Interactions in Lidocaine-Carboxylic Acid-Based Deep Eutectic Solvents: Implications for Cobalt Extraction

Abstract

The limited understanding of intermolecular interactions in deep eutectic solvents (DESs) has restricted their rational design and broader application. In this study, a series of hydrophobic DESs (HDESs) were prepared using lidocaine as the hydrogen bond acceptor and various carboxylic acids as hydrogen bond donors. Their physicochemical properties, including density, viscosity, and thermal stability, were systematically characterized. Interactions between components were evaluated through excess molar volume, viscosity deviation, and Grunberg–Nissan parameters. Strong hydrogen bonding between lidocaine and carboxylic acids was confirmed, which weakened with increasing alkyl chain length of the acids. Furthermore, as the acid content in the mixture increased, lidocaine’s ability to disrupt the intrinsic hydrogen-bonding network of the carboxylic acids decreased, thereby weakening the hydrogen bonding interactions between the components. The extraction capability of the HDESs for cobalt ions was evaluated in aqueous systems. Cobalt, a key material in lithium-ion batteries and advanced alloys, is in rising demand due to clean energy development. The lidocaine/decanoic acid (1:2) system exhibited nearly 100% extraction efficiency, surpassing conventional extractants. The hydrophobic nature of the HDESs facilitated effective phase separation and reduced solvent loss. These findings provide theoretical insights and design principles for developing high performance HDESs tailored for environmentally friendly metal recovery, particularly in battery recycling and treatment of industrial wastewater.

1. Introduction

In recent years, deep eutectic solvents (DES) have emerged as environmentally friendly alternatives to ionic liquids [1], demonstrating broad application prospects in catalysis [2], electrochemistry [3], and separation science [4] due to their customizable properties, low toxicity, and simple preparation [5,6]. Typically, DESs are formed through intermolecular interactions between hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), resulting in customizable physicochemical properties that can be precisely tailored by adjusting the composition and molar ratio of their constituents [7].

The first DES, reported in 2001, was inherently hydrophilic [8]. However, conventional hydrophilic DES (e.g., water–choline chloride systems) face challenges in phase-separation-dependent applications such as liquid–liquid extraction, where their miscibility with water leads to component loss and recovery difficulties [9]. To address these limitations, hydrophobic deep eutectic solvents (HDES) have been developed. Van et al. [10] pioneered the preparation of HDES using long-chain carboxylic acids and quaternary ammonium salts, establishing a non-aqueous system through the deliberate selection of hydrophobic components (e.g., long-chain carboxylic acids, quaternary amines). These systems demonstrate unique advantages in metal ion extraction and organic pollutant adsorption. The distinctive characteristics of HDES [11], including density, viscosity, acidity, polarity, and volatility, coupled with their exceptional extraction capabilities for various target analytes [12], have attracted significant scientific interest. For instance, Tereshatov et al. [13] employed menthol-based HDES, consisting of a quaternary ammonium salt/menthol as the HBA and a carboxylic acid as the HBD, to selectively extract indium from hydrochloric and oxalic acid media. Subsequent studies have further broadened the scope of HDESs in valuable metal recovery(

Table 1. Representative studies on HDES-based metal extraction.

HBA/HBD Composition	Target Metal Ion(s)	Extraction Medium	Extraction Efficiency	Study/Ref.
Aliquat336/L-menthol (3:7)	Fe3+ Mn2+ Co2+	HCl solution	~99%	Milevskii et al. [14]
TOPO/Decanoic acid	La3+ Ce3+	Battery acid leachate	96–98%	Cruz et al. [15]
Choline chloride/Citric acid (2:1)	Co2+	LiCoO2	98%	Peeters et al. [16]
Di-(2-ethylhexyl) phosphate/1-butanol (2:1)	Co2+	HCl solution	95.64%	Liu et al. [17]
Polyethylene glycol 400/p-Toluene sulfonic acid (1:1)	Co2+ Mo2+	Spent catalyst	93% of Co/87% of Mo	Ebrahimi et al. [18]
Choline chloride/Urea (1:2)	Pb2+ Co2+ Ni2+ Mn2+	Oil samples	>95%	Soylak et al. [19]

). Milevskii et al. [14] achieved 99% recovery rates for Fe(III), Mn(II), and Co(II) from hydrochloric acid solutions using Aliquat336/L-menthol HDES (3:7 molar ratio), demonstrating successful metal stripping into aqueous phase for both metal recovery and solvent regeneration. Cruz et al. [15] developed a trioctylphosphine oxide-decanoic acid HDES system that effectively separated lanthanides from battery acid leachate. Under an aqueous-to-HDES mass ratio of 1:8, the system achieved high extraction efficiencies (96% for La and 98% for Ce) with remarkable separation factors (763 for La/Ni and 1149 for Ce/Ni).

Lidocaine has demonstrated remarkable capabilities in metal extraction. For instance, Dannie et al. [20] successfully prepared HDESs using lidocaine and decanoic acid at various molar ratios for the removal of alkali and transition metal ions from the aqueous phase. They found that cobalt could be extracted with a high distribution coefficient, achieving an extraction efficiency greater than 98%. Ola et al. [21] used the same HDES system (lidocaine and decanoic acid) to separate iron from manganese. Depending on the pH, extraction could occur either through exchange with lidocaine or via ion pair formation with decanoic acid, with the latter being more favorable as it limits contamination of the aqueous phase. In this study, the authors achieved high iron extraction efficiency (approximately 100% for 10 mmol·L⁻¹ Fe) and a high separation factor for manganese (separation factor > 100). Shaibuna et al. [22] synthesized a series of HDESs with varying molar ratios of lidocaine/ibuprofen and heptanoic acid for the efficient removal of heavy metals such as Cd(II), Pb(II), and Ni(II) from wastewater. Following parameter optimization, the lidocaine–heptanoic acid system (1:1 molar ratio) exhibited excellent extraction performance (95–97%) in both single-ion and mixed-ion solutions. The presence of basic functional groups in lidocaine facilitates selective coordination or ion-exchange interactions with metal ions [23]. However, studies have revealed that during the extraction process, lidocaine can readily undergo protonation by interacting with protons (H⁺) in the aqueous phase, resulting in enhanced hydrophilicity and migration into the aqueous phase. This leads to the irreversible loss of active HDES components and compromises the hydrophobic stability of the system, significantly limiting its reusability. As a result, enhancing the hydrophobicity and structural stability of HDESs has become a critical challenge.

