Experimental Evaluation of CO₂ Absorption and Thermophysical Properties of TBAB-Based Deep Eutectic Solvents with Amine and Acid Donors

Abstract

Carbon dioxide emissions from fossil fuel burning remains a severe environmental challenge that needs to be addressed. Deep eutectic solvents (DESs) have emerged as promising alternatives to conventional alkanolamines for CO₂ capture applications due to their lower volatility and reduced corrosion potential. In this work, two tetrabutylammonium bromide (TBAB)-based systems were synthesized using different hydrogen bond donors: 2-amino-2-methyl-1-propanol (AMP) at a 1:1 molar ratio and p-toluenesulfonic acid (PTSA) at a 1:2 molar ratio. FTIR spectroscopic analysis confirmed that TBAB-AMP (1:1) forms a true DES through hydrogen bonding interactions, whereas TBAB-PTSA (1:2) undergoes proton transfer to form an ionic salt. CO₂ solubility measurements were conducted using the pressure drop method up to 15 bar at 30 °C. The TBAB-AMP system exhibited a CO₂ uptake of 0.194 mol CO₂/mol DES at 14.7 bar, approximately 2.5-fold higher than the TBAB-PTSA system, which achieved 0.079 mol/mol at 14.5 bar. Critical and thermophysical properties were estimated using the modified Lydersen–Joback–Reid, Lee–Kesler, and Haghbakhsh group-contribution methods. Viscosity measurements conducted from 30 to 50 °C revealed that TBAB-AMP exhibited significantly lower viscosity, ranging from 163 to 46 mPa·s, compared to TBAB-PTSA, which showed viscosity values between 536 and 155 mPa·s. The superior CO₂ capture performance of the amine-functionalized DES was attributed to favorable hydrogen-bonding interactions, lower viscosity, which enabled better mass transfer, and enhanced chemical affinity toward CO₂ through carbamate formation.

1. Introduction

Many countries restrict the atmospheric release of greenhouse gases due to their negative effects on the environment and public health. These gases include carbon dioxide (CO₂), sulfur oxides (SO_x), nitrogen oxides (NO_x), hydrogen sulphide (H₂S), and ammonia (NH₃). Mitigating greenhouse gases is one of the world’s most pressing challenges because of their substantial impact in climate change and global warming. Fuel significantly contributes to environmental pollution in the petrochemical industry due to its composition of several aromatic compounds, particularly nitrogen- and sulfur-containing aromatics, which emit toxic chemicals into the atmosphere upon combustion. The characteristics of fossil fuels can influence both climate and human health [1].

Over the past 10 years, renewable energy technology has advanced significantly. For wind and solar energy technologies, the net energy cost of power has decreased by 66% and 85%, respectively. This signifies that, only a decade ago, the mWh cost of solar power was about sixfold more. Furthermore, by 2040, fossil fuels are expected to account for 78% of global energy consumption. Carbon capture, storage, and utilisation of fossil-based emissions are essential as a transition strategy while renewable energy technologies mature and replace fossil fuels. In a nation like India, transportation and power production account for 45% of the nation’s overall greenhouse gas emissions [2,3,4,5]. Flue gas with up to 10–15 volume percent [6] CO₂ poses a serious risk to both the environment and public health. Therefore, it is crucial to create biodegradable absorbents with high capacity and low volatility to remove CO₂ from flue gas.

Amine solutions are the most commonly utilized reversible solvents for industrial CO₂ capture operations. However, economic and environmental concerns—including equipment corrosion, amine volatility losses, and high regeneration energy costs—necessitate the development of alternative solvents with lower volatility and reduced energy requirements [7].

Ionic liquids have attracted considerable attention as promising materials for gas separation applications, including CO₂ capture. They offer potential advantages over conventional solvents in terms of capacity, selectivity, regenerability, and thermal stability. Numerous ionic liquids have been investigated as CO₂ capture sorbents [8,9,10,11,12,13]. Further, ref. [14] developed imidazolium ILs for the chemical absorption of CO₂ with an amino-grafted cation. As the anion is required for carbon capture, several task-specific ILs have been produced across the world. As a result, in recent years, an increasing number of functional ILs have been developed, including those based on acids, amines, imidazoles, and carboxyls. Although functional ILs have a high CO₂ capacity even at low CO₂ partial pressures, they nevertheless have significant limitations, such as high viscosity, difficult synthesis processes, and high cost [15,16,17,18,19].

Deep eutectic solvents (DESs) have generated a lot of scientific attention in recent years. These materials are produced by eutectic mixing of a hydrogen bond acceptor (HBA), commonly a quaternary ammonium salt, with a hydrogen bond donor (HBD), resulting in a combination with a melting point significantly lower than either component. Abbott et al. defined “deep eutectic solvent” and divided DESs into four groups depending on their composition [20]. Type I DESs consist of quaternary ammonium salts with metal chlorides, Type II combines quaternary ammonium salts with metal chloride hydrates, Type III pairs quaternary ammonium salts with hydrogen bond donors (such as alcohols or amines), and Type IV comprises metal salts with hydrogen bond donors. n contrast to traditional ionic liquid synthesis, DES preparation is relatively straightforward and cost-effective. These fluids can be suggested as excellent carbon capture sorbents due to their favourable profiles. With great overall kinetics (25.2 wt percent uptake within 2.5 min), recyclability, and an unexpected gravimetric uptake of 33.7 wt percent, a DES composed of Monoethanolamine hydrochloride and ethylenediamine is demonstrated. The provided DES also performs sustainably when water is present, shows a respectable tolerance to temperature rise, and has a low heat of absorption. This suggests that even if a new chemical bond between DES and CO₂ forms, ingested CO₂ hardly affects the water solubility of DESs [21,22,23,24]. Further, applications of DES in biological matrices are linked to the experimental specifications and characteristics of the most commonly used matrices in clinical and toxicological analysis [25].

The development of innovative functionalized ionic liquids for CO₂ capture requires an examination of the thermodynamic characteristics of CO₂-functionalized ionic liquid systems. Investigations into the thermodynamic characteristics of CO₂ chemical absorption by functionalised ILs have attracted significant attention to date. For instance, to examine the thermodynamic characteristics of CO₂ in ILs functionalised with amines, Brennecke et al. proposed the “deactivated IL” and “two-reaction” models. According to Wang et al., entropic effects led to anion-functionalised [P66614][p-AA] and [P66614][p-ANA] exhibiting higher absorption capacities and lower reaction enthalpies. Through the “deactivated IL” model, Hu et al. thoroughly investigated the thermodynamic characteristics of CO₂ capture in low-viscosity fluorine-substituted phenolic ILs. Therefore, it is crucial to expand the thermodynamic study of various synthesised DES types [14,26,27,28,29].

