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Influence of the anionic structure and central atom of a cation on the properties of LCST-type draw solutes for forward osmosis

Thermo-responsive ionic compounds were synthesized to examine if they have a powerful ability to draw solutes for forward osmosis (FO). The investigated compounds were tetrabutylammonium benzenesulfonate, tetrabutylphosphonium benzenesulfonate, tetrabutylammonium 2-naphthalenesulfonate, and tetrabutylphosphonium 2-naphthalenesulfonate (abbreviated as [N4444][BS], [P4444][BS], [N4444][NS], and [P4444][NS]). The lower critical solution temperature (LCST) characteristics of the materials that formed the monocyclic aromatic compound [BS] were not confirmed; however, the LCSTs of others that formed the bicyclic aromatic compound [NS] were confirmed to be approximately 37 °C ([N4444][NS]) and 19 °C ([P4444][NS]) at 20 wt% in aqueous solutions; this is valued in reducing the energy required for recovery of the draw solute. In addition, it suggests that ammonium-based ionic compounds have a higher recovery temperature than phosphonium-based ionic compounds. When an active layer was oriented to a draw solution (AL-DS mode) and using 20 wt% aqueous [N4444][NS] draw solution at room temperature, water and reverse solute fluxes were about 3.07 LMH and 0.58 gMH, respectively. Thus, this is the first study to investigate structural transformations of the anion and central atom of the cation and to examine prospective draw solutes of the FO system in this series.


Introduction
Rapid population growth, rapid industrialization, and limited freshwater resources have led to serious concerns regarding energy crises and the scarcity of clean water worldwide. Potable water supplies and the development of energy sources are codependent and have emerged as crucial requirements in our community. Accordingly, various water treatment technologies have been developed, and their commercial feasibility has been demonstrated by categorizing them as distillation, 1 membrane separation, 2,3 adsorption, 4 coagulation, occulation, 5 and biological processes. 6 Among them, membrane desalination technology has been referred to as a distinguished methodology, which ameliorates the aforementioned problems owing to its lower energy consumption and higher efficiency. The method is also advantageous in terms of its selectivity, scalability, simple procedure, and facile operation. [7][8][9] As an emerging process, forward osmosis (FO) does not require external pressure to operate this osmosis system owing to a membrane-based process. To drive this process, it relies solely on the natural energy of osmotic gradient differences between the draw and feeds solution, which are on either side of a semipermeable membrane, thus demonstrating higher energy efficiency. [10][11][12][13] FO is a two-step process that consists of osmotically diluting the concentrated draw solute and re-concentrating the dilutive draw solute. In the rst step, spontaneous movement of water occurred across the FO membrane. The second step involves regenerating the diluted draw solute to its initial state using an external stimulus. However, the FO process still presents some challenging issues that need to be addressed or improved for the commercialization of the process. 14,15 The primary impediment to the FO technology is the recovery and reuse of draw solutes in the second step, which is considered to be a potentially energy-intensive process. Therefore, it is necessary to discover a draw solute for being eco-friendly, which can easily be separated from water in the second step and can bring about a great osmotic pressure in the water permeation step. 11,16 Several researchers have found a variety of draw solutes that can come in two primary categories: non-stimuli-responsive draw solutes and stimuli-responsive draw solutes. Nonstimuli-responsive draw solutes include a myriad of polymers, 17 gases or volatile compounds, 18 inorganic compounds, 19,20 organic compounds such as nutrient compounds, [21][22][23] and organic salts. 24 However, to date, none of these classes of draw solutes can be considered as an excellent choice or a perfect t; moreover, non-stimuli-responsive draw solutes can render the overall process costly. This is because no noticeable change occurs in their water affinity aer stimulation, which necessitates high energy in the draw solute recovery step. Stimuli-responsive draw solutes are generally closer to this target, thereby facilitating the ease of regeneration compared with non-stimuli-responsive draw solutes. 25 Stimuli-responsive draw solutes can be inferred as compounds with water affinities that can undergo instantaneous changes in response to external stimuli such as magnetic; 26,27 electric eld; 28 changes in temperature, 29 pH, 30 gas, 31 salt, 32 and light. 33 A thermo-responsive property is attractive to draw solute application when separating the draw solution into pure water and draw solute owing to its simplicity and the possibility of utilizing heat sources, such as geothermal, solar thermal energy, and low-grade waste heat from industrial sources. 11 The thermo-responsive material has various forms in forward osmosis eld as draw solute, including polymers, 34-36 magnetic nanoparticles, 37-39 hydrogels, [40][41][42][43] and ionic liquids (ILs). [44][45][46][47][48] In particular, the study of the structures and thermo-responsive properties of ILs has grown exponentially over the past few decades. ILs constitute appropriate draw solutes owing to their easy recovery processes, continuous recycling, and sufficient osmotic pressures generated by the ionic groups, which result in greater water ux. ILs have attracted considerable research interest owing to plentiful structural possibilities, which can be realized by the tweak in the cation-anion combination. 49,50 In addition, ILs and water can form a homogenous solution and can exhibit unique phase behavior based on the temperature, resulting in a solubility limit. In general, two classes of IL-water mixtures exist and are known as the lower critical solution temperature (LCST) and upper critical solution temperature (UCST) classes. 51,52 In the phase separation of the LCST-type, the mixture is miscible at low temperature and becomes immiscible above the LCST point, whereas in the phase separation of the UCST-type, the mixture becomes immiscible at low temperature and becomes miscible above the UCST point. 53 The result of the phase separation is the formation of IL-rich-and water-rich phases. A recent study reported the application of a series of thermo-responsive IL materials to draw solutes. As reported by Liu et al., 47 LCST-type ILs, including monocationic tetrabutylphosphonium hydrogen maleate (P1Mal), monocationic tetrabutylphosphonium p-toluenesulfonate (P1TSO), dicationic tetrabutylphosphonium p-toluenesulfonate (P2TSO), and dicationic tetrabutylphosphonium trimethylbenzenesulfonate (P2TMBS), have been investigated to see whether it is possible to potentially act as draw solutes for FO. Although they are potentially suitable for use as draw solutes, research on thermoresponsive IL draw solutes is still limited, and thus, continued search for other effective thermo-responsive ILs and systematic investigations are needed to study their applicability in FO. Based on the abovementioned ndings, an ionic compound structure consisting of monocationic and monoanionic forms capable of inducing water permeation appears to be advantageous for efficient FO performance. Additionally, to improve the energy efficiency in the second step, a suitable thermoresponsive ionic compound can be tailored by a suitable selection of cations and anions to approximately tune the phase separation temperature to the room temperature.
In this study, four new classes of ammonium-and phosphonium-based ionic compounds with anions of different hydrophilicity, including benzenesulfonate (BS) and 2-naphthalenesulfonate (NS), were synthesized to examine as a powerful draw solute. Further, the applicability of these ionic compounds as draw solutes for FO was intensively investigated in terms of both the thermo-recovery properties and FO performance, thereby providing fresh ideas and guidance toward the development of prospective draw solutes.