To address these issues, researchers have proposed optimizing the structural components of HDESs (i.e., the hydrogen bond acceptor or donor) to modulate interfacial behavior and improve chemical stability. Long-chain carboxylic acids [24] (e.g., decanoic acid, lauric acid) have shown great potential as hydrogen bond donors (HBDs), as their extended alkyl chains enhance hydrophobicity and effectively suppress miscibility with the aqueous phase [25]. Furthermore, the carbon chain length, degree of branching, and substituent groups on the carboxylic acid significantly influence the polarity, viscosity, and intermolecular interactions of the HDES, thereby enabling fine-tuning of extraction performance. Nevertheless, the synergistic mechanisms involving hydrogen bonding, van der Waals forces, and hydrophobic interactions within HDESs remain insufficiently understood.

In this study, lidocaine is employed as the hydrogen bond acceptor (HBA), and four carboxylic acids—8-methylnonanoic acid (Versatic 10), decanoic acid, dodecanoic acid (lauric acid), and tetradecanoic acid (myristic acid)—are selected as HBDs to construct a series of HDES systems at various molar ratios. To elucidate the influence of hydrogen bonding on the physicochemical properties of these systems, we determined the density and viscosity of each HDES, calculated excess molar volumes, viscosity deviations, and Grüneisen–Nissan interaction parameters, and analyzed the systems using Fourier-transform infrared (FT-IR) spectroscopy. These analyses provide theoretical support for optimizing HDES performance and broadening their potential applications. Additionally, the synthesized lidocaine-based HDESs were applied to extract cobalt (Co²⁺) ions from aqueous solutions, with a particular focus on how inter-component interactions affect extraction efficiency. The experimental results are interpreted through structure–property and structure–activity relationships, offering new insights into the rational design of high-performance HDESs. The recyclability of the developed HDESs was also evaluated to assess their suitability for sustainable metal recovery applications.

2. Materials and Methods

2.1. Reagents and Chemicals

Lidocaine (99%) and cobalt(II) chloride hexahydrate (99%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Decanoic acid (98%), lauric acid (99%), and myristic acid (99%) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Versatic 10 (100%) was supplied by Kopper Chemical Industry Corp., Ltd. (Chongqing, China). Deionized water was used in all experiments.

2.2. HDES Preparation

The HDES preparation procedure was conducted as follows: First, the selected HBD and HBA were combined in a reaction vessel at predetermined molar ratios. The vessel was then placed on a thermostatic heating magnetic stirrer ((Wiggens Technology Co., Ltd., Beijing, China) and homogenized under continuous stirring at 500 rpm and a constant temperature of 343.15 K (70 °C) until a transparent, homogeneous liquid formed. After preparation, the resulting liquid was stored at ambient temperature (25 °C) for 24 h to assess stability. Systems exhibiting phase transitions (e.g., liquid-to-gel or liquid-to-solid transformations) during this period were discarded due to insufficient stability, while those retaining liquid phase integrity were retained for subsequent experimental studies.

2.3. Investigation of HDES Physicochemical Properties and Intermolecular Interactions

The viscosity characteristics of synthesized HDES systems were systematically characterized across a temperature range of 20–90 °C using a rotational viscometer (Shanghai Pingxuan Scientific Instrument Co., Ltd., China) (measuring range: 1–1 × 10⁶ cP; accuracy: ±1%). As a critical physical property, the high viscosity of HDES is primarily attributed to the intricate hydrogen-bonding networks formed between their components [26]. To elucidate the molecular interaction mechanisms and quantify interaction strengths among HDES components, the viscosity deviation (Δη) was calculated to characterize the non-ideal mixing behavior of the systems. Δη is defined as the difference between the experimentally measured viscosity and the ideal linearly weighted value, expressed by the following equation:

$$\Delta \eta =\eta -x_1{\eta }_1-x_2{\eta }_2$$

(1)

In the equation, η₁ and η₂ represent the viscosities of the HBA and HBD, respectively, while η denotes the viscosity of the prepared DES. The variables x₁ and x₂ correspond to the mole fractions of the HBA and HBD, respectively.

The Grüneisen–Nissan interaction parameter (d) serves as an approximate measure of the interaction strength between components in the mixture. This parameter can be calculated using the following relationship:

$$d={\displaystyle \frac{\mathit{ln}\eta -x_1\mathit{ln}{\eta }_1-x_2\mathit{ln}{\eta }_2}{x_1{x}_2}}$$

(2)

The strength of intermolecular interactions can be assessed based on the following criteria: When Δη > 0 and d > 0: indicates strong interactions between components. When Δη < 0 and d > 0: suggests weak interactions between components. When Δη < 0 and d < 0: implies negligible or no interactions between components.

The density of HDES was determined via the isothermal dilution method: a 50 mL volumetric flask was placed in a precision thermostatic water bath (±0.1 °C), and the average value was obtained from three independent measurements.

The density of DES is significantly influenced by the structural properties of the HBD and HBA. The excess molar volume (V^E), defined as the difference between the molar volume of the real mixture formed by pure components and that of an ideal solution, serves as a critical parameter for evaluating intermolecular interactions. V^E is calculated as follows:

$$V^E={\displaystyle \frac{X_1{M}_1+X_2{M}_2}{\rho }}-{\displaystyle \frac{X_1{M}_1}{{\rho }_1}}-{\displaystyle \frac{X_2{M}_2}{{\rho }_2}}$$

(3)

In the equation, X₁ and X₂ represent the mole fractions of the HBA and HBD, respectively; ρ denotes the density of the DES, while ρ₁ and ρ₂ are the densities of the HBA and HBD, respectively.