Deep eutectic solvents (DESs) have emerged as promising alternatives to ionic liquids for CO₂ capture due to their low cost, biodegradability, and simple preparation. Early investigations by Li et al. demonstrated that ChCl:urea (1:2) exhibited maximum CO₂ capacity (xCO₂ = 0.309 at 12.5 MPa, 313.15 K), with solubility increasing with pressure and decreasing with temperature [30]. Systematic studies on glycol-based systems showed ChCl:ethylene glycol and ChCl:glycerol achieving solubilities up to 3.13 and 3.69 mol·kg⁻¹, respectively, at 303.15 K [31,32]. Li et al. reported ChCl:triethylene glycol (1:4) as the most effective among phenol and glycol-based DESs, while azole-functionalized systems (acetylcholine chloride:imidazole) demonstrated enhanced capacities (H_m = 1.91–2.32 MPa·kg·mol⁻¹) [33,34].

Hydrophobic DESs have attracted attention for water-resistant applications. Zubeir et al. measured CO₂ solubilities in decanoic acid-based DESs with tetraalkylammonium salts, finding that TBAB:decanoic acid (1.5:1) exhibited Henry’s constants (5.90 MPa at 298.15 K) comparable to fluorinated ILs without hydrolysis concerns [35]. Ali et al. synthesized 17 DESs and discovered that amine-functionalized systems (ChCl:monoethanolamine, 1:6) achieved xCO₂ = 0.1096, with 90% physical absorption requiring lower regeneration energy than aqueous MEA [36]. Luo et al. conducted comprehensive process simulations that confirmed DESs’ advantages of negligible vapor pressure and low ecotoxicity relative to conventional solvents [37].

CO₂ absorption in DESs consistently follows physical dissolution, with negative enthalpies (from −10 to −20 kJ·mol⁻¹) and solubilities correlatable with extended Henry’s law (AAD < 2%). These systems demonstrate absorption capacities comparable to or exceeding many ionic liquids while offering superior sustainability profiles for industrial CO₂ capture applications [33,34,38].

In this study, two TBAB-based systems were synthesized using different hydrogen bond donors—2-amino-2-methyl-1-propanol (AMP) at a 1:1 molar ratio and p-toluenesulfonic acid (PTSA) at a 1:2 molar ratio, to investigate their CO₂ capture performance. FTIR spectroscopy was employed to distinguish actual DES formation from ionic salt formation. CO₂ solubility measurements were conducted at 30 °C up to 15 bar using a SETARAM PCT-PRO apparatus. Critical properties were estimated using modified Lydersen–Joback–Reid and Lee–Kesler methods. Thermophysical properties were estimated using the Haghbaksh group contribution method. FTIR was performed to validate hydrogen bonding between the DES constituents. Temperature-dependent viscosity measurements from 30 to 50 °C were performed using an Anton Paar MCR 102e rheometer. The relationship between molecular structure, thermophysical properties, and CO₂ absorption performance was systematically evaluated.

2. Materials and Methods

2.1. Materials

Table 1. List of chemicals ^a used in this study.

Chemical Name	CAS No.	Purity	Supplier
Tetrabutylammonium bromide	40360	≥99%	Molychem (Mumbai, India)
Aminomethylpropanol	124-68-5	≥95%	Molychem
p-toluenesulfonic acid	24057-28-1	≥98%	S D Fine-Chem Limited (Mumbai, India)
Argon	7440-37-1	≥99.999%	Purshottam Gas Suppliers (Raebareli, India)
Carbon dioxide	124-38-9	≥99.999%	Purshottam Gas Suppliers
Nitrogen	7727-37-9	≥99.999%	Purshottam Gas Suppliers
Helium	7440-59-7	≥99.999%	Purshottam Gas Suppliers

^a Chemicals have been provided, and their purity was stated by the manufacturer.

lists the chemical names, purity levels, CAS numbers, and supplier names. The DES components (i.e., HBA and HBD) were dried overnight at 60 °C in a vacuum oven to remove moisture.

2.2. Synthesis of DES

DES mixtures were prepared by mixing HBA, such as tetrabutylammonium bromide, and two distinct HBD: Aminomethylpropanol (AMP) and p-toluene sulphonic acid at the appropriate HBA/HBD molar ratio, i.e., 1:1, and then heating the mixtures at 80 °C until a clear homogeneous liquid was formed. The obtained DESs were then dried at 60 °C for eight hours prior to use [20]. The chemical name, CAS number, sample purification method, purity, and supplier for the preparation of DESs are shown in Table 1, and

Table 2. List of DESs synthesized.

DES	HBA	HBD	Molar Ratio
DES1	Tetrabutylammonium Bromide	AMP	1:1
DES2	Tetrabutylammonium Bromide	p-toluene sulfonic acid	1:2

lists the DESs synthesized in this work.

2.3. Critical Properties Computation

The critical properties of each hydrogen bond donor and acceptor were determined using the Modified Lydersen–Joback–Reid (LJR) technique. Valderrama et al. devised the modified LJR approach by combining the Lydersen and Joback–Reid methods [39,40,41]. The modified LJR approach was developed to assess the critical properties of ionic liquids. It is designed explicitly for high-molecular-weight molecules. The critical parameters of HBD and HBA were estimated using the following equations:

$${\mathrm{T}}_{\mathrm{b}}\left(\mathrm{K}\right)=198.2+\sum \mathrm{n}\Delta {\mathrm{T}}_{\mathrm{b}}$$

(1)

$${\mathrm{T}}_{\mathrm{C}}\left(\mathrm{K}\right)={\displaystyle \frac{{\mathrm{T}}_{\mathrm{b}}}{\left[\mathrm{A}+\mathrm{B}\sum \mathrm{n}\Delta {\mathrm{T}}_{\mathrm{c}}-{\left(\sum \mathrm{n}\Delta {\mathrm{T}}_{\mathrm{c}}\right)}^2\right]}}$$

