Physicochemical Properties and Structural Effects of 1‑Ethyl-4-Alkyl-1,2,4-Triazolium Acetylalaninate Ionic Liquids

Abstract

Two 1-ethyl-4-alkyl-1,2,4-triazolium acetylalaninate ionic liquids, [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala], were synthesized through sequential N-alkylation, anion exchange, and acid–base neutralization steps. The standard addition method was applied to correct for trace moisture interference, and anhydrous density, surface tension, and refractive index were measured across the temperature range of 288.15–323.15 K. Volumetric properties, surface thermodynamic parameters, molar refraction, and polarity scales were derived from experimental data using classical thermodynamic relationships. Comparison with acetylglycinate homologues and non-acetylated analogues indicates that elongation of the cationic alkyl chain generally decreases density, surface tension, refractive index, and polarity while increasing molecular volume and isobaric thermal expansion coefficient under the tested conditions. The additional methyl group on the alaninate anion correlates with reduced interionic cohesion and overall polarity whereas acetylation of the anion corresponds to a strengthened hydrogen-bonding network relative to unmodified alaninate ionic liquids. This study addresses the lack of systematic cationic chain length investigations for triazolium acetyl-amino acid ionic liquids, offering new dual cationic–anionic structure–property insights into acetylalaninate homologues beyond existing short-chain reports.

1. Introduction

Ionic liquids (ILs), commonly defined as molten salts with melting points below 373.15 K, have received extensive research attention as tunable functional materials, owing to their low volatility, structural versatility, and broad liquid-state temperature window. − Their use has expanded from green reaction media to thermal energy storage, gas separation, and electrochemical systems, creating sustained demand for accurate thermodynamic data and quantitative structure–property correlations. − Among available cationic frameworks, 1,2,4-triazolium-based ILs have emerged as promising alternatives to conventional imidazolium analogues. Imidazolium cations carry acidic C2 protons that can undergo carbene formation under alkaline conditions; by contrast, 1,2,4-triazolium cations exhibit improved chemical stability and structural robustness, which supports their use in systems requiring long-term operation under chemically demanding conditions. −

Amino acid-based ILs, which incorporate naturally derived amino acid anions, constitute a category of biocompatible ILs associated with low toxicity and inherent functional groups. , However, unmodified amino acid ILs typically exhibit limited thermal stability, which constrains their use in high-temperature applications. Acetylation of the amino group on amino acid anions can partially mitigate this limitation: the introduced amide moiety increases thermal decomposition temperature and introduces additional hydrogen-bonding sites, allowing for tuned physicochemical and solvation properties. In recent years, acetyl-amino acid ILs have drawn increasing attention as sustainable heat transfer fluids, owing to their favorable thermal stability, low corrosivity, and tunable thermophysical properties. Previous work has reported enhanced heat storage density and thermal conductivity for acetyl-amino acid ILs relative to their nonacetylated counterparts, supporting their potential in low-to-medium temperature thermal energy storage and circulation cooling systems. ,

To date, research on triazolium-based acetyl-amino acid ILs remains incomplete. Available reports mainly cover short-chain butyl homologues and anionic acetylation effects, , while cationic side-chain length regulation over a full set of basic physicochemical properties (density, surface tension, refractive index, and derived thermodynamic parameters) for long-chain pentyl/hexyl homologues has not been systematically established. Recent work on acetylglycinate triazolium ILs preliminarily revealed cationic chain length effects, but parallel data for acetylalaninate systems are still lacking, and the dual cationic–anionic structure–property rules remain to be consolidated. This data gap restricts targeted molecular design and process optimization of triazolium acetyl-amino acid ILs for practical applications.

Accurate measurement of fundamental properties, including density, surface tension, and refractive index, forms a basis for evaluating application performance and developing thermodynamic models. , For hydrophilic amino acid-derived ILs, uptake of trace moisture from the ambient environment can introduce measurable deviations in property measurements. The standard addition method has accordingly been developed as a validated protocol to correct for water-induced bias and extract intrinsic anhydrous property values. − Derived parameters, including volumetric, surface, and polarization properties calculated from experimental measurements, provide insight into interionic interactions, molecular packing behavior, and interfacial microstructures. These parameters reflect the relative strength of electrostatic, hydrogen-bonding, and van der Waals interactions within IL systems and also supply input data for process simulation and semi-empirical property prediction frameworks. −

Building on earlier investigations of acetylglycinate triazolium ILs, , the present work extends the study system to acetylalaninate homologues with pentyl and hexyl cationic side chains. Two target ILs were synthesized via a three-step synthetic route, and their anhydrous density, surface tension, and refractive index were determined across 288.15–323.15 K using the standard addition method. Derived thermodynamic parameters, including molar volume, isobaric thermal expansion coefficient, lattice energy, molar refraction, and polarity scales, were calculated using classical thermodynamic and semi-empirical equations. The effects of cationic alkyl chain length, anionic alkyl substitution, and anion acetylation on physicochemical properties were examined through comparison with acetylglycinate homologues and non-acetylated alaninate ILs. The results add to the thermodynamic database for triazolium ILs and offer quantitative reference for structural optimization of acetyl-amino acid IL systems for targeted applications.

2. Experimental Section

2.1. Reagents

The CAS registry numbers, purities, and suppliers of all reagents used in this study are listed in Table .