The variation in excess molar volume (V^E) can be attributed to molecular interactions, structural and geometric effects, and changes in free volume [27]. The sign and magnitude of V^E reflect volume contraction (negative V^E) or expansion (positive V^E) during mixing. Generally, deviations in V^E are primarily determined by specific interactions between different molecules (e.g., dipole–dipole interactions, van der Waals forces, or hydrogen bonding) and the adjustable modulation of molecular gaps governed by physical forces. Strong directional interactions such as dipole–dipole forces, van der Waals interactions, or hydrogen bonding contribute to negative V^E, while dispersion forces and electrostatic repulsion are responsible for positive V^E.

Thermal stability: the thermal decomposition behavior and stability of different HDES were investigated using thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹.

Differential scanning calorimetry (DSC) analysis was performed to evaluate the thermal behavior of the individual components and the synthesized HDESs. Approximately 5–10 mg of each sample was accurately weighed and sealed in standard aluminum pans. The experiments were carried out on a DSC instrument (Model Q2000, TA Instruments, New Castle, DE, USA) under a constant nitrogen flow to prevent oxidative degradation. The temperature program ranged from −60 to 200 °C with a constant heating rate of 10 °C min⁻¹. All measurements were conducted in triplicate to ensure reproducibility. The resulting thermograms were analyzed to assess phase transitions and to evaluate the impact of HDES formation on the thermal properties of the components.

FT-IR analysis: To verify intermolecular interactions within the deep eutectic solvents, Fourier-transform infrared (FT-IR) spectroscopy was conducted on the synthesized HDES, pure HBD, and pure HBA using a Tensor27 spectrometer (Shanghai RANCHAO Optoelectronic Technology Co., Ltd., Shanghai, China). The KBr pellet method was employed for sample preparation. To prevent moisture absorption by KBr during testing, the pellet preparation process was carried out under an infrared heating lamp. For the pure HBA and HBD, a small amount of sample was mixed with finely ground KBr powder, thoroughly homogenized by grinding, and pressed into transparent pellets. The observed shifts in characteristic absorption bands (e.g., O–H, N–H, or C=O stretching vibrations) were analyzed to identify hydrogen bonding and other intermolecular interactions.

¹H NMR analysis: To further confirm the successful formation of the hydrophobic deep eutectic solvents (HDESs) and investigate their hydrogen-bonding interactions, proton nuclear magnetic resonance (¹H NMR) spectroscopy was performed using a Bruker Avance III HD 500 MHz NMR spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany). The spectra of the synthesized HDESs and their individual components were recorded after dissolving the samples in deuterated chloroform (CDCl₃). All measurements were conducted at room temperature with 16 scans to ensure adequate signal quality.

2.4. Extraction Experiment

A pipette was used to measure specified volumes of the aqueous solution and organic phase, which were then added into a 20 mL centrifuge tube at an organic-to-aqueous (O/A) phase ratio of 1:1 (8 mL each). The tube was placed in a centrifuge (Shanghai CHUBO Laboratory Equipment Co., Ltd., Shanghai, China) for phase separation (3000 rpm, 5 min). After centrifugation, a clear interface formed: the upper layer comprised the organic phase, and the lower layer was the aqueous phase. The aqueous phase was carefully removed, and the organic phase was decanted from the top for subsequent stripping and recycling. The metal concentration in the raffinate was quantified using an atomic absorption spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The extraction procedure is illustrated in Figure 1.

3. Results

3.1. Preparation of HDES

Lidocaine (HBA) and four carboxylic acids-decanoic acid, dodecanoic acid, myristic acid, and Versatic 10 (HBDs)-were used to prepare a series of HDESs at various molar ratios, as listed in

Table 2. List of the lidocaine-based hydrophobic HDES.

Abbreviation	HBA	HBD	HBD:HBA Molar Ratio	Phase State at Room Temperature
Lid/DecA-HDES(1:1)	Lidocaine (Lid)	Decanoic acid (DecA)	1:1	Homogeneous transparent liquid
Lid/DecA-HDES(1:2)		Decanoic acid (DecA)	1:2	Homogeneous transparent liquid
Lid/DecA-HDES(1:3)		Decanoic acid (DecA)	1:3	Homogeneous transparent liquid
Lid/DecA-HDES(1:4)		Decanoic acid (DecA)	1:4	Homogeneous transparent liquid
Lid/DodeA-HDES(1:1)		Dodecanoic acid (DodeA)	1:1	Homogeneous transparent liquid
Lid/DodeA-HDES(1:2)		Dodecanoic acid (DodeA)	1:2	Homogeneous transparent liquid
Lid/DodeA-HDES(1:3)		Dodecanoic acid (DodeA)	1:3	Gel-like
Lid/DodeA-HDES(1:4)		Dodecanoic acid (DodeA)	1:4	Gel-like
Lid/MyrA-HDES(1:1)		Myristic acid (MyrA)	1:1	Homogeneous transparent liquid
Lid/MyrA-HDES(1:2)		Myristic acid (MyrA)	1:2	Gel-like
Lid/MyrA-HDES(1:3)		Myristic acid (MyrA)	1:3	Gel-like
Lid/MyrA-HDES(1:4)		Myristic acid (MyrA)	1:4	Gel-like
Lid/Versatic10-HDES(1:1)		Versatic 10 (Versatic10)	1:1	Homogeneous transparent liquid

. In a typical procedure, the HBA and HBD were accurately weighed according to the desired molar ratio, with a total combined mass of 20 g. The mixture was placed in a glass vial and heated to 343.15 K under magnetic stirring at 500 rpm for 30 min or until a visually homogeneous and transparent liquid was obtained. The resulting liquids were then stored at ambient temperature (273.15 °C) for 24 h to evaluate their phase stability. Systems that remained as homogeneous liquids were selected for further analysis, while those showing gelation or crystallization were excluded. Homogeneity was confirmed visually under ambient light; a system was deemed homogeneous if it exhibited no phase separation, turbidity, or suspended particulates after 24 h of standing.