(2)

$$\mathrm{Pc} (\mathrm{bar})={\displaystyle \frac{\mathrm{M}}{[\mathrm{C}+\sum \mathrm{n}∆{\mathrm{P}}_{\mathrm{c}}{]}^2}}$$

(3)

$${\mathrm{V}}_{\mathrm{c}} ({\mathrm{cm}}^3/\mathrm{mol})=\mathrm{D}+\sum \mathrm{n}∆{\mathrm{V}}_{\mathrm{c}}$$

(4)

$${\mathrm{Z}}_{\mathrm{c}}={\displaystyle \frac{{\mathrm{P}}_{\mathrm{c}}{\mathrm{V}}_{\mathrm{c}}}{\mathrm{R}{\mathrm{T}}_{\mathrm{c}}}}$$

(5)

$$\omega ={\displaystyle \frac{{(\mathrm{T}}_{\mathrm{b}}-43){(\mathrm{T}}_{\mathrm{c}}-43)}{{(\mathrm{T}}_{\mathrm{c}}-{\mathrm{T}}_{\mathrm{b}}){(0.7\mathrm{T}}_{\mathrm{c}}-43)}}\log \left({\displaystyle \frac{{\mathrm{P}}_{\mathrm{c}}}{{\mathrm{P}}_{\mathrm{b}}}}\right)-{\displaystyle \frac{{(\mathrm{T}}_{\mathrm{c}}-43)}{{(\mathrm{T}}_{\mathrm{c}}-{\mathrm{T}}_{\mathrm{b}})}}\log \left({\displaystyle \frac{{\mathrm{P}}_{\mathrm{c}}}{{\mathrm{P}}_{\mathrm{b}}}}\right)+\mathrm{l}\mathrm{o}\mathrm{g}\left({\displaystyle \frac{{\mathrm{P}}_{\mathrm{c}}}{{\mathrm{P}}_{\mathrm{b}}}}\right)-1$$

(6)

where constants A = 0.5703, B = 1.021, C = 0.2573, and D = 6.75, M is the molecular mass (g/mol) of the molecule, n is the number of groups present in the molecule, and $\Delta {\mathrm{T}}_{\mathrm{b}}$, $\Delta {\mathrm{T}}_{\mathrm{c}}$, $∆{\mathrm{P}}_{\mathrm{c}}$, and $∆{\mathrm{V}}_{\mathrm{c}}$ epitomise group contribution of atoms in the molecule to their boiling point (K), critical temperature (K), critical pressure (bar), and critical molar volume (cm³/mol), respectively [39].

To test the consistency of estimated properties (T_b, T_c, and V_c), the density model proposed by Valderrama et al. was used [40].

Density Model for testing [41]:

$$\rho =\left({\displaystyle \frac{\mathrm{E}}{\mathrm{F}}}\right)+\left({\displaystyle \frac{2}{7}}\right)\left({\displaystyle \frac{\mathrm{E}\ln \mathrm{F}}{\mathrm{F}}}\right)\left({\displaystyle \frac{\mathrm{T}-{\mathrm{T}}_{\mathrm{b}}}{\mathrm{T}-{\mathrm{T}}_{\mathrm{c}}}}\right)$$

(7)

where

$$\mathrm{E}=\mathrm{a}+{\displaystyle \frac{\mathrm{b}\mathrm{M}}{{\mathrm{V}}_{\mathrm{c}}}}$$

(8)

and

$$\mathrm{F}=\left({\displaystyle \frac{\mathrm{c}}{{\mathrm{V}}_{\mathrm{c}}}}+{\displaystyle \frac{\mathrm{d}}{\mathrm{M}}}\right){\mathrm{V}}_{\mathrm{c}}^{\mathsf{\delta }}$$

(9)

where a = 0.3411, b = 2.0443, c = 0.5386, d = 0.0393 and δ = 1.0476.

The Lee–Kesler mixing rule was used to estimate the critical properties of DESs, based on the predicted critical properties of HBD and HBA [42]. The following are the equations that were used:

$${\mathrm{T}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}=\left({\displaystyle \frac{1}{{\mathrm{V}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}^{0.25}}}\right){\sum }_{\mathrm{i}}{\sum }_{\mathrm{j}}{\mathrm{x}}_{\mathrm{i}}{\mathrm{x}}_{\mathrm{j}}{\mathrm{V}}_{\mathrm{c}\mathrm{i}\mathrm{j}}^{0.25}{\mathrm{T}}_{\mathrm{c}\mathrm{i}\mathrm{j}}$$

(10)

where

$${\mathrm{T}}_{\mathrm{c}\mathrm{i}\mathrm{j}}={\left({\mathrm{T}}_{\mathrm{c}\mathrm{i}}{\mathrm{T}}_{\mathrm{c}\mathrm{j}}\right)}^{0.5}{\mathrm{k}}_{\mathrm{i}\mathrm{j}}$$

(11)

$${\mathrm{V}}_{\mathrm{c}\mathrm{i}\mathrm{j}}=0.125{\left({\mathrm{V}}_{\mathrm{c}\mathrm{i}}^{(1/3)}+{\mathrm{V}}_{\mathrm{c}\mathrm{j}}^{(1/3)}\right)}^3$$

(12)

$${\mathrm{V}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}={\sum }_{\mathrm{i}}{\sum }_{\mathrm{j}}{\mathrm{x}}_{\mathrm{i}}{\mathrm{x}}_{\mathrm{j}}{\mathrm{V}}_{\mathrm{c}\mathrm{i}\mathrm{j}}$$

(13)

and

$${\mathrm{P}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}=\left(0.2905-0.085{\omega }_{\mathrm{D}\mathrm{E}\mathrm{S}}\right){\displaystyle \frac{\mathrm{R}{\mathrm{T}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}}{{\mathrm{V}}_{\mathrm{c}\mathrm{D}\mathrm{E}\mathrm{S}}}}$$

(14)

where

$${\omega }_{\mathrm{D}\mathrm{E}\mathrm{S}}={\sum }_{\mathrm{i}}{\mathrm{x}}_{\mathrm{i}}{\omega }_{\mathrm{i}}$$

(15)

T_cDES (K), V_cDES (cm³/mol), and P_cDES (bar) represent the critical temperature, volume, and pressure of DESs, respectively. ${\omega }_{\mathrm{D}\mathrm{E}\mathrm{S}}$ and k_ij are acentric factor and binary interaction parameters of DESs. For mixtures of nonpolar compounds, k_ij values vary from 1.0 to 1.3, while for polar substances, k_ij values range from 0.95 to 1.06.

