Influence of Alkyl Chain Length in Cation of Imidazolium-Based Ionic Liquids in Restructuring Water

Abstract

The effect of alkyl chain length of a series of ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride ([C _n mim]­Cl, where n = 2, 4, 8, 12, and 14) on the ability to restructure water in aqueous media has been systematically investigated. When water and solutes interact, different types of clusters form, and the strength and number of hydrogen bonds in these clusters vary with the concentration of solutes. The hydrodynamic diameter, measured using dynamic light scattering, reveals monomodal distributions at low concentrations and temperatures (20–30 °C), while multimodal distributions at high concentrations, particularly for n ≥ 8. The limiting partial molar volume at infinite dilution increases significantly with alkyl chain length (∼130 cm³ mol^–1 for [C₂mim]Cl to ∼330 cm³ mol^–1 for [C₁₄mim]­Cl). The density of the IL-water system increases with concentration for shorter alkyl chains, whereas an opposite trend is observed for longer chains. The asynchronous 2D NIR spectra reveal that short-chain ILs exhibit structure-making behavior driven by strong electrostatic hydration, whereas long-chain analogues induce structure-breaking through hydrophobic association. Solutions with high concentrations exhibit greater temperature sensitivity, and a distinct dichotomous behavior appears beyond 50 °C for all the IL solutions. This work enhances understanding of IL–water interactions and may contribute to the rational design of alternative solvents with reduced volatility compared to conventional organic solvents.

Introduction

Ionic liquids (ILs) have emerged as auspicious materials in contemporary science owing to their distinctive features, including nonflammability, minimal vapor pressure, high ionic conductivity, wide range of solubility, and exceptional electrochemical, chemical, and thermal stability. ILs are substances entirely composed of ions having a melting point lower than 100 °C. ILs remain liquid at low temperatures since their large, asymmetric, and charge-delocalized ions produce low lattice energy and poor crystal packing. Owing to their one-of-a-kind properties like low volatility, recyclability, and tunable properties, they are widely used in chemical synthesis, − analytical chemistry, separation science, green chemistry, , catalysis, − electrochemistry, , and energy storage. , ILs can reduce the environmental impact of industrial processes, but a clearer molecular-level understanding is needed to design them effectively. While modifying the cation and anion allows task-specific properties, controlling their behavior through structure alone is often difficult. Besides, ILs suffer from high viscosity, low ionic conductivity due to ion pairing, and expensive synthetic steps, which limit their hands-on applications. Mixing ILs with suitable solvents offers a practical way to fine-tune their properties, enabling better performance in targeted applications. When deliberately mixed with ILs, water can serve as a functional fluid in bioscience. − ILs are hygroscopic materials; even hydrophobic ILs have been shown to absorb a certain quantity of atmospheric water. − The densities, viscosities, conductivities, and polarities of ILs can all change significantly when water and other polar solvents are added. − By combining ILs with molecular solvents, not only can the properties be precisely tailored, but the high costs of ILs can also be reduced.

Water is the most abundant liquid on the Earth and has been extensively studied, yet a definitive theory explaining its structural changes and bonding remains elusive. Water structure refers to the three-dimensional hydrogen bonding network, which forms a complex structural organization called clusters. In pure and solutions of water, continuous cluster destruction and formation are observed. One cluster contains at least six water molecules. Many factors, like pressure, temperature, and additives, affect the structure of water. Water molecules constantly reorient and form or break hydrogen bonds (HBs) on the femtosecond to picosecond scale. Each molecule can form up to four HBs, yet their strengths and geometries fluctuate continuously. Water simultaneously contains transient regions with both ice-like ordered and liquid-like disordered structures, and its arrangement changes dramatically with impurities, surfaces, confinement, or ionic solutes. Hence, it is still a challenge for the scientific community to fully comprehend water structure.

The addition of ions to water changes both the packing density and average HB length of water. Although there is no precise definition for the terms “structure-breaker” and “structure-maker”, they are nearly always used to refer to the effects of various ions on the nearby water network. − For instance, Hribar et al. inferred that small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking HBs. But large ions (chaotropes) are water structure breakers, in general.

ILs are potential candidates for restructuring water. ILs containing the 1-alkyl-3-methylimidazolium cation ([C _n mim]⁺) have drawn significant attention in recent years. With longer alkyl chains, these cations display hydrogen-bond disruption behavior; for example, 1-decyl-3-methylimidazolium bromide ([C₁₀mim]­Br) forms aggregates in water at low concentrations and lyotropic mesophases at high concentrations. The aggregation behavior of 1-alkyl-3-methylimidazolium bromides (n = 4, 6, 8, 10, 12) and [C₄mim]­BF₄ in water has been investigated using various techniques, including conductivity, volume measurements, fluorescence probes, and NMR spectroscopy. , [C₄mim]Br did not form aggregates, while [C₆mim]Br and [C₄mim]­BF₄ formed weak aggregates above their critical aggregation concentrations (CACs). ILs with alkyl chains longer than six carbons (n > 6) showed strong aggregation, forming micelles above their CACs. Conductivity and surface tension measurements of aqueous [C₄mim]­BF₄, [C₈mim]­Cl, and [C₈mim]­I show that these salts act as surfactants, forming aggregates above 0.12, 0.10, and 0.81 mol L^–1, respectively.

ILs and their clusters in water have been vastly studied through different experimental techniques such as IR spectroscopy, − NMR spectroscopy, , microwave spectroscopy, , X-ray diffraction, , neutron diffraction, , electron spectroscopy, , 2D correlation spectroscopy, and dielectric relaxation. , Theoretical studies have also been carried out using ab initio calculations with different basis sets, along with simulations involving Monte Carlo and MD methods. −

An analysis of the interactions between [C₄mim]­BF₄ and water has been conducted using 2D correlation spectroscopy. The introduction of water causes the 3D network of the IL to break down into ionic clusters of smaller dimensions, and the bond of IL-water strengthens, while the cohesive force between the anions and cations decreases. Wu and co-workers accomplished a study comparing the interactions of two ILs, [C₄mim]­BF₄ and 1-allyl-3-methylimidazolium chloride, in binary systems with water using near infrared (NIR) spectroscopy. They found that the arrangement of anions relative to cations in these ILs significantly affected how they interacted with water, which was evident in the NIR spectra of the two systems. They also performed a series of experiments, including dynamic light scattering (DLS), transmission electron microscopy (TEM), and static light scattering (SLS) on aqueous solutions of [C₄mim]­BF₄. Their findings revealed that at a 1.0 mol L^–1 concentration (X _IL = 0.018), the IL forms vesicle-like structures that resemble hollow spheres. These clusters are about 121.7 nm in radius, with water molecules binding to the BF₄ ^– anions that surround the bigger cations.

Although ILs have attracted considerable attention because of their unique physicochemical properties, their molecular-level behavior in aqueous solutions is still not fully understood. Previous studies have primarily focused on molecular dynamics (MD) simulations, conventional vibrational spectroscopy, or the physicochemical properties of individual ILs. For example, Bhargava and Klein investigated the influence of alkyl chain length on the structure and dynamics of aqueous 1-n-alkyl-3-methylimidazolium bromide ([C _n mim]­Br, n = 2, 4, 6, and 8) solutions using MD simulations. Similarly, Feng and Voth examined the effects of cation alkyl chain length and anion identity on the nanostructure of IL–water mixtures through MD simulations. They used [C₄mim]­BF₄, [C₈mim]­BF₄, and [C₈mim]Cl for this purpose. Kumar et al. studied cation–anion–water interactions in several aqueous imidazolium ILs like [C₄mim]­CH₃OSO₃, [C₄mim]­C₈H₁₇OSO₃, [C₄mim]­BF₄, and [C₈mim]­Cl using FTIR spectroscopy. While these studies have substantially improved our understanding of IL–water interactions, they primarily provide information on solution structure, aggregation, or hydrogen-bond interactions at fixed conditions and do not establish how the hydrogen-bond network of water evolves with both alkyl chain length and temperature. Systematic comparative investigations across a homologous series of imidazolium-based ILs with chloride as the anion over a broad temperature range, using complementary thermodynamic (volumetric and viscometric) and spectroscopic (NIR and photon correlation spectroscopy (PCS)) techniques, remain limited.

