Studies on Physico-chemical Properties and Wetting Behavior of Protic Ionic Liquid Solutions

Abstract

Introduction The compounds that are composed of ions having melting point below 1000C are termed as ionic liquids. (Welton et al., 1999; Sirengo et al. 2023; Boamah et al. 2023). Unlike electrolytes which requires a solvent for dissociation, ionic liquids are self-dissociated (Singh et al., 2015; Sirengo et al. 2023; Boamah et al. 2023; Giernoth 2010). Ionic liquid has gain importance because of its unique chemical and physical properties. Compare to mineral acids and base, ionic liquid possesses low vapour pressure, non-flammable, non-volatile at ambient conditions, less corrosive and show excellent lubrication as well as hydraulic properties (Welton et al., 1999). Due to these properties ILs have been used alternative solvents to harsh volatile organic solvents (Singh et al., 2015; Boamah et al. 2023). Thus, they have been considered as green solvent. Broadly ILs can be characterized as aprotic IL and protic ILs, the later having labile proton (Singh et al., 2015; Boamah et al. 2023). The typical neutralization reaction between Bronsted acid and Bronsted base can form the PIL with melting point below 100 oC. The interactions among the components and steric factors may contribute in lowering the melting point of resultant PILs. As compared to AILs, PILs have better thermal stability and low volatility and thus are recently being explored for industrial applications such as electrolyte in electrochemical devices, super capacitor or batteries. Besides electrochemical applications, the possible multiple interactions, chemical activity and ease of synthesis, PILs are being used as solvent, catalyst as well as component for drug delivery systems (Greaves, & Drummond 2008; Shang et al., 2017; Greaves, & Drummond 2015). The application of PILs require precise determination of thermodynamic or physicochemical properties. Generally, the PIL are being used along with water as a solvent to overcome the issues related to high viscosity. The problems related to drug solubility or extraction have also been addressed by replacing commonly used harsh solvents by ILs (Greaves, & Drummond 2008; Shang et al., 2017; Greaves, & Drummond 2015). Recently, ILs have been used as active reagent, emulsifier, antisolvent or solvent for synthesis of theranostics, improvement or modification of drug delivery system. For such application or use of PIL for extraction for separation, the knowledge about their properties in pure form or in mixture with solvent (water) is desirable. The interfacial properties are particularly useful when IL have been employed as emulsifier in drug loading applications (Armand et al. 2009; Ghatee et al. 2012; Han et al. 2021; Xiao et al. 2023). The way a liquid interact with a solid surface in ambient condition is described by measuring the wetting behaviour. The liquid-surface-gas interactions causing the balance of interfacial energies due to cohesive or adhesive forces are studied by measuring contact angle (CA). Typically, water is used as probe liquid for wetting characterization but recently IL have also been utilized a probe liquid (Gao & McCarthy., 2007). For various applications of ionic liquids their knowledge of physicochemical properties is thus important. For instance, application of ionic liquids as lubricants or solvents for electrochemical energy systems, requires an understanding of ionic liquid-surface interactions (interfacial properties). The physical and chemical properties of pure ionic liquid or its combination with other solvents/solutes are needed to be studied for application as working fluid, gas adsorption and heat transport (Bordes et al., 2018; Tsuchitani et al., 2017; Wang et al., 2017; Warsi, Islam, Alam, & Ali, 2020; Han et al. 2021). Thus, studies of different physicochemical properties of ionic liquid (binary/ternary mixtures) by altering temperature and concentration provide information required to investigate the link between the nature of intermolecular/interfacial interactions, governing the alteration of solvent structure i.e. solvation, and also the wetting properties (Amirchand, Gardas, & Singh, 2022; Marsh et al. 2004; Meine, Benedito, & Rinaldi, 2010). In this regard we have measured density and speed of sound of binary and ternary aqueous solutions containing PILs at different concentrations and temperatures. The binary aqueous solutions of PIL with water was studied and ternary aqueous solutions with various solutes (vitamins/drugs) were studied. The apparent molar volume V2,and apparent molar isentropic compression Ks,2,, infinite dilution partial molar volumes (Vo and Ko), volume of transfer and expansion coefficients were evaluated and interpreted to understand various interactions occurring in solutions. The surface tension and excess molar volume, Vm E only for binary aqueous mixtures and the wetting properties was determined to understand the nature of interfacial interactions. The knowledge regarding the physicochemical and interfacial properties are highly important from applied and fundamental aspects.