3.2. Physicochemical Properties of HDES

During the HDES screening process, it was observed that not all HDES formulations exist in stable states. Alterations in the molar ratios and structural configurations of HBDs and HBAs directly influence the physicochemical properties and intermolecular interaction strengths of synthesized HDES. To investigate the structure-property relationships of HDES, it is imperative to systematically characterize the physicochemical properties of these HDES systems.

3.2.1. Density of HDES

The densities of lidocaine-carboxylic acid-based HDES were measured at atmospheric pressure across a temperature range of T = 293.15–363.15 K.

As shown in Figure 2, all HDES systems exhibited significant temperature-dependent density behavior, characterized by a linear decrease in density with increasing temperature, ranging from 0.91 to 0.98 g/cm³. This trend arises from increased intermolecular voids at elevated temperatures, leading to reduced density, consistent with the volumetric expansion characteristics of typical fluids [22]. At a constant temperature (293.15 K), the densities of HDES formed between lidocaine and carboxylic acids with distinct carbon-chain structures followed this order: Lid/MyrA-HDES(1:1)(ρ = 0.95 g/cm³) < Lid/DodeA-HDES(1:1) (ρ = 0.96 g/cm³) < Lid/DecA-HDES(1:1) (ρ = 0.97 g/cm³) < Lid/Versatic10-HDES(1:1) (ρ = 0.98 g/cm³). This sequence underscores the significant influence of the carboxylic acid structure on HDES density. Specifically, increasing the degree of branching in the HBD (from MyrA to Versatic 10) reduces molecular packing efficiency and increases molar volume, which is consistent with the principles of free volume theory and steric hindrance effects [28]. Furthermore, the density of Lid/DecA-HDES was found to decrease with increasing carboxylic acid content. At 293.15 K, the density dropped from 0.97 g/cm³ to 0.94 g/cm³ as the molar fraction of decanoic acid increased. This reduction is attributed to the lower intrinsic density of the carboxylic acid relative to lidocaine. As the proportion of the carboxylic acid increases, the overall density of the HDES approaches that of the pure acid, thereby decreasing the density of the mixture.

3.2.2. Viscosity of HDES

The temperature-dependent viscosity of lidocaine-based carboxylic acid HDES systems was systematically investigated using a rotational rheometer. The experimental design included two dimensions: (1) fixing the hydrogen bond donor (DecA) while varying the molar ratio (1:1–1:4), with viscosity measured over the temperature range of 293.15–343.15 K; and (2) fixing the molar ratio (1:1) while altering the structure of the HBD (DecA, DodeA, MyrA, Versatic 10), with measurements extended to a higher temperature region of 343.15–363.15 K.

As shown in Figure 3, the viscosity of all HDES systems exhibited strong temperature sensitivity. Below 323.15 K, viscosities decreased rapidly, with most lidocaine-carboxylic acid HDES systems exceeding 200 cP. Above 323.15 K, the rate of viscosity decline slowed. This behavior is attributed to the tight molecular packing and reduced free volume at lower temperatures. As the temperature increases, molecular kinetic energy rises, enhancing relative motion between molecules and thereby reducing dynamic viscosity [29].

In the molar ratio effect study (Figure 3a), viscosity under isothermal conditions (298 K) displayed a non-monotonic trend: Lid/DecA(1:2) (η = 350.05 cP) > Lid/DecA(1:3) (η = 302.13 cP) > Lid/DecA(1:1) (η = 281.03 cP) > Lid/DecA(1:4) (η = 198.97 cP). This pattern reflects a dynamic balance between enhanced hydrogen-bond network formation and dilution effects. At a 1:2 ratio, the increased DecA content promotes a denser hydrogen-bonding network, raising viscosity. However, further increase in DecA (≥1:3) introduces a dilution effect that weakens intermolecular interactions, leading to a reduction in viscosity.

Further structural effect analysis (Figure 3b) demonstrated that, at a fixed 1:1 molar ratio, donor carbon-chain architecture significantly influenced rheological behavior: Lid/DecA(1:1) (η = 11.59 cP) < Lid/DodeA(1:1) (η = 13.57 cP) < Lid/MyrA(1:1) (η = 14.11 cP) < Lid/Versatic10(1:1) (η = 14.70 cP). This trend indicates that the carbon-chain architecture of the HBD significantly influences HDES rheology. Viscosity increased with longer or more branched alkyl chains. In particular, the branched structure of Versatic 10 resulted in higher viscosity than its linear counterparts, likely due to decreased molecular packing efficiency and stronger steric hindrance, which may enhance intermolecular hydrogen-bonding interactions [30].

3.2.3. Thermal Stability of HDES

Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability of lidocaine-based HDESs. Figure 4 presents the thermogravimetric (TG) curves of these HDES systems. The single-step mass loss observed in the curves indicates structural stability and strong intermolecular interactions between components, suggesting that the HDES components behave as a unified entity during thermal degradation.

The structural characteristics of HDES components significantly influenced their thermal decomposition temperatures. When varying the chain length of the HBD, the onset decomposition temperature decreased with increasing HBD chain length. For molar ratio variations, the highest onset decomposition temperature occurred at a higher lidocaine content (X_Lid = 0.5), with mass loss initiating at 200 °C. As the lidocaine content decreased, the onset decomposition temperature gradually declined, reaching 155 °C at HBD:HBA molar ratio of 1:4. Notably, the thermal decomposition temperatures of HDES were lower than those of the pure raw components, likely due to hydrogen bonding interactions disrupting the original crystalline lattice structures of the individual components, rendering the HDES more thermally labile.