Reagents and instrumentation
Tetrabutylphosphonium bromide, tetrabutylammonium bromide, sodium benzenesulfonate, and sodium 2-naphthalenesulfonate were purchased from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan). Dichloromethane and anhydrous magnesium sulfate were purchased from DaeJung Chemicals & Metals Co. Ltd (Siheung, Republic of Korea). All reagents and solvents were no longer puried when used in synthesis. Distilled water was produced using the Human Power I + Scholar type (Humancorp, Seoul, Republic of Korea).
To identify the synthesized structure, proton nuclear magnetic resonance ( 1 H-NMR; Agilent, MR400 DD2) and Fourier transform infrared (FT-IR; Thermo Fisher Scientic, NICO-LET iS20) spectroscopy was performed. 1 H-NMR (400 MHz) spectra were obtained using deuterium oxide (D 2 O). FT-IR spectroscopy was performed under the attenuated total reection (ATR) mode at wavenumbers ranging from 4000 to 670 cm −1 . Conductivity measurements were performed using a conductivity meter (METTLER TOLEDO, Seven2Go Pro). An osmometer (KNAUER, SEMI-MICRO OSMOMETER K-7400) uses the freezing point depression method to measure the osmotic pressure of the solution.
For the LCST characterization, the aqueous solution was analyzed by turbidity measurements (l ¼ 650 nm, UV-Vis; EMCLAB Instruments GmbH, EMC-11D-V) coupled with a temperature controller (Misung Scientic. Co., Ltd, TC200P). Also, Fourier Transform-Nuclear Magnetic Resonance Spectrometer (FT-NMR; 600 MHz, JEOL, JNM ECA-600) is used for obtaining temperature-variable 1 H-NMR spectra. Water and reverse solute uxes were determined from height differences between the solution levels on either side of a custom-made Ushaped tube before and aer the experiment. The value of reverse solute ux was determined from the difference in the conductivity of the feed solution measured by a conductivity meter (METTLER TOLEDO, Seven2Go Pro) before and aer the experiment. A mixture of tetrabutylammonium bromide (3.22 g, 10 mmol) and sodium benzenesulfonate (3.60 g, 20 mmol) in a molar ratio of 1 : 2 was dissolved in distilled water (25 mL) in a 250 mL atbottom ask, and the mixing was carried out by magnetic stirrer for 24 h at room temperature. The crude product was extracted three times with dichloromethane, washed three times with distilled water, and dried with the addition of anhydrous magnesium sulfate. Aer ltering out the drying agent, the solvent was eliminated using a rotary evaporator at 50 C. The concentrated product was dried overnight in a vacuum oven at 80 C. [

