Thermophysical and Transport Properties of Ethanolammonium Carboxylate Protic Ionic Liquids in Water and Dimethyl Sulfoxide

Abstract

Volumetric and transport properties of protic ionic liquids (PILs) in molecular solvents provide essential insights into solute–solute and solute–solvent interactions in solution. In this study, the density and electrical conductivity of binary mixtures of ethanolammonium carboxylate PILs, ethanolammonium acetate, propionate, butyrate, and pentanoate with water and dimethyl sulfoxide (DMSO) were measured across the entire mole fraction range at 298 K. Excess molar volumes (V ^E) were derived from density (ρ) data and correlated using the Redlich–Kister polynomial. The V ^E values were negative over the full composition range for both PIL + water and PIL + DMSO systems, indicating strong ion–dipole interactions, extensive hydrogen bonding, and efficient molecular packing. Increasing the alkyl chain length of the PIL anion decreased the magnitude of the density and V ^E, highlighting the role of hydrophobicity in reducing packing efficiency and solvation. To account for differences in solvent size, the excess molar volumes were normalized with respect to the molar volume of the mixture (V _m). The normalized values (V ^E/V _m) indicate that the larger absolute contractions observed in aqueous systems are associated with both stronger interactions and the smaller molar volume of water relative to DMSO. In addition to specific conductivity (κ), molar conductivity (λ_m) was evaluated to better assess ionic dissociation independent of solvent molar volume effects. The λ_m analysis revealed enhanced dissociation in solvent-rich regions and a progressive decrease at higher PIL mole fractions due to increased ion association and reduced mobility. Overall, the results demonstrate that molecular interactions, volumetric behavior, and charge transport in PIL–solvent mixtures are governed by the combined effects of the solvent polarity, hydrogen-bonding ability, solvent molar volume, and anion hydrophobicity.

1. Introduction

Ionic liquids (ILs) are salts composed entirely of ions and are generally defined as materials with melting points below 100 °C. Unlike conventional inorganic salts such as sodium chloride, ILs typically consist of bulky, asymmetric ions, most often organic cations paired with organic or inorganic anions, which disrupt efficient crystal packing and lower the melting point. Over the past two decades, ILs have transformed several domains of chemistry and materials science owing to their exceptional features, including negligible vapor pressure, high thermal and chemical stability, wide liquidus range, high ionic conductivity, and remarkable ability to solvate both organic and inorganic substances. , These unique properties are consistent with a complex interplay of Coulombic forces, hydrogen bonding, and van der Waals interactions between their cationic and anionic constituents. Importantly, the physicochemical properties of ILs are highly tunable through an appropriate choice of cation–anion combinations, enabling the design of task-specific ionic liquids for targeted applications.

Protic ionic liquids (PILs) constitute an important subclass of ILs formed via a simple stoichiometric proton-transfer neutralization reaction between a Bronsted acid and a Bronsted base. , This straightforward synthesis contrasts with the multistep quaternization and anion-exchange processes required for aprotic ionic liquids (APILs). PILs offer several advantages including ease of preparation, lower cost, and often lower toxicity. As a result, PILs have been extensively explored across diverse areas, including organic synthesis, catalysis, extraction and separation, CO₂ capture, electrochemistry, pharmaceuticals, biomass conversion, and other industrially relevant fields. − In many applications, binary mixtures of ILs with molecular solvents perform better than the pure ILs. , The addition of a molecular solvent can reduce viscosity, tune polarity, enhance solvation ability, and improve ionic mobility, making such mixtures valuable in extraction processes and as tunable electrolytes for batteries, fuel cells, and supercapacitors. , Significantly, mixing ILs with solvents modulates ion–ion, ion–solvent, and solvent–solvent interactions, enabling the design of versatile and adaptive solvation environments.

The molecular interactions in IL + solvent mixtures have been extensively investigated using experimentally measured properties such as density, viscosity, ultrasonic velocity, refractive index, and electrical conductivity, from which various thermodynamic and transport parameters are obtained. − These evaluated parameters provide insights into the structural organization, solvation dynamics, and ion mobility within mixed systems. Previous studies have shown that negative excess molar volumes (V ^E) in PIL + water mixtures arise from strong hydrogen-bonding interactions and efficient packing between water molecules and PIL ions. In contrast, positive V ^E values in PIL + DMSO systems reflect weakened specific interactions and disruption of the DMSO dipolar association network.¹⁹ Similarly, electrical conductivity trends indicate that solvents with high dielectric constants and strong hydrogen-bonding capability enhance ionic dissociation and mobility.²⁰ In contrast, solvents with low polarity or high viscosity impede charge transport. Thus, key molecular factors, including dielectric constant, solvatochromic parameters, viscosity, hydrogen-bond donating ability (α), and hydrogen-bond accepting ability (β), collectively govern the behavior of IL-based mixtures.

Recent studies on ammonium-based protic ionic liquids have highlighted the strong influence of cation–anion structure on their thermophysical behavior and solvent interactions. , Yunus et al. reported density, viscosity, and CO₂-absorption characteristics of ammonium PILs containing acetate and butyrate anions, demonstrating that short-chain carboxylate anions promote high polarity and strong hydrogen-bonding interactions. In contrast, longer alkyl chains introduce hydrophobic effects that alter the liquid structure and compressibility. Ethanolammonium acetate (EOAA) has been exceptionally well explored, with mixture studies showing its strong association with polar solvents such as DMSO and n-butanol and its ability to form extensive hydrogen-bond networks, which significantly modify conductivity and solvation behavior. , Thermophysical property measurements on structurally related ethanolammonium carboxylates further indicate that increasing anion chain length systematically affects density, molecular packing, and solvent structuring. However, despite these advances, comprehensive experimental data on density, excess molar volume, and electrical conductivity for ethanolammonium carboxylate PILs, especially propionate, butyrate, and pentanoate, across full composition ranges in both water (a protic, strongly hydrogen-bonding solvent) and DMSO (a polar aprotic, strong hydrogen-bond acceptor) remain scarce.