To evaluate the thermal behavior and phase properties of the synthesized lidocaine-based HDESs, DSC measurements were extended to a wider temperature range from −60 to 200 °C (Figure 5). The DSC thermograms of the individual components-lidocaine and decanoic acid-exhibited sharp endothermic peaks, indicating well-defined melting transitions associated with their crystalline nature.

In contrast, the DSC thermogram of the lidocaine–decanoic acid HDES showed a complete absence of any distinct endothermic or exothermic peaks throughout the entire temperature range. This indicates that no classical phase transitions (e.g., melting or crystallization) occurred, suggesting that the original crystalline structures of the components were disrupted during HDES formation. The result implies the formation of a homogeneously mixed, amorphous or glass-like phase. Such thermal behavior is characteristic of DESs or HDESs, in which strong non-covalent interactions—primarily hydrogen bonding and van der Waals forces—suppress crystallization and stabilize a single-phase system. Although these systems do not display the classical eutectic melting peak due to their non-equilibrium nature, the observed thermal signature supports the formation of a stable, molecularly integrated HDES rather than a simple physical mixture [31,32].

3.3. Investigation of Intermolecular Interactions in HDES

3.3.1. Excess Molar Volume

The temperature-dependent excess molar volume (V^E) of lidocaine-carboxylic acid HDES systems, calculated using Equation (3), is illustrated in Figure 6. The study revealed significant negative deviations from ideality(V^E < 0) for all systems, which thermodynamically indicates the presence of strong intermolecular interactions within the HDES [33]. According to solution theory, the magnitude of negative deviation(|V^E|) is positively correlated with the strength of attractive forces between the components. The experimental data demonstrated large |V^E| values, suggesting the formation of robust non-covalent interaction networks between lidocaine and carboxylic acid donors, with hydrogen bond-driven molecular recognition mechanisms serving as the primary driving force [34].

Further structure–property relationship studies highlighted the regulatory role of HBD structural features on intermolecular forces. Increasing the alkyl chain length of linear HBDs (C10 → C14) progressively reduced the |V^E| deviation, indicating that hydrophobic chain elongation weakens effective hydrogen-bond formation. Notably, at 343.15 K, branched HBDs introduced steric hindrance, decreasing |V^E| by 4.97 m³/mol compared to their linear isomers, confirming the critical influence of molecular geometry on hydrogen-bond network construction [35]. The HBD:HBA molar ratio exhibited distinct stoichiometric effects on hydrogen-bond synergy. The maximum negative deviation occurred at a Lid:DecA = 1:1 ratio. At 343.15 K, |V^E| reached 5.14 m³/mol, signifying the formation of a densely crosslinked hydrogen-bond network. This phenomenon is likely attributed to the stereo complementarity between HBA and HBD and the saturation of multiple hydrogen-bonding sites.

3.3.2. Viscosity Deviation and Interaction Force Parameters

The viscosity deviation analysis based on Equation (1) requires pure substance viscosity data as a reference. Given that lidocaine’s melting point (343.15 K) precludes viscosity measurements in its solid state, this study calculated viscosity deviations within the temperature range of 343.15–363.15 K (at 5 K intervals). As shown in Figure 7, all lidocaine-carboxylic acid-based HDES systems exhibited significant positive viscosity deviations (Δη) across the experimental temperature range, with the maximum value observed for Lid/Versatic10-HDES(1:1) (Δη_max = 10.05 Cp). This result hydrodynamically confirms the presence of intermolecular interactions between components. Notably, as the temperature increased to 363.15 K, Δη decreased to 3.74 cP (a reduction of 62.79%), yet remained positive, indicating that thermal perturbation weakens but does not fully disrupt the intermolecular interaction networks. This phenomenon aligns with the combined action mechanism of viscosity deviation, where its magnitude is governed by molecular configuration compatibility, van der Waals forces, and hydrogen-bonding synergy [36].

To quantitatively elucidate the nature of intermolecular forces, the Grüneisen–Nissan interaction parameter (d) was further employed to characterize and compare the interaction strengths between HBD and HBA in HDES. As shown in Figure 8, all HDES systems exhibited positive d-values (d = 4.48–12.04) across the experimental temperature range, following a temperature-dependent decay trend. Specifically, at a fixed temperature (e.g., 343.15 K), the d-values ranked as Lid/DecA-HDES (6.67) > Lid/DodeA-HDES (5.44) > Lid/Versatic10-HDES (5.12) > Lid/MyrA-HDES (4.48). This sequence reveals two critical structure–activity relationships: increasing the linear carbon chain length of HBDs (DecA-C10 → MyrA-C14) reduced d-values by 19.3%, indicating that hydrophobic chain elongation weakens effective contact between HBDs and HBAs, while branched HBDs (Versatic10) decreased d-values by 23.24% compared to their linear isomeric counterparts (DecA), confirming that steric hindrance significantly disrupts hydrogen-bond network formation. These conclusions thermodynamically and kinetically corroborate prior findings from excess molar volume studies, which demonstrated weakened hydrogen bonding with increased chain length/branching, collectively validating that molecular structural engineering can precisely modulate intermolecular interaction strengths in HDES.