2.4. Physical Properties of DESs

The physical properties, such as density $\left({\rho }_{\mathrm{D}\mathrm{E}\mathrm{S}}\right)$ and refractive index ${(\mathrm{n}}_{\mathrm{D}\mathrm{E}\mathrm{S}})$ of DESs was determined using the group contribution model proposed by [43]. The models used are shown in Equations (16)–(21).

$${\mathrm{X}}_1={\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{A}}{\sum }_{\mathrm{i}=1}^{\mathrm{p}}{\mathrm{k}}_{\mathrm{i}}{(∆{\mathrm{X}}_{1,\mathrm{i}})}_{\mathrm{H}\mathrm{B}\mathrm{A}}+{\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{D}}{\sum }_{\mathrm{i}=1}^{\mathrm{q}}{\mathrm{l}}_{\mathrm{i}}{(∆{\mathrm{X}}_{1,\mathrm{i}})}_{\mathrm{H}\mathrm{B}\mathrm{D}}$$

(16)

$${\mathrm{X}}_2={\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{A}}{\sum }_{\mathrm{i}=1}^{\mathrm{p}}{\mathrm{k}}_{\mathrm{i}}{(∆{\mathrm{X}}_{2,\mathrm{i}})}_{\mathrm{H}\mathrm{B}\mathrm{A}}+{\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{D}}{\sum }_{\mathrm{i}=1}^{\mathrm{q}}{\mathrm{l}}_{\mathrm{i}}{(∆{\mathrm{X}}_{2,\mathrm{i}})}_{\mathrm{H}\mathrm{B}\mathrm{D}}$$

(17)

where

X₁ and X₂ = density ($\rho )$ and refractive index (n) values of HBA and HBD;

$(∆{\mathrm{X}}_{1,\mathrm{i}})$ and $(∆{\mathrm{X}}_{2,\mathrm{i}})$ = group contributions of “i” in the HBA and HBD molecules

${\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{A}} \mathrm{a}\mathrm{n}\mathrm{d} {\mathrm{m}}_{\mathrm{H}\mathrm{B}\mathrm{D}}$ = normalised number of moles;

$\mathrm{p} \mathrm{a}\mathrm{n}\mathrm{d} \mathrm{q}$ = total number of HBA and HBD functional groups.

$${\rho }_{\mathrm{D}\mathrm{E}\mathrm{S}}(\mathrm{g}/{\mathrm{cm}}^3)={\left({\displaystyle \frac{{\rho }_1}{{\mathrm{M}}_{\mathrm{w}\mathrm{D}\mathrm{E}\mathrm{S}}}}\right)}^{-0.2045}+{\left({\displaystyle \frac{{\rho }_2}{{\mathrm{M}}_{\mathrm{w}\mathrm{D}\mathrm{E}\mathrm{S}}}}\right)(\mathrm{T}}^{-0.6785})+0.2818$$

(18)

$${\mathrm{n}}_{\mathrm{D}\mathrm{E}\mathrm{S}}={\left({\mathrm{n}}_1\right)}^{-0.3597}+\left({\mathrm{n}}_2\right){\mathrm{T}}^{-1.8254}+1.3695$$

(19)

$$C_{pDES}(\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K})={C_{p1}}^{0.8653}+{(C}_{p2})\left(T^{-0.4528}\right)+341.4081$$

(20)

$${\sigma }_{\mathrm{D}\mathrm{E}\mathrm{S}}(\mathrm{m}\mathrm{N}/\mathrm{m})={\sigma }_1-\left({\sigma }_2\right){\mathrm{T}}^{0.0115}+40.8235$$

(21)

Furthermore, using the refractive index value, molar free volume was calculated using the Lorentz–Lorenz relation as shown in Equation (22) [44,45]:

$${\mathrm{V}}_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}\left({\mathrm{c}\mathrm{m}}^3/\mathrm{m}\mathrm{o}\mathrm{l}\right)={\mathrm{V}}_{\mathrm{M}}\left({\displaystyle \frac{3}{{\mathrm{n}}_{\mathrm{D}\mathrm{E}\mathrm{S}}^2+2}}\right)$$

(22)

where

$${\mathrm{V}}_{\mathrm{M}} \mathrm{i}\mathrm{s} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r} \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{D}\mathrm{E}\mathrm{S}\left({\mathrm{c}\mathrm{m}}^3/\mathrm{m}\mathrm{o}\mathrm{l}\right)={\displaystyle \frac{{\mathrm{M}}_{\mathrm{w}\mathrm{D}\mathrm{E}\mathrm{S}}}{{\rho }_{\mathrm{D}\mathrm{E}\mathrm{S}}}}$$

(23)

2.5. Gas Solubility Measurements

The pressure drop method was employed to determine the solubility of CO₂ in the synthesised systems. PCTPro by SETARAM (Model D6350) was used for the experiments. Although the apparatus is rated up to 250 bar, measurements were conducted up to 15 bar at a constant temperature of 30 °C. Three gases were utilised during the experiments: CO₂ (purity ≥ 99.999%) as the test gas, argon (purity ≥ 99.999%) for calibration, and nitrogen (purity ≥ 99.999%) to provide an inert medium.

Approximately 5 g of each DES sample was loaded into the sample holder equipped with nickel 60 µm filter elements to ensure leak-tight operation. Prior to each measurement, the sample was degassed under vacuum at 30 °C for 2 h to remove any dissolved gases and moisture. CO₂ was then introduced incrementally from the calibrated reservoir into the sample chamber. At each pressure step, the system was allowed to equilibrate for 2 h, and equilibrium was considered achieved when the pressure change was less than 0.02 bar over a 15 min interval. The quantity of CO₂ absorbed was calculated from the pressure drop and known system volumes using the ideal gas equation corrected for real gas behavior. All measurements were performed in duplicate, and the average values are reported. The estimated uncertainty in CO₂ solubility measurements was ±3% based on replicate measurements and pressure gauge accuracy (±0.01 bar).

3. Results and Discussion

3.1. FTIR Analysis

The formation of a DES is commonly validated by spectroscopic evidence of strong hydrogen-bonding interactions between HBA and HBD, accompanied by the absence of complete proton transfer or salt formation. In the present study, FTIR spectroscopy was employed as a primary tool to assess whether such interactions are established in the investigated systems.