In this work, the structure-breaking and structure-making capacity of the ions of a series of ILs (Figure ), 1-ethyl-3-methylimidazolium chloride ([C₂mim]­Cl), 1-butyl-3-methylimidazolium chloride ([C₄mim]­Cl), 1-methyl-3-octylimidazolium chloride ([C₈mim]­Cl), 1-dodecyl-3-methylimidazolium chloride ([C₁₂mim]­Cl), and 1-tetradecyl-3-methylimidazolium chloride ([C₁₄mim]­Cl) was focused and the effect of ILs on the water at different concentrations and temperatures was investigated.

1.

Structure of [C₂mim]­Cl, [C₄mim]­Cl, [C₈mim]­Cl, [C₁₂mim]­Cl, and [C₁₄mim]­Cl.

The viscosity, density, and refractive index of aqueous IL solutions at different concentrations were measured at various temperatures. Thermodynamic parameters, including apparent molar volume, enthalpy change, entropy change, and Gibbs free energy change for viscous flow, were calculated from the measured values. In order to estimate the degree of hydration of ions, PCS was used to measure the size of the aggregates in IL-water binary mixtures. Near-infrared (NIR) spectroscopy, together with two-dimensional (2D) correlation analysis, was used to directly probe changes in the HB network of water induced by the ILs. The ultimate goal has been to evaluate the influence of alkyl chain length in imidazolium cations of ILs in restructuring water.

Experimental Section

[C₂mim]Cl and [C₄mim]Cl were received from Sigma-Aldrich; [C₈mim]Cl and [C₁₄mim]Cl were received from Macklin; and [C₁₂mim]Cl was received from Dun & Bradstreet, and all were used without further purification. Stock solutions were prepared by dissolving the essential amount of material in deionized water using volumetric flasks. The solutions were sonicated for 20 min to ensure homogeneity. The highest concentration of each IL solution was prepared first and then sequentially diluted to obtain the anticipated lower concentrations (Table S1). Experiments were conducted from 20 to 65 °C at 5 °C intervals.

The densities of the solutions were determined using an Anton Paar vibrating tube density meter (Model: DMA 4500) based on oscillating U-tube method. The density meter was calibrated using dry air and ultrapure water. The consistency of density measurements was up to ±10^–5 gcm^–3. Viscosities were measured using a Lovis 2000 M/ME Microviscometer based on the rolling ball principle (accuracy ±10^–6 mPa·s). A 1.59 mm capillary and a gold ball (max deviation 0.2%) were used, with the capillary calibrated using S3 and N26 standard oils. Refractive indices were measured using an Anton Paar Abbemat 300 refractometer with temperature controlled to ±0.1 K by an integrated thermostat. The measurements achieved a resolution of ±10^–5. With a Zetasizer Nano ZS90 (ZEN3690, Malvern Instruments Ltd., UK), the hydrodynamic diameters (D _h) of the various aggregates in aqueous solutions at various concentrations were measured using the DLS. The measurements were conducted at a constant scattering angle of 90° using a He–Ne laser beam with a wavelength of 632.8 nm. A glass cell was used as sample holder. Cluster sizes were determined from intensity measurements. The instrument was controlled by built-in thermostats maintaining the temperature within ±0.01 K of the set value. NIR spectra were recorded using a Frontier FT-NIR spectrometer (PerkinElmer, UK). The spectra were collected over the wavenumber range of 10,000–4000 cm^–1 at a spectral resolution of 4 cm^–1, with 16 scans accumulated for each spectrum. Spectral acquisition and data processing were performed using Spectrum software (version 10.4.4, PerkinElmer, UK). The acquired NIR spectra were further analyzed by generalized 2D correlation spectroscopy.

Results and Discussion

Density Profiles

Density is a measure of how much matter is packed into a given space. The density of shorter-chain ILs like [C₂mim]Cl and [C₄mim]Cl solutions (Figures and S1) increases almost linearly with concentration at all temperatures, as these ILs are denser than water and promote strong ion–water interactions through hydration, leading to compact structures. [C₈mim]Cl shows a similar but weaker trend, indicating reduced ion–solvent interactions due to its longer alkyl chain. At higher temperatures, a slight density decrease with concentration is observed for [C₈mim]­Cl. In contrast, for [C₁₂mim]Cl and [C₁₄mim]­Cl, density decreases with increasing concentration at all temperatures. Despite being denser than water, ILs with longer alkyl chains exhibit pronounced surface activity owing to their amphiphilic nature. , Their extended hydrophobic chains weaken ion–water interactions and disrupt the water HB network, thereby promoting hydrophobic association, which contribute to a reduction in the bulk density of the solution. In all systems, density decreases with temperature due to the reduction in the number and strength of HBs and enhanced molecular motion (Figure S2).

2.

Variation of density of aqueous solutions of (a) [C₂mim]­Cl, (b) [C₈mim]­Cl, and (c) [C₁₄mim]Cl with concentration at different temperatures.

Volumetric Analysis

Apparent molar volume (Φ_v) is a key parameter for understanding solute behavior in solution and is sensitive to both temperature and concentration. It denotes the difference between the overall volume of the solution and the volume of the unaltered solvent for each mole of solute present. The values for Φ_v under constant temperature, pressure, and composition were determined using

$${\mathrm{\Phi }}_v=\frac{M}{\rho }-\frac{1000(\rho -{\rho }_0)}{m\rho {\rho }_0}$$ 1

where M represents the molar mass of solute in g mol^–1, m is the molal concentration of solutions in mol kg^–1, ρ and ρ_o denote the density of solutions and pure solvent in gcm^–3, respectively.

The limiting partial molar volume (Φ_v ⁰) of the solutions and experimental slope (S _v) are determined by using the Redlich–Meyer first-order equation

$${\mathrm{\Phi }}_{\mathrm{v}}={\mathrm{\Phi }}_{\mathrm{v}}^0+S_{\mathrm{v}}m$$ 2

here Φ_v (cm³ mol^–1) represents the limiting partial molar volume at infinite dilution, reflecting solute–solvent interactions. The Φ_v ⁰ values were obtained by linearly extrapolating the data to zero molality. The slope, S _v, is the volumetric pairwise interaction coefficient and provides insight into solute–solute interactions. However, as all the solutes are electrolytes in this study, the more accurate extended second-order Redlich−Mayer limiting law for volumes has been used to achieve a more reliable fit and accurately describe the concentration dependence of the system, i.e.

$${\mathrm{\Phi }}_{\mathrm{v}}={\mathrm{\Phi }}_{\mathrm{v}}^0+S_{\mathrm{v}}m+K_{\mathrm{v}}{m}^2$$ 3

where K _v is the second-order concentration-dependent term, which depends on temperature. The coefficients of determination (R ²) obtained from fitting the experimental data to the extended Redlich–Meyer equation at different temperatures are listed in Table S7.

