Adsorption of Dimethyl Phthalate and Its Isomers on Nitrogen-Doped Activated Carbon: A DFT Study

Abstract

Dimethyl phthalate (DMP) is an environmental contaminant known for its endocrine-disrupting properties, and its removal poses a critical environmental challenge. In this paper, we present a theoretical study of the adsorption of the DMP molecule and its isomers on pristine and nitrogen-doped graphitic surfaces, which represent the pore walls of nanoporous activated carbon, using density functional theory (DFT) calculations. Detailed wavefunction analyses were performed to elucidate the nature of adsorption on the AC surfaces. Our results reveal that nitrogen doping improves phthalate adsorption in the following order: AC-Pristine < AC-NH₂ < AC-Graphitic-N < AC-Graphitic-2N. This enhancement arises from changes in charge distribution that introduce electrostatic interactions between the COOCH₃ groups of the molecules and nitrogen-doped atoms on the AC surface. This study provides mechanistic insights into DMP adsorption on nitrogen-doped AC and offers rational guidelines for designing efficient carbon-based adsorbents for the removal of phthalate esters from contaminated water and the environment.

1. Introduction

Phthalates, or phthalic acid esters, are a family of synthetic compounds widely used as additives in the polymer industry to enhance the flexibility, transparency, and durability of plastic materials, particularly polyvinyl chloride (PVC). Numerous studies have demonstrated that phthalates are endocrine disruptors, capable of interfering with human and animal hormonal systems, as well as increasing the risks of metabolic disorders and cancers [1,2]. This is why several agencies have classified certain phthalates as hazardous substances to the environment [3]. Due to their extensive use in consumer products (toys, food packaging, medical devices, cosmetics), phthalates have become environmental contaminants [4]. Among the various phthalate esters, dimethyl phthalate (DMP) is one of the most widely used, frequently employed in personal care products, insect repellents, and plastic packaging. Their presence has been detected in virtually all environmental compartments (surface waters, groundwater, sediments, soils, and air) [5,6]. A recent study reported total phthalate concentrations of up to 3769 ng/L in a polluted environment [7]. Similarly, investigations have revealed that phthalate concentrations in wastewater can reach up to 83.500 µg/g (equivalent to 83,500,000 ng/L) [8], indicating a high ecological risk.

Among the treatment techniques for contaminated water, adsorption onto activated carbon (AC) is widely recognized as one of the most effective and economically viable methods. AC has a high specific surface area and well-developed microporosity, which confers excellent adsorption capacity for organic compounds [9]. However, a thorough understanding of the interaction between phthalates and the carbon surfaces is essential for optimizing treatment processes and developing next-generation adsorbents. The adsorption of aromatic esters onto AC has been the subject of various studies. Paredes-Doig et al. [10] studied the adsorption of benzoic acid, salicylic acid, and phenol onto AC prepared from chestnut shells, showing that the order of removal capacity (salicylic acid > benzoic acid > phenol) is correlated with both the structural properties of the carbon and the electronic adsorbate–adsorbent interactions. Similarly, Cotoruelo et al. [9] studied the adsorption of acetaminophen, salicylic acid, and benzoic acid onto AC derived from lignin, highlighting the crucial role of surface oxygen groups. Recently, Li et al. [11] brought new insights into the adsorption of benzoic and phthalic acids onto functionalized carbon nanotubes, highlighting the role of hydrogen bonds. Although their study focused on carboxylic acids, the findings are relevant to understanding how functional groups on carbon surfaces interact.

Although previous theoretical studies have focused on the adsorption of carboxylic acids and simple aromatic compounds onto carbon surfaces, the systematic study of phthalate esters on nitrogen-doped carbon surfaces remains largely unexplored. Furthermore, the influence of positional isomerism of ester groups on adsorption behavior has not yet been investigated.

However, these studies remain largely experimental, and the theoretical aspects of dimethyl phthalate (DMP) adsorption onto carbon surfaces remain underexplored, particularly the influence of heteroatom doping on adsorption performance and the role of non-covalent interactions at the molecular scale. The relative positions of the ester groups significantly influence molecular symmetry, dipole moment, and adsorption behavior on carbon surfaces [12]. Recent advances have shown that nitrogen doping of AC significantly enhances adsorption performance. Liu et al. [13] demonstrated that nitrogen doping modifies the electronic structure of the carbon surfaces, creating Lewis basic sites that improve affinity for polar pollutants. Various nitrogen configurations have been explored, including pyridinic, graphitic, amino, and doubly graphitic nitrogen sites, each exhibiting distinct electronic properties and interaction capabilities with adsorbate molecules.