Considering these observations, the present study aims to elucidate the nature and strength of molecular interactions in binary mixtures of ethanolammonium carboxylate protic ionic liquids with water and dimethyl sulfoxide (DMSO). The chosen PILs, namely, ethanolammonium acetate, ethanolammonium propionate, ethanolammonium butyrate, and ethanolammonium pentanoate, contain multiple hydrogen-bonding sites on both cation and anion. Water, a highly polar protic solvent, can engage in extensive hydrogen bonding, while DMSO, though also highly polar, behaves as a strong hydrogen-bond acceptor. These contrasting solvent characteristics allow for a systematic comparison of PIL–solvent interactions. Therefore, density (ρ) and electrical conductivity (κ) of the PIL + water and PIL + DMSO mixtures were measured across the whole composition range at 298 K. These measurements provide insight into ion dissociation, solvent structuring, hydrophobic effects of anion chain length, and overall solvation behavior within PIL + solvent systems, contributing to a deeper understanding of their potential utility in chemical, biological, and industrial applications.

2. Experimental Section

2.1. Materials

Analytical reagent (AR) grade chemicals used for the synthesis of PILs and for the preparation of binary mixtures are given in Table . All chemicals were used as received without further purification. The ethanolammonium carboxylate PILs were synthesized via a solvent-free acid–base neutralization reaction between equimolar amounts of ethanolamine and the corresponding carboxylic acids. The acid was added dropwise to ethanolamine with continuous stirring, and the reaction mixture was kept in an ice bath throughout the addition. After the addition was complete, the mixture was stirred at room temperature for an additional 24 h to ensure complete proton transfer and formation of the ionic liquid. The synthesized PILs were subsequently dried in a vacuum oven for 36 h and immediately transferred to a desiccator containing anhydrous calcium chloride and phosphorus pentoxide (P₂O₅) to prevent exposure to moisture. The structures of the cations and anions of synthesized PILs are shown in Figure . Karl–Fischer titration (Veego/Matic EX automatic titrator) was performed to determine the water content in all synthesized PILs, and the moisture level in each sample was found to be below 110 ppm. The structures and purity of the synthesized PILs were confirmed by ¹H NMR spectroscopy, which verified the absence of unreacted amines or acids and showed chemical shifts consistent with the expected protic ionic liquid structures. The ¹H NMR spectra are provided in Figures S1–S4. All mass measurements were performed using a Mettler Toledo ML204/A01 analytical balance with a readability of ± 0.1 mg.

1. Chemical Name, Abbreviation, CAS No., Mass Fraction, and Source of Chemical Used.

chemical name	abbreviation	CAS no.	mass fraction purity	source
dimethyl sulfoxide	DMSO	67-68-5	≥0.99	Actylis Chemicals
ethanolamine		141-43-5	≥0.98	Otto Chemicals
acetic acid		64-19-7	≥0.99	SD Fine Chemicals
propanoic acid		79-09-4	≥0.99	SD Fine Chemicals
n-butyric Acid		4107-92-6	≥0.99	Loba Chemie
pentanoic acid		109-52-4	≥0.99	Avra Chemicals
ethanolammonium acetate	[EtA][Ace]		≥0.99	synthesized in lab
ethanolammonium propionate	[EtA][Pro]		≥0.98	synthesized in lab
ethanolammonium butanoate	[EtA][But]		≥0.99	synthesized in lab
ethanolammonium pentanoate	[EtA][Pen]		≥0.98	synthesized in lab

^aUsed as received from manufacturers without further purification.

^bThe purity of synthesized PIL was estimated by ¹H NMR spectral analysis.

1.

Structures of (a) ethanolammonium cation, (b) acetate, and (c) propionate. (d) butyrate and (e) pentanoate anions in the studied protic ionic liquids.

2.2. Experimental Methods

2.2.1. Density Measurements

The densities of the studied PILs and their binary mixtures with water and DMSO were measured at 298 K by using a high-precision single-capillary pycnometer. First, the exact volume of the pycnometer was determined using the known density of water at 298 K and the experimentally measured mass of water filled up to the calibration mark. For each mixture composition, the mass of the sample (m) was recorded using a Mettler Toledo analytical balance with a readability of 0.0001 mg, and the density (ρ) was calculated from the relation, ρ = m/V. At least four replicate mass measurements were performed for every mixture to ensure reproducibility. To verify the reliability of the experimental setup, the densities of n-butanol, DMSO, ethylene glycol, and acetonitrile were measured at 298 K and compared with literature data (Table S1, Supporting Information). Excellent agreement with the reported values confirmed the accuracy of the method. The estimated uncertainty in the density values was within ±6 × 10^–4 g cm^–3.

2.2.2. Conductance Measurements

Electrical conductivity measurements were carried out using a Systronics conductometer 306. The instrument was calibrated according to the manufacturer’s instructions using a standard 0.01 mol dm^–3 KCl solution. After the conductivity cell and temperature probe were immersed in the standard solution, the CAL function was activated, and the conductometer was used to automatically determine the cell constant as 0.9923 cm^–1 at 298 K. The specific conductivity (κ) of each PIL + solvent mixture was obtained by multiplying the measured conductivity by the calibrated cell constant. Before each measurement, the mixture was equilibrated in a thermostat at 298 K for 15 min. The percentage error in specific conductivity was estimated using ±(S/κ ®) × 100, where κ̅ is the mean of at least five measurements and S is the corresponding standard deviation. The overall uncertainty in the conductivity values was approximately ±0.15%. Throughout all measurements, the temperature of the water bath was maintained at 298 ± 0.4 K.