3.3.3. FT-IR Analysis

Fourier-transform infrared (FT-IR) spectroscopy provided direct evidence for elucidating the hydrogen-bonding network and molecular interaction mechanisms in lidocaine-carboxylic acid HDES. Figure 9 compares the evolution of characteristic vibrational peaks for pure decanoic acid, pure lidocaine, and their composite systems at varying molar ratios (1:1–1:4) under 293.15 K. The HDES spectra appear as superimpositions of the spectra of the individual components, with subtle but notable differences. Specifically, peak broadening and shifting are observed in the HDES spectra, while no new peaks emerge, indicating the presence of hydrogen bonding without the formation of new chemical species. Notably, the frequency shifts of the carbonyl (C=O) stretching vibration strongly correlate with hydrogen-bond strength: the C=O peak of pure decanoic acid is located at 1699.53 cm⁻¹ [37], while in HDES systems with molar ratios of 1:1 and 1:2, this peak red-shifts to 1692.11 cm⁻¹ ($\overline{\nu }$= −7.42 cm⁻¹) and 1693.70 cm⁻¹ ($\overline{\nu }$= −5.83 cm⁻¹), respectively. These observations align with the classical theory that hydrogen bonding reduces bond force constants [38]. Equation (4) This phenomenon indicates that at low hydrogen bond donor (HBD) ratios (1:1–1:2), the lone pair electrons on lidocaine’s nitrogen atom form strong N···H–O hydrogen bonds with the hydroxyl group of decanoic acid. These interactions surpass the strength of decanoic acid’s intrinsic O–H···O dimeric hydrogen bonds, leading to electron density redistribution in the C=O bond and a consequent weakening of bond strength. When the HBD ratio increases to 1:3–1:4, an anomalous blue shift in the C=O peak is observed. This reveals a dynamic reconfiguration mechanism of hydrogen-bonding patterns: excess decanoic acid promotes the formation of self-associated dimers, where O–H···O hydrogen-bond networks partially replace the heterotypic N···H–O interactions with lidocaine. Self-association reduces electron density in the π* antibonding orbital of the C=O bond, enhancing bond order and increasing bond force constants. Thus, HDES systems with Lid/DecA molar ratios of 1:1 or 1:2 exhibit the strongest intermolecular interactions.

$$\overline{\nu }={\displaystyle \frac{1}{2\mathsf{\pi }\mathrm{c}}}\sqrt{{\displaystyle \frac{\mathrm{K}}{\mathsf{\mu }}}}$$

(4)

c is the wavenumber r, μ is the reduced mass of the atoms, and K is the bond force constant of the chemical bond.

Figure 10 presents the Fourier-transform infrared (FT-IR) spectra of lauric acid (DodeA), myristic acid (MyrA), Versatic 10 (neodecanoic acid), and their corresponding HDES systems: Lid/DodeA-HDES(1:1), Lid/MyrA-HDES(1:1), and Lid/Versatic10-HDES(1:1). The hydrogen bonding between lidocaine and carboxylic acids primarily involves the nitrogen atom (N) of lidocaine and the hydroxyl group (–OH) of the carboxylic acid. Notably, the C–N stretching vibration peaks of lidocaine in HDES systems broaden or disappear, likely due to intermolecular interactions. As previously explained, hydrogen bonding primarily affects the bond force constant of the C=O group adjacent to the -OH group. The C=O stretching vibration peaks of pure lauric acid, myristic acid, and Versatic 10 are observed at 1701.22 cm⁻¹, 1701.94 cm⁻¹, and 1698.89 cm⁻¹, respectively. In their corresponding HDES systems, these peaks red-shift to 1694.78 cm⁻¹, 1696.73 cm⁻¹, and 1692.89 cm⁻¹, with red shifts of 6.44 cm⁻¹, 5.21 cm⁻¹, and 6.00 cm⁻¹, respectively. These reductions in bond force constants confirm strong intermolecular interactions. Notably, the extent of red shifts exhibits significant structural dependence: Lid/DodeA-HDES (Δν = −6.44 cm⁻¹) > Lid/Versatic10-HDES (Δν = −6.00 cm⁻¹) > Lid/MyrA-HDES (Δν = −5.21 cm⁻¹), all of which are smaller than the red shift observed in the lidocaine-decanoic acid HDES system (Δν = −7.42 cm⁻¹), all of which are smaller than the red shift observed in the lidocaine-decanoic acid HDES system (7.42 cm⁻¹). This trend reveals two key patterns: increasing the carbon chain length of linear carboxylic acids (C10 → C14) reduces the red shift magnitude by 29.78%, indicating that elongation of hydrophobic alkyl chains decreases the probability of effective molecular contact, thereby weakening hydrogen-bond interactions, while branched carboxylic acids (Versatic10) exhibit a 19.2% smaller red shift compared to their linear DecA counterparts (Δν = −7.42 cm⁻¹), attributed to steric hindrance effects, which diminish hydrogen-bond strength in HDES systems formed with branched acids compared to those with lidocaine and decanoic acid.

3.3.4. NMR Analysis

In the ¹H NMR spectra of lidocaine-based HDESs prepared with decanoic acid (C10), lauric acid (C12), and myristic acid (C14), notable differences were observed compared to the spectra of their individual components (Figure 11). Specifically, no new signals appeared in any of the HDES systems, indicating that no covalent chemical reactions occurred during their formation. This strongly suggests that the HDESs were formed through physical, non-covalent interactions. Similar conclusions were drawn by Florindo et al. [39], who reported that the formation of DESs is governed by intermolecular forces, particularly hydrogen bonding, without the formation of new chemical species.

Although no new peaks emerged, slight downfield shifts were observed in certain proton signals, particularly in regions corresponding to the lidocaine moiety (e.g., aromatic and amide protons). These shifts suggest changes in the local electronic environment due to molecular interactions between lidocaine and the fatty acid components. According to prior studies [40], such deshielding effects are often associated with hydrogen bonding, where electron-withdrawing groups alter the electron density surrounding nearby protons.

The combination of unaltered signal patterns (i.e., no new peaks) and subtle but consistent chemical shift changes provides strong evidence that the prepared systems are homogeneous eutectic mixtures stabilized by non-covalent interactions. These results confirm that the HDESs were successfully formed as single-phase systems through physical processes, which is consistent with previously reported behavior for similar DES systems [41].