In this context, we present a theoretical study of the adsorption of DMP and its isomers, dimethyl isophthalate (DMI) and dimethyl terephthalate (DMT), onto pristine and nitrogen-doped AC surfaces, using density functional theory (DFT) calculations. The objective is to present several unique contributions: (i) a systematic comparison of three DMP isomers (ortho, meta, para) on nitrogen-doped AC; (ii) a comprehensive analysis of different nitrogen-doping configurations (pristine, pyridinic-N, graphitic-N, NH₂, graphitic-2N); (iii) a multi-technique approach combining DFT with IRI, ESP, and QTAIM analyses to elucidate adsorption mechanisms; and (iv) the identification of graphitic-2N as the optimal doping configuration for enhanced phthalate adsorption. The results will provide rational guidelines for the design of nitrogen-doped carbon adsorbents to remove ubiquitous environmental contaminants, such as phthalates, efficiently.

2. Computational Methods

The geometries of the DMP molecule and its isomers, DMI and DMT, adsorbed on pristine and nitrogen-doped AC surfaces were calculated using DFT with the r2SCAN-3c functional as implemented in the ORCA code, which is suitable for non-covalent interactions (NCI) [14,15,16,17,18]. To determine the conformations of the systems, a structural search was conducted by varying the distance separation between the adsorbate and the carbon surface across all possible configurations (parallel, perpendicular, and orientations with ester groups directed toward the surface). This method has been successfully validated for adsorption studies of molecules on a substrate and provides good results.

For most molecular systems adsorbed on carbonaceous surfaces, NCI dominates the molecule/surface interaction, such as van der Waals (vdW) interactions, $\pi$-$\pi$ stacking, electrostatic interactions (ESIs), hydrogen bonding (HB), etc., in which the adsorbates do not disrupt the graphitic structure of the carbon surface [9,11,12,19,20]. The interaction energies for specific NCI were obtained by spatially integrating the electron density descriptor $\mathrm{sign}\left({\lambda }_2\right)\rho$ over the basins associated with each contact, as defined by the Independent Gradient Model (IGM) analysis [21], which provides a real-space segmentation of the interaction region in the system.

For NCI analysis, we used the orca_2mkl utility from the ORCA code [15]. The wave function was converted to the .wfn format, which serves as the input file for the Multiwfn program [22], to perform the interaction region indicator function (IRI), electrostatic potential (ESP), and quantum atoms-in-molecules (QTAIM) [23,24,25,26,27]. Graphical representations of the IRI isosurface, ESP maps, critical points (CPs), and bond critical points (BCPs) were generated using Visual Molecular Dynamics (VMD) software [28].

3. Results and Discussion

3.1. Geometrical Structures of Pristine AC and Nitrogen-Doped AC

The AC model used in this study represents the plane of graphitic pore walls in nanoporous activated carbon, which is the preferred adsorption site for aromatic molecules. Furthermore, full nanopore confinement effects are not explicitly modeled; previous studies have shown that adsorption on flat graphitic surfaces correlates well with adsorption in pores where confinement effects are minimal [29,30]. The interaction energies obtained represent the molecule/surface interaction at the pore wall surface, which is the dominant contribution to the overall adsorption in nanoporous carbons. However, the present study focuses on the chemical doping effects at the surface, which are expected to be the dominant factor in determining adsorption performance.

According to previous studies, carbon materials are generally modeled with seven to eleven aromatic rings [30,31,32]. In this study, the AC model includes five aromatic carbon rings along the x direction and six along the y direction, where the lateral size exceeds the dimension of the DMP molecule and its isomers (6.80–9.80 Å) to minimize edge effects and accurately describe the interaction between the molecule and the AC surfaces [10,29]. Consequently, the adsorbed planar molecules interact primarily with the planar region of the surface through $\pi$-$\pi$ interactions, which are stronger than the non-covalent interactions present at the surface edges. Five nitrogen-doped AC models were used: AC-Pristine, AC-Pyridinic-N, AC-Graphitic-N, AC-NH₂, and AC-Graphitic-2N (Figure 1) [13,33,34,35,36,37,38,39,40].

