Cation-Dependent Role of Water on the Dynamics and Ionic Conductivity of Levulinate Based Ionic Liquids

1. Introduction

Ionic liquids (ILs) are salts composed entirely of ions, characterized by negligible vapor pressure, wide electrochemical windows, and high thermal and chemical stability [1]. These properties, combined with the ability to tailor cation and anion structures, make ILs attractive for diverse applications [2], particularly in energy-related devices where they function as electrolytes in batteries, supercapacitors, and other electrochemical systems [3,4,5,6,7,8,9,10]. Structural tunability allows rational design of ILs with optimized macroscopic properties for specific technological needs.

A critical factor affecting IL performance is water, which is often present due to the hygroscopic nature of most ILs [11,12,13]. Even small amounts of water can significantly influence viscosity, glass transition temperature (Tg) and ionic conductivity [14,15,16,17,18,19,20,21,22,23,24,25,26]. Water can act as a plasticizer, lowering Tg and enhancing ionic mobility, but the microscopic effects depend on its organization within the ionic matrix, including ion hydration, cluster formation, or partitioning into polar and nonpolar domains [23,25,27]. This complex interplay often leads to system-dependent behavior, including acceleration of ionic motion or partial decoupling of translational ionic motion from structural relaxation.

In this study, we investigate the impact of absorbed moisture on Tg, ionic conductivity, and molecular dynamics in two levulinate based ILs: 1-ethyl-1-methylpyrrolidinium levulinate and 1-butyl-1-methylpyrrolidinium levulinate. Because the cations differ only in the length of the alkyl chain attached to the pyrrolidinium nitrogen, while anion remains the same, we can directly assess how this variation influences water-ion interactions and structural organization within the ionic matrix. By combining differential scanning calorimetry (DSC) with broadband dielectric spectroscopy (BDS), we aim to establish clear correlations between cation structure, water-induced plasticization, and ionic mobility, providing insight into the molecular mechanisms governing hydrated ILs and informing the design of IL-based electrolytes for energy applications.

2. Materials and Methods

2.1. Materials and Sample Preparation

The ionic liquids studied are 1-ethyl-1-methylpyrrolidinium levulinate, [C2C1Pyr]Lev, and 1-butyl-1-methylpyrrolidinium levulinate, [C4C1Pyr]Lev. These ionic liquids share a common levulinate (Lev) anion, while their cations differ in the length of the alkyl chain attached to the nitrogen atom of the pyrrolidinium ring. The chain consists of two carbons (C2C1Pyr) in the first case and four carbons (C4C1Pyr) in the second. The preparation of the materials is described in [28].

Two sets of samples were prepared: those maintained in a desiccator over P2O5 for an extended period, and those exposed to ambient conditions (temperature 25-30 °C and relative humidity 50-60%) for five days. The water content of each sample was determined by weighing the sample before and after the differential scanning calorimetry measurements, during which water evaporation was observed. The water percentages reported hereafter refer to the water content relative to the dry mass and were calculated using the following equation

$$h_w={\displaystyle \frac{m_h-m_d}{m_d}}100$$

where mh is the mass of the sample before water evaporation and md is the mass of the sample after. Hereafter, the samples will be referred to as hw%, where hw represents the water percentage by weight. For the ionic liquid [C2C1Pyr]Lev, samples with water concentrations of 9% and 59% were prepared, while for [C4C1Pyr]Lev, samples with water concentrations of 10% and 44% were prepared.

2.2. Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) measurements were conducted using a TA Q200 instrument (TA Instruments). The samples were placed in aluminum pans and sealed with a crimp press. The mass of both the samples and the pans was measured using a high-precision balance. Subsequently, the pans were pierced to allow for water evaporation and placed into the calorimeter. The experimental protocol involved cooling the sample from ambient temperature down to -150 °C at a rate of -10 °C/min, followed by an isothermal hold for 2 minutes, and then heating the sample up to 200 °C at a rate of 10 °C/min.

2.3. Broadband Dielectric Spectroscopy (BDS)

Dielectric measurements were performed using an experimental setup consisting of an Alpha-Analyzer frequency response analyzer and a Novocool temperature control system (Novocontrol Technologies). The samples were placed in a liquid dielectric cell, consisting of a Teflon cylinder with metal electrodes at the top and bottom. The cell had a thickness of 2 mm and an electrode diameter of 11 mm. An alternating voltage of 1 Vrms was applied across the electrodes. Isothermal measurements were carried out in the frequency range of 0.1 Hz to 1 MHz and over a temperature range of -100 °C to 60 °C, with a temperature step of 10 °C.

