Lipophilicity of Phosphonium Ionic Liquids As

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Trends in Hydrophilicity/Lipophilicity of Phosphonium Ionic Liquids as Determined by Ion-Transfer Electrochemistry T. Jane Stockmann, Ryan Guterman, Paul J. Ragogna, and Zhifeng Ding Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03031 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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Trends in Hydrophilicity/Lipophilicity of Phosphonium Ionic Liquids as Determined by Ion-Transfer Electrochemistry T. Jane Stockmann,a,b Ryan Guterman,b,c Paul J. Ragogna,b and Zhifeng Dingb,* a

Sorbonne Paris Cité, Paris Diderot University, Interfaces, Traitements, Organisation et Dynamique des Systèmes, CNRS-UMR 7086, 15 rue J.A. Baïf, 75013 Paris, France. b

Center for Advanced Materials and Biomaterials Research (CAMBR) Department of Chemistry, The University of Western Ontario, Chemistry Building, 1151 Richmond Street, London, Ontario N6A 5B7, Canada c Max Planck Institute for Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam, Germany

1 ACS Paragon Plus Environment

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Abstract Ionic liquids (ILs) have become valuable new materials for a broad spectrum of applications including as additives or components for new hydrophobic/hydrophilic polymer coatings. However, fundamental information surrounding IL molecular properties is still lacking. With this in mind, the micro-interface between two immiscible electrolytic solutions (micro-ITIES), for example between water|1,2-dichloroethane, has been used to evaluate the hydrophobicity/lipophilicity of 10 alkylphosphonium ILs. By varying the architecture around the phosphonium core, chemical differences were induced, changing the lipophilicity/hydrophilicity of the cations. Ion transfer (IT) within the polarizable potential window (PPW) was measured to establish a structure-property relationship. The Gibbs free energy of IT and the solubility of their ILs were also calculated. Phosphonium cations bearing either three butyl, or three hydroxypropyl groups with a tunable fourth arm; the latter displayed a wide variety of easily characterizable IT potentials. The tributylphosphonium ILs however, were too hydrophobic to undergo IT within the PPW. Utilizing a micro-ITIES (25 µm diameter) housed at the tip of a capillary in a uniquely designed pipette holder, we were able to probe beyond the traditional potential window and observe ion transfer of these hydrophobic phosphonium ILs for the first time. A similar trend in lipophilicity was determined between the two subsets of ILs by means of derived solubility product constants. The above results serve as evidence towards the validation of this technique for the evaluation of hydrophobic cations that appear beyond the conventional PPW and of the lipophilicity of their ILs.


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Introduction Ionic liquids (ILs), often composed of alkylammonium/alkylphosphonium or imidazolium cations with melting points below 100°C, have been used in applications as alternative solvents/solid state support in lithium batteries,1-2 for micelle formation,3 solar cells,4 as surfactants,5 for ion extraction,6-7 and in polymerizable coatings.8-11 Their properties include negligible vapour pressure, good conductivity, and high thermal stability. The unique aspect of ILs is their tunability; that is, significant changes in physicochemical properties can be achieved through minor variations to substituents or by pairing different anions and cations together. One property of particular interest is hydrophobicity,12 which can influence an ILs suitability towards various coating applications8-10 and in biphasic, water|IL (w|IL) metal ion extraction.6, 13-14 Solid-fluid-vapour (so-called three phase) contact angles have been used to probe the surface characteristic (wet-ability or ‘non-stick’ properties) of solids,1516

while a variation of this technique, water contact angles (WCA) measurement, has

recently been adapted to estimate or quantify surface (e.g. coating) hydrophobicity.9,

17

The WCA method involves either coating a suitable material (e.g. cloth) with the IL9 or preparing the finished fully polymerized coating or self-assembled monolayer (SAM).17 A water droplet is mechanically dispensed onto the surface and the contact angle is then measured using a CCD – or equivalent – camera and accompanying software. This methodology has many advantages as it inherently describes surface effects, such as roughness, which are often synergistic towards creating water repellent materials. However, it has a significant disadvantage in gaining molecular information specifically: hydrophobicity/hydrophilicity information obtained through WCA measurements is a function of the surface morphology, environmental conditions, and packing of the polymers/molecules at the interface. To gain quantitative information about the molecular species independently, an alternative technique must be used. Ion-transfer electrochemistry at the interface between two immiscible electrolyte solutions (ITIES), utilizing water|1,2-dichloroethane (w|DCE)18-22 – as well as a variety of other interfaces,22-26 offers a complimentary technique to WCA and presents a unique opportunity to measure quantitatively the hydrophobicity of charged species. The term “hydrophobic” is preferred to “lipophilic” to designate ion-transfer electrochemistry since