1. CAS Registry Number, Purity, and Source of the Reagents.

reagent	CAS Reg. No	purity	source
bromoethane	74-96-4	99%	Shanghai Macklin Biochemical Co., Ltd.
1-bromopentane	110-53-2	99%	Shanghai Macklin Biochemical Co., Ltd.
bromohexane	111-25-1	99%	Shanghai Macklin Biochemical Co., Ltd.
N-Acetyl- l -alanine	97-69-8	99%	Shanghai Macklin Biochemical Co., Ltd.
methanol	67-56-1	99%	Tianjin Fuyu chemical Co., Ltd
sodium methoxide	124-41-4	99%	Shanghai Macklin Biochemical Co., Ltd.
acetonitrile	75-05-8	99%	Tianjin Fuyu chemical Co., Ltd
dichloromethane	75-09-2	99%	Tianjin Fuyu chemical Co., Ltd
deionized water			manufactured by laboratory

2.2. Preparation

In this work, the ILs [Taz­(2,n)]­[Acala] (n = 5, 6) were prepared via a three-step route of N-alkylation, anion exchange, and acid–base neutralization, following a previously reported protocol for triazolium amino acid ILs. , Identical reaction conditions were used for both homologues to ensure structural consistency.

First, the 1-ethyl-1,2,4-triazole intermediate was synthesized by selective monoalkylation of 1,2,4-triazole. A mixture of 1,2,4-triazole, sodium methoxide, and methanol (molar ratio 1:1:1) was stirred at 25 °C to form a homogeneous solution, then bromoethane was added dropwise and refluxed at 65 °C for 48 h. After the reaction, volatile impurities and unreacted raw materials were removed by vacuum distillation to obtain purified 1-ethyl-1,2,4-triazole. Subsequently, the intermediate underwent a second N-alkylation with 1-bromopentane and 1-bromohexane (molar ratio 1:1.2) to yield [Taz­(2,5)]­[Br] and [Taz­(2,6)]­[Br] precursors, respectively. For anion conversion, bromide precursors were eluted through a pretreated anion exchange resin column to fully replace Br^– with OH^–. The obtained triazolium hydroxide solutions were mixed with equimolar N-acetyl- l -alanine and stirred at 25 °C for 48 h for neutralization. Crude products were purified by liquid–liquid extraction with a dichloromethane/acetonitrile mixed solvent. Finally, all samples were dried under vacuum at 55 °C for 48 h to remove residual solvent and adsorbed water, yielding homogeneous pale yellow liquids with high purity. The overall isolated yields after the three-step synthesis were approximately 73% for [Taz­(2,5)]­[Acala] and 69% for [Taz­(2,6)]­[Acala]. The synthetic route is shown in Figure .

1.

Synthesis of the novel ILs [Taz­(2,n)]­[Acala] (n = 5, 6).

2.3. Characterization

The ILs [Taz­(2,n)]­[Acala] (n = 5, 6) were characterized by ¹H NMR, ¹³C NMR, FT-IR spectroscopy, and thermogravimetric analysis (TGA) (Figures S1–S8 and Table S1). The water content was determined by a Karl Fischer moisture titrator (Model ZSD-2).

The NMR spectra showed clean baselines without impurity peaks, indicating high purity. For both ILs, downfield signals at δ_H = 9.32–10.82 corresponded to triazolium ring protons, and carbon signals at δ_C = 143 – 145 confirmed the triazolium cation skeleton. Carbonyl carbon signals at δ_C = 168 – 176 were attributed to the acetylalaninate anion, verifying successful ion combination. The extra carbon signal at δ_C = 25.36 for [Taz­(2,6)]­[Acala] arose from the additional hexyl methylene group, consistent with their homologous structures.

The FT-IR spectra confirmed functional group integrity. Broad bands at 3279–3418 cm^–1 were assigned to overlapping O–H and N–H stretching of [Acala]^‑, indicating inherent hygroscopicity, with weak water-induced pseudobands near 650 cm^–1. Absorption peaks at 1651–1655 cm^–1 originated from coupled CO stretching, triazolium skeleton vibration, and N–H in-plane bending. Saturated C–H stretching of alkyl chains appeared at 2877–2965 cm^–1. Characteristic R–CH vibration at 1394–1399 cm^–1 and C–C–H/N–C–H bending at 1163 cm^–1 further confirmed intact amino acid functional groups.

The TGA results showed favorable thermal stability: [Taz­(2,5)]­[Acala] was stable below 178.49 °C, and [Taz­(2,6)]­[Acala] maintained integrity up to 177.56 °C. The slight decrease in decomposition temperature for the hexyl homologue was attributed to longer alkyl chains weakening intermolecular interactions. No obvious low-temperature weight loss occurred, indicating negligible volatile impurities. Combined with clean spectral features, both ILs were confirmed to have sufficient purity for subsequent experiments.

2.4. Determination of Physicochemical Properties

Owing to the polar amino and carboxyl moieties on the acetylalaninate anion, the as-prepared ILs [Taz­(2,n)]­[Acala] (n = 5, 6) possess inherent hygroscopicity and readily form hydrogen bonds with trace moisture, rendering complete water removal via conventional vacuum drying impractical. To eliminate the systematic deviation caused by residual water, the standard addition method (SAM) was adopted for property quantification. − Five parallel samples with stepwise increasing water mass fractions (ranging from ∼5 × 10^–3 to ∼15 × 10^–3) were prepared by gravimetric addition of ultrapure water. Each sample was sealed and equilibrated for at least 12 h under constant temperature to ensure uniform water distribution before measurement. The density (ρ), surface tension (γ), and refractive index (n _D) of IL samples with stepwise increasing water contents were measured at 5 K increments across the temperature range of 288.15–323.15 K. For all temperature points, the measured property showed a linear correlation with water mass fraction (r ² > 0.99 for all datasets). The property values of anhydrous ILs were extrapolated from the intercept of linear fitting between each parameter and water mass fraction.