3. Results and Discussion

3.1. DSC Results

Figure 1 presents the DSC thermograms of [C2C1Pyr]Lev and [C4C1Pyr]Lev at different hydration levels. Upon heating, all samples exhibit a step-like change in the heat flow in the low temperature region (-100 to -50 °C), attributed to the glass transition. At higher temperatures (around 100 °C), a pronounced endothermic peak corresponding to water evaporation is observed. In both ionic liquids, increasing water content shifts the glass transition to lower temperatures, indicating a clear plasticizing effect of water. Simultaneously, the evaporation peak shifts to lower temperatures and increases in magnitude with increasing hydration, reflecting the higher amount of absorbed water and its modified thermal stability within the ionic matrix.

Table 1 summarizes the glass transition temperature (Tg), the heat capacity change at Tg (ΔCp), the enthalpy of water evaporation (ΔH), and the peak evaporation temperature (Tmax). For [C2C1Pyr]Lev, Tg decreases from -73 °C to -92 °C as the water content increases from 9 wt% to 59 wt% confirming the pronounced plasticizing effect of water. A corresponding increase in ΔCp is observed, consistent with enhanced molecular mobility and increased configurational freedom in the hydrated samples. ΔH increases with water content, reflecting the larger amount of absorbed water, while Tmax decreases from 130 °C to 95 °C. The reduction in Tmax suggests stronger water–ionic liquid interactions at lower hydration levels and progressively weaker binding as the water content increases.

A similar trend is observed for [C4C1Pyr]Lev, where Tg decreases from -76 °C to -85 °C as the water content increases from 10 wt% to 44 wt%. The corresponding decrease in Tmax from 119 °C to 92 °C again indicates a weakening of specific water-ionic liquid interactions at higher hydration levels. Notably, the magnitude of Tg depression is smaller, and the Tmax of the dry sample is lower compared to [C₂C₁Pyr]Lev, suggesting differences in the manner by which water interacts with the cation structure and modifies the local organization of the ionic matrix.

It should be noted here that a previous study on [C4C1Pyr]Lev reported melting and crystallization at 41.6 °C and -33.3 °C, respectively, with a glass transition observed only during the first heating cycle and disappearing after dehydration at elevated temperatures [28]. In contrast, the hydrated samples investigated in the present work exhibit exclusively glass transitions, with no evidence of crystallization. This behavior suggests that the presence of sufficient water suppresses crystallization, likely by disrupting the regular packing of ions and modifying intermolecular interactions within the ionic matrix.

For both ionic liquids investigated, water acts as a plasticizer, progressively lowering the glass transition temperature. Such Tg depression upon hydration has been widely reported for ionic liquids and is generally associated with reduced viscosity and weakened ion–ion interactions [25,26,29,30,31], although opposite trends can occur in systems with less hydrophilic anions [32]. DSC measurements reveal that the structure of the cation strongly influences the degree of plasticization, with Tg depression being more pronounced for [C₂C₁Pyr]Lev than for [C₄C₁Pyr]Lev. The heat capacity change at Tg (ΔCp) increases with rising water content in both ILs, showing no significant cation dependence. In contrast, the peak evaporation temperature (Tmax) is consistently lower for [C₄C₁Pyr]Lev than for [C₂C₁Pyr]Lev across all hydration levels, indicating stronger water–ionic liquid interactions in [C₂C₁Pyr]Lev. Spectroscopic evidence further indicates that water induces microstructural heterogeneity rather than acting as a simple diluent, and that alkyl-chain length modulates water accommodation, influencing both polar and nonpolar domains [29,33]. In line with these observations, the stronger Tg depression in [C₂C₁Pyr]Lev likely arises from direct water–cation interactions, whereas in [C₄C₁Pyr]Lev, water primarily perturbs alkyl-chain packing. Overall, the combined effect of network softening and hydration-induced structural reorganization underlies the observed cation-dependent thermal behavior of these levulinate-based ionic liquids.