Review of Literature Protic ionic liquids are a subclass of ionic liquids that have attracted much attention due to their unique properties. Few investigations, according to a different study, have examined the volumetric behaviour of protic ionic liquids when shortchain hydroxylic compounds like water, methanol, and ethanol were present (Greaves, & Drummond 2008; Shang et al., 2017; Greaves, & Drummond 2015). Santos et al., 2016 synthesized bis (2-hydroxyethyl) ammonium acetate, propionate, and butanoate protic ionic liquids and determined their densities at six different temperatures. The Mattedi-Tavares-Castier equation of state was used for fitting the densities, and it produced good correlation with the experimental results. A maximum density was observed for synthesized bis (2-hydroxyethyl) ammonium butanoate [BHEA][Bu] and water at intermediate composition for each measured temperatures. Pinheiro et al., 2018 investigated various volumetric and transport properties for binary aqueous ionic liquids. Density and viscosity data of water and binary mixtures of protic ionic liquids, 2-hydroxydiethanolamine formate (2-HDEAF), 2- hydroxydiethanolamine acetate (2-HDEAA), were measured at T = (293.15 to 343.15) K and atmospheric pressure. The Redlich-Kister polynomial equation was used to fit all excess properties and found a negative behaviour for both ionic liquids + water mixtures, which is caused by the shrinkage of the mixtures. Tomas et al., 2022, experimentally obtained the density and electrical conductivity to evaluate the effect of side alkyl chain length of different ionic liquids on interactions with ethylene glycol (EG) at temperature range (278.15 - 313.15) K. Ion- dipole interactions on the solvation process decreases with side chain length on ionic liquid while solvophobic solvation increases. The impact of ion-ion and ion-solvent interactions on thermodynamic characteristics was discussed in relation to alkyl chain length. The cation with the longer side chain (n = 4, 6) was structure maker whereas with shortest side chain (n =0, 1) was a structural breaker. The ion association constants value of IL in water is lower than in EG. Thermodynamic and transport properties of thiocyanate based ionic liquids which are low toxic and fluorine free were studied by Shekaari et al., 2020. The density, speed of sound and viscosity of solutions have been determined in this regard and these properties have been used to calculate the apparent molar volume, and apparent molar isentropic compressibility and viscosity B-coefficient. The results showed the dominant interactions were hydrophobic kind of interactions. The solvation behaviour of monosaccharides in aqueous ammonium based protic ionic liquid solutions at temperatures (288.15 to 318.15) K was investigated by Singh et al., 2016. Thermodynamic equations were applied to obtain the values of apparent molar volume, partial molar volume, enthalpy of dilution. NMR analysis has also been employed for the system in order to give justifications about the type of interactions present in the solute. Different properties such as apparent molar isentropic compressibilites, apparent molar volumes were calculated for D(-)-ribose in aqueous solution of ammonium based protic ionic liquid ( 3-hydroxypropylammonium acetate) at various temperature and atmospheric pressure by Singh et al., 2015 and moreover various other parameters such as expansion coefficients, hydration numbers, pair and triplet interaction coefficients were also discussed. The nature of interactions between 1,8-diazabicyclo[5.4.0]undec-7-en-8-ium trifluroacetate [DBUTFA] in two organic solvents viz. n,n-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at different temperature were studied by using DFT intra-ionic and interionic interaction between ions of PIL and organic solvents. The experimental results on density and speed of sound showed good agreement with computational data (Singh et al., 2015). The imidazolium based ionic liquids that have functionalized large alkyl side chains and benzyl groups engage in more energetic interactions with fresh graphite surfaces. To evaluate the affinity of liquid towards carbon allotropes, the interfacial energy appears to be an appropriate descriptor. To investigate the ordering of ions around the graphite surface molecular dynamics simulations were used by Bordes et al., 2018 and also concluded that ordering of ions on the graphite surface was increased due to the presence of large side alkyl chain on cation of ionic liquid. Herrera et al., 2015 formed nanodroplets of different sizes by using amino acid based ionic liquid (1-ethyl-3-methylimidazolium glycine) on sheets of graphene. The interaction energy between ion-graphene was find out as a function of droplets size along with the ions arrangement at the interface. The effects of adsorption on fluid structure as well as the mechanism and strength of ion-graphene interactions are examined in relation to the wetting of graphene by the ionic liquid. Literature review suggest the notable reports on studies of ionic liquid in presence of various solvents/solutes. Mostly the work has been performed by considering aprotic ionic liquid and the studies on protic ionic liquid are scant. In this regard we have studied and evaluated the thermodynamic and interfacial properties of mixed protic ionic liquid solutions.