Figure shows that the Φ_v of [C₂mim]Cl decreases systematically with increasing concentration at all temperatures as the solvated ions reduce the volume of solution, giving a compact and denser structure. This negative slope indicates strong solute–solvent interactions, particularly ion–dipole interactions between the imidazolium cation or Cl^– anion and water molecules. As concentration increases, crowding effects and partial solute–solute associations diminish the available free volume, resulting in lower Φ_vs. In the case of [C₄mim]­Cl, a similar trend is observed (Figure S3). The Φ_v decreases with concentration. However, the overall Φ_v values are higher than those of [C₂mim]­Cl, reflecting the effect of the longer alkyl chain, which adds extra hydrophobic volume. The decrease in the Φ_v with concentration suggests that solute–solvent interactions still play a dominant role, but steric contributions from the butyl chain weaken the structuring effect of water relative to [C₂mim]­Cl. For [C₈mim]­Cl, the Φ_v values are considerably larger due to the extended alkyl chain. Unlike [C₂mim]Cl and [C₄mim]­Cl, the decrease in Φ_v with concentration is less pronounced and even shows an increase at higher concentrations and temperatures, suggesting a balance between hydrophilic (imidazolium headgroup) and hydrophobic (octyl chain) effects.

3.

Variation of apparent molar volume of aqueous solutions of (a) [C₂mim]­Cl, (b) [C₈mim]­Cl, and (c) [C₁₄mim]Cl with concentration at different temperatures.

For the long-chain homologues, [C₁₂mim]­Cl and [C₁₄mim]­Cl, Φ_v values are the highest among the studied ILs, and the trend with concentration becomes increasing. This behavior suggests that hydrophobic interactions dominate, where alkyl chains promote aggregation in water. , Such aggregation reduces the number of favorable ion–solvent contacts, leading to a weaker concentration dependence of Φ_v. Therefore, short-chain ILs enhance water structure and reduce Φ_v, while long-chain ILs disrupt the hydrogen-bond network. The intermediate IL ([C₈mim]­Cl) exhibits mixed behavior, showing electrostatic hydration at low concentrations but hydrophobic dominance at higher levels.

Figures and S4 show that the Φ_v of all IL solutions increases with temperature up to 50 °C, after which anomalous behavior appears. This anomaly arises from extensive disruption of the HB network of water, indicating a structural crossover between two distinct structural states of water, with a narrow intermediate switching region. Notably, at all concentrations, hydrogen bonding weakens with increasing temperature, causing thermal expansion and an overall increase in solution volume.

4.

Variation of apparent molar volume of aqueous solutions of [C₈mim]Cl with temperature at different concentrations.

The positive Φ_v values (Table ) indicate strong water–ion solvation. The Φ_v values increase with alkyl chain length (peaking with [C₁₄mim]­Cl) and rising temperature (293–323 K), consistent with thermal expansion and hydrophobic volume contributions. However, this trend reverses above ∼323 K. The subsequent decrease in Φ_v suggests that at high temperatures, solute–solvent interactions weaken while hydrophobic aggregation becomes more dominant. These illustrate the effect of electrostatic interaction, hydrogen bonding, and hydrophobic interaction on the volumetric behavior of aqueous ILs.

1. Limiting Partial Molar Volume Φ_v of Aqueous Solutions of (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, (d) [C₁₂mim]­Cl, and (e) [C₁₄mim]Cl at Different Temperatures .

temperature, T (K)	Φv (cm3 mol–1)
	[C2mim]Cl	[C4mim]Cl	[C8mim]Cl	[C12mim]Cl	[C14mim]Cl
293	132.74 ± 0.67	165.42 ± 0.36	224.73 ± 0.46	292.17 ± 0.78	325.23 ± 0.18
298	133.16 ± 0.64	165.95 ± 0.34	225.74 ± 0.53	293.39 ± 0.88	326.46 ± 0.20
303	133.60 ± 0.63	166.52 ± 0.38	226.71 ± 0.48	294.65 ± 0.66	327.82 ± 0.16
308	134.03 ± 0.63	166.99 ± 0.29	227.80 ± 0.46	296.05 ± 0.98	329.16 ± 0.14
313	134.44 ± 0.62	167.59 ± 0.29	228.81 ± 0.38	297.48 ± 0.93	330.47 ± 0.23
318	134.88 ± 0.61	168.16 ± 0.35	229.93 ± 0.31	298.82 ± 0.97	331.88 ± 0.23
323	135.23 ± 0.59	168.67 ± 0.28	230.82 ± 0.19	300.00 ± 1.00	333.12 ± 0.24
328	133.90 ± 0.43	167.57 ± 0.50	226.16 ± 1.17	295.53 ± 2.17	328.77 ± 1.44
333	134.45 ± 1.14	167.06 ± 0.74	223.67 ± 1.88	293.24 ± 3.02	326.59 ± 2.24
338	138.95 ± 1.03	167.23 ± 0.83	223.26 ± 2.28	293.17 ± 3.22	326.65 ± 2.46

^aThe uncertainties correspond to the 95% confidence intervals of the fitted coefficients.

Negative S _v values reflect weak solute–solute interactions and strong hydration (solvent compression). As shown in Table , short-chain ILs ([C₂mim]­Cl, [C₄mim]­Cl) exhibit consistently negative S _v values up to ∼323 K, indicating that these ions are well accommodated by the solvent. , Beyond ∼323 K, S _v becomes slightly positive, suggesting that thermal disruption of the water lattice facilitates weak solute–solute attraction through hydrophobic aggregation. In contrast, long-chain ILs ([C₁₂mim]­Cl, [C₁₄mim]­Cl) exhibit large positive S _v values across the entire temperature range, confirming persistent hydrophobic aggregation by long alkyl chain. While [C₈mim]Cl acts as a crossover system, S _v shows a transition from negative to positive values around 308–313 K. This marks a change from predominantly hydrated ions to strong hydrophobic association at higher temperatures. Thus, [C₈mim]Cl represents a transition between structure-making and structure-breaking behavior.

2. Volumetric Pairwise Interaction Coefficient S _v of Aqueous Solutions of (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, (d) [C₁₂mim]­Cl, and (e) [C₁₄mim]Cl at Different Temperatures .

temperature, T (K)	S v (cm3 kg mol–2)
	[C2mim]Cl	[C4mim]Cl	[C8mim]Cl	[C12mim]Cl	[C14mim]Cl
293	–1.56 ± 1.08	–2.67 ± 0.53	–4.87 ± 3.17	6.95 ± 5.25	5.16 ± 1.22
298	–1.47 ± 1.03	–2.53 ± 0.51	–3.46 ± 3.68	7.25 ± 5.87	5.75 ± 1.29
303	–1.43 ± 1.01	–2.47 ± 0.57	–1.83 ± 3.35	7.87 ± 4.45	5.94 ± 1.09
308	–1.40 ± 1.00	–2.12 ± 0.44	–0.96 ± 3.17	7.07 ± 6.59	6.14 ± 0.90
313	–1.36 ± 1.00	–2.18 ± 0.44	0.27 ± 2.60	6.16 ± 6.25	6.45 ± 1.51
318	–1.36 ± 0.99	–2.14 ± 0.53	0.88 ± 2.13	6.17 ± 6.52	6.49 ± 1.51
323	–1.26 ± 0.95	–1.98 ± 0.43	2.52 ± 1.32	6.47 ± 6.73	6.95 ± 1.60
328	0.38 ± 0.69	–0.48 ± 0.75	24.27 ± 8.13	27.17 ± 14.59	27.34 ± 9.51
333	0.10 ± 1.82	0.61 ± 1.11	37.89 ± 13.01	40.09 ± 20.24	40.11 ± 14.80
338	–4.35 ± 1.65	1.06 ± 1.25	44.00 ± 15.79	44.66 ± 21.59	44.49 ± 16.26

^aThe uncertainties correspond to the 95% confidence intervals of the fitted coefficients.