The density tests were performed on an Anton Paar DMA 5000M vibrating-tube densimeter. The surface tension data were collected by using a DP-AW maximum bubble pressure tensiometer (Nanjing Sangli, China). The refractive index measurements were conducted with a WZS-1 Abbe refractometer at the sodium D line wavelength. All sample cells were fitted with circulating thermostatic jackets to maintain a precise temperature control throughout the measurement process. Prior to formal testing, all instruments were calibrated following standard protocols: ultrapure water and dry air served as reference materials for densimeter calibration, while certified glass blocks were used for refractometer verification. ,

Each reported value corresponds to the average of three parallel measurements. The standard uncertainties were estimated as 0.05 kg·m^–3 for density, 0.0001 N·m^–1 for surface tension, and 0.0016 for refractive index. The corresponding expanded uncertainties at the 95% confidence level (coverage factor k ≈ 2) were U(ρ) = 0.10 kg·m^–3, U(γ) = 0.0002 N·m^–1, and U(n _D) = 0.0032.

3. Results and Discussion

3.1. Density and Volumetric Properties

The density values of [Taz­(2,n)]­[Acala] (n = 5, 6) extrapolated via the standard addition method are summarized in Table . At all tested temperatures from 288.15 to 323.15 K, the density of both samples exhibits an excellent linear correlation with water mass fraction (r ² > 0.99), verifying that the standard addition method can effectively eliminate systematic deviation caused by trace moisture in this hydrophilic acetyl-amino acid IL system. The intercepts of these linear fits provided the experimental density values for anhydrous ILs, depicted in Figure .

2. Density (ρ / kg·m^–3) Values of ILs [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala] Containing Various Mass Fractions of Water (w ₂) at T = (288.15–323.15) K .

[Taz(2,5)][Acala]
T / K	103 w 2 = 5.35	103 w 2 = 7.58	103 w 2 = 9.82	103 w 2 = 12.18	103 w 2 = 15.02	103 w 2 = 0	r	sd × 10–6
288.15	1109.302	1109.279	1109.254	1109.235	1109.196	1109.35	0.999	1.89
293.15	1105.793	1105.771	1105.748	1105.720	1105.684	1105.84	0.996	3.89
298.15	1102.282	1102.254	1102.222	1102.189	1102.149	1102.41	0.999	1.22
303.15	1098.764	1098.727	1098.691	1098.650	1098.601	1098.72	0.999	4.54
308.15	1095.241	1095.183	1095.122	1095.072	1095.002	1095.37	0.999	6.29
313.15	1091.661	1091.569	1091.460	1091.351	1091.204	1091.92	0.999	7.16
318.15	1088.160	1088.112	1088.063	1088.012	1087.948	1088.28	0.999	2.00
323.15	1084.636	1084.584	1084.531	1084.475	1084.407	1084.77	0.998	5.84
[Taz(2,6)][Acala]
T / K	103 w 2 = 5.89	103 w 2 = 8.29	103 w 2 = 10.25	103 w 2 = 12.68	103 w 2 = 14.91	103 w 2 = 0	r	sd × 10–5
288.15	1088.381	1088.319	1088.234	1088.170	1088.089	1088.58	0.994	1.50
293.15	1084.936	1084.872	1084.798	1084.737	1084.655	1085.12	0.995	1.21
298.15	1081.477	1081.417	1081.348	1081.288	1081.213	1081.65	0.996	1.02
303.15	1078.015	1077.960	1077.895	1077.840	1077.769	1078.18	0.995	1.04
308.15	1074.552	1074.458	1074.468	1074.421	1074.375	1074.66	0.997	0.53
313.15	1071.083	1071.038	1070.981	1070.937	1070.875	1071.22	0.993	1.07
318.15	1067.551	1067.516	1067.487	1067.454	1067.422	1067.63	0.999	0.13
323.15	1064.079	1064.049	1064.029	1064.001	1063.978	1064.14	0.999	0.24

^aThe standard uncertainties u for temperature and pressure are u(T) = 0.01 K and u(p) = 0.01 MPa, respectively. The combined expanded uncertainty for density and surface tension is U (ρ) = 0.10 kg·m^–3 with a 0.95 level of confidence (k ≈ 2).

2.

Plot of density (ρ) of anhydrous ILs [Taz­(2,n)]­[Acala] (n = 4, 5, 6), [Taz­(2,5)]­[Ala], and [Taz­(2,n)]­[Acgly] (n = 5, 6)¹⁷ versus temperature (T). [Taz­(2,5)]­[Acala]: ρ = 1311.53–0.70162 T, r ² = 0.99, sd = 0.69; [Taz­(2,6)]­[Acala]: ρ = 1289.87–0.69843 T, r ² = 0.99, sd = 0.38.

As shown in Figure , the density of both ILs decreases monotonically with increasing temperature. This trend can be attributed primarily to intensified thermal motion of ion pairs, which widens interionic spacing and reduces molecular packing efficiency. To clarify structure–property correlations, three reference systems were compared: [Taz­(2,4)]­[Acala] with shorter cationic chains, [Taz­(2,n)]­[Acgly] series with shorter anionic alkyl chains, and [Taz­(2,5)]­[Ala] with non-acetylated anions. At 298.15 K, the density follows the order [Taz­(2,4)]­[Acala] > [Taz­(2,5)]­[Acala] > [Taz­(2,6)]­[Acala]. This descending trend of density with increasing cationic alkyl chain length is consistent with the trend observed in acetylglycinate analogues. , Each additional cationic methylene group increases ionic volume, loosens packing arrangements, and weakens electrostatic interactions, thus lowering density. For anionic effects, [Taz­(2,5)]­[Acala] has slightly lower density than [Taz­(2,5)]­[Acgly] due to the extra anionic methylene but higher density than [Taz­(2,5)]­[Ala]. This dual effect arises because the acetyl group strengthens interionic hydrogen bonding to improve packing efficiency, while anionic alkyl extension causes volume expansion that partially offsets densification.