3.2. DRS Results

3.2.1. Dielectric Spectra

Figure 2 presents representative dielectric spectra for [C₂C₁Pyr]Lev at 59% water content over the temperature range from -100 to -30 °C. Panels (a–e) show the real (ε′) and imaginary (ε′′) parts of the dielectric function, the derivative of the real part (εderiv′′, free from DC conductivity contributions [34], the real conductivity (σ′), and the imaginary part of the electric modulus (M′′), all plotted as a function of frequency.

At -100 °C, a temperature below Tg, two peaks are observed in the imaginary part of the dielectric function ε′′ (Figure 2b), indicated by arrows on the corresponding plot.

These peaks correspond to two secondary dielectric relaxation processes. The contribution of each mechanism is manifested as a step-like change in the real part of the dielectric function ε′ (Figure 2a), and as a shoulder or peak in the plots of the derivative εderiv′′ (Figure 2c) and the imaginary part of the electric modulus M′′ (Figure 2e). As the temperature increases, these mechanisms shift toward higher frequencies. The faster process, also visible in the dry sample, is attributed to localized ion motions, consistent with previous reports in the literature [35,36], whereas the slower process appears only in the hydrated sample and is associated with water. A detailed analysis of these secondary relaxations, however, lies beyond the scope of the present study. As the temperature increases above Tg (>-90 °C), a strong increase in σ′ (Figure 2d) is observed at low frequencies, forming a nearly frequency-independent plateau that grows with temperature. Simultaneously, a pronounced step in ε′, a peak in ε′′deriv, and a shoulder in M′′ appear in the same frequency range, also shifting to higher frequencies with temperature. This process corresponds to the main relaxation (α-relaxation or conductivity relaxation), associated with the motion of ions. In ε′′, the α-relaxation is masked at low frequencies by the linear increase due to DC conductivity. At frequencies lower than those of the main relaxation deviations in ε′, εderiv′′, and σ′ are attributed to electrode polarization [37].

At higher temperatures (not shown), the dielectric spectra become increasingly dominated by DC conductivity and electrode polarization. A similar pattern is observed for hydrated [C₄C₁Pyr]Lev, with secondary relaxations below Tg (with the exception that only a single process is detected at the lowest water content) and a dominant α-relaxation above Tg, although the characteristics of these processes differ from those of [C₂C₁Pyr]Lev, indicating that the underlying ionic dynamics and charge transport mechanisms are preserved across both systems.

3.2.2. Temperature - Water Dependence of Dynamics and Conductivity

The main focus of the present work is the investigation of the effect of water on the DC conductivity and the primary α-relaxation of [C₂C₁Pyr]Lev and [C₄C₁Pyr]Lev ionic liquids. To extract the peak frequencies and relaxation strengths of the α-relaxation, fitting procedures were applied to the ε′′ vs. frequency plots in selected regions around the ε′′deriv peaks. For this purpose, the following equation was used to model the data:

$${\mathsf{\epsilon }}^{\mathrm{\text{'}}\mathrm{\text{'}}}=\mathsf{A}{\mathrm{f}}^{-1}+\mathrm{I}\mathrm{m}\left[{\mathsf{\epsilon }}_{\mathrm{\infty }}+{\displaystyle \frac{\mathsf{\Delta }\mathsf{\epsilon }}{{\left(1+{\left(\mathrm{j}\mathsf{\omega }\mathsf{\tau }\right)}^{\alpha }\right)}^{\mathsf{\beta }}}}\right]$$

(1)

The first term of equation (1), where A is a constant, accounts for the contribution of DC conductivity to ε′′, while the second term corresponds to the imaginary part of the Havriliak–Negami equation [38] and describes the contribution of the main α-relaxation. Δε is the dielectric strength (dielectric increment) of the main relaxation, ε_∞ is the dielectric constant at the high-frequency limit, τ is a characteristic relaxation time related to the peak frequency (f_m) of ε′′, and α, β are shape parameters with values in the range 0-1.

Figure 3 presents the Arrhenius plots of the main relaxation, illustrating its temperature dependence for the ionic liquids [C₂C₁Pyr]Lev and [C₄C₁Pyr]Lev. The frequencies (f_m) of the ε′′ peaks at various temperatures, obtained from the best fit of equation (1), were used to construct the plots. As shown in Figure 3, the main relaxation of [C₂C₁Pyr]Lev samples with 9% and 59% water content, as well as [C₄C₁Pyr]Lev with 44% water content, follows Vogel–Fulcher–Tammann (VFT) behavior [39,40,41]. An exception is observed for [C₄C₁Pyr]Lev with 10% water content, which exhibits Arrhenius-type behavior.