FO performance
The water ux is an important consideration in the FO process, and it was assessed through the FO test. The FO test was carried out in a custom-designed FO setup that contained U-shaped glass tubes facing L-shaped glass tubes. A semi-permeable membrane (thin-lm composite FO membrane (TFC), diameter of 2.06 cm) was developed by Hydration Technologies Inc. (HTI, Albany, Oregon, United States) and was centrally placed in a circular channel between the glass tubes. The entire setup was exposed to air at room temperature. When the system was operated, membrane orientation consist of two modes both the active layer facing the draw solution (AL-DS) and the active layer facing the feed solution (AL-FS). Under the AL-DS mode, one tube was dosed with a draw solution facing the active layer of the membrane, and another tube was dosed with distilled water as the feed solution. Under the AL-FS mode, the active layer of the membrane was placed opposite that in the AL-DS mode. The draw and feed solutions were simultaneously stirred by a magnetic bar using a solenoid (AS ONE, OCTOPUS CS-4) to well blend the existing draw solution and fresh water transferred from the feed solution. The water ux (J w ) was used to quantify the amount of water owing in direction of the draw solution by measuring the height difference of the draw solution before and aer FO. The water ux (J w , L m −2 h −1 , LMH) was calculated as shown in eqn (1): where DV denotes the change in volume of the draw solution over time (Dt), and A represents the effective membrane area (3.32 Â 10 −4 m 2 ). The reverse solute ux (J s ) was obtained by analyzing the quantity of the draw solute that permeated through the FO membrane by means of total dissolved solids (TDS) in the feed solution. In addition, the conversion factor between the TDS (mg L −1 ) and electrical conductivity (mS cm −1 ) was 0.64. 54 The change in the conductivity and volume of the feed solution before and aer FO was measured to calculate the reverse solute ux (J s , g m −2 h −1 , gMH) using the following eqn (2): where DC denotes the change in concentration of the feed solution, DV represents the volume change, and Dt is time during FO.  62 (peak b, c), and 1.94-2.23 (peak d)); the phenyl groups of [BS] − (d ¼ 7.49-7.75 (peak e), and 7.75-7.84 (peak f)); and the naphthyl groups of [NS] − (d ¼ 7.61-7.75 (peak e), 7.81-7.91 (peak h), 7.95-8.13 (peak f), and 8.32-8.42 (peak g)); the integral ratio of each peak is ideally presented as a ratio of the predicted number of hydrogens in each chemical environment.

Results and discussion
FT-IR spectroscopy was also utilized to conrm that the aforementioned series were synthesized or not, and the FT-IR spectra are presented in Fig. 3. The FT-IR spectra of all materials in this series exhibit the vibrational C-H stretching peak of the aromatic ring at approximately 3052-3062 cm −1 . 55 The characteristic peaks related to the C-H stretching vibration and -CH 2bending vibration of the alkyl chain can be observed at approximately 2870-2959 cm −1 and 1464-1488 cm −1 , respectively. 56

Electrical conductivity
Electrical conductivity quanties the extent to which a material conducts electricity and is mostly affected by the ionic concentrations and type of ions. In addition, small ions tend to demonstrate less resistance and attract polar water molecules more strongly, resulting in freely moving hydrated ions. As the extent of hydration (which depends on the size of the ions) increases, the ionic conductivity is enhanced. [59][60][61] Typically, the electrical conductivity tends and the osmotic pressure trend have similarities with respect to the degree of ion generation 62 The electrical conductivities of all draw solutions whose concentrations are 5, 10, 15, and 20 wt% were measured at room temperature.
According to Fig. 4   In terms of the structural difference between cations and anions with respect to their size, quaternary ammonium cations and benzene sulfonate anions are smaller than quaternary phosphonium cations and naphthalene sulfonate anions, respectively. Therefore, small ions, that is, quaternary ammonium cations and benzene sulfonate anions induce relatively high electrical conductivities.