Based on the experimental density data, the molar volume (V) and molecular volume (V _m, defined as the volume occupied by a single cation–anion pair) were calculated by eq and eq , respectively. ,

$$V=\frac{M}{\rho }$$ 1

$$V_{\mathrm{m}}=\frac{V}{N}=\frac{M}{\rho N}$$ 2

where M is the molar mass of the IL, ρ is the experimental density of the anhydrous sample, and N is the Avogadro constant. The results of V and V _m at 298.15 K are listed in Table .

3. Values of Density (ρ), Molar Volume­(V), Molecular Volume (V _m), Isobaric Thermal Expansion Coefficient (α_p), Standard Entropy (S ⁰), and Lattice Energy (U _POT) for ILs [Taz­(2,n)]­[Acala] (n = 4, 5, 6), [Taz­(2,5)]­[Ala], and [Taz­(2,n)]­[Acgly] (n = 5, 6) at 298.15 K ^{, ,}

ILs	ρ/kg·m–3	104 V /m3·mol–1	V m /nm	104αp / K–1	S 0 /J·K–1·mol–1	U POT /kJ·mol–1
a [Taz(2,4)][Acala]	1115.96	2.549	0.4232	6.25	557	416
[Taz(2,5)][Acala]	1102.41	2.701	0.4495	6.40	590	410
[Taz(2,6)][Acala]	1081.65	2.888	0.4796	6.49	627	404
b [Taz(2,5)][Ala]	1054.20	2.296	0.3814	6.74	505	427
c [Taz(2,5)][Acgly]	1113.32	2.554	0.4241	6.17	558	416
c [Taz(2,6)][Acgly]	1100.27	2.712	0.4503	6.27	591	410

^{a , ,} Derived from previous works, ,, respectively.

^bThe combined expanded uncertainties (k ≈ 2, 0.95 confidence level) for derived volumetric properties are U(V _m) = 4 × 10^–5 nm.

^c U(V) = 2 × 10^–8 m³·mol^–1, U(α_p) = 8 × 10^–6 K^–1, U(S ⁰) = 0.05 J·K^–1·mol^–1, and U(U _POT) = 0.01 kJ·mol^–1.

The calculation results reveal that molecular volume increases linearly with the number of carbon atoms in the cationic side chain. For the [Taz­(2,n)]­[Acala] homologous series, the average volume contribution per additional –CH₂– group on the cation is 0.0282 nm³, comparable to the 0.0289 nm³ average value derived from the [Taz­(2,n)]­[Acgly] series. This value also matches well with the methylene contribution on the acetyl-amino acid anion and the average of 0.0277 nm³ reported for 79 imidazolium-based ILs. , These findings indicate that the volume increment of methylene groups is barely affected by their location (cation or anion) or the cationic backbone type, providing a practical reference for estimating molecular volume of triazolium IL homologues. Relative to [Taz­(2,5)]­[Ala], [Taz­(2,5)]­[Acala] possesses a larger molecular volume due to the steric effect of the acetyl moiety, despite the denser packing induced by enhanced hydrogen bonding.

According to the thermodynamic definition, the isobaric thermal expansion coefficient (α_p) is expressed as eq :

$${\alpha }_{\mathrm{p}}=\frac{1}{V}{\left(\frac{\partial V}{\partial T}\right)}_P=-\frac{1}{\rho }{\left(\frac{\partial \rho }{\partial T}\right)}_P=-{\left(\frac{\partial \ln \rho }{\partial T}\right)}_P$$ 3

The absolute value of the slope from linear fitting of ln ρ against thermodynamic temperature T corresponds to α_p, as compiled in Table and Figure . The obtained α_p values for [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala] fall in the range of 6.40 × 10^–4 to 6.49 × 10^–4 K^–1, slightly higher than those of the [Taz­(2,n)]­[Acgly] series, and lie between those of aqueous inorganic salt solutions and common organic molecular solvents. Comparative analysis shows that α_p increases slightly with cationic alkyl chain elongation, as longer flexible alkyl chains enhance the thermal expansibility of the system, a trend also observed in acetylglycinate analogues. , Meanwhile, the α_p values of acetylated ILs are lower than those of the corresponding non-acetylated [Taz­(2,n)]­[Ala] homologues, which can be ascribed to the stronger hydrogen-bonding network formed by acetyl groups that restricts thermal volume expansion.

3.

Plot of lnρ of anhydrous ILs [Taz­(2,n)]­[Acala] (n = 4, 5, 6), [Taz­(2,5)]­[Ala], and [Taz­(2,n)]­[Acgly] (n = 5, 6)¹⁷ versus temperature (T ). [Taz­(2,5)]­[Acala]: lnρ = 7.1959–6.40 × 10^–4 T, r ² = 0.999, sd = 2.38 × 10^–6; [Taz­(2,6)]­[Acala]: lnρ= 7.1797–6.49 × 10^–4 T, r ² = 0.999, sd = 1.91 × 10^–6.

On the basis of Glasser’s theory, the standard entropy (S ⁰) and lattice energy (U _POT) of [Taz­(2,n)]­[Acala] (n = 5, 6) at 298.15 K were estimated from molecular volume data, as formulated in eqs and :

$$S^{°}=1246.5V_{\mathrm{m}}+29.5$$ 4

$$U_{\mathrm{P}\mathrm{O}\mathrm{T}}=1981.2V^{-1/3}+103.8$$ 5

The calculated results are summarized in Table , alongside reference data for [Taz­(2,4)]­[Acala], [Taz­(2,5)]­[Ala], and [Taz­(2,n)]­[Acgly] (n = 5, 6) from earlier reported work. ,,

For the [Taz­(2,n)]­[Acala] homologous series, the S ⁰ increases linearly from 557 to 627 J·K^–1·mol^–1 as the cationic side chain extends from butyl to hexyl. The average contribution of each additional –CH₂– group to S ⁰ is calculated to be 35 J·K^–1·mol^–1, close to the 36 J·K^–1·mol^–1 value obtained from the [Taz­(2,n)]­[Acgly] series and previous anionic methylene data. , This consistency confirms that the entropy gain from methylene insertion is largely independent of its location on the cation or anion. The continuous rise in S ⁰ indicates that longer alkyl side chains increase conformational freedom and reduce structural ordering in the IL bulk.