3. Results and Discussion

3.1. Density, Entropy, and Lattice Energy of Neat PILs

The density (ρ) and molar volume (V _m,PIL) of studied pure ethanolammonium carboxylate PILs at 298 K are summarized in Table . The experimentally measured densities show slight deviations from the literature values reported for ethanolammonium acetate, propionate, butyrate, and pentanoate. − These minor discrepancies likely arise from differences in experimental techniques, water content, and the degree of drying of the ionic liquids before measurement. Overall, the data are in good agreement with previously reported values, confirming the reliability of the present measurements. The densities of the neat ethanolammonium carboxylate PILs decrease systematically with increasing alkyl chain length of the anion in the order [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen]. This trend reflects the combined effects of increasing molecular volume and reduced packing efficiency associated with the progressive hydrophobic expansion of the anion. Short-chain carboxylate anions enable closer ion–ion packing and stronger electrostatic stabilization, resulting in higher densities, whereas longer alkyl chains introduce steric hindrance and increase free volume, leading to lower densities. The observed behavior is consistent with earlier reports on homologous ionic liquid series and confirms that anion size and hydrophobicity play a dominant role in governing the volumetric properties of neat PILs. ,

2. Data of Experimental and Literature Density (ρ), Molar Volume (V _m,PIL), Molecular Volume (V _molc), Standard Entropy (S ⁰), and Lattice Potential Energy (U _POT) for PILs at 298 K .

PIL	ρ (g·cm–3)	ρlit (g·cm–3)	106·V m,PIL (m3·mol–1)	V molc (nm3)	S 0 (J·K–1·mol–1)	U POT (kJ·mol–1)
[EtA][Ace]	1.1533	1.1298, 1.1512, 1.1462, 1.1490	105.03	0.1744	246.9	612.8
[EtA][Pro]	1.1127	1.1096, 1.1211	121.47	0.2017	280.9	598.7
[EtA][But]	1.0739	1.0614, 1.0567	138.92	0.2307	317.0	586.2
[EtA][Pen]	1.0434	1.0381 1.0454	156.42	0.2597	353.3	575.7

^aStandard uncertainty is u(T) = 0.4 K combined uncertainty in density is u _c(ρ) = 5 × 10^–4 g·cm^–3.

^bRef .

^cRef .

^dRef .

^eRef .

^fRef .

^gRef .

^hRef .

The standard molar entropy (S ⁰) values were estimated by using the Glasser volume-based thermodynamic relationship, which provides an empirical correlation between entropy and molecular volume for ionic compounds and ionic liquids. Although this approach does not replace direct calorimetric measurements, it is widely accepted for comparative evaluation of structural and size-dependent entropy trends in ionic liquid systems. Accordingly, the standard entropy (S°) of each PIL was calculated from its molecular volume (V _molc) using the Glasser equation eq

$$S^{°}({\mathrm{JK}}^{-1}{\mathrm{mol}}^{-1})\approx 1246V_{\mathrm{molc}}+29.5$$ 1

The calculated S° values are remarkably high compared to those of typical inorganic salts (e.g., $S_{\mathrm{NaCl}}^0$ = 72.1 J·K^–1·mol^–1, $S_{\mathrm{KCl}}^0$ = 82.6 J·K^–1·mol^–1, and $S_{{\mathrm{BaCl}}_2}^0$ = 123 J·K^–1·mol^–1) and even higher than those for many high-melting organic salts. , Such large entropic values reflect extensive structural disorder in the liquid state of these PILs. This defining characteristic is responsible for their low melting points and liquid nature at ambient temperatures. A clear trend is observed in which the standard entropy (S°) increases systematically with the alkyl chain length of the carboxylate anion. This corresponds to an increase in molecular volume and enhanced hydrophobic contributions from the longer alkyl chains. The higher disorder associated with bulkier anions results in increased configurational freedom within the liquid, thereby increasing entropy. The lattice potential energy (U _POT), i.e., crystal energy of the PILs, was calculated using eq .

$$U_{\mathrm{POT}}(\mathrm{kJ}·{\mathrm{mol}}^{-1})\approx 2I\{{\alpha }^{\prime }(V_{\mathrm{molc}}{)}^{-1/3}+{\beta }^{\prime }\}$$ 2

where α′ and β′ are fitting coefficient, ionic strength (I) = 1, and the molecular volume (V _molc ) presented in nm³. The values of α′ and β′ have been recently used as 83.3 kJ mol^–1 nm and 157.3 kJ mol^–1, respectively, by Gutowski et al. U _POT decreases as the anion chain length increases, indicating weakening of the cation–anion cohesive forces. This reduction in the lattice energy, combined with the increase in entropy, strongly supports the notion that hydrophobic expansion in the anion disrupts efficient ion packing and reduces electrostatic stabilization. Consequently, these PILs exhibit significantly lower lattice energies than fused inorganic salts, for example, much smaller than fused CsI (613 kJ·mol^–1), which explains their ability to remain liquid at room temperature.

3.2. Excess Molar Volumes of PIL + Water and PIL + DMSO Systems

The densities (ρ) of the binary mixtures of ethanolammonium acetate [EtA]­[Ace], ethanolammonium propionate [EtA]­[Pro], ethanolammonium butyrate [EtA]­[But], and ethanolammonium pentanoate [EtA]­[Pen] with water and DMSO were measured over the entire composition range at 298 K to understand the molecular interactions operating in these systems. Protic ionic liquids are generally completely miscible with high-dielectric constant solvents. In agreement with this, all investigated PILs were found to be fully miscible with water (ε = 78.23) and DMSO (ε = 46.45). The experimental values of ρ and κ are summarized in Tables and . A clear dependence of density on anion hydrophobicity is observed. Figure presents the density (ρ) variations of ethanol ammonium carboxylate PILs with water and DMSO as a function of the mole fraction of ionic liquid at 298 K. The results reveal that both the nature of the solvent and the alkyl chain length of the carboxylate anion play significant roles in governing the volumetric behavior of these systems. For all PILs studied, the density of PIL + water mixtures increase with the PIL mole fraction. A pronounced rise is observed in the low-composition region (x ₂ ∼ 0.10–0.20), indicating strong ion–dipole interactions and extensive hydrogen-bond formation between the PIL ions and the surrounding water molecules.

