Interactions of alkyltriphenyl phosphonium based ionic liquids with block copolymer microstructures: A multitechnique study

Interactions of alkyltriphenyl phosphonium based ionic liquids with block copolymer microstructures: A multitechnique study

Journal Pre-proof Interactions of alkyltriphenyl phosphonium based ionic liquids with block copolymer microstructures: A multitechnique study Bijal V...

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Journal Pre-proof Interactions of alkyltriphenyl phosphonium based ionic liquids with block copolymer microstructures: A multitechnique study

Bijal Vyas, Sadafara A. Pillai, Debes Ray, Vinod K. Aswal, MuRong Wang, Li-Jen Chen, Pratap Bahadur PII:

S0167-7322(19)33072-7

DOI:

https://doi.org/10.1016/j.molliq.2019.112341

Reference:

MOLLIQ 112341

To appear in:

Journal of Molecular Liquids

Received date:

2 June 2019

Revised date:

2 December 2019

Accepted date:

16 December 2019

Please cite this article as: B. Vyas, S.A. Pillai, D. Ray, et al., Interactions of alkyltriphenyl phosphonium based ionic liquids with block copolymer microstructures: A multitechnique study, Journal of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.112341

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© 2019 Published by Elsevier.

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Interactions of Alkyltriphenyl Phosphonium Based Ionic Liquids with Block Copolymer Microstructures: A Multitechnique Study Bijal Vyas1, Sadafara A. Pillai2*, Debes Ray3, Vinod K. Aswal3, Mu-Rong Wang4, Li-Jen Chen4 and Pratap Bahadur1, 1 2

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India School of Sciences, P.P. Savani University, Surat 394125, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

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Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

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*Corresponding Author: Dr. Sadafara A. Pillai

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Email: [email protected]; [email protected]

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Tel: +919913337995

Bijal Vyas: [email protected] Debes Ray: [email protected] V. K. Aswal: [email protected] Mu-Rong Wang: [email protected] Li-Jen Chen: [email protected] Pratap Bahadur: [email protected]

Declaration of Interests: None

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Abstract: Here we report comprehensive analysis on the influence of different alkyltriphenyl phosphonium (CnTPPBr) (with varying alkyl chain lengths ) based cationic surfactants displaying analogical characteristics to ionic liquids (IL) on the physicochemical properties of an ethylene oxide–propylene oxide (EO–PO) triblock copolymer, Pluronic®, P103 (EO17-PO60-EO17 ; M. Wt-3250) using several techniques viz. cloud point (CP), viscosity, small angle neutron scattering (SANS), dynamic light scattering (DLS) and high sensitivity differential scanning calorimetry (HSDSC). The CP results indicate notable increase in values with the increase in

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alkyl chain length of IL. A significant decrease in the micelle size was witnessed from the DLS

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and SANS results. The presence of sodium bromide (NaBr), however, counteracted the disruptive effect of ILs and promoted micellar growth. The critical micelle temperature (CMT)

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results attained using HSDSC are also reported. The information acquired from the present study will be highly beneficial in understanding the behavior of Pluronic ® micelles in the presence of

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CnTPPBr ILs and for their effective utilization in various industrial applications.

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Keywords: Pluronics; micellization; CMT; IL; SANS; HSDSC

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1. Introduction Pluronics® are commercially available poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) block copolymers and have been marketed for more than five decades. Both the constituting blocks being chemically different, Pluronics® display amphiphilic properties. The performance of Pluronics® in aqueous environment largely depends on the length of PEO blocks. Hence, some are hydrophilic (PEO ≥70%), some are moderately hydrophobic (PEO ≤70% and >40%) while some are very hydrophobic (PEO ≤ 20%) in nature. The amphiphilic properties of Pluronics® have made them useful in diversified fields such as nanoparticle synthesis [1, 2],

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Drug delivery [3-7], cosmetics [8], dispersants for inks and pigments [9] and in perovskite solar

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cells, dye sensitized solar cells and in energy storage devices[10-14] to name a few. ILs being green solvents has been of interest in the past few decades. The steric hindrance

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arising due to the larger dimensions of the constituting ions allows them to exist as liquid at temperatures below 100 ◦C. These are considered as green solvents due to their thermal stability,

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high conductance, easy recyclability and non-inflammability [15-17]. These are known to