K _v is the second-order concentration-dependent term in the extended Redlich–Meyer equation, which accounts for the nonlinear contribution to apparent molar volume. Whereas S _v accounts for pairwise interactions of solutes with one another, K _v emphasizes the higher-order interactions, aggregation, or departures from ideality that play an increasingly important role at high concentrations of the solute. Referring to Table below, it can be seen that K _v for [C₂mim]Cl and [C₄mim]Cl is almost zero, indicating a nearly linear variation in Φ_v. This reflects their short alkyl chains, which interact weakly and do not promote aggregation extensively. For [C₈mim]­Cl, [C₁₂mim]­Cl, and [C₁₄mim]­Cl, the magnitude of the K _v values is larger. These K _v values reflect that, beyond pairwise attraction, packing inefficiencies and steric hindrance also affect the structure, leading to unfavorable higher-order interactions.

3. Second-Order Interaction Coefficient K _v of Aqueous Solutions of (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, (d) [C₁₂mim]­Cl, and (e) [C₁₄mim]Cl at Different Temperatures .

temperature, T (K)	K v (cm3 kg2 mol–3)
	[C2mim]Cl	[C4mim]Cl	[C8mim]Cl	[C12mim]Cl	[C14mim]Cl
293	0.00 ± 0.36	0.16 ± 0.16	10.5 ± 4.65	–6.28 ± 7.42	–2.78 ± 1.70
298	–0.02 ± 0.34	0.14 ± 0.16	8.99 ± 5.39	–6.47 ± 8.31	–3.13 ± 1.80
303	–0.02 ± 0.34	0.15 ± 0.17	7.05 ± 4.91	–7.27 ± 6.30	–3.18 ± 1.51
308	–0.03 ± 0.33	0.06 ± 0.13	5.96 ± 4.65	–6.21 ± 9.33	–3.24 ± 1.25
313	–0.03 ± 0.33	0.10 ± 0.14	4.32 ± 3.82	–5.05 ± 8.85	–3.41 ± 2.10
318	–0.03 ± 0.33	0.10 ± 0.16	3.45 ± 3.12	–5.12 ± 9.22	–3.42 ± 2.10
323	–0.05 ± 0.32	0.07 ± 0.13	1.32 ± 1.94	–5.37 ± 9.52	–3.70 ± 2.23
328	–0.45 ± 0.23	–0.26 ± 0.23	–21.71 ± 11.92	–26.33 ± 20.65	–23.94 ± 13.23
333	–0.38 ± 0.60	–0.51 ± 0.34	–36.09 ± 19.08	–39.44 ± 28.65	–36.63 ± 20.57
338	0.74 ± 0.55	–0.62 ± 0.38	–42.80 ± 23.15	–43.85 ± 30.56	–40.68 ± 22.61

^aThe uncertainties correspond to the 95% confidence intervals of the fitted coefficients.

To understand the solute–solvent interactions existing in the solution, the limiting apparent molar expansivity, Φ_E ⁰, can be used.

$${\mathrm{\Phi }}_{\mathrm{E}}^0=\frac{\mathrm{d}{\mathrm{\Phi }}_{\mathrm{v}}^0}{\mathrm{d}T}$$ 4

The caging phenomenon or packing effect, which further implies interactions between solute and cosolute molecules, may give rise to the positive values. The solute molecules are described by their positive and negative values of the Φ_E ⁰, respectively, as being kosmotropic and chaotropic. Two significant components of the expansivity can be used to represent the impact of temperature on the value of Φ_E ⁰

$${\mathrm{\Phi }}_{\mathrm{E}}^0={\mathrm{\Phi }}_{\mathrm{E}}^0(\mathrm{S}\mathrm{t}\mathrm{r})+{\mathrm{\Phi }}_{\mathrm{E}}^0(\mathrm{E}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t})$$ 5

where Φ_E ⁰(Str) represents changes in the liquid structure of the solvent and Φ_E ⁰(Elect) represents changes in electrostriction (changes in the hydration around solute molecules).

Hepler utilized the general thermodynamic equation to elucidate the ability of a solute to create or disrupt structures in an aqueous system

$$\frac{\mathrm{d}{\mathrm{\Phi }}_{\mathrm{E}}^0}{\mathrm{d}T}=\frac{{\mathrm{d}}^2{\mathrm{\Phi }}_{\mathrm{v}}^0}{\mathrm{d}{T}^2}$$ 6

A solute serves as a structure maker when the second-order differentiation of the limiting apparent molar volume with respect to temperature yields a negative value. The coefficient of thermal expansion, α, can be calculated using the following relationship, which elucidates solute–solvent and solute–solute interactions. Small values of α indicate weak solute–solvent interactions, whereas large values indicate strong solute–solvent interactions.

$$\alpha =\frac{1}{{\mathrm{\Phi }}_{\mathrm{v}}^0}{\left[\frac{\mathrm{d}{\mathrm{\Phi }}_{\mathrm{v}}^0}{\mathrm{d}T}\right]}_P$$ 7

The first derivative of Φ_v is called limiting apparent molar expansivity, Φ_E . Positive values of Φ_E indicate the structure-making tendency of additives, while negative values indicate structure-breaking tendencies. The Φ_E values can give an idea about the thermal effect on the structure modification of water. From Table S2, it is clear that with increasing temperature, the three-dimensional structure of water is broken beyond 323 K. Hepler’s constant helps in determining whether a solute has a structure-breaking or a structure-making tendency. According to Hepler, a positive value indicates that the solute disrupts the water structure, while a negative value suggests that the solute enhances water structure. It is apparent from Table S3 that the structure-breaking tendency is increasing with the length of the alkyl chain as the positive entries are increasing. Near ambient temperature (293–313 K) values are small, indicating weak net temperature sensitivity of the expansivity. A pronounced temperature dependency is observed at elevated temperatures.

The values of α describe the relative change in volume with respect to temperature at constant pressure. In aqueous IL solutions, α provides insight into the extent to which solute–solvent interactions alter the intrinsic expansivity of the system. Positive values indicate normal thermal expansion, while negative values indicate unusual contraction caused by the structural reorganization of solute–solvent complexes. Table S4 shows that, at lower temperatures (293–318 K), α values are positive, which reflects normal expansion of the solutions with heating. The magnitude of α increases with alkyl chain length up to [C₈mim]­Cl, consistent with stronger hydrophobic contributions to structural sensitivity. At high temperatures, all ILs exhibit negative α, indicating anomalous contraction of the solutions. This contraction corresponds to a cooperative restructuring of the hydrogen-bonded network of water and IL aggregates.

Viscometric Analysis

Viscosity is the extent to which a fluid resists flowing, caused by internal friction between its moving layers, and it determines how “thick” or “thin” the fluid is. Viscosity increases with concentration for all ILs due to stronger intermolecular interactions and reduced molecular mobility (Figure S5). Short-chain ILs show moderate viscosity growth, while long-chain ILs exhibit a steep increase caused by hydrophobic aggregation and microheterogeneity. In contrast, viscosity decreases with temperature for all systems as thermal motion weakens intermolecular forces and disrupts hydrogen bonding (Figure S6). This thermal effect is also evident from Φ_v . The viscosity data of aqueous solutions help to assess solute–solute and solute–solvent interactions using the Jones–Dole equation

$$\frac{{\eta }_{\mathrm{r}}-1}{\surd C}=A+B\surd C$$ 8

where η_r is the relative viscosity of the aqueous salt solution, and C is the molarity of the solution. The coefficients of determination (R ²) for the fits of the experimental data to the Jones–Dole equation at different temperatures are listed in Table S8. The solute–solute interactions are indicated by the Jones–Dole constants A, which is known as Falkenhagen coefficient, and the structural modification brought about by the solute–solvent interaction is indicated by B. , The B coefficient indicates the influence of hydrodynamic shape and size, solvation characteristics, and structural elements.