For both ionic liquids at different hydration levels, the main relaxation shifts to higher frequencies at a given temperature as the water content increases across the entire temperature range. This plasticizing effect of water on the main relaxation is likely related to changes in the viscosity of the samples. According to the Debye–Stokes–Einstein relation, the rotational relaxation time of a particle in a liquid medium is directly proportional to viscosity. Although deviations from this relationship have been reported in ionic liquids [42], it has been shown to hold approximately in similar systems [43,44]. Therefore, the observed increase in the peak frequency f_m of the main relaxation with increasing water content may be attributed to the corresponding decrease in viscosity.

For the ionic liquid samples exhibiting VFT behavior (Figures 3), the best fit to the experimental results was performed using the empirical equation:

$${\mathrm{f}}_{\mathrm{m}}={\mathrm{f}}_{\mathrm{o}}{\mathrm{e}}^{-{\displaystyle \frac{\mathrm{D}{\mathrm{T}}_{\mathrm{o}}}{\mathrm{T}-{\mathrm{T}}_{\mathrm{o}}}}}$$

(2)

which often describes the non-Arrhenius temperature dependence dynamics of glassy systems. In Equation (2), To is the Vogel temperature, typically located a few tens of Kelvin below the material’s glass transition temperature. fo is the pre-exponential factor, which corresponds to an inverse attempt frequency, and D is the so-called strength parameter [45]. Parameter D is used to distinguish between strong and fragile glass formers and serves as a measure of the deviation from Arrhenius behavior. Alternatively, the fragility index m is also used for the classification of glass formers. Both parameters are connected via the relation m=16+590/D [46]. Higher values of m (lower values of D) correspond to more fragile temperature characteristics and stronger deviations from Arrhenius behavior. The fits in Figure 3 are extrapolated to logf_m=−2.79, which corresponds to the characteristic time τ=100 s. The temperature at which the relaxation time of the main α-relaxation reaches 100 s is commonly defined as the dielectric glass transition temperature (T_gdiel) [47].

As shown in Figure 3, the Tgdiel values for the [C₂C₁Pyr]Lev samples are nearly identical, whereas those for [C₄C₁Pyr]Lev are slightly higher (by approximately 3–4 K). This indicates that, in [C₂C₁Pyr]Lev, the mobility of the ions involved in the main relaxation closely follows the mobility of the bulk ionic liquid. In contrast, the small shift observed for [C₄C₁Pyr]Lev points to a slight decoupling between ionic motion and structural relaxation. Consequently, the observed increase in ion mobility with increasing hydration is primarily attributed to the corresponding decrease in T_g. Moreover, for [C₄C₁Pyr]Lev, increasing the water content from 10% to 44% has a more pronounced effect on the shift of the main relaxation frequency f_m than the corresponding shift observed for [C₂C₁Pyr]Lev. This behavior is consistent with the stronger influence of water on the T_g of [C₄C₁Pyr]Lev.

For the [C4C1Pyr]Lev sample with 10% water content, the experimental data were best described using the Arrhenius equation:

$${\mathrm{f}}_{\mathrm{m}}={\mathrm{f}}_{\mathrm{o}}{\mathrm{e}}^{-{\displaystyle \frac{\mathrm{E}}{\mathrm{k}\mathrm{T}}}}$$

(3)

where f_o is a pre-exponential factor and E is the activation energy.

Table 2 summarizes the parameters obtained from fitting Equation (2) to the experimental data. The Vogel temperature, T₀, falls within a narrow range of 135–140 K for all three samples: 9% and 59% water content in [C₂C₁Pyr]Lev, and 44% water content in [C₄C₁Pyr]Lev. The lowest fragility index, m = 49, is observed for the sample with the lowest water content (9% [C₂C₁Pyr]Lev). In contrast, the two samples with higher water contents exhibit larger and nearly identical fragility indices (m = 66 and 67), indicating a stronger deviation from Arrhenius behavior. For the [C₄C₁Pyr]Lev sample containing 10 wt% water, the activation energy associated with the main relaxation process was determined using Equation (3), yielding a value of E = 0.80 eV.