Research Gap and Significance The studies on physicochemical properties such as volumetric properties (apparent and partial molar volumes, apparent and partial molar isentropic compressibilities, partial molar volumes of transfer, thermal expansion coefficient, etc.) and interfacial properties (surface tension, contact angle, interfacial tension and surface free energy for IL-surfaces) of mixed PILs solutions are required for understanding the nature of interactions occurring in solutions as well as how these mixtures can be employed for surface wettability alterations (Herrera et al., 2015; Bordes et al., 2018; Bahadur et al. 2015; Das, Swami & Gardas 2023). The literature survey suggests that for targeted applications of PILs, the knowledge and understanding of various interactions (solute-solute, solute-solvent, IL-solvent, ILsurface, etc.) are desirable. The studies of interactions as a function of concentration and temperature can be studied by measuring and analysis of thermodynamic and interfacial properties. Protic ionic liquids can be synthesised with ease and have been studied owing to their promising properties. These tuneable solvent systems can be suitably synthesized by appropriate combination of anions and cations. The PIL with water or organic solvents have been used for making bi-phasic system for separation and purification. Thus, task-specific IL based aqueous bi-phasic systems (ABS) have been used for separation and purification of antioxidants, alkaloids, flavonoids, antibiotics and functionalized amino acids (Pedro et al., 2020, McQueen, & Lai 2019, Ventura et al., 2017). The nature of interactions between the extractants and analytes, thus influences the separation/purification process (Ventura et al., 2017, McQueen, & Lai 2019). For instance, the strong interaction between the imidazolium cation (aromatic π-cloud) with aromatic and polarizable solutes, enhances the separation ability of IL based ABS (Ventura et al., 2017; Bogdanov & Svinyarov, 2013; Bogdanov et al., 2012). Various reports are available for the synthesis and characterization of ILs and their interactions with solutes (saccharides, proteins, amino acid, etc.) and solvents (polar and non-polar). Most of the work has been done on aprotic and that too with imidazolium based ILs. However, still there is lack of data on the physicochemical properties of mixed protic IL solutions containing active pharmaceutical agents or drug molecules (Pedro et al., 2020; McQueen, & Lai., 2019). Furthermore, the interfacial studies (wetting) are also missing in literature for PIL solutions. Knowledge about the specific interactions dominating in mixed IL systems will help in designing drug delivery system and solvent system for liquid-liquid extraction of active pharmaceutical ingredients.

Objectives 1. To synthesize and characterize Protic ionic liquids. 2. To evaluate physicochemical properties of binary mixtures (Protic ionic liquid + solvent). 3. To study the solvation behaviour of vitamins in aqueous solutions of Protic ionic liquids. 4. To examine the volumetric properties of pharmaceuticals in aqueous Protic ionic liquid solutions. 5. To study the interfacial properties of Protic ionic liquid solutions at solid surfaces. 6. Studies on optimization of Protic ionic liquid solutions for smart surfaces.