In contrast, the U _POT shows a monotonic decreasing trend with increasing side-chain length, dropping from 416 kJ·mol^–1 for [Taz­(2,4)]­[Acala] to 404 kJ·mol^–1 for [Taz­(2,6)]­[Acala]. Each methylene group reduces the lattice energy by approximately 6 kJ·mol^–1, which is comparable to the 6 – 8 kJ·mol^–1 range observed in the acetylglycinate series and conventional imidazolium ILs. , The relatively low lattice energy compared with inorganic molten salts accounts for the room-temperature liquid state of these title ILs.

Notably, the [Taz­(2,5)]­[Acala] shows a higher S ⁰ but lower U _POT than both [Taz­(2,5)]­[Acgly] and the non-acetylated [Taz­(2,5)]­[Ala]. , This phenomenon arises from combined volume and interaction effects: the acetyl group and anionic methylene both enlarge ionic volume and increase conformational disorder, which dominates entropy increase and lattice energy reduction; while the introduced amide moiety adds hydrogen-bonding sites to strengthen interionic cohesion, this effect is insufficient to offset the lattice energy decline from volume expansion. This volume-dominated variation aligns well with the core assumption of Glasser’s theory, which correlates lattice energy primarily with ionic volume.

3.2. Surface Tension and Surface Properties

The surface tension (γ) values of [Taz­(2,n)]­[Acala] (n = 5, 6), extrapolated from standard addition method measurements, are compiled in Table . Over the temperature range of 288.15–323.15 K, the γ exhibits an excellent linear dependence on water mass fraction for both homologues (r ² > 0.99), corroborating that the standard addition method reliably eliminates systematic bias from residual moisture in these hydrophilic acetyl-amino acid ILs. The intercepts of these linear fits provided the experimental γ values for anhydrous ILs, depicted in Figure . With rising temperature, the γ of both samples decreases monotonically. This behavior arises from thermally intensified molecular motion that weakens cohesive forces between interfacial ion pairs, thus lowering the surface free energy per unit area. The temperature-dependent trend is fully consistent with those reported for [Taz­(2,n)]­[Ala] alanine-based and [Taz­(2,n)]­[Acgly] acetylglycinate triazolium ILs. ,

4. Surface Tension (10³ γ / N·m^–1) Values of ILs [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala] Containing Various Mass Fractions of Water (w ₂) at T = (288.15–323.15) K .

[Taz(2,5)][Acala]
T / K	103 w 2 = 5.35	103 w 2 = 7.58	103 w 2 = 9.82	103 w 2 = 12.18	103 w 2 = 15.02	103 w 2 = 0	r	sd × 10–2
288.15	44.5	44.9	45.2	45.5	45.9	43.9	0.999	1.23
293.15	44.2	44.6	44.9	45.2	45.6	43.6	0.999	1.23
298.15	43.9	44.3	44.6	44.9	45.3	43.3	0.999	1.23
303.15	43.6	44.0	44.3	44.6	45.0	43.0	0.999	1.23
308.15	43.2	43.6	43.9	44.2	44.6	42.6	0.999	1.23
313.15	42.8	43.2	43.5	43.8	44.2	42.2	0.999	1.23
318.15	42.4	42.8	43.1	43.4	43.8	41.8	0.999	1.23
323.15	42.1	42.5	42.8	43.1	43.5	41.5	0.999	1.23
[Taz(2,6)][Acala]
T / K	103 w 2 = 5.89	103 w 2 = 8.29	103 w 2 = 10.25	103 w 2 = 12.68	103 w 2 = 14.91	103 w 2 = 0	r	sd × 10–2
288.15	41.3	41.6	41.8	42.1	42.3	40.6	0.998	3.05
293.15	41.0	41.3	41.5	41.8	42.1	40.3	0.998	3.05
298.15	40.7	41.0	41.2	41.5	41.8	40.0	0.998	3.05
303.15	40.4	40.7	40.9	41.2	41.5	39.7	0.998	3.05
308.15	40.0	40.3	40.5	40.8	41.1	39.3	0.998	3.05
313.15	39.6	39.9	40.1	40.4	40.7	38.9	0.998	3.05
318.15	39.2	39.5	39.7	40.0	40.3	38.5	0.998	3.05
323.15	38.9	39.2	39.4	39.7	40.0	38.2	0.998	3.05

^aThe standard uncertainties u for temperature and pressure are u(T) = 0.01 K, u(p) = 0.01 MPa, respectively. The combined expanded uncertainty for density and surface tension is U(γ) = 0.2 × 10^–3 N·m^–1 with a 0.95 level of confidence (k ≈ 2).

4.

Plot of surface tension (γ) of anhydrous ILs [Taz­(2,n)]­[Acala] (n = 4, 5, 6), [Taz­(2,5)]­[Ala], and [Taz­(2,n)]­[Acgly] (n = 5, 6) versus temperature (T). [Taz­(2,5)]­[Acala]: γ = 64.2–7.024 × 10^–2 T, r ² = 0.996, sd = 1.72 × 10^–3; [Taz­(2,6)]­[Acala]: γ = 60.9–7.024 × 10^–2 T, r ² = 0.996, sd = 1.72 × 10^–3.