Figure 4 presents the dielectric relaxation strength, Δε, of the main relaxation as a function of temperature for the [C₂C₁Pyr]Lev and [C₄C₁Pyr]Lev samples at different hydration levels. For the [C₂C₁Pyr]Lev samples, Δε decreases with increasing temperature. In contrast, the [C₄C₁Pyr]Lev sample containing 10 wt% water exhibits an increase in Δε with rising temperature. Moreover, the absolute values of Δε in this sample are significantly higher than those of the 44 wt% [C₄C₁Pyr]Lev sample, where Δε decreases with temperature, following the same trend observed for the [C₂C₁Pyr]Lev systems. It should be noted that the dielectric relaxation strength, Δε, is related to the number density of mobile ions and therefore provides insight into changes in ionic mobility and charge carrier concentration [48].

Figure 5 shows the Arrhenius representations of the dc conductivity, highlighting its temperature dependence for the [C₂C₁Pyr]Lev and [C₄C₁Pyr]Lev ionic liquids. The dc conductivity values, σ_dc, were extracted from the frequency-independent plateaus observed at each temperature.With the exception of the 10 wt% [C₄C₁Pyr]Lev sample, which follows Arrhenius behavior, the remaining three ionic liquid samples exhibit a non-Arrhenius temperature dependence that is well described by the empirical Vogel–Fulcher–Tammann (VFT) equation:

$${\mathsf{\sigma }}_{\mathrm{d}\mathrm{c}}={\mathsf{\sigma }}_{\mathrm{o}}{\mathrm{e}}^{-{\displaystyle \frac{\mathrm{D}{\mathrm{T}}_{\mathrm{o}}}{\mathrm{T}-{\mathrm{T}}_{\mathrm{o}}}}}$$

(4)

Figure 5 includes the corresponding fits of Equation (4) for the three samples displaying VFT behavior, while for the 10 wt% [C₄C₁Pyr]Lev sample, the best fit using the Arrhenius expression ${\sigma }_{dc}={\sigma }_o\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{E}{kT}})$ is presented. The fits (shown in Figure 5) are extrapolated to σ_dc ≈ 10⁻¹⁵ S/cm. When ionic motion is coupled to the structural (α) relaxation, σ_dc is expected to approach this characteristic value at the glass transition temperature, Tg [49].

Table 2 summarizes the parameters obtained from fitting Equation (4) to the experimental data. The Vogel temperature, T₀, lies in the range 143–147 K for the three samples: 9% and 59% water content in [C₂C₁Pyr]Lev and 44% water content in [C₄C₁Pyr]Lev. The fragility indices (m) calculated from Equation (4) (Table 2) are higher for all ionic liquids than the corresponding m values derived from Equation (2) for the main relaxation dynamics. The lowest fragility index is observed for the sample with the lowest water content (9% [C₂C₁Pyr]Lev), with m = 64. The two samples with higher water contents exhibit larger and nearly identical fragility values (m = 85 and 86). For the 10 wt% [C₄C₁Pyr]Lev sample, the activation energy associated with dc conductivity, σ_dc, was determined to be E = 0.90 eV. In the hydrated ionic liquids examined in the present study, increasing water content results in higher fragility values. In contrast, the opposite trend has been reported for hydrated BMIMCl systems, where higher fragility values were observed at lower water contents [50]. This difference suggests distinct modes of water interaction and structural organization across different ionic liquid systems.

The ionic liquid samples [C₂C₁Pyr]Lev containing 9% and 59% water and [C₄C₁Pyr]Lev containing 10% and 44% water exhibit dc conductivity values at their respective glass transition temperatures of 0.8 × 10⁻¹³ S/cm, 2.4 × 10⁻¹³ S/cm, 1.8 × 10⁻¹³ S/cm, and 7.4 × 10⁻¹³ S/cm, respectively. These values, particularly for [C₄C₁Pyr]Lev with 59% water content. are significantly higher than the characteristic value of ~10⁻¹⁵ S/cm expected when ionic motion is fully coupled to structural relaxation. This finding indicates that although structural mobility is effectively frozen at T_g (as determined by DSC), a residual ionic mobility persists and continues to contribute to charge transport. Therefore, the translational ionic motion in the hydrated ionic liquids studied here appears to be partially decoupled from the structural relaxation process.

Similar decoupling behavior between ionic transport and structural (α) relaxation has been reported in several ionic liquid systems [42,51,52,53]. It should be noted, however, that in lidocaine-based ionic liquids no decoupling between ionic motion and structural relaxation was observed over a range of water contents [25]. These contrasting findings further emphasize that the interaction and structural organization of water within ionic liquids are highly system-dependent, leading to distinct coupling or decoupling behaviors between translational ionic motion and structural relaxation.