3. Mole Fraction (x ₂) of PIL, Density (ρ), Excess Molar Volumes (V ^E), and Specific Conductivity (κ) for the Systems of PIL with Water at 298 K.

x 2	ρ (g·cm–3)	V E (cm3·mol–1)	κ(mS·cm–1)	x 2	ρ (g·cm–3)	V E (cm3·mol–1)	κ (mS·cm–1)
[EtA][Ace]+water				[EtA][Pro]+water
0	0.9971	0.000	0.000	0	0.9971	0.000	0.000
0.0858	1.0822	–0.708	60.654	0.1008	1.0768	–0.794	40.422
0.1753	1.1149	–0.939	31.610	0.2030	1.0996	–1.051	19.541
0.2706	1.1307	–0.988	17.558	0.2986	1.1070	–1.071	10.105
0.3527	1.1382	–0.956	8.727	0.4062	1.1107	–1.011	5.691
0.4557	1.1440	–0.874	6.396	0.4975	1.1131	–0.967	3.454
0.5669	1.1468	–0.684	2.931	0.6040	1.1142	–0.852	2.617
0.6758	1.1493	–0.529	2.696	0.7039	1.1136	–0.631	1.960
0.7908	1.1511	–0.345	2.520	0.8028	1.1143	–0.521	0.909
0.9005	1.1521	–0.141	0.536	0.9085	1.1128	–0.190	0.477
1	1.1533	0.000	0.380	1	1.1127	0.000	0.511
[EtA][But]+water				[EtA][Pen]+water
0	0.9971	0.000	0.000	0	0.9971	0.000	0.000
0.0940	1.0597	–0.793	31.463	0.1022	1.0497	–0.909	25.840
0.1929	1.0749	–1.082	16.690	0.1973	1.0545	–1.117	14.190
0.2919	1.0792	–1.172	8.613	0.3016	1.0551	–1.216	7.658
0.3907	1.0799	–1.147	5.634	0.3977	1.0541	–1.224	4.163
0.4907	1.0802	–1.107	3.007	0.5020	1.0523	–1.136	2.652
0.5947	1.0792	–0.961	1.101	0.5893	1.0510	–1.049	2.006
0.6919	1.0782	–0.803	1.043	0.6998	1.0492	–0.878	1.221
0.7978	1.0770	–0.587	0.700	0.8042	1.0476	–0.673	0.799
0.8965	1.0751	–0.278	0.616	0.8985	1.0458	–0.408	0.549
1	1.0739	0.000	0.234	1	1.0434	0.000	0.254

4. Mole Fraction (x ₂) of PIL, Density (ρ), Excess Molar Volumes (V ^E), and Specific Conductivity (κ) for the Systems of PIL with DMSO at 298 K.

x 2	ρ (g·cm–3)	V E (cm3·mol–1)	κ (mS·cm–1)	x 2	ρ (g·cm–3)	V E (cm3·mol–1)	κ (mS·cm–1)
[EtA][Ace]+DMSO					[EtA][Pro]+DMSO
0.0000	1.0954	0.000	0.000	0.0000	1.0954	0.000	0.013
0.0923	1.1044	–0.105	2.537	0.1262	1.1011	–0.170	2.282
0.2154	1.1141	–0.158	3.535	0.2186	1.1039	–0.234	3.147
0.3201	1.1216	–0.203	4.554	0.3138	1.1067	–0.320	3.474
0.4119	1.1276	–0.242	4.142	0.4091	1.1087	–0.360	2.878
0.5063	1.1328	–0.240	3.622	0.5050	1.1098	–0.339	2.311
0.6066	1.1378	–0.222	2.692	0.6023	1.1107	–0.313	1.932
0.7001	1.1421	–0.201	1.987	0.7019	1.1110	–0.229	1.581
0.8001	1.1455	–0.108	1.317	0.8008	1.1113	–0.155	1.066
0.9062	1.1492	–0.033	0.764	0.9084	1.1116	–0.065	0.721
1.0000	1.1533	0.000	0.380	1.0000	1.1127	0.000	0.311
[EtA][But]+DMSO				[EtA][Pen]+DMSO
0.0000	1.0954	0.000	0.000	0.0000	1.0954	0.000	0.013
0.1060	1.0950	–0.261	1.931	0.1158	1.0886	–0.358	1.790
0.2096	1.0921	–0.316	2.408	0.2206	1.0817	–0.519	2.126
0.3037	1.0899	–0.363	2.253	0.3169	1.0760	–0.622	1.954
0.4055	1.0880	–0.436	1.883	0.4133	1.0702	–0.636	1.747
0.4958	1.0861	–0.459	1.581	0.5068	1.0654	–0.648	1.406
0.5984	1.0832	–0.388	1.292	0.6089	1.0606	–0.627	1.116
0.6993	1.0809	–0.330	1.064	0.6979	1.0567	–0.588	0.879
0.7979	1.0788	–0.269	0.713	0.8000	1.0516	–0.384	0.604
0.8981	1.0760	–0.101	0.468	0.9036	1.0469	–0.163	0.424
1.0000	1.0739	0.000	0.244	1.0000	1.0434	0.000	0.214

2.

Densities (ρ) for the mixtures of (a) PILs + water and (b) PILs + DMSO as a function of the composition expressed in the mole fraction (x ₂) of PIL at 298 K for (i) violet box solid → [EtA]­[Ace], black cross mark symbol → [EtA]­[Pro], blue triangle up solid → [EtA]­[But], and red circle solid → [EtA]­[Pen]. The solid black line illustrates the trend of the experimental data.

The [EtA]­[Ace] + water system exhibits comparatively higher densities across the entire composition range, consistent with the strong hydrophilic character and effective hydration of the acetate anion. The present density values are in good agreement with previously reported data by Augusto et al., further supporting the reliability of the measurements. In contrast to water, the PIL + DMSO mixtures exhibit two markedly different compositional responses depending on the nature of the anion. For the acetate-based PIL, the density increases steadily with increasing mole fraction of PIL. This monotonic rise suggests favorable ion–dipole interactions between the acetate anion and DMSO, enabling efficient packing of PIL ions within the DMSO network. The small anion size and low hydrophobicity allow acetate and propionate to integrate well into the highly polar environment of DMSO, thereby minimizing structural disruption and limiting the introduction of free volume. Systems containing butyrate and pentanoate exhibit a continuous decrease in density with increasing PIL concentration. This decreasing trend reveals that, at higher mole fractions, disruption of the local DMSO–ion structure becomes significant. The increasing alkyl chain length of the anions introduces additional hydrophobic bulk, steric hindrance, and greater free volume, all of which lead to progressively less efficient packing.