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modify solvent properties of water. In aqueous environment, ILs with shorter alkyl chain lengths remain non surface active while the longer alkyl chain endows them with amphiphilic character

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and imparts surface activity. ILs bearing long alkyl chains have combined properties of ILs and surfactants and are hence named as surface active ionic liquids (SAILs). These have been widely

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investigated for their adsorption characteristics and micelle formation in water [18-21]. Phosphonium salts falling under the category of low melting compounds, several studies

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classifying them as ILs have been reported [22-24]. Among all those, our attention was drawn by amphiphilic 1-alkyltriphenyl phosphonium bromides (CnTPPBr ) due to their classic features such as improved solubilizing capacity coupled with a low critical micelle concentration (CMC) as compared with the known ammonium analogues [25, 26]. As a classic surfactant, organization of CnTPPBr in water has been well investigated as an individual [25-27], in binary [28, 29] and in ternary mixtures [30] by several different methods. However, a very few of them have reported them as IL [31]. Individually, surface activity and the association behavior of Pluronics® [32-36] and ILs [19, 37-42] are well documented in the literature. But studies defining the effect of ILs on micellization of Pluronics® are very few, which may open new possibilities in designing polymeric materials [43-51]. Liu et al.[50] examined the effect of different 1-alkyl-33

Journal Pre-proof methylimidazolium bromides on Pluronics® L64 and F68 micelles and reported that the organization of the formed aggregates strongly depended on the hydrophobicity of IL and copolymer/IL concentration. Zheng et al.[43] scrutinized the effect of 1-butyl-3-methylimidazolium bromide on P104 micelles using several techniques and suggested that the added IL promotes micellization at lower concentrations while at higher concentrations disrupts micelles and enhances the critical micelle concentration (CMC). Parmar et al. [49] examined the effect of 1-alkyl 3-methyl imidazlolium tetraflouroborates on P103 micelles and concluded that the presence of ILs hindered micellization. Guo and co-workers[43] witnessed lower CMTs in the

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presence of 1-butyl-3-methyl-imidazolium bromide for Pluronic® P104. Reddy and Venkatesu [52] compared the effect of ILs incorporated with different counterions on the CMT of a

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Pluronic® block copolymer and noted the weak ion pair interactions within the ILs and the size

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and charge of anions to be responsible for reducing the CMT of the copolymer. Vekariya et al. in two independent studies on Pluronics® F127[47] and P123[53] checked the effect of alkyl chain

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length, head group, anions and concentration of various pyridinium based ILs and found

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substantial drop in micellar size and aggregation number. Khan et al.[54] witnessed significant drop in CMT of a triblock copolymer in the presence of different cholinium based ILs. Pillai et al. [55] scrutinized the influence of different 1-alkyl-3-methyl imidazolium based ILs on the

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micelles of a moderately hydrophobic star block copolymer and reported a decrease in the CMT at lower concentrations of ILs. Conversely, CMT increased at higher concentrations. Recently,

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He et al.[56] checked the influence of ethyl ammonium nitrate (EAN) and 1-butyl-3-

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methylimidazolium tetrafluoroborate (BmimBF4) on micellization of Pluronic® P123 and noted that EAN promotes micellization while BmimBF4 demotes it. From the above survey it appears that presence of imidazolium and pyridinium based ILs significantly affect the copolymeric micelles. However, there are no reports defining the role of alkyl triphenylphosphonium (CnTPPBr) based ILs on Pluronics®. Taking inspiration from this, in the present study we have tried to elucidate the influence of different alkyl (C 10-C16) triphenylphosphonium based ILs on the micelles of P103 using small angle neutron scattering (SANS), dynamic light scattering (DLS), high sensitivity differential scanning calorimetry (HSDSC) and cloud point (CP). The effect of alkyl chain length in the presence/absence of salt on the micelles of P103 has been discussed.

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2. Experimental 2.1 Materials: The copolymer P103 was received from BASF Corp. Parsippany, NJ, USA as a gift sample. The ILs namely (1-decyl) triphenyl phospomium bromide C10TPPBr) , (1-dodecyl) triphenyl phospomium bromide (C12TPPBr), (1-tetradecyl) triphenyl phospomium bromide (C14TPPBr) and (1-hexadecyl) triphenylphospomium bromide (C16TPPBr) were purchased from Sigma–Aldrich and used without any further purification . Sodium bromide (99.8%) was

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purchased from LOBA. Double distilled water was used for all the experiments except SANS

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where D2O was used as a solvent.