The dc conductivity at 25 °C, a key parameter for practical ionic liquid applications, increases with increasing water content in both ionic liquid systems. For the [C₂C₁Pyr]Lev ionic liquid, the conductivity rises by approximately one order of magnitude as the water content increases from 9% to 59%, reaching values on the order of 10⁻⁴ S/cm (9% sample) and 10⁻³ S/cm (59% sample). In the case of the [C₄C₁Pyr]Lev ionic liquid, the effect of water is even more pronounced. The 44% sample exhibits a conductivity at 25 °C of about 10⁻³ S/cm, nearly two orders of magnitude higher than that of the 10% sample (≈10⁻⁵ S/cm). Thus, the enhancement of conductivity induced by water is significantly stronger in [C₄C₁Pyr]Lev than in [C₂C₁Pyr]Lev. As discussed in [Sippel 2015] the conductivity at 25 °C is governed by both the glass transition temperature (Tg) and the fragility index (m). In agreement with this relationship, the 10% [C₄C₁Pyr]Lev sample—characterized by a relatively high T_g, Arrhenius-type temperature dependence, and consequently a low m value—exhibits the lowest dc conductivity at 25 °C. In contrast, the 59% [C₂C₁Pyr]Lev sample, which combines a lower T_g with a higher fragility index, shows the highest dc conductivity at the same temperature.

4. Conclusions

In the present work the effect of water on the dynamics and conductivity of [C2C1Pyr]Lev and [C4C1Pyr]Lev ionic liquids was investigated. It was found that the water content is a key factor controlling the dynamics and conductivity of ionic liquids, with effects that depend critically on the cation structure. The main results are summarized as follows:

Plasticizing Effect: For both ionic liquids, water acts as a plasticizer, effectively decreasing the glass transition temperature (Tg). The cation structure significantly influences the degree of Tg plasticization, which is more pronounced in the [C2C1Pyr] cation compared to [C4C1Pyr]. This suggests that the alkyl chain length of the cation plays a critical role in water organization and its interaction with the ionic liquid matrix.

Relaxation Dynamics: The temperature dynamics of the main relaxation and DC conductivity exhibit Vogel–Fulcher–Tammann (VFT) behavior in all samples, with the exception of the [C4C1Pyr]Lev sample with 10% water content, which follows Arrhenius behavior.

Dielectric Strength: The dielectric strength (Δε) of the main relaxation decreases with increasing temperature for most samples. However, the [C4C1Pyr]Lev sample with 10% water content exhibits an increase in Δε with temperature.

Ion Mobility and Coupling: The mobility of ions participating in the main relaxation is consistent with the bulk mobility of the ionic liquid. At Tg, as extracted from DSC measurements, the DC conductivity values for all samples are noticeably higher than the expected value of 10⁻¹⁵ S/cm. This discrepancy indicates that while structural mobility is frozen at Tg, there is still notable ionic movement, proving that translational ionic motion in these hydrated systems is not well coupled to the structural relaxation measured by DSC.

Fragility Index: In all studied samples, higher water content leads to increased fragility index (m) values.

Conductivity at 25 °C: The DC conductivity at room temperature is determined by the synergy between Tg and the fragility index m. The 10% [C4C1Pyr]Lev sample (high Tg, Arrhenius behavior, lowest m) exhibits the lowest conductivity at 25 °C, whereas the 59% [C2C1Pyr]Lev sample (low Tg, high m) shows the highest conductivity.

In conclusion, water content is a critical parameter that drastically affects the dielectric and electrical properties of ionic liquids. The molecular structure of the ionic liquid components plays a decisive role in both the organization of water and the nature of the resulting interactions.

These results can be explained by the differing roles of water in the two ionic liquids. In [C₂C₁Pyr]Lev, water integrates into the ionic network, supporting structural integrity and enabling efficient ion transport. In [C₄C₁Pyr]Lev, water disrupts the packing of the long alkyl chains, reducing the number of ions actively participating in the main relaxation and partially decoupling ionic motion. This molecular-level distinction accounts for the observed differences in Δε, Tg, fragility, and conductivity, highlighting the crucial influence of cation architecture on hydration effects. These insights provide mechanistic guidance for the rational design of IL-based electrolytes under non-anhydrous conditions.