3.4. Study on HDES Extraction of Cobalt from Aqueous Phase

To validate the application of lidocaine-carboxylic acid HDES in aqueous metal extraction, experimental evaluations were conducted on the extraction of cobalt (Co²⁺) ions from aqueous solutions using the synthesized HDES systems. Studies revealed that lidocaine undergoes protonation under acidic conditions to form lidocaine hydrochloride, which exhibits high aqueous solubility (50 mg·mL⁻¹). Consequently, during metal extraction, protonated lidocaine migrates to the aqueous phase, initiating an ion-exchange process with metal ions. The specific mechanisms are as follows [42]:

$${\mathrm{DecAH}}_{\mathrm{org}}+{\mathrm{Lid}}_{\mathrm{org}}\to {\mathrm{DecA}}_{\mathrm{org}}^{-}+{\mathrm{LidH}}_{\mathrm{org}}^{+}$$

(5)

$$2{\mathrm{LidH}}_{\mathrm{org}}^{+}+\mathrm{M}2+{{\mathrm{Cl}}_2^{-}}_{\mathrm{aq}}\to 2\mathrm{LidH}+{\mathrm{Cl}}_{\mathrm{aq}}^{-}+{\mathrm{M}}_{\mathrm{org}}^{2+}$$

(6)

In the HDES system, lidocaine abstracts hydrogen ions from carboxylic acids via hydrogen bonding, forming lidocaine hydrochloride. This process increases the population of carboxylate anions (${\mathrm{DecA}}_{\mathrm{org}}^{-}$) in the organic phase, which readily coordinate with metal ions in the aqueous phase, thereby facilitating efficient extraction.

3.4.1. Effect of Hydrogen Bond Donor on HDES Extraction of Cobalt

A study on the synergistic extraction performance of cobalt in strongly acidic media (pH = 1, Figure 12) revealed the intrinsic relationship between HDES structural design and extraction efficiency. Experimental data demonstrate that at an organic-to-aqueous phase volume ratio (O/A) of 1:1, linear-chain HDES systems (Lid/DecA, Lid/DodeA, Lid/MyrA) achieved Co²⁺ extraction efficiencies exceeding 98.13%, while the branched-chain Lid/Versatic10-HDES system exhibited only 31.17% efficiency. Notably, when the O/A ratio decreased to 1:5, distinct responses emerged: Lid/DecA-HDES maintained a high extraction efficiency of 95.85%, whereas Lid/DodeA-HDES and Lid/MyrA-HDES declined to 58.64% and 20.08%, respectively, representing reductions of 40.39% and 78.05%. This decline in extraction efficiency aligns with prior findings on hydrogen-bond strength: excess molar volume analysis indicated that the DecA system exhibited the largest negative deviation, and FT-IR characterization confirmed its maximum C=O red shift (Δν = −7.42 cm⁻¹), collectively validating that robust hydrogen-bond networks effectively stabilize the chelation environment for [CoCl₄]²⁻ complexes. In contrast, the cobalt extraction capacity of Lid/Versatic10-HDES(1:1) remained largely unaffected by changes in the O/A ratio. This is hypothesized to arise from Versatic10’s stronger hydrogen ion dissociation capability compared to other HBDs. During extraction, competition between hydrogen bonding (Lid/Versatic10) and Versatic10’s inherent hydrogen ion dissociation disrupts coordination dynamics, resulting in stable but low extraction efficiency (−30%), rendering it unsuitable for practical cobalt recovery.

The pH evolution patterns summarized in

Table 3. Effect of A/O on the aqueous equilibrium pH by lidocaine-carboxylic acid deep eutectic solvent extractant.

A/O	Equilibrium pH
	Lid/DecA-HDES(1:1)	Lid/DodeA-HDES(1:1)	Lid/MyrA-HDES(1:1)	Lid/Versatic10-HDES(1:1)
1	6.61	6.62	6.44	2.26
2	6.41	5.72	5.42	0.74
4	6.21	5.6	5.21	0.75
4	6.04	5.52	4.14	0.82
5	5.89	5.2	2.1	0.86

reveal distinct proton transfer mechanisms among different HDES types. For the Lid/Versatic10-HDES system, the equilibrium pH increased from 1.00 to 2.26 (ΔpH = +1.26) at an aqueous-to-organic (A/O) ratio of 1:1. However, when A/O ≥ 2:1, the pH reversed to 0.74–0.86, suggesting that hydrogen bonding between lidocaine and the branched carboxylic acid (Versatic10) competes with the intrinsic proton dissociation of the acid itself. This competition likely destabilizes the extraction process, rendering the system unsuitable for further investigation. In contrast, linear-chain HDES systems (DecA, DodeA, MyrA) consistently exhibited equilibrium pH values higher than the initial pH, with pH decreasing as the A/O ratio increased. For example, in Lid/DecA-HDES, the pH decreased from 6.61 to 5.89 with increasing A/O ratio. This behavior arises from the cation-exchange mechanism dominant in these systems: during extraction, lidocaine binds to hydrogen ions in the aqueous phase to form water-soluble lidocaine hydrochloride, reducing free H⁺ concentration and thereby elevating the equilibrium pH. According to Le Chatelier’s principle, increasing the initial aqueous pH enhances the reaction equilibrium toward metal ion coordination, improving extraction efficiency.

3.4.2. HBA Molar Ratio on HDES Extraction of Cobalt

As previously established, Lid/DecA-HDES exhibits optimal cobalt extraction performance. The extraction mechanism involves hydrogen-bond-driven proton transfer: Lid interacts with DecA to abstract hydrogen ions, enhancing the acid’s extraction capacity, while protonated lidocaine undergoes ion exchange with Co²⁺ to complete the extraction process. The extraction efficiency of Lid/DecA-HDES correlates with both intermolecular interaction strength and the effective concentration of active extractants. Variations in the HBD:HBA molar ratio alter these interactions and the lidocaine content, thereby modulating extraction performance. To investigate this, cobalt extraction efficiencies were evaluated for Lid/DecA-HDES systems with molar ratios ranging from 1:1 to 1:4 under ambient conditions using a 1 g/L metal ion solution.