(CnTPPBr)

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Scheme 1 Structural Formula of (a) Pluronic® P103 and (b) alkyl triphenylphosponium bromide

2.2 Methods: 2.2.1 Cloud Point Cloud point (CP) measurements for aqueous 5% (w/v) P103 solutions with varying concentrations of NaBr)/ CnTPPBr were determined by gently heating the sample in a thin 20ml glass vial immersed in a beaker filled with water. The temperature of the sample was maintained by using a magnetic stirrer equipped with a hot plate. The temperature of the sample was increased gradually at a rate of 1◦C min-1. The temperature at which the turbidity first appeared 5

Journal Pre-proof was considered as CP of the measured sample. The measurement values were repeated twice to check the reproducibility (reproducible up to ±0.5◦C). 2.2.2 Dynamic light scattering: Dynamic light scattering (DLS) measurements were performed to measure the average size of the micelles at a fixed scattering angle of 173o using Malvern Zetasizer Instrument (NanoZS 4800,UK) at 25◦C. The incident beam was generated using He–Ne laser light source at a 633 nm. All samples were filtered using micron Millipore nylon filters. Each measurement was

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repeated twice and average values were considered. Experimental results are reproducible within

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±1 nm.

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2.2.3 High sensitivity differential scanning calorimetry:

High sensitivity differential scanning calorimetry (VP-DSC, MicroCal Inc., Amherst,

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MA) was carried out to determine the CMT of the copolymer in the presence/absence of C12TPPBr/ NaBr. The experiment was performed within the temperature range of 5 to 120 ◦C

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and the scanning rate of the experiment was set at 20 ◦C /h. The sample solution was filled in the sample cell while the reference cell was filled with deionized water. The sample was equilibrated

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for at least 20 min before each measurement. A progressive baseline was attained by the extrapolation of pre and post transitional portions of the transitional peak. The area between the

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HSDSC thermogram and the progress baseline was integrated and defined as the heat of

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micellization (△H) [57-59]. To determine the free energy change (△G) and entropy change (△S) of micellization, mass action model was used [58]. 2.2.4 SANS

SANS measurements were carried out to understand the effect of IL, C12TPPBr on Pluronic® P103 micelles in the presence/absence of salt (NaBr) at the Dhruva reactor, BARC, Mumbai, India. The samples were prepared in D2O by dissolving calculated amounts of surfactants. Thick quartz tubes (5 mm) with Teflon seal were used for sample holding and the temperature was maintained at 30 ◦C. The scattered neutrons were detected using a 1m long linear position sensitive detector. The data were recorded in the Q range of 0.015–0.35 Å−1. Correction of measured data was done by using standard protocols for the background, empty 6

Journal Pre-proof cell contribution, sample transmission and normalized to absolute scattering cross-section. The corrected intensity is fitted using the following expression for theoretical differential scattering cross-section per unit volume:

(1) where n denotes the number density of particles, V is Volume fraction, (ρp–ρs)2 is the contrast factor (ρpand ρs are the scattering length densities of the particle and solvent, respectively), P(Q)

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is the intraparticle structure factor depends on the shape and size of the particles, S(Q) is the interparticle structure factor, which depends on the interaction and ordering of particles. B is a

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constant term representing the incoherent background, which is mainly due to the hydrogen present in the sample. The analysis of SANS data has been carried out using spherical particle

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model with hard sphere structure factor with the help of the SASFIT program [60].

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3. Results and Discussion

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3.1 Micellar behavior of Pluronic® in water

Pluronics® possess unique structural features, resembling those of amphiphiles. These

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thermoresponsive block copolymers fabricated with distinct PEO and PPO blocks exhibit special features based on the length of the constituting blocks and self-assemble to form micelles in

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selective solvents. The process of micellization and the morphologies assumed by the Pluronics® micelles has been well described in literature [33, 61-64]. These copolymers are available in a

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wide range of molecular weights and PEO/PPO ratio. At low temperatures, both the blocks remain completely soluble but become insoluble above a temperature called critical micelle temperature (CMT) and trigger micellization. The length of the PEO and PPO blocks plays an essential role in micellization and it has been noted that copolymers with shorter PEO chains (except PEO< 30%) usually form micelles at low temperatures and those with longer PEO chains in constitution generally observe a delay in micelle formation and usually form micelles at higher temperatures [58].