As shown in Figure 1, the FTIR spectrum of the TBAB-AMP (1:1) system provides strong evidence for DES formation. The hydroxyl and amine stretching vibrations of AMP exhibited pronounced broadening and a clear shift toward lower wavenumbers in the 3600–3300 cm⁻¹ region when compared to pure AMP. These spectral changes are characteristic of extensive hydrogen bonding between the HBD (AMP) and the HBA (TBAB), primarily via the bromide anion. Importantly, no new absorption bands attributable to ammonium (NH₃⁺) bending vibrations were observed in the 1600–1500 cm⁻¹ region, indicating the absence of full proton transfer from AMP. The preservation of the alkyl C-H vibrational features of TBAB further confirms that the quaternary ammonium salt remains structurally intact. Collectively, these observations satisfy the established spectroscopic criteria for DES formation, validating that the TBAB-AMP (1:1) mixture constitutes a hydrogen-bonded deep eutectic solvent rather than a simple salt or physical mixture.

In contrast, the TBAB-PTSA (1:2) system does not fulfill the defining criteria of a deep eutectic solvent. The FTIR spectrum of this mixture showed the disappearance or significant attenuation of the broad sulfonic acid O-H stretching band (3000–2500 cm⁻¹), accompanied by the persistence and slight shifting of the characteristic sulfonate S=O stretching vibrations in the 1150–1180 cm⁻¹ and 1020–1050 cm⁻¹ regions. These spectral features unambiguously indicate proton transfer from p-toluenesulfonic acid to the counterion environment, resulting in the formation of a tetrabutylammonium p-toluenesulfonate salt. Such acid–base neutralization and salt formation are inconsistent with the hydrogen-bond-dominated interaction framework required for DES classification.

Therefore, based on FTIR spectroscopic analysis, the TBAB-AMP (1:1) system is validated as a deep eutectic solvent formed through strong, cooperative hydrogen bonding without proton transfer, whereas the TBAB-PTSA (1:2) system is best described as an ionic salt system rather than a deep eutectic solvent. These findings highlight the critical role of functional group chemistry in governing DES formation and provide a spectroscopic basis for distinguishing true DES systems from simple acid–base salts.

3.2. Critical Properties of DES

Critical Properties of deep eutectic solvents were calculated by combining the modified Lyderson–Joback–Reid method and Lee–Kesler method [40,41,42,46]. Critical properties of DES are shown in

Table 3. Critical properties of DES.

DES	VcDES (cm3/mol)	TcDES (K)	ωDES	PcDES (Bar)
DES1	460.30	677.48	0.73	28.00
DES2	634.16	897.27	0.66	27.53

.

3.3. Physical Properties of DES

The thermophysical properties of the investigated TBAB-based systems are summarised in

Table 4. Physical properties.

DES	ρDES	nDES	CpDES	uDES	σDES	Vm	Vfree
	g/cm3		J/mol-K	m/s	mN/m	cm3/mol	cm3/mol
DES1	1.07	1.48	610.08	1740.68	49.18	156.93	112.25
DES2	1.08	1.49	420.20	4758.04	35.74	157.29	111.83

. The TBAB-AMP (1:1) system exhibited a density of 1.07 g·cm⁻³ and a refractive index of 1.48, indicative of a compact hydrogen-bonded network. The relatively high heat capacity (610.08 J·mol⁻¹·K⁻¹) and elevated surface tension (49.18 mN/m) suggest strong intermolecular interactions, consistent with deep eutectic solvent behavior and extensive hydrogen bonding. The speed of sound in TBAB-AMP (1740.68 m/s) was significantly lower than that in TBAB-PTSA (4758.04 m/s), reflecting differences in the molecular structure and interaction strength between the two systems.

In contrast, the TBAB-PTSA (1:2) system displayed a lower heat capacity (420.20 J·mol⁻¹·K⁻¹) and reduced surface tension (35.74 mN/m), reflecting stronger ionic character and restricted molecular motion. The exceptionally high speed of sound (4758.04 m/s) in the TBAB-PTSA system is consistent with its ionic salt character, where strong electrostatic interactions result in a higher elastic modulus and acoustic velocity. The comparable densities (1.07–1.08 g·cm⁻¹) and similar molar volumes (156.93–157.29 cm³/mol) indicate dense molecular packing in both systems. However, the slightly lower free volume in TBAB-PTSA (111.83 cm³/mol vs. 112.25 cm³/mol) suggests diminished space for molecular rearrangement, supporting the conclusion that this system behaves as an ionic salt-dominated liquid rather than a classical deep eutectic solvent.

3.4. Viscosity of DES

Viscosity is a characteristic that defines a fluid’s resistance to flow. Compared to organic solvents, deep eutectic solvents (DESs) have significantly higher viscosity akin to ionic liquids (ILs), hence presenting challenges in handling, filtering, pumping, and stirring. Consequently, examining the viscosity of DESs is essential. The viscosity of DES was determined using an Anton Paar Rheometer MCR 102e. The temperature dependency of viscosity was checked from 30 °C to 50 °C. The viscosity of the DES showed an exponential decline with increasing temperature, as shown in Figure 2 and

Table 5. Viscosity of DES at different temperatures.

DES1		DES2
Temperature (°C)	Viscosity (mPa·s)	Temperature (°C)	Viscosity (mPa·s)
30	163.15	29.98	535.78
34.54	124.74	34.54	392.36
38.97	90.757	38.97	295.5
43.37	66.511	43.36	195.19
47.8	51.582	47.8	159.23
50.01	45.669	50.02	154.65

[47]. This might be explained by the weakening of the hydrogen bond between HBD and HBA as the temperature rises. TBAB-PTSA had higher viscosity compared to TBAB-AMP. This may be because, at higher temperatures, the intermolecular attraction forces weaken, increasing molecular mobility and kinetic energy, which aid in reducing viscosity [48]. It was also observed that the higher the viscosity, the lower the CO₂ solubility in DES. Hence, the lower solubility of TBAB-PTSA compared to TBAB-AMP.

The Arrhenius equation model, a two-parameter model denoted by Equation (24), was used to validate the experimental viscosities of the DESs, as shown in Figure 3 [49]. The fitting parameters of the Arrhenius equation of DESs are shown in

Table 6. Arrhenius equation fitting parameters of DESs.