At 298.15 K, the γ follows the order [Taz­(2,4)]­[Acala] > [Taz­(2,5)]­[Acala] > [Taz­(2,6)]­[Acala], mirroring the descending trend in acetylglycinate analogues. , Each additional cationic methylene enhances the hydrophobicity of the cationic moiety. From a microscopic interfacial perspective, flexible long alkyl chains tend to orient toward the gas phase and preferentially accumulate at the gas–liquid interface, while charged triazolium backbones remain more distributed in the bulk phase. , This surface enrichment of alkyl groups dilutes charged moieties at the interface and attenuates electrostatic cohesion between surface ion pairs, leading to a gradual decrease in surface tension with increasing chain length. This interfacial segregation behavior has been directly verified by neutron reflectivity experiments and molecular dynamics simulations for multiple IL families, , supporting the variation trend observed in this work. For anionic variation, the [Taz­(2,5)]­[Acala] shows slightly lower surface tension than [Taz­(2,5)]­[Acgly] due to the extra anionic methyl group but remains higher than its non-acetylated counterpart [Taz­(2,5)]­[Ala]. This dual effect indicates that acetylation strengthens the interfacial hydrogen-bonding network to elevate cohesive energy while anionic alkyl extension adds hydrophobic character to partially reduce surface tension, aligning with previous observations on acetyl-amino acid ILs. ,,

The surface entropy (S _a) and surface energy (E _a) are two key surface thermodynamic quantities: the former describes the temperature response of surface tension, while the latter characterizes the intermolecular interaction strength at liquid surfaces. Both quantities can be quantified using the following equation:

$$E_{\mathrm{a}}=\gamma -T{\left(\frac{\partial \gamma }{\partial T}\right)}_P=\gamma +S_{\mathrm{a}}T$$ 6

The calculated S _a and E _a values at 298.15 K are presented in Table . The S _a values of [Taz­(2,n)]­[Acala] are around 70.2 mJ·K^–1·m^–2 for n = 5 and 6, highly consistent with those of the [Taz­(2,n)]­[Acgly] series and non-acetylated analogues, indicating that the temperature sensitivity of surface tension is barely affected by side-chain length or acetylation within this IL family. , In contrast, the E _a shows pronounced structural dependence: it declines with increasing cationic alkyl chain length and also decreases slightly from acetylglycinate to acetylalaninate anions, while anion acetylation causes a distinct increase relative to non-acetylated ILs. , This variation matches the trend of bulk intermolecular interactions reflected by lattice energy, confirming that surface properties are intrinsically governed by interionic force strength. Furthermore, the E _a remains nearly constant across the tested temperature range, attributed to the compensatory effect of surface entropy on temperature-induced energy changes.

5. Values of the Surface Entropy (S _a) and Surface Energy (E _a) of ILs [Taz­(2,n)]­[Acala] (n = 5, 6) in the Range of T = (288.15–323.15) K .

T / K	[Taz(2,5)][Acala]		[Taz(2,6)][Acala]
	S a / mJ·K–1·m–2	E a / mJ·m–2	S a / mJ·K–1·m–2	E a / mJ·m–2
288.15	70.2	64.1	70.2	60.8
293.15	70.2	64.2	70.2	60.9
298.15	70.2	64.2	70.2	60.9
303.15	70.2	64.3	70.2	61.0
308.15	70.2	64.2	70.2	60.9
313.15	70.2	64.2	70.2	60.9
318.15	70.2	64.1	70.2	60.8
323.15	70.2	64.2	70.2	60.9

^aThe combined expanded uncertainties (k ≈ 2, 0.95 confidence level) for derived surface properties are U(S _a) = 2.0 mJ·K^–1·m^–2 and U(E _a) = 0.3 mJ·m^–2.

3.3. Refractive Index and Polarization Properties

The refractive index (n _D) is a fundamental optical property tightly linked to molecular polarizability and molecular packing density of liquid materials. The n _D values of [Taz­(2,n)]­[Acala] (n = 5, 6), extrapolated via the standard addition method at the sodium d-line wavelength, are summarized in Table . Consistent with ρ and γ, the n _D shows a linear negative correlation with water mass fraction at each temperature (r ² > 0.99), verifying the suitability of the extrapolation protocol for this property in hydrophilic ILs. The intercepts of these linear fits provided the experimental n _D values for anhydrous ILs, depicted in Figure .

6. Refractive Index (n _D) Values of ILs [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala] Containing Various Mass Fractions of Water (w ₂) at T = (288.15–323.15) K .

[Taz(2,5)][Acala]
T / K	103 w 2 = 5.35	103 w 2 = 7.58	103 w 2 = 9.82	103 w 2 = 12.18	103 w 2 = 15.02	103 w 2 = 0	r	sd × 10–5
288.15	1.4869	1.4864	1.4861	1.4856	1.4852	1.4880	0.998	4.58
293.15	1.4859	1.4854	1.4850	1.4845	1.4840	1.4871	0.999	2.24
298.15	1.4845	1.4840	1.4835	1.4830	1.4824	1.4859	0.997	6.48
303.15	1.4832	1.4827	1.4823	1.4818	1.4812	1.4845	0.996	7.79
308.15	1.4820	1.4815	1.4811	1.4807	1.4801	1.4832	0.993	9.28
313.15	1.4808	1.4803	1.4799	1.4795	1.4791	1.4819	0.997	5.90
318.15	1.4799	1.4795	1.4791	1.4787	1.4782	1.4810	0.996	6.10
323.15	1.4787	1.4783	1.4779	1.4775	1.4770	1.4798	0.997	5.10
[Taz(2,6)][Acala]
T / K	103 w 2 = 5.89	103 w 2 = 8.29	103 w 2 = 10.25	103 w 2 = 12.68	103 w 2 = 14.91	103 w 2 = 0	r	sd × 10–2
288.15	1.4845	1.4838	1.4832	1.4825	1.4816	1.4861	0.999	4.93
293.15	1.4831	1.4825	1.4819	1.4812	1.4804	1.4846	0.999	3.24
298.15	1.4818	1.4812	1.4806	1.4801	1.4793	1.4832	0.999	6.27
303.15	1.4808	1.4803	1.4797	1.4791	1.4784	1.4822	0.999	3.55
308.15	1.4797	1.4792	1.4786	1.4780	1.4773	1.4811	0.999	3.55
313.15	1.4785	1.4780	1.4775	1.4769	1.4763	1.4797	0.999	3.70
318.15	1.4775	1.4770	1.4765	1.4759	1.4753	1.4787	0.999	3.70
323.15	1.4763	1.4758	1.4754	1.4749	1.4743	1.4775	0.999	3.62

^aThe standard uncertainties u for temperature and pressure are u(T) = 0.01 K and u(p) = 0.01 MPa, respectively. The combined expanded uncertainty for density and surface tension is U(n _D) = 0.0032 with a 0.95 level of confidence (k ≈ 2).