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Fig 1 (a) Surface tension (b) HSDSC themogram (5% w/v) (c) SANS curve (5% w/v) of

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P103 at 30 ◦C

In order to understand the micellization behavior of P103, surface tension and HSDSC

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measurements were performed and the results are portrayed in Fig.1. The CMC for P103 as realized from the figure is 0.16 % w/v (Fig.1a). Similarly, as evident in Fig. 1 (b), the CMT

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appears at 21.2 ◦C (discussed in detail in the HSDSC section) which is comparable with

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previously reported values using other methods [65, 66]. The SANS measurements were also performed (Fig 1 (C)) to define different micellar parameters namely core radii (Rc), hard sphere

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radii (Rhs), volume fraction (Ø ) and aggregation number (Nagg). Our fits to SANS data revealed that 5% w/v) P103 exhibited spherical morphology with Rc =57.1 Å, Rhs = 86.3 Å and Ø = 0.08 at 30 ◦C with Nagg of 135 which is closer to the reported value [49].

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Pluronics® being thermoresponsive in character, temperature plays an essential role in in

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micelle formation and in altering the properties of the micellar system. Likewise, the presence of salt entropically favors the dehydration of the copolymeric chains and promotes aggregation. Hence, it would be interesting to note the simultaneous effect of temperature and salt on the micelles of P103. With a similar approach, the effect of salt (NaBr) and temperature was checked on the micelles of 5% (w/v) P103 using DLS. It can be observed that at 30 ◦C, in line with the previous reports, the apparent hydrodynamic diameter (Dh) of P103 micelle is ~18.2 nm[49]. However, with the increase in temperature as evident in Fig. 2, the micelle size increased for all the concentrations of salt. A slight initial decrease in micellar size with the addition of salt is mainly due to the breaking of hydration layers around the copolymeric units involved in micelle formation. However, for the higher salt concentrations the micelle size increased because of enhanced hydrophobicity experienced by the copolymeric units and their increased participation 8

Journal Pre-proof in aggregation eventually leading to the formation of larger micelles with high aggregation numbers as also observed with SANS (discussed in SANS section). Temperature has a similar dehydrating effect and it has been seen that increase in temperature even in the absence of salt, as evident in Fig. 2, leads to the dehydration of copolymeric units which in turn enhances their hydrophobicity and provokes more copolymeric units to aggregate and participate in micelle

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Fig 2 Size distribution curves for 5%P103 in the presence of salt at different temperatures .

To confirm the effect of temperature/salt and micellar growth, relative viscosity for aqueous solutions of 5% (w/v) P103 were measured as a function of temperature in the presence and absence of 1 M NaBr as shown in Fig. 3. As apparent from the figure, the viscosity of copolymer solution remains almost constant in the absence of salt up to 50 ◦C. However, in the presence of salt, a steep increase in the viscosity above 35 ◦C is noted. This is because salt has an analogous effect on micelles like temperature. In both the cases dehydration of PEO blocks of the copolymer units occurs. As a result, more void is created due to the compaction of the copolymer units and accordingly more copolymer units aggregate to form larger micelles. In the absence of salt, only temperature induced dehydration prevails in the micelles. As a consequence, higher temperatures are required. While in the presence of salt, the salt and the

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Journal Pre-proof temperature both are responsible for causing dehydration of the PEO units. As a result, at lower temperatures, micellar growth is noticed. A similar effect of salt and temperature has been

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reported for different Pluronics ® and their star shaped analogues Tetronics®[67-69].

Fig. 3 Relative viscosity of 5% (w/v) P103 aqueous solutions as a function of temperature in the

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presence and absence of 1 M NaBr.