The density trends observed for PIL+ solvent mixtures (see Figure ) reflect not only specific ion–solvent interactions but also the intrinsic densities of the pure protic ionic liquids. Among the studied systems, the neat PIL densities follow the order [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen], owing to the increasing alkyl chain length and associated decrease in molecular packing efficiency. Consequently, mixtures containing the intrinsically denser [EtA]­[Ace] exhibit a more pronounced increase in density with increasing PIL mole fraction in both water and DMSO. In aqueous mixtures (Figure a), the addition of dense PILs initially leads to a sharp rise in density, which is further amplified by strong ion–dipole interactions and hydrogen bonding with water. For PILs with longer alkyl chains, the lower intrinsic density counterbalances the effect of PIL addition, resulting in weaker density enhancement and, at higher mole fractions, a slight decrease due to steric effects and reduced packing efficiency. In DMSO (Figure b), the role of intrinsic PIL density becomes even more evident. While [EtA]­[Ace] and [EtA]­[Pro] show a continuous increase in density with composition, reflecting both its higher neat density and favorable ion–solvent interactions, mixtures containing [EtA]­[But] and [EtA]­[Pen] display decreasing density trends as PIL concentration increases. This behavior arises from the lower intrinsic densities of these PILs combined with disruption of the DMSO solvent structure and increased free volume associated with longer alkyl chains. Thus, the observed density behavior in both solvent systems results from a balance between the inherent density of the ionic liquid components and composition-dependent molecular interactions.

The distinct density trends in water and DMSO underscore the dominance of solvent polarity and hydrogen-bonding capabilities in dictating structural organization within these mixtures. Water, with its higher polarity and superior hydrogen-bonding ability, consistently promotes compact structuring regardless of IL composition or anion chain length. DMSO, although highly polar, exhibits a more sensitive structural response: it accommodates short-chain PILs efficiently but becomes increasingly destabilized in the presence of longer-chain anions.

Overall, the density data clearly demonstrate that (i) density decreases systematically with increasing anion hydrophobicity; (ii) water induces tighter molecular packing than DMSO; and (iii) long-chain PILs introduce significant free volume in DMSO, leading to reduced densities with increasing composition.

To further interpret molecular packing and structural effects, the excess molar volume (V ^E) of each system was calculated from experimental density data using eq .

$$V^{\mathrm{E}}=\frac{x_1{M}_1+x_2{M}_2}{\rho }-\frac{x_1{M}_1}{{\rho }_1}-\frac{x_2{M}_2}{{\rho }_2}$$ 3

where x ₁ and x ₂ are mole fractions of components 1 and 2, respectively. ρ₁, ρ₂, and ρ are the densities of pure components 1, 2, and their mixtures, respectively. M ₁ and M ₂ are the molecular weights of components 1 and 2, respectively. The excess molar volumes V ^E for all PIL + water and PIL + DMSO mixtures are listed in Tables and and plotted in Figure . As shown in the figure, V ^E values remain negative across the entire composition range for every system studied. Negative deviations indicate volume contraction upon mixing, implying strong attractive interactions between PIL ions and solvent molecules, leading to more efficient packing than expected for an ideal mixture. ,, Similar trends have been previously reported for ammonium-based ionic liquids in dipolar protic and aprotic solvents. Such as water, DMSO, DMF, and NMP, highlighting the characteristic behavior of hydrogen-bonding and ion–dipole-rich systems. ,− All systems exhibit a distinct minimum in V ^E. For aqueous mixtures, the minimum typically appears within (x ₂ ≈ 0.2–0.4), whereas for the DMSO systems, the minimum shifts toward higher PIL content (x ₂ ≈ 0.4–0.6). This shift indicates that DMSO requires a higher PIL concentration before significant disruption of ion–ion interactions occurs. This composition region corresponds to the most substantial disruption of ion–ion interactions and the most excellent formation of PIL–solvent hydrogen-bonded complexes. After the minimum point, the magnitude of V ^E gradually decreases as the system becomes richer in PIL and the self-association of ions becomes stronger.

3.

Excess molar volumes (V ^E ) for the mixtures of (a) PILs + water (b) PILs + DMSO as a function of the composition expressed in the mole fraction (x ₂) of PIL at 298 K. Symbols represent violet box solid → [EtA]­[Ace]: black cross mark symbol → [EtA]­[Pro]; blue triangle up solid → [EtA]­[But]; and red circle solid → [EtA]­[Pen]. The symbols represent experimental values, and the solid black line indicates fit obtained using the Redlich–Kister equation.