5.

Plot of refractive index (n _D) of anhydrous ILs [Taz­(2,n)]­[Acala] (n = 4, 5, 6) and [Taz­(2,n)]­[Acgly] (n = 5, 6) versus temperature (T). [Taz­(2,5)]­[Acala]: n _D = 1.5576–2.4095 × 10^–4 T, r ² = 0.997, sd = 1.51 × 10^–3; [Taz­(2,6)]­[Acala]: n _D = 1.5554–2.4119 × 10^–4 T, r ² = 0.997, sd = 1.53 × 10^–3.

As the temperature increases from 288.15 to 323.15 K, the refractive index of both samples decreases linearly. This phenomenon stems from the thermal expansion of the liquid: elevated temperature widens intermolecular spacing and reduces polarizable units per unit volume, lowering the bulk refractive index. Comparative analysis across reference ILs reveals well-defined structural effects. At 298.15 K, the n _D follows the sequence [Taz­(2,4)]­[Acala] > [Taz­(2,5)]­[Acala] > [Taz­(2,6)]­[Acala], in line with the trend observed in the acetylglycinate series. , Each cationic methylene reduces the refractive index, as volume expansion from alkyl chain elongation outweighs the incremental gain in molecular polarizability, leading to a net decrease in per-unit-volume polarizability. Notably, [Taz­(2,n)]­[Acala] exhibits slightly higher n _D than [Taz­(2,n)]­[Acgly] at the same cation length, as the anionic methylene adds more polarizability than the volume increase it introduces.

Based on the Lorentz–Lorenz relation, which bridges the macroscopic refractive index and microscopic molecular polarizability, the molar refraction (R _m) and molar polarizability (α_m) are derived as follows:

$$R_{\mathrm{m}}=\frac{{n_{\mathrm{D}}}^2-1}{{n_{\mathrm{D}}}^2+2}V=\frac{4\pi N}{3}{\alpha }_{\mathrm{m}}$$ 7

where V is the molar volume of the IL and N is the Avogadro constant.

The calculated R _m and α_m values at 298.15 K are listed in Table . Both parameters show negligible temperature dependence across the tested range, confirming that molar refraction is an intrinsic property characterizing induced polarization capacity of IL molecules. With cationic alkyl chain elongation, both R _m and α_m increase linearly, as each methylene contributes a fixed increment to total molecular polarizability, a pattern also observed in acetylglycinate homologues. , These polarization parameters reflect the electronic structural characteristics of the ILs and provide a theoretical basis for predicting surface tension, solubility, and other properties via semi-empirical models.

7. Values of the Molar Polarization (R _m) and Molar Polarizability (α_m) of ILs [Taz­(2,n)]­[Acala] (n = 5, 6) in the Range of T = (288.15–323.15) K .

T / K	[Taz(2,5)][Acala]		[Taz(2,6)][Acala]
	R m / cm3·mol–1	1024αm / cm3	R m / cm3·mol–1	1024αm / cm3
288.15	77.24	30.62	82.68	32.78
293.15	77.29	30.64	82.82	32.83
298.15	77.33	30.66	82.91	32.87
303.15	77.45	30.7	82.97	32.89
308.15	77.54	30.74	83.05	32.92
313.15	77.59	30.76	83.13	32.95
318.15	77.71	30.81	83.27	33.01
323.15	77.79	30.84	83.37	33.05

^aThe combined expanded uncertainties (k ≈ 2, 0.95 confidence level) for derived polarization properties are U(R _m) = 0.4 cm³·mol^–1 and U(α_m) = 2 × 10^–25 cm³.

3.4. Polarity and Solvation Characteristics

The polarity is a critical property that determines the solvation capacity and application scope of ILs. To accurately characterize the polarity of the title systems, two complementary scales were employed: the new polarity scale (δ_μ) and the polarity coefficient (P), both derived from the partition of molar vaporization enthalpy into nonpolar and polar contributions. ,

According to the Lawson–Ingham relation, the nonpolar contribution to vaporization enthalpy (Δ_g H _m,n ), originating from induced dipole interactions, can be estimated from refractive index and molar volume:

$${\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mathrm{n}}^0=C\left(\frac{{n_{\mathrm{D}}}^2-1}{{n_{\mathrm{D}}}^2+2}\right)V$$ 8

where C = 1.297 kJ·cm^–3 is an empirical constant for organic liquids. The polar contribution (Δ_g H _m,μ ) from permanent dipole interactions is then obtained by subtracting the nonpolar part from the total vaporization enthalpy (Δ_g H _m ):

$${\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mu }^0={\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m}}^0-{\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mathrm{n}}^0$$ 9

And the values of Δ_g H _m can be estimated by using the Kabo empirical equation:

$${\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m}}^0=11.21(\gamma V^{2/3}{N}^{1/3})+2.4$$ 10

The Kabo equation correlates molar vaporization enthalpy with surface tension and molar volume and has been widely validated in various polar IL systems including amino acid-based and imidazolium ILs. ,, Its reliability for hydrophilic acetyl-amino acid ILs has also been confirmed in previous triazolium IL studies, , ensuring the validity of polarity calculation in this work.