As shown in Figure 13, the extraction efficiency for cobalt decreased from 99.49% to 48.79% as the Lid:DecA molar ratio shifted from 1:1 to 1:4 at pH = 1. This decline aligns with calculated intermolecular interaction strengths, which initially weakened and then slightly intensified with increasing DecA content, a result of complex hydrogen-bond network dynamics. Systems with molar ratios of 1:1 and 1:2 achieved near-complete metal extraction (>99%), while higher DecA ratios (1:3, 1:4) led to reduced efficiency due to weaker intermolecular interactions and diminished lidocaine availability.

The equilibrium pH of the raffinate (

Table 4. Effect of the mole ratios on the aqueous equilibrium pH by lidocaine-carboxylic acid deep eutectic solvent extractant.

pH	Equilibrium pH
	Lid/DecA-HDES(1:1)	Lid/DecA-HDES(1:2)	Lid/DecA-HDES(1:3)	Lid/DecA-HDES(1:4)
1	6.4	6.17	5.25	4.56
2	6.45	6.48	5.44	5.05
4	6.61	6.41	5.47	5.24
4	6.58	6.46	5.62	5.40
5	6.60	6.46	5.74	5.48

) exhibited a consistent decrease with higher DecA content, though all values remained above the initial aqueous phase pH. This trend arises because lidocaine migrates to the aqueous phase, binding free H⁺ ions as lidocaine hydrochloride, thereby reducing aqueous acidity. According to Le Chatelier’s principle, elevating the initial pH shifts the equilibrium toward metal coordination, enhancing extraction efficiency. These findings underscore the critical role of balanced HBD:HBA ratios in optimizing HDES performance for cobalt recovery.

3.4.3. Regeneration Performance of HDES

The recyclability of HDES is a critical metric for assessing its practical applicability in industrial settings. Stripping experiments were conducted on metal-loaded Lid/DecA-HDES systems using 0.1 M oxalic acid, with results summarized in

Table 5. Extraction rate, stripping rate and regeneration extraction rate of cobalt by HDES.

	Lid/DecA-HDES(1:1)	Lid/DecA-HDES(1:2)	Lid/DecA-HDES(1:3)	Lid/DecA-HDES(1:4)
Extraction/%	99.74	99.61	92.11	60.41
Stripping/%	27.46	76.21	84.4	90.85
Regeneration/%	99.44	98.98	91.24	59.11

.

As the lidocaine content decreased, the stripping efficiency gradually increased. The extraction-efficient Lid/DecA-HDES(1:1) exhibited a 72.28% metal retention rate (stripping efficiency: 27.46%), while the less efficient Lid/DecA-HDES(1:4) achieved a significantly higher stripping efficiency of 90.85%. This trend inversely correlates with hydrogen-bond network strength, as stronger networks hinder metal release during stripping.

Mechanistic analysis of the lidocaine-carboxylic acid HDES extraction process reveals that lidocaine binds with H⁺ to form water-soluble lidocaine hydrochloride, facilitating ion exchange with metal ions. While this process reduces the effective HDES component concentration, regenerated HDES systems retained superior cobalt extraction performance, demonstrating their potential for cyclic reuse.

3.4.4. Comparison of HDES Extraction Systems with Conventional Extraction Systems

As a novel extractant in solvent extraction, HDES eliminate the need for diluents, thereby mitigating environmental pollution associated with traditional diluent usage. This study conducted a comparative evaluation of cobalt extraction between conventional diluent-based systems and HDES systems. Given that Lid/DecA-HDES(1:2) demonstrated high extraction efficiency (99.61%) and significant stripping efficiency (76.21%) for 1 g/L cobalt solutions, this system (Lid:DecA = 1:2) was selected for comparative experiments, with results illustrated in Figure 14.

Initially, decanoic acid dissolved in kerosene was tested for cobalt extraction, revealing negligible extraction capacity (<5%), confirming that carboxylic acids alone lack sufficient coordination activity. Subsequently, a 1:2 molar mixture of lidocaine and decanoic acid dissolved in kerosene (diluent) demonstrated enhanced extraction efficiency (>60%), attributable to hydrogen bonding between lidocaine and decanoic acid. This interaction increases the effective concentration of active extractants, improving metal coordination.

In contrast, the Lid/DecA-HDES(1:2) system achieved near-quantitative cobalt extraction (>99%). This superior performance is hypothesized to arise from stronger and more organized hydrogen-bond networks in HDES compared to diluent-based mixtures. The absence of kerosene in HDES eliminates solvent-induced dilution effects, enabling dense molecular packing and amplified synergistic interactions between lidocaine and decanoic acid. These factors collectively enhance ligand accessibility and ion-exchange efficiency, underscoring HDES as a sustainable, high-performance alternative to traditional solvent extraction systems.

4. Conclusions

This study systematically investigated the physicochemical properties of lidocaine-carboxylic acid-based HDES and their application in cobalt extraction. Through density, viscosity, and thermal stability measurements, combined with calculations of excess molar volume, viscosity deviation, and Grüneisen–Nissan interaction parameters, the intermolecular interaction mechanisms within HDES were elucidated. FT-IR analysis revealed hydrogen bonding between the nitrogen atom of lidocaine and the carboxylic hydroxyl group of the acids. The interaction strength between components initially weakened and then strengthened with increasing carboxylic acid content, attributed to competitive hydrogen-bonding equilibria. Furthermore, elongation of carboxylic acid carbon chains was found to weaken intermolecular interactions, while the introduction of branching further reduced interaction strength, likely due to enhanced hydrogen ion dissociation in branched carboxylic acids, which disrupts hydrogen-bond formation. In application studies, lidocaine-decanoic acid HDES (Lid/DecA-HDES(1:2)) demonstrated exceptional performance in aqueous cobalt extraction, achieving 99.61% extraction efficiency and 76.21% stripping efficiency, with robust regenerability. These results validate its potential as a high-performance extractant for cobalt recovery, offering a sustainable and efficient solvent system for critical metal recycling.