A pronounced solvent effect is observed. For all PILs, the magnitude of negative V ^E is significantly greater in water than in DMSO. This can be attributed to the higher dielectric constant, greater hydrogen-bond-donor capability, and lower viscosity of water. Water effectively screens cation–anion electrostatic interactions, replaces intraionic hydrogen bonds, and forms strong PIL–water hydrogen-bonded complexes. These processes reduce the free volume and enhance structural compaction, resulting in more negative V ^E values. Although DMSO is an excellent hydrogen-bond acceptor, its inability to donate hydrogen bonds, coupled with its greater viscosity and stronger intrinsic molecular organization, results in comparatively weaker structural rearrangement. Consequently, DMSO-based mixtures display smaller negative excess volumes and minima occurring at higher x ₂. The magnitude of negative V ^E increases systematically with increasing hydrophobicity of the anion and follows the order: [EtA]­[Ace] < [EtA]­[Pro] < [EtA]­[But] < [EtA]­[Pen], in both solvents. The figures clearly show that [EtA]­[But] and [EtA]­[Pen] exhibit deeper minima and larger negative deviations than [EtA]­[Ace] or [EtA]­[Pro]. This behavior suggests that longer alkyl chains enhance solvophobic interactions and induce tighter packing of surrounding solvent molecules. In aqueous systems, hydrophobic hydration around the alkyl chain contributes significantly to volume contraction, which is why the difference among PILs is more pronounced in water than in DMSO. Additionally, the −OH and −NH moieties of the ethanolammonium cation can form multiple hydrogen bonds with solvent molecules, further strengthening ion–dipole interactions and reducing the molar volume. Overall, the negative V ^E values and their minima demonstrate that mixing of PILs with water and DMSO leads to strong heterointeractions, reduced free volume, and efficient molecular packing. The PIL + water mixtures show greater contraction than the corresponding PIL + DMSO mixtures, confirming more substantial structural reorganization and solvation effects in water. The density trends observed in the DMSO mixtures support the V ^E analysis. For [EtA]­[Ace] + DMSO and [EtA]­[Pro], density increases with increasing x ₂, consistent with strong attractive interactions and adequate packing, leading to contraction. In contrast, for [EtA]­[But] and [EtA]­[Pen], density decreases with increasing PIL concentration. This decrease reflects the dominance of hydrophobic chain effects and the progressive introduction of larger molar-volume ions, resulting in less compact structural organization at higher x ₂. Thus, the contrasting density trends within the DMSO systems align with the hydrophobicity-dependent variation observed in V ^E minima. The experimental V ^E data were correlated using the Redlich–Kister polynomial expression:

$$Y^{\mathrm{E}}=x_1{x}_2\sum _{i=0}^n{a}_i(x_1-x_2{)}^i$$ 4

where Y ^E represents V ^E, and a _i are the adjustable parameters obtained through a least-squares fitting procedure. The fitted coefficients and standard deviations are reported in Table . The goodness of fit for the experimentally determined excess molar volumes was evaluated using the residual standard deviation (σ), calculated according to eq .

$$\sigma ={\left[\frac{\sum (V_{\exp }^{\mathrm{E}}-V_{\mathrm{cal}}^{\mathrm{E}}{)}^2}{(n-p)}\right]}^{1/2}$$ 5

where $V_{\exp }^{\mathrm{E}}$ and $V_{\mathrm{cal}}^{\mathrm{E}}$ are the experimental and Redlich–Kister polynomial-calculated values of the excess molar volume, respectively. Here, n represents the total number of data points, and p is the number of adjustable parameters. The values of σ for all studied systems are listed in Table and lie in a range of 0.010–0.034. These low σ values indicate an excellent fit of the experimental excess molar volume data to the Redlich–Kister equation.

5. Estimated Parameters of Redlich–Kister Equation and Standard Deviation, σ, for the Systems of PILs with Molecular Solvent.

systems	a 0	a 1	a 2	a 3	a 4	σ
[EtA][Ace] + water	–3.202	2.533	–1.508	2.606	–2.193	0.019
[EtA][Pro] + water	–3.796	1.764	–2.373	3.296	–0.888	0.034
[EtA][But] + water	–4.326	1.728	–1.991	3.089	–1.197	0.017
[EtA][Pen] + water	–4.604	1.561	–1.641	2.608	–3.756	0.021
[EtA][Ace] + DMSO	–0.980	–0.096	0.464	0.909	–0.284	0.010
[EtA][Pro] + DMSO	–1.407	0.416	0.878	0.030	–0.791	0.010
[EtA][But] + DMSO	–1.802	0.010	0.357	–1.380	–0.808	0.011
[EtA][Pen] + DMSO	–2.607	–0.089	–1.299	1.768	1.883	0.015

In addition to differences in intermolecular interactions, the magnitude of the excess molar volume is influenced by the overall molar volume of the mixture (V _m). Since, water has a much smaller intrinsic molar volume (∼18.0 cm³ mol^–1) compared to DMSO (∼71.3 cm³ mol^–1), and therefore, volumetric deviations upon mixing with PILs appear more pronounced in aqueous systems when expressed in absolute terms. To account for this size effect and enable a more meaningful comparison between the two solvent systems, the excess molar volumes were normalized with respect to the molar volume of the mixture (V ^E/V _m). The normalized plots (see Figure ) demonstrate that a significant part of the smaller absolute V ^E values observed for PIL + DMSO mixtures originates from dilution by the larger solvent molar volume rather than solely from weaker intermolecular interactions. Importantly, even after normalization, the PIL + water systems continue to exhibit more negative deviations than the corresponding DMSO mixtures. This confirms that, beyond volumetric dilution effects, stronger ion–solvent interactions, consistent with the higher dielectric constant and superior hydrogen-bond donating ability of water, promote more efficient packing and greater volume contraction in aqueous PIL mixtures. Hence, normalization of V ^E reinforces the conclusion that both solvent size and ion–solvent interaction strength govern the observed volumetric behavior. Molecular simulation studies support the present experimental interpretations by showing that variations in ion structure and solvent environment strongly affect molecular organization, even though direct molecular-level evidence cannot be obtained from macroscopic properties. ,

4.

Normalized excess molar volumes (V ^E/V _m) for the mixtures of (a) PIL + water and (b) PIL + DMSO as a function of the mole fraction (x ₂) of PIL at 298 K. Symbols represent violet box solid → [EtA]­[Ace]; black cross mark symbol → [EtA]­[Pro]; blue triangle up solid→ [EtA]­[But]; and red circle solid → [EtA]­[Pen].

3.3. Specific Conductivity of PIL + Water and PIL + DMSO Mixtures

The electrical conductivities of pure protic ionic liquids (PILs) and their binary mixtures with water and dimethyl sulfoxide (DMSO) were measured over the entire composition range. The variation of electrical conductivity (κ) with mole fraction of PIL (x ₂) is presented in Figure . In both solvent systems, the conductivity of the PIL + solvent mixtures are significantly higher than those of the pure PILs, indicating an enhanced degree of ionic dissociation upon mixing with the solvent.