To predict electrostatic reactive sites in molecule/surface interactions, we investigated ESP interactions, in which regions of positive electrostatic potential interact with regions of negative potential, bringing the molecule closer to the surface and minimizing the total energy. The structures and isosurface maps of the ESP for nitrogen-doped AC surfaces are shown in Figure 1f,j. Qualitative analysis reveals that the introduction of nitrogen atoms can modify the local electrostatic potential of AC. As shown in Figure 1f, the surface ESP minima of pure AC are located above and below the benzene rings, while the maxima appear near the hydrogen atoms. For functionalized activated carbon surfaces, significant variations in electrostatic potential are observed in the vicinity of doped nitrogen atoms (Figure 1g–j), thus creating sites on the AC surfaces and increasing their adsorption affinity.

3.2. Geometrical Structures of DMP and Its Isomers (DMI and DMT)

The optimized structures of DMP molecule and its isomers (DMI and DMT) in the gas phase are shown in Figure 2. All these molecules contain 24 atoms (C₁₀H₁₀O₄) and consist of a benzene ring bearing two methoxycarbonyl (-COOCH₃) groups; DMP has the ester groups in the ortho position (1,2) (Figure 2a), DMI in the meta position (1,3) (Figure 2b), and DMT in the para position (1,4) (Figure 2c). Thus, the distances between the opposite ends of the DMP, DMI, and DMT molecules are 6.80 Å, 9.70 Å, and 9.80 Å, respectively. The ESP maps shown in Figure 2d–f provide valuable insights into the charge distribution onto the surfaces. For DMP isomers, the negative electrostatic potential is predominantly localized around the oxygen atoms of the methoxycarbonyl groups. A comparison of these ESP maps reveals subtle differences among the three isomers. DMT exhibits the most symmetrical charge distribution due to its para configuration, with two equivalent negative regions located at the ends of the molecule (Figure 2f). DMP indicates a more asymmetric distribution due to the proximity of the two ester groups, creating a combined negative region on one side of the molecule (Figure 2d). DMI presents an intermediate behavior (Figure 2e). These ESP maps indicate that the oxygen atoms are electron-rich sites, capable of interacting with the positively charged regions of the nitrogen-doped AC surface, while the aromatic rings of the molecules can interact by $\pi$-$\pi$ stacking with the graphitic domains of the surface.

Possible interactions between DMP isomers and nitrogen-doped AC include:

1.

$\pi$-$\pi$ stacking between the aromatic ring of the molecules and the graphitic aromatic rings of AC.

2.

Electrostatic interactions guide the molecule and nitrogen-doped AC to bind by aligning donor and acceptor electron regions.

3.3. Geometrical Structures of the DMP Molecule and Its Isomers on Nitrogen-Doped AC

Adsorption energy is a crucial parameter for understanding the adsorption mechanism. It is one of the most common methods for studying adsorption strength, as it quantitatively describes the interactions between the molecule and the surface in the adsorbate–adsorbent system [9,11,12].

The adsorption energy ($E_{ads}$) is defined as:

$$E_{ads}=E_{system}-(E_{adsorbate}+E_{surface})$$

(1)

where $E_{system}$ is the total energy, $E_{adsorbate}$ is the energy of the isolated molecule, and $E_{surface}$ is the energy of the pristine or doped AC surface. More negative values indicate stronger adsorption.

It should be noted that our model represents the graphitic pore walls of nanoporous AC rather than the full three-dimensional pore structure. However, the relative trends in adsorption energies for different doping configurations are expected to remain valid, as the chemical effects of nitrogen doping on the surface electronic structure are the dominant factor determining the adsorption enhancement. This is consistent with experimental studies showing that nitrogen doping enhances adsorption across a range of pore sizes [13].

The interaction between dimethyl phthalate isomers and AC involves multiple conformations, each exhibiting a different energy level [12]. In doped AC studies, the doping of hetero-atoms or functional groups at different positions significantly influences adsorption behavior [13]. Therefore, determining adsorption conformations is essential for understanding the adsorption behavior of DMP and its isomers on AC and for analyzing the contributions of each effect to the overall adsorption process. To achieve this, we performed a conformational search of the interaction sites between the AC models and the molecules, as well as energy calculations for the optimal structure.

