA]Cl at 1560-1630, 975-1025, 10301070, and 600-650 cm-1 are characteristic of the aromatic C=C stretching, the in-plane trigonal ring deformation, -C-N stretching, and antisymmetric skeletal deformation modes, respectively. The typical features of the aromatic -CH stretching are observed above 3000 cm-1.

Fig. 1. Raman spectra of BTEG along with the individual constituents, [BTPA]Cl and EG in the frequency window of (A) 400-1800 and (B) 2600-3200 cm-1 at 298 K.

The formation of BTEG results in distinct red-shifts of the characteristic symmetric and antisymmetric C-N stretching modes shown in Fig. 2B and C, respectively, as well as of the ring-deformation mode, see Fig. 2A. These observations indicate the presence of strong hydrogen bonds between [BTPA]Cl and EG during the formation of DES nanostructures. This agrees well with similar observations that were reported for choline chloride-based DESs.24,27,45 On the other hand, the in-plane ring deformation mode along with the aromatic C=C and C-H stretching frequencies of BTEG shift towards higher frequency in comparison to the neat [BTPA]Cl (see Figures 3A-C). These changes in the characteristic vibrational modes of [BTPA]Cl suggest the disruption of intermolecular π-π stacking interactions during the formation of BTEG. Similar blue-shifted vibrational modes in a DES in comparison to the individual component were observed earlier in the eutectic mixture of malic acid-proline, and lidocaine-ibuprofen.8,16



Fig. 2. Comparative Raman spectra of BTEG along with the individual constituents, [BTPA]Cl and EG in the frequency window of (A) 600-640, (B) 1010-1070, and (B) 1200-1240 cm-1 at 298 K.

Fig. 3. Comparative Raman spectra of BTEG along with the individual constituents, [BTPA]Cl and EG in the frequency window of (A) 985-1020, (B) 1570-1630, and (C) 3035-3090 cm-1 at 298 K.

3.2. Hydrogen-bonding in aqueous solutions To further analyze the nature of the intermolecular associations in the synthesized DES, the Raman spectra of BTEG during the gradual addition of water were studied. For this purpose, D2O was used to avoid an overlap of the broad water signature with the CH stretching modes of interest. In this subsection, we focus on the interactions from the DES perspective. Figure 4A shows the full spectra as an overview. The Raman signatures of the ν(Cring-H) and ν(Calkyl-H) modes along with the aromatic ν(C=C) modes of the BTEG in the solutions are enlarged in Figures 4B-C. Moreover, the ring deformation and ν(C-N) modes in the eutectic mixtures are Page 8 of 31 ACS Paragon Plus Environment

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shown in Figures 5A-C. It is noteworthy that the vibrational bands of BTEG are slightly blueshifted upon the addition of water. The [BTAP]Cl and EG molecules in the eutectic mixture (i.e., BTEG) are associated through a hydrogen-bonding network, which can be represented as (Cring−H)[BTAP]Cl···(O-H)EG and (Calkyl−H)[BTAP]Cl···(O-H)EG. Thus, the intermolecular hydrogen-bonds between the constituents of BTEG become successively weaker with increasing water concentration. This is indicated by the observed blue-shift as a result of the strengthening of the corresponding vibrational bonds.46 The observed phenomena suggest an arrangement of the water molecules near to the aromatic benzene ring of the BTEG. The oxygen atom of water molecules seem to transfer charge of their lone-pair of electrons to the benzene ring. The establishment of such charge-assisted hydrogen-bonding interactions between BTEG and water explains the deviation of the Raman bands from their ideal state.47,48

Fig. 4. Raman spectra of BTEG at different volume fraction of water in the frequency window of (A) 400-3500, (B) 2820-3130, and (C) 1570-1620 cm-1 at 298 K.