5.

Specific conductivity (κ) for the mixtures of (a) PILs + water and (b) PILs + DMSO as a function of the composition expressed in the mole fraction (x ₂) of PIL at 298 K. Symbols represent violet box solid → [EtA]­[Ace]; black cross mark symbol → [EtA]­[Pro]; blue triangle up solid → [EtA]­[But]; red circle solid → [EtA]­[Pen]. The solid black line illustrates the trend in the experimental data.

The studied PILs consist of the ethanolammonium cation, which contains −OH and −NH groups capable of forming hydrogen bonds with solvent molecules and carboxylate anions of varying alkyl chain lengths that modulate hydrophobicity. In aqueous mixtures, the specific conductivity values follow the decreasing order [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen]. This trend corresponds to the increasing hydrophobicity of the anions, which reduces the ion–solvent interactions and consequently lowers the number of free charge carriers. Water is a polar protic solvent with a high dielectric constant (ε = 78.4) and low viscosity (η = 0.89 cP). These properties strongly favor ionic dissociation and enhance ion mobility. The high dielectric constant significantly reduces the Coulombic attraction between ethanolammonium cations and carboxylate anions, leading to a higher degree of dissociation. The low viscosity of water further facilitates the mobility of ions, thereby increasing electrical conductivity. The solvation capability of water is reflected by its solvatochromic parameters. $E_N^T$ = 1, α = 1.121 (hydrogen-bond donating ability), and β = 0.50 (hydrogen-bond accepting ability). ,, The high α value indicates that water can effectively donate hydrogen bonds to the basic sites of the PILs, such as carboxylate oxygen atoms. Similarly, the ethanolammonium cation can form hydrogen bonds through its −OH and −NH groups with water molecules, thereby enhancing solvation and dissociation. Therefore, PIL + water systems exhibit markedly higher conductivities than the corresponding PIL + DMSO systems.

Dimethyl sulfoxide (DMSO) is a polar aprotic solvent characterized by a moderate dielectric constant (ε = 46) and relatively higher viscosity (η = 1.99 cP). Its solvatochromic parameters are $E_N^T$ = 0.44, α = 0.00, and β = 0.76. The absence of hydrogen-bond donating ability (α = 0) and the relatively high viscosity limit ionic dissociation and ion mobility in the PIL + DMSO mixtures. Although DMSO can accept hydrogen bonds from the −OH and −NH groups of the ethanolammonium cation (due to its β value), its overall solvation power toward ionic species is weaker than that of water. Consequently, the conductivities of the PIL + DMSO systems are lower than those of the corresponding aqueous mixtures but follow the same decreasing trend with anion hydrophobicity: [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen]. In aqueous media, the specific conductivity (κ) increases sharply with the PIL concentration within the studied composition range. Because the lowest mole fraction investigated is x ₂ = 0.10, no conductivity maximum is experimentally observed for the PIL and water mixtures. The apparent "peak" at low x ₂ in the plotted curves arises solely from the graphical convention of assigning the conductivity of pure water as zero to maintain curve continuity; pure water, in fact, has a finite but very small conductivity.

For the PIL + water systems, the variation of specific conductivity (κ) with composition indicates that the true conductivity maximum occurs at mole fractions lower than those investigated in the present work. As shown in Figure a, within the experimentally accessible range (x ₂ ≥ 0.10), κ decreases monotonically with an increase in PIL content. This behavior arises from the progressive reduction in water content, which weakens the dielectric screening of cation–anion interactions and increases the medium’s viscosity, thereby reducing ion mobility. − The conductivity data for the [EtA]­[Ace] + water system were compared with previously reported values by Augusto et al. The literature study covers lower PIL mole fractions and reports a conductivity maximum in the solvent-rich region. Although a direct point-to-point comparison is not possible due to differences in the investigated composition intervals, the present results are consistent with the reported trend. Notably, our conductivity values at overlapping compositions closely follow the descending trend of the literature data beyond the maximum, supporting the reliability and consistency of the present measurements.

The agreement with literature data further supports the interpretation that the apparent peak in specific conductivity (κ) is located at very low PIL concentrations, where high dielectric screening by water promotes extensive ion dissociation and high ionic mobility. At higher PIL mole fractions, enhanced ion pairing dominates, leading to the observed monotonic decrease in conductivity. This behavior is characteristic of aqueous protic ionic liquid systems and reflects the delicate balance between ion dissociation and transport limitations imposed by a reduced solvent content. Water has an exceptionally high dielectric constant and a strong hydrogen-bond donating ability, thereby initially promoting extensive ion dissociation by weakening Coulombic attraction within ethanol ammonium carboxylate ion pairs as the mole fraction of PIL increases. , Enhanced ion pairing and a reduced polarity environment limit ion mobility, leading to the observed decrease in conductivity. Across all compositions in water, the conductivity decreases with hydrophilicity and weakened ion–solvent interactions as the alkyl chain length of the anion increases.

The conductivity trends in DMSO differ substantially from those in water. In PIL+ DMSO mixtures, specific conductivity (κ) first increases with PIL concentration, reaches a broad maximum in the intermediate composition region (x ₂ ≈ 0.2–0.4), and then decreases at higher PIL fractions. This maximum reflects the balance between the increasing number of charge carriers at a low PIL content and growing ion association as the mixture becomes richer in PIL. DMSO, a polar aprotic solvent, possesses a moderate dielectric constant and a relatively high viscosity, lacks hydrogen-bond donating ability, yet is a strong hydrogen-bond acceptor. These properties limit its capacity to disrupt ion pairs as effectively as water and reduce ionic mobility. Consequently, the conductivity maxima occur at higher PIL mole fractions and have lower magnitudes than those anticipated for aqueous systems. Nevertheless, the anion-dependent conductivity sequence [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen] is preserved in DMSO, again highlighting the influence of anion hydrophobicity and reduced solvent accessibility.