3.2. Phase behaviour and micellar characteristics of aqueous 5%P103 in the presence of IL 3.2.1 Cloud Point

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The EO-PO block copolymers like other nonionic surfactants, depending on the lengths of their constituting blocks, undergo phase separation at different temperatures. [49, 70]. It has

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been commonly observed that the copolymers with longer PEO chains undergo phase separation

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at higher temperatures while those with shorter chains display clouding at lower temperatures [71]. For instance, L92 constituted with 80% PPO display clouding at temperatures as low as 15oC while F127 incorporated with 70% PEO undergoes phase separation at temperatures above the boiling point of water. Clouding is supposed to be due to breaking up of hydrogen bonds between water and hydrophilic moieties of the copolymer and generally an increase in temperature or presence of additives stimulates disruption of bonds to occur at a faster pace until the copolymer becomes completely insoluble.

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Fig. 4 Effect of alkyl chain length in the absence (a), and presence (b) of 0.1 M NaBr and (c) the effect of

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C16TPPBr concentration in the absence () , 0.1 (), 0.5 () and 1.0 () M on 5% (w/v) P103

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micelles.

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The effect of varying alkyl chain length of different CnTPPBr based ILs has been checked on the clouding behavior of 5% (w/v) P103 aqueous solutions as shown in Fig.4 (a). As

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evident from the figure, C10TPPBr with shortest alkyl chain shows minimal increase in CP. However, CP clearly increases in the presence of C12TPPBr and C14TPPBr. The maximum

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change can be noted in the presence of C16TPPBr, the IL with longest alkyl chain. The increase in CP can be mainly correlated with the orientation of IL monomers in the hydrophobic core of

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micelle in a way that the hydrophobic tails remains occupied within the core while their charged head groups reside at the interface. The orientation of ILs with longer alkyl chains in the core is

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more favored and accordingly more number of IL units get accommodated. Further, such an orientation of IL develops charge on the micelle and causes repulsion between the constituting P103 units thereby resisting the formation of compact P103 micelles [49, 72]. As clouding is mainly due to the dehydration of copolymer chains, the arrangement of IL in the micelles induces an opposite effect and rehydrates the copolymer. As a result, higher temperatures are required to reach the CP. A similar trend has been noted by Umapathi et al. [73] for the increasing chain lengths of the imidazolium based ILs on the micelles of Pluronic® F108. The effect of alkyl chain length was also checked in the presence of salt, NaBr as presented in Fig. 4(b). The general trend noted in CP is analogous to that observed in the absence of salt. The CP of 5% (w/v) P103 in the presence of 0.2 mM C16TPPBr is 98 ◦C. However, it was interesting to find that the presence of 0.1 M NaBr depresses the CP to ~94 ◦C while higher 11

Journal Pre-proof temperatures of CP are (97 ◦C) achieved for higher concentration of IL (0.5 mM). This is essentially due to the “salting out” effect which breaks down the hydration layers formed around the PEO shell in micelles which eventually dehydrates it and reduces the solubility of the copolymer units. Hence, counteracts the hydrating effect observed due to the presence of IL. Further, the presence of salt neutralizes the charge developed on micelles due to the incorporation of IL which induces more copolymer units to aggregate and hence improves the overall hydrophobicity of the micelle and accordingly reduces the CP. The study based on the effect of salt was further extended by examining the CP of 5%

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(w/v) P103 aqueous solutions as a function of C16TPPBr concentration in the presence of

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different concentrations (0.1, 0.5 and 1.0 M) of salt (Fig. 4c) and a very interesting behavior was noted. As noted in the previous discussion (Fig. 4b), the simultaneous presence of salt and IL

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creates a complex situation since salt favors aggregation and generates hydrophobicity in micelles, while IL tends to hydrate the copolymeric chains and leads to demicellization. Thus, it

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can be expected that it is the concentration of the IL /salt that plays a decisive role in prevailing

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the effect. Accordingly, in the present case it can be observed from the figure that at lower salt concentrations, the hydrophobicity generated in the micelle is low. Hence, CP sufficiently increases even with low concentrations of IL. On the contrary, at higher concentrations of salt,

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the CP of the copolymer (in the absence of IL) drops down significantly and doesn’t change with the addition of low concentrations of IL. Hence, for a sufficient increase in CP higher

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concentrations of IL are required. A similar effect of salt and ionic surfactants has been noted by

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Bharatiya et al.[72] on the CP of the copolymers P123 and F127.