All investigated imidazolium chloride solutions exhibit negative A values (Table S5) across the studied temperature range, indicating predominantly repulsive solute–solute interactions due to strong ion hydration and effective solvent screening. The magnitude of A becomes more negative as the alkyl chain length increases, reflecting stronger hydrophobic interactions that push the charged centers apart and weaken electrostatic attraction. A is less negative with increasing temperature in small chain lengths due to weakening of the hydration shell. In larger chain lengths, however, A increases in negative value because of increased hydrophobicity between the two molecules. This trend is in agreement with the observed increase in Φ_v observed for longer alkyl chains. The Jones–Dole constant B reflects the structural impact of the solute ions on the surrounding solvent. As presented in Table S6, all investigated imidazolium chloride solutions exhibit positive B values over the temperature range 293–338 K, indicating the presence of ion–solvent interactions in the solutions.

Values of dB/dT are better decisive factors for determining the structure-making or breaking nature of any solute than the B-coefficient. A negative value of dB/dT indicates a structure-making property, and a positive value indicates a structure-breaking property. As shown in Table , the values of dB/dT for [C₂mim]­Cl, [C₄mim]­Cl, and [C₈mim]Cl are negative, which implies that ILs with shorter alkyl chain length act as structure makers. But the positive values of dB/dT for [C₁₂mim]­Cl, and [C₁₄mim]Cl indicate that a longer alkyl chain in the imidazolium ion functions as a structure breaker, a trend consistent with the corresponding Hepler’s constant values.

4. Derivative of B with Respect to Temperature, dB/dT for the Viscosity of Aqueous Solutions of (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, (d) [C₁₂mim]­Cl, and (e) [C₁₄mim]Cl at Different Temperatures.

temperature, T (K)	dB/dT (dm3 mol–1 K–1)
	[C2mim]Cl	[C4mim]Cl	[C8mim]Cl	[C12mim]Cl	[C14mim]Cl
293	–0.0028	–0.0100	–0.0043	0.0139	0.0140
298	–0.0032	–0.0094	–0.0038	0.0124	0.0126
303	–0.0033	–0.0078	–0.0013	0.0160	0.0165
308	–0.0026	–0.0072	–0.0028	0.0152	0.0143
313	–0.0027	–0.0070	–0.0035	0.0064	0.0077
318	–0.0021	–0.0062	–0.0023	0.0039	0.0157
323	–0.0018	–0.0057	–0.0029	0.0023	0.0160
328	–0.0017	–0.0053	–0.0046	0.0036	0.0074
333	–0.0017	–0.0048	–0.0090	0.0029	0.0041
338	–0.0025	–0.0045	–0.0107	0.0013	0.0022

Thermodynamic Parameters

Using the absolute reaction rate theory and partition functions, Eyring and co-workers derived a correlation between viscosity and the free energy of activation. The change in free energy of activation for viscous flow has been computed according to Eyring equation

$$\mathrm{\Delta }G=RT\ln \frac{\eta V}{hN}$$ 9

where ΔG stands for the Gibbs free energy of activation for viscous flow, h for Planck’s constant, N for Avogadro’s number, V for molar volume of solution, R for molar gas constant, and η for observed viscosity in Pa·s (kg m^–1 s^–1).

The change in entropy of activation for viscous flow, ΔS, is calculated by the equation

$$\mathrm{\Delta }S=-\frac{\mathrm{d}\mathrm{\Delta }G}{\mathrm{d}T}$$ 10

The enthalpy of activation of viscous flow, ΔH, is obtained by using the thermodynamic equation

$$\mathrm{\Delta }H=\mathrm{\Delta }G+T\mathrm{\Delta }S$$ 11

ΔG increases with IL concentration for all systems, indicating stronger molecular interactions that hinder molecular motion (Figure S7). The positive value of ΔG specifies that the system is nonspontaneous and some energy is required to perform work. The flow rate of liquid is determined by the capability of molecules to go through the hole, and the readiness of the liquid to create the hole, which is controlled by ΔG. Long-chain ILs show higher ΔG than short-chain ILs due to hydrophobic interactions. With rising temperature, ΔG decreases (Figure S8) because thermal energy lowers the barrier to flow by disrupting hydration structures. This decrease is less pronounced for long-chain ILs, which maintain stronger aggregation and thus resist flow more than short-chain ILs. In Figure , all IL solutions show positive ΔS values, indicating increased disorder compared to the pure solvent. For [C₂mim]­Cl, entropy decreases at high concentrations due to stronger hydration and increased structural ordering. For [C₄mim]­Cl, entropy rises because the longer hydrophobic chain disrupts structure. [C₈mim]Cl shows a balance between hydrophilic and hydrophobic hydration, with hydrophilic effects dominating at low concentrations and hydrophobic effects at high concentrations. In the case of longer ILs, effective hydrophobic hydration causes a decrease in entropy owing to more structural order surrounding the long alkyl chain. This occurs due to an increase in entropy with temperature (Figure S9). Hydrophobic hydration dominates at low temperature, while hydrophilic interaction increases with temperature. The ΔH values increase with increasing concentration in the cases of [C₂mim]Cl and [C₄mim]­Cl, which suggests enhanced solute–solvent and solute–solute interactions that demand additional energy to break viscous forces (Figure S10). For [C₈mim]­Cl, however, the increase is smaller as both electrostatic and hydrophobic interactions are of comparable magnitude. Long-chain ILs ([C₁₂mim]­Cl, [C₁₄mim]­Cl) show nearly constant or slightly decreasing ΔH with concentration. All ILs have positive ΔH, confirming that flow is an endothermic process. With increasing temperature, ΔH decreases (Figure S11) as hydration structures weaken and IL-water networks loosen, reducing the energy needed for viscous flow.

5.

Variation of change in entropy of activation for viscous flow of aqueous solutions of (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, and (d) [C₁₄mim]Cl with concentration at different temperatures.

Dynamic Light Scattering Measurements

Particle size analysis can directly provide information about the arrangement of the solutes in water and the different clusters that might form. Figure shows that in short-chain IL solutions, cluster size increases with concentration, reflecting their structure-making tendency. [C₈mim]Cl exhibits intermediate behavior. In long-chain ILs, limited hydration and strong structure-breaking effects result in predominantly small clusters. The shorter-chain ILs, [C₂mim]Cl and [C₄mim]­Cl, predominantly form aggregates with hydrodynamic diameters of approximately 300 nm at 0.25 M, which gradually increase to around 1000 nm at 2.0 M. In addition, a weak population of smaller aggregates (∼20 nm) appears at high concentrations. In contrast, the long-chain ILs, [C₁₂mim]Cl and [C₁₄mim]­Cl, exhibit a dominant peak centered at approximately 1 nm, accompanied by a weaker population near 200 nm that progressively diminishes with increasing concentration. The intermediate-chain IL, [C₈mim]­Cl, displays characteristics of both regimes: the intensity of the ∼1 nm peak increases with concentration, while the large aggregates simultaneously grow from approximately 300 to 700 nm. These observations suggest that the aggregation behavior evolves systematically with alkyl chain length, reflecting the increasing contribution of hydrophobic interactions and the resulting changes in the mesoscale organization of the IL–water mixtures.