Figure 3 demonstrates the optimized chemical structures of DMP molecule and its isomers (DMI and DMT) adsorbed on distinct AC surfaces (AC-Pristine, AC-Pyridinic-N, AC-Graphitic-N, AC-NH₂, and AC-Graphitic-2N) as determined by DFT calculations. For each molecule, the most stable adsorption configuration corresponds to a parallel orientation on the AC surface, due to $\pi$-$\pi$ stacking interactions.

Table 1. Adsorption energies, their variation as a percentage change ($\Delta (\%)$) relative to pristine AC, and equilibrium distances for DMP and its isomers (DMI and DMT) adsorbed on AC-Pyridinic-N, AC-Graphitic-N, AC-NH₂, and AC-Graphitic-2N surfaces, calculated using the ORCA code with the DFT method at the r2SCAN-3c level. A positive $\Delta (\%)$ indicates an increase of adsorption energy (more negative adsorption energy), while a negative $\Delta (\%)$ indicates a decrease of adsorption energy. $\Delta (\%)={\displaystyle \frac{E_{ads}^{doped}-E_{ads}^{pristine}}{|E_{ads}^{pristine}|}}\times 100$.

Molecules	DMP	DMI	DMT
$E_{\mathrm{ads}}$ (kcal/mol) and $\Delta (\%)$
AC-Pristine	−19.73 (—)	−21.53 (—)	−21.09 (—)
AC-Pyridinic-N	−19.11 (−3.2%)	−20.69 (−3.9%)	−19.81 (−6.1%)
AC-Graphitic-N	−21.46 (+8.7%)	−22.64 (+5.2%)	−21.94 (+4.0%)
AC-NH2	−19.67 (−0.3%)	−21.68 (+0.7%)	−20.98 (−0.5%)
AC-Graphitic-2N	−21.64 (+9.7%)	−22.68 (+5.3%)	−22.74 (+7.8%)
Distance (Å)
AC-Pristine	3.13	3.31	3.29
AC-Pyridinic-N	3.37	3.27	3.21
AC-Graphitic-N	3.22	3.28	3.32
AC-NH2	3.24	3.26	3.24
AC-Graphitic-2N	3.22	3.19	3.29

presents the calculated adsorption energies ($E_{ads}$) and the equilibrium distances (d) between the benzene ring of the adsorbate molecule and the AC surface. The adsorption energies of DMP molecule and its isomers on nitrogen-doped AC-Graphitic are lower (more negative) than those of AC-Pristine, AC-Pyridinic-N, and AC-NH₂ (Table 1). Thus, among the three isomers, DMI and DMT are the most stable across all surfaces studied due to their favorable molecular symmetry and charge distribution. In contrast, AC-Pyridinic-N and AC-NH₂ have unstable structures due to weak electrostatic interactions between the functional groups and nitrogen atoms. This demonstrates, both qualitatively and quantitatively, that parallel stacking on the graphitic terrace is the dominant adsorption mode, consistent with studies of phenol adsorption on AC surfaces [9,10,12,29].

In the most stable structures, DMP/AC-Graphitic-2N, DMI/AC-Graphitic-2N, and DMT/AC-Graphitic-2N, the cyclic group occupies a top site on the surface to preserve the molecular geometry (Figure 3). The adsorption energies for these stable structures, DMP, DMI, and DMT on AC-Graphitic-2N, including $\pi$-$\pi$ stacking, electrostatic, and van der Waals interactions, are −21.64 kcal/mol, −22.68 kcal/mol, and −22.74 kcal/mol, respectively. The distances between the AC-Graphitic-2N substrate and the plane of the DMP, DMI, and DMT molecules are 3.22 Å, 3.19 Å, and 3.29 Å, respectively (Table 1).

Among the nitrogen-doped AC, we observed that AC-Graphitic-2N exhibits the lowest adsorption energies, suggesting that nitrogen doping of AC-Graphitic improves the adsorption performance of DMP and its isomers by up to 2.00 kcal/mol (+9.7%). Moreover, these results are in good agreement with the ESP analysis of isolated dimethyl phthalate (Figure 2) and doped AC surfaces (Figure 1), which showed that, in addition to $\pi$-$\pi$ interactions, the electron-rich regions of the COOCH₃ groups (ESP minima) interact with the positive potential of nitrogen atoms on the doped AC.