DES	ηarr (mPa·s)	Ea (kJ/mol)	R2
DES1	1.25 × 10−7	52.86	0.999
DES2	3.48 × 10−7	53.29	0.985

.

$$\eta ={\eta }_{\mathrm{a}\mathrm{r}\mathrm{r}}{\mathrm{e}}^{{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}}$$

(24)

The activation energies of both systems are within the same range, indicating that the molecular rearrangement requires a similar amount of energy in both cases. Nonetheless, the key difference in the mechanism between DES1 and DES2 lies in the nature of interactions in the two systems:

In DES1, hydrogen bonds are defined as ephemeral and directed, exhibiting typical lifetimes of 1.5–20 picoseconds [50]. Throughout that period, the bonds are nearly perpetually disrupted and re-established, thereby permitting the molecules to move freely. This elucidates the low viscosity of the solution, measured at 163 mPa·s at 303 K.

Conversely, the interaction between the cation and anion in DES2 is significantly more robust and enduring. The system exhibits reduced directionality, resulting in both species attracting oppositely charged neighbors, which establishes a network of long-range Coulombic contacts that maintain a consistent resistance to flow. This elucidates the elevated viscosity in this instance, measured at 536 mPa·s at 303 K. The viscosity increase, reaching 3.3 times its standard value, directly influences CO₂ mass transfer by regulating diffusion rates and accessibility to reactive sites [51,52].

3.5. CO₂ Absorption in DES

Figure 4 and

Table 7. Experimental CO₂ solubility data at 30 °C.

Pressure (Bar)	Moles of CO2/Moles of DES	Mol Fraction of CO2
DES1
1.012	0.135	0.119
2.273	0.136	0.120
3.447	0.139	0.122
4.692	0.141	0.124
5.975	0.144	0.126
7.350	0.145	0.127
8.586	0.148	0.129
9.823	0.150	0.130
11.104	0.155	0.134
12.327	0.161	0.139
13.478	0.168	0.144
14.073	0.182	0.154
14.727	0.194	0.163
DES2
1.225	0.030	0.029
2.542	0.040	0.038
3.894	0.044	0.042
5.179	0.049	0.047
6.361	0.053	0.050
7.532	0.057	0.054
8.693	0.060	0.056
10.148	0.063	0.060
11.308	0.066	0.062
12.478	0.068	0.064
13.872	0.071	0.067
14.536	0.079	0.073

show the CO₂ absorption data at 30 °C for both DESs across a pressure range of 1–15 bar. DES1 (TBAB-1AMP) absorbed 0.135 mol CO₂/mol DES at 1 bar and increased steadily to 0.194 mol/mol at 14.7 bar, corresponding to mole fractions of 0.119 and 0.163, respectively. In comparison, DES2 (TBAB-2PTSA) absorbed only 0.030 mol/mol at 1.2 bar and reached 0.079 mol/mol at 14.5 bar (mole fraction 0.073). This represents roughly a 2.5-fold difference in capacity at similar pressures. The data reveal two distinct absorption patterns: DES1 reaches 70% of its total capacity at 1 bar before gradually increasing at higher pressures, while DES2 shows a continuous, slow increase across all tested pressures.

These contrasting behaviors can be explained by examining how the molecular structure, as confirmed by FTIR spectroscopy (Section 3.1), influences CO₂ interactions. TBAB-AMP is a true deep eutectic solvent where hydrogen bonds between the bromide anion and the hydroxyl/amine groups of AMP create a dynamic, flexible network. TBAB-PTSA underwent complete proton transfer to form a tetrabutylammonium p-toluenesulfonate ionic salt with a rigid electrostatic lattice. This structural difference has cascading effects on every aspect of CO₂ absorption. The free primary amine in TBAB-AMP interacts with CO₂ through a well-known carbamate reaction pathway which begins when the amine compound attacks CO₂ to create a zwitterionic intermediate (R-NH₂⁺-COO⁻) that undergoes stabilization through proton transfer to a second base which can be either another amine group or the bromide anion, resulting in the formation of the stable carbamate (R-NHCOO⁻) [53]. The presence of methyl groups next to the amine group creates steric hindrance, leading AMP to react in a 1:1 stoichiometry; primary amines without such hindrance typically show a 2:1 ratio, resulting in double the CO₂ absorption capacity per amine group. The hydroxyl group in AMP contributes to the stability of the carbamate through intramolecular hydrogen bonding and to proton shuttling during absorption [54]. This explains why 0.135 mol CO₂ per mol DES was captured at just 1 bar; the chemical reaction proceeds without requiring high pressure. The additional 0.059 mol/mol gained from 1 to 15 bar results from the physical dissolution of CO₂ into the hydrogen-bonded matrix, which depends on pressure according to Henry’s law. This dual-mode absorption-chemical at low pressure, physical at higher pressure, makes TBAB-AMP particularly suited for post-combustion capture where flue gas CO₂ partial pressures are only 0.03–0.20 bar [55,56].

TBAB-PTSA offers none of these advantages. The sulfonic acid has completely deprotonated to form sulfonate anions (−SO₃⁻), which are charge-saturated species lacking a nucleophilic character and therefore are unreactive toward CO₂ [26]. The ionic structure formed by the electrostatic pairing of tetrabutylammonium cations with sulfonate anions is rigid and densely packed, preventing molecular movement while providing minimal void space for gas accommodation. The 0.030 mol/mol absorbed at 1 bar and the linear pressure dependence indicates purely physical absorption, which occurs when CO₂ molecules fill all available tiny spaces within the ionic lattice. The viscosity results from Section 3.4 show that TBAB-AMP reached 163 mPa·s, compared to TBAB-PTSA, which reached 536 mPa·s at 30 °C, resulting in a 3.3-fold difference that directly impacts gas diffusion rates and mass transfer. The transient nature of hydrogen bonds in TBAB-AMP produces lower viscosity because these bonds continuously break and reform, allowing molecular motion [50,57]. The electrostatic interactions in the TBAB-PTSA system extend over long distances, which results in increased viscosity because the molecules remain in relatively fixed positions [24]. Such mechanistic differences are quantitatively supported by the Arrhenius activation energy data in Section 3.4, which shows that TBAB-PTSA has a much higher energy barrier for viscous flow compared to TBAB-AMP, thereby indicating that more energy is needed to break the strong electrostatic interactions that are stable over time, while less energy is required to break transient hydrogen bonds. Slow diffusion creates two challenges: it requires a long equilibration time and prevents the full use of all available absorption sites, resulting in poor performance of TBAB-PTSA [58,59,60].