Fig. 5. Raman spectra of BTEG at the different volume fraction of water in the frequency window of (A) 610-640, (B) 990-1020, and (C) 1020-1060 cm-1 at 298 K. Page 9 of 31 ACS Paragon Plus Environment


3.3. The water state in hydrated DES In the next step, the aqueous systems are analyzed from the water perspective. The state of water in confined environments has been extensively studied in the past because of the close relationship with many applications in chemistry and biology. Water confined in DES nanostructures was found to play an important role, e.g., in protein folding15 and enzymatic reactions.8 In order to obtain molecular-level information about the confined water in the DES we consider the Raman spectra and compare the results with the frequently studied IL-water systems. It is well-known that the vibrational bands of liquid water in the range of 3000-3800 cm-1 originate from the O-H stretching modes. They are very sensitive to perturbations, e.g., to modifications in the hydrogen bonding network due to the presence of additives and contaminants, but also owing to temperatures. To avoid misinterpretation from the overlapping contributions of the C-H and O-H modes of BTEG and water in the 3100-3800 cm-1 region, we have used D2O instead of H2O.49 This is a very common protocol and red-shifts the O-H stretching band by about 1000 cm-1 into a region where basically no other vibrational signatures occur. Figure 6A illustrates the evolution of the O-D Raman band in the 2100-2700 cm-1 region with increasing water concentration. The overall broad band can be interpreted in terms of multiple sub-peaks, which originate from water or D2O molecules in different bonding states. These states range from weakly or non-hydrogen-bonded water to fully tetrahedrally bonded water: The stronger the hydrogen bonding the smaller the vibrational frequency. Hence, the shape of the band or the de-convolved sub-peaks provide comprehensive information about the hydrogen-bonding network, e.g. about the populations of the individual sub-groups of water molecules and the disruption or formation of any hydrogen-bonding associations. The spectra in Figure 6A suggest significant deviations between the confined water in BTEG and bulk water.50 In the presence of small amounts of BTEG (BTEG:D2O = 1:9, 2:8 and 3:7), an increase of the relative intensity of the higher frequency wing of the band suggests an increase of the weakly hydrogen bonded population of water molecules. The fraction of tetrahedrally arranged water molecules decreases upon further addition of BTEG. This is indicated by the apparent reduction in intensity at the low-frequency wing of the band.


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Fig. 6. Raman spectra of BTEG at different volume fraction of water in the O-D frequency region of (A) 2100-2700 cm-1 at 298 K. Gaussian de-convoluted bands showing various states of water (green, magenta, and blue lines indicate network, intermediate and multimer water, respectively) at a particular ratio of DES and water, such as (B) 1:9, (C) 4:6, and (D) 8:2.

In order to obtain a more quantitative picture, we de-convolved the O-D Raman bands across the whole range of studied compositions. The O-D bands of interest are decomposed into the three Gaussian components in agreement with the previous literature.51–56 The results of this decomposition are shown in Figures 6B-D. Three different water populations can be identified as network water (NW), multimer water (MW) and intermediate water (IW). They manifest as sub-peaks at 2280-2380, 2550-2700, and 2400-2540 cm-1, respectively.57–59 The NW molecules are organized in a tetrahedrally ordered hydrogen-bonding network to form an ice-like structure, whereas the non-H-bonded or weakly H-bonded water molecules are designated as MW. The IW molecules are involved in a distorted hydrogen-bonding network with neighboring molecules other than water. The three water states are found in all the BTEG-water mixtures, but their fractions vary significantly as can be seen from the relative intensities of the sub-peaks. The molecular fraction of one population is calculated as the ratio between the area of the Gaussian peak associated with the water population and the total area of the three



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Gaussians. The resulting fractions are illustrated in Figure 7A. In addition, the binary mixtures of EG and water were studied to obtain further insights.