6.

Size distribution of aggregates formed in (a) [C₂mim]­Cl, (b) [C₄mim]­Cl, (c) [C₈mim]­Cl, (d) [C₁₂mim]Cl and (e) [C₁₄mim]Cl aqueous solutions at 25 °C.

Figure S12 shows that in [C₂mim]Cl solutions, cluster size increases from 300 to 1000 nm with concentration at low temperatures, reflecting a structure-making effect driven by strong solute–solvent interactions as inferred from volumetric and viscometric analysis. At high temperatures, however, cluster size first grows and then decreases as large aggregates break into smaller ones, indicating greater temperature sensitivity in concentrated solutions. This trend, based on the refractive index and S _v values, results from increased solute–solute interactions that reduce water structure. A similar trend is seen for [C₄mim]Cl (Figure S13), indicating that both ILs behave like structure-makers at low temperatures but not at high temperatures. Figure S14 reveals that [C₈mim]Cl exhibits a bimodal size distribution, with small hydrodynamic species (∼1 nm) and large species (∼300 nm) appearing at higher concentrations. This behavior suggests a transition from hydration-dominated interactions to hydrophobically driven molecular association with increasing concentration. This intermediate behavior arises from the interplay of hydrophobic interaction and electrostatic interaction in accordance with the observed S _v, α, and ΔH. On the other hand, Figures S15 and S16 show that [C₁₂mim]Cl and [C₁₄mim]Cl mainly exhibit the formation of smaller clusters of ∼1 nm that increase as the concentration increases, owing to their low hydration and high structure-breaking nature, as is evident from dB/dT. With increasing concentration, the appearance of larger hydrodynamic entities suggests that long-chain ILs not only affect water structuring but also increasingly favor hydrophobic interactions. This phenomenon is also observed in Figures S22 and S23, where it was also noted that concentrated solutions exhibit temperature sensitivity.

Temperature is an important factor that impacts the stability of aggregates. At low temperatures, ranging from 20 to 30 °C, the formation of aggregates becomes more stable and prominent, particularly for ILs with long chains. However, when the temperature increases to 50–60 °C, the size and intensity of aggregates become smaller. In certain instances, heating may cause recombination without full separation, resulting in changes in the positions of peaks. At elevated temperatures, the greater kinetic energy of small clusters may also lead to aggregation to form large clusters. Figures S17–S21 clearly show that at low concentrations and low temperatures (20–30 °C), there is a narrow distribution of the clusters, where small aggregates dominate. However, long alkyl chains have already shown some signs of mesoscale clustering. As the concentration rises, a second major peak (>100 nm) is formed, resulting in multimodality caused by microheterogeneity. There is a structural change window between 40 and 50 °C, during which polydispersity becomes higher. The small-aggregates peak moves to a higher aggregation number, and a large-clusters peak emerges continuously. This is in line with a partial dehydration of head groups and a stronger hydrophobic force, favoring the formation of nanometer-sized aggregates. Above 55 °C, small cluster modes prevail, suggesting that clusters are heat-labile, whereas large cluster modes dominate for long-chain systems, proving their stability to temperature changes.

NIR Spectroscopic Analysis of IL-Water Systems

NIR spectroscopy is a powerful technique for probing the characteristics of HBs and has been widely employed to investigate hydrogen-bonded systems. , Some computational approaches have also been developed to complement NIR spectroscopy, providing molecular-level insights into the structure and dynamics of hydrogen-bonding networks. , NIR spectroscopy offers several advantages over conventional mid-infrared (MIR) spectroscopy for investigating hydrogen-bonded systems. In the overtone region, absorption bands arising from monomeric and hydrogen-bonded species are more distinctly resolved. This facilitates the identification of weakly associated species and free OH groups, which are often difficult to detect in the fundamental IR region. Unlike the sharp and intense absorption bands in the MIR region, NIR spectra are characterized by broader and weaker bands. Their higher reflectance and enhanced sensitivity to hydrogen-bonding environments make them particularly valuable for probing the structure of water and other hydrogen-bonded systems.

Raw NIR spectra of pure water were recorded over the 7500–4500 cm^–1 spectral region at temperatures ranging from 20 to 75 °C in 5 °C increments. For an isolated water molecule, the fundamental vibrational frequencies are 3651.7 cm^–1 (υ₁, symmetric stretching), 1595.0 cm^–1 (υ₂, bending), and 3755.8 cm^–1 (υ₃, asymmetric stretching). Combination bands involving the bending and stretching vibrations, together with their overtones, occur at higher frequencies and are observed in the NIR region. The assignment of the individual spectral bands of pure water in the NIR region has been well established. , As shown in Figure S24, within the 5400–4600 cm^–1 region, the absorption intensity in the lower-wavenumber region decreases with increasing temperature. In contrast, the higher-wavenumber side exhibits an increase in absorption accompanied by a shift of the band maximum toward higher wavenumbers. These spectral changes indicate progressive weakening of the hydrogen-bonding network, resulting in an increased population of weakly hydrogen-bonded species and monomeric water molecules at high temperatures. The spectra reveal an isosbestic point at 5161 cm^–1, marking the intersection of the low- and high-wavenumber regions. This isosbestic point suggests the presence of a dynamic equilibrium of hydrogen-bonded clusters of water molecules.

The present analysis focused on the 4700–5500 cm^–1 spectral region, where the dominant absorption originates from the second-order combination band of the O–H asymmetric stretching and bending vibrations. NIR spectra of aqueous solutions of ILs (Figure S25) were recorded at different concentrations of ILs. For deeper insight, 2D correlation spectra were obtained from raw data. The concentration-dependent synchronous 2D NIR correlation spectra of aqueous imidazolium chloride solutions are presented in Figure . In synchronous spectra, the diagonal autopeaks represent spectral regions that undergo the largest concentration-induced changes, while the off-diagonal cross-peaks indicate spectral bands varying simultaneously during the perturbation.

7.

Concentration-dependent Synchronous 2D NIR correlation spectra of aqueous solutions of (a) [C₂mim]­Cl, (b) [C₈mim]­Cl, and (c) [C₁₄mim]­Cl.

For [C₂mim]Cl (Figure a), a single, intense, and nearly symmetric autopeak centered around 5160 cm^–1 dominates the spectrum with only weak off-diagonal features. The autopeaks are positive, and these autopeaks indicate that the changes of these peaks are simultaneous. The localized correlation pattern indicates that concentration mainly perturbs one predominant water population without inducing extensive redistribution among different hydrogen-bonding environments. The synchronous spectrum of [C₈mim]Cl (Figure b) remains dominated by an autopeak near 5160 cm^–1, although the correlation contours become broader and slightly asymmetric. The emergence of additional correlation features indicates that concentration influences multiple hydrogen-bonded water species simultaneously. Compared with [C₂mim]­Cl, the longer octyl chain introduces significant hydrophobic interactions while the imidazolium headgroup continues to participate in strong hydration. A substantially different correlation pattern is observed for [C₁₄mim]Cl (Figure c). Besides the intense autopeak near 5160 cm^–1, a pronounced off-diagonal positive correlation extending toward the lower-wavenumber region becomes evident. The increased intensity and spatial distribution of the cross-peaks demonstrate that concentration induces cooperative changes among several hydrogen-bonded water populations rather than affecting a single hydration environment.

The asynchronous 2D correlation spectra shown in Figure provide complementary information regarding the sequence of concentration-induced spectral changes. Unlike synchronous spectra, asynchronous cross-peaks arise only when different spectral components respond to the external perturbation at different rates. These spectra reveal the dynamic evolution of distinct hydrogen-bonded water species during the progressive addition of IL.