The thermophysical properties in Table 4 provide additional detail for this explanation. TBAB-AMP has a marginally higher free volume (112.25 vs. 111.83 cm³/mol), indicating voids between molecules where CO₂ can dissolve. Free-volume theory predicts greater gas solubility in systems with more open structures [61]. Heat capacity is notably different: 610.08 J·mol⁻¹·K⁻¹ for TBAB-AMP versus 420.20 J·mol⁻¹·K⁻¹ for TBAB-PTSA. The exothermic nature of amine-CO₂ reactions releases between 40 and 60 kJ/mol of energy, and the higher heat capacity of DES1 means it experiences smaller temperature increases during absorption, which helps maintain CO₂ solubility, as solubility decreases with increasing temperature [27]. The surface tension values of 49.18 mN/m for TBAB-AMP and 35.74 mN/m for TBAB-PTSA demonstrate that TBAB-AMP possesses stronger hydrogen bonding cohesive forces, which enable it to maintain dissolved CO₂ and carbamate compounds in the DES matrix. Table 8 compares the performance of synthesized DESs with the literature. The TBAB:AMP (1:1) system exhibited a superior CO₂ absorption capacity of 0.194 mol/mol at 303.15 K and 14.7 bar, which is significantly higher than conventional physical absorption-based systems. The TBAB:AMP system showed better performance than TBAB:Ethylene Glycol (0.057 mol/mol) and TBAB:Diethylene Glycol (0.110 mol/mol), because it used AMP functionality to achieve chemical fixation [62,63,64]. The TBAB:AMP system demonstrated a capacity that was almost 7 times that of the standard ChCl:Urea system (0.028 mol/mol) [36,47].

The TBAB:AMP system showed a viscosity of 163.15 mPa·s at 303.15 K, which is favorable compared to other high-performance solvents. While ChCl:Ethylene Glycol glycol-based systems display lower viscosity of approximately 25.72 mPa·s, they exhibit absorption capacities below 0.06 mol/mol [36,65]. Systems with moderate capacity exhibit excessive viscosity, including TBAB:Decanoic Acid (900 mPa·s) and ChCl:Urea (447 mPa·s) [35,66]. The TBAB:AMP system thus provides an ideal solution that combines amine-based solvent capacity with lower viscosity, surpassing most ionic liquids and DESs, resulting in improved mass transfer and pumping efficiency.

From an application standpoint, TBAB-AMP shows promising behaviour for post-combustion CO₂ capture because it maintains high capacity at the low partial pressures typical of flue gas (0.1–0.15 bar, corresponding to 0.135 mol/mol at 1 bar). TBAB-PTSA, despite sharing the same quaternary ammonium cation, captures negligible CO₂ under these conditions. This demonstrates that simply choosing a TBAB-based system is not enough; a hydrogen bond donor must provide reactive functionality, form a true DES rather than an ionic salt, and not introduce excessive viscosity. The study found that viscosity and CO₂ loading display an inverse relationship: 163 mPa·s yielded 0.194 mol/mol, while 536 mPa·s yielded 0.079 mol/mol at maximum pressure. The development of DESs for CO₂ capture should focus on two essential elements: viscosity reduction and chemical functionality.

Table 8. Comparison of CO₂ absorption capacity and viscosity of the synthesized DESs with those reported in the literature.

DESs	Molar Ratio	T (K)	P (bar)	CO2 Capacity (mol/mol)	Viscosity (mPa·s) at 303.15 K	Reference
TBAB:AMP	1:1	303.15	14.7	0.194	163.15	This work
TBAB:PTSA	1:2	303.15	14.5	0.079	535.78	This work
TBAB:Decanoic Acid	1:2	298.15	10	0.11	900	[35,66]
TBAB:Diethylene Glycol	1:2	303.15	13.9	0.110	149.24	[62,63]
TBAB:Ethylene Glycol	1:2	303.15	12.8	0.057	77.27	[62,64]
ChCl:Urea	1:2	298.15	10	0.028	447	[36,47]
ChCl:MEA	1:6	303.15	1	0.123	350	[36,67]
ChCl:Ethylene Glycol	1:2	303.15	12.5	0.055	25.72	[36,65]

Table 8. Comparison of CO₂ absorption capacity and viscosity of the synthesized DESs with those reported in the literature.

DESs	Molar Ratio	T (K)	P (bar)	CO2 Capacity (mol/mol)	Viscosity (mPa·s) at 303.15 K	Reference
TBAB:AMP	1:1	303.15	14.7	0.194	163.15	This work
TBAB:PTSA	1:2	303.15	14.5	0.079	535.78	This work
TBAB:Decanoic Acid	1:2	298.15	10	0.11	900	[35,66]
TBAB:Diethylene Glycol	1:2	303.15	13.9	0.110	149.24	[62,63]
TBAB:Ethylene Glycol	1:2	303.15	12.8	0.057	77.27	[62,64]
ChCl:Urea	1:2	298.15	10	0.028	447	[36,47]
ChCl:MEA	1:6	303.15	1	0.123	350	[36,67]
ChCl:Ethylene Glycol	1:2	303.15	12.5	0.055	25.72	[36,65]

3.6. CO₂ Absorption Kinetics

In order to both measure the capture dynamics and check the validity of the reaction mechanism, the time-resolved absorption profiles were analyzed with the help of a macroscopic reactive absorption model. The experimental uptake data was fitted by means of a pseudo-first order mass transfer Equation (25) that looks like [68]:

$${\displaystyle \frac{\mathrm{d}\mathrm{C}}{\mathrm{d}\mathrm{x}}}={\mathrm{k}}_{\mathrm{o}\mathrm{b}\mathrm{s}}\mathrm{P}({\mathrm{C}}_{\mathrm{e}}-{\mathrm{C}}_{\mathrm{o}})$$

(25)

where C_o (mol CO/mol DES) is the instantaneous CO₂ loading in the liquid phase, C_e (mol CO/mol DES) is the equilibrium loading at CO₂ partial pressure P (bar), and k_obs (min) is the overall rate constant. Here, k_obs include the volumetric mass transfer coefficient k_La, and, where chemical reaction occurs, an enhancement factor arising from the reaction kinetics [69]. The driving force stands for the difference from thermodynamic equilibrium and decreases gradually as the liquid phase saturation occurs. The parameters of the eq 25 were computed from the plot of concentration of CO₂ (mol/mol) vs. time (min⁻¹). The extracted kinetic parameters for both systems are summarized in

Table 9. Reactive absorption kinetic parameters for CO₂ in DES1 and DES2 at 303 K.