Fig. 7. Various molecular states of water extracted from the de-convoluted plots as a function of water content in (A) BTEG, and (B) EG. The Raman spectra for the aqueous EG solutions are provided in the Supporting Information (Figure S2). The analysis of the individual vibrational signatures of EG with the gradual addition of water can be discussed with the help of Figures S3 and S4. The O-D bands of water are found to be blue-shifted in the presence of EG (Figure S2B), similar to the behavior of the BTEG-water systems. In analogy to the above analysis, the O-D Raman bands for the EGwater systems were decomposed, see Figure S5, and the derived population fractions are shown in Figure 7B. The comparison between the BTEG-water and EG-water mixtures reveals that the contributions of NW and MW molecules increase with the addition of water for both systems. On the other hand, the IW fraction decreases more monotonously in BTEG-water system, while a plateau between 4:6 and 6:4 is observed in EG-water. A similar behavior was observed earlier for aqueous mixtures with a number of protic ILs including 1ethylimidazolium

bis(trifluoromethane-sulfonyl)imide,

[C2HIm][TFSI],

and

1-

ethylimidazolium trifluoromethanesulfonate, [C2HIm][TfO].19 Interestingly, the relative contribution of IW molecules for hydrated BTEG was found to be comparable with hydrated [C2HIm][TfO], but higher than hydrated [C2HIm][TFSI] upon an increase in water content. This indicates that water coordinates in a different manner with the ILs and DES.


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Fig. 8. Comparative Raman spectra of hydrated BTEG and EG with bulk water in the O-D frequency region at a different volume fraction of BTEG or EG and water, such as (A) 2:8 and (B) 8:2. The differences between BTEG and EG are further highlighted in Figures 8A and B. Specifically, the effects in the water-rich region (BTEG or EG:D2O = 2:8) and the DES or EGrich region (BTEG or EG:D2O = 8:2) are compared. In the water-rich region, a slight blue-shift of the O-D Raman band with respect to bulk D2O is found for both systems. This indicates a perturbation of the hydrogen-bonding environment in the bulk water. In the BTEG-water system, this blue-shift is more pronounced and comes along an enlarged intensity of the highfrequency side of the band. As aforesaid, this indicates the increased presence of non- or weakly-hydrogen bonded water molecules in BTEG. The presence of non-polar moieties such as the benzene ring and the propyl groups of [BTPA]Cl in BTEG can explain this observation. The data also suggest that the hydrogen-bonding between water and the hydroxyl groups of EG is weaker than the interactions in bulk water. It is also worth mentioning that the O-D bands for the aqueous DES systems are slightly blue-shifted compared to aqueous EG systems at lower water content. This further supports our hypothesis that the average hydrogen bond strength among the DES molecules is weaker than that of EG molecules.

3.4. Nano-confinement of DES inside the reverse micellar interface



The previous sections aimed at developing a detailed understanding of the behavior of the DES in its neat form and in aqueous mixtures. Both systems could be studied under highly defined conditions. Micro-structured systems such as micelles and vesicles are more difficult to control. Nevertheless, such self-assembled materials are of great practical interest, e.g., as alternative templates for the synthesis of nanostructured materials and drug delivery systems. On the other hand, ILs have been extensively used in another class of organized self-assembled systems, i.e., non-aqueous RMs, which have been developed as potential drug delivery carrier for poorly soluble drugs molecules.33,60 DESs as an alternative of ILs have not been utilized in the formulation of non-aqueous RMs despite exhibiting several potential advantages. In this section, we aim at testing the ability of BTEG to formulate non-aqueous RMs. Moreover, the question of how the nature of the entrapped BTEG impacts the interactions within the RM interface and potentially modifies the interfacial droplet structure in comparison to the well-known IL-based RMs. Cy and Tween-80 (vide Figure 9A) were chosen as the oil phase and surfactant, respectively. The solubilization efficiency of BTEG in the mixture of the oil and the surfactant was determined to confirm the formation of self-assembled DES-in-oil RMs. In this context, the maximum amount of BTEG required for the formation of clear and stable ternary RMs in terms of the molar ratio R of BTEG to surfactant was evaluated and compared to the well-known AOT (dioctyl sodium sulfosuccinate)/Cy-based systems.