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an emphasis is on the solvation change upon transfer of the charged species from one phase to the other.18 Developments in biphasic electrochemistry are continuously being reviewed and a few contemporary examples have been included.18-19, 27-28 This form of electrochemistry offers insight into the molecular interactions of species with water and an organic solvent, hydrophobic hydration process. Previously, we have utilized electrochemistry to assess heavy metal extractions in nuclear waste remediation.6, 12-13, 2930

These hydrophobic hydration processes should be distinguished from those called

hydrophobic interactions, where oily molecules (or non-polar groups) are brought into contact with each other in water, as recently highlighted by Ben-Amotz.31 The watermediated interaction describes water’s ability to either aid or hinder aggregation. The above perspectives provide a means of analyzing intermolecular lipophilicity, along with intramolecular interactions that can be used to discern molecular folding. It should be stressed that the Pratt-Chandler model,32 as well as the work herein, is fundamentally different from strategies that seek to analyze aggregates – as an infinite plane or indeed an entirely separate phase – as shown recently for biomolecules (e.g. proteins33 or phospholipid bilayers34) and polymer coatings.8, 10-11, 35 Schwartz et al. recently probed hydrophobic interactions using trajectories of fluorophore-labeled amphiphilic molecules at a hydrophobic methylated fused silica/water interface.36 We have also used some phosphonium ILs and their photopolymers, ILs 4, 5 and 6 in Table 1, as ion-exchange surfaces,8 scaffolds for the synthesis of gold nanoparticles,37 and antibacterial coatings,38 which are relevant to the hydrophobic hydration. IL 4 showed the highest hydrophilicity, and ILs 5 and 6 as polymer coating precursors illustrated increased hydrophilicity. Since the ion-electrochemistry has been utilized to investigate ILs with moderate hydrophobicity cations and anions, including imidazolium,39 it is expected that it would give us some insight into the hydrophobicity trend. This has become a powerful motivation for finding alternatives to mitigate the industrial or commercial use of such materials and prevent further environmental contamination. It is desirable to establish a structure-hydrophobicity property relationship by means of the Ion-transfer electrochemistry. Table 1 lists the ILs evaluated in this paper. Phosphonium ILs were chosen as the focus owing to their high electrochemical and thermal stability.40 Cations of high


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hydrophilicity were examined and direct comparisons between one pair of fluorinated and non-fluorinated ILs are made. Table 1: Structural list of quaternized phosphonium ionic liquids (ILs) tested for lipophilicity; the first six ILs have been divided into two groups R-tributylphosphonium and R-tris(1-hydroxypropyl)phosphonium with anions –B(C6F5)4 and I−, respectively unless otherwise noted. The last four are with verity of both cations and anions as listed. The R groups are given on the middle column.

Fluorinated materials have been used commercially for many years in non-stick, water and oil repellent materials. However these compounds, along with their derivatives, have been found to bioaccumulate in the environment as well as biomagnify up the food chain, persisting in top predators.41 Critically, fluorinated compounds, unlike other halogenated reagents, tend to reside in protein rich tissues including the liver and nervous system.42 Evidence exists for these compounds inhibiting proper prenatal development in rats;42 in


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this way, fluorinated alkyl compounds have been linked to neurological disorders such as Parkinson’s disease and dementia. Furthermore, the Gibbs free energy of both cations and anions in these ILs provides an estimate of the key thermodynamic relationship, Ksp of the ILs in water. Thus, through facile electrochemistry measurements, the quantitative assessment of IL lipophilicity can be obtained and comparisons can be made. The lipophilic performance of a variety of highly fluorinated polymerizable phosphonium salts, used in photopolymeric systems, had been evaluated by means of solubilities of monomer ILs in various polar and non-polar solvents and WCA measurements for their photopolymers.35 This led to the analysis of a suite of phosphonium ILs (Table 1) with varying molecular architecture to understand the limits of this technique, as well as provide a deeper understanding between the molecular structure of phosphonium ILs and their lipophilicity. Experimental Chemicals and preparation. All reagents were used as purchased without further purification, unless otherwise noted. All compounds were synthesized under a N2 atmosphere or prepared in a nitrogen-filled MBraun Labmaster 130 glove box. Solvents were purchased from Caledon and dried using an MBraun Solvent Purification System. Lithium chloride, lithium iodide, lithium bromide, lithium nitrate, lithium sulfate monohydrate,