DES	P (bar)	kobs (min−1)	R2
DES1	1.04	134.99 ± 4.78	0.918
	7.36	666.33 ± 34.24	0.881
DES2	1.23	2.24 ± 0.04	0.924
	7.55	50.06 ± 0.25	0.999

.

As shown in Figure 5, at both pressures, the profiles exhibit a steep initial rise that progressively flattens as the loading approaches C_e, reflecting the diminishing driving force (C_e-C_o) as amine sites become progressively occupied. The overall rate constant increased from k_obs = 134.99 ± 4.78 min⁻¹ at 1.04 bar to 666.33 ± 34.24 min⁻¹ at 7.36 bar, a 4.9-fold pressure-induced enhancement consistent with a bimolecular reaction in which both CO₂ fugacity and local amine concentration govern the absorption rate [70].

Figure 6 shows the corresponding profiles for DES2 at 1.23 and 7.55 bar. The kinetics are markedly slower: at 1.23 bar, the loading climbs gradually over nearly 2 min toward a plateau of 0.030 mol/mol, while at 7.55 bar, an equilibrium is reached in under 0.05 min. The rate constant rises from k_obs = 2.24 ± 0.04 min⁻¹ at 1.23 bar to 50.06 ± 0.25 min⁻¹ at 7.55 bar, with the near-perfect fit at the higher pressure indicating a well-defined, kinetically uniform absorption regime dominated by physical dissolution [11].

Figure 7 places the pressure dependence of k_obs for both systems on a common axis. TBAB-AMP maintains a decisive and consistent advantage over TBAB-p-TSA—approximately 60-fold at ~1 bar and 13-fold at ~7.5 bar. Since both DES share an identical TBAB cation framework and were measured under the same conditions, this systematic difference is mechanistic rather than physical in origin [71].

DES1 incorporates 2-amino-2-methyl-1-propanol, whose primary amine undergoes nucleophilic addition to CO₂ to form a carbamic acid intermediate that deprotonates to a stable carbamate [72]:

R-NH₂ + CO₂ ⇌ R-NH-COO⁻ + H⁺

The bromide anion in TBAB further facilitates proton transfer during carbamate formation, accelerating the reaction in the non-aqueous environment [69]. The implied enhancement factor for DES1 falls in the range 10²–10³, placing the system firmly in the fast pseudo-first-order regime where CO₂ is consumed near the gas–liquid interface rather than diffusing into the bulk [73,74]. DES2, by contrast, contains para-toluenesulfonic acid as the hydrogen bond donor—a species with no nucleophilic site for covalent CO₂ reaction and absorbs solely through physical dissolution and weak dipolar interactions, which are intrinsically slower [11]. The initial absorption rate (r_o) for DES1 at 1.04 bar (r_o = 17.87 mol CO₂/(mol DES·min)) is 224-fold higher than that of DES2 at comparable pressure (0.080 mol CO₂/(mol DES·min)), quantifying this mechanistic gap directly.

The concurrent enhancement of equilibrium capacity (0.194 vs. 0.079 mol/mol at ~15 bar) and absorption kinetics (k_obs up to 666 vs. 50 min⁻¹) in DES1 constitutes a defining signature of chemically reactive absorption—reactive systems break the capacity-kinetics trade-off inherent to physical solvents by introducing a fast parallel chemical pathway that elevates both simultaneously [75]. Taken together with the isotherm data, these results provide strong, self-consistent evidence that amine-mediated carbamate formation is the primary CO₂ capture mechanism in DES1.

4. Conclusions

Two TBAB-based solvents with different hydrogen bond donors were synthesized and characterized through analytical techniques: TBAB-AMP (1:1) and TBAB-PTSA (1:2). FTIR spectroscopic analysis provided conclusive evidence that TBAB-AMP forms a proper deep eutectic solvent through hydrogen bonding interactions between the bromide anion and the hydroxyl/amine groups of AMP, whereas TBAB-PTSA undergoes complete acid–base neutralization to form an ionic salt. This fundamental structural distinction determined all subsequent performance characteristics. CO₂ solubility measurements demonstrated that the TBAB-AMP DES achieved significantly higher absorption capacity, capturing 0.194 mol CO₂/mol DES at 14.7 bar compared to the TBAB-PTSA salt system, which absorbed only 0.079 mol/mol at 14.5 bar. The 2.5-fold enhancement in capacity was attributed to multiple factors working in concert: the primary amine functionality in AMP enables chemical absorption through carbamate formation with favorable 1:1 stoichiometry due to steric hindrance, the hydrogen-bonded DES network provides accessible pathways for CO₂ diffusion and dissolution, and the substantially lower viscosity (163 mPa·s versus 536 mPa·s at 30 °C) facilitates rapid mass transfer. Critical and thermophysical properties were estimated using group contribution methods. Temperature-dependent viscosity measurements from 30 to 50 °C confirmed an inverse relationship between viscosity and CO₂ solubility, with TBAB-AMP maintaining lower viscosity across the entire temperature range. The thermophysical property analysis revealed that TBAB-AMP has a higher free volume (112.25 cm³/mol), higher heat capacity (610.08 J·mol⁻¹·K⁻¹), and higher surface tension (49.18 mN/m) than TBAB-PTSA, all of which contribute to better CO₂ accommodation and thermal management during the exothermic absorption process. Notably, TBAB-AMP achieved 70% of its maximum capacity at just 1 bar, making it particularly suitable for post-combustion CO₂ capture from flue gas, where partial pressures are typically 0.1–0.15 bar. The results demonstrate that amine-functionalized deep eutectic solvents offer significant advantages over acid-based ionic salt systems for CO₂ capture applications. The study also highlights the critical importance of proper system classification, distinguishing true DESs from ionic salts formed through proton transfer. This distinction fundamentally determines absorption performance. Future research should focus on evaluating the regeneration characteristics of TBAB-AMP, measuring desorption kinetics and energy requirements, assessing long-term stability through multiple absorption–desorption cycles, and exploring the effects of water and other flue gas components on absorption performance to fully establish the practical viability of this system for industrial CO₂ capture.