Fig. 9. (A) Chemical structure of Tween-80, and (B) the solubilization capacity of BTEG in different reverse micellar systems (RMs) at 298 K. Page 14 of 31 ACS Paragon Plus Environment

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Figure 9B summarizes the maximum amount of BTEG solubilized in various RMs at a particular surfactant concentration of 0.1 mol dm-3. The measured Rmax values for both surfactant-based systems vary significantly. The Tween-80/Cy RMs are capable of dissolving a higher amount of BTEG compared to the AOT/Cy RMs. This result suggests that the spontaneous curvature along with the elasticity (or, rigidity) of the interfacial film and the interdroplet interactions play a significant role in the solubilization process.61–63 The lower solubilization capacity of the AOT/Cy system compared to Tween-80/Cy is characteristic of a stronger interaction between the RM droplets. This is due to the ionic nature of the polar head group in AOT, which results in an inflation of the droplet coagulation followed by phase separation.62,63 To increase the BTEG solubility in AOT-based RMs, we have further investigated their solubilization efficiency in two different non-polar solvents, benzene (Bz) and n-hexane (Hx). It is observed that BTEG is soluble in all formulated AOT-based RMs. However, the solubilization efficiency follows the order AOT/Hx < AOT/Bz < AOT/Cy. Assuming that Cy penetrates more to the surfactant palisade layer compared to Bz and Hx, a rigid interface and higher solubilization capacity can be anticipated.64 The total surfactant concentration also has a small effect on the solubilization efficiency of BTEG (R = 1.45-1.50) in the Tween-80/Cy mixture, which is a strong indication for the formation of the microscopic structures between DES and Tween-80 in the organic solvent, Cy. In order to further assess the role of BTEG on the solubilization behavior of RMs, we have tested another synthesized DES (which is composed of [BTPA]Cl and phenol, i.e., BTPH).44 The maximum solubilization capacity of BTPH (Rmax) was found to be approximately 0.5 at a surfactant concentration of 0.2 mol dm-3. The viscosity value of BTPH (381.5 mPa·s) was found to be higher than that of BTEG (229.2 mPa·s) at 298 K.44 This implies a greater extent of intermolecular interactions in neat BTPH compared to BTEG. The greater hydrogen bonding associations between the constituents of BTPH compared to the constituents of BTEG result in a rigid DES/oil interfacial layer for Tween-80/Cy/BTEG RMs due to the stronger interaction between Tween-80 and individual constituents of BTEG. Subsequently, a higher solubilization efficiency of BTEG was evidenced in Tween-80/Cy RMs compared to BTPH. Similar behavior was observed earlier for Tween-80/Cy RMs, when the two ILs choline hexanoate, [Chl][Hex], and N-methyl2-pyrrolidonium hexanoate, [NMP][Hex] were entrapped in RMs.65 Dynamic light scattering is a powerful technique to test whether RM system or a bi-continuous structureless microemulsion is formed. In the present work, DLS has been applied to DES-inPage 15 of 31 ACS Paragon Plus Environment


oil RMs for the first time to find out whether the DES is effectively entrapped or just dispersed in the surfactant/organic solvent mixture without forming any self-organized microstructure. It has long been accepted that an increase in the droplet size in a surfactant/organic solvent mixture with the gradual addition of the polar solvent implies the formation of RMs. This is recognized as the swelling law of RMs.66 The obtained size distributions of the RM samples measured at two different temperatures of 298 and 313 K are illustrated in Figures 10A and B. An increase in the droplet size is evidenced with the increase in BTEG content at both temperatures. This points to a successful entrapment of BTEG into the surfactant layer and the formation of RM system. The swelling behavior of these systems is found to be directly proportional to the added volume of BTEG. This is in agreement with earlier studies of traditional RMs.67 The entrapped BTEG molecules can construct hydrogen-bonded associations with the Tween-80 inside the RMs, which results in an increase in the cross-section area of the polar head group of Tween-80 and subsequently leads to larger RM droplet sizes. Similar features were observed for EG/sodium bis-(2-ethylhexyl) phosphate (NaDEHP)/nheptane RMs.68 Furthermore, a broader size distribution was noticed for the RMs with the higher amount of BTEG from Figures 10A-B similar to the reported IL-in-oil RMs.69,70

Fig. 10. Size distribution plots of Tween-80/Cy reverse micellar systems at different molar ratios of BTEG to Tween-80 (R) and at a particular temperature of (A) 298, and (B) 313 K.