8.

Concentration-dependent Asynchronous 2D NIR correlation spectra of aqueous solutions of (a) [C₂mim]­Cl, (b) [C₈mim]­Cl, and (c) [C₁₄mim]­Cl.

For [C₂mim]Cl (Figure a), the asynchronous spectrum exhibits a relatively simple cross-peak pattern consisting of one dominant positive and one negative correlation feature. The asynchronous spectrum is antisymmetric about the diagonal, and its intensity provides information about the successive or sequential change at two different wavenumbers. Figure a shows two peaks at around 5030 cm^–1 and 5200 cm^–1, indicating out-of-phase spectral changes at these wavenumbers. Above the diagonal, the sign of the cross-peak is positive, and below the diagonal, the sign is negative, which reveals that the band shifts to lower wavenumbers with increasing concentration. So the number and strength of HBs among water molecules increase with increasing concentration of [C₂mim]­Cl.

The asymmetric asynchronous spectrum becomes considerably more pronounced for [C₈mim]Cl (Figure b), where stronger positive and negative cross-peaks are observed between the high- and low-wavenumber regions. For [C₁₄mim]Cl (Figure c), the asynchronous spectrum displays the largest and most intense cross-peaks, revealing highly heterogeneous spectral evolution. Above the diagonal, the sign of the cross-peak is negative, while below the diagonal, the peak is positive for both [C₈mim]Cl and [C₁₄mim]­Cl. This reveals that the band shifts to higher wavenumbers with increasing concentration since the HBs between the water molecules become weaker. This suggests that the number of free water molecules is increasing while the HBs between water molecules are diminishing.

According to Noda’s generalized 2D correlation theory, the combined interpretation of the synchronous and asynchronous spectra enables the order of spectral variations to be established. The increasing complexity of the asynchronous maps from [C₂mim]Cl to [C₁₄mim]Cl clearly demonstrates that elongation of the alkyl chain progressively changes the restructuring mechanism from localized hydration of individual ions to large-scale reorganization of the water HB network. These spectroscopic observations provide direct molecular-level evidence for changes in the HB network of water, thereby supporting the interpretations derived from the volumetric and viscometric analyses. Together, these complementary results confirm the structure-making behavior of short-chain ILs and the aggregation-driven structure-breaking behavior of their long-chain counterparts.

Role of Alkyl Chain-Length of Imidazolium Cation in Restructuring Water

[C₂mim]Cl and [C₄mim]Cl solution densities change nearly linearly with respect to concentration over all experimental temperatures. This is because of the higher intrinsic densities of these solutions along with good hydration, which leads to compact arrangements. However, [C₈mim]Cl exhibits a similar but less pronounced pattern due to the decrease in ion–solvent interactions resulting from chain extension. The density of this IL decreases slightly as the temperature increases. [C₁₂mim]­Cl and [C₁₄mim]Cl exhibit a different behavior. Though both have densities higher than that of water, their densities decrease with increasing concentrations due to their large hydrophobic tails, which hinder HB formation and cause clustering. Observations can also be supported by apparent molar volume studies [C₂mim]­Cl and [C₄mim]Cl demonstrate a steady decline in Φ_v with increasing concentration, thus showing strong ion–dipole interaction and low free volume. In spite of the extra hydrophobic contribution from the octyl group in [C₈mim]­Cl, the compound [C₈mim]Cl shows mixed characteristics. The Φ_v value increases with concentration and temperature due to an opposing effect of both hydrophobic and hydrophilic interactions. In the case of [C₁₂mim]Cl and [C₁₄mim]­Cl, Φ_v is found to be maximum and dependent on concentration, which suggests that these species are predominantly associated with hydrophobic forces. This trend of increased structure disruption with increase in chain length has been substantiated with Hepler’s constant, the derivative of Jones–Dole constant B with respect to temperature, and DLS measurements. These macroscopic trends are corroborated by the NIR spectroscopic results, which show that short-chain ILs preserve a larger population of strongly hydrogen bonded water species, whereas increasing alkyl chain length progressively shifts the spectral intensity toward higher wavenumbers, indicating weakening of the hydrogen bond network and an increase in less-associated water species.

The synchronous and asynchronous 2D NIR correlation spectra provide molecular-level insight into this restructuring process. Short-chain ILs exhibit sequential spectral changes characteristic of hydration-dominated water reorganization, whereas long-chain ILs show a markedly different sequence consistent with increasing hydrophobic influence on the water network. The spectroscopic evidence therefore complements the volumetric and viscometric analyses by directly demonstrating that water restructuring evolves from predominantly electrostatic hydration in [C₂mim]Cl and [C₄mim]Cl to increasingly hydrophobic interactions as the alkyl chain length increases. The intermediate behavior of [C₈mim]Cl observed from Φ_v is likewise reflected in the NIR spectra, indicating a gradual transition between these two interaction regimes.

The trend of higher Φ_v values for larger alkyl chains is also apparent, with [C₁₄mim]Cl exhibiting the highest values due to its higher hydrophobic volume. Moreover, S _v provides a clear distinction between the different systems. [C₂mim]Cl and [C₄mim]Cl exhibit negative S _v values until ∼323 K, indicating strong solute–solvent interactions. But at higher temperatures, S _v becomes slightly positive as hydrogen bonding weakens and hydrophobic aggregation begins. For [C₁₂mim]Cl and [C₁₄mim]­Cl, S _v remains strongly positive at all temperatures, confirming persistent hydrophobic aggregation of long chains. Solute–solute interactions increase with increasing chain length in the imidazolium ion and the temperature for all IL solutions, which is backed by S _v and A-coefficient data. [C₈mim]Cl shows crossover behavior, with negative S _v at low temperatures, approaching zero near 308–313 K, and becoming strongly positive at high temperatures, indicating a transition to aggregation-driven behavior. These intermediate behaviors of [C₈mim]Cl are also supported by DLS analysis, α values, and ΔH values. K _v values reveal that in long-chain ILs, steric hindrance and packing inefficiencies further weaken favorable interactions, leading to second-order interactions. But short-chain ILs show almost linear behavior with concentration and temperature.

The structure-breaking tendency increases with the increase in temperature at all concentrations of all ILs due to the reduction in the number and strength of HBs in solutions. The molecular movements, including translational, rotational, and vibrational dynamics of water molecules, increase as the temperature rises. Concentrated solutions of all ILs exhibit pronounced temperature sensitivity, as evidenced by the DLS and refractive index measurements. From the values of Φ_v ⁰, α, and Φ_v, it is apparent that all the ILs show a dichotomous behavior beyond 50 °C due to the extensive disruption of HBs in water structure.

Thus, all the analysis demonstrates that short-chain ILs ([C₂mim]­Cl, [C₄mim]­Cl) act as structure makers by enhancing water structuring, long-chain ILs ([C₁₂mim]­Cl, [C₁₄mim]­Cl) act as structure breakers by disrupting hydrogen bonding, while [C₈mim]Cl occupies an intermediate regime, showing both structure-making and structure-breaking tendencies depending on conditions (Scheme ).

1. Proposed Molecular-Level Interactions of (a) [C₂mim]­Cl, (b) [C₈mim]­Cl, and (c) [C₁₄mim]Cl with Water in Aqueous Solutions.