1,2-dichloroethane

(DCE,

>99.0%),

dichloromethane,

tetradecylammoniumm tetrakis(para-chlorophenyl)borate (TDATPBCl, product #87255), tetramethylammonium chloride (N(CH3)4Cl, >99.0%), and tetramethylammonium iodide (N(CH3)4I, 99%) were purchased from Sigma-Aldrich Canada (Mississauga, ON). Iododecane (98%) and iodohexane (98%) were purchased from Alfa Aesar, while 1H,1H,2H,2H-perfluorohexyl

iodide

(95%)

was

purchased

from

Fluoroflash.

Tetraoctylphosphonium chloride (P8888Cl), tributylphosphine, and tris(3-hydroxypropyl)phosphine were generous gifts from Cytec Industries Inc. (Niagara Falls, ON). Potassium tetrakis(pentafluorophenyl)-borate (K(B(C6F5)4) (>99%) was purchased from Boulder Scientific Company (Longmont, CO). The IL, tetraoctylphosphonium tetrakis(pentafluorophenyl)borate (P8888(B(C6F5)4) was prepared by facile metathesis in 6 ACS Paragon Plus Environment

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dichloromethane (DCM) as has been described elsewhere.12 Nuclear Magnetic Resonance (NMR) spectroscopy was conducted on a Varian INOVA 400 MHz spectrometer (400.09 MHz for 1H, 161.82 MHz for decoupled 31P[1H], and 376.15 MHz for 19F). All 1H spectra were referenced relative to tetramethylsilane (CDCl3; 1H δH = 7.26 ppm and CO(CD3)2; 1

H δH = 2.04). The chemical shifts for decoupled

31

P[1H] NMR spectroscopy were

referenced using an external standard (85% H3PO4; δP = 0). The chemical shifts for decoupled

19

F[1H] NMR spectroscopy were also referenced using an external standard

(trifluorotoluene; δF= −63.9 ppm). Mass spectrometry for the phosphonium salts was recorded in both positive and negative ion modes using electrospray ionization (ESI) Micromass LCT spectrometer. Phosphonium salts 1-6 were synthesized using either tributylphosphine or tri(hydroxypropyl) phosphine and a stoichiometric excess of the alkyl halide in either acetonitrile or DMF. The solution was stirred for 24 hours before purification and isolation. Ion-exchange reactions were performed by adding solid potassium tetrakis(pentafluorophenyl) borate to a solution containing a phosphonium salt in DCM. The mixture was stirred for 24 hours prior to purification. Purity of the ionexchange product was determined by silver nitrate tests and by mass spectrometry (absence of (2M+I)- clusters in TOF-MS-ES+ spectra). Synthesis of the phosphonium salts 4, 5, and 6 is described elsewhere;8,

43

the preparation and characterization of the

others in Table 1 are described in Supporting Information. Micropipettes. Micropipettes were fabricated in-house using a method described elsewhere;12-13, 29-30 however, a brief outline has been provided in Supporting Information. Electrochemical Instrumentation. Electrochemical measurements were carried out using a Modulab System (Ametek Advanced Measurement Technology, Farnborough, UK) that was equipped with a femtoammeter and specially designed electrochemical cell apparatus as described elsewhere.12-13 A micro-interface was employed that required only a two-electrode system – no ohmic compensation was necessary; however, the Modulab does possess a positive feedback loop for signal correction. The following electrochemical cells were used: 2.5 mM N(CH3 )4 I Ag AgI

2.5 mM LiI (aq)

5 mM IL AgB(C6 F5 ) 4 Ag

(Cell 1)

( DCE )


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Ag

AgI

2.5 mM IL 2.5 mM LiI (aq)

5 mM P8888 B(C6 F5 ) 4

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AgB(C6 F5 ) 4 Ag

(DCE)

(Cell 2) 3.4 mM N(CH3 ) 4Cl Ag AgCl

2.5 mM LiCl (aq)

5 mM IL AgB(C6 F5 ) 4 Ag

(Cell 3)

( DCE )

2.5 mM IL Ag Ag 2SO4 5 mM Li 2SO 4 5 mM P8888B(C6 F5 ) 4 AgB(C6 F5 ) 4 Ag (aq)

(Cell 4)