Unfortunately, the droplet sizes for the Tween-80/Cy RMs were not reported earlier using DES within a similar range of R. However, we can compare our results with the previously reported IL-based systems, such as [NMP][Hex]/Tween-80/Cy RMs, where the droplet sizes were found Page 16 of 31 ACS Paragon Plus Environment

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to be ~70 and 108 nm at similar values of R = 0.3 and 0.7, respectively.65 In order to explain the smaller droplet size of DES-based RMs in comparison to the IL-based system, we have to consider the effective packing parameter of the surfactant, p (= v/alc), where a is the surfactant head group area, and lc and v are the length and volume of the hydrocarbon chain, respectively.71 Usually, the droplet size increases when the p values of the surfactant become small. Factors that contribute to the decrease in v or to the increase in a will result in a decrease of the p value. In cationic and anionic RMs, the droplet size can be controlled by the interaction of the entrapped polar solvent and the respective surfactant.66 A strong interaction between the entrapped polar solvent and the polar head group of the surfactant yields a larger droplet size, as a increases and p decreases. The small droplet sizes obtained for BTEG entrapped in Tween80/Cy RMs compared with the values reported for [NMP][Hex] entrapped in the similar system can be explained by considering a weaker BTEG-Tween-80 interaction present at the RM interface than the [NMP][Hex]-Tween-80 interaction. Thus, an alteration in the type and extent of the interactions with the entrapped polar solvent (i.e., electrostatic for IL, whereas Hbonding for DES) promotes remarkable changes in the RM interface.

To design a new DES-based RM system, it is essential to know the characteristic nature of the fluid. This can be achieved by rheological measurements, which provide information regarding the inter-droplet interactions inside the RM system.65 Generally, RM systems are recognized as Newtonian and low viscous liquids. To understand the influence of the DES on the flowcharacteristics of the RM system, the shear rate dependent viscosity was measured as a function of BTEG content, R (= 0.3, 0.7 and 1.3). The results are shown in Figure 11A and in the Supporting Information (Figure S6). Only a small variation of the shear viscosity (η) with the gradual increase in shear rate (γ) at different temperatures is observed. This is very close to the idealized Newtonian fluid behavior, where the η values are found to be fully independent of γ.72 In the further analysis, the apparent viscosity (ηa) of the RM systems was calculated from the Herschel-Bulkely equation,73,74 𝑑𝛾 𝑛

(2)

𝜏 = 𝜏𝑎 + 𝜂𝑎( 𝑑𝑡 )

where, dγ/dt, n, τ, and τa are the shear rate, H-B index, shear stress, and apparent yield stress, respectively. For the Newtonian flow behavior, the n value can be considered as equal to 1.75 The representative plots of the shear stress (τ) versus shear rate (γ) for other systems have been provided in Figure 11B and Figure S6. They show a linear relationship between the τ and γ Page 17 of 31 ACS Paragon Plus Environment


(with R2 values ≥ 0.99). The ηa values were obtained from the slopes of the linear plots. For BTEG-based RMs with R = 0.3, 0.7, and 1.3 they were found to be 1.18, 1.79, and 3.51 mPa.s, respectively. Thus, a large decrease in viscosity is observed for nano-confined DES compared to the bulk DES (280 mPa.s).44 With an increase in BTEG content (R), the viscosity for RM systems was found to be increased. This can be further correlated with the larger droplet size and/or inter-droplet interactions at higher R values.76 The comparison of the ηa values of the DES-based and IL-based RMs (both with Tween-80 and Cy) shows distinct differences. The ηa values of the BTEG-based RMs are found to be lower than those of the IL ([NMP][Hex])based RMs (ηa = 4.29 and 9.50 mPa.s at R = 0.3 and 0.7, respectively).65 Stronger dropletdroplet interactions in the presence of [NMP][Hex] can explain this observation. The ions in ILs are generally held by electrostatic interactions whereas the constituents in BTEG are dominated by hydrogen-bonding associations. Therefore, a stronger interaction between [NMP][Hex] and Tween-80 is expected due to their more polar nature in comparison to the BTEG-Tween-80.77 The viscosity of DESs depends on the constituent salt and the HBD along with the propensity towards their associations via hydrogen bonds.