Comparison of the measured conductivities for the [EtA]­[Ace] + DMSO system with literature data reveals close agreement in both low and high x ₂ regions. In the intermediate composition range (x ₂ ≈ 0.1–0.4), the present conductivities are slightly higher than the reported values; however, the published data are discontinuous in this interval, averting a direct point-to-point comparison. Overall, the observed conductivity behavior underscores the dominant influence of solvent polarity, hydrogen-bonding characteristics, and anion hydrophobicity. Water, by virtue of its high polarity and substantial hydrogen-bond donation, promotes extensive ion dissociation and enhanced ion mobility. In contrast, DMSO provides weaker solvation of the ionic species and supports lower conductivities. Increasing the alkyl chain length of the anion progressively shields the ionic core, reduces solvent–ion interactions, and encourages ion pairing, thereby diminishing conductivity. Collectively, these trends demonstrate that electrical conductivity in PIL–solvent systems occurs from a delicate interplay between ion dissociation, solvation strength, and viscous resistance within the medium.

To further elucidate the extent of ionic dissociation in PIL–solvent mixtures and to account for differences in solvent molar volumes, the molar conductivity (λ_m) was calculated and is reported in Table S2 (Supporting Information). The variation of λ_m with the PIL mole fraction is presented in Figure . Unlike specific conductivity, molar conductivity normalizes charge transport per mole of electrolyte and thus provides a more direct measure of ionic dissociation and effective ion mobility. The λ_m values are highest in the solvent-rich region, particularly for aqueous mixtures, indicating a high degree of ionic dissociation at low PIL mole fractions. In water-rich systems, charge transport arises predominantly from vehicular diffusion of ionic species (i.e., physical translational motion of protonated ions through the liquid), with an additional possible contribution from Grotthuss-type proton hopping (successive proton transfer along a hydrogen-bonded network without net molecular diffusion). − The extensive hydrogen-bond connectivity of water may facilitate rapid proton transfer at low PIL concentrations, thereby enhancing molar conductivity. As the PIL content increases, disruption of the continuous water hydrogen-bond network, increased ion association, and reduced solvent mobility progressively suppress proton hopping, and charge transport becomes dominated by the vehicular motion of ethanolammonium cations and carboxylate anions_. In DMSO, the absence of hydrogen-bond donating ability and the higher viscosity result in lower molar conductivities, reflecting weaker ionic dissociation and stronger ion pairing. Across the entire composition range in both solvents, the molar conductivity follows the order [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen], demonstrating that increasing anion hydrophobicity reduces the degree of dissociation and effective charge transport.

6.

Molar conductivity (λ_m) for the mixtures of (a) PILs + water and (b) PILs + DMSO as a function of the composition expressed in the mole fraction (x ₂) of PIL at 298 K. Symbols represent violet box solid → [EtA]­[Ace]; black cross mark symbol → [EtA]­[Pro]; blue triangle up solid → [EtA]­[But]; red circle solid → [EtA]­[Pen]. The solid black line illustrates the trend in the data.

Overall, the combined density, excess molar volume, specific conductivity, and molar conductivity analyses demonstrate strong ion–solvent interactions in PIL + solvent mixtures, governed primarily by solvent polarity and anion hydrophobicity. At low PIL mole fractions, enhanced ion dissociation and efficient solvent structuring around ionic species lead to increased density and high conductivities, as further supported by elevated molar conductivity values. Normalization of excess molar volumes highlights that the more pronounced volumetric contraction in aqueous systems arises not only from stronger interactions but also from the smaller molar volume of water relative to DMSO. At higher PIL concentrations, ion–ion association limit density changes and lowers conductivity, especially for longer alkyl chain anions. Water, owing to its high dielectric constant and hydrogen-bond donating capability, consistently promotes greater ion dissociation and mobility than DMSO, resulting in higher molar and specific conductivities across the composition range. Collectively, these findings demonstrate that molecular organization and charge transport in ethanolammonium-based PIL mixtures are controlled by a balance of solvent hydrogen-bonding strength, solvent molar volume effects, and anion structural characteristics.

4. Conclusions

The thermophysical and transport properties of ethanolammonium-based protic ionic liquids in water and DMSO were investigated to elucidate the roles of solvent polarity and anion structure on molecular interactions. The large standard entropies and low lattice energies of the studied neat PILs indicate that hydrophobic anion expansion disrupts efficient ion packing and electrostatic stabilization, leading to structural disorder and their liquid nature under ambient conditions. Density measurements showed an initial rise with PIL addition, followed by either saturation or a slight decrease at higher mole fractions, reflecting a transition from intense ion–solvent interactions to increased ion–ion association and self-aggregation in PIL-rich mixtures. The density systematically decreased with an increasing alkyl chain length of the carboxylate anion, confirming the role of hydrophobicity in reducing packing efficiency and solvent structuring. All of the mixtures exhibited negative excess molar volumes, confirming volume contraction and efficient packing upon mixing. Normalization of V ^E by the solvent molar volume (V ^E/V _m) showed that the more pronounced contraction in aqueous systems reflects both stronger interactions and the smaller molar volume of water relative to DMSO. Electrical conductivity and molar conductivity analyses demonstrated enhanced ionic dissociation and mobility in solvent-rich regions, followed by increased ion association at higher PIL contents. Water consistently promoted greater dissociation and charge transport than DMSO, while increasing anion hydrophobicity reduced solvation efficiency and ionic mobility in the order [EtA]­[Ace] > [EtA]­[Pro] > [EtA]­[But] > [EtA]­[Pen]. Overall, these results show that volumetric behavior and charge transport in PIL + solvent mixtures are governed by the combined effects of solvent hydrogen-bonding ability, solvent size, and anion hydrophobicity, providing valuable insight for tailoring PIL-based systems for separation, electrochemical, and green solvent applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c13309.

• Figure S1 to S4:¹H NMR spectra of synthesized PILs; Table S1: Comparison of density of molecular solvents; Table S2: Molar conductivities of PIL + mixture (PDF)

The authors declare no competing financial interest.