Osmotic pressure
During FO, a driving force for water transport into draw solution is the osmotic pressure gradient between the feed and draw solutions. The osmotic pressure is of vital importance in predicting water ux and is mostly relevant for its concentration, which is dened by the van't Hoff equation [eqn (3)], as shown below: 16 where p denotes the osmotic pressure, C i represents the molar concentration of solute i in the diluted solution, R denotes the gas constant, and T indicates the absolute temperature of the solution.   1093, 333, and 294 mOsmol kg −1 , respectively, when they have the concentration increased as 20 wt%. The osmotic pressure is a colligative property that is inextricably linked with the number of solute particles in the solution and increases with the increasing concentration of the draw solution, as expected. Water solubility and molecular weight play important roles in generating osmotic pressure, which implies that increasing the molecular polarity and decreasing the molecular weight leads to a rise in the osmotic pressure. The osmotic pressures of ionic compounds formed by the quaternary ammonium cations are higher than those of the ionic compounds formed by the quaternary phosphonium cations at the same concentration. This is because the electronegativity difference between nitrogen and carbon is larger than that between phosphorus and carbon. 63 In addition, the osmotic pressures of ionic compounds that form naphthalene sulfonate anions are lower than those of ionic compounds that form benzene sulfonate anions. This is because the benzene moiety is relatively smaller than the naphthalene moiety, and it is known that a smaller hydrophobic motif increases water solubility. Moreover, solutes with lower molecular weights can generate higher osmotic pressure because they have a signicantly higher number of ions than solutes with higher molecular weights at equal masses. Such solutes can induce a relatively higher osmotic pressure than ionic compounds formed by quaternary phosphonium cations or naphthalene sulfonate anions.

Recovery properties
In a study on the practical application of the FO process, the main point is whether energy was reduced or not in the recovery step, and thus recovery properties of the draw agent are crucial to the feasibility of FO process. The LCST denotes the phasetransition temperature, and a monophasic and transparent solution is observed below the LCST, whereas a biphasic and opaque solution is observed above the LCST. The LCST can help achieve efficient recovery through thermally triggered transformations to make the separation from the draw solution into draw solutes and pure water. [NS] solution were measured using UV-Vis spectrophotometry in combination with a temperature controller, and temperature point at which the transmittance was 50% was dened as the LCST phasetransition temperature by Fig. 6 presenting transmittance changes according to increase in temperature.
As depicted in Fig. 6    O at the temperature range from 10 to 60 C and from 30 to 80 C, respectively, in consideration of respective phase transition temperature. Note that all the signals are normalized using the integrated intensity of the solvent (D 2 O) peak as a reference because the solvent peak does not shi during the entire procedure. As depicted in Fig. 7   the AL-FS mode, the water ux of [N 4444 ][NS] decreases to 1.09, 1.43, 1.61, and 1.65 LMH, respectively, at the abovementioned concentrations. The AL-DS mode suffered the less severe effects of dilutive internal concentration polarization (ICP), which reduces the effective osmotic pressure driving force (Dp eff ) across the membrane. Moreover, concentrative ICP does not increase owing to the use of distilled water as a feed stream in the AL-DS mode. Thus, the overall Dp eff in the AL-DS mode is relatively superior to that in the AL-FS mode, and consequently, exhibits much better water ux values than the AL-FS mode. 71 When the membrane is used in both two modes, the water permeability of [N 4444 ][NS] was increased following an increase of [N 4444 ][NS] concentration. The reason for this phenomenon is that the driving force increases coming from an increasing amount of draw solute in solution; hence, the water ux becomes higher too.
Reverse solute ux is an inevitable performance-degrading property that has a negative impact on healthy FO operation. In an ideal semipermeable membrane, the diffusion of draw solute would not occur into the feed solution; however, a realistic membrane would unavoidably lead to movement of draw solute passing through the membrane. An occurrence of reverse solute ux is motivated by the concentration gradient of a draw solute existing between both solution sides, and the corresponding value of [N 4444 ][NS] is measured simultaneously to measure the water ux. The reverse solute ux was determined from the TDS of the feed solution, analyzed using the conductivity meter before and aer the FO process, which indicates the amount of draw solute that permeates FO membrane. The results indicate that [N 4444 ][NS] has a lower propensity to migrate towards the feed solution, and its reverse solute ux is observed to be ca. 0.38 and 0.58 gMH and ca. 0.37 and 0.48 gMH at concentrations between 5 and 20 wt% under the AL-DS and AL-FS mode, respectively.

Conclusions
In this study, a series, which is composed of a combination of cations (tetrabutylammonium or tetrabutylphosphonium) and anions (benzenesulfonate or 2-naphthalenesulfonate), was synthesized to examine its applicability as a draw solute in FO. Tetrabutylammonium  , not only to recover the draw solute but also to transport water for the acquisition of freshwater product from the feed solution. In terms of the water ux, [N 4444 ][NS] has values of approximately 3.07 LMH and 1.65 LMH at 20 wt% in the AL-DS and AL-FS mode, and in terms of reverse solute ux, it has a value of approximately 0.58 gMH and 0.41 gMH under the same conditions, respectively. In addition, this series, which demonstrated LCST characteristics, improved FO efficiencies due to its characteristic recovery temperature near room temperature, and consequently, the solutes required lower amounts of energy for recovery than other draw solutes. As a result, this research is of help in study of designing and synthesizing thermo-responsive ionic compounds for draw solute application.

Conflicts of interest
There are no conicts to declare.