Fig. 11. Variation of (A) viscosity (η) and (B) shear stress (τ) versus steady shear rate (γ) plots for BTEG/Tween-80/Cy RMs at different molar ratios of BTEG to surfactant (R) as a function of temperature.

The determination of the experimental viscosity of DES-based RMs is generally useful because of their applicability in various fields. However, no appropriate models to envisage the Page 18 of 31 ACS Paragon Plus Environment

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viscosity of these complex systems based on DESs exist to date. Low viscous liquids are usually more desirable for their easy handling as a solvent for biomolecular extraction or as a catalyst. The development of suitable models requires experimental data obtained under systematic variations of parameters. As an important step in this direction, we investigate the temperature-dependent characteristics of the droplet size and flow behavior. The thermal stability of formulated RMs is a key parameter for their practical use and it is highly desirable to optimize it, e.g. for performing chemical reactions and nanomaterial synthesis at elevated temperature using these systems as templates.32 Therefore, it would be fascinating and of paramount importance to explore the thermal stability of BTEG-entrapped Tween-80-based RM systems. The temperature-dependent variation in size and the polydispersity index (PDI) of BTEG-in-Cy RMs at different R values along with their corresponding size distribution plots are shown in the Supporting Information (Figures S7A-B and Figure S8). The figures reveal that the droplet sizes of BTEG-based RMs decrease with increasing temperature. Moreover, the droplet size distribution is nearly monodisperse as evident from the low PDI values. The mean droplet size decreases from 65 to 53 nm for BTEG-based Tween-80/Cy RMs at a constant R of 0.70 when the temperature increases from 298 K to 313 K. In the [NMP][Hex]-based RM system the size decreases from 135 to 75 nm.65 This observation further implies that the DESbased RMs retain their structural integrity more efficiently compared to IL-based RMs across the studied temperature range. We have also performed rheological measurements of BTEG/Tween-80/Cy RMs in the same temperature range (298-313 K) that was employed in the DLS measurements (Figure S6). The apparent viscosity (ηa) of these systems was again calculated from the τ vs. γ plots. Figure 12A illustrates the results a function of temperature for different R values. The viscosity is found to decrease with increasing temperature due to the weakening of the interactions between the constituents inside the RM droplets. From the temperature-dependent viscosity values, the activation energy (Ea) can be calculated by using the following Arrhenius-type equation;65 𝐸𝑎

η = A𝑒

𝑅𝑇

(3)

where A and R are the pre-exponential factor and universal gas constant, respectively. For the RM systems, the Ea can be represented as the required energy for the movement of individual droplets in a macromolecular crowded environment of RMs, which is an indirect measure of interactions between the discrete droplets.78



Fig. 12. (A) Apparent viscosity (ηa) of BTEG/Tween-80/Cy RMs at different temperatures, and (B) Plot of viscosity as a function of temperature for similar systems at the different molar ratio of IL to Tween-80 (R). The plot of the logarithmic value of η against 1/T (see Figure 12B) was used to determine the Ea for these RMs at different R values, which were found to be 30.59, 29.18 and 28.16 kJ/mol at a particular R of 0.30. 0.70, and 1.30, respectively. On the other hand, the Ea value for the neat BTEG was reported to be 44.9 kJ/mol.44 The higher value of Ea for the neat BTEG implies the presence of a greater extent of intermolecular interactions compared to the nano-confined DES. Similarly, the Ea value for neat IL obtained from the solvation and rotational relaxation time was found to be lower compared to the IL-in-Bz RMs .79,80 To gain further insights into the DES-based RMs, we studied these systems using Raman spectroscopy. The aim was to investigate the intermolecular interactions between the solute and solvent along with the structure and dynamics of the molecules. The Raman technique is perfectly suited for this purpose as it provides a non-invasive “molecular fingerprint” of the RM system. This may enable understanding the molecular interaction mechanisms between the individual constituents (i.e., BTEG and Tween-80) in the formulated RMs. Figures 13A-B show the Raman spectra of ‘dry’ Tween-80/Cy RMs (i.e., R = 0) along with the DES entrapped RMs at different R values (= 0.3, 0.7 and 1.3) in the region of 500-1800 and 2600-3200 cm-1, respectively. The vibrational signatures of BTEG are marked by an asterisk (*) in Figure 13A.