The adsorption energies found for AC-Pyridinic-N and AC-NH₂, which are less stable compared to doped nitrogen AC-Graphitic, can be explained by the dominance of $\pi$-$\pi$ stacking during adsorption, which prevents the charge distribution on the N atoms from aligning optimally with COOCH₃ groups of the molecule.

On the other hand, the combination of $\pi$-$\pi$ stacking and electrostatic interactions, in the adsorption of the DMP molecules and their isomers on AC-graphitic-N or AC-graphitic-2N, explains why these systems exhibit superior adsorption performance.

Our calculated adsorption energies for DMP and its isomers on pristine and doped-nitrogen AC, ranging from –19.73 to –22.74 kcal/mol, are in good agreement with DFT values reported in the literature for similar organic pollutants. For example, a recent DFT study on the adsorption of trichlorophenol on nitrogen-doped AC reported adsorption energies ranging from –17.1 to –23.5 kcal/mol [41], which is remarkably consistent with our findings. Similarly, DFT calculations on ethane adsorption on AC models using the M06-2X method reported an adsorption energy of –24.44 kcal/mol, while MP2 calculations gave –17.49 kcal/mol [42]. These values bracket our results and confirm that the physisorption regime for organic pollutants on carbon surfaces typically falls within this energy range. Furthermore, the equilibrium distances reported in the trichlorophenol study (3.28–3.30 Å) [41] are also in excellent agreement with our calculations (3.13–3.37 Å). The consistency between our results and these literature values confirms that the adsorption of DMP on activated carbon is governed by physisorption, primarily through $\pi$-$\pi$ stacking interactions, which are the dominant mechanism for the adsorption of aromatic hydrocarbons on carbon surfaces [42]. Overall, our results are fully consistent with the existing literature, confirming that $\pi$-$\pi$ stacking is the primary interaction mechanism and that nitrogen doping enhances adsorption through additional electrostatic contributions, without altering the physisorption nature of the process.

To confirm the relevance of our surface models, we performed additional calculations on a larger 7 × 7 cyclic model. The adsorption energies differed by less than 0.5 kcal/mol between the two calculations (5 × 6 vs. 7 × 7 rings), confirming that edge effects are negligible and that the 5 × 6 model is sufficient to accurately describe the adsorption process.

3.4. Analysis of Weak Interactions

To provide an intuitive and quick understanding of how the different parts of the system interact, we have used the IRI function, derived from RDG, which effectively reveals regions of weak interactions and chemical bonds. This is very useful for greatly facilitating the study of interactions within chemical systems [43,44].

Figure 4 shows the interactions in the conformations of DMP, DMI, and DMT molecules adsorbed on pristine and nitrogen-doped AC surfaces. Weak interactions are represented in green, blue regions indicate covalent interactions between atoms, and red indicates steric repulsion within aromatic rings. These results show that NCI dominates the interactions between the pristine Ac substrate and the molecules, including $\pi$-$\pi$ stacking and vdW forces.

In addition, when nitrogen atoms are added to the surface, the green region of the NCI between the molecule and the doped AC surface increases, particularly around COOCH₃ groups, due to electrostatic interactions between the oxygen atoms of these groups and the nitrogen atoms of the AC surfaces, especially for the AC-Graphitic-2N and AC-Graphitic-N systems. These results qualitatively confirm the positive influence of nitrogen doping of AC on the adsorption of DMP, DMI, and DMT molecules.

The IRI scatter plots (Figure 5) provide quantitative confirmation of the interaction types observed in the isosurface maps. Three distinct regions can be identified based on the $\mathrm{sign}\left({\lambda }_2\right)\rho$ values:

• Covalent bonds: $\mathrm{sign}\left({\lambda }_2\right)\rho$ values in the range of −0.050 to −0.025 a.u., corresponding to the blue regions in Figure 4.
• Non-covalent interactions: $\mathrm{sign}\left({\lambda }_2\right)\rho$ values in the range of −0.025 to +0.015 a.u., corresponding to the green regions between the molecules and the AC surfaces ($\pi$-$\pi$ stacking, van der Waals, and electrostatic interactions).
• Steric repulsion: $\mathrm{sign}\left({\lambda }_2\right)\rho$ values around +0.022 a.u., corresponding to the red spindle-shaped regions within the aromatic rings.