( DCE )

where IL indicates the ionic liquid of interest as provided in Table 1, while P8888(B(C6F5)4 refers to the IL tetraoctylphosphonium tetrakis(pentafluorophenyl)borate. The micro-ITIES was continuously monitored through the use of a CCD camera (Moticam, Hong Kong) attached to a 12× zoom lens assembly (Navitar, Rochester, NY), which was also used to characterize the size of the interface. Figure S1, in the Supporting Information (SI), illustrates typical cyclic voltammograms (CVs) obtained using Cell S1 and S2, with Li2SO4 and LiI, respectively, used as supporting electrolytes in the aqueous phase and tetraoctylphosphonium tetrakis(pentafluorophenyl)borate (P8888(B(C6F5)4) as the organic phase supporting electrolyte. Inset in Figure S1 is a photo obtained of the tip of the micro-pipette, which is filled with an aqueous solution and immersed in the organic phase. The interfacial diameter was determined to be ~25 µm through optical measurements using the Moticam software.


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Results and Discussion

Figure 1: Schematic illustration of phosphonium ion transfer approach for the two groups of ions, R-tris(1-hydroxypropyl)phosphonium (A) and R-tributylphosphonium (B), initially dissolved in the aqueous and organic phases, respectively. Arrows indicate the initial direction of the potential dependent ion transfer. Two main groups of phosphonium ILs were synthesized, including tributyl- and tris(3hydroxypropyl)phosphonium with the fourth substituent varied as detailed in Table 1. The ILs featuring the tris(3-hydroxypropyl) groups exhibited favourable solubility in water and were tested using biphasic electrochemistry by dissolution in the aqueous phase, while the R-tributylphosphonium ILs (paired with B(C6F5)4–) were dissolved in the DCE phase. Figure 1 depicts the initial configuration for both cases. The R-tributylphosphonium series, including 1a, 2a, and 3a, along with tetraoctylphosphonium, 7 (see Table 1) were investigated using Cell 1. No IL cation transfer was observed within the PPW, as it is limited by the transfer of the aqueous phase supporting electrolytes Li+ and I−, from w to o, at positive and negative potentials, respectively. Figure S1 in the SI illustrates the typical PPW observed through cyclic voltammetry with either Li2SO4 or LiI as the aqueous supporting electrolyte and P8888(B(C6F5)4) as the organic phase supporting electrolyte, was well as N(CH3)4+ simple ion transfer. Taking advantage of the high stability of the micro-ITIES electrolytic cell and the low current required by a micro-interface, the region beyond the typical PPW was probed, as was recently demonstrated.29-30 Figure 2 illustrates the CV obtained using Cell 1 with the IL 3a dissolved in the organic phase. During the forward scan, from the initial 9 ACS Paragon Plus Environment

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potential of 0.000 V to 0.487 V, the transfer of N(CH3)4+ was visible with a peak potential at 0.188 V. During the reverse scan, a large peak-shaped wave with a peak potential of approximately −0.416 V has been attributed to iodide transfer from w to o. The peak shaped wave is a result of the micropipette internal geometry; i.e. the micropipette capillary.6,

44

Owing to the small volume of solution inside the micro-channel, species

within rapidly transfer and are depleted, generating a sharp increase in the current signal, which ultimately peaks with a subsequent decay in current; this is often referred to as ‘linear diffusion’. During the reverse scan, the sigmoidal wave is similar to metalelectrolyte electrochemistry at a disk shaped ultramicroelectrode (UME); species in the surrounding solution diffuse from a large – relative to the electrode size – hemispherical volume surrounding the micro-ITIES.19 This generates a rise followed by a steady state current and is sometimes referred to as hemispherical diffusion. These data are in good agreement with voltammetry at a liquid|liquid micro-interface held at the tip of a pulled pipette as first described by Girault et al.45 The potential scale was calibrated using the TATB,46 or Parker’s assumption, along with the addition of tetramethylammonium iodide (N(CH3)4I) to the aqueous phase. The transfer of N(CH3)4+ was employed as an internal reference, with a formal IT potential of 0.160 V.47 The half-wave potential, Δ owφ1/ 2 , was determined from the peak shaped wave and the potential at the peak maximum, Δowφ p , within the CV through the following,48-49

Δowφ1/2 = Δowφ p ± (0.028 V)/zi

(1)

The relationship between the peak current and the half-wave potential was developed for a large (millimeter) sized metal-electrolyte interface; therefore, its implementation here is a convenient estimation. Moreover, and to the best of our knowledge at the time of publication, a similar numerical treatment of the CV profile – as that performed by Nicholson and Shain48-49 for large interfaces – has not been presented for IT at a microITIES housed at the tip of a pulled pipette.