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Fig. 13. Raman spectra of Tween-80/Cy reverse micellar systems at the different molar ratio of IL to Tween-80 (R) in the frequency window of (A) 500-1800, and (B) 2600-3200 cm-1 at 298 K. The asterisk in panel (A) indicates the vibrational modes of BTEG.

Figures 14A-C show the Raman spectra of the asymmetric -C=O stretching mode (C=O, 17101760 cm-1), the overlapping modes of C-OH torsion (HO-C-C), the out-of-plane bending of OC-O (γOCO), the -CH2 torsion (HC) (780-910 cm-1), and the C-H stretching (CH) vibrations (2800-3080 cm-1) of the Tween-80/Cy/DES RMs with varying BTEG content, respectively. A red-shift in the C=O mode of Tween-80 is noticed upon the gradual addition of BTEG to the Tween-80/Cy RMs. The incorporated BTEG inside the RMs can interact with the carbonyl moiety of the Tween-80 through hydrogen bonding, which in turn weakens the -C=O bond. Consequently, the -C=O stretching band shifts towards a lower wavenumber. A similar observation was made in EG/NaDEHP/n-heptane RMs.68 On the other hand, a blue-shifted hydrogen bond is evidenced for the CH, HO-C-C, γOCO, and HC modes of Tween-80 when the BTEG content increases.81 The formation of organized Tween-80/Cy RMs can be hypothesized as a result of the strong hydrogen-bonded associations among the Tween-80 molecules through the terminal -OH moiety. In addition, a weak hydrogen bond between the -C-H moieties of one Tween-80 and the oxygen atom of POE units of other Tween-80 molecules would enhance this effect. When BTEG is entrapped inside the RMs, it can disrupt these hydrogen bond associations and thereby shift the C-H stretching bands of Tween-80 further towards a higher frequency. This interaction might be the driving force for solubilizing BTEG into the core of the Tween-80 aggregates. Similar behavior was observed for 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-p-xylene RMs.82 Page 21 of 31 ACS Paragon Plus Environment


Fig. 14. Raman spectra of Tween-80/Cy reverse micellar systems at the different molar ratio of IL to Tween-80 (R) in the frequency window of (A) 1710-1760, (B) 780-930, and (C) 27503060 cm-1 at 298 K. The asterisk in panel (C) indicates the vibrational modes of BTEG.

Finally, the vibrational modes of the entrapped DES were compared with those of the bulk DES. Figures 15A-C show the characteristic spectral signatures as a function of R. The signatures correspond to the ring deformation, the -C-N symmetrical, and -NC4 asymmetrical stretching along with the -CH3 asymmetrical deformation, and the -CH2 twisting modes, respectively. They predominantly arise from the [BTPA]Cl of the entrapped DES. The results were further compared with the bulk DES and the [BTPA]Cl salt. The diagrams of Figure 15 show that all the stretching vibrations of BTEG are slightly blue-shifted with respect to the bulk state when entrapped in RMs. The classical formation of an A-X···H hydrogen bond can be considered as the lengthening of the A-X bond and the subsequent shift of the vibrational A-X bond frequency towards a lower wavenumber and vice versa.46 Our result indicates a disruption of hydrogen-bonding interactions between [BTPA]Cl and EG molecules when BTEG is incorporated in RMs due to the association with the Tween-80 moiety. All the vibrational modes of [BTPA]Cl are also found to be significantly red-shifted in DES in comparison to their salt form, which clearly indicates the strong intermolecular association between [BTPA]Cl and EG during the formation of the DES.83 Further, the microstructures of BTEG inside the RMs can be examined by probing the -C-H stretching vibration of the entrapped BTEG, which is also highly sensitive to the formation of weak hydrogen bonds (i.e., -C−H···O). Figure 15D shows the red-shifted (-C-H)aromatic stretching vibration of the BTEG entrapped in RMs in comparison with its bulk signature. The red-shift evidences the BTEG’s ability to form hydrogen bonds with Tween-80 in the formulated RMs. Page 22 of 31 ACS Paragon Plus Environment

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Fig. 15. Raman spectra of Tween-80/Cy reverse micellar systems at the different molar ratio of IL to Tween-80 in the frequency window of (A) 610-630, (B) 1010-1050, (C) 1200-1225, and (C) 3040-3110 cm-1 at 298 K.