Experimental The synthesis of ammonium based protic ionic liquids was carried out. The ethanolammonium based protic ionic liquids were synthesized by neutralization reaction of selected Bronsted acid and Bronsted base in appropriate ratios. The generalized reaction for the synthesis of PIL is shown in the scheme I. NH3 + HO -O O NH2 HO O R OH Base Carboxylic acid Protic Ionic Liquid + R Scheme I. Neutralization reaction between Bronsted acid and Bronsted base for the formation of PILs. Three PILs namely ethanolammonium acetate [EAAc], ethanolammonium butyrate [EABu] and c) ethanolammonium hexanoate [EAHx] were synthesized and characterized by 1H NMR and FT-IR spectroscopic analysis. The physicochemical properties and wetting behaviour of synthesized PILs and their aqueous mixtures were carried out. The binary mixture of PILs with water was studied as a function of concentration and temperature. The physicochemical studies of mixed aqueous solutions containing PIL and various solutes (vitamins/pharmaceutical agents) were also studied as a function of concentration and temperatures (298.15 to 313.15) K at 0.1 MPa. The solvation behaviour of PILs in water and various vitamins/pharmaceutical in aqueous PIL solutions was analyzed in terms of possible interactions occurring in solutions. Further, the wetting behaviour of PIL and their aqueous solution was also studied. The work done has been presented in thesis by considering the following chapters: 1. Introduction 2. Review of Literature 3. Experimental 4. Results and Discussion 5. Conclusions 6. References

CHAPTER I Introduction In this chapter, the introduction regarding the ionic liquid and its classification as aprotic and protic ionic liquid have been described. The enormous applications of ILs (pure or their mixtures) have been highlighted. Concise and systematic scrutiny was done on protic ionic liquids and their applications in separation or extraction or drug delivery systems. The challenges associated with industrial application of IL is also described. The role of surface-active molecules in modulating the surface wettability is discussed. Further, the introduction regarding various equations used for analyzing the physiochemical properties and wetting studies are also described. The use of IL as probe liquid for wetting characterization is also highlighted. The surfaces based on wetting characteristics have also been classified. Different parameters have been determined and discussed to explain the nature of interactions occurring in mixed aqueous solutions as well as the interactions between solid-liquid interface is also included.

CHAPTER II Literature Review The literature review on physicochemical studies of mixed solutions of IL have been included. The detailed literature survey is included to understand the literature gap and importance of determination of physicochemical/interfacial properties is included. Further, volumetric properties of aqueous and mixed aqueous solutions of PIL with water or various solutes (vitamins/drugs) in presence of water was surveyed. The important parameters such as apparent molar volumes, apparent molar isentropic compressibilities, volumes of transfer, Helper constant, wetting parameters (contact angle or contact angle hysteresis) as mentioned in previous publications has been provided in this chapter.

CHAPTER III Experimental Information regarding the source, purity and CAS number of chemicals used has been included in this chapter. The synthesis and characterization of PILs have been mentioned. The specifications of density and speed of sound analyzer, drop shape analyzer, FT-IR, NMR, and UV-Visible spectrophotometer used in this work is included in this chapter. The sample preparation and analysis has also been described.

CHAPTER IV Results and Discussion In this chapter we have provided the findings and analysis of the experimental work done. The physicochemical and interfacial properties of aqueous and mixed aqueous solutions have been provided. The whole study included in this thesis and their discussion are divided into Sections I-IV. Physicochemical properties at (298.15 to 313.15) K and interfacial properties at 298.15 K of aqueous PIL solutions has been provided and discussed in Section I. Section II delas with the investigations on volumetric properties of vitamins in aqueous PIL solutions at different concentrations and temperatures. The solvation behavior of pharmaceutical drugs in aqueous PIL solutions have been investigated and provided in Section III. Section IV deals with the wetting studies of PIL and their mixtures with aqueous solutions on various typical substrates.

Section I. Volumetric properties of aqueous PIL solutions. The density, and speed of sound, u of aqueous ammonium based protic ionic liquid solutions were measured at different concentration and temperature ranging from 298.15-313.15 K. Also surface tension , was measured at 298.15 K. Using these experimental data of densities and speed of sound, derived parameters such as apparent molar volume and apparent molar isentropic compressibility were calculated. Further the excess molar volumes were evaluated and the data was fitted to Redlich-Kister equation to analyse the kind of interactions. Redlich-Mayer type of equation was used to estimate the infinite dilution partial molar volume. Surface tension decrease as the anionic chain length of PILs increases. Hydrophilic type of interactions exists between the water and PILs molecules. As the chain length increases the interactions between the PILs and water gets weakens. Also, interfacial properties result strongly influences the hydrophobicity of alkyl chain. Surface tension decrease as the anionic chain length of PILs increases. Hydrophilic type of interactions exists between the water and PILs molecules.