The new polarity scale (δ_μ), defined as the square root of polar vaporization enthalpy per unit volume, is calculated by:

$${\delta }_{\mu }=\sqrt{\frac{{\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mu }^0({\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m}}^0-RT)}{{\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m}}^{0}V}}$$ 11

To further eliminate volume effects and reflect the intrinsic polarity of ionic species, the polarity coefficient P was introduced as the ratio of polar to nonpolar contributions:

$$P=\sqrt{\frac{{\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mu }^0}{{\mathrm{\Delta }}_{\mathrm{g}}^{\mathrm{l}}{H}_{\mathrm{m},\mathrm{n}}^0}}$$ 12

The calculated polarity parameters at 298.15 K are listed in Table . Both δ_μ and P consistently show that polarity decreases with elongation of the cationic alkyl side chain for the [Taz­(2,n)]­[Acala] homologous series. Each additional methylene group increases the hydrophobic alkyl moiety proportion and dilutes the charged polar groups, thus reducing the overall polarity of the system. For anionic structural effects, the [Taz­(2,5)]­[Acala] exhibits lower polarity than both [Taz­(2,5)]­[Acgly] and non-acetylated [Taz­(2,5)]­[Ala]. , The extra anionic methyl group enhances nonpolar contribution and weakens permanent dipole proportion, while acetyl substitution replaces the highly polar primary amino group with an amide moiety, both driving down overall polarity. This trend aligns with the hydrophobic enhancement effect of alkyl extension and acetylation reported in triazolium IL studies. , The polarity values of the title ILs are comparable to common imidazolium ILs such as [C₃mim]­[Thr] (δ_μ ≈ 16.37 J^1/2·cm^–3/2), indicating favorable solvation capacity for both polar and moderately nonpolar substances.

8. Values of Vaporization Enthalpy (Δ_g H _m ), the Contribution Part from the Induced Dipole Moment to Vaporization Enthalpy (Δ_g H _m,n ), the Contribution Part from the Average Permanent Dipole Moment to Vaporization Enthalpy (Δ_g H _m,μ ), New Scale of Polarity (δ_μ), and Polarity Coefficient (P) for [Taz­(2,n)]­[Acala] (n = 4, 5, 6) and [Taz­(2,n)]­[Acgly] (n = 5, 6) at T = 298.15 K ^,

ILs	Δg H m / kJ·mol–1	Δg H m,n / kJ·mol–1	Δg H m,μ / kJ·mol–1	δμ / J1/2·cm– 3/2	P
[Taz(2,4)][Acala]	176.3	95.72	80.55	17.65	0.917
[Taz(2,5)][Acala]	173.8	100.08	73.72	16.40	0.858
[Taz(2,6)][Acala]	167.9	107.52	60.34	14.35	0.749
[Taz(2,5)][Acgly]	166.0	94.44	71.52	16.16	0.870
[Taz(2,6)][Acgly]	165.1	99.71	65.35	15.41	0.810

^{a ,} Derived from previous works, , respectively.

^bThe combined expanded uncertainties (k ≈ 2, 0.95 confidence level) for derived polarity properties are U(Δ_g H _m ) = 0.8 kJ·mol^–1, U(Δ_g H _m,n ) = 0.5 kJ·mol^–1, U(Δ_g H _m,μ ) = 1.0 kJ·mol^–1, U(δ_μ) = 0.12 J^1/2·cm^–3/2, and U(P) = 0.007.

These structure–property relationships provide practical guidance for targeted molecular design of triazolium acetyl-amino acid ILs. For heat transfer fluid applications, moderate elongation of the cationic alkyl chain can reduce density and surface tension to improve fluid flow performance while anion acetylation maintains a strong hydrogen-bonding network to support thermal stability. For liquid–liquid extraction and reaction media applications, system polarity can be finely tuned by adjusting cationic side-chain length to match the polarity of target solutes or reactants, enabling optimized solvation and separation efficiency.

4. Conclusions

In this work, two long-chain 1-ethyl-4-alkyl-1,2,4-triazolium acetylalaninate ionic liquids (ILs) ([Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala]) were synthesized and systematically characterized for anhydrous density, surface tension, and refractive index across 288.15–323.15 K using the standard addition method. Key volumetric, surface, polarization, and polarity parameters were derived via classical thermodynamic relationships. Structural analysis shows that cationic alkyl chain elongation reduces density, surface tension, refractive index, and overall polarity while increasing molecular volume and thermal expansion coefficient. Anionic methyl substitution weakens interionic cohesion and lowers polarity whereas anion acetylation strengthens the hydrogen-bonding network and improves thermal stability relative to non-acetylated alaninate ILs. These dual cationic–anionic regulation rules are consistent with those in acetylglycinate homologues, and the methylene group contributions to molecular volume and standard entropy are nearly independent of its location on cations or anions. The obtained thermophysical data fill the gap in long-chain triazolium acetyl-amino acid IL systems, and the established structure–property relationships can guide rational design of functional ILs for heat transfer fluids, green reaction media, and mild biomolecule extraction applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c08470.

• Data on the spectroscopic and thermal characterization for [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala]; ¹H NMR, ¹³C NMR, and FT-IR spectroscopies; and thermogravimetry of [Taz­(2,5)]­[Acala] and [Taz­(2,6)]­[Acala] (PDF)

Jing Qiao: validation, writingoriginal draft, and investigation. Kunhao Liang: conceptualization, methodology, and writingreview and editing. Lanju Liang: supervision, resources, and funding acquisition. Haiyun Yao: visualization and data curation. Xuefeng Wang: writingreview and editing. Weiyao Xu: resources and project administration. Dawei Fang: methodology and supervision.

The authors declare no competing financial interest.