4. Summary In summary, we have investigated the intermolecular associations of a deep eutectic solvent (BTEG) based on [BTPA]Cl and EG using Raman spectroscopy. The red and blue-shifted vibrational signatures of [BTPA]Cl compared to the eutectic mixture of [BTPA]Cl-EG indicate strong hydrogen-bonding interactions between the individual components along with the disruption of intermolecular interactions between like species ([BTPA]Cl or EG). The effect of water on the cooperative hydrogen-bonding network in BTEG was studied within a wide range of water content. Since the mixture of EG and [BTPA]Cl forms BTEG through intermolecular hydrogen bonding, the nano-structured behavior of aqueous EG was also investigated and compared with the DES. The spectroscopic signatures reveal that the interionic

hydrogen-bonds

between

the

constituents

of

BTEG

[apparent

as

(Cring−H)[BTAP]Cl···(O-H)EG and (Calkyl−H)[BTAP]Cl···(O-H)EG] become successively weaker with increasing water concentration. For the first time, we examined Raman spectra of archetypal Page 23 of 31 ACS Paragon Plus Environment


BTEG-water blends to probe the molecular state of water in these systems. Various types of water molecules were found to be located inside the DES nano-structure in the form of strongly-bonded and weakly-bonded water molecules. The balance of strong and weak associations changes with the degree of hydration. However, a difference in hydrogen-bonding between BTEG-water and EG-water was evidenced along with the states of water from the O-D Raman bands. A non-ionic surfactant (Tween-80) and cyclohexane as a model oil were employed to study the formation of DES-based RMs. Their microstructural characteristics were analysed and compared to IL-based RMs. To the best of our knowledge, the DES-in-oil RMs are reported for the first time in the open literature, where BTEG was utilized as the polar component instead of water or IL or any conventional polar solvents. Both DLS and rheological measurements support the formation of RMs with swelling and Newtonian-flow characteristics, where BTEG has been solubilized effectively in the reverse micellar corona. However, altered interfacial characteristics have been observed from the IL-based RMs in terms of solubilization behavior, droplet size, and flow characteristics. Eventually, the Raman spectra provided information about the detailed interaction mechanism between the entrapped BTEG and Tween-80. This manifested as blue- and red-shifted vibrational signatures of Tween-80 and BTEG. The results offered new molecular-level insights into the nano-domain structures of the DES, which may help in interpreting the dynamics of DES in various confined environments and/or complex systems. A comprehensive understanding of such systems will be the basis for developing a detailed model to predict their behavior and open up new possibilities for practical applications.

Acknowledgments SP and RLG are thankful to IIT Madras for financial support through the Institute Research and Development Award (IRDA): CHY/15-16/833/RFIR/RAME. KK acknowledges UGC, Govt. of India for the financial support in the form of Dr. D. S. Kothari Post-Doctoral Fellowship.


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For Table of Contents Use Only Molecular Level Insights into the Microstructure of a Hydrated and Nanoconfined Deep Eutectic Solvent Somenath Panda,1† Kaushik Kundu,2† Johannes Kiefer,3 Siva Umapathy,2 and Ramesh L. Gardas1* 1

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India. 2

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.

3

Technische Thermodynamik, University of Bremen, 28359 Bremen, Germany. †

S. P and K. K have contributed equally to this work.

Raman spectroscopy, dynamic light scattering, and rheometry are employed to understand the molecular interactions, microstructures, and macroscopic behavior of the hydrated and nano-confined deep eutectic solvent (benzyltripropylammonium chloride/ethylene glycol).


Molecular-Level Insights into the Microstructure of a Hydrated and

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