Upon nitrogen doping, the non-covalent interaction range (green region) increases significantly, particularly for the AC-Graphitic-2N and AC-Graphitic-N configurations. This increase is attributed to additional electrostatic interactions between the oxygen atoms of COOCH₃ groups and the positively charged nitrogen atoms on the doped AC surfaces, which supplement the $\pi$-$\pi$ stacking interactions present in the pristine system.

While the overall shape of the IRI scatter plots (Figure 5) appears similar across systems, quantitative differences are observed in the width and intensity of the non-covalent interaction region:

• For AC-Graphitic-2N, the non-covalent interaction region shows a broader distribution, approximately 10–15% higher integrated intensity compared to pristine AC. This reflects electrostatic interactions between the COOCH₃ groups and nitrogen atoms.
• For AC-Graphitic-N, the integrated intensity in the non-covalent region increases by approximately 5–10% compared to pristine AC.
• For AC-Pyridinic-N and AC-NH₂, the non-covalent region shows minimal change or a slight decrease in integrated intensity, consistent with the weaker adsorption energies observed for these configurations.
• The covalent bonding region (from −0.050 to −0.02 a.u.) remains essentially unchanged across all systems, as intramolecular bonds and surface bonds on the activated carbon are not significantly affected by adsorption.

These quantitative differences in IRI scatter diagrams confirm that nitrogen doping enhances non-covalent interactions between molecules and AC surfaces through additional electrostatic contributions.

3.5. AIM Topological Analysis

The most common analytical method in AIM is topological electron density analysis. The characteristics of weak interactions and bonds can be studied through the locations of bond critical points (BCPs) and critical points (CPs) identified in this topological analysis.

The BCPs and CPs between dimethyl phthalate and doped AC surfaces are shown in Figure 6. These results clearly reveal multiple interaction pathways between the molecules and the AC surfaces. The consistent characteristics of $\rho$ and ${\nabla }^2\rho$ confirm the presence of $\pi$-$\pi$ stacking and electrostatic interactions. Importantly, additional bond paths are observed between the oxygen atoms of the COOCH₃ groups and the nitrogen atoms on AC surfaces, particularly for the DMI/AC-Graphitic-2N and DMT/AC-Graphitic-2N systems, the electron density at these BCPs ($\rho$(BCP)) is 0.65 × 10⁻² a.u. and 0.57 × 10⁻² a.u., respectively, vs. 0.38 × 10⁻² a.u. of bond paths between the oxygen atoms in COOCH₃ groups and the carbon atoms of the pristine AC surface. These higher $\rho$(BCP) values indicate stronger electrostatic interactions between the adsorbate and the nitrogen, which findings suggest that the formation of electrostatic bonds between dimethyl phthalate and nitrogen-doped AC-Graphitic enhances the adsorption capacity.

In all studied systems, the properties at the BCPs indicate that the interactions between dimethyl phthalate and the doped AC surfaces are primarily $\pi$-$\pi$ stacking and electrostatic interactions.

4. Conclusions

The adsorption of dimethyl phthalates (DMP, DMI, and DMT) on models of the graphitic pore walls of nanoporous nitrogen-doped AC was investigated using DFT calculations at the r2SCAN-3c level. The results indicate that nitrogen doping alters the local electrostatic potential of AC and increases its physisorption capacity for dimethyl phthalate, primarily through $\pi$-$\pi$ stacking and electrostatic interactions. Energy decomposition analysis, IRI isosurface maps, IRI scatter diagrams, and AIM analysis reveal that the electrostatic component of nitrogen-doped AC plays a crucial role in the overall physisorption process. The adsorption intensity follows the order: AC-Pristine < AC-NH₂ < AC-Graphitic-N < AC-Graphitic-2N. Thus, the DMT and DMI molecules display the highest binding due to their molecular symmetries and charge distributions. This study demonstrates that combining $\pi$-$\pi$ stacking with electrostatic interactions is a promising strategy for enhancing the adsorption capacity of phthalate esters on carbon surfaces.