Section II. Volumetric and spectroscopic analysis of vitamins in aqueous PIL solutions. The present work aims to study the interactions of different vitamins (viz. Lascorbic acid, nicotinic acid and thiamine hydrochloride) in aqueous PILs solutions at four different co-solute concentration (0.05,0.10,.0.15,0.20 mol.kg-1) and temperatures (298.15-313.15) K. Various interactions occurring between the solute (vitamins), co-solute (PILs) and solvent (water) were analyzed using various volumetric and spectroscopic studies. The infinite dilution partial molar volume V2 º for non-electrolyte type of vitamins such as L- ascorbic acid and nicotinic acid was determined using method of least square equation while for electrolyte type of vitamin (thiamine hydrochloride) was determined using Masson equation. The transfer values increase for vitamins as the PILs concentration increases which indicates the hydrophilic-hydrophilic and ion-hydrophilic interactions strengthen between vitamins and PIL in aqueous solutions. Negative transfer volumes at lower concentration of EABu and EAHx suggests the presence of hydrophobic-hydrophobic and ionhydrophobic types of interactions among vitamins and PILs cations/ anions. Transfer volume for all vitamins decreases as the anionic chain length increases. The ΔtV2 º of vitamins follows trends thiamine hydrochloride > L-ascorbic acid > nicotinic acid, indicating stronger interactions for thiamine hydrochloride-PIL systems. Hyperchromic shift was observed in the UV−visible absorption spectra for ternary solutions of all vitamins and aqueous PILs solutions signifies the presence of hydrogen bonding type of interactions.

Section III. Volumetric and spectroscopic analysis of drugs in aqueous PIL solutions. The study focuses to evaluate the interactions occurring in ternary mixtures with different drug molecules (betaine hydrochloride, ketorolac tromethamine and caffeine) as solute PILs as co-solutes in aqueous solutions at temperatures between 298.15-313.15 K and at 0.1 MPa using volumetric and UV- Visible spectroscopy studies. Masson equation was used to determine the infinite dilution values for electrolyte type of solute such as betaine hydrochloride and ketorolac tromethamine whereas method of least square equation used for non-electrolyte solute caffeine. The ΔtV2º of vitamins follows the order: betaine hydrochloride > ketorolac tromethamine > caffeine. Indicating the weaker interactions of caffeine with PIL as compared to other studied solutes. Hyperchromic shift was observed for all drug molecule in presence of aqueous PILs solutions which indicate the hydrogen bonding.

Section IV. Wetting behaviour of PIL and its solutions on typical and smart total wetting surfaces. The synthesized PIL and their aqueous mixtures were studied to evaluate the interfacial properties. How PIL interact with typical and fabricated smart surfaces have been studied. The wetting behaviour as a function of anion chain length was studied. Contact angle of PIL was determined on glass, PTFE (polytetrafluorethylene), PMMA [Poly(methyl methacrylate)], charcoal based surfaces and PC (polycarbonate). The results indicate that increase in PIL anionic chain length showed drastic influence on wetting behaviour.

CHAPTER VI Conclusions In this chapter the summary and conclusion of the entire work on physicochemical and interfacial properties involving newly synthesized PILs have been included. The conclusion on experimental results and analysis of thermodynamic parameters (apparent molar volume V2,and apparent molar isentropic compression Ks,2,, infinite dilution partial molar volumes (Vo and Ko), volume of transfer and expansion coefficients) for various aqueous mixture of PIL and vitamins/drugs have been described. The wetting studies and conclusion regarding the same have been provided. The future perspectives of utilizing the binary or ternary aqueous PIL solutions for various applications and their important significance in industrial and academic purposes have also been emphasized.

CHAPTER VII References At the end of the thesis, the references have been provided in this chapter.