3.2.2. Dynamic Light Scattering:

To evaluate the size of the micellar aggregates in the copolymer-IL mixed system, DLS measurements were performed. The apparent hydrodynamic diameter (Dh) values as a function of IL concentration is shown in Fig. 5(a). In the absence of ILs, the size of P103 micelles at 30◦C was ~18.5 nm which is in consistency with our previous result [49]. With the addition of ILs, however, Dh values decreased. These values decreased almost linearly with the increasing concentration of ILs and the effect became more significant with increase in alkyl chain length of the ILs.

It can be concluded that studied ILs interact readily with the polymer and are 12

Journal Pre-proof incorporated into micelles which eventually decreases micellar size. This trend is in accordance with the CP results (Fig. 4a). Vekariya et al.[47] observed a similar effect of ILs on F127 micelles and noted a significant drop in the micellar size using DLS. Likewise, Parmar et al. [49] observed a drop in size of P103 micelles due to the presence of different 1-alkyl-3methylimidazolium tetrafluoroborate based ILs. This decrease in size can be associated with the penetration of ILs in the core and charge development on micellar surfaces which promotes micellar breakdown. The effect becomes more pronounced for ILs with higher alkyl chain lengths.

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The effect of alkyl chain length on DLS was also checked in the presence of salt (0.1 M

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NaBr) as shown in Fig. 5(b). As portrayed in figure and in analogy to the CP results, the size of micelles decreases linearly with the increase in IL concentration and the effect becomes more

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pronounced for higher alkyl chain lengths. Though, in the presence of salt, the micellar size is higher in comparison to the results obtained in the absence of salt. As discussed in the previous

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sections, the presence of salt breaks the hydration layers around the copolymeric chains which

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improves the hydrophobicity and promotes micellar growth. Further, the presence of salt neutralizes the charge on the polymeric micelles developed as a result of IL incorporation which endorses more copolymeric units to participate in micelle formation. A competition prevails

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between both the additives leading to opposite effects. As a result, the micellar size though

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decline in the presence of IL, the values achieved are higher than those in the absence of salt.

Fig. 5 Effect of alkyl chain length on the CP of 5% (w/v) P103 in the absence (a) and presence of 0.1 M NaBr. (c) Effect of different NaBr concentrations on the CP of 5% (w/v) P103 in the presence C16TPPBr.

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Journal Pre-proof The effects of different salt concentrations were also checked on the P103-IL aqueous system as shown in Fig. 5(c). It can be seen that in the absence of salt, the size of P103 micelles drops significantly. However, the presence of salt moderated the influence of IL. The drop in micellar size slowed down significantly with 0.05 M and 0.1M NaBr. While in the presence of 0.5 and 1 M NaBr, the micellar size remained unchanged and decreased slightly at higher concentrations of IL. This suggests that the “salting out” effect dominates over the hydrating effect of IL at lower concentrations of IL. However, as the concentration of IL increases, the hydrating effects of IL commands over the “salting out” effect and leads to micellar break down

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and consequently drop in micellar size is noted.

3.2.3 Small angle neutron scattering:

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The results of CP and DLS clearly revealed disruptive role of ILs on copolymer micelles. The increase in the alkyl chain length favored demicellization while the presence of salt

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moderated the effect. In order to verify the same, SANS measurements were performed. The

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SANS scattering curves for the pure 5% (w/v) P103 and its solutions in the presence of ILs are presented in Fig. 6. The model fitting of SANS data for the copolymer at a concentration of 5% (w/v) and in the presence of ILs showed that spherical micelles were formed. Table 1 shows the

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various important model parameters such as core radius (Rc), hard sphere radius (Rhs)

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polydispersity (δ) and aggregation number (Nagg).

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Fig. 6 SANS Curves for 5% (w/v) P103 in different solution conditions.

The effect of IL concentration was checked on 5% P103 solutions using SANS. It can be

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seen that for C10TPPBr (Fig. 6 a) as well as C16TPPBr (Fig. 6b), the increase in the concentration of IL has significant effect on P103 micelles. Further, the correlation peak in both the cases shifts to high Q region with a considerable decrease in intensity. This is attributed to the presence of IL which altered the intermicellar interactions as discussed in the previous sections and therefore decreased the micellar size. It is also reflected from the parameters shown in Table 1. The key parameters Rc, Rhs and Nagg decrease significantly pointing the formation of smaller micelles at higher IL concentrations.

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Table 1. SANS parameters for 5% (w/v) P103 in the presence of ILs and NaBr at 30 ◦C

Aggregation Hard Volume number sphere System fraction radius, Nagg Nagg ϕ Rhs, Å P103 IL 5% P103 57.1 86.3 0.08 135 Effect of Concentration of ILs 5% P103+2mM C10TPPBr 56.6 98.2 0.09 130 25 5% P103+10mM C10TPPBr 50.5 94.4 0.15 89 84 5% P103+20mM C10TPPBr 44.8 84.6 0.17 59 112 5% P103+2 mM C16TPPBr 57.8 93.9 0.07 138 26 5% P103+20 mM C16TPPBr 44.0 87.6 0.19 54 101 Effect of Alkyl Chain Length of ILs (CnTPPBr,n=10,12,14,16) 5% P103+2 mM C10TPPBr 56.6 98.2 0.09 130 25 5% P103+2 mM C12TPPBr 53.0 103.7 0.11 107 20 5% P103+2 mM C14TPPBr 54.3 103.6 0.18 115 22 5% P103+2 mM C16TPPBr 57.8 93.9 0.07 138 26 5% P103+20 mM C10TPPBr 44.8 84.6 0.17 59 112 5% P103+20 mM C12TPPBr 45.6 85.7 0.17 62 116 5% P103+20 mM C14TPPBr 45.2 89.1 0.18 59 111 5% P103+20 mM C16TPPBr 44.0 87.6 0.19 54 101 Effect of Salt 5% P103+ 0.1M NaBr 59.1 85.5 0.16 150 5% P103+ 0.5M NaBr 58.7 93.8 0.19 147 5% P103+20 mM C16TPPBr 44.0 87.6 0.19 54 101 5% P103+20 mM 52.3 114.0 0.04 90 170 C16TPPBr+0.25 M NaBr 5% P103+20 mM 53.4 120.4 0.04 96 181 C16TPPBr+0.50 M NaBr 5% P103+20 mM 54.8 161.0 0.05 104 195 C16TPPBr+1.00 M NaBr

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Core radius, Rc, Å

The effect of alkyl chain length and concentration of IL has also been demonstrated. Fig. 6(c) shows the effect of ILs at lower concentrations and Fig. 6 (d) at higher concentrations. In both the cases, the intensity decreases and the correlation peak shifts towards the higher Q values. The fitted parameters as presented in Table 1, Rc and Rhs decrease with higher alkyl chain lengths further confirming the disruptive effect of ILs. The Nagg decreases for C10TPPBr and C12TPPBr but increases for higher alkyl chain lengths. This is attributed to the fact that mixed 16

Journal Pre-proof micelle formation is favored for ILs with higher alkyl chain lengths. Hence, Nagg does not only represent the number of copolymer units involved in micelle but also shows a count of IL in the mixed micelles. The presence of salt, NaBr, in the micellar system counteracts the hydrophilicity generated in the presence of ILs. Fig. 6 (e) shows the distribution curves for the copolymer in the presence of variable concentrations of IL and NaBr. The intensity and the correlation peak decreases significantly in the absence of salt, however, with the addition of salt in the P103-IL complex system, there is an increase noted in the intensity and a notable shift in correlation peak towards

of

lower Q values. The results are in line with the results derived from the previous sections.

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3.2.4 HSDSC:

The HSDSC scans for 5% (w/v) P103 micelles in the presence of IL and salt are shown in

-p

Fig. 7. The asymmetric shape of the curves with a sharp leading edge followed by a declining tail is an indication of association transition [58, 59, 74, 75]. It can therefore be anticipated that the

re

HSDSC thermograms represent aggregation of copolymer chains in response to the temperature

lP

driven increase in hydrophobicity of the PPO blocks in the copolymer chains. Table 2 shows some key parameters determined from the HSDSC thermograms. Generally, CMT can be ascertained from three different methods: the onset temperature (Tonset), the inflection point

na

temperature (Tinf), and the peak maximum temperature (Tm)[58]. However, it was revealed by Batsberg et al. [57] that the polydispersity of Pluronic® has a strong influence on Tonset and Tinf.

ur

Therefore, the peak maximum temperature, Tm has been adopted as CMT in this study.

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Table: 2 Thermodynamic parameters for 5% (w/v) P103 in the presence of IL and salt as calculated from HSDSC thermogram. S,

Tonset,

Tinf,

Tm,

H,

G,

(◦C)

(◦C)

(◦C)

kJ/mol

kJ/mol

0

18.0

18.6

21.2

-

-

-

0.1 M NaBr

16.2

17.0

20.3

338.4

-20.9

1.2

20mM C16TPPBr

11.0

13.5

17.4

-

-

-

14.8

16.1

20.2

-

-

-

[Additive]

20mM C16TPPBr + 0.1 M NaBr

17

kJ/molK

Journal Pre-proof As evidenced from the calculated parameters and the HSDSC thermogram portrayed in Fig. 7, the CMT of 5% (w/v) P103 is 21.2 ◦C. In the presence of salt, as discussed in the previous sections, a more hydrophobic environment is created. Hence, the CMT decreases in the presence of salt, as expected. It is interesting to see that the addition of 20mM C16TPPBr in the copolymer solution decreases the CMT. This is the presence of IL promotes the core of the copolymeric units to aggregate at a lower temperature. Reddy and Venkatesu [52] observed a similar decline in CMT of P104 in the presence of different 1-butyl-3-methylimidazolium ILs. Likewise, Pillai et al. [55] observed a decrease in CMT in the presence of different 1-alkyl-3-methyl imidazolium

of

based ILs on the CMT of Tetronic ® T1304 which has similar EO-PO blocks in their constitution but a branched structure unlike the linear form of Pluronics®. Surprisingly, the CMT of the

ro

micellar system in the presence of 20mM IL and 0.1 M NaBr was found to be 20.2 ◦C , almost

-p

remains intact as that in the presence of 0.1 M NaBr. This can be explained with the results of Li et al. [76] who confirmed that for the sodium dodecyl sulfate(SDS) and Pluronic ® F127 system,

re

different binding and aggregation processes take place including induced micellization, growth

lP

of mixed micelles, breakdown of mixed micelles and binding of SDS to monomeric F127. Thus, it can be anticipated that the presence of IL though favors mixed micelle formation, in the presence of salt, complex formation between the IL and copolymer is disturbed. As a result, the

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ur

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CMT remains almost unaltered and a slight decrease is noted.

Fig.7 HSDSC thermograms showing the effect of 0.1 M NaBr and 20mM C 16TPPBr on the CMT of 5%P103.

18

Journal Pre-proof Conclusion: The effect of different CnTPPBr based ILs (C12-C16) was checked on the phase behavior and micellization of an EO-PO linear block copolymer, P103 using several different techniques. The aqueous solutions of 5% w/v P103 were characterized and it was established that spherical micelles with the average size of ~18.2 nm, with CMC ~ 0.16% w/v and CMT ~ 21.2◦C occur in ambient conditions. The presence of salt plays a constructive role in the micellization of copolymeric systems. Hence, the effect of salt was checked in the presence and absence of ILs. It was found that the presence of ILs improved the hydration of the copolymeric chains; improve

of

their solubility in the aqueous media and their CP. The effect gets more pronounced at higher

ro

concentrations of ILs and with the increase in alkyl chain length. Likewise, the results from DLS suggest a steep decline in micellar size in the presence of ILs. However, the effect gets milder in

-p

the presence of salt. SANS revealed the occurrence of spherical micelles and variations in the micellar size were in line with the DLS results. A reverse trend in CMT was however noted

re

using HSDSC. The presence of IL favored the aggregation of the copolymer and lowered the

lP

CMT values in the absence of salt. However, the addition of salt disturbed the IL-copolymer complex formation and thus increased the CMT in comparison to the system without salt. The present study dealing with results ascertained from several techniques provides in-depth

na

knowledge in the modulation of P103 solutions in the presence of ILs and the results will

Acknowledgements:

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probably be helpful in finding applications of P103-IL mixed micelles for diverse uses.

References:

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PB thanks University Grants Commission, India for Emeritus fellowship.

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Journal Pre-proof

Highlights:  Effect of several alkyltriphenyl phosphonium based ILs was checked on P103 micelles.  The data on cloud point, CMT (HSDSC) and micelle size (DLS & SANS) are presented.

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 Presence of IL alters cloud point, CMT, aggregation number and micellar size.

28