Molecular-Level Interactions of Ionic Liquids with Water

Scheme illustrates the proposed concentration-dependent structural evolution of aqueous solutions containing 1-alkyl-3-methylimidazolium chloride ILs inferred from the combined volumetric, viscometric, NIR spectroscopic, and DLS results. The molecular-level interactions are governed by a competitive interplay between the hydrophilic cationic head groups and the hydrophobic alkyl chains of varying lengths. For short-chain homologues such as [C₂mim]Cl and [C₄mim]­Cl, the ILs function as structure-making solutes. As concentration increases from 0.25 to 2.00 M, the ions draw water molecules into a progressively tighter and more compact hydrogen-bonded network through enhanced hydration. This results in a higher packing density of water molecules around the ions, emphasizing the importance of ions in making the solution stable. This is reflected by the density, apparent molar volume, and viscosity results. The corresponding asynchronous 2D NIR analysis confirms that the spectral changes are primarily associated with hydration-induced rearrangement of the water network rather than extensive disruption, consistent with the structure-making behavior inferred from the thermodynamic properties.

2. Proposed Molecular-Level Interactions of Ions of [C₂mim]­Cl, [C₈mim]­Cl, and [C₁₄mim]Cl with Water at Various Concentrations of ILs.

In sharp contrast, homologous ions such as [C₁₂mim]­Cl and [C₁₄mim]Cl have always been found to be structural disruptors. Long hydrophobic chains in these ions interfere with the tetrahedral hydrogen bonding arrangement of water molecules, causing a reduction in hydration. Unlike the ILs with compact packing, the extended IL chains inhibit the structuring of water even at fairly dilute levels. Consistent with this behavior, the NIR spectra show a gradual shift in spectral intensity toward higher wavenumbers, indicating a reduction in the population of strongly hydrogen-bonded water species and an increase in weakly hydrogen-bonded or less-associated water molecules. The DLS measurements simultaneously reveal the appearance of progressively larger hydrodynamic species with increasing concentration. Existing at the intermediate stage between these two classes of ILs, [C₈mim]Cl exhibits an exceptional dual behavior through the crossover phenomenon. The phenomenon occurs due to the delicate balance between the hydrophilic head and the hydrophobic tail. In situations where the concentration and temperature levels are low, the structural preferences of the head dominate. On the other hand, as the concentration is increased, hydrophobic interference caused by the octyl group dominates, thereby making the IL function as a disruptor rather than an organizer. The above situation clearly brings into focus the fact that the hydrophobic and hydrophilic interaction ratio determines the structure of the mixture depending on IL concentration.

The temperature dependence of the aqueous systems for [C₂mim]­Cl, [C₈mim]­Cl, and [C₁₄mim]Cl is shown in Scheme below, showing the role of temperature in the competition between water molecules and ions. In the case of the short-chained [C₂mim]Cl and [C₄mim]­Cl, heating from 20 to 65 °C results in the gradual dehydration of the hydration shell surrounding each molecule. The reason for this trend is due to the thermal breakdown of HBs, thereby lowering the structuring of the solvent and making interactions between solutes more pronounced. At high temperatures and concentrations, the systems start showing the presence of micelle-like regions in the solution; the system undergoes transformation from a state having high hydration to a solute–solute region.

3. Proposed Molecular-Level Interactions of Ions of [C₂mim]­Cl, [C₈mim]­Cl, and [C₁₄mim]Cl with Water at Different Temperatures.

[C₈mim]Cl is an important intermediary. High temperatures lead to increased disruption in the hydration layers. Due to the reduction in strength caused by heat, the solvent–solvent interactions begin to prevail over hydrogen bonding. Thus, in the presence of high temperatures and high concentrations, [C₈mim]Cl is capable of forming its own micelles just like its longer-chain analogs. This is due to the fact that the heat supplied offers the required push for overcoming the weak structural tendencies of the cationic headgroup. In the case of the long-chain [C₁₄mim]Cl homologues, where the substance is a structure breaker that causes breaking of the tetrahedral configuration of the water molecule, temperature acts as an exacerbating factor. This is because the rise in temperature level leads to more damage to the structured form of water. This enhances the interactions between the solute particles. The long-chain molecule [C₁₄mim]Cl forms clusters that resemble micelles due to its greater tendency to aggregate as opposed to the short-chain molecules. This shows that whereas concentration is the factor that determines the first stage of IL-water complexes (as shown in Scheme ), temperature is the determining factor in the dissolution process as well as hydrophobic association.

Conclusions

There is a definitive trend between structure-maker and structure-breaker for the chain length investigation of alkyl side chains attached to the imidazolium cation group. Short-chain ILs like [C₂mim]­Cl and [C₄mim]Cl fall under structure-making ILs. These ions help to structure water via ion–dipole interactions. On the other hand, the long-chain counterparts like [C₁₂mim]­Cl and [C₁₄mim]Cl serve as structure breakers, breaking the tetrahedral hydrogen bonding network of water and allowing them to aggregate in the form of micelles. In intermediate chain lengths, [C₈mim]Cl demonstrates both types of behavior; that is, it changes from structure-making to structure-breaking tendencies, depending upon temperature and concentration, which results from the balance between its hydrophilic headgroup and hydrophobic tail.

Temperature brings yet another dimension to IL-water interactions. When temperatures are low to moderate, most ILs act as structuring agents because of the cooperative hydration and organization that occurs in the water network via solvent–solute interactions. A transition occurs above ∼323 K. The HBs are broken, solute–solute interactions become weaker, and solute–solute associations dominate. The shift is most pronounced for [C₈mim]­Cl, whose temperature dependence is very strong, changing from negative S _v at low temperatures to highly positive S _v at high temperatures. Beyond 50 °C, all the investigated ILs exhibit dual behavior because of the breakdown of the HBs within the water structure. With an increase in temperature, solute–solute interaction increases in all the ILs under study.

In contrast to regular salts, ILs provide an interesting platform whereby the interactions between the solute and the solvent or between solutes can be optimized within the aqueous system. Unlike regular salts, whose main role is structure-making due to electrostatic interactions with water, ILs exhibit more varied behavior. Modifications in chain length in the cation and counteranion, as well as variations in concentration and temperature, can manipulate water structures. The versatility of these changes enables ILs not just to modify the water structures, but also to become molecular tools in aqueous systems.

Consequently, the results of this study have established ILs to be a very diverse group of solutes that lie somewhere in between regular salts and molecular solvents. This is because of their ability to be designed in such a way as to provide specific solutions to certain chemical problems. Through careful selection of the right ILs in particular concentrations, reactions can be guided in such a way that improved yields are obtained, or even new products can be formed.

Data supporting this article have been included as part of the Supporting Information. Further data may be available upon request.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05508.

• Variation of density (ρ), dynamic viscosity (η), refractive index, apparent molar volume (Φ_v), free energy (ΔG), entropy (ΔS), and enthalpy (ΔH) changes of activation for viscous flow as a function of temperature and concentration for the binary mixtures of water and IL. Calculated ${\mathrm{\Phi }}_{\mathrm{E}}^0,\frac{\delta {\mathrm{\Phi }}_{\mathrm{E}}^0}{\delta T},\alpha ,A,\text{and}B$ for IL-water binary mixtures at different temperatures. Coefficients of determination (R ²) for the fits of experimental data to the Redlich-Mayer and Jones–Dole equations. Size distribution of aggregates formed in IL-water binary mixture at various temperatures and concentrations. NIR spectra of pure water and IL-water systems (PDF)

§.

(M.H.) Department of Chemistry, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Kawsar Ahmmed: Conceptualization, conducting experiments, data interpretation and analysis, writingoriginal draft preparation, reviewing and editing; Mohammad Hossain: Conceptualization, writingreview and editing; Md. Abu Bin Hasan Susan: Conceptualization, writingreview and editing, supervision, funding acquisition, resources.

The authors declare no competing financial interest.