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Figure 2: Cyclic voltammogram acquired using Cell 1 with 3a as the IL. Instrument parameters included a scan rate of 0.020 V·s−1, an initial potential of 0.000 V, and a potential range from 0.487 to −0.602 V.


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Figure 3: Differential pulse voltammograms recorded using Cell 1 with ILs 7, 1a, 2a, and 3a. Instrument parameters included a step, pulse amplitude, pulse period, and pulse width of 4 mV, +/−50 mV, 0.1 s, and 0.05 s, respectively, with a potential range from approximately 0.000 and −0.750 V. However, in order to discern if other ions may be transferring, differential pulse voltammetry (DPV) was employed with the following instrument parameters: 4 mV, +/−50 mV, 0.1 s, and 0.05 s step, pulse amplitude, pulse period, and pulse width, respectively, along with a potential range between 0.000 V and −0.800 V, approximately. Figure 3 illustrates overlaid DPV curves obtained for separate experiments given that the IL in Cell 1 has been varied such that 7, 1a, 2a, and 3a are represented by black, red, purple, and green traces, respectively. Traces acquired when scanning from positive to negative potentials elicited the largest variation in peak potentials and were poorly resolved; therefore, traces obtained in the negative scan direction were used for all further analysis unless otherwise stated. The half-wave potential was calculated through the following:44, 50

Δowφmax = Δowφ1/2 +

RT zi F

Dw ΔE − Do 2

(2)

where Δ owφmax is the potential at the peak maximum, Dα is the diffusion coefficient in phase α, and ∆E is the pulse amplitude. If the diffusion coefficients in the two phases are assumed to be equal in each phase, equation 1 can be reduced to a simple relation. The hysteresis observed in Figure 3 between the forward and reverse peak potentials may be the result of water molecules interacting with the hydrophilic moieties on the IL cations. The hysteretic phenomena has been reported at the solid/IL interface and is normally where IL cations and anions form organized layers that undergo potential dependent rearrangement.51 Indeed, it has been shown that cations adsorb on a negatively biased electrode52 (vice-versa for IL anions) and have non-trivial consequences with regards to the theory of the electric double layer when applied to solid electrode/IL and liquid|liquid53 systems; particularly, in terms of restructuring/reorganization.54 While IL concentrations employed here are modest when compared to a bulk IL phase, it is likely that this phenomena persists and may be similar to that of charged phospholipids 12 ACS Paragon Plus Environment

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monolayers (likely, without protonation events), undergoing adsorption followed by ion transfer;55-56 thereby, contributing to the observed hysteresis. In all four cases, the sweep was initiated at 0.000 V as shown and a negative peak current was subsequently observed at −0.265 V and has been attributed to the transfer of I− from w to o. This provides a formal IT potential for I− of −0.290 V ( ΔowφIo−' ) calculated from equation 2, which is in fair agreement with that determined by Abraham and Danil De Namor57 (−0.254 V) as well as Samec et al.58 (−0.342 V). The former literature value was calculated based on solubility data whilst the latter was determined through a rigorous numerical approach based on the CV edge of scan profile for a large (millimeter scale) ITIES electrolytic cell.58 ΔowφIo−' is also in fair agreement with that reported by Girault et al.59 who demonstrated a range of formal IT values for I−, from −0.320 to −0.340 V, dependent on which supporting electrolyte was present in the organic phase while using a microhole apparatus. To the best of our knowledge, this is the first time the IT of I− has been observed through voltammetric techniques at conventional, supporting electrolyte concentrations. Little variation, ±10 mV, in ΔowφIo−' was observed for traces obtained in the negative scan direction, which is in good agreement with the results of Girault’s group.59 After I− transfer, a second peak was observed. The peak potential at current maximum varied from −0.530 to −0.490, −0.530, and −0.640 V for curves 7, 1a, 2a, and 3a, respectively, which were taken to be the IL cation transfer from o to w. Using these peak maxima, the formal IT potentials were calculated to be −0.560, −0.515, −0.550, and −0.660 V for the cations of ILs 7, 1a, 2a, and 3a, respectively. Similar to the previous case, proceeding to negative potentials attracts the cation (dissolved in DCE) and causes it to transfer from o to w. The less applied potential required the more hydrophilic the ion; therefore, a trend of increasing cation hydrophilicity can be deduced such that 3a

Lipophilicity of Phosphonium